Estimating the benefits of agroforestry to European wildlife.

The term agroforestry is used to denote practices in which the cultivation of trees is integrated either with the rearing of livestock (in which case it is called silvopasturalism) or other plant crops (silvoarablism). This is a traditional practice across much of Europe, where methods such as grazing livestock in orchards are very widespread, with newer methods being developed more recently, such as short-rotation coppicing being carried out alongside rows of other crops. Systems in which productive trees are grown around the edges of fields are also sometimes considered to be agroforestry, although in these cases the trees are managed separately to the other produce, and may be under separate ownership.

 

Pigs grazing in an open Oak forest system in Spain, a system known as a 'dehesa'. Álvarez (2016).

Europe has suffered particularly severe losses of biodiversity compared to other parts of the world, and this is particularly severe in areas where intensive agriculture is prevalent. Agroforestry promotes a more diverse landscape than arable monoculture, potentially resulting in higher biodiversity. Quantifying the benefits of this could potentially lead to the system being more heavily prioritised under the European Common Agricultural Policy or any successor system.

Agroforestry systems have been well studied in tropical environments, where the evidence suggests that the system offers significant advantages in biodiversity preservation over intensive monocultural systems, but nevertheless tends to lead to reduced biodiversity compared to both primary and secondary forests. The system is less well studied in temperate regions, with most studies tending to concentrate on single groups of Animals, such as Birds or Insects. This leaves the benefits of such systems in Europe somewhat unclear, particularly as the definitions of agroforestry can vary, leading to differences in what systems are included in studies, making comparisons between studies difficult. 

 

Hazel short rotation coppice system alongside crops in Suffolk, UK. Smith et al. (2014).

In a paper published in the journal BMC Ecology and Evolution on 23 October 2021, Anne‑Christine Mupepele of Nature Conservation and Landscape Ecology and Biometry and Environmental System Analysis at the University of Freiburg, and Matteo Keller and Carsten Dormann, also of Environmental System Analysis at the University of Freiburg, present the results of a meta-analysis which combined results from a number of studies of agroforestry systems across Europe.

Mupepele et al. sought to answer three questions, 'What is the effect of agroforestry on biodiversity relative to forests, pastures, cropland or abandoned, shrub-encroached agroforestry?', 'Is the effect of agroforestry on biodiversity influenced by environmental variables, specifically the kind of agroforestry system (silvopasture or silvoarable), sampling method, the specific measure of biodiversity, sampling year, country, climate and the reference used?' and 'How strong and robust is the underlying evidence of these results?'

To which end they located 1411 previous studies of agroforestry systems in Europe, 50 of which were eventually included in the study, representing 69 individual agroforestry sites. Each of these had a direct comparison of a type of agroforestry (silvoarable or silvopastoral) to forests, cropland, pasture, and/or abandoned agroforestry systems.

 

Map of Europe with the number of effect sites per country. Mupepele et al. (2021).

The studies included in the analysis covered sites across Europe where agroforestry systems have been studied between 1984 and 2019. The majority of these sites were caried out in Iberia and the Mediterranean region, with twelve studies from Spain, eight from Portugal, five from Italy, one from France and one from Turkey. Temperate central Europe was represented by six studies from the UK, four from Romania, two each from France, Germany, and Switzerland, and one each from Belgium and northern Italy. The northern boreal region was represented by four studies from Sweden and two from Finland.

Thirty six of the included studies looked at silvopastoral systems, with thirty six studies looking at 52 sites, while silvoarable systems were the subject of thirteen studies looking at seventeen sites. The biodiversity of agroforestry was most commonly compared to that of pasture (23 sites), or forests (21 sites), then abandoned agroforestry systems (thirteen sites) and cropland (12 sites).

 

Sheep grazing in a plantation of Pine and Eucalyptus in Spain. Monica Pelliccia/Mongabay.

The different studies measured biodiversity in different ways, and concentrated on different groups. In order to make a comparison between these diverse studies, Mupepele et al. divided the measured wildlife into five groups, Arthropods, Birds, Bats, Plants, and 'Fungi plus Lichens and Bryophytes', Most of the included studies measured biodiversity at the 'species richness level', although other measures were used.

Mupepelele et al.'s results showed no overall benefit for biodiversity compared to the average derived from all systems. However, silvoarable systems were found to host considerably more biodiversity than other croplands, although they generally hosted less biodiversity than forests. Silvopastoral systems produced less clear results, with measures often producing conflicting results in different studies (i.e. one study might show higher Avian biodiversity in a silvopastoral system than a forest, while another showed the reverse.

Birds and Artropods were typically found at higher levels of diversity in agroforestry envoronments than other systems, Where the original group sorted Arthropods into different groups (e.g. Bees, Beetles and Spiders', then this biodiversity increassed, although this was across all environments, with no change in the beneficial effect of agroforestry.

 

Cereal crops grown alongside trees in Bedfordshire, UK. Agroforestry Research Trust.

Mupepele et al. note that the quality of the studies they were referencing varied somewhat, with some using replicated experimentation with clear controls, whilst others were more observational in nature. To compensate for this, they tried applying a statistical weighting method that gave more value to the more statistically strong studies, but found this made no difference to the overall result. They also carried out funnel plot and Egger’s regression tests for undetected biases in their data, but did not find bias was a problem.

A previous  meta-analysis led by Mario Torralba of the Department of Geosciences and Natural Resource Management at the University of Copenhagen found that agroforestry had a much stronger impact on biodiversity, which caused Mupepele et al. to consider the differences between their findings and that of the earlier study. They note that Torralba et al.'s study was published in 2016, and contained the results from two studies published in 2015 on the benefits of agroforestry in Mediterranean ecosystems, both of which produced very strong positive results, and that if these were excluded from Torrialba et al.'s data then the result was closer to that of Mupepele et al. who included several post 2015 studies with less clear results.

