--> Complex Response of Fluvial Systems to Extreme Global Warming at the Paleocene-Eocene Boundary

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Complex Response of Fluvial Systems to Extreme Global Warming at the Paleocene-Eocene Boundary

Abstract

How does global warming change the frequency and intensity of extreme weather events? The response to this question is partly preserved in the geological record. 56 Ma ago, global temperatures increased during the Paleocene-Eocene Thermal Maximum (PETM), leading to a major biotic turnover, but how this event affected the nature of extreme events remains unknown. On several continents, fluvial systems with sinuous channels within fine-grained floodplains suddenly transformed at the P-E boundary into apparently coarser-grained braid plains with frequent lateral migrations, washing their muddy floodplains to the seas. This landscape transformation has been related to aridification and intensification of extreme precipitation implying transport of coarser material and deeper flow depths as a result of P-E global warming, with important implications for predicting the consequences of current global change. Here we test this hypothesis by quantifying the grain-size distribution and the magnitude of grain-size change and flow depths across the PETM at a representative P-E locality in Northern Spain. We find that the mean size of pebbles in transport remained similar to, or even smaller than, pre-PETM conditions, and that flow depths also remained similar or slightly smaller. Thus, if seasonal and extreme events of precipitation did occur, our data imply primarily a planform response, with a shift from a few to multiple active channels constantly reworking their own floodplains. Alternatively, our data could document that global warming in this area did not lead to more intense rainfall and flooding events but instead lead primarily to vegetation deterioration due to extended drought periods, triggering floodplain instability and dramatic landscape transformation. This emphasizes the vulnerability of vegetation and its importance for the dynamics of landscape surface systems. Eventually, both hypothesis are not mutually exclusive and could have combined into a positive feedback loop whereby greater floodplain reworking arised from open vegetation, greater channel mobility and loss of clays. Our finding thus implies a complex response of fluvial systems, even to extreme events, illustrating the challenge of inverting climate records from fluvial deposits.