Wednesday, 22 January 2020

Fluctuations in mercury and organic carbon in the peatlands of southwest China before the End Permian Extinction.

Carbon has two stable isotopes, carbon¹² and carbon¹³, of which plants preferentially incorporate carbon¹² into their tissues as it requires less energy to fix; this means that sediments with a high plant-derived carbon content (such as coal bed) will tend to be enriched in carbon¹² relative to sediment without (such as marine limestones). Furthermore, an increase in carbon in the atmosphere from burning plant matter, either as forests or coal beds, will tend to lead to an increase in the relative amount of carbon¹² in all sediments, known to geochemists as a positive organic carbon isotope excursion, whereas an increase in atmospheric carbon from other sources, such as volcanic eruptions, will tend to lead to a a drop in carbon¹² in all sediments, or a negative organic carbon isotope excursion.The Permian-Triassic mass extinction was the most severe extinction event of the Phanerozoic, both in marine and terrestrial settings, but the relative timing of these crises is debated. A negative carbon isotope excursion in both carbonate and organic matter is seen at the main extinction horizon and is usually attributed to release of volcanic carbon. Most proposed kill mechanisms for the Permian-Triassic mass extinction are linked to the effects of Siberian Traps eruptions. A spike in mercury concentrations observed at the onset of the Permian-Triassic mass extinction, thought to be derived from Siberian eruptions, provides a chemostratigraphic marker in marine records. A similar mercury enrichment event has also been documented in contemporaneous terrestrial sediments. Marine records show widespread environmental instability prior to the Permian-Triassic mass extinction, and a new study of the Sydney Basin (New South Wales, Australia), suggests that the collapse of southern high-latitude floras occurred significantly before the onset of marine extinctions roughly coincident with onset of northern high latitude marine stress.

In a paper published in the journal Geology on 3 January 2020, Daoliang Chu of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Stephen Grasby of the Geological Survey of Canada, Haijun Song, also of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Jacopo Dal Corso of the School of Earth and Environment at the University of Leeds, Yao Wang, again of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, Tamsin Mather of the Department of Earth Sciences at the University of Oxford, Yuyang Wu, Huyue Song, Wenchao Shu, and Jinnan Tong, once again of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, and Paul Wignall, also of the School of Earth and Environment at the University of Leeds, evaluate the timing and nature of the terrestrial crisis at the End of the Permian in southwest China by examining variations in fossil charcoal abundance from paleo–tropical peatlands to explore changes in wildfire occurrence and the carbon-isotope composition of land plant cuticles, charcoal, and bulk organic matter to track changes in the isotopic composition of atmospheric carbon dioxide. In addition they investigated sedimentary mercury concentrations, and the integration of their record with carbon isotope values permits chemostratigraphic correlation of terrestrial and marine records.

Chu et al. examined the continental Permian-Triassic transition in cored borehole ZK4703, drilled 15 km south of Fuyuan County in Yunnan Province, China, and the Chinahe outcrop section, 30 km southeastern of Xuanwei City, both from the border area between western Guizhou and eastern Yunnan in southwestern China. Latest Permian to earliest Triassic terrestrial strata in this region include, in ascending order, the fluvial-coastal swamp facies of the Xuanwei and Kayitou Formations. The former consists of sandstone, mudstone, and common coal beds. The associated plant fossils belong to the Gigantopteris flora and include Pecopterids (Tree Ferns), Gigantopterids (a morphologically advanced group of Permian Vascular Plants that disappeared in the End Permian Extinction), Lycopsiales (Giant Club Mosses), and Equisetales (Horsetails) taxa, collectively regarded as tropical rainforest-type vegetation. The Kayitou Formation (latest Permian to earliest Triassic age) is similar to the underlying Xuanwei Formation, but lacks coal and is shale dominated. Previous studies showed that the loss of the Gigantopteris flora occurred in the lowest Kayitou Formation.

Late Permian to Early Triassic palaeogeographic map showing locations of the ZK4703 core (25.54151°N, 104.28994°E) and the Chinahe section (26.13077°N, 104.35637°E) in southwestern China, and the Meishan section, south China. Chu et al. (2020).

Organic carbon isotopes, charcoal abundance, fossil plant ranges, total organic carbon, total sulphur concentrations, and aluminium and Mercury contents were assessed through the Permian-Triassic transition in the ZK4703 core and at the Chinahe outcrop. To avoid facies variation issues, only mudstone samples were processed for charcoal. Some charcoal was examined under scanning electron microscope to confirm identification. To ensure that the charcoal concentrations were not affected by variations in the nature or abundance of organic material, its abundance was normalised to phytoclast abundance and total organic carbon.

 Location map of the studied section. Chinahe section (26.13077°N, 104.35637°E) is located in the Chinahe Viliage of the Tianba town, Xuanwei City. ZK4703 core (25.54151°N, 104.28994°E), drilled in Anzichong Viliage of Dahe Town, Qujing City. Chu et al. (2020).

The proportion of carbon¹² drops sharply in the lower part of the Kayitou Formation at Chinahe, both in organic matter and charcoal (a negative organic carbon isotope excursion). At the same time bulk organic matter and palynomorphs (pollen fossils) from the ZK4703 core section also show a drop in carbon¹² values.

Cuticle and charcoal particles under binocular microscope and scanning electron microscope. Chu et al. (2020).

The abundant, peat-forming Gigantopteris flora is seen at six levels in the Xuanwei Formation at Chinahe, and is dominated by well-preserved, large leaves. Both diversity and abundance of this flora decline drastically at the very top of the formation at a level that corresponds to the onset of the negative negative organic carbon isotope excursion. Thereafter, the flora consists of a monotonous assemblage of small plants, mostly Annalepis and Peltaspermum.

 Typical Gigantopteris flora from the Xuanwei Formation of the Chinahe section. (A) Gigantopteris dictyophylloides; (B) Annularia pingloensis; (C) Lobatannularia sp.; (D) Pecopteris marginata; (E) Gigantonoclea guizhouensis; (F) Pecopteris sp.; (G) Compsopteris contracta; (H) Abundant plant leaf fossils preserved on the same bedding surface. Chu et al. (2020).

At Chinahe, the charcoal abundance is less than 300 particles per 100 g rock prior to the a negative organic carbon isotope excursion, but rises briefly above background levels during the onset of the excursion (1524 particles per 100 g at 25 m log height), and ranges from 400 to 1600 particles per 100 g in the 4 m interval of the uppermost part of the Xuanwei Formation to lower part of the Kayitou Formation. Similarly, in the ZK4703 record there is a sharp increase in charcoal abundance, from under 400 particles per 100 g below 15 m, to over 2400 particles per 100 g above 16.5 m height at the base of the Kayitou Formation. Scanning electron microscope observation shows that the charcoal preserves anatomical details and has similar preservation and structures with variable size, indicating minimal transport sorting. The reported variations in charcoal abundance do not appear to be an artifact of preservation or changes in terrestrial organic delivery, because variations in preserved phytoclasts (microscopic plant fragments) and total organic carbon do not vary with charcoal abundance.

The plant fossil ranges and species richness of the Chinahe section. (1) Peltaspermum sp.; (2) Annalepis sp.; (3) Compsopteris contracta; (4) Fascipteris densata; (5) Cladophlebis permica; (6) Annularia pingloensis; (7) Compsopteris sp.; (8) Lobatannularia heianensis; (9) Pecopteris marginata; (10) Lobatannularia cathaysiana; (11) Pecopteris guizhouensis; (12). Rajahia guizhouensis; (13) Gigantonoclea sp.; (14) Stigmaria sp.; (15) Gigantonoclea guizhouensis; (16) Gigantopteris dictyophylloides; (17) Pecopteris sp.. Chu et al. (2020).