Properly done, meta-analyses can provide a powerful tool for understanding ecological systems in ways not possible from individual studies or unsystematic literature searches. However, the robustness of these results is dependent on the methods used to analyse the data, and in particular the use of weighting to take into account the quality of the studies being referenced. This needs to be done carefully, as failure to apply the right weighting can often lead to very different results. This said, applying weighting to Mupepele et al.'s results resulted in no significant change in the outcome of the study, which strongly supports the robustness of their findings. 

The application of repeated meta-analyses to the same data set can reveal changes over time, as new studies add to the overall picture, dampening the results from atypical studies that might have a profound impact on a smaller data-set. By building a cumulative model in which data were added in chronological order, Mupepele et al. were able to demonstrate that the impact of agroforestry upon biodiversity remained essentially unchanged over time, despite the presence of some anomalous data. They do, however, note that silvoarable systems make up a relatively small proportion of the whole, and that the addition of a higher proportion of studies of these systems in future might change the results of the meta-analysis.

 

Merino Sheep under a Cork Oak in a montado silvopastoral system in Portugal. European Agroforestry Foundation.

The ability to reproduce results is an important principle in science, but can be difficult in fields like ecology, which look at complex natural systems, no two of which are ever completely the same. Mupepele et al.'s results differed strongly from the earlier results of Torralba et al., resulting in their drawing different conclusions; Torralba et al. concluded that agroforestry has a general positive impact upon biodiversity, while Mupepele et al. concluded that this benefit was only clear when agroforestry was compared to croplands, despite both studies having used much of the same data. Mupepele et al. note that Torralba et al. included hedgerows and woody riparian buffers to agricultural land as agroforestry, while Mupepele et al. excluded them on the basis that they are not emplaced for silvicultural purposes (i.e. the trees used in these settings are grown for their value as boundaries, not as a crop in themselves). Neither did Torralba et al. include data from studies which suggested agroforestry had a negative impact on biodiversity. Mupepele et al. believe that scientists should be very clear about what data they are including in meta-analyses, the criteria for choosing this data, and the reasons to do so, in order to help policy-makers judge the significance of different studies. 

Mupepele et al. conclude that silvoarable systems produce an increase in biodiversity compared to conventional croplands, particularly with regard to Birds and Arthropods, but that this increase is not large, and there was no overall positive benefit of agroforestry to all other settings. Notably, silvopasturalism showed no clear benefit over either forestry or conventional pasturelands. Where previous studies have produced enthusiastic support for agroforestry, and strongly suggested these systems are linked to a significant increase in biodiversity, Mupepele take a more cautious approach, noting that relatively few studies find an unqualified link between agroforestry and increased biodiversity, and that literature reviews and meta-analyses need to be careful to include both the positive and negative impacts of systems when drawing on data from multiple studies. Nevertheless, they do conclude that agroforestry can have a positive impact on biodiversity under some circumstances, as well as providing carbon sequestration and other ecosystem services, and that a better understanding of how these systems work could lead to more informed future decisions by policy makers.

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Vishnuonyx neptuni: A new species of Otter from the late Miocene Hammerschmiede Fauna of Bavaria.

The Hammerschmiede locality in Bavaria, Germany, has been producing Late Miocene Vertebrate and Invertebrate fossils for almost five decades. The fossils are predominantly found within a series of river channels, and date to between 11.62 and 11.44 million years ago. Over 130 species of Vertebrate have been recorded from this location, several of which are unknown from any other location, most notably the Ape, Danuvius guggenmosi. Despite this high diversity, to date only two species of Carnivorans have been described from Hammerschmiede, the Palm Civets Semigenetta sansaniensis and Semigenetta grandis. 

In a paper published in the Journal of Vertebrate Palaeontology on 16 September 2021, Nikolaos Kargopoulos of the Department of Geosciences at Eberhard Karls University of Tübingen, Alberto Valenciano of the Departamento de Ciencias de la Tierra, and Instituto Universitario de Investigación en Ciencias Ambientales de Aragón at the Universidad de Zaragoza, Panagiotis Kampouridis, also of the Department of Geosciences at Eberhard Karls University of Tübingen, and Thomas Lechner and Madalaine Böhme, also of the Department of Geosciences at Eberhard Karls University of Tübingen, and of the Senckenberg Centre for Human Evolution and Paleoenvironment, describe a new species of 'Bunodont' Otter from the Hammerschmiede locality.

The phylogeny and classification of Otters, Lutrinae, is generally poorly resolved, with the relationship of fossil groups such as the Potamotheriinae and Bunodonts to modern Otters remaining unclear. Bunodont Otters are a (probably paraphyletic) group of large and very large fossil Otters found across Eurasia, Africa, and North America. The classification of this group is itself poorly resolved, with uncertainty about which fossils should be included within it, with one genus, Enhydrictis, having been recently reclassified and no longer thought of as an Otter at all. 

The new species is placed in the genus Vishnuonyx, which contains previously described species from the Miocene of South and Southeast Asia and the Miocene and Pliocene of Africa, and given the specific name neptuni, in reference to Neptune, the Roman god of the seas. The species is described from a right hemimandible, with a first premolar alveolus (canine tooth socket) and the second, third and fourth premolars and first molar intact. A left hemimandible and a number of isolated teeth are also referred to the species. All were recovered from river channel deposits at Hammerschmiede between 2011 and 2020, during excavations carried out by palaeontologists from Eberhard Karls University of Tübingen.

