LAUSR

Abstract The late Albian–early Cenomanian oceanic anoxic event 1d (OAE-1d; ~103 to 99.5 Ma) represents a global interval of enhanced organic carbon burial due to widespread oxygen deficiency in the water column and/or increased primary productivity. The biostratigraphy and geochemistry of organic-rich sediments in the La Grita Member (Capacho Formation) in southwestern Venezuela were studied to document the paleoenvironmental conditions that governed the deposition of this succession during OAE-1d. Carbon-isotope (δ13Ccarb and δ13CCorg) chemostratigraphy and biostratigraphic constraints show that the La Grita Member spans the late Aptian period (Rotalipora appenninica Zone), and that OAE-1d is well recorded in this succession. A prominent increase in total organic carbon (TOC) values (up to 10%) is clearly recorded through the onset of OAE-1d, coinciding with a prominent change in overall redox-sensitive proxies suggesting that the La Grita Member sediments accumulated under anoxic (and possibly euxinic) bottom-water conditions in a partially restricted basin. The detrital proxies suggest that the greenhouse climate prevailing during OAE-1d induced significant acceleration of the hydrologic cycle and an increase in continental chemical weathering rates. Overall, the carbon and oxygen isotope records, TOC contents, as well as the detrital proxies show cyclic variation during the late Albian stage. These variations support the hypothesis that orbital forcing likely also modulated the equatorial monsoonal activity during the OAE-1d.