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 large-scale climate change during the Miocene. This study investigates the latest Oligocene to middle