Mudstone total organic carbon concentrations are relatively high in the Xuanwei Formation and modestly enriched concentrations persist into the lower part of Kayitou Formation before dropping at the 27 m log height at Chinahe. Both overall mercury levels and the mercury-total organic carbon ratio rise above background levels immediately above the interval with the onset of the negative organic carbon isotope excursion and increased charcoal abundance. High overall mercury levels and the mercury-total organic carbon ratios can also be observed at higher stratigraphic levels, with a peak value at 19.75 m in the ZK4703 core, which is about 50 times background levels. Overall mercury levels and the mercury-total organic carbon ratio drop to the previous baseline values above 37 m at Chinahe and 25 m in ZK4703. The weak correlation between overall mercury levels and the mercury-total organic carbon ratios suggests that the mercury fluctuations are not affected by changes in total organic carbon. Additionally, the ZK4703 core has low total sulphur contents which show no significant covariation with mercury values. Correlation between Aluminium and Mercury concentrations is also weak, indicating that mercury fluctuations are not controlled primarily by clay content, even if some mercury is probably adsorbed onto clay. Nonetheless, there is secular variability in the mercury/aluminium ratio, with very low background mercury/aluminium values below and above the mercury anomaly and enriched mercury/aluminium values within the interval.

Volcanic emissions represent one of the largest natural inputs of mercury to the atmosphere, and the mercury enrichment seen in many marine Permian-Triassic boundary sequences is thought to record large-scale Siberian Traps eruptions. Volcanic mercury emissions from this source may have been up to 10 000 milligrammes per year (roughly 14 times natural background levels). Thermogenic release of mercury from baking of organic-rich sediments on contact with Siberian Traps intrusions is another potential source of mercury. Terrestrial plants constitute a large mercury reservoir, and so wildfires can also contribute significantly to mercury fluxes to the atmosphere and freshwater environments such as those studied by Chu et al., who propose that the mercury spikes observed in terrestrial and marine successions provide a useful correlative tool between terrestrial and marine records along with carbon isotope ratios.

The onset of the main phase of marine extinctions in South China, at the top of the Clarkina yini Zone, correlates with a peak in mercury concentrations and mercury/total organic carbon ratios, while a second phase of extinctions at the top of the Isarcicella staeschi Zone corresponds to a rise in mercury concentrations and mercury/total organic carbon ratios that peaks in the following Isarcicella isarcica Zone. The relative magnitude of these peaks varies between sections: at Meishan, South China, the lower mercury/total organic carbon ratios peak is the largest, whereas at Guryul Ravine, Kashmir, the second peak is larger. Levels of organic carbon began to decline somewhat before the marine extinctions in the Clarkina changxingensis Zone.

In the terrestrial sections of southwestern China, the floral mass extinction (and charcoal peak) starts with the onset of the negative organic carbon isotope excursion. Mercury concentrations begin to slightly rise at the same time, while mercury/total organic carbon ratios shows a sharp spike at the minimum of the negative organic carbon isotope excursion. The mercury and mercury/total organic carbon ratios peak 4–6 m above the terrestrial extinction level and likely correlate with the rise in mercury/total organic carbon ratios values seen at the end–Isarcicella staeschi Zone that saw diverse taxa disappear from Triassic oceans. Thus, the terrestrial crisis seen in equatorial sections of China appears to predate the main marine extinction phase (which occurred near the low point of negative organic carbon isotope excursion), and likely dates to the late Clarkina changxingensis Zone.

Chu et al.'s results demonstrate that a synchronous onset of the negative organic carbon isotope excursion is present in the bulk organic matter, cuticle, and charcoal carbon isotope records from terrestrial settings. Changes in organic carbon values of land plant cuticles record changes in atmospheric CO₂. Chu et al. suggest that the observed negative organic carbon isotope excursion in cuticles and fossil charcoal reflects an injection of carbon¹³-depleted emissions associated with the Siberian Traps. Interestingly, in our study, the peak in mercury concentrations and mercury/total organic carbon ratios also occurred after the onset of the negative carbon isotope excursion, suggesting a decoupling between the carbon and the mercury records that could result from the source of these two elements, be it volcanic, thermogenic, continental runoff, wildfire, or a combination of different reservoirs. Such decoupling deserves further investigation because it suggests different mechanisms of carbon and mercury release and/or processing in End Permian environments.

Studies have shown insignificant or constant fractionation of carbon isotopes during the burning process, and so charcoal carbon isotope ratios are a direct record of the original wood tissue carbon isotope ratios. The gradual decrease in the proportion of carbon¹³ in terrestrial plant-derived charcoal in the studied successions indicates, as discussed also for cuticle carbon isotope ratios, a change in the proportion of carbon¹³ in the original peat and vegetation due to changes in the proportion of carbon¹³ in the atmospheric CO₂. The charcoal carbon¹³ negative shift is coeval with an increase in charcoal abundance, i.e., with increased wildfire activity suggesting that the latest Permian forests experienced recurring wildfires and regrowth while the atmosphere became more carbon¹³ depleted. Additionally, burning of terrestrial plant biomass can also increase the emission of mercuty into the atmosphere, and then this mercury can be scavenged and buried in sediments.

The intensification of wildfire activity at the time of terrestrial mass extinction provides evidence of the harmful climatic changes in the lead-up to terrestrial crisis. The Gigantopteris coastal swamp flora thrived in humid, warm equatorial locations and was unlikely to have been adapted to intense levels of wildfire, as evidenced by the low charcoal abundance prior to the extinction interval. Thus, the increased wildfires suggest a transition to more unstable conditions punctuated by dry periods that would have been detrimental to coastal swamp floras. 

Chu et al.'s study sheds new light on the temporal links between the deterioration in the terrestrial environment and floral extinction, and the geochemical changes that mark the Permian-Triassic mass extinction. Their terrestrial mercury record from the Permian-Triassic transition shows a sharp peak contemporaneous with the disappearance of Permian flora that correlates with marine mercury records. Carbon isotope data from cuticles and fossil charcoal, thought to reflect changes in the carbon isotope composition of the atmosphere, show a negative carbon isotope excursion during the terrestrial flora mass extinction interval. However, this was prior to the increase in mercury concentrations. Charcoal abundance shows that the floral extinctions coincided with an increase of wildfire activity and the carbon-cycle disruption. This likely reflects a change from persistent humidity to an unstable climate with frequent drought episodes. The temporal relationships between the events show that terrestrial disruption occurred shortly (but measurably) before the marine crisis.

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Monday, 20 January 2020

Seven missing following avalanche on Mount Annapurna, Nepal.

Seven people are missing following a landslide on Mount Annapurna, Nepal, on Saturday 18 January 2020. The missing are described as three South Korean tourists, two men and a woman in their fifties, plus a woman in her thirties, plus three Nepalese tour guides. Several other parties of hikers who were in the area at the time were evacuated safely, but rescuers were unable to locate the missing party on Saturday, and could not return to the area due to poor weather, until late on Sunday, when it was found that their last known location was covered by snow five metres deep. It is thought unlikely that they have survived.

Rescue workers on Mount Annapurna, Nepal, on 18 January 2020. Phurba Ongel Sherpa/AP.

Avalanches are caused by the mechanical failure of snowpacks; essentially when the weight of the snow above a certain point exceeds the carrying capacity of the snow at that point to support its weight. This can happen for two reasons, because more snow falls upslope, causing the weight to rise, or because snow begins to melt downslope, causing the carrying capacity to fall. Avalanches may also be triggered by other events, such as Earthquakes or rockfalls. Contrary to what is often seen in films and on television, avalanches are not usually triggered by loud noises. Because snow forms layers, with each layer typically occurring due to a different snowfall, and having different physical properties, multiple avalanches can occur at the same spot, with the failure of a weaker layer losing to the loss of the snow above it, but other layers below left in place - to potentially fail later.

Diagrammatic representation of an avalanche, showing how layering of snow contributes to these events. Expedition Earth.

