Chemical weathering controls on marine life extinction during the latest Ordovician – CHAMELEON

About 445 million years ago, the end of the Ordovician witnessed the second largest biotic crisis in Earth’s history, most likely caused by widespread marine anoxia. This mass extinction coincided with major climate change, culminating with the growth and demise of extensive ice sheets in southern high latitudes. This period can be regarded as an analogue for the current biodiversity crisis; Late Ordovician sedimentary sequences hence represent unique archives for investigating the response of marine ecosystems to extreme climate change. Enhanced continental weathering linked to sustained volcanism and the advent of the first land plants on the continents is suspected to have played a crucial role in the end-Ordovician crisis, via the export of substantial amounts of phosphorus to the ocean – a key nutrient that controls marine productivity, possibly leading ultimately to oxygen depletion in the water column. Major deglacial events associated with massive meltwater release to the ocean may have also possibly affected marine ecosystems at that time, via their presumed impact on marine productivity, ocean circulation et oxygenation.

In this project, we propose to use geochemical tracers for sediment provenance and chemical weathering ? based on the measurement of neodymium (Nd) and hafnium (Hf) isotopes in fine-grained sediments ? for testing these hypotheses. Unlike other proxies, the new ‘DeHf-clay’ proxy can trace the degree of alteration of apatite and other phosphate minerals on continents, hence offering unique opportunity to investigate the role of continental weathering in driving past marine biotic crises. Sedimentary sections from key reference sites from paleo-tropical (Scotland, Canada, China) and southern high-latitude regions (Armorican Massif, Morocco, Sardinia) will be investigated to reconstruct the evolution of terrestrial weathering, marine productivity and ocean redox changes in response to ice-sheet fluctuations. Finally, a modelling study will be conducted to further explore the putative links between continental weathering, pulses of meltwater release and marine life extinction during the latest Ordovician.