Chemical weathering during the PETM: Insights from stable and radiogenic isotopes in the Spanish Pyrenees

Abstract

We explore the feedback mechanisms at play during the climate recovery of the Paleocene Eocene Thermal Maximum (PETM) in a continental section of the Spanish Pyrenees, the Esplugafreda section. We combine isotopic and mineralogical proxies to assess the changes in the chemical weathering intensity and regime and explore their implications in this mid-latitude catchment. The changes in the clay mineral assemblages in two size fractions (<0.5 µm and 0.5-2 µm) were examined as paleoclimatic proxies. We also used these fractions to study the combined Sm-Nd and Lu-Hf radiogenic isotope systems to constrain the clay provenance and chemical weathering intensity. Furthermore, we measured lithium isotopes in the bulk clays to assess local changes in the weathering regime and clay neoformation during the PETM.The clay mineralogy reveals an interplay between detrital clay input during extreme events and more hydrolyzing conditions reflected in the presence of pedogenetic smectites during the onset and body of the PETM. The extent to which the clay samples deviate from the clay array (ΔεHf) show no variations throughout the section, indicating no increase in the chemical weathering intensity. Nonetheless, the δ7Li values show a distinct negative excursion during the recovery of the PETM in the Esplugafreda section. This excursion is coeval with an increase in the proportions of illite and chlorite relative to smectite, an indicator of increasing physical erosion.Our results suggest that the low surface reactivity of the floodplain deposit sediments resulted in negligible variations in the chemical weathering intensity recorded in the clays. Yet, the floodplain soils saw more fractionated lithium isotope compositions, contemporary with an increase in the detrital nature of the clay minerals. Climate change in the region was characterized by a substantial temperature increase with massive but short precipitation events, resulting in efficient sediment transport. Although weathering in the Pyrenees was kinetic-limited, both chemical weathering and physical erosion played a role in the climate recovery to pre-PETM conditions. Rapid climatic changes and their impact on the geological record are of utmost importance for understanding ongoing global warming and the associated feedback mechanisms on environmental conditions.