Understanding ocean chemistry in the Western Interior Seaway during the Cenomanian–Turonian Extinction Event.

The boundary between the Cenomanian and Turonian stages of the Cretaceous Period is marked by a mass extinction event that saw the demise of a quarter of the marine invertebrates present at the onset of the crisis, combined with carbon, oxygen, and sulphur isotope levels, the deposition of a thick (up to 3 m in places), organic-rich, black shale in many ocean basins, and the onset of a greenhouse climate, known as the Cretaceous Climatic Maximum, which peaked in the early Turonian, then gradually cooled off over the remaining 24 million years of the Cretaceous. Numerous causes have been proposed for the Cenomanian–Turonian Extinction, but the most likely is thought to be massive volcanic emplacements, possibly in the Caribbean Large Igneous Province, which injected large amounts of carbon dioxide, hydrogen sulphide, and sulphur dioxide, as well as a variety of metal compounds, into the ocean-atmosphere system. Increasing atmospheric carbon dioxide would have led to higher global temperatures and higher precipitation on land, which in turn would have led to higher erosion on land, more nutrients being washed into the oceans, and vast Algal Blooms, which would be recorded as a higher burial rate for organic carbon, causing the global carbon isotope excursion, which can be observed from both black shales and carbonate rocks spanning the Cenomanian–Turonian boundary. At the time much of the world's ocean system was dominated by shallow, epicontinental seas (i.e. seaways covering continents in the already warm Cretaceous world), which would have quickly become stagnant when these Algal Blooms were combined with a combination of an injection of oxygen consuming metals and a break-down in ocean circulation caused by the rising temperatures, resulting in large portions of the global ocean becoming anoxic and hostile to multicellular life. The widespread occurrence of black shales at the Cenomanian–Turonian boundary is thought to be a reflection of this. Curiously, however, these phenomena are not recorded in all sequences spanning the Cenomanian–Turonian boundary, with many shallow marine environments (which would be predicted to be the most severely impacted by such events) seemingly unaffected. This variability, with the event leaving a strong signal in some sequences, a light one in others, and being totally absent in some places, leads to the conclusion that the 'global event' may in fact have been a series of overlapping local occurrences, driven by multiple factors rather than a single change in global atmospheric composition.

In a paper published in the journal Scientific Reports on 30 June 2021, Rob Forkner of the Deep Time Institute, Jeremy Dahl of Biomarker Technologies, Inc., and the Stanford University Institute for Materials and Energy Sciences, Andrea Fildani, also of the Deep Time Institute, Silvana Barbanti, also of Biomarker Technologies, Inc., Inessa Yurchenko of the Department of Geological Sciences at Stanford University, and Mike Moldowan, again of the Deep Time Institute, present the results of a study of the USGS Portland-1 core, which was drilled in Colorado, and which includes a section of the Greenhorn Formation including the Cenomanian–Turonian boundary.

 

Palaeogeographic map of North America during Oceanic Anoxic Event 2. The location of the Portland-1 core as well as active volcanic centres are shown. Forkner et al. (2021).

Forkner et al. sampled the core through the Cenomanian–Turonian boundary interval (as determined by the isotope excursion), as well as on either side, for organic geochemical analyses. They initially targeted layers with high organic carbon which were thick enough to determine if reworking or bioturbation had occurred, although this severely limited the number of suitable layers, with the effect that samples were taken at intervals of between 3 and 12 cm across the boundary interval, and 20 cm or more outside this interval. The samples were round segments 2-3 cm in diameter and 1 cm thick taken from the larger core, which were first tested for rock richness and maturity (the extent to which rocks have been heated, altering organic molecules preserved within them), before the most suitable samples were selected for analysis by gas chromatography–mass spectrometry.

Molecular fossils, or biomarkers, are recognisable fragments of molecules synthesised by biological organisms, which can be used to determine the presence and abundance of groups of organisms. Forkner et al. analysed biomarkers from the Portland-1 core across the Cenomanian–Turonian boundary, thereby obtaining a series of snapshots of the water column ecology, which were used to develop a new molecular stratigraphy for the boundary, thereby deriving a wealth of new information with regard to the biota, depositional environment, and the multiple drastic environmental changes that occurred before, during, and after the Cenomanian–Turonian Extinction Event.

A geological examination was used to establish a lithological sequence of events (i.e. changes in the rock type being laid down over time, which would have related to local environmental conditions), using photographs to cover those sections of the core which have previously been heavily sampled by previous workers. This enabled the comparison of similar facies ( specified characteristics, which can be any observable attribute of rocks), in order to correlate changes in the biota in intervals with similar climatic and environmental conditions. This selection process meant that effectively only the finest grained, dark mudrocks were sampled, as these gave the greatest opportunity to detect changes in the water column biota uninfluenced by sedimentary conditions.

The data obtained from the Portland-1 core indicates that, in this area of the Cretaceous Western Interior Seaway at least, conditions in the water column during the Cenomanian–Turonian boundary event (sometimes known as the Cenomanian–Turonian Ocean Anoxic Event) conditions do not appear to have been particularly anoxic. In fact, the sediments laid down across the boundary appear to have been laid down in a more oxygenated environment than wither the sediments above the boundary or those below it, something which has been noted at other locations in the Western Interior Seaway, and coeval shallow water deposits from the Tethys Ocean. The sediments laid down before the boundary layers are predomanenty finely laminated, whereas those across the boundary interval are mostly heavily bioturbated, suggesting a thriving benthic community living within them.

