Paleoclimate interpretation using stable carbon and oxygen isotopes from caliche in the Blackwater Draw and Blanco formations



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Stable carbon and oxygen isotopic ratios were measured for caliche horizons in the Blackwater Draw Formation, at Blanco Canyon, in order to determine the paleoclimatic history of the Southern High Plains for approximately the past 1.6 Ma Carbon isotopes were analyzed to determine the proportion of C4 to C3 vegetation, and possible soil conditions at the time of carbonate formation. It was found to be quantitatively difficult to determine the fraction of c4 to c3 vegetation once supported by the Blackwater Draw Formation paleosols, however it has likely varied from less than 50% C4 prior to 1.6 Ma up to as much as 80% C4 in more recent times. Modeling of paleosol conditions suggested that the caliches formed under low soil C02 concentrations, which resulted from sparse surface vegetation giving rise to low soil respiration rates and allowing for a higher influence of atmospheric C02 • Oxygen isotopes were analyzed in order to calculate paleometeoric water compositions and to determine primary sources for precipitation on the Southern High Plains. Measured ~180 compositions suggest that the oxygen isotopic composition of precipitation has not changed drastically over the past 1.6 Ma and modem precipitation for the Southern High Plains is more depleted in 180 relative to values calculated in this study. It is suggested that pre-Holocene atmospheric circulation patterns, over the Southern High Plains, were similar as today but allowed for a higher influence of precipitation from Gulf moisture.