Does burial diagenesis reset pristine isotopic compositions in paleosol carbonates?

摘要

Sedimentological study of early Oligocene continental carbonates from the fluvial Dagshai Formation of the Himalayan foreland basin, India resulted in the recognition of four different types namely, soil, palustrine, pedogenically modified palustrine and groundwater carbonates. Stable oxygen and carbon isotopic (δ~(18)O and δ~(13)C) analyses of fabric selective carbonate microsamples show that although the pristine isotopic compositions are largely altered during deep-burial diagenesis, complete isotopic homogenization does not occur. δ~(18)O and δ~(13)C analyses of ~200 calcrete and palustrine carbonates from different stratigraphic horizons and comparison with δ~(18)O of more robust bioapatite (fossil vertebrate tooth) phase show that dense micrites (~>70% carbonate) invariably preserve the pristine δ~(18)O value (mean) of ~-9.8‰, while altered carbonates show much lower δ~(18)O value ~-13.8‰. Such inhomogeneity causes large intra-sample and intra-soil profile variability as high as >5‰, suggesting that soils behave like a closed system where diagenetic overprinting occurs in local domains. A simple fluid-rock interaction model suggests active participation of clay minerals to enhance the effect of fluid-rock ratio in local domains during diagenesis. This places an upper limit of 70% micrite concentration above which the effect of diagenetic alteration is minimal. Careful sampling of dense micritic part of the soil carbonate nodules, therefore, does provide pristine isotopic composition and it is inappropriate, as proposed recently, to reject the paleoclimatic potential of all paleosol carbonates affected by burial diagenesis. Based on pristine δ~(13)C value of -8.8±0.2‰ in soil carbonates an atmospheric CO_2 concentration between ~764 and ~306ppmv is estimated for the early Oligocene (~31Ma) Dagshai time. These data show excellent agreement between two independent proxy records (viz. soil carbonate and marine alkenone) and support early Oligocene survival of the Antarctic ice sheet.