 

USGS Portland-1 core lithologic section, carbon isotope profile and RockEval data. Facies Explanation: (1) Peloidal/foraminiferal, packstone/grainstone; (2) Bioturbated peloidal packstone; (3) Bioturbated peloidal wackestone; (4) Skeletal grainstone; (5) Rippled mudstone; (6) Silty laminated mudstone; (7) Diffusely laminated mudstone; (8) Massive mudstone; (9) Bentonite. Samples were limited to facies (7) and (8). The occurrence of bioturbated peloidal carbonates during the Oceanic Anoxic Event positive carbon isotope excursion indicates that the environment at the time of deposition was oxygenated and supported a diversity of tropical marine life. Note that the core is measured in imperial units as the Portland-1 core and core photos are curated with imperial measurements. This reference is preserved here in the case that the reader wishes to cross-reference these results to the Portland-1 core. Radio-isotopic measurements from bentonites A, B, and C, along with biostratigraphy and correlation of depositional cycles to orbital timescales have produced an average sedimentation rate of 0.93 cm/per thousand years during the Oceanic Anoxic Event positive carbon isotope excursion. The interval of samples with the greatest flux in measured biomarker concentration occurs from about 473 feet (144 m) to about 479 feet (146 m), in the central portion of the Oceanic Anoxic Event positive carbon isotope excursion. Sample spacing in this interval is somewhat irregular in order to stay within the same depositional facies, but varies between 3 and 12 cm indicating that rapid flux in organic geochemical composition of analysed sediments over periods approximately 3–15 thousand years. The carbon isotopic excursion (CIE) that defines the Oceanic Anoxic Event is shown on the carbon¹³ as a proportion of total carbon (δ¹³C) track and highlighted in blue on all compound tracks. Hydrogen Index (HI) is generally negatively correlated with depositional environment oxygen concentrations, thus supporting the trend of Oceanic Anoxic Event positive carbon isotope excursion oxygenation. Oxygen Index (OI) generally correlates positively with depositional environment oxygen concentrations, and again provides evidence for oxygenation during the Oceanic Anoxic Event positive carbon isotope excursion. Forkner et al. (2021).

The geochemical analysis of samples extracted from the core supports the geological analysis. The 'Hydrogen Index', which derives from the proportion of total organic carbon made up of hydrocarbons, is generally negatively correlated with the oxygen concentration in the depositional environment, i.e. the Hydrogen Index tends to go up when there is less oxygen and down when there is more oxygen. In the Portland-1 core the Hydrogen Index above the Cenomanian–Turonian boundary layer averages at 509, during the boundary the average fell to 177, and below the boundary the average rose again, to 423, supporting the idea that oxygen levels in the water column rose rather than fell during the boundary interval. The 'Oxygen Index', derived from the purporting of carbon dioxide to total organic carbon, is positively correlated with the level of oxygen in depositional environment (i.e. the Oxygen Index goes up when the amount of oxygen present in the depositional environment goes up). In the Portland-1 core the Oxygen Index above the boundary layer averages 16, in the boundary layer averages 28, and below the boundary layer averages 15, again suggesting a rise in oxygen levels across the boundary interval.

A number of biomarkers also strongly imply a rise in oxygen levels during the Cenomanian–Turonian boundary interval. The Gammacerane Index is derived from the ratio of the biomarker gammacerane (derived from bacterivorous Ciliates) to hopane (derived from Bacteria), is associated with stratification in the water column, with high levels of gammacerane typically indicating highly saline or reducing conditions. In the Portland-1 core the Gammacerane Index drops to its lowest level during the Cenomanian–Turonian boundary interval, implying conditions became less reducing (generally a sign of higher oxygen levels). The Homohopane Index is derived from the proportion of C₃₅ hopanes (hopane molecules with 35 carbon atoms) to the total C₃₁-C₃₅ hopanes (hopanes with between 31 and 35 carbon atoms). This idex also tends to rise with reducing conditions, and again has its lowest valuse in the Cenomanian–Turonian boundary interval in the Portland-1 core, again suggesting that the enviroment became less reducing during this interval. The proportions of A-oleanane relative to oleanane and 17α-diahopane relative to 17α-hopane are also thought to be indicative of higher oxygen levels, since both A-oleanane and 17a-diahopane require oxygen for their producers (Bacteria and Flowering Plants, respectively), to form them from their precursors, oleanene and hopane. The levels of A-oleanane and 17a-diahopane remain constant throughout the section, but the levels of oleanene and hopane fall during the Cenomanian–Turonian boundary layers, so that the proportion of A-oleanane and 17a-diahopane rise, presumably indicative of a rise in oxygen. 

 

Compound tracks through the Cenomanian–Turonian boundary layers relating to oxygenation before, during, and after the event. Gammacerane and Homohopane Indexes, which are affected by sediment redox conditions, show a significant decrease and the ratios related to 17α-Diahopane exhibit an increase with striking fluctuations within the Cenomanian–Turonian boundary layers. These broad scale changes reflect an overall increase in oxygenation during the Cenomanian–Turonian boundary interval, with periods of reducing conditions punctuated through the event. The relative preservation of des-A-oleanane revealed by the des-Aoleanane/oleanane ratio could be a function of oxidation. Forkner et al. (2021).

Given the generally healthy ecosystem recorded in the sediments of the Portland-1 core across the Cenomanian–Turonian boundary, and the geochemical evidence for a healthy, well-oxygenated water column, biomarkers associated with primary Algal production would be expected to increase across the Cenomanian–Turonian boundary layer. However, a range of such biomarkers, including cholestanes, ergosteranes and C₂₇, C₂₈, and C₃₀ steranes, undergo fluctuations in the boundary layer, with a general decrease in levels. This would appear to represent a deteriorating environment, with repeated stressful intervals.

While this decrease in steranes implies a drop in Algal productivity across the Cenomanian–Turonian boundary, levels of hopanes, indicative of Bacterial productivity, remain relatively high, although again they undergo some severe fluctuations during the boundary interval, suggesting that environmental conditions were fluctuating in the water column. 