 

The lower dentition of Vishnuonyx neptuni. (A) SNSB-BSPG 2020 XCIV-0301, right hemimandible (holotype; original specimen and screenshots of its 3D model) in labial (A1), (A4), lingual (A2) and occlusal (A3) views; (B) GPIT/MA/16733, left hemimandible in labial (B1), lingual (B2), (B4) and occlusal (B3(, (B5) views; (C) SNSB-BSPG 2020 XCIV-1301 right p4 in labial (C1), lingual (C2) and occlusal (C3) views. Screenshots of the 3D models not in scale. Kargopoulos et al. (2021).

The third premolar of Vishnuonyx neptuni is distinctly asymmetrical, with the bak of the tooth being visibly lager than the front. The main cusp has three crista (ridges) deriving from it, one at the front, one at the back, and one on the inside edge. The inside crysta expands to form part of the wall of the tooth, which hosts one of the three roots of the tooth.

The upper carnassial tooth (front molar, modified to have a cutting edge which occludes the cutting edge on the lower tooth, a diagnostic trait of Carnivorans) has moderate wear on its cutting edge. The tooth has a strong cingulum (convex protuberance below the crown). The tooth has a high paracone (front outer) cusp, which is attached to a ridge a metastyle (ridge on the outer surface). There is no carnassial notch (notch used to hold bone for a crushing bite, a common Carnivoran trait). The protocone, parastyle, and paracone cusps form a line, with the hypocone outside this, behind the paracone. 

 

The upper dentition of Vishnuonyx neptuni. (A) SNSB-BSPG 2020 XCIV-1022, left P3 in occlusal (A1), labial (A2) and lingual (A3) views; (B) GPIT/MA/17347, right P4 (original specimen and screenshots of its 3D model) in occlusal (B1), (B4), labial (B2), (B5) and lingual (B3), (B6) views; (C) SNSB-BSPG 2020 XCIV-1552, left M1 (original specimen and screenshots of its 3D model) in occlusal (C1), (C4), labial (C2), lingual (C3), mesial-occlusal (C5) and mesial (C6) views. Abbreviations: meta, metacone; metal, metaconule; lp, lingual platform; para, paracone; paral, paraconule; prot, protocone; protl, protoconule. Kargopoulos et al. (2021).

The upper molar is complete with some wear to the inner sides of the paracone and metacone. A distinct cingulum is present, although this is less developed towards the forepart of the toorh. The tooth is nearly rectangular in outline, and somewhat slim, although the outer suface is slightly higher at the fore than at the rear. The inner cusps are roughly the same height, and connected by a low crest with a notch towards its middle. Behind these two further cusps, the protoconule and metaconule rear cusps are connected by another ridge.

Neither of the two hemimandibles is complete. The right specimen preserves part of the socket of the canine, part of the angular process (bottom part of the back of the jaw) and part of the masseteric fossa (muscle attachment above the angular process), and everything in between. The muscle attachment is deepest at the top, the hind part of the hemimandible bent inwards, and the angular process small and hook-like. The left mandible is less well preserved, with only part of the canine socket, the cheek teeth and part of the masseteric fossa muscle attachment, however, on this specimen the attachments for the masseter pars superficialis and pars profunda muscles can be seen.

The canine is absent in both hemimandibles. The second premolar is two-rooted and has a distinct cingulum (convex protuberance beneath the crown), particularly on the inner side and towards the rear. The long axis of this tooth is not in line with the long axis of the tooth row. The third and fourth premolars are high and pointed, with their largest cusps towards the middle and inclined backwards. In the third premolar only a single cusp is present, and the tooth has a wrinkled surface. The fourth premolar is taller than the third (and the first molar), and has a second, smaller cusp behind the first. The first molar is broad, with a talonid (crushing region) covering about a third of its surface. The first cusp is wider than this crushing area. This tooth also has wide cingulum. 

The dentition of Vishnuonyx neptuni differs considerably from that of modern Otters, but fits well with the genus Vishnuonyx, which has been confidently assigned to the group based upon more complete material. This genus has previously been described from Asia and Africa, with the Asian specimens being older and less robust than the African specimens; Vishnuonyx neptuni, the first described European member of the genus, appears to be intermediate between these groups in both age and size.

 

Temporospatial distribution of the known species of the genus Vishnuonyx. Kargopoulos et al. (2021).

The oldest known member of the genus, Vishnuonyx maemohensis, comes from Mae Moh in Thailand, and is dated to the Middle Miocene, between 14.2 and 13.2 million years old. Slightly younger is the late Middle Miocene Vishnuonyx chinjiensis from the Siwalik Hills of northwest India, which is thought to be between 13.8 and 12.7 million years old. A second specimen assigned to Vishnuonyx chinjiensis has been described from Ngorora in western Kenya, this specimen being about 12 million years old. Also known from Africa is the Latest Miocene Vishnuonyx? angololensis from Lower Nawata in Lothagam, near the southwestern shores of Lake Turkana, Kenya, which is dated to between 7.3 and 6.6 million years ago. Finally, an Early Pliocene specimen Vishnuonyx sp. has been recorded from Ethiopia and is thought to be between 5.2 and 4.85 million years old. The HAM 4 fossiliferous layer at Hammerschmiede, which produced Vishnuonyx neptuni, has been dated to 11.44 million years ago, making Vishnuonyx neptuni slightly younger than Vishnuonyx chinjiensis.

