Platinum-group elements record both weathering changes and volcanic input through the Pliensbachian–Toarcian transition at Peniche, Portugal

Abstract

Large igneous provinces (LIPs) are thought to be the drivers of numerous Mesozoic episodes of dramatic environmental perturbation due to massive influx of volcanically sourced CO₂ and subsequent enhanced global weathering rates. Two such episodes studied here are the Pliensbachian-Toarcian (Pl/To) event, closely followed by the Toarcian oceanic anoxic event (T-OAE), both of which have been temporally linked to eruption phases of the Karoo and Ferrar LIPs (KFLIP). Here we present high-precision platinum-group element (PGE) data for the sedimentary succession spanning across the Pl/To and the T-OAE from the Toarcian Global Stratotype Section and Point (GSSP) at Peniche, Portugal. The concentrations of PGEs peak across both Pl/To and the T-OAE, and correlate with Hg enrichment and negative carbon-isotope excursions reported from these intervals. They are also positively correlated with siliciclastic content (e.g., Al, Zr and Ti) and weathering proxy variations from these intervals (i.e., ¹⁸⁷Os/¹⁸⁸Os and ⁸⁷Sr/⁸⁶Sr), although there are still enrichments in PGE/Al at both intervals. Pd/Ir and Pt/Ir ratios are slightly higher than in continental crust (∼30–80 for Peniche samples and ∼ 22 for UCC), but similar to basalts from KFLIP. These observations, together with Hg concentrations, ¹⁸⁷Os/¹⁸⁸Os and ⁸⁷Sr/⁸⁶Sr ratios, are best explained by elevated continental weathering and additional volcanic input. These results support the hypothesis that KFLIP eruptions were responsible for both Pl/To and T-OAE, and further show that PGEs are a powerful proxy for fingerprinting volcanism and weathering.