PALEOCLIMATE RECORDS FROM LATE PENNSYLVANIAN PALEOSOLS: A COMPARATIVE APPLICATION OF MULTIPLE GEOCHEMICAL PROXIES

A series of Late Pennsylvanian, compound paleosols in central Texas, U.S.A., record changes in hydrology and climate during the Pennsylvanian/Permian glacially driven eustacy. Paleosols were first measured and described in the field, and then sampled for thin-section, bulk geochemical, and stable isotope analysis. The paleosols range from 1-3 m in thickness, and are typically overlain by marine flooding (transgressive) surfaces overlain by marine strata, The lower portions of the paleosols exhibit angular blocky ped structure, high-chroma colors, and stage 2 to 3 pedogenic calcite nodules indicating well-drained soil conditions. Nodules show internal septarian and circumgranular cracks as evidence for shrinkage associated with desiccation. Pedogenic slickensides, sepic-plasmic microfabric, and pedogenic calcite attest to initial well-drained conditions with associated shrink-swell processes, whereas gley features, high organic C preservation, and pedogenic siderite in the upper portions of the paleosols are evidence for more poorly drained soil conditions, which were superimposed on initially well-drained soils. For both paleosols the shift from well-drained to poorly drained soil conditions during formation is possibly related to climate change, however both paleosols are overlain by marine carbonates, therefore it is also possible that the changes reflect rising water table caused by relative rise in sea-level. Late Pennsylvanian pCO₂ levels are thought to be comparable to modern values. From this study we estimate paleoatmospheric pCO₂ as 740 ppmV during the time of soil formation using δ¹³C of non-luminescent pedogenic micrite, δ¹³C of paleosol organic matter, and the carbonate-CO₂ paleobarometer. These estimates are slightly higher than those reported by other researchers (~300-400 ppmV), but are still low compared to the most of the Phanerozoic. Paleoprecipitation was estimated using the CALMAG proxy for bulk soil geochemical data and yielded MAP values of 1300 to 1400 mm/yr, except for one red paleosol with extensive pedogenic carbonate development, for which estimates were lower (800-900 mm/yr). Paleotemperatures estimated using the δ¹⁸O of carbonate are 19 to 24^oC, and we plan to use the δ¹⁸O values of pedogenic siderite to independently test these paleotemperature estimates.