This suggests that the genus Vishnuonyx originated in Southeast Asia, and migrated westwards through the Indian subcontinent, reaching Europe and East Africa by the end of the Middle Miocene, with the genus persisting in East Africa into the Pliocene. Vishnuonyx neptuni is sufficiently different from other members of the genus to imply a reasonably long period of reproductive isolation, leading Kargopoulos et al. to reason that its ancestors reached Europe before 11.5 million years ago.

Vishnuonyx is thought to have had a semi-aquatic lifestyle similar to that of modern Otters, suggesting that it is most likely to have dispersed from Southeast Asia into South Asia, Europe, and Africa, by water. During the Middle Miocene the Central Paratethys Ocean separated East Africa and Arabia from southern Asia, with the Eastern Paratethys, separated from the Cenral Paratethys by the Araks Straight, forming a connection with Central Europe. This would have provided a plausible route for the westward expansion of the genus Vishnuonyx.

 

The proposed dispersal path of Vishnuonyx from South Asia towards Central Europe and East Africa during the Konkian around 13 million years ago (late Badenian, early Serravalian). Kargopoulos et al. (2021).

The teeth of Otters are adapted for two slightly different diets; catching slippery Fish, and breaking open hard shells. All living Otters pursue both these feeding methods to some extent, although they vary in the extent to which they depend on each method, which is reflected in their dental morphology. In species which primarily feed on hard-shelled prey such as Clams or Crustaceans, the molars tend to be broad and flat with well-developed crushing areas, whereas those that specialise in catching Fish tend to have high pointed cusps useful for catching slippery prey. The teeth of Vishnuonyx neptuni are apparently more adapted towards a piscivorous diet, with high, pointed premolars and a strong cutting blade on the molar. This is at odds with other 'Bunodont' Otters, which appear to have been primarily durophages specialising in hard prey, and suggests an ecology similar to that of the modern Amazonian Giant Otter, Pteronura brasiliensis.

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Eruption on Mount Aso, Japan.

Mount Aso, a volcanic caldera on central Kyūshū Island, Japan, erupted at about 11.45 am local time on Wednesday 20 October 2021, according to the Japan Meteorological Agency. The eruption produced a column of gas and ash about 3.5 km high from its No. 1 Nakadake Crater, a pyroclastic flow was observed on its western flank, and cinder falls recorded as far as 900 m from the crater. The eruption occurred without warning, and 16 climbers were on the volcano when it happened, although all have been accounted for and are uninjured.

 

An ash column over Mount Aso, Kyūshū Island, Japan, on 20 October 2021. AP.

The Aso Caldera is the largest in Japan, and one of the largest in the world, being approximately 25 km in diameter. The caldera is thought to be the result of a series of four massive eruptions, the first of which took place around 300 000 years ago, and the last around 90 000 years ago. The caldera contains five smaller summits in a complex at its centre, the highest of which Mount Taka, rises 1592 m above sea-level. These are also a series of hot springs within the caldera, which is a popular tourist resort and part of the Japanese Geoparks Network. Despite the site's dramatic history modern eruptions tend to be quite small, and there are no records of any historical fatalities connected with the volcano.

 

Eruption on Mount Aso, Japan, on 20 October 2021. Tikara Zukeyama/Instagram.

Japan has a complex tectonic environment with four plates underlying parts of the Islands; in addition to the Pacific in the east and the Othorsk in the North, there are the Philipine Plate to the south and the Eurasian Plate to the West. Kyūshū Island lies at the northeast end of the Ryukyu Island Arc, which sits on top of the boundary between the Eurasian and Philippine Plates. The Philippine Plate is being subducted beneath the Eurasian Plate, in the Ryukyo Trench, to the Southeast of the Islands. This is not a smooth process, with the two plates continuously sticking together then breaking apart as the pressure builds up, leading to frequent Earthquakes in the region.

 

The movement of the Pacific and Philippine Plates beneath eastern Honshu. Laurent Jolivet/Institut des Sciences de la Terre d'Orléans/Sciences de la Terre et de l'Environnement.

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Investigating the drivers of methane release from cold seeps on the Southern Hydrate Ridge off the coast of Oregon.

Cold seeps release methane along most coastal margins, either as gas dissolved in water or as bubbles which rise through the water column. Bubbles of methane released from cold seeps have been observed rising hundreds of meters through the water column in the Guaymas Basin in the Gulf of California, on the continental rise off the coast of the Carolinas, along the Cascadia Margin off the coast of British Colombia and the northwestern US, in the Okinawa Trough in the East China Sea, in the Black Sea, and in the Gulf of Mexico, as well as at an experimental station in Monterey Bay. When emitted at depths of less than a hundred metres, methane bubbles typically reach the surface and release their gas into the atmosphere, while bubbles released at greater depths the methane in them is lost into the water column as they rise. However, in areas where stable gas hydrates exist at deeper depths, then the bubbles may have a hydrate coating, which slows dissolution of their gas content, allowing bubbles from greater depths to reach the surface. 

This gas is thought to be of minor significance compared to other sources of carbon entering the atmosphere, but the amount gas produced, and how this varies both geographically and over time, has been the subject of very little investigation. The amount of gas released at different sites is known to vary considerably, which is thought to be related to factors such as the subsurface structure and the source of the methane. External factors are also thought to affect the amount of methane being released, such as variations in hydrostatic pressure caused by the action of tides, swell, or storms, changes in sea temperature linked to seasons or longer term changes, seismic tremors, and even to isostatic rebound.