Mount Annapurna is a 55 km long massif reaching 8091 m above sealevel at its highest point, with sixteen more peaks that reach over 6000 m above sealevel, making it the tenth highest mountain in the world. Annapurna was the first mountain over 8000 m to be climbed (by Maurice Herzog in 1950), and is popular with both climbers and trekkers. Nonetheless, the mountain is considered to be one of the most dangerous high mountains in the world, with 61 known fatalities since 1990 (exceeded only by Mount Kangchenjunga, also in Nepal), including 43 people killed in a single storm in October 2014, which triggered a series of avalanches in areas popular with tourists.

The South Face of Mount Annapurna. Prajwal Mohan/Wikimedia Commons.

Nepal is located entirely within the Himalayas, a range of mountains formed by uplift associated with the collision between the Indian and Eurasian tectonic plates. The Indian Plate is moving northwards at a rate of 5 cm per year, causing it to impact into Eurasia, which is also moving northward, but only at a rate of 2 cm per year. When two tectonic plates collide in this way and one or both are oceanic then one will be subducted beneath the other (if one of the plates is continental then the other will be subducted), but if both plates are continental then subduction will not fully occur, but instead the plates will crumple, leading to folding and uplift (and quite a lot of Earthquakes). The collision of the Indian and Eurasian plates has lead to the formation of the Himalayan Mountains, the Tibetan Plateau, and the mountains of southwest China, Central Asia and the Hindu Kush.

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Three killed by explosion at chemical plant in Tarragona, Spain.

Three men have died and several more remain in serious conditions following an explosion at a chemical plant in the city of Tarragona in the Catalonia Region of Spain on Tuesday 14 January 2020. Two of the men worked at the plant, one of whom was killed instantly and the other died of his injuries the following day/ However, the third man, a 59-year-old identified only as Sergio, died in his home about 2.5 km from the blast, when the building was hit by the lid of the vessel in which the explosion occurred, described as a metal plate measuring 122 by 165 centimetres, and weighing about a tonne.

The trajectory followed by a metal plate thrown from an explosion at a chemical plant in Catalonia on 14 January 2020, killing a man 2.5 km from the site. El País.

The plant where the explosion occurred was manufacturing ethylene glycol, C₂H₄O, a highly volatile gas used primarily in the manufacture of ethylene glycol, which it the major component of most commercially available anti-freezes and a precursor of polyester. Ethylene oxide is an explosive gas at normal pressures and temperatures above -10°C, and is usually stored under pressure as a liquid. However, should a vessel containing ethylene oxide develop a leak, or otherwise be compromised, the escaping gas will remain explosive at even low concentrations in the atmosphere.

Fire caused by an explosion at a chemical plant in Taragona, Spain, on 14 January 2020. Reuters.

As well as being highly explosive, ethylene oxide is also toxic and carcinogenic, which led to residents of the area being ordered to remain inside their homes until it was certain that there was no release of toxic material, although the efficient combustion of the chemical proved to have made the incident safe quite quickly. In addition to the fatalities, a further seven workers at the plant were injured in the incident, with described as remaining in a serious condition at Barcelona’s Vall d’Hebron Hospital. Local civil defence authorities have expressed extreme concern that they were not informed of the event by the site's management, but rather were called by residents of the area, delaying access to the site by firefighters, who needed to ascertain the nature of the incident before entering the facility.

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Sunday, 19 January 2020

Magnitude 6.4 Earthquake in western Xinjoang Province, China.

The China Earthquake Networks Center recorded a Magnitude 6.4 Earthquake at a depth of 16 km in the Tian Shan Mountains of northwestern Xinjiang Province, China, slightly after 9.20 pm local time (slightly after 1.20 pm GMT) on Sunday 19 January 2020. There are no reports of any damage or injuries associated with this event, but it was felt across a wide area of Xinjiang Province and neighbouring Kyrgyzstan.
The approximate location of the 19 January 2020 Xinjiang Earthquake. USGS.
The Tian Shan Mountains stretch for 2500 km across Xinjiang, Kazakhstan, Kyrgyzstan and Uzbekistan. The Tian Shan are part of the Himalayan Orogenic Belt, mountains in Central Asia pushed upwards by the collision of India and Asia. The Indian Plate is currently pushing into the Eurasian Plate from the south at a rate of 3 cm per year. Since both are continental plates, which do not subduct, the Eurasian Plate is folding and buckling, causing uplift in the Himalayas and other mountains of Central Asia. This is not a smooth process, the rocks will remain effectively stationary for log periods of time while pressure builds up, then give suddenly, releasing large amounts of energy in the form of Earthquakes.

The movement of India relative to Asia, and the blocks within the eastern part if the Eurasian Plate. University of Wollongong.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

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The Bayelsa State Oil and Environmental Commission's Interim Report on oil pollution in Bayelsa State, Nigeria.

Few countries on the face of the planet have suffered more from oil pollution than Nigeria. Over the last half century, as many as ten million barrels of oil have been spilled across the country, equivalent to a spill similar in size to the Exxon Valdez catastrophe, which devastated the coast of Alaska. every single year for the last fifty years. Furthermore, few parts of Nigeria have suffered worse pollution than Bayelsa State. Bayelsa is one of Nigeria’s main oil producing states, accounting for almost a quarter of its onshore crude oil production, and approximately a third of its oil wealth. It is home to one of Africa’s most diverse ecosystems, a rich but fragile tapestry of wetlands and mangrove swamps. Despite its immense oil reserves, Bayelsa’s people are poor, with the state scoring lower on the United NationsHuman Development Index than any other Nigerian state. Almost three quarters of Bayelsa’s two million people rely on fishing or farming to support themselves. Since oil was first pumped in 1956 by Shell, Bayelsa has suffered a pollution catastrophe on a barely imaginable scale. Exact numbers are hard to come by. However, analysis suggests that if Bayelsa’s share of oil spilled is the same is as its share of oil pumped, as much as a barrel of oil may have been spilled for every man, woman and child living in Bayelsa today. The impact has been devastating. The environmental damage has been tremendous and unique ecosystems have been destroyed. The health of hundreds of thousands of people has been affected by the contamination of the water they drink, the land they grow food on and the air they breathe. Estimates suggest that the pollution could be responsible for as many as 16 000 infant deaths in one year alone.

The Bayelsa State Oil and Environmental Commission was established by Henry Seriake Dickson, Governor of Bayelsa State on 26th March 2019. Its mission is to gather and assess the evidence on the scale and impact of oil spills and associated pollution in Bayelsa state and develop recommendations to ensure existing pollution is cleaned up and future pollution is prevented. The Commission is made up of Dr John Sentamu, Archbishop of York, His Excellency John Kufuor, former President of Ghana, Baroness Valerie Amos, former Under-Secretary General of Humanitarian Assistance at the United Nations, Dr Kathryn Nwajiaku-Dahou of the Department of Politics and International Relations at the University of Oxford, Michael Watts, Professor of Geography at the University of California, Berkeley, Dr Anna Zalik, Associate Professor at the Faculty of Environmental Studies at York University, Dr Isaac ‘Asume’ Osuoka of Social Action International, Professor Engobo Emeseh, Head of School of Law at the University of Bradford, and Roland Hodler, Professor of Public Economics at the University of St Gallen

Since its launch, the Bayelsa State Oil and Environmental Commission has conducted a deep assessment of the evidence on oil pollution in Bayelsa. It has conducted an extensive review of the available literature and documents, has commissioned detailed studies on aspects of the impact of pollution and has conducted over 500 meetings with stakeholders, both internationally and within Nigeria. The Commission's work has been supported by an international network of environmental scientists and forensic experts, a local expert research team and a network of civil society actors with a long track record of documenting oil related environmental damage in the Niger Delta.

The Commissioners have undertaken three visits to the state, held town hall meetings with representatives from eight Local Government Areas (Brass, Ekeremor, Southern Ijaw, Obgia, Kolokuma/Opokuma, Sagbama, Yenagoa), conducted on-site visits with a local team of researchers.

