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Biscuits help resolve climate change controversy

28 January 2015

About 56 million years ago, a large amount of carbon dioxide was added – by natural means – to the atmosphere. Major climatic disruption resulted, together with changes in ocean chemistry and marine biology. But did the CO2 emission occur over just a few years, for example due to cometary impact (faster than we are releasing CO2 now), or over millennia (slower than today’s emission rates)? That scientific controversy, important for projecting the scale of consequences of human-driven CO2 emissions, has now been resolved, by the detailed study of ‘biscuits’ in samples from marine mud and clay deposited on the sea floor.

Analysis of seafloor sediments laid down in the geological past provides scientists with direct evidence of dramatic natural changes in oceanic, atmospheric, climatic and ecological conditions, that have, at several times, resulted in worldwide species extinctions. Whilst there is scientific agreement that an asteroid’s impact on Earth around 65 million years ago caused the demise of the dinosaurs and marine life forms such as ammonites, much less is known about the causes and timescale of events around 9 million years later, at the boundary between the Paleocene and Eocene geological epochs.  

A crucial issue is whether the layering in sediment cores from New Jersey, reflecting conditions during that period, might be annual. In 2013, a high-profile publication claimed that it was, hence implying a near-instantaneous release to the atmosphere of thousands of billions of tonnes of carbon, as either CO2 or methane. Such an interpretation has major implications for our understanding of climate processes and their effects, including the tolerance of marine organisms to very rapid ocean acidification. Other scientists disagreed about the time-scale of the carbon release, but conclusive evidence one way or another has, until now, been lacking. 

Professor Paul Pearson (Cardiff University) suspected that the apparently annual layers were an artefact of the sampling process. Sediment cores are extracted by cylindrical drills, and a combination of spinning and slippage can result in ‘biscuiting’ or ‘discing’, shown by internal cracks or breaks in the core samples. This process has been observed previously, for example in sediment cores obtained from the Deep-Sea Drilling Project. In the Paleocene-Eocene boundary cores from New Jersey, around 13 such layers (each 2 - 3 cm thick) occurred over the period of the carbon chemistry changes; re-examination of the cores by Pearson and his colleague Dr Ellen Thomas (Yale University) showed evidence of concentric grooves characteristic of biscuiting.  

But could such biscuiting arise from pre-existing weaknesses in the core, with the underlying cause being an annual periodicity in sedimentation? There was no evidence for such a process in the New Jersey cores, and in addition Pearson and Thomas re-examined the abundance of microscopic marine fossils (foraminifera) within the cores. If the layers had been formed annually, the accumulation rate of these micro-fossils would have had to have been many times higher than the fastest rate observed in similar environments today, i.e. on the continental shelf, with rapid sediment accumulation. Since that is an unrealistic scenario, the inescapable conclusion is that the changes took place over thousands of years, consistent with Earth system models, rather than a decade or so.

“Whilst the initial mis-interpretation of the layering was an easy mistake to make, the resolution of this issue has profound implications” said Professor Pearson, “The link between carbon dioxide emissions and ocean acidification crucially depends on the rate at which atmospheric changes occur; at slower rates, there is much more chemical buffering because carbon compounds move from the atmosphere into surface ocean waters and then into the deep sea, where they dissolve limestone. Furthermore, slower rates may enable some evolutionary adaptation by marine organisms. Whilst we can now be more confident in interpreting the past, these re-analyses increase our concern over what might happen in the future”.

Dr Phil Williamson, Science Coordinator of the UK Ocean Acidification (UKOA) research programme that funded the re-analyses, commented: “At the Paleocene-Eocene boundary, global temperatures rose by at least 5°C, many common deep-sea calcifying organisms became extinct, and the subsequent recovery from ocean acidification lasted around 100,000 years. There is now no doubt that current human-driven increases in carbon dioxide are occurring very much more rapidly than they did then; thus it is near-inevitable that, without urgent and very ambitious mitigation actions, consequences for the future ocean will be very much more severe”.

The UKOA programme is jointly funded by the Natural Environment Research Council (NERC), the Department for Environment, Food and Rural Affairs (Defra) and the Department for Energy and Climate Change (DECC). In addition to research on past ocean acidification events and their impacts, UKOA has supported fieldwork in the Arctic and Antarctic; experimental studies on cold-water corals and a wide range of other organisms; and modelling studies, to improve projections for future ocean acidification and its socio-economic consequences under different climate change scenarios.