High-resolution marine osmium and carbon isotopic record across the Aptian–Albian boundary in the southern South Atlantic: Evidence for enhanced continental weathering and ocean acidification

Abstract

The Early Cretaceous Aptian–Albian boundary (AAB) interval (∼113 Ma) was marked by severe environmental perturbations, including Oceanic Anoxic Event (OAE) 1b, global warming, and a major species turnover of planktonic foraminifera. Most paleoenvironmental studies across the AAB have focused on Tethyan sedimentary sequences deposited at low latitudes, but, because of the scarcity of well-dated sedimentary sequences across the AAB at high latitudes, the global extent of these environmental perturbations remains uncertain. Deep Sea Drilling Project (DSDP) Site 511 was drilled on the Falkland Plateau, southern South Atlantic, and is one of the few sedimentary sequences at southern high latitudes to record an almost continuous AAB interval. Previous studies documented the planktonic foraminiferal changes across the AAB at DSDP Site 511, but chemostratigraphic information has been limited. We reconstructed an osmium and carbon isotopic stratigraphy to further constrain the position of the AAB. We also inferred the paleo-marine redox conditions at southern high latitudes by combining chemical and lithological information. We found an abrupt negative δ13Ccarb shift coincident with the first occurrence of Microhedbergella renilaevis, an event that marks the base of the Albian Stage. This negative δ13Ccarb shift has also been observed across the AAB interval in the Tethyan and Atlantic regions. The fact that the osmium isotopic ratios show a pronounced radiogenic shift after the AAB is consistent with osmium isotopic variations detected in the Tethys and Pacific regions. Because the Os shift corresponded to a warming event during OAE1b in the low-latitude Atlantic and Tethyan regions, we interpret this radiogenic osmium isotopic shift as a response to increased continental weathering caused by global warming. A large marcasite crystal in the AAB interval at DSDP Site 511 suggests that the southern Atlantic Ocean may have been oxygen-depleted with a low pH caused by contemporaneous global environmental perturbations.