Hydrate Ridge forms part of the Cascadia Subduction Zone, an anticlinal ridge on the accretionary wedge with a north-south orientation. This ridge has two distinct areas of cold seeps, known as the Northern and Southern Hydrate Ridges, both of which have extremely large methane hydrate deposits close to the sediment surface, which lead to persistent gas emissions in the form of bubble plumes, as well as extensive chemosynthetic ecosystems and authigenic carbonates. Seismic profiling of the Southern Hydrate Ridge has found a reflective surface, thought to be the base of the gas hydrate zone, at 125 m beneath the sediment surface. This hydrate zone overlies a structure called Seismic Horizon A, along which methane-rich fluids appear to migrate from the underlying accretionary wedge. Bubble plumes are extremely visible here, and have not been shown to fluctuate with tidal cycles, as is the case on the Northern Hydrate Ridge. Instead, the rate of bubbling appears to follow a cycle unrelated to the tides. It has been suggested that this might be caused by the formation of gas hydrate seals close to the surface, which remain in place until the amount of gas accumulated in the sediment beneath them overcomes their resistance, then reform when gas emissions tail off sufficiently, a cycle which can take several years to complete. More recent surveys have established that while bubbling is not constant on the Southern Hydrate Ridge, it does appear to occur consitently at the same sites on the ridge, and can vary on an hourly timescale. This unexpected behaviour has led to a need for a more organised long-term monitoring of this system in order to understand it.

The systematic monitoring of activity has been carried out at several cold seeps, including a study on Clayoquot Slope, off the coast of Canada on the northern Cascadia Subduction Zone which lasted over a year, although the equipment used in that study was only able detect gas emissions when bottom currents were flowing in certain directions, hampering understanding of the system.

In a paper published in the journal Geochemistry, Geophysics, Geosystems on 21 September 2021, Yann Marcon of the Center for Marine Environmental Sciences and Department of Geosciences at the University of Bremen, Deborah Kelley of the School of Oceanography at the University of Washington, Blair Thornton of the Centre for In Situ and Remote Intelligent Sensing at the University of Southampton, and the Institute of Industrial Science at the University of Tokyo, and Gerhard Bohrmann, also of the Center for Marine Environmental Sciences and Department of Geosciences at the University of Bremen, present the results of a project that carried out systematic monitoring of the Southern Hydrate Ridge between July and November 2018.

Marcon et al. monitored gas emissions on the Southern Hydrate Ridge with the Southern Hydrate Ridge Rotating Sonar, which consists of a multibeam echosounder mounted on a rotator, with a rotating range of 360°. In addition they used a single-beam scanning-sonar connected to the Ocean Observatories Initiative's Regional Cabled Array to provide a finer detail monitoring gas releases at Einstein’s Grotto, one of the methane vents on the Southern Hydrate Ridge, and photo cameras also connected to the Regional Cabled Array to provide visual ground-truthing information about the dynamics and strength of bubble release, at Einstein’s Grotto and the Summit-A vent area. 

 

Top left: Location of Southern Hydrate Ridge. Top right: Map of the primary infrastructure of the Ocean Observatories Initiative's Regional Cabled Array observatory. Bottom: Overview map of the Southern Hydrate Ridge summit with the location of the Southern Hydrate Ridge Regional Cabled Array fibre optic cables, junction boxes and monitoring instruments. Shaded areas show the location of the main known vents. Marcon et al. (2021).

The bathymetry (depth of water) of the area was established in 2008 by the Autonomous Underwater Vehicle Sentry, operating from the Research Vessel Thomas G. Thompson, during preparation for the laying of the Ocean Observatories Initiative's Regional Cabled Array. A 3D photomosaic of the area was developed from imagery gathered by the University of Tokyo's Autonomous Underwater Vehicle AE2000f during the Schmidt Ocean Institute's FK180731 #Adaptive Robotics expedition.

A conductivity, temperature, and pressure (CTP) probe equipped with a dissolved oxygen optode sensor was used to monitor the conductivity, temperature, pressure, and dissolved oxygen concentration of the bottom water at one minute intervals. High-frequency tidal seafloor pressure was recorded with a tsunami pressure sensor. Current velocities were monitored with an upward-looking acoustic Doppler current profiler. Seismic data was collected by three ocean-bottom seismometers connected to the Regional Cabled Array. Wave height data was obtained from the National Data Buoy Center of the National Oceanographic and Atmospheric Administration.

Seafloor pressure at the Southern Hydrate Ridge was found to vary considerably with the tide, and was sensitive to the neep/spring tide cycle, with pressure varying by 1.4 dbar during neap tides and up to 3.8 dbar during spring tides. The seafloor temperature varied between 3.8°C and 4.5°C over the course of the study (June-November 2018) and between 3.7°C and 4.7°C over the longer period June 2018-June 2020, showing no long-term warming or cooling trend. Seafloor salinity varied between 34.26 to 34.37 psu over the period June-November 2018, while dissolved oxygen varied between 0.24 and 0.30 ml/L.

The Southern Hydrate Ridge Rotating Sonar carried out scans every two hours during the four month period of the experiment (with some gaps due to technical problems), producing a total of 888 readings. Of these, 886 (99.8%) detected gas bubbles, suggesting that bubbling was more-or-less constant. However, the amount of bubbling varied continuously, with peaks and troughs on a twice daily basis. These peaks and troughs appear to become larger at the spring tides and weaker at the neap tides, although the short duration of the experiment combined with the gaps in the data make it difficult to be completely confident about this.

