Eocene to Miocene terrigenous inputs and export production: geochemical evidence from ODP Leg 177, Site 1090

Abstract

Changes in ocean circulation and climate during the Cenozoic led to the development of the Antarctic Circum-polar Current (ACC) and permanent Antarctic ice sheets. However, the timing of the opening of the Drake Passage and the establishment of the ACC is poorly constrained. We present geochemical proxies of terrigenous inputs and export production at a single site in the southeastern Atlantic Ocean that suggest the Drake Passage had opened to intermediate and deep water by 32.8 Ma. Two separate styles of sedimentation and geochemical composition of terrigenous material are observed, with the change in sediment geochemical characteristics occurring at 32.8 Ma. Middle to late Eocene records are highly variable, while latest Eocene to early Miocene records exhibit cyclic variations in elemental records and terrigenous material. Based on Al/Ti ratios, metal sources change from continental crust to oceanic crust sources at the Eocene/Oligocene transition, which we suggest reflects a change in deep-water circulation. Phosphorus/metal ratios indicate that there are two distinct intervals of enhanced export production, one in the middle Eocene and one throughout the Eocene/Oligocene transition. Elevated Al/Ti ratios, greater than any lithic source, in the middle Eocene provide evidence of particulate scavenging and thus increased export production. Barium ratios further support changes in productivity in the middle Eocene and at the Eocene/Oligocene transition. Permanent changes in the Ba concentration record in the early Oligocene further support a change in deep-water circulation due to the opening and deepening of the Drake Passage at 32.8 Ma.