Iron Isotope Evidence for Heightened Primary Productivity Triggered by Hydrothermally Sourced Nutrients During Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (OAE2, ∼94 Ma) represents a major global cycle perturbation in the mid-Cretaceous greenhouse that is commonly attributed to increased biological primary productivity stimulated by elevated nutrients. However, whether increased nutrients were sourced from submarine volcanism or enhanced continental weathering remains elusive. Here we present a high-resolution whole-rock Fe isotope record of the expanded OAE2 interval at Gongzha section in southern Tibet to trace iron cycling through the biological, geological and geochemical processes across OAE2. δ⁵⁶Fe_T data show three notable negative shifts, corresponding to episodes of intensified volcanism as documented by the osmium isotope record, with the largest shift followed by a prolonged positive δ⁵⁶Fe_T excursion coeval with the positive δ¹³C shift of OAE2, compatible with a causal link between hydrothermally sourced iron and biological uptake. Similar δ⁵⁶Fe_T patterns are observed in other OAE2 sections of different settings, implying submarine volcanism as the driving force for initiating OAE2.