CENOZOIC TERRESTRIAL CHEMICAL WEATHERING RECORDS FROM NORTH AMERICAN PALEOSOLS

Three newly compiled records of chemical weathering from North American paleosols allow for a more in-depth examination of the role that climatic and tectonic changes in the Cenozoic have made on the silicate weathering cycle.  A long-standing model links Cenozoic climatic cooling to CO₂ drawdown in response to increased rates of silicate weathering due to the uplift of the Himalayas beginning ~40 Ma ago.  The strontium isotopic record in marine sediments shows a pronounced increase in radiogenicity over the past 150 Ma, rather than just the past 40 Ma, well prior to the uplift of the Himalayas.  There were also comparable changes in the record earlier in the Phanerozoic that were not accompanied by CO₂ drawdown or climatic cooling.  Evidence from modern streams indicates that much of the radiogenic Sr is being weathered from carbonates rather than silicates, chemical reactions that result in no net drawdown of CO₂.  Furthermore, the new long-term records of chemical weathering from North America are inconsistent with the timing of the uplift-drawdown hypothesis, and with the monotonic CO₂ decrease it would predict.  Chemical index of alteration (CIA) values in Oregon, Nebraska, and Montana track global climatic records (e.g., marine oxygen isotopes) to varying degrees, but broadly speaking chemical weathering was strongest when the global climate was warm (Eocene and Middle Miocene) and weakest when the global climate was cool (Oligocene-Early Miocene, Late Miocene-present).  Differences between the records are attributable to changing orographic conditions/rain shadows locally.  Leaching of Ca and Mg is relatively unaffected by either climatic or tectonic changes and is consistently more important in the cumulative silicate weathering budget than the weathering of Na and K.  Leaching of Na and K is unaffected by tectonic changes, but variable according to climatic regime with significantly less leaching occurring during cool climatic periods.  Calculations of CO₂ consumption due to weathering suggest that paleosol proxies for chemical weathering intensity track independent proxies of atmospheric CO₂ levels.