Bubbling appears to be possible at any recorded pressure on the Southern Hydrate Ridge, although it was more pronounced when the pressure was low or falling. Two thirds of all peaks in activity occurred at times when the pressure was below average. The majority (65-70%) of the remaining third of peaks happened when the tide was decreasing, or changing (18-20%), with only a small proportion (15%) occurring while the tide was rising. However, there were some anomalies with this, most notably a large bubble release in July 2018m which occurred during a rising tide when the water pressure was above average.

 

Temporal variations of the Southern Hydrate Ridge Rotating Sonar backscatter magnitude and bottom pressure between 6 and 22 July 2018 (top plots) and between 19 October and 8 November 2018 (bottom plots). The backscatter magnitude non-linearly reflects the strength of the gas bubble emissions. The bottom pressure plot shows the local mixed tidal regime with diurnal and semi-diurnal constituents, as well as the fortnightly neap/spring tidal cycles. Bubble release is commonly stronger during ebb tide, and possibly also during spring tidal phases. However, some ebullition events do not correlate with the tide and may be triggered by local accumulation of pressurized free gas in the subsurface; the prominent peak observed on 18 July 2018 corresponded to the reactivation of the Summit-A vent after a very short venting interruption of about 4 hours; it did not affect the other vents at the Southern Hydrate Ridge summit and happened during flood tide within a neap tidal phase, hinting at shallow, local changes in the sediments. Marcon et al. (2021).

Eight different centres of bubbling were identified on the Southern Hydrate Ridge, with an average of four active at any given time. Five main clusters of activity were identified; each of these being 10-30 m across, and capable of producing more than one stream of bubbles at a time. These were named Smokey Tavern, Einstein's Grotto, Summit-A, Summit-B, and Summit-C. Bubbling at all of these sites was frequent, but not continuous, with sites remaining dormant for days at a time and all sites being active at the same time being a rare occurrence. As well as these main sites of activity around the summit of the Southern Hydrate Ridge, Marcon et al. detected at least six smaller venting sites, further away from the summit and much less active, which they named Smokey Tavern West, Summit-D, Far NE, Far S, Summit South, and Summit SW. Unlike the main sites, these sites were only sporadically active, and would often remain dormant for months between bouts of venting.

 

(a) Location of flare base points recorded with the Southern Hydrate Ridge Rotating Sonar between 8 July and 8 November 2018; the base points are grouped into clusters marking the location of the different Southern Hydrate Ridge vent sites. (b) Location of the main and periphery vents overlain on the photomosaic; the main vent sites are all located on areas covered with microbial mats. (c) Close-up view of the Southern Hydrate Ridge Rotating Sonar location and the Summit-A vent, with the 3D photomosaic in the background; a depression on the seafloor from Ocean Drilling Program drill site 1949 (ODP Leg 204) can be seen in the top-left corner as well as in the bathymetric data. (d) Close-up view of the 3D photomosaic at the Smokey Tavern vent showing the distribution of the microbial mats and the domed, collapsed and hummocky areas. Marcon et al. (2021).

The seafloor around the main vents is highly uneven, and covered by white microbial mats. These areas have numerous up-domed mounds, which extend laterally by as much as 35 m, with hummocky, jagged-edged depressions eaten into them, which are probably caused by the more-or-less constant venting of gas from these areas. The venting of gas does appear to be strongly tied to these areas, with venting restricted to them at all sites except Smokey Tavern, where it occurs across the entire microbial mat-covered dome structure. In the peripheral venting area this kink appears to be broken, with dark sediments and white areas of microbial matting visible, but venting not connected to these.

The sites closest to directly underneath the Southern Hydrate Ridge Rotating Sonar (Summit A and Summit B) appeared to be the most active, with sites moving away from this area being progressively less active. This might indicate that the sonar was better act detecting gas emissions directly beneath it and less good at detecting them further away. Alternatively, it might be a reflection the fact that the sonar was placed directly over the highest point of the Southern Hydrate Ridge, so that the most active points were those in the shallowest water, with sites becoming progressively less active as the water grows deeper.

None of the vents was active all the time, although venting over the wider Southern Hydrate Ridge area was continuous. During the period 6-22 July 2018, The Summit A vent was active 75% of the time, Einstein's Grotto was active 69% of the time, Smokey Tavern was active 65% of the time, Summit B and Summit C were active 54% of the time, Summit D 22% of the time, Far NE 9%, Far S 2%, and Smokey Tavern West was active 1% of the time, while Summit South and Summit SW were completely inactive. During the period 19 October-8 November 2018, the Smokey Tavern vent was active 44% of the time, Summit B 30% of the time, Summit C 70%, Summit D 23%, and Smokey Tavern West was active 6% of the time. A sudden decrease in activity at one site was usually accompanied by an equally sudden increase at another. For example, a pause in venting at Summit A on 10 July 2018 was marked by a sudden increase at Summit C. On 12 July activity at Summit C fell off again, and activity resumed at Summit A and Summit B. During the period 19 October-8 November 2018, Summit B was only active when Summit C was inactive, and vice versa. These synchronous changes in activity were frequent enough that they are unlikely to be coincident. Their timing also appears to be linked to the daily tide cycles. 

Southern Hydrate Ridge Rotating Sonar magnitude data from 19 October to 8 November 2018 for individual plume clusters. Einstein's Grotto and Summit-A are not shown because of the restricted 245° scanning sector of the Southern Hydrate Ridge Rotating Sonar after 10 October 2018. Far NE, Far S, Summit South, and Summit SW are not shown because they were inactive over this period. The vertical axis is logarithmic to facilitate visualisation of low magnitude variations. Absolute magnitude values cannot be compared between the clusters due to a distance bias and are not shown. Marcon et al. (2021).

