Weathering flux and CO 2 consumption determined from palaeosol sequences across the Eocene–Oligocene transition

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Rates of base cation depletion are determined for late Eocene and early Oligocene alluvial palaeosols (Pacific Northwest of North America) from pedogenic mass balance geochemistry and time estimates of palaeosol sequences. Pedogenic strain (volume change) and mass transfer of base cations, silica, and other pertinent elements are calculated from the geochemical differences between palaeosol horizons and parent material. Parent material compositions are determined by identification of the least-altered alluvial strata in the sequence. Time spans of sequences of palaeosols are estimated from 40Ar/ 39Ar age dates of interbedded tuff beds and from time of formation estimates of palaeosols based on their degree of development. Palaeosol sequences from the mid-latitude setting of central Oregon document a significant decrease in the degree of weathering of individual palaeosols across the Eocene–Oligocene transition as the climate became cooler and drier. This decrease in the degree of weathering is accompanied by an increase in the accumulation rate of alluvial soils, as indicated by sedimentation rates of stratigraphic packages. According to these calculations and estimates, the weathering flux of base cations in the John Day Basin, measured in grams per 1000 years per square centimetre, increased across the Eocene–Oligocene transition. These results, when attributed to climate change, indicate that weathering flux at this local increased as climate changed from warm, humid late Eocene conditions to cooler, drier Oligocene conditions. Furthermore, the extrapolation of these weathering flux results to other mid-latitude settings with similar climate change history may help to explain the severe climate change of the Eocene–Oligocene boundary. It is proposed that, in some mid-latitude settings, greater quantities of less-weathered silicate material were exposed during the early Oligocene than during the late Eocene, thus causing an increase in flux of weathered product. Increased weathering flux during this time in certain regions of the mid-latitudes may have increased both CO 2 consumption and bicarbonate transport to the ocean and contributed to atmospheric pCO 2 drawdown and global cooling.