Silicate weathering in the semi-arid Southern Pyrenees during the PETM: lithium isotope evidence

<p>The Palaeocene-Eocene Thermal Maximum (PETM), a hyperthermal event <span class="inline-formula">∼</span> 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 (<span class="inline-formula"><i>δ</i>⁷</span>Li) 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.</p> <p>We investigated two continental sections in the Southern Pyrenees. In the Esplugafreda section, we measured <span class="inline-formula"><i>δ</i>⁷</span>Li values as a silicate weathering proxy and <span class="inline-formula"><i>ε</i>_Nd</span> values as a provenance proxy in the clay minerals. In the Rin section, we characterised the PETM locally by analysing <span class="inline-formula"><i>δ</i>¹³</span>C values in organic matter and examined the clay mineralogy in the paleosols, as well as measuring <span class="inline-formula"><i>δ</i>⁷</span>Li values in clays and carbonate nodules to trace silicate weathering. In the Esplugafreda section, we observe temporally stable <span class="inline-formula"><i>ε</i>_Nd</span> values, while the <span class="inline-formula"><i>δ</i>⁷</span>Li<span class="inline-formula">_clays</span> record shows two small positive excursions, one during the Pre-Onset Excursion (<span class="inline-formula">∼</span> 0.7 ‰) and a second during the body of the PETM (<span class="inline-formula">∼</span> 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, kaolinite, and chlorite, consistent with a seasonal climate in the region, and we find a positive <span class="inline-formula"><i>δ</i>⁷</span>Li<span class="inline-formula">_clays</span> excursion of <span class="inline-formula">∼</span> 0.8 ‰.</p> <p>The combined <span class="inline-formula"><i>δ</i>⁷</span>Li<span class="inline-formula">_clays</span> and <span class="inline-formula"><i>ε</i>_Nd</span> records indicate increased clay formation and increased silicate weathering fluxes in the semi-arid Pyrenees, while the sediment provenance was stable. The <span class="inline-formula"><i>δ</i>⁷</span>Li 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 fluxes, despite a comparatively high weathering intensity, evolved towards higher weathering fluxes with enhanced erosion during the PETM.</p>