Fluctuating biomarker ratios are highly indicative of an unstable environment, with fluctuating primary productivity. In order to further explore this, Forkner et al. examined three further examples. The hopane/sterane ratio reflects the levels of both heterotrophic and photosynthetic Bacteria to primary producers including marine Algae and terrestrial Plants. The 3β-methylhopane/dinosteranes ratio compares the ratio of 3β-methylhopane, derived from aerobic methanotrophs and fermentative Bacteria, to the ratio of dinosteranes, which are almost exclusively derived from Dinoflagellates. The 3β-methyl-24-ethylcholestane/4α-methyl-24-ethylcholestane ratio compares a reworked sterane to one produced by marine Algae. All of these ratios record a significant drop in primary production across the Cenomanian–Turonian interval, but with rapid fluctuations which appear to be unrelated to any change in the lithology, and which Forkner et al. suggest might be related to short-term anoxia events not recorded in the rock record.

 

Compound tracks through the Cenomanian–Turonian boundary interval relating to productivity before, during, and after the event. For the main the Cenomanian–Turonian boundary interval section, the concentration of biomarkers derived from algae such as C₂₇, C₂₈, and C₃₀steranes (24-n-propylcholestanes), and 4α-methyl-24-ethylcholestane 20R decreases significantly relative to concentrations of those derived from bacteria, which increase moderately with variations. This suggests that the record of productivity variability we interpret is reliable and is not simply a record of poor preservation of the organic fraction. Of note is the interval in all track from about 473 feet (144 m) to about 479 feet (146 m) where organic geochemical measurements return the most erratic results. By applying the most recently published timescales through this interval, it is possible to calculate that the productivity cycles of biomarker decline and recovery can vary from about 26 thousand years at the shortest to about 130 thousand years at the longest. These productivity cycles occur within individual lithofacies internal to single lithocycles. Fornkner et al. (2021).

Isoprenoids can be derived from chlorophyll side chains produced by photosynthetic Algae, although other organisms do produce them, including Cyanobacteria and some Archaeans. Notably, head-to-head isoprenoids such as biphytane are produced by marine Archaea. Unfortunately, isoprenoids are found at very low concentrations throughout the core, although some fluctuations can be observed during the Cenomanian–Turonian boundary layers, and biphytane and its diagenetic products are only ever found at trace levels, and often drop below detectability levels. Studies of the Cenomanian–Turonian layer in other sections have suggested that there might have been blooms of opportunistic Archaea during this interval, but this cannot be detected in the Portland-1 core.

A number of biomarkers remain relatively constant on either side of the Cenomanian–Turonian boundary layers, but fluctuate greatly during the boundary interval. Previous work on the Portland-1 core and other cores from the same area have been studied extensively to determine sedimentation rates. These studies have led to a calculated sediment accumulation rate of 0.93 cm per thousand years in the upper part of the Cenomanian–Turonian boundary, using calculations that include radiometric dates from bentonite layers, and orbital time scales. Calibrating this with the observed fluctuations in biomarker ratios suggests that productivity fluctuations were occuring on cyclical scales of between 26 and 90 thousand years, with individual biological collapses happening in as little as 3200 years.

One possible cause of influxes into the shallow, enclosed, Western Interior Seaway during the Cenomanian–Turonian boundary period that has been previously suggested is sea level rises, Evidence for such changes has been found in the Tethys Ocean, where repeated cycles of carbonate platforms being replaced by deeper sediments have been observed. Such an increase in water volume would provide relief from the effects of stagnation, but cannot explain the drastic changes in water column ecology observed by Forkner et al., which appear to happen on a much finer scale. Forkner et al. were unable to find any references to previous examples of such rapid changes in organic composition of a rock sequence, particularly one one independent of changes in the lithology, and conclude that the drivers these changes were clearly independent of the drivers of lithology changes.

Studies of the lithology and astronomical forcing cycles recorded in the Portland-1 core have previously concluded that the carbonate-mudstone cycle here (which would have been driven by changes in sealevel) would have lasted about 100 000 years. The biological productivity cycle, however, is clearly working on a much shorter, more erratic, and independent timescale. Forkner et al. cannot rule out the possibility that some shorter Milankovitch-band processes was occurring in the Western Interior Seaway, but the irregular nature of the cycles makes this unlikely, as does the fact that they are not seen outside the Cenomanian–Turonian boundary interval.

Forkner et al. suggest that these biomarker cycles may have been driven by changes in primary productivity (i.e. photosynthesis), and the subsequent decay of organic matter. During the intervals on either side of the boundary, more reducing conditions prevailed, probably due to higher rates of organic decay. During times of extreme stress within the boundary period, the amount of primary production dropped, and there was a drop in the amount of organic matter in the water column, and therefore the amount of decay that could occur. However, the driver of this stress, and the cause of the erratic cycle it was following, remain unclear.

A cause of environmental stress unrelated to sealevel changes or orbital forcing and operating on rapid and erratic timescales could plausibly be volcanism. Volcanism has the potentially to disrupr Algal productivity rapidly, and would not cause any change in the depositional environment (which changes in sealevel related to Milankovitch forcing would do). A number of other cores from the Western Interior Seaway (including Eagle Ford, Boquillas, and Bouldin Flags) contain numerous bentonite layers (caused by volcanic ash falling on water then settling to the bottom), which have been used to determine the age of the sediments and the rate at which they were accumulating. Volcanism can impact Algal productivity in a number of ways, from lowering light levels to altering the pH of the water. While this cannot be proven from Forkner et al.'s current work, it would potentially have left detectable signs which could be revealed by future investigations. Either way, these findings appear to support the idea that the Cenomanian–Turonian Extinction Event was caused by a prolonged breakdown in environmental conditions rather than a single catastrophic event.

