Source-to-sink weathering response to the Paleocene-Eocene Thermal Maximum (PETM) in the Southern Pyrenees

Global warming and the associated hydrological cycle variations are known to disrupt the weathering regime over geological timescales. Enhanced weathering and erosion, which constitute denudation, are important feedback mechanisms for regulating Earth’s temperature over multi-million-year timescales. Weathering can draw down CO2 from the atmosphere, while enhanced physical transport can accelerate organic carbon sedimentation and sequestration. This study aims to uncover changes to the denudation regime accompanying a massive climatic disturbance in deep time, the Paleocene-Eocene Thermal Maximum (PETM). The global warming of 5-8 °C due to the PETM has been documented to have increased the magnitude and intensity of precipitation events in the Spanish Pyrenees. But how did weathering respond to such a climatic and hydrological disturbance? We investigated the lithium (Li), hafnium (Hf), and neodymium (Nd) isotopic composition of the <2 mm clay size-fraction in three sections in the Spanish Pyrenees, from source to sink: the Esplugafreda, Campo, and Zumaia localities. The Li isotope record at Esplugafreda in the fluvial domain shows a positive δ7Li excursion during the onset and body of the event and a negative excursion during the PETM recovery, with no variation in the ΔεHf, i.e., εHf corrected for provenance changes with the εNd record. The Campo coastal section shows a negative δ7Li excursion during the body of the event. In the Zumaia deep marine section, the body of the event was characterized by a positive δ7Li excursion, coeval with a negative excursion in ΔεHf. These results suggest a relative decrease in weathering (W) to denudation (D = W+E, where E is erosion) during the PETM. The terrestrial section (Esplugafreda) indicates a local decrease in clay formation relative to erosion (E). The coastal section (Campo), which integrates a larger catchment area, seems to record an absolute increase in weathering. Finally, the “sink” deep-marine section (Zumaia) appears to indicate a relative decrease in regional weathering to denudation (W/D), consistent with the positive Li isotope and negative ΔεHf excursions. The source-to-sink approach suggests that weathering in the Pyrenees increased during the PETM but that physical erosion increased even more, hence controlling the denudation regime in the region. These changes imply a trend towards a kinetically-limited weathering regime in the region, with local variations in weathering efficiency.