SR ISOTOPE RATIOS AND TRACE ELEMENT CONCENTRATIONS OF PEDOGENIC CALCITE RECORD ENVIRONMENTAL CONDITIONS IN WEST TEXAS PALEOSOLS SPANNING THE K-T BOUNDARY

A succession of carbonate bearing paleosols that span the K-T boundary in west Texas contain chemical and isotopic signatures that reflect conditions during pedogenesis. The Mid-Maastrichtian and Late Maastrichtian greenhouse events have been recognized at this location based on positive excursions of both carbon and oxygen isotope ratios in pedogenic calcite. These positive isotope excursions have been interpreted as indicating both warmer temperatures and higher atmospheric pCO₂. To further investigate environmental conditions through this time period, we have measured both trace element concentrations and Sr isotope ratios in these soil calcites.

Sr isotope ratios of the pedogenic carbonates are surprisingly nonradiogenic and average 0.7067 while exhibiting a range of values between 0.7060 and 0.7072. This is in marked contrast to the Sr ratio of the parent material which has ⁸⁷Sr/⁸⁶Sr > 0.710. The Sr isotope ratios in the pedogenic carbonates exhibit positive excursions through both green house events. These Sr ratios appear to be controlled by the intensity of weathering of the parent material versus the weathering of a nonradiogenic allochthonous source – probably dust. During the greenhouse events, the increased weathering of parent materials apparently resulted in higher Sr ratios in the pedogenic carbonate although Sr ratios throughout the section are dominated by the nonradiogenic Sr contributed by the dust.

Trace element concentrations of pedogenic carbonates also change systematically across the greenhouse events. Pedogenic carbonates have elevated Mg concentrations (>2000 ppm) during both green house events indicating that soil minerals were experiencing increased weathering during these time. Redox sensitive elements U, Fe, and Mn behave coherently (U decreases to <0.3 ppm while Fe and Mn increase to > 1000 ppm) across the Late Maastrichtian greenhouse event suggesting that soils were experiencing relatively reducing conditions during this time period.

This study illustrates that pedogenic carbonates can preserve a record of soil water chemistry which in turn is controlled by environmental conditions. In this example, paleosol carbonate records increased weathering of soil minerals during the Mid-Maastrichtian and Late Maastrichtian greenhouse events.