EFFECTS OF GLOBAL WARMING ON HYDROLOGICAL CYCLE AND CHEMICAL WEATHERING RATES; THE PALEOCENE-EOCENE THERMAL MAXIMUM PERSPECTIVE

In less than 300 yrs our atmosphere may be heading towards a climate state that has not existed on Earth since the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago. A climate state far beyond the boundaries of all modern and Quaternary icehouse-world observations and calibrations, where not all aspects of climate change processes and consequences are yet well understood. This paper focuses on the effects of the PETM global warming to the hydrological cycle. We present results of detailed stratigraphic, sedimentologic, d¹³C, ichnological and palaeosol study of river sediments from the Western Interior of US. We compare the results to literature data globally. Our detailed dataset indicates a significant increase in precipitation peakedness in the Uinta Basin, Utah. This increase in climate seasonality is evidenced by changes in ancient river dynamics, nature of paleosols and the length of vertical terrestrial burrows. We document extreme precipitation events that caused catastrophic terrestrial flooding events, with intermittent, in some cases, multi-year droughts. Comparing this dataset to literature data reveals that such highly seasonal or intensely monsoonal conditions were not unique for the Uinta Basin, but occurred widely across the Western Interior of US, and in Europe in river systems with mountainous hinterland. In contrast to the modern climate conditions, where monsoonal climates occur in sub-tropical zone, the PETM examples come from mid-latitudes. Thus, our results suggest that increase in precipitation peakedness and monsoon intensification rather than increase in total average humidity was the effect of the PETM warming on the hydrological cycle, and that the monsoon belt may have expanded. This is in contrast to the existing global PETM climate models. Moreover, our data on river sediment petrography from the Uinta Basin does not indicate loss of feldspar or lithic fragment content during the PETM. In contrast to what has been predicted by existing models, where increase in average humidity has been linked to increased chemical weathering rates of silicate minerals. We conclude that increased precipitation peakedness restrained chemical weathering, due to the sustained intermittent droughts and the increased physical erosion during extreme precipitation events.