In addition to meeting with communities who have experienced the direct impacts of oil pollution, the Bayelsa State Oil and Environmental Commission has also met with professionals (legal, health, oil servicing companies) civil society organisations, and officials at all levels of government (notably at the Ministry of the Environment, and representatives of regulatory institutions responsible for assessment working in and on Bayelsa (Bayelsa State Ministry of the Environment and state level staff of the Nigerian National Oil Spill Detection and Response Agency). The Commission has also received a written submission from one of the three main international oil companies operating Joint Ventures in the state (Shell Nigeria). Efforts are ongoing to secure oral and written testimony from all international oil companies operating in the state.

The Bayelsa State Oil and Environmental Commission will publish its full report in 2020. It will present the Commission’s findings on the scale and scope of oil pollution in Bayelsa and its causes and will lay out proposals for the remediation of the damage done-both environmental and social-as well as recommendations for changes to the legal, policy and regulatory framework to prevent further pollution in future. However, it published an Interim Report on 1 November 2019, laying out its concerns about oil pollution in Bayalsa State.

Members of a community affected by oil pollution in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

Thousands of communities and countless people in Bayelsa Stae have been cast into poverty with their livelihoods destroyed, due to pollution related problems. Communities have been de-stabilised and their cohesion undermined by disputes and competition for resources arising from oil extraction which have been sharpened by spills and their impacts. The cost-in terms of environmental degradation and human suffering has been vast. And it is rising every day.

Furthermore, the individuals and communities affected have found it almost impossible to win redress for their suffering. The oil companies have not done enough to put right the damage that has been caused and the cost and process involved have prevented communities from pursuing legal action.

The Bayelsa State Oil and Environmental Commission has been set up to document this continuing catastrophe and to devise solutions that will help bring an end to pollution crisis and the suffering it has caused. It brings together leaders from the world of government, non-governmental organisations and faith communities with international experts.

One of the Bayelsa State Oil and Environmental Commission’s priorities has been to hear from the people who have been directly affected by the oil contamination. All too often their voices have been ignored. Over six months in 2019, the Bayelsa State Oil and Environmental Commission visited pollution sites across Bayelsa, held meetings in several communities and taken testimony from over 500 victims of oil pollution.

Boatman in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

Oil spills and the associated pollution have had untold effects on the environment of Bayelsa State. The loss of habitat and biodiversity, including mangrove swamps as a result of oil spills as well as acid rain damage from gas flaring, has been enormous. Huge swathes of fragile wetlands have been destroyed or put at risk. Water courses that local people rely on for fishing have been contaminated. Farmland has been tainted. There have even been reports of seismic tremors that communities believe are caused by oil extraction.

The members of the Bayelsa State Oil and Environmental Commission have observed many of these problems first-hand. On the Commission’s visit to the Brass Canal, members of the Commission witnessed polluted effluent being dumped directly into the waterway. During another field visit, the Commission witnessed polluted topsoil being dug up and burnt in a pit. Throughout, the Commission has heard repeated testimony on the indiscriminate dumping of waste and the impact it has had. 

A dwelling close to a polluted waterway in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

Oil contamination has tainted the farmland people grow their food on, the water they drink and fish in and even the very air they breathe. The health implications have been complex, and often devastating. For instance, research has found that people living near pollution sites have been progressively exposed to elevated levels of heavy metals such as chromium, lead and mercury in their blood stream, leading to increased risk of diseases ranging from Alzheimer’s and Parkinson’s diseases to cancer, diabetes and kidney damage.

The contamination of crops and fish by oil spills has shown to increase the outbreak of diarrhoea in Bayelsa and the bioaccumulation of heavy metals in food as well as affecting food quality. The presence of oil has also resulted in a substantial increase in the prevalence of childhood malnutrition in the affected communities. More broadly, evidence from across Nigeria suggests that high levels of pollution have also contributed to significant increases in child mortality.

Research has also highlighted that communities living near oil impacted areas frequently consume drinking water with high levels of pollution. The list of health effects is long, and their impact will be felt for years, maybe decades to come. What the Commission has seen is just the tip of the iceberg. But it is clear that many people in Bayelsa have suffered life-changing health consequences as a result of oil pollution.

One participant from the Aghoro 1 Community in the Ekeremor Local Government Area noted that a spill occurred in May 2018 and that the community recorded deaths and the destruction of mangrove creeks and farmlands. Letters were written to Shell through the community interface coordinator, but Shell did not respond.

Villagers in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

Another member of the same community stated 'when the spill occurred, it was a thing of battle for us in the environment. We really suffered it. Our houses were nearly set ablaze. The spill spoiled the water. We could not bathe or drink the water. The spill killed the Fish in the river. This caused a lot of sicknesses in the community and it killed a lot of people. Many children died because of the spill. We cannot do otherwise than to starve. We waited for relief materials and only few people received it. No food, we have been starving up until this time.'

In Oporoma Community in Southern Ijaw participants noted that there is a high prevalence of ailments in the community, ranging from skin rashes, respiratory illnesses, pneumonia, and growths in female genitals with causes unknown. There are also no near functional health facilities which can be accessed by the community.

About three quarters of Bayelsa’s population relies on farming and fishing to survive. Not only their health, but also their livelihood are at risk when oil pollution poisons their environment. Through its hearings and evidence gathering, the Commission has come across case after case where individuals and communities have lost their livelihoods and, in some cases, reduced to destitution as the result of oil spills.

Oil spills have destroyed many community’s ability to make a living from the land they farm and the water courses they fish. And even where pollution has been more limited, communities have often failed to reap the benefit of oil extraction.

While oil extraction has generated substantial profits for oil companies and tax revenues for the Federal Government, host communities rarely see the benefits. There are often no alternative livelihoods or development benefits. There is widespread under-investment and lack of jobs in communities that host oil company activities and oil companies’ investment in local communities has been infrequent and sporadic.

Projects are often not completed and offers to upskill and train those in the local communities are not fulfilled, leaving many in those communities frustrated and angry at the economic exclusion by oil companies.

One witnes in the Brass Local Government Area stated 'Agip has been here for so many years. They say they have improved on employment level. I am a woman. I was born and bred here. I have lived all my life. This is my London. After graduation they do not employ us. They invite us for interviews, but no job. They employ few labourers and cleaners. There was a time they met the different Amas of Twon to bring in some females for training to be employed. These ladies went for the training. At the end, they brought females from outside. They employed only one or two, just to fulfil all righteousness. When we wanted to cause trouble, the military men were given orders to kill at sight all those who are causing trouble. They have employed people outside this island. But not of the host community'.

Farmers in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

Both oil company activity and the competition for resources they have created have undermined community stability and cohesion. As a result, communities that play host to oil activities have often been plagued by conflict, violence and exploitation of vulnerable groups, including children. Oil companies often chose to work with certain groups within communities which exacerbates internal divisions, pitting communities against each other for much needed funds and to be represented during the remediation process.

If the damage done by oil pollution is clear, the path to redress, remediation and compensation often is not. Victims are often unable to pursue their claims due to process, time and most importantly cost. Even where they do, the time taken by the judicial process and potential issues with independence of the judiciary often makes gaining recompense and justice almost impossible. The deck all too often feels stacked against the people who have suffered.

One lawyer representing communities in the area stated 'we are pursuing on the payment that Agip refuses to pay, they have appealed to the Supreme Court with a motion for leave at the Court Of Appeal. For a date to be given, it takes a number of days. These are indigenous communities. I have to finance these matters. Some of these communities cannot pay a surveyor, environmentalist and a valuer.'

Another lawyer observed 'You discover that all the oil companies have their offices in Port Harcourt or Lagos. Engaging a lawyer to go to Port Harcourt is difficult. If you do not have money you cannot handle these cases. Most of the communities do not have a dime to assist you. It is impossible. I have declined a number of cases. There was a directive that they should have their head offices in areas where they operate. Until recently, we never had a Federal High Court. We had to go to Port Harcourt which was a fundamental problem. We also had issues with the filing fees, 50 000 to 100 000 naira (US$140-280) before you could file. These are subsisting fishermen. The burden is transferred to the lawyer and to the agent. Even where you have the Power Of Attorney and you cannot go further, you settle for pittance.'

