Testing the control of weathering on CO2 - Evidence from extreme climate events (NERC Grant NE/I020571/2)

Published papers for NERC grant NE/I020571/2. Grant award abstract: How does the Earth's climate recover from events of rapid and extreme global warming or cooling? Why have the huge fluctuations in atmospheric CO2 in the geological past not caused runaway climate effects, making the Earth become Venus- or Mars-like? Silicate weathering of the continents is the main CO2 removal process, and therefore a dominant long-term climate control mechanism. However the debate on what controls silicate weathering, and therefore atmospheric CO2, is still contentious and ongoing. A correct understanding of the controls on weathering, and its link to atmospheric CO2 levels is critical, because 1) it is possible that weathering is the process that has kept Earth's climate in the relatively narrow bounds required for life over the past several hundred million years; 2) it is impossible to decipher the causes and consequences of long-term climate variations through Earth's history without accurate weathering data, which in turn impacts on our understanding of current climate; 3) comprehension of climate systems leads to more accurate modelling of future climate change; 4) rapid global climate change inevitably leads to large mass extinctions. Therefore it is important to unravel the link between extinctions and the Earth's climate systems, including CO2 control. Lithium isotopes have gained much interest over the past few years because large variations in the Li isotope ratio in rivers and clays are caused by silicate weathering processes. Furthermore, unlike tracers of weathering used previously, Li isotopes also respond to the intensity of weathering, and therefore can be linked directly to weathering rates. This is critical, because for the first time is gives us a window into the variation of weathering rates through time, which in turn means we can use the Earth's past climate variations as a natural laboratory. Three of the largest climate fluctuations and mass extinctions in Earth's history will be examined and modelled, primarily using Li isotopes, but also several other tracers, which will serve to reveal information on marine and volcanic conditions at the time. These geological periods (the end-Ordovician glaciation (450 Ma (million years ago)), the Permo-Triassic event (251 Ma) and the Cenomanian-Turonian Ocean Anoxic Event (94 Ma)), represent times when rapid warming or cooling of Earth's climate occurred, resulting in the extinction of up to 90% of life on Earth. Samples from these time periods exist in the form of marine calcium carbonate. This was precipitated (either inorganically, or via various life forms) in the oceans at the time, and provides a record of ocean chemistry, which in turn is directly linked to the atmospheric conditions. Analysing Li isotopes is a complex procedure, and will be undertaken at Oxford University. Collaborations will exist with Prof. Jan Veizer (Ottawa University) and Dr. Christoph Korte (Copenhagen University), who are specialists in the studied time periods, with Prof. Andy Ridgwell (Bristol University), who is an expert climate modeller, and with Prof. David Harper, who is an expert in mass extinctions. By understanding weathering and climatic responses to periods of rapid global warming and cooling we will gain critical information on Earth's climate feedbacks, and on processes that led to the extinction of vast proportions of the biosphere.