Contrasting Climate and Weathering Processes During Two Hyperthermal Events Preceding the K/Pg Boundary, Dawson Creek Section, Big Bend National Park, Texas

Two successive greenhouse events immediately precede the K/Pg boundary, and the record of this climate change is well preserved in paleosols of the Dawson Creek section in Big Bend National Park, Texas, USA. Based on stable carbon and oxygen isotope excursions within pedogenic calcite nodules, the Mid-Maastrichtian event (MME) and the Late-Maastrichtian event (LME) are both characterized by a punctuated and short-lived increase in temperature of 5 to 7 °C and an increase in atmospheric pCO2 of up to 4 times preindustrial levels. Even though these two hyperthermal events exhibit a similar magnitude of temperature and atmospheric change, the climate and weathering response to these two events is distinctly different. Mean annual precipitation (MAP), determined from transfer functions applied to bulk paleosol elemental chemistry, indicates that the MME had MAP of 1200 mm/year whereas the LME was distinctly dry with MAP as low as 300 mm/year. The response of soil weathering intensity to these two hyperthermals is also distinctly different as interpreted from the chemistry of pedogenic calcite. For example, K and Rb concentrations are depleted in calcite nodules within the MME and testify to intense weathering resulting in the complete dissolution of feldspars. Additionally, MME pedogenic carbonates record a non-radiogenic Sr isotope ratio indicative of vigorous weathering of the volcanics that make up a significant component of the mudrocks. In contrast, an elevated weathering intensity of soil minerals during the LME is not observed. The cause of both the MME and the LME have been attributed to separate phases of Deccan Traps eruptions. However, based on the observed difference in terrestrial environmental conditions during these two hyperthermal events, it appears that the driving mechanisms are distinctly different.