Sedimentary chromium isotopic compositions across the Cretaceous OAE2 at Demerara Rise Site 1258

• > 1‰ coherent negative δ53Cr excursion during peak OAE2.
• The δ53Cr is coincident with a large positive carbon isotope excursion.
• The δ53Cr excursion is most easily explained by increased euxinic at the expense of reducing conditions.

In order to advance our understanding of the emerging chromium (Cr) isotope system as a paleoredox proxy, we measured the 53Cr/52Cr of black shales deposited before, during, and after Cretaceous Oceanic Anoxic Event 2 (OAE2). We observed a > 1‰ coherent negative 53Cr/52Cr excursion and significant drawdown in Cr enrichments during OAE2 coincident with a large positive carbon isotope excursion. Our observed negative 53Cr/52Cr excursion during OAE2 is most easily linked to an increase in the ratio of euxinic to reducing conditions. Additional work on other OAE sections is needed to determine the spatial significance of this Cr isotope trend.