The Cenomanian–Turonian boundary reflects a profound change in environmental conditions on a global level, although the record of this varies from location to location. Geochemical examination of organic molecular fossils preserved in the Portland-1 core, a nearly-continuous core of sediment recovered from the Western Interior Seaway of North America, shows a record of extreme environmental variability not previously observed, with a generally higher level of oxygenation than before the crisis, prior to previous assumptions about a single, massive, anoxia event driving the crisis. Instead, the environment seems to have undergone a series of smaller, but still significant, crises, operating on an irregular cycle divorced from Milancovich forcing and sealevel changes. Forkner et al. hypothesise that this could have been driven by volcanic episodes, which tend to be erratic in their timing. A single, massive, volcanic event has previously been suggested as a possible cause of the Cenomanian–Turonian Extinction Event, injecting vast amounts of material into the atmosphere and oceans, and leading to significant changes in seawater pH, global temperature, and the hydrological cycle, but Forkner et al.'s evidence points towards a more prolonged period of change, with periods of high organic productivity punctuated by sudden collapse. This repeated stressing of the marine environment would have challenged the biota of these ecosystems, potentially causing the observed extinctions.

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Looking for a connection between the Columbia River Flood Basalts and the Middle Miocene Climate Optimum.

Flood basalts are large volumes of basalt erupted from deep mantle plumes, which cover large areas of land with volcanic basalt rock in a relatively short time, geologically speaking (hundreds of thousands to millions of years). Like other volcanic eruptions they also release climate altering carbon dioxide and sulphur dioxide gasses, and do so in very large volumes. This is thought to have a very profound effect on the climate, and flood basalts are often closely linked to mass extinction events, for example the Siberian Traps Flood Basalt is associated with the End Permian Extinction, the Mid-Atlantic Flood Basalt with the End Triassic Extinction, and the Deccan Traps Flood Basalt with the End Cretaceous Extinction. The youngest known flood basalt is also the youngest; the Columbia River Flood Basalt, in the Pacific Northwest of the United States, erupted between 17 and 5 million years ago, from a north-south trending fissure system than covered parts of eastern Washington, eastern Oregon, western Idaho, and northern Nevada. This is coincident with a wider outbreak of volcanism in the area, associated with the subduction on the Cascade Arc, which fuelled volcanism in central Oregon and northern Nevada, and the Yellowstone–Snake River Plain Hotspot, which fuelled volcanism in western Idaho and eastern Wyoming. For this reason, plus difficulties constraining the dates of the basalt eruptions, the Columbia River system is not universally accepted as a true-mantle-plume-driven flood basalt. Nor is the Columbia River Flood Basalt associated with an extinction event, though its onset does appear to be correlated with the Middle Miocene Climate Optimum, 16.5 million years ago, a period of rapid warming associated with a spike in atmospheric carbon dioxide, that saw the retreat of the Polar Ice Caps, and major climatic perturbations in many areas.

In a paper published in the journal Science Advances on 19 September 2018, Jennifer Kasbohm and Blair Schoene of the Department of Geosciences at Princeton University publish a new chronological sequence for the Columbia River Flood Basalts, based upon Uranium-Lead Zircon dating.

Zircons are volcanic minerals that form within molten rock as it cools. Like other such minerals, zircons will incorporate some elements present in the melt, but not others, notably they will incorporate uranium, but not lead. This is important because uranium is an unstable element, and over time undergoes fission, with the uranium atoms breaking down to produce, amongst other things, lead atoms. This is very useful because nuclear fission occurs at a steady rate, unaffected by conditions such as temperature or pressure, so that by comparing the proportion of uranium to lead within a zircon crystal.

One problem with this is the remarkable stability of zircon crystals. Zircons form in cooling magma and volcanic melts where the right elements are present, but unlike other such minerals can survive being reheated past the temperature at which they originally formed. This enables zircons to survive the melting of rock along subductive plate margins, and subsequent re-eruption through volcanoes along these margins. The durability of zircons has proved extremely useful to scientists studying the early evolution of the Earth, as the oldest minerals on the planet are zircons far older than their host rocks, but is a serious problem in an area like the Columbia River Flood Basalts, where material from a deep mantle plume has erupted through recent volcanic rocks associated with a subductive plate margin, where most of the material being subducted was older volcanic rock, something that has made it very hard to date the Columbia River sequence.

In order to resolve this problem Kasbohm and Schoene took samples from eight horizons within the Colombia River Sequence, then tested a large number of zircons (20-40) from each sample, selecting the youngest age present in each sample to represent that part of the sequence. Using this method, they were able establish that the upper 72% of the Steens Basalt, the oldest rocks of the sequence, erupted between 16 653 000 and 16 589 000 years ago, the overlying Imnaha Basalt erupted around 16 572 000 years ago, the Grande Ronde Basalt, which overlies the Imnaha Basalt, finished erupting around 16 066 000 years ago, and the lowest 77% of the Wanapum Basalt, close to the top of the sequence, had erupted by 15 895 000 years ago.

Map of Columbia River Flood Basalts and regional volcanism. The map shows the areal extent of each formation of the Columbia River Flood Basalts, and the legend provides the volume contribution of each formation. Stars represent geochronology sample collection sites; dashed lines enclose areal extent of source dike swarms. The Prineville Basalt and Picture Gorge Basalt are coeval with the Grande Ronde Basalt, represent 1.4% of the total Columbia River Basalt volume, and are grouped with the Grande Ronde Basalt for all volume estimates presented here. Kasbohm & Schoene (2018).

This implies that 95% of the total volume of the Columbia River Flood Basalts erupted within 758 000 years, between 16 653 000 and 15 895 000 years ago. If correct, this means that the Columbia River Basalts were erupted at a remarkably high rate even for flood basalts, with peak rates of between 20 and 40 cubic kilometres of material per year being produced. This compares to estimates of peak eruption levels for the Deccan Traps Flood Basalts of between one and two cubic kilometres per year, for the Central Atlantic Flood Basalts of three to five cubic kilometres per year, and for the Siberian Traps Flood Basalts of between one and four cubic kilometres per year.

Such a large volume of volcanic material being erupted over such a short time would have had a very strong effect on the atmosphere, potentially doubling the amount of carbon dioxide in a relatively short period of time. This correlates well with the onset of the Middle Miocene Climate Optimum at 16.5 million years ago. The Middle Miocene Climate Optimum was followed my the Middle Miocene Disruption, which began around 14 million years ago, when falling carbon dioxide levels led to plunging temperatures, and an extinction event that removed many warm adapted groups from higher latitudes. This suggests that the rapid eruption of volcanic material created a spike in atmospheric carbon dioxide that lasted around a million years after the eruptions ceased.

