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 per mil 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.

Age model is constructed using a sedimentation rate of 0.9 cm/kyr (Sageman et al., 2006 (doi:10.1130/G22074.1) and MacLeod et al., 2008 (doi:10.1130/G24999A.1)). The authigenic fraction is estimated using crustal Cr/Ti values of 0.03 (Methods section).

Supplement to: Wang, Xiangli; Reinhard, Christopher T; Planavsky, Noah J; Owens, Jeremy D; Lyons, Timothy W; Johnson, Thomas M (2016): Sedimentary chromium isotopic compositions across the Cretaceous OAE2 at Demerara Rise Site 1258. Chemical Geology, 429, 85-92