Barremian to Aptian Os isotopic record of a pelagic deep-sea chert (Goshikinohama-bedded chert) in a Japanese accretionary complex

Abstract

The early Aptian oceanic anoxic event (OAE) 1a was one of the largest OAEs in the Cretaceous, and the organic-rich sediments have been reported from various sedimentation depths worldwide. Because the sedimentation age of this organic-rich interval corresponds to the radiometric ages of the Ontong Java Nui (OJN) basalts, their causal linkage has been debated. The Goshikinohama-bedded chert is a Cretaceous sedimentary sequence deposited at depths deeper than the carbonate compensation depth (∼4000 m) in the pelagic Pacific. Although it covers the Barremian to Aptian, its stratigraphic relation to the OAE1a has not been fully constrained. Here, we present the 187Os/188Os record of the Goshikinohama-bedded chert to further constrain the OAE1a interval. As a result, 187Os/188Os showed several shifts to unradiogenic values, which may be correlated to the early Aptian OAEs. However, the 187Os/188Os values of the Goshikinohama-bedded chert are less radiogenic than the previously reported Barremian to Aptian 187Os/188Os values. The less radiogenic trend may represent the high contribution of extraterrestrial Os in the sediments or the influence of hydrothermal fluid from the underlying Valanginian basaltic rocks. Because of such unknown factors, there remain uncertainties in the stratigraphic correlations between the Goshikinohama-bedded chert with other sections.