Using lake sediments to understand past climate
Past climates can be understood by examining geochemical and biological information that is recorded in sedimentary archives. The vast majority of climate research on ancient sedimentary rocks has been conducted on material deposited in marine environments. In this unusual study, published in Nature Geoscience, postgraduate student Weimu Xu (St Peter’s 2012) has examined sediments deposited in a former lake.
Geographical map of the Early Jurassic (Toarcian: 180 million years ago), with the position of major lake systems that formed at that time
Weimu studied sediments from one of the largest lakes in Earth history, which formed rapidly due to Early Toarcian (Early Jurassic) climate change, around 183 million years ago. Warming atmospheric temperatures due to the massive release of greenhouse gases such as carbon dioxide and methane led to increased rainfall in certain continental interiors. The lake, which formed in the Sichuan Basin (China), was more than twice the size of England and 3–4 times the size of Lake Superior (presently the largest lake on Earth); it was only one of several that formed on the Eurasian continent at that time. An accelerated hydrological cycle operating in this region promoted an increased supply of nutrients to the lake, encouraging biological productivity, which resulted in the deposition of dark-coloured organic-rich sediments.
Outcrop photo of Early Jurassic (Toarcian) marine black shales in northern Italy (Dogna).
Weimu’s research demonstrates that continental interiors responded rapidly to global climatic change. It further shows that the major lake systems that formed at that time operated in a similar way to the oceans, with synchronous deposition of organic-rich sediments in both settings. In the marine realm, these phenomena have been referred to an ‘Oceanic Anoxic Event’: geochemical and climatic changes that we now know were not limited only to the oceans. The large-scale draw-down of carbon dioxide from the atmosphere not only into marine deposits but also into lacustrine sediments aided subsequent global climatic recovery.
This research, conducted as part of Weimu’s DPhil project, was funded by Shell, as part of the Oxford–Shell Research Collaboration: http://shell.earth.ox.ac.uk/ together with researchers at the University of Exeter, British Geological Survey, University of Durham, University of Bristol and Shell Global Solutions International.
Paper: Carbon sequestration in an expanded lake system during the Toarcian oceanic anoxic event, was published in Nature Geoscience, doi: 10.1038/NGEO2871 by Weimu Xu, Micha Ruhl, Hugh C. Jenkyns, Stephen P. Hesselbo, James B. Riding, David Selby, B. David A. Naafs, Johan W.H. Weijers, Richard D. Pancost, Erik W. Tegelaar, Erdem F. Idiz
Image Top Left: Outcrop photo of Early Jurassic lacustrine (lake) black shales that formed in the Tarim Basin lake.