Grain Size Distribution and Organic Matter Content from Paleosols That Span the Cretaceous-Tertiary Boundary, Big Bend, Texas

This research project investigates climate change across the Cretaceous-Tertiary (K-T) boundary, a time period that encompasses one of the major extinction events in Earth history. By characterizing the grain size distribution in a succession of paleosols, it is possible to infer temporal trends in the intensity of soil-mineral weathering which can lend insight into ancient climatic conditions. Additionally, paleosol organic matter content was quantified to see if this attribute could be used as an indicator of the abundance of ancient plant cover, which is probably controlled by climate.

The study area is within the Tornillo Basin of Big Bend National Park in west Texas. This study examined 43 paleosols spanning the late Cretaceous and early Tertiary with the specific goal of characterizing grain size distribution and organic matter content in B-horizons of paleosols associated with two previously identified periods of elevated temperature and atmospheric carbon dioxide content (green house events).

The most common grain size within the paleosols is fine silt (2 to 15.6µm) which makes up about 68% of the sediment particles. On average, about 8% of the paleosol sediments fall within the clay size range (<2 µm), 22 % are coarse silt, and less than 2% of the particles are within the very fine and fine sand fraction (62.5 - 250 µm). Organic carbon abundance varies between 0.07 and 0.27 weight percent.

The abundance of the finest grain-size fractions (< 7.8 µm) increase during the green house events. This is most likely due to increased mineral weathering, a process that produces fine-grained weathering products. Therefore, grain size in paleosols appears to be a good proxy for constraining ancient climatic conditions. In contrast, organic matter abundance shows a weak and inconsistent relationship to the climate events and thus is of limited help in deciphering ancient climates.