Fisherwoman in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

For over 50 years, oil company activity and its associated impacts have caused untold devastation across Bayelsa State. Hundreds of thousands of people in Bayelsa have been forced to live on contaminated land, drink and fish in contaminated water and breathe contaminated air. Mortality and morbidity rates have risen sharply, as has the incidence of chronic disease, in communities without the resources to cope. Countless lives and livelihoods are being destroyed. Thousands of communities and tens if not hundreds of thousands of people have seen their land and fishing grounds poisoned. Neo-natal death and child malnutrition has risen, and hundreds of thousands have been forced into abject poverty.

The problem has been ignored for too long. And even when the world has paid attention, little has happened. Previous reports have merely sat on the shelf, gathering dust. Action is needed and needed now. The oil companies are beginning to divest from onshore projects in favour of offshore deep-water sites where returns are higher, and risks of environmental damage and social unrest are lower. Time is running out to hold them to account for the legacy of pollution and suffering they are leaving behind. But there is an opportunity for real change.

The Bayelsa State Oil and Environmental Commission’s final report will be published in 2020. It will tackle the causes of this catastrophe and lay out measures to remediate the damage that has already been done and to ensure further spills can be avoided.

Damage caused by oil operations in Bayelsa State, Nigeria. Bayelsa State Oil and Environmental Commission.

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Saturday, 18 January 2020

Looking for the source of the Australasian Strewn Field Tektites.

The Australasian Strewn Field, a horizon of glassy clasts ('tektites') quenched from molten ejecta of a bolide impact about 790 000 years ago, extends across about ⅒th of the Earth’s surface, from Indochina to East Antarctica and from the Indian to western Pacific Oceans. The northwestward increase in both the abundance and the size of tektite specimens points to the impact site being in eastern central Indochina. This is within the region of Muong Nong-type tektites, the least streamlined, most volatile-rich, most siliceous, and largest of the ejected melt fragments. Their high silica content, relict grains, and other chemical characteristics indicate primarily quartz-rich coarse siltstone to fine sandstone target rocks, perhaps of Jurassic age. Concentrations of microtektites and iridium in contemporaneous marine sediments more than a 1000 km away from the impact region yield very poorly constrained estimates of crater diameter, ranging from 15 to 300 km. Given these large crater sizes, it is remarkable that the many searches of the past half-century have yielded neither a definitive impact site nor a proximal ejecta blanket. This failure implies either that a crater never formed, or that either burial or erosion has obscured it.

In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on  January 2020, Kerry Sieh and Jason Herrin of the Earth Observatory of Singapore at Nanyang Technological University, Brian Jicha of the Department of Geoscience at the University of Wisconsin–Madison, Dayana Schonwalder Angel, James Moore, and Paramesh Banerjee, also of the Earth Observatory of Singapore at Nanyang Technological University, Weerachat Wiwegwin of the Department of Mineral Resources at the Ministry of Natural Resources and Environment of Thailand, Vanpheng Sihavong of the Department of Geology and Mines at the Ministry of Energy and Mines of the Lao People’s Democratic Republic, Brad Singer, also of the Department of Geoscience at the University of Wisconsin–Madison, Tawachai Chualaowanich, also of the Department of Mineral Resources at the Ministry of Natural Resources and Environment of Thailand, and Punya Charusiri of the Department of Geology at Chulalongkorn University, present evidence for the Australasian Strewn Field Tektite Impact Crater being located beneath the Bolaven Volcanic Field of southern Laos.

The Bolaven Plateau Volcanic Field likely buries the impact crater that produced the tektites of the Australasian Strewn Field. It is the only adequately large and thick postimpact deposit on the Khorat Plateau, the largest region of plausible target rocks. It is also the only thick, postimpact deposit within the inner Muong Nong strewn field, the region containing exclusively nonaerodynamically shaped Muong-Nong–type tektites (circumscribed by the blue ellipse). (Inset) Finds of Australasian Tektites and Microtektites (white dots) define an asymmetric strewn field (blue). Sieh et al. (2020).

The most promising place to look for either an eroded or a buried crater is within the largest, contiguous expanse of fine grained, siliclastic (sedimentary rock largely composed of silica compounds) Mesozoic sedimentary rocks in the region, the Khorat Plateau. However, obscuration by erosion within this 155 000 km² region of predominantly gentle topography is not plausible. The crater rim is likely to have risen more than 100 m above the target surface, but post impact erosion of the region by the Mekong River and its tributaries has been far less than this. This is clear from the facts that tektites occur in situ primarily on gentle surfaces no more than a few tens of meters above modern nearby streams and that pre-impact basalt flows cover surfaces that are only about 50 m above the stream bed of the modern Mekong River.

Evidence that erosion does not obscure the impact crater. Height of pre-impact surfaces and lava flows above nearby modern stream channels at our sites in eastern Thailand and along the Mekong River and a major tributary. These demonstrate that post-impact incision by the Mekong River system into the rocks and sediments of the Khorat Plateau is too slight to have obliterated the primary features of a large impact crater. Squares indicate post-impact lava flows; circles indicate tektite site. Sieh et al. (2020).

Moreover, field examinations of candidates for an eroded crater, several large, circular, low-relief features in central Laos and northern Cambodia, have shown that these are, instead, eroded synclines in Mesozoic rock (i.e. folds in the rock that projected upwards, but which have eroded away, leaving exposures that cut through the same layers on either side, sometimes resembling a crater). Likewise, examination of a proposed crater in northeast Cambodia revealed that it is, in fact, the top of a granitoid pluton surrounded by a resistant contact-metamorphic quartzite aureole derived from surrounding Mesozoic sandstone. Another candidate crater in southern China appears to have a similar origin.

Burial of the impact crater might also seem unlikely, because adequately wide and thick post impact sedimentation is nearly absent on the Khorat Plateau. The only notable exception is an extensive basaltic volcanic field centred on the Bolaven Plateau in Southern Laos. Siah et al. present evidence below that this thick pile of volcanic rocks does indeed bury the site of the impact.

The 6000 km² Bolaven Plateau rises about 1 km above the Khorat Plateau in Southern Laos, east of the Mekong River. Fine-grained, nearly flat-lying Mesozoic quartz sandstones and mudstones underlie this elevated surface and crop out almost continuously around its cliffy perimeter. Judging from the nearly vertical pitches at the top of this perimeter cliff and from outcrops on the plateau, the uppermost 200–250 m of the Mesozoic bedrock comprise massive to cross-bedded fluvial sandstones. Gentler slopes below indicate that friable mudstones dominate the underlying 250 m. If the Australasian bolide struck the Bolaven Plateau, this 500 m thick sandstone–mudstone sequence would have comprised much of the impacted target rock.

The Bolaven volcanic field covers much of the Bolaven Plateau and spills over its kilometre high flanks to the floodplain of the Mekong River. The perimeter cliffs of the plateau expose nearly flat-lying Mesozoic sandstones and mudstones like those inferred from tektite composition to be the dominant target rocks of the Australasian impactor. Argon⁴⁰-argon³⁹ ages on lavas appear as dots coloured according to age. Siah et al. (2020).

However, a basaltic volcanic field that covers an area of about 5000 km² caps these rocks and spills down the flanks of the plateau. Structure contours drawn on the Mesozoic bedrock/basalt contact by interpolating under the lavas between bedrock outcrops allowed Siah et al. to create an isopach map of the volcanic field. From this they calculate a volcanic volume of about 910 km³. In the vicinity of the summit region of the volcanic field, the basalt is up to 300 m thick. This extent and thickness are great enough to hide a crater up to 15 km in diameter and with a rim that rises up to a couple hundred meters above the bedrock surface.

