INFLUENCE OF CHANGING HYDROLOGY ON PEDOGENIC CALCITE PRECIPITATION IN VERTISOLS, DANCE BAYOU, BRAZORIA COUNTY, TEXAS, USA: IMPLICATIONS FOR ESTIMATING PALEOATMOSPHERIC (P)CO_2

摘要

The delta(~13)C values of pedogenic (soil-formed) calcite preserved in the sedimentary record have been used to estimate atmospheric pCO_2 using the paleosol calcite paleobarometer. A fundamental assumption for applying this paleobarometer is that atmospheric CO_2 concentrations have a direct influence on the measured pedogenic calcite delta~(13)C values as a result of open-system exchange between atmospheric and soil-respired CO_2. Here we address the timing of calcite precipitation in relation to the soil saturation state and soil atmosphere connectivity in a modern Vertisol (smectitic, clay-rich soil, seasonally saturated) in Brazoria County, Texas, U.S.A. Luminescent phases of calcite growth, under cathodoluminescence microscopy, have more negative delta~(13)C values (delta~(13)C = 11.1 parts per thousand VPDB +/- 0.78 1 sigma) than the non-luminescent phases (delta~(13)C = 2.53 parts per thousand VPDB +/- 1.41 1 sigma). The luminescent phase of calcite formed during the water-saturated portion of the year, thereby minimizing the incorporation of atmospheric CO_2, and negating its use for pCO_2 estimations. The non-luminescent phase formed during the well-drained portion of the year when atmospheric C(O)2 mixed with soil-respired CO_2 and is therefore useful for pCO_2 estimation. From these results we present a model to independently test the saturation state of a paleosol at the time of pedogenic carbonate precipitation. Finally, we calculate soil-respired CO_2 concentrations that are an order of magnitude lower than those that are typically assumed in the soil-carbonate paleobarometer equation.