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Looking for the eastern margin of the Palaeo-Tethys Ocean.

The Palaeo-Tethys Ocean ran separated the continent of Gondwana from the landmasses that would become Europe and Asia during the Palaeozoic Era, opening during the Middle Cambrian and eventually closing with the formation of the Supercontinent of Pangea during the Permian/Triassic. The western part of this ocean is reasonably well understood, but the eastern extent is less well known, as much of the geology of the region has been distorted and overwritten by the Himalayan Orogeny, as the Indian Plate has impacted Eurasia from the south. The ocean probably passed along the southern margin of the South China Block (also known as the Yangtze Plate) (i.e. the southeastern coast of China), producing an area of rifting similar to that seen beneath the Red Sea today, but there is little evidence to support this.

In a paper published in the journal Acta Geologica Sinica on 27 February 2018, Hu Lisha, of the Collage of Marine Geosciences at the Ocean University of China, and the Laboratory for Marine Geology at the Qingdao National Laboratory for Marine Science and Technology, Du Yuansheng of the State Key Laboratory of Biogeology and Environmental Geology at the China University of Geosciences, and Xu Yajun, Wang Zhiwan, and Wang Chenghao, also of the Collage of Marine Geosciences  at the Ocean University of China, describe the discovery of a volcanic tuff (ash) layer from Banchen in the Qinzhou area of the Guangxi Zhuang Autonomous Region of southeastern China, could provide evidence for subduction along the southern margin of the South China Block during the Palaeozoic.

Hu et al. report the discovery of a green tuff layer between layers of Devonian siliceous rocks, from which zircons were extracted for uranium/lead dating. Zircon is a mineral formed by the crystallisation of cooling magmas. When it forms it often contains trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead (which has a much lower precipitation temperature) will not have been present in the original lava, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

(a) Tectonic framework of the East Asia; (b) Simplified geological map of the Qinfang Trough and location of the study area; (c) Photo for the Late Devonian chert and tuff; (d) Concordia diagram and cathodoluminescence (CL) images of representative zircons for the tuff sample. Hu et al. (2018).

Eighteen zircons were subjected to this analysis. Seven of them produced ages older than 600 million years, whereas eleven produced ages of between 380 and 350 million years, consistent with a Devonian or Carboniferous age for the tuff layer. The presence of much older zircons in the sample does not undermine this, as zircons are extremely tough, and are known to be able to endure repeated cycles of subduction and volcanic eruption, nor is the broad spread of ages found in the Palaeozoic zircons, as ash deposits often contain mineral grains aggregated over a long period before being erupted.

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Jovian InfraRed Auroral Mapper discovers new volcanic field on Io.

Io is the innermost of the four Galilean Moons of Jupiter (the four large moons discovered by Galileo Galilei in January 1610), and is one of the most distinctive bodies in the Solar System, with a surface dominated by a series of extensive volcanic fields. The volcanism is thought to be caused by tidal forces, as Io is pulled by the gravitational forces of both Jupiter and the other large Galilean Moons, deforming and heating the moon's interior. This has led to a body unlike any other in the Outer Solar System, with no significant ice or hydrocarbon deposits (presumably lost due to the heat of the volcanic activity) and a silicate rock surface surrounding an iron or iron-sulphur core.

The Galilean Moon Io, as imaged by the Galileo Spacecraft in 1995. NASA/JPL/University of Arizona/Wikimedia Commons.

In a press statement released on 13 July 2018, scientists from NASA described the discovery of a new volcanic field on Io, close to the moon's South Pole and about 300 km from the nearest previously discovered field. This was revealed in an image of Io taken by the Jovian InfraRed Auroral Mapper instrument on the Juno Spacecraft during a flyby on 16 December 2018. 

This annotated image highlights the location of the new heat source close to the south pole of Io. The image was generated from data collected on 16 December 2017, by the Jovian Infrared Auroral Mapper (JIRAM) instrument aboard NASA's Juno mission when the spacecraft was about 470 000 kilometres from the Jovian moon. The scale to the right of image depicts of the range of temperatures displayed in the infrared image. Higher recorded temperatures are characterised in brighter colours – lower temperatures in darker colours. NASA/JPL/Caltech/Southwest Research Institute/Agenzia Spaziale Italiana/Insituto Nazionale di Astrofisica/Jovian Infrared Auroral Mapper .

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Using mercury to assess the role of Central Atlantic Magmatic Province volcanism in the End Triassic Extinction.

The End Triassic Extinction event is considered to be the fourth of the Big Five extinction events recorded in the fossil record of the Phanerozoic Eon. On land it wiped out many large Vertebrate groups, paving the way for the Dinosaur-dominated ecosystems of the Jurassic and Cretaceous, but its effects were more severely felt in the oceans, with the total collapse of almost all carbonate reef ecosystems and the extinction of the Conodonts, a jawless Vertebrate group that first appeared in the Cambrian. The extinction is associated with a sharp rise in carbon dioxide levels, thought to have caused significant ocean acidification. This has in turn been linked to the eruption of the Central Atlantic Magmatic Province, the volcanic event that began the break-up of the supercontinent of Pangea and the formation of the Atlantic Ocean, which occurred in three or four phases over a period of about 700 000 years. However all known rocks directly associated with the Central Atlantic Magmatic Province were laid down in either terrestrial or deep marine environments, making it hard to make a direct link between the timing of these volcanic deposits and the collapse of shallow marine reef ecosystems.

