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October 5-7
DDC:
550 - Earth sciences
Abstract:
Over geologic timescales, removal of atmospheric CO2 by weathering of silicate rocks balances CO2 input from the Earth´s interior. The coincidence of global cooling and the rise of mountain belts during the late Cenozoic has led geologists to suggest feedbacks between these two events. A centerpiece of t5his hypothesis was partially founded on observations of a young (0-5 My) 4-fold increase in global sedimentation rates, which seemed like a clear proxy for increased denudation and uplift of uplands. Over the same time interval, the radiogenic seawater Sr isotope record seemed to confirm enhanced global chemical weathering. However, paleo-ocean pH records provide no evidence for reduced atmospheric CO2 concentrations over this period. Here, we provide evidence that this increase in global erosion rate and weathering flux has never occurred. First, the sedimentary record is biased towards measuring decreasing rates of erosion and sedimentation with increasing age due to observational biases and the stochastic nature of erosion and sediment transport. Second, new estimates of the global erosion rate extrapolated from hundreds of field measurements of loads and cosmogenic nuclides in bedload of modern rivers match those expected from the Phanerozoic stratigraphic record after correction for time scale bias. Third, we provide evidence that Quaternary glaciers of Arctic areas were much less erosive than commonly assumed. Fourth, we recast the ocean dissolved Be/9Be isotopic system as a weathering proxy that spans the last ~12 Ma. This proxy indicates stable weathering flux magnitudes during the late Cenozoic. These independent observations show neither clear evidence for increased erosion nor for a pulse in weathering fluxes to the ocean. We conclude that processes different from an increase in global denudation has caused Cenozoic global cooling, and that global cooling had no profound effect on global, spatially and temporarily averaged weathering rates.
