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 oceanic temperatures, leading to widespread oceanic anoxia which is reflected in the fossil record as the Early Jurassic Toarcian Extinction Event. Extinction events are common in the fossil record, and are widely used as proxies for environmental change. However it is far from clear if all organisms in the marine community were affected in the same way or at the same time, with the potential that sudden changes in fossil community makeup could be misleading if not interpreted correctly.
In a paper published in the journal Geology in March 2015, Silvia Danise of the School of Geography, Earth and Environmental Sciences at Plymouth University, Richard Twitchett of the Department of Earth Sciences at the Natural History Museum and Crispin Little of the School of Earth and Environment at the University of Leeds describe a study of benthic (bottom dwelling) and nektonic (water column dwelling) invertebrates across a 1.7 million year section in the Cleveland Basin, North Yorkshire, UK, spanning the Early Jurassic Toarcian Extinction Event.
Danise et al. compared benthic and nektonic diversity levels to variations in oxygen, carbon, strontium, sulphur and molybdenum isotope ratios and total organic carbon levels in deposits of the Whitby Mudstone Formation, which was laid down in the Cleveland Basin during the Early Toarcian, when it was located at latitudes of between 30˚ and 40˚ north in part of the Laurasian Seaway.
Study area (Cleveland Basin, UK) and location map of the sections that form the composite stratigraphy. Maximum intertidal rock exposure shown in white. Danise et al. (2015).
Molybdenum isotope levels are a proxy for ocean anoxia, while carbon isotope levels reflect changes in the rate at which organic matter is produced and buried and strontium isotope levels reflect weathering of rocks on land. Oxygen isotope ratios are used to determine temperatures, and sulphur isotope levels reflect productivity by sulphate reducing Bacteria, which thrive under anoxic conditions, but also require phosphorus from organic matter.
Danise et al. found that extinction and diversity levels in benthic and nektonic communities across the Toarcian Extinction Event responded to different isotope changes, suggesting that they were driven by different processes. Following the initial extinction levels diversity levels recovered most quickly in nektonic communities, reaching maximum diversity when the temperature was warmest and sea levels were highest, which is likely to reflect high phytoplankton productivity, followed by a recovery in oxygen levels in the water column.
Benthic communities took longer to recover and were dominated first by Bositra radiata, which is interpreted as an epifaunal (surface-dwelling, non-burrowing) filter feeder tolerant of low oxygen levels, then by a slightly more diverse fauna dominated by Pseudomytiloides dubius with occurrences of Bositra buchii and Meleagrinella substriata, which are all also is interpreted as an epifaunal filter feeder tolerant of low oxygen levels.
Bositra radiata shells from the Toarcian Oceanic Anoxic Event. Palaeoenvironmental Change.
Prior to the extinction event the most abundant nektonic group were Belemnites (an extinct group of Squid-like Cephalopods with internal shells) of the genus Passaloteuthis, which is interpreted to have lived in deep, cool waters, while following the event these were replaced with members of the genus Acrocoelites, interpreted to have lived in shallower, warmer waters, which may also be a reaction to low oxygen levels in deeper waters. These Belemnites were eventually replaced by Ammonites (Cephalopod Molluscs with chambered external shells).
Specimen of the deep water Belemnite Passaloteuthis bisulcata from before the Toarcian Extinction Event. Wikimedia Commons.
The strongest correlation between isotope ratios and extinctions was seen for molybdenum, with 40% of benthic species turnover relating directly to this variable, suggesting that in the Early Jurassic as now the main factor determining diversity in benthic marine communities was dissolved oxygen levels. Following the main extinction event diversity was also correlated with variations in strontium and carbon isotope levels, which correlate to levels of erosion and organic matter burial.
For Nektonic communities molybdenum isotope ratios are still important, but strontium isotope ratios become more important, suggesting that diversity is more strongly related to terrestrial erosion levels. During the Toarcian atmospheric carbon dioxide levels are thought to have risen sharply due to emissions from the volcanoes of the Karoo-Ferrar Large Igneous Province, leading to an increase in dissolved carbon dioxide in rainwater (acid rain), which would in turn lead to higher rates of weathering on land. This would lead to increased levels of nutrients entering the marine environments, driving productivity in surface waters. However the same atmospheric carbon dioxide would also have raised temperatures in both the atmosphere and oceans (global warming) which would have led to lower oxygen levels in the water column (warm water can hold less dissolved oxygen than cool water), particularly in deeper waters which cannot replenish their oxygen from the atmosphere.
Interestingly oxygen isotope levels did not appear strongly correlated with extinction rates, despite the fact that these are thought to reflect atmospheric temperature, the factor which is thought to have driven ocean anoxia. Danise et al. suggest that this is because oxygen isotope levels are obtained from Belemnite shells, and we may not fully understand how their physiology and behaviour changed in response to environmental changes during the extinction event.
Finally Danise et al. suggest that this decoupling between factors affecting benthic and nektonic communities may be found in other extinction events where it has not previously been looked for, and that it may also be seen in modern faunas faced with similar environmental stresses, given that Jurassic marine invertebrate faunas are thought to be quite close to modern faunas in composition.
The Toarcian Oceanic Anoxic Event is an extinction event that took place in the Early Jurassic, about 183 million years ago. It took place in four phases, thought to have been related to Milankovitch Cycles. During each phase the temperature of the global ocean is thought...
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