In a paper published in the journal Nature Comunications on 6 April 2016, Alyson Thibodeau of the Department of Earth Sciences at the University of Toronto and the Department of Earth Sciences at Dickinson College, Kathleen Ritterbush of the Department of Geology and Geophysics at the University of Utah, Joyce Yager and Joshua West of the Department of Earth Sciences at the University of Southern California, Yadira Ibarra of the Department of Earth System Science at Stanford University, David Bottjer and William Berelson, also of the Department of Earth Sciences at the University of Southern California, Bridget Bergquist, also of the Department of Earth Sciences at the University of Toronto and Frank Corsetti, again of the Department of Earth Sciences at the University of Southern California, examine mercury concentrations across the Triassic-Jurassic boundary in shallow marine deposits at Muller Canyon in Nevada as a proxy for volcanism.

Mercury reaches the Earth's surface almost exclusively through volcanism, with major eruptions leading to significant amounts of the volatile metal entering the atmosphere, and mercury in rocks being found predominantly in volcanic deposits, and fluvial and oceanic sediments derived from such rocks. Atmospheric mercury tends to bind strongly to organic material and clay particles, tending to find its way into marine sediments within at most a few million years of erupting.

The Muller Canyon rock sequence comprises a Late Triassic Bivalve-dominated carbonate reef, which is succeeded by an Early Jurassic sedimentary sequence dominated by siltstone, with rare microscopic Gastropods and Sponge spicules. Importantly the last occurrence of the Ammonite Choristoceras crickmayi, which is used to mark the end of the Triassic, occurs seven meters below the first occurrence of the Ammonite Psiloceras spelae, which is used to mark the beginning of the Jurassic, suggesting that a good sequence across the Triassic-Jurassic boundary has been preserved.

Thibodeau et al. found that mercury levels rose sharply with the onset of the extinction event, and while they drop back after this they remain significantly higher than pre-extinction levels throughout the strata with an impoverished fauna, with several smaller peaks in mercury concentration during this time. Only when mercury levels drop back to pre-extinction levels does the biota begin to recover, and a significant Jurassic ecosystem starts to develop.

Summary of key features of the Triassic-Jurassic interval, Muller Canyon, Nevada. Panels compare (a) Mercury chemostratigraphy, (b) Ammonite species diversity; (c) benthic palaeoecology and microfacies; and (d) ecosystem state for Muller Canyon, Nevada, in association with lithology and key dates. These comparisons show that significant biotic recovery follows the mercury anomalies and provide evidence that biotic recovery began after the cessation of Central Atlantic Magmatic Province magmatism. Thibodeau et al. (2016).

This strongly suggests that the End Triassic Extinction was associated with the onset of volcanism in the Central Atlantic Magmatic Province, and that the fauna did not begin to recover until after volcanism ceased. This contrasts with previous studies, which have shown the End Triassic extinction as an abrupt event, hard to reconcile with the more drawn-out Central Atlantic Magmatism. Furthermore, while the initial extinction event was accompanied by a spike in carbon dioxide levels, likely to have caused a significant ocean acidification event, the ocean fauna remained impoverished significantly after carbon dioxide levels had returned to normal, not recovering for around two million years after the initial event, or a million years after the end of volcanism, compared to a predicted recovery timescale of 10-100 000 years for an ocean acidification event.

This suggests that while ocean acidification may have played a role in the initial extinction event, it was not the only factor involved. Thibodeau et al. note that at their highest mercury levels in the Muller Canyon deposits reach 600 parts per billion relative to organic carbon. This compares to levels of around 200 parts per billion in sediments in parts of the San Fransisco Bay area considered to be heavily polluted today as a result of the use of mercury in gold mining in California in the nineteenth century. Mercury is a powerful neurotoxin, and known to be harmful to a wide range of organisms, being highly deleterious to modern ecosystems at high levels. However Thibodeau et al. do not go as far as to claim that mercury was directly responsible for the extinction event rather than an indicator of a volcanically induced deteriorating ecosystem.

See also...

Evidence for a Middle Permian extinction event from Spitzbergen Island.                      In recent years considerable evidence has been exposed in South China for a major extinction event in the Middle Permian. This event, known as...

Evidence of an Ice Age at the start of the Middle Jurassic.                                           The Jurassic lasted approximately 65.3 million years, from about 201.3 million years ago till about 145 million years ago, and is generally considered to have been...

The reaction of marine invertebrates to global warming during the Early Jurassic Toarcian Extinction Event.                                        About 183 million years ago a major eruptive episode in the Karoo-Ferrar Large Igneous Province of South Africa lead to an abrupt rise in global atmospheric and...

 

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Fissure eruptions from the southern polar region of Saturn's moon Enceladus.

Observations of Saturn's moon Enceladus by the Cassini space probe have revealed a number of sulci (linear structures) in the southern polar region. These have been named the Alexandria, Baghdad, Cairo and Damascus Sulci, or informally the 'Tiger Stripes'. A number of jets of water vapour and ice particles have been seen emerging from these sulci, and infrared images suggest that they are warmer than the surrounding terrain.

In a paper published in the journal Nature on 7 May 2015, Joseph Spitale of the PlanetaryScience Institute, Terry Hurford of NASA's Goddard Space Flight Center, Alyson Rhoden of the Applied Physics Laboratory at Johns Hopkins University, Emily Berkson of the Rochester Institute ofTechnology and Symeon Platts of the Film and Television Department at the University of Arizona discuss the results of a re-examination of a series of images of Enceladus taken by the Cassini Space Probe in 2010.

Spitale et al. found that rather than discrete and isolated jets issuing from particular points on the sulci, there were in fact curtains of material being expelled all along the length of these feature, strongly supporting the idea that these are fissures from which material from the interior of the moon is escaping.

 Curtain eruptions issuing from fissures in the southern polar region of Saturn's moon Enceladus. Cassini/JPL/NASA.

The 'jets' could be observed as discrete brighter features within these curtains, however attemps to calculate the precise position of these features by triangulation from different images failed, and proved more inaccurate when more widely spaced observations were used. Such widely spaced observations would be expected to give a more precise estimate of the position of an object, which suggests that these features are in fact illusionary; their position appears to vary depending on the position of the observer (like a rainbow). It is therefore concluded that the jets do not represent areas where material is being expelled at a higher rate than elsewhere on these fissures.