Detailed map of the Bolaven Plateau region. Screenshot of detailed geological map of the Bolaven Plateau and surroundings created by analysis of Shuttle Radar Topography Mission 30 m topography and follow-up fieldwork. In preparation for collection of samples for geochronology and to evaluate opportunities to test the Bolaven impact hypothesis in the field. Siah et al. (2020).

Siah et al. offer four tests of the hypothesis that the Australasian impact crater lies buried beneath the basaltic lavas of the Bolaven Plateau. First, they examined published geochemical analyses of the tektites to test whether or not they could include a basaltic component. If the bolide that created the Australasian tektites impacted a location that had a cover of mafic lavas, then the Bolaven volcanic field would be a prime candidate for the impact site. Second, they used the argon⁴⁰-argon³⁹ dating method to date many of the flows comprising the volcanic field, to see if they both antedate and postdate the impact. The presence of basalts older than the impact date would imply a contribution to the ejected materials. Basalts younger than the impact would need to wholly mantle the proposed impact site. Third, they conducted a field program of gravity measurements to see if there is a gravity anomaly that would reflect a large buried crater. And fourth, they search for coarse proximal ejecta with shocked quartz, as evidence for an impact site under the basalts of the plateau.

Thickness of basalts of the Bolaven volcanic field. Thickness of basalt flows, constructed by subtracting elevations of the contact between basalt and underlying Mesozoic bedrock from elevations of the top of the basalts. Note the 25 km wide basaltic cap atop the plateau and the thick western-canyon fill and northern fans. Black dots are locations of our gravity measurements, and the black ellipse indicates the proposed crater location, from analysis of the gravity field. Shading is for lighting from both 0° and 90°. The sharp discontinuity just inside the southern, western and northern perimeters of the lavas is an artifact of the way the base of the basalt has been contoured, it was assumed that elevations were uniform on each side and that there is a 50 m step up along that line. Siah et al. (2020).

Previous investigations have observed magnesium concentrations in Australasian Tektites higher than are typical in siliciclastic sediments and have proposed a Mafic (iron-rich volcanic) component within the target rocks. Sporadic enrichment in nickel cobalt, and chromium, without a concomitant enrichment of highly siderophile elements ('iron-loving' elements; i.e. elements which will dissolve in iron, such as  ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum, and gold), also points to a mafic terrestrial source.

To examine further the possible presence of mafic rocks at the impact site at the time of impact, Siah et al/ performed an analysis of published major-element compositions of 241 Australasian Tektites from various locations. They found that over 90% of the observed chemical variation can be readily explained by mixing of Mesozoic sequences of the Khorat Plateau with Bolaven basalts. with more distal tektites, such as Australites, tending toward higher proportions of basalt.

Histogram of relative contributions of Bolaven basalt and Bolaven Mesozoic rocks to best-fit solutions for 241 Australasian Tektites. Major-element compositions of tektites are consistent with a modal average of 30-40 weight percent addition of a basaltic component to a fluvial sandstone precursor. A small group of Australites are consistent with higher basalt contributions in the range 55-70% by weight. Siah et al. (2020).

Variations in the strontium isotopic composition of Australasian Tektites also show mixing of a low-strontium, high strontium⁸⁷/strontium⁸⁶ end member with a high-strontium, less-radiogenic component (strontium isotopes vary a great deal with local geology and hydrology, and are useful for determining the origin of displaced rocks), again consistent with an admixture of Mesozoic bedrock with Bolaven volcanics and their weathering derivatives. Similarly, the more basalt-like strontium component is expressed to a greater extent in more distal tektites, the Australites in particular.

Characteristically high levels of the beryllium isotope berylium¹⁰ in Australasian tektites is also noteworthy because it places considerable constraint on their genesis. Beryllium¹⁰ is formed when cosmic rays hit oxygen or nitrogen molecules in the atmosphere, causing them to lose protons and neutrons, a process called spallation. Elevated ¹⁰berylium implies that the impacted rocks contained a significant fraction of materials exposed to near-surface conditions within the few million years prior to the impact.

Schematic depiction of tektite trajectories out of the impact crater offer an explanation for distal tektites having a higher component of basalt than proximal tektites. Cross-section of impact crater depicts basalt and basalt-derived saprolites overlying a laterite surface and a Mesozoic fluvial sequences of sandstone and mudstone. Trajectory (1) would be the path of the most-distal ejecta, Australites and Antarctic Microtektites. Their chemistry is closest to that of the basalt-rich layers. The ratio of basalt to bedrock along Trajectory (1) is high relative to the ratio along Trajectories (2) and (3). Trajectory (2) represents the paths of intermediate-range ejecta, such as splash form tektites from Indonesia, the Philippines, southern China, and greater Indochina. Trajectory (3) represents the paths of the most-proximal ejecta, the Muong Nong Tektites, found predominantly in southern Laos and eastern Thailand. Siah et al. (2020).
Siah et al. observe that basalts on the plateau weather largely to clay-rich saprolite (granite-derived soil) within a couple of hundred thousand years. These clayey layers are well-suited for absorption and retention of meteoric berylium¹⁰, which, unlike berylium¹⁰ produced in minerals and commonly used in determination of surface-exposure ages, forms by spallation of nitrogen and oxygen in the atmosphere and precipitates onto and into surface layers. Stacking of successive basalt flows would create a sequence of berylium¹⁰-enriched saprolites many times thicker than what could be achieved on the erosional surface cut into the Mesozoic sandstones of the Bolaven Plateau. Thus, we propose that a stack of weathered pre-impact basalt flows accounts for the anomalously high berylium¹⁰ concentrations observed in Australasian Tektites. As with the geochemical trends described above, the increasing enrichment of berylium¹⁰ with distance from Indochina  is consistent with ejection trajectories that yield a greater basaltic component farther from the impact.

If lavas bury the impact crater, they must be younger than the impact. Conversely, if there is a component of basalt in the tektites, thee must also be flows there that antedate it. Radioisotopic dating of flows on the plateau offers a test whether both of these two requirements are met.

Argon⁴⁰-argon³⁹ dating relies on determining the ratio of non-radioactive argon⁴⁰ to radioactive argon³⁹ within minerals from igneous or metamorphic rock (in this case volcanic ash) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of argon⁴⁰ to argon³⁹ is constant in the atmosphere, where argon³⁹ is formed by the spallation of argon⁴⁰ at a constant rate, and this ratio will be preserved in a mineral at the time of crystallisation. No further argon³⁹ will enter the mineral from this point, but argon⁴⁰ is produced by the decay of radioactive potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.
Three published argon⁴⁰-argon³⁹ dates for the Bolaven lavas, ranging from 16 million to 50 000 years old, span the impact date. However, these dates are too few and too far from the proposed impact site to test either hypothesis.

The youthful appearance of the volcanic landforms in the vicinity of the summit and down most of the northern and southern flanks of the plateau suggests that most exposed flows are Late Quaternary in age. Most of the exposed flows and cinder cones of the Bolaven field do not exhibit appreciable erosion, despite the region’s heavy tropical rainfall (about 150 cm per year). Canyons erode into only restricted, steep portions of the western and southern flanks of the field. Moreover, large tracts of the northern and southern field sport very thin tropical soils and exhibit scant saprolitization (granite weathering).

Photographs of several exposures of the young lava flows. (a) Exposure of 779 000 year old basalt flow in a quarry near the western edge of the Bolaven plateau. Most of the exposure is thick, clayey saprolite derived from the basalt flow, of which only un-weathered core stones remain. This flow is incised several hundred meters by streams that plunge over the western flank of the plateau. (b) Un-weathered core stones from this 3 m high outcrop near the northwestern edge of the Bolaven plateau yielded a date of 215 000 years before the present. Clayey saprolite comprises most of the outcrop. (c) Basalt boulders at the flow front of a very young flow on the northern edge of the Bolaven volcanic field. Presence of boulders at the surface indicates scant soil development and perhaps a Holocene age. (d) Collapsed lava tube near the terminus of the most sparsely vegetated flow, at the southern edge of the Bolaven volcanic field. This flow yielded the youngest argon⁴⁰-argon³⁹ plateau ages, about 27 000 years before the present. Siah et al. (2020).