A closer examination three vents at Einstein's Grotto over a 24 hour period found that, while they were all bubbling continuously throughout the day, the amount of outflow from them varied considerably over the day, with the bubbles undergoing sudden peaks in activity, followed by slower tailing off, several times per day. Furthermore, these peaks in activity always occured when the tide was high or waning.

Cameras directed at the Summit A and Einstein's Grotto vents were able to confirm that the events detected by sonar were in fact bubble releases. The one directed at Einstein's Grotto further revealed that the depression from which the bubbles were escaping was growing slightly wider with each bubble-release event, probably due to the loss of gas hydrates within the sediment, and that during major bubble releases this was accompanied by slumping of the walls of the depression and the collapse of overhanging sections. In some cases, hydrates were exposed by sediment loss, and then rapidly lost. Currents on the seafloor also appeared to contribute to the rate of erosion, by slowly breaking up blocks of sediment. The video films also revealed that events recorded as coming from a single vent could in fact come from different locations within the vent at different times, and that sometimes the location of bubble releases shifted in response to sediment movements. On occasions bubbles were seen emerging from as many as four different locations within a vent.

It was only possible to measure the velocity of bubble streams when they were visible to a camera and immediately adjacent to a measuring scale, but those that could be measured rose at speeds of between 18 and 34 cm per second, with an average speed of 25 cm per second.

The camera directed at the Einstein's Grotto vent detected a significant gas blowout between 0.46 and 1.16 am on 23 July 2018, with a sudden release of a significant volume of gas, along with ejected sediment, which led to a change in the seafloor topography. The event happened about midway through a rising tide, with gas release being week, but active, both before and after the event. 

 

A pressure outburst was documented by the CAMDSB103 camera on 23 July 2018. The camera images show sediment resuspension shortly (0–30 min) after the outburst. Images taken after visibility improved show significant seabed changes including the presence of a large well-lithified sediment block into the collapsed area subsequent to the blow out. Scanning-sonar scans recorded before and after the event show that the seabed morphology at the location of the outburst changed over an area of at least 3 m². The hummocky area east of the sonar is part of the Einstein's Grotto vent. The range of the sonar scans is 20 m. The laser pointers on the camera images are 10 cm apart. In the difference plot, blue and red colours show negative and positive differences respectively. Marcon et al. (2021).

Turbulence in the upper part of the water column was relatively high throughout the study, with currents having an average velocity of 50 cm per second. The depth of this layer varied with season and time of day, between about 300 and about 400 m. Below this the water was much calmer, with velocities below 15 m per second, although here too there were daily and seasonal variations. Currents on the seafloor generally run from the northwest. Upwelling was also a significant component of bottom currents, with upwelling currents of up to 5 cm per second rising for about 300 m (i.e. to a depth of 470 m), where they generally became undetectable, although some reached as high as 400-350 m below the surface, by which time they had lost much of their initial velocity. These upwelling flows were intermittent, generally appearing when the prevailing northwest current was either weak or reversed, and appeared to be linked to episodes of bubbling, although this could be an indicator that the northwest current was deflecting bubbles away from the sonar beam.

Contrary to expectations, no connection was found between bubble outbursts and seismic events. A number of such events were measured during the study period, none of which were linked to conspicuous changes in bubble activity.

Wave height was found to be strongly seasonal in the study area, with the monthly average wave height between October and April exceeding 3 m and individual waves reaching close to 10 m, whereas between May and September the monthly average remained below 2 m, and the highest recorded waves little over 3 m. No correlation was found between wave height and bubble activity.

Macron et al.'s study shows that the Southern Hydrate Ridge has a number of vents (where 'vent' is defined as 'a distinct area of the seafloor where gas ebullition recurs') and that each vent has a number of bubble outlets, with vents capable of producing bubbles from several bubble outlets at once, often at different rates of activity.

Five main vents, and six peripheral vents were detected, a significant increase on the number found by previous, ship-based surveys, although it is possible that others exist and were missed by the study. Macron et al. note that bubble events have been recorded at two other sites on the Southern Hydrate Ridge (named Central and Pinnacle), that were outside the scope of their survey.

The discovery that there are multiple bubble-producing sites within each vent, which may be active at different times and at different rates at the same time, is new. This potentially presents additional challenges for ship-based surveys, as snapshots of bubbling from different locations might not be recognised as the same vent, particularly if the origin of bubbles is further complicated by varying currents.

The main vents all appear to be up-domed and hummocky areas covered by microbial mats. Such areas have previously been linked to the presence of gas hydrates at shallow depths within the sediment, something which was confirmed by Macron et al.'s study. The hummocky topography seen at these locations appears to be directly linked to bubble-venting, which causes changes the seafloor morphology quite rapidly at vent sites.

The loss of gas hydrate deposits is likely to be largely due to direct disolution of gas into non-methane saturated water, without the formation of bubbles. The vent zone of the Southern Hydrate Ridge lies within the gas hydrate stability zone (approximately between 500 and 900 m below sea level), so the excess loss of gas hydrates through bubble formation is unlikely in shallow sediments here. The depressions and hummocky surfaces observed at the site could as easily be formed by hydrate dissolution as by bubble formation, particularly at a site being regularly scoured by currents of non-methane saturated water. Based upon this, Macron et al. suggest that the bubbles observed venting on the Southern Hydrate Ridge are not being formed by the disassociation of gas hydrates, but rather are bubbles of trapped within the sedimenr that are released into the water.