See also...

Methane storms as a possible cause of Titan’s equatorial dune fields.                        Observations by the Cassini Space Probe have revealed vast dune fields, similar to those observed...

The origin of mini-jets in Saturn's F Ring.     The F Ring is the outermost and thinest of Saturn's rings. It was discovered by the Pioneer 11 Spacecraft in 1979, and has been studied by NASA's Cassini...

Cassini Probe finds 400 km river on Titan. NASA scientists have released an image of a 400 km long river on Saturn's moon Titan, imaged by the Cassini Space Probe during a flyby on 26 September 2012. The river meanders for some distance across...

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(Relatively) recent volcanic activity in the Southern Highlands of Mars.

The planet Mars formed about 4.5 billion years ago, and is thought to have been highly volcanically active in its early history. However due to its smaller size it is thought to have cooled much more rapidly than the Earth, with volcanism ceasing in most areas by the end of the Noarchian Epoch (about 3.7 billion years ago), with activity persisting in areas such as the Hesperian ridged plains and Tyrrhenus and Hadriacus Montes till about 3.5 billion years ago. The youngest lavas in the two main Martian volcanic provinces, Tharsis and Elysium are thought to be low-viscosity basaltic lavas, i.e. lavas that reached the surface in a hot, runny state then cooled rapidly on exposure to the atmosphere. More viscous, evolved lavas (lavas that have risen to the surface slowly, cooling and losing minerals which crystalize at high temperatures) are thought to be rare on Mars.

In a paper published in the journal Earth and Planetary ScienceLetters on 1 April 2015, Petr Brož of the Institute of Geophysics of the Academy of Sciences of the Czech Republic and the Institute of Petrology and StructuralGeology at Charles University in Prague, Ernst Hauber of the Institute of Planetary Research, Thomas Platz of the Institute of Geological Sciences at the Freie Universität Berlin and the Planetary Science Institute and Matt Balme also of the Planetary Science Institute, and of the Open University, describe a series of small-scale volcanic edifices in the Terra Sirenum region of the Martian Southern Highlands, which appear to have been formed by the action of viscous, highly evolved lavas during the mid-Amazonian Epoch, i.e. less than a billion years ago.

The Terra Sirenum is a highland region within the proposed former Eridania paleolake, crossed by east-west trending radial graben-systems (longitudinal depressions caused by stretching and thinning of the Martian crust) which may be associated with unexposed volcanic dykes (intrusive volcanic lavas running forming horizontal channels), derived from Arsia Mons 3700 km away, as well a series of wrinkle-ridges interpreted as fault-propagation folds associated with deformation caused by contraction of the crust.

Regional map of part of the southern hemisphere on Mars. The cyan colordelineates the extent of the proposed Eridania Lake based on the 1100m contour. Position of investigated area is marked by dashed box and clearly the area lies inside the proposed borders of the former lake. Brož et al. (2015).

The putative volcanic structures lie within an unnamed depression measuring approximately 150 km by 30 km, with all bar one of these within a further depression located inside this. The volcanic structures comprise three volcanic domes (A, B & C), all located within the inner depression, with A and B located on different sinuous wrinkle-ridges, which rise tens of meters above the surrounding plains, and two volcanic cones (T1 &T2), with Cone T1 sitting outside the inner depression on the rim of an ancient, highly eroded impact crater, which is roughly 110 km in diameter, and Cone T2 within the inner depression. In addition there are four flows of apparent volcanic material associated with these structures (Flows 1-4). Flow 1 may originate from Cone T1, though this area is also intersected with other flows, thought to be related to a 6 km diameter impact crater rather than any volcanic structure, making its interpretation complex; Flow 2 is elongate and appears to originate from Dome A, Flow 3 is roughly circular and surrounds Dome B and Dome C, though its precise origin is unclear, and Flow 4 is again elongate and appears to originate from Cone T2.

THEMIS-IR daytime (a), nighttime (b) and interpretational map of the study area (c). The thermal contrast between the two upper images (a, b) suggests the presence of flow structures associated with cones and domes. Note the wrinkle ridges crossing the basin. The cross-section shows the stratigraphic relations of the investigated edifices with underlying units. Brož et al. (2015).

Cone T1 is about 3 km in diameter and about 230 m in height. It is breached to the south, giving rise to a flow apron 8.5 km wide and 100-130 m thick, which partially covers the lower flanks of the cone. Two small impact craters, 1.3 and 0.8 km in diameter, are located on this flow apron.

(a)    Cone T1 with associated flow apron. (c) Detail of Cone T1 central vent from flow aprons material erupted to the surface. Brož et al. (2015).

Cone T2 is about 3 km in diameter and 290 m in height, and breached to the east, giving rise to a flow apron measuring 12.5 km from north to south, with four distinct lobes. The margins of these lobes appear to overlap, suggesting that the apron was formed by multiple eruptive episodes. The source of Flow 4 lies beneath these aprons; Flow 4 appears to originate from Cone T2 and flows roughly 50 km to the southwest, measuring 1-8 km wide and 30-50 m thick; it appears to have originally flowed to the west then been deflected southward when it encountered a wrinkle-ridge.

(b) Cone T2 with associated flow apron. (d) Detail of Cone T1 central vent from flow aprons material erupted to the surface. (e) Edge of overlapping flow aprons. Brožet al. (2015).

Dome A is a 2.5 km diameter mound located on top of a wrinkle ridge. Flow 2, which is about 5 km long and about 300 m wide appears to originate from this dome and spread to the southeast.

 Image of Dome A with marked Mars Orbiter Laser Altimeter Precision Experiment Data Records (MOLA PEDRs) and associated topographic profiles. Part of the Dome A seems to collapse and propagate in eastern direction. Brož et al. (2015).