Siah et al. dated 37 exposed flows via argon⁴⁰-argon³⁹ incremental-heating experiments. The dating strategy was two-pronged: They targeted a suite of lavas that spans the spectrum of geomorphologically young to old flows––that is, from those that exhibit little to no erosion or soil formation to those that are highly dissected and saprolitized and lack preserved upper-flow surfaces. They also focused on flows near the summit region, at and adjacent to the proposed epicentre of the impact. All analyses are of ground mass, so the argon⁴⁰-argon³⁹ dates reflect the time since cooling of the flows. None of the lavas contain significant excess argon and all of the samples produced dates consistent with the argon present having an atmospheric origin.

The dates show that eruptions occurred over a sustained period of time, from about 16 million years ago to about 27 000 years ago. Fourteen samples antedate the impact, 21 postdate it, and two are approximately contemporaneous with the impact.

All twelve dates from lavas in the summit region and directly above the proposed crater are distinctly younger than the date of the impact. Moreover, all but two of the dated flows within 8 km of the hypothetical crater perimeter are younger than or close to the date of the impact, ranging from about 51 000 to about 779 000 years old. The two exceptions are these are: (i) An approximately 1.26 million year date about 7 km west of the inferred crater rim and buried about 55 m below a nearby surface flow with a date indistinguishable from the 790 000 years ago date of the impact, and (ii) an approximately 12 million year date from a 200-m-wide mound of highly weathered basalt about 6 km southeast of the inferred crater rim. This volcanic lump may be the remnant of a small scoria (basaltic lava) cone serendipitously left uncovered by the ejecta blanket and post impact lavas.

The fact that all of the dates from lava flows above the proposed crater and most dates nearby are younger than the impact lends support to the hypothesis that Bolaven lavas fill the impact crater and completely obscure it. Conversely, pre-impact ages for many flows on the periphery of the volcanic field, imply that there are basaltic lavas, now buried beneath the young summit lavas that were impacted by the bolide, as reflected especially in the chemistry of the more distal tektites.

Geophysists can use the local gravity fields to determine the nature of buried rocks and other structures. This is because gravity related directly to mass, so denser matter (high mass to volume) excerpts a higher gravitational pull than less dense (low mass to volume) matter. Local gravitational fields can be described as being positive (indicating dense buried matter) or negative (indicating low density buried matter) relative to the surrounding area.
If there were a large crater buried beneath the summit region of the Bolaven Plateau, it would likely be apparent in the local gravity field. For example, if dense basalt fills the portion of the crater that is below the plane of the eroded bedrock surface, then it would manifest in a positive gravity anomaly consistent with its horizontal dimensions. Alternatively, if loose impact debris fills this lower part of the crater, the gravity field would exhibit a negative anomaly. If this part of the crater fill is a combination of basalt and impact debris, then the sign of the anomaly would depend on which of the deposits were prevalent. The presence of an ejecta blanket, perhaps 100 or 200 meters thick at the crater rim and lying atop the pre-impact surface, should manifest as a negative anomaly.

In search of such an anomaly, Siah et al. measured gravity at 404 locations, focused upon the summit region of the volcanic field but extending well beyond the Bolaven Plateau’s perimeter, to constrain the regional gravity signal. The gravity map obtained exhibits a regional southwest-to-northeast negative gradient ornamented with several smaller anomalies on the plateau.

(a) Bouguer gravity map of the Bolaven Plateau and surrounding region that assumes a uniform density typical for sandstone (2400 kg m³) and a terrain correction up to 170 km. Smaller anomalies on the plateau interrupt the pronounced regional southwest-to-northeast gradient. This map reflects the subtraction of the -53 mGal average value of all points, so that the map contours centre on a value of zero. Contour interval is 1 mGal. Black dots indicate all but the most distal of the 404 measurement locations. Contour interval is 1 mGal, and thicker contours appear at intervals of 4 mGal. Regions further than ~5km away from any measurements are grey. Proposed crater location outlined by black ellipse. (b) Regional map, where bounds of Figure (a) are marked with a red box. Siah et al. (2020).

Of particular interest is a 20 km wide, roughly 8 mGal (milligal) anomaly in the summit region of the volcanic field. Siah et al. processed the Bouguer gravity field to account for contributions from basalt flows and low-density components in the western-canyon fill and northern fan to yield a gravity field, in which a large negative anomaly still remains within the region of the suspected impact crater.

Fans removed map generated by the removal of regional gradient using a best-fit bi-linear ramp and from removal of the western- canyon and northern-flank anomalies by assuming substantial amounts of low- density basalt-derived alluvium and saprolite in those locales. Siah et al. assume a material density of 1800 kg m-3 for these materials, based upon an initial density 2400 kg m³ and 25% unfilled pore space. The best fit for north-flank fan thicknesses result from simultaneous minimization of the L2 norm of (BR + correction terms + ramp) using a simulated annealing method and 1000 starting search points. Proposed crater location outlined by black ellipse. Siah et al. (2020).

Most of that remaining negative 6 mGal anomaly disappeared when iah et al. replaced a portion of the basalt with a 100 m- hick, elliptical lens of low density breccia within an elongated crater that is about 13 km wide and 17 km long. An ellipticity of around 30% would correspond to an impact angle of about 10°. Of course, the use of this simple lens of low-density material would be a simplification of the actual geometry of materials related to the proposed crater.

Cross-section through the summit region of the Bolaven Plateau volcanic field that assumes a buried crater with a geometry nearly identical to that published for the similar-sized Ries Crater in southern Germany. No vertical exaggeration; horizontal lines are at 200 m intervals. Siah et al. (2020).

The gravity anomaly cannot reflect the presence of a volcanic caldera, buried beneath the lavas, because calderas are features associated with large crustal magma reservoirs beneath composite volcanoes. The Bolaven field and similar intraplate volcanic fields comprise scattered, low-eruption-volume scoria cones and flows that reflect the rise of individual batches of magma. The absence of long-lived, localised composite volcanoes on the Bolaven Plateau implies the absence of an underlying crustal magma reservoir. Thus, the presence of large crustal magma volumes characteristic of calderas is unlikely. Further evidence against the existence of a buried caldera is the lack of Quaternary Age ignimbrite (volcanic rock consisting essentially of pumice fragments) or high-porosity volcanoclastic deposits in the region of the Bolaven Plateau. These are commonly associated with composite volcanoes and calderas.

Another unlikely explanation for the gravity anomaly is a maar (broad, low-relief volcanic crater caused by a phreatomagmatic eruption), a common feature of volcanic fields, characterised by craters with floors lying 5 to 400 m below the pre-eruptive surface, surrounded by a ring of ejecta and sourced from a low-density diatreme (volcanic pipe formed by a gaseous explosion). Maar craters are, however, typically only about a kilometre in diameter, considerably smaller than the dimensions of the anomaly Siah et al. observe.

A fourth positive, and perhaps definitive test of the Bolaven crater hypothesis would be discovery of proximal ejecta. Lavas mantle most of the western half of the Bolaven Plateau, however, so one might consider the search for an ejecta blanket to be a fool’s errand. Of great significance, then, is one small pie-shaped piece of oddly rilled terrain 10–20 km southeast of the summit of the volcanic field. This patch is the largest area near the summit that has escaped volcanic burial. Streams there flow southeastward in 40 to 50 m deep valleys, away from the summit and with much closer spacing than drainages either atop or cutting into the lavas. In the two places where Siah et al could gain access through the thick jungle vegetation, the stream beds are flowing on in situ, flat-lying sandstone bedrock. This suggests that the rills have cut through a loose, 40 to 50 m-thick deposit to its basal contact with bedrock.