Previous studies of the Southern Hydrate Ridge have captured only snapshots of bubbling activity there, suggesting that bubbling was either 'off' or 'on' at different times. This is consistent with the idea that methane was being delivered to the system from a reservoir more deeply buried gas at Horizon A, which was escaping intermittently through a system of cracks in the overlying rock. Macron et al.'s study contradicts this model, showing that bubbling is more-or-less constant on the Southern Hydrate Ridge, although different parts of the system are active at different times, and that this activity is strongly linked to bottom pressure, and thereby to the tides. However, Macron et al. cannot rule out the possibility that the vents are connected in some way, or that there may be some form of interaction between them.

Tidal modulation of methane vent systems on accretionary wedges has previously been suggested, but until now there has not been sufficient evidence to support this. Notably, video evidence from the Northern Hydrate Ridge appeared to show a correlation, but was not sufficient on its own to prove the link. Therefore the long-term sonar study carried out by Macron et al. at the Southern Hydrate Ridge clearly establishes a relationship between tides and bubbling for the first time. A relationship has also been suggested between tides and bubbling at cold seep sites, which appears to have been supported by observations off the coast of Canada.

Macron et al. suggest that this link is related to the effect that pressure has upon pore spaces within sediments, which are pushed shut at higher pressures (i.e. when the tide is high) and pulled open at lower pressures, allowing gas bubbles trapped within the sediment (where the water is methane-saturated) to escape. At spring tides, the pressure loading on the seafloor would be higher than at neep tides, causing the dilation of pore-spaces deeper into the sediments, which would in turn result in more bubbles being released, something line with Macron et al.'s observations. 

However, this does not appear to be the only factor at play; Macron et al.'s theory can explain why vents tail of activity after outbursts, when they might be expected to have reduced numbers of bubbles within their sediments, but not what causes the reactivation of individual vents after periods of inactivity. Furthermore, a number of significant bubble outbursts were recorded at times which appeared to show no connection to tidal cycles. Another problem with this hypothesis is the absence of any connection with seismic activity. Earthquakes would be expected to cause changes in pore spaces, which should have resulted in bubble releases, but despite several Earthquakes being recorded during the study period, none of them was linked to any change in bubble-activity.

One possibility previously suggested is that gas hydrates accumulating in pore spaces trap a layer of gas below the surface sediments, in Horizon A, which is released when the gas pressure builds up to a point where it overcomes the resilience of the hydrates. This would explain how vents become dormant and then reactivate after a period of inactivity, but the time scale predicted for this model does not fit Macron et al.'s data. Under this model the vents should be inactive for years at a time, whereas the observed quiet periods lasted from a few hours to a few months. Macron et al. suggest that possibly the basic premise of this hypothesis is correct, but that the gas is being trapped much closer to the surface by a less substantial hydrate lid, enabling smaller, more frequent bubble-outbursts from beneath a lid more easily overcome.

This would allow bubble release to be controlled at a local level, will small pockets of trapped gas building up within sediments in the gas hydrate solubility zone, beneath shallow blockages in the sediment pore-system. Overcoming these shallow blockages would require much less pressure to build up, with the increased pressure from tidal forces often being sufficient to trigger outbursts. 

The perpheral sites, where venting occurs but is far less frequent, may be receiving lower levels of gas from the underlying feeder sediments, or may be acting as pressure-release valves for a larger gas system beneath the gas hydrate solubility zone, venting less frequently as is predicted by the deep-reserve, large hydrate-lid model. 

The strong bottom currents observed at the Southern Hydrate Ridge also ought to play a role in the gas hydrate dissolution cycle, since these currents constantly bring in new water unsaturated in methane, leading to more rapid hydrate dissolution. 

Macron et al. note that strong upwelling currents are often observed at the Southern Hydrate Ridge, and that these tend to coincide with outbursts of bubble activity. They suggest that the upwelling currents may in fact be driven by the gas bubbles dragging water along with them as they rise. Curiously, however, the upwelling component of the system appears to be more strongly tied to the tidal system than the bubble activity is, with upwellings tending to stop during rising tides, while bubble activity frequently continues. Both upwelling currents and bubbles appear to reach depths slightly above the gas hydrate stability zone, which is consistent with the two phenomena being linked and the bubbles being protected by a hydrate-skin. The height to which both bubbles and upwelling currents rise varies seasonally, which is likely to be linked to seasonal water stratification.

Methane bubbling from the Southern Hydrate Ridge is a more-or-less constant phenomenon, although the location of the bubbling does vary over time. A number of vents are present, with the controls on bubbling at each of these apparently being independent. The continuous venting of bubbles appears is linked to the slow erosion of sedimentary structures on the seafloor, with occasional more substantial outbursts provoking more substantial overturn. The timing of bubble venting appears to be linked to tidal cycles, and is probably driven by hydrostatic pressure, although some bubble releases appear to be random and unconnected to tidal cycles.

Bubbling is more prevalent at low and falling tides, when pressure is low or falling, and this is more notable around spring tides, when pressure changes are more marked. At these times pore spaces in the sediment are more open, which could conceivably be related to outbursts of bubbling. However, bubbling also occurs at other times, which may indicate that pressure within the sediment is also a significant influencing factor. 

The main vents on the Southern Hydrate Ridge were active for between 50 and 75% of the time, although the level of activity at 'active' vents varied considerably in intensity, and could come from multiple locations within the vents. This makes it hard to estimate the amount of methane being released by the Southern Hydrate Ridge, and Macron et al. were unable to estimate this, despite having undertaken the most detailed study of the system to date. Based upon their findings they do not feel it would be reasonable to extrapolate data from a single vent across the whole system, instead any attempt to estimate the amount of methane being released would require long-term monitoring of the entire system. 

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