Monday 30 September 2013

Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event.

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 to have risen abruptly by as much as 13℃, leading to a depletion in oxygen levels in the oceans, followed by an extinction event. Warmer waters are less able to retain oxygen, and this is thought to have been made worse by an increase in runoff from the continents due to higher rainfall, and a breakdown in ocean currents caused by the warming of deep oceanic waters. Each of these phases is marked by a distinct shift in Carbon, Oxygen and Strontium isotope ratios, an extinction event in the fossil record and the deposition of vast amounts of organic matter which has led to extensive hydrocarbons deposits at these levels in many places around the world. The Early Jurassic was considerably warmer than today, and it is thought that the warmest points on the Milankovitch Cycles (which are driven by cyclic variations in the Earth's orbit) raised global temperatures above a tipping point which led to runaway warming.

In a paper published in the journal Geology on 6 September 2013, Bryony Caswell of the School of Environmental Sciences at the University of Liverpool and Angela Coe of the Department of Environment, Earth and Ecosystems at the Centre for Earth, Planetary, Space and Astronomical Research at the The Open University, examine the behavior of populations of two species of opportunistic Bivalve, Bositra radiata and Pseudomytiloides dubius during the Toarcian Oceanic Anoxic Event in deposits at Whitby near North Yorkshire, England.

During the run up to the initial event the deposits were dominated by the Bivalve Bositra radiata, a Posidoniid Clam related to modern Scallops, quickly came to dominate to fauna, forming monospecific pavements and growing to sizes not achieved prior to the onset of the event. 

Bositra radiata shell pavement from Hawsker Bottoms, North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

During the remaining three events the Inoceramid Clam Pseudomytiloides dubius (also related to modern Scallops) quickly came to dominate the faunas, again reaching larger sizes during the event. The two species appear to compete during the onset of the third event, but Pseudomytiloides dubius quickly came to dominate, suggesting that the two species competed for some resource (probably food) and that Pseudomytiloides dubius was able to outcompete Bositra radiata.

Pseudomytiloides dubius from Port Mulgrave in North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

Bositra radiata and Pseudomytiloides dubius are both thought to have been opportunistic species. Both were small (even when reaching exceptional sizes), and are thought to have had short generation times and high larval production rates, probably comparable to the modern Mulinia lateralis, a small Surf Clam which can breed at two months old and seldom lives more than two years, which does well in anoxic conditions. Such organisms can thrive in environments even where they are exterminated each year, since they will have time to reproduce and can recolonize from adjacent areas.

The approximate location of the study area. Google Maps.

The Toarcian Oceanic Anoxic Event is of particular interest today as we are living in time when rising global temperatures, combined with local anoxic zones in parts of the oceans caused by excess runoff due to human activities, for example in the Black Sea and the Gulf of Mexico. This makes it important to understand how organisms cope (or don't) with such events, and what the long-term impact on the global biosphere is likely to be, particularly if these events become larger and more frequent.

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