The looser material that forms the closely spaced rills is exposed in two road cuts. The better-exposed of these displays well a fining upward breccia of angular sandstone and mudstone clasts (clasts are individual fragments of rock, in this instance mudstone). The lowest bed in the exposure is a monolithologic, cobbly, and bouldery fine-sandstone breccia, in a matrix of angular sand grains and pebbles. The overlying bed is a monolithologic, cobbly, boulder mudstone breccia. These two breccia beds are remarkable for the fact that large domains within the outcrop comprise cobbles and boulders that fit together like jigsaw-puzzle pieces. Overlying the mudstone breccia is a thin sandstone breccia. Overlying and in sharp contact with this is a massive coarse silt to very fine sand, which we ascribe to deposition as a loess-like bed from a convecting cloud produced by the impact, similar to the genesis proposed for deposits in eastern Thailand.

Sandstone boulders within an impact-breccia deposit ∼20 km southeast of the centre of the proposed crater contains abundant planar fractures in quartz crystals. Boulders shattered in situ at the end of ballistic trajectories from the proposed crater. Siah et al. (2020).

The sedimentological and stratigraphic nature of the lower three beds in this outcrop is consistent with the rapid accumulation of clasts at the end of ballistic trajectories from the impact crater. The angular nature of both framework and matrix indicates that they did not experience rounding during transport, as would be expected if they arrived as bed load in a very powerful stream. The jigsaw-puzzle-like fitting of neighbouring clasts in the coarse, lower two beds is wholly unlike outcrops of intensely weathered siliclastic bedrock elsewhere on the plateau. These jigsaw patterns imply that the boulders shattered at the site and support the argument that they could not have been transported to the site in a debris flow or weathered in situ. Moreover, the size of the sandstone boulders implies very high energies of emplacement, far greater than are plausible in the small neighbouring stream, which has no large tributaries and descends only about 100 m from its headwaters 4 km upstream.

Roadcut exposure of impact ejecta exposes a fining-upward, irregularly bedded sandstone and mudstone breccia. The color-coded boulders and cobbles in the lower part of the outcrop are sandstone (pink) and mudstone (green). The sandstone and mudstone clasts broke apart in situ. This fact supports our argument that they landed as ballistic clasts and were not deposited as part of a debris-flow, landslide or avalanche. The unexposed base of the breccia lies between the base of the roadcut and in situ Mesozoic fluvial sandstone that crops out in the streambed about 10 m below. This unexposed section may contain basaltic debris. Siah et al. (2020).

Siah et al. argue that this outcrop exposes part of the ejecta blanket that surrounds the impact site. The fact that the thick mudstone breccia overlies the sandstone breccia supports this claim, because inversion of the ordering of the Mesozoic stratigraphy (layers) of the plateau, roughly 200 m of sandstone overlying about 250 m of mudstone, is what one would expect in the ejecta blanket. They estimate the impact velocity for these boulders to be about 450 metres per second, assuming that they exited the crater at an angle between 45° and 60°. If these beds represent inverted stratigraphy of the target rocks, one would expect a breccia bed comprising basalt debris directly beneath the sandstone breccia layer and atop bedrock. Unfortunately, the several meters of section between the sandstone bed and bedrock is not exposed, but there are rare, loose basalt clasts atop bedrock in a nearby stream bed that might have come from that unexposed, basal part of the breccia.

Discovery of shocked quartz within the sandstone boulders would provide an independent test of whether this outcrop represents a part of the eject blanket, Not finding evidence of shocked quartz would support the argument that this is not an impact deposit, even though its sedimentological nature and setting preclude any other origin that we can imagine. Petrographic examination reveals that the quartz grains in the sandstone boulders do indeed have planar fractures like those caused by high-velocity impacts, but quartz grains in the underlying bedrock do not.

First Siah et al. examined differences in the petrographic textures of the in situ bedrock and the overlying ejecta deposit. The bedrock comprises dominantly angular to sub-angular medium-size grains of quartz sand with little to no matrix. In contrast the boulder from the ejecta deposit consists principally of very fine to fine-grained angular quartz sand floating in a nondescript, clayey matrix. We are uncertain whether this texture is an original depositional texture or the result of post depositional comminution (fragmentatiion) and weathering, but the latter is a possibility.

(a) Texture of sandstone sample from the bedrock. The rock comprises a clast supported, well sorted coarse-grained quartz sandstone. Yellow arrow shows typical planar fractures of the bedrock. (b) Example of irregular planar fractures crossing grain boundaries in the bedrock. (c) Textures in the sandstone boulders in the ejecta deposits. The rock is matrix supported, with smaller sub-angular quartz grains that are spatially separated. (d) Example of planar fractures associated with shock metamorphism.(e) Example of planar fractures similar to the ones observed in the bedrock, non shock related. Siah et al. (2020).

Fractures appear in the quartz grains of both the bedrock and the boulders. However, fractures in the bedrock grains do not have the characteristics of shock-related fractures: They are curved and display variations in thickness. They are not evenly distributed within a grain and in some cases continue across grain boundaries. Similar fractures exist in some grains of the sandstone boulders.

However, Siah et al. also observe grains in the boulders that display very distinct, parallel, planar fractures about 2 μm wide and 2–18 μm apart. These do not cross grain boundaries.

In some of these crystals Siah et al. measured the apparent polar angles between the index plane of the planar fractures and the c axes (in describing the geometry of crystals three axes are used, a, b, and c) of the host quartz grains, to see if they are within the range of crystallographic orientations proposed by others to be typical of shock-related features. most of the calculated polar angles cluster around 52° and 66°, which could correspond to Miller Index planes, notably {10 11}, that are typical in shocked quartz grains. The predominance of fractures with these crystallographic orientations have sometimes been used to distinguish high-pressure (10–35 GPa) planar deformation features. Compelling evidence that the fractures in the Bolaven Plateau quartz grains are planar deformation features would require measurement of 100 or so samples. Siah et al. have elected not to do such a comprehensive test, since they have so many other lines of evidence that the deposit is part of a proximal ejecta blanket, i.e., the poorly sorted nature of the deposit itself, the angularity of both its large and small clasts, and the proximity of the deposit to the proposed crater rim. Siah et al. hypothesise that these quartz grains do indeed reflect the weak (less than 10–20 GPa) shocking that one would expect in rocks that originated near the perimeter of the excavating crater.

Microphotograph in plane-polarized light of parallel planar fractures (upper left to lower right) in 1 quartz crystal. The fractures are well-defined and do not cross grain boundaries. Siah et al. (2020).

One other site that might contain proximal ejecta is on the northeast flank of the suspected crater. Strewn on a small, faulted inselberg (isolated hill or mountain) of Mesozoic bedrock surrounded by basalt are large blocks of cross-bedded Mesozoic fluvial sandstone. None of these large boulder slabs are overturned, but the extreme discordance of dips and strikes from boulder to boulder is consistent with their having been thrown out of the crater onto the crater rim. Alternatively, these loose blocks might have been dislodged by the passage of impact shock waves or normal faulting underfoot during collapse of the impact crater rim. Similar outcrops of very large boulders occur on the crest of a ridge 1 or 2 km southeast of the proposed crater rim.

Four lines of evidence imply strongly that the impact that produced the vast Australasian Strewn Field lies beneath young lavas of the Bolaven Volcanic Field in Southern Laos. First, the Mesozoic siliclastic rocks and young overlying pre-impact basalts of the plateau are consistent with tektite geochemistry and relict mineralogy. Second, exposed lava flows above and near the hypothesised crater are younger than the 790 000 years before present date of the impact. Third, a negative gravity anomaly at the summit region of the volcanic field is of a dimension and magnitude consistent with the presence of low-density clastic deposits associated with an impact crater. Finally, an outcrop 10–20 km from the proposed impact site consists of brecciated sandstone and mudstone boulders that appear to have shattered in situ during ballistic emplacement. Planar deformation features in quartz grains within 1 of the boulders imply shock metamorphism that supports this interpretation.

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