Stable isotope geochemistry of caliche in the Blackwater Draw Formation, southern High Plains, Texas
Stable carbon and oxygen isotopic ratios were measured for soil carbonate and organic matter from three successive caliche horizons in the type section of the Blackwater Draw Formation in northern Lubbock County, Texas. These data allow for interpretation of the Pleistocene paleoclimatic history of the Southern High Plains. Carbon isotopes were measured to determine the fraction of C4 to C3 vegetation that grew on the soil surface during the time that the Blackwater Draw Formation caliches formed. The caliche horizons probably formed under low soil CO2 concentrations and high atmospheric CO2 input leading to a higher difference between carbonate and organic matter ä^13C values than is typical of modem soils. The fraction of C4 vegetation increased, and the climate became progressively warmer as the Blackwater Draw Formation profile aggraded over the past 2 million years. Oxygen isotopes were analyzed in order to interpret the paleometeoric water compositions and possible temperatures under which the Blackwater Draw Formation accumulated. Oxygen isotopic values suggest that the ä^18O of meteoric waters from which the caliche precipitated has remained relatively constant for the past 2 million years, and that modem precipitation is more depleted in ä^18O relative to the values calculated for the Blackwater Draw Formation at Lubbock.