Variations in Miocene phytoplankton growth rates in the southwest Atlantic: Evidence for changes in ocean circulation

Changes in ocean circulation are often credited as the primary control on l arge-scale climate change during the Miocene. This study investigates the l atest Oligocene to middle Miocene evolution of Southern Ocean circulation b y evaluating stable isotopic trends of shallow- and deep-dwelling planktoni c foraminifera, as well as epsilon(p) records reconstructed from the carbon isotopic composition of diunsaturated alkenones in the southwestern Altant ic Ocean (Deep Sea Drilling Project site 516). Changes in epsilon(p) at sit e 516 closely paralleled the opening and deepening of the Drake Passage as inferred from seafloor magnetic anomalies. A large negative shift in epsilo n(p) at similar to 20.3 Ma is interpreted to reflect an increase in upper w ater column nutrient concentrations, caused by the onset or strengthening o f the Antarctic Circumpolar Current (ACC). Measurable alkenone concentratio ns disappear by similar to 17 Ma, prior to a collapse in surface-to-thermoc line delta(18)O and delta(13)C gradients. This is interpreted as reflecting a severe decrease in mixed layer nutrient Concentrations and reduced proto -Antarctic Intermediate Water influence. The delta(18)O gradient was reesta blished by 14.5 Ma, coincident with the hypothesized East Antarctic ice she et expansion, suggesting a direct relationship between increased strength o f the ACC and the largest climate shift of the middle Miocene.