Silicate Weathering in the Semi-Arid Southern Pyrenees During the PETM: Lithium Isotope Evidence

Abstract. The Palaeocene-Eocene Thermal Maximum (PETM), a hyperthermal event ~56 Ma ago, allows the Earth system response to abrupt climate change to be explored. Recent investigations link the PETM with a negative lithium isotope (δ⁷Li) excursion, interpreted as an increase in continental silicate weathering fluxes, which can regulate Earth’s surface temperature over geological timescales. However, the silicate weathering response under different climatic regimes has yet to be constrained. Here we aim to address the chemical weathering response to the PETM in the semi-arid Southern Pyrenees, and to explore how different archives (i.e., clays and carbonate nodules) record the weathering changes.

We investigated two continental sections in the southern Pyrenees. In the Esplugafreda section, we measured δ⁷Li values as a silicate weathering proxy and ε_Nd values as a provenance proxy in the clay minerals. In the Rin section, we characterised the PETM locally by analysing δ¹³C values in organic matter and examined the clay mineralogy in the paleosols, as well as measuring δ⁷Li values in clays and carbonate nodules to trace silicate weathering. In the Esplugafreda section, we observe temporally stable ε_Nd values, while the δ⁷Li_clay record shows two small positive excursions, one during the Pre-Onset Excursion (~0.7‰) and a second during the body of the PETM (~0.8‰). In the Rin section, the PETM is characterised by a negative carbon isotope excursion of 2.8‰. The clays consist mostly of illite/smectite, illite, and kaolinite, consistent with a seasonal climate in the region, and we find a positive δ⁷Li_clay excursion of ~0.8‰.

The combined δ⁷Li_clays and ε_Nd records indicate increased clay formation and increased silicate weathering fluxes in the semi-arid Pyrenees, while the sediment provenance was stable. The δ⁷Li values in the carbonate nodules indicate more variability, potentially due to clay contamination. Constrained by the bedrock type of dominantly reworked sediments and the seasonal precipitation regime, the initially low weathering rate, despite a comparatively high weathering intensity, evolved towards a higher weathering rate with enhanced erosion during the PETM.