Dome B is approximately 5 km by 7 km and 390 m in height, being highest in the north. It is located on top of a wrinkle-ridge, and appears to extend some way to the south along the crest of this ridge. It is irregular in shape, with four deeply incised valleys dividing the dome into four portions. It is surrounded by flow features, some of which are shared with Dome C, superimposed upon surrounding terrain which appears to be a single ancient unit with a dense covering of impact craters. The western margin of the flow apron surrounding Dome B has a 3 km impact crater, largely infilled with debris; several smaller impact craters are also located on this apron, and on Dome B itself.

Image of Dome B with close up details. (a) Detail of Dome B and surrounding flow and marked position of HiRISE image ESP_033977_1385 with marked positions of MOLA PEDRs and associated topographic profiles. (b) Detail of HiRISE image showing the contact between the northwestern edge of the dome and the underlying unit. Large boulders forming the dome itself and aeolian deposits at the top of the dome together with modification by gully activities are clearly visible. (c) Detail of the bedrock on which Dome B is superposed as exposed by an ∼80mhigh scarp. Pristine fracture morphologies suggest ongoing scarp erosion. Note that the talus is mainly formed by fine-grained material and small amounts of boulders less than ∼6metres large (marked by gray arrows), larger blocks are missing. The tensile fractures parallel to the scarp in the capped unit (marked by white arrows) are similar to fractures associated with rotational block-fall landslides known from Earth. (d) Detail of HiRISE image showing another part of the scarp. Again, no large blocks of fallen rocks are visible and the talus is composed mainly of finer particles and smaller boulders (marked by gray arrows). Two white arrows indicate small grooves, which might have formed by ongoing aeolian erosion. These grooves show that the exposed material is susceptible to erosion. Brožet al. (2015).

Dome C is roughly 3.5 km by 6 km and 530 m in height, again being higher closer to its northern margin.

Image of Dome C and surrounding flow. (a) The dome edifice is clearly surrounded by a flow structure that has steep edges, as demonstrated by the shadows on the southern margin. White arrow marks scarp on the edge of Flow 3. Position of MOLA PEDRs marked by lines and HiRISE image ESP_026474_1385 marked by dashed rectangle. (b) Detail of HiRISE image covering part of dome flanks (left part of image b) and flow structure (right part). Note large boulders forming the dome itself and aeolian deposits forming Transverse Aeolian Ridges (marked by white arrow). (c) The edge of the flow structure on the border with surrounding older, flat layer. The margin of the internal flow is formed by large-scale boulders that are partly covered by aeolian material (marked by white arrow). The older unit contains small Transverse Aeolian Ridges. Brož et al. (2015).

The ages of deposits on other planets are usually worked out by measuring the density of impact craters on those deposits, since impacts are thought to have occurred at a steady rate for much of the history of the Solar System. However none of the volcanic deposits examined by Brož et al. were extensive enough to use of this method. Instead the ages of the deposits were worked out by their stratigraphic relationships with other deposits for which age estimates were available; this is possible because a rock formation must be younger than a bed lying beneath it and older than a bed laid down on top of it, enabling geologists to estimate the age of geological strata in the absence of direct data.

Using this method it was determined that the flow apron associated with Cone T1 is approximately 600-800 million years old, while Flow 1 (also associated with Cone T1) is approximately 560-760 million years old. The flow apron around Cone T2 is estimated to be 370-570 million years old, while Flow 4, which originates beneath this flow, is 420-520 million years old. It was not possible to obtain dates for the dome structure, but these are thought to share a common origin with the cone structures, are therefore thought to be of similar ages.

Absolute model ages of (a) the crater’s ejecta and Flow 1, and (b) Cone T2, and Flow 4. (a) The cumulative crater size-frequency curves indicate an absolute model age 660  ± 100 Ma for Flow 1 and 700  ± 100 Ma for the crater’s ejecta. (b) The cumulative crater size-frequency curves indicate an absolute model age of 470  ± 100 Ma for Cone T2 and 470  ± 50 Ma for Flow 4. Note that for Flow 4 a resurfacing correction was applied to exclude larger craters, which underlie, and therefore predate Flow 4. Note the panels above the cumulative crater size-frequency plots represent the randomness analyses. Brož et al. (2015).

A spectrographical analysis of the mineral composition of exposed boulders on Domes D and C and Flow 3 suggests that these have compositions with low olivine abundances and moderate levels of pyroxene. Such mineral compositions are typical for terrestrial lava domes, and are associated with mature lavas; lavas that have been cooling for some time close to the surface before being extruded and have a distinctive mineralogical composition because of this (on smaller, chillier Mars such cooling may have occurred deeper within the planet). This is quite different from previous small volcanic structures recorded on Mars, such as scoria cones and tuff rings, which on Earth are typically formed by the eruption of younger, less viscous lavas. The margins of the flows are also quite steep, in places up to 20°, which is also indicative of more viscous mature lavas, and quite different from previously observed basalt flows in the Tharsis or Elysium volcanic provinces, which show very gentle flank slopes.

See also…

Hydrated silicate minerals in the Mariner Valley, Mars.

Hydrated minerals (minerals containing water) are considered to be evidence of the former presence of liquid water on Mars. They have been observed at a...

 

 

Interpreting landslide deposits in the Mariner Valley, Mars.

Landslides on Mars typically have much greater runout distances than those on Earth, due to the planets lower gravity and thinner atmosphere. This can lead to areas of layered deposits from different landslides quite distant from the source, particularly within the larger canyons on Mars. Since it is possible to produce approximate ages for such deposits based upon the number of impact...

 

Ice in Martian impact craters.

The surface of Mars has been observed continuously by the Mars Orbiter Camera from 1997 to 2006 and the Mars Reconnaissance Orbiter since 2006. During the time that these observations have been occurring around 200 new impact craters have been observed on the surface of the planet; most of them in dusty areas, where they are easily detected due to the dark blast patterns that surround fresh impacts in these...

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