Antarctic subtropical humid episode at the Paleocene-Eocene boundary: Clay-mineral evidence

Abstract

Clay-mineral assemblages from East Antarctica have been analyzed at high stratigraphic resolution (20 to 1 ka) throughout the interval from 55.6 to 55.0 Ma, which includes the terminal Paleocene isotopic excursion in Ocean Drilling Program Site 690B on Maud Rise (lat 65°S) in the Weddell Sea region. Changes in the clay associations reflect a major increase in chemical weathering caused by increased temperature and/or rainfall in at least this sector of East Antarctica for a brief (270 ka) interval in the latest Paleocene. This represents the most intense warming known for the Cenozoic. This high-latitude climatic episode is recorded synchronously by stable isotopes and clay minerals. A progression is evident in the clay assemblages during the latest Paleocene that apparently reflects changing relations between temperature and precipitation. This sequence began with the interval of rapid temperature increase that marks the beginning of the oxygen isotopic excursion. A brief increase in kaolinite at the inception of the excursion suggests a temporary increase in year-round precipitation in Antarctica, in response to an increased continent-to-ocean temperature gradient. This kaolinite spike was followed by almost total dominance by smectite for the remainder of the isotopic excursion (∼120 ka), suggesting that warmer Southern Ocean surface temperatures of 18 to 22 °C were associated with seasonal precipitation (alternating wet and dry seasons). Claymineral variations on Antarctica during the isotopic excursion reflect a tight coupling between oceanic and continental climate change. Immediately following the excursion at ∼55.22 Ma, kaolinite percentages increased to values similar to modern subtropical-tropical areas for ∼150 ka, a remarkable event for the Antarctic. Abundant kaolinite suggests perennial rainfall and minimum soil temperatures of 15 °C during at least part of the year. The kaolinite increased during a time of lower Southern Ocean surface-water temperatures, suggesting increased atmospheric heat transport toward the poles. A temporary change in atmospheric circulation is suggested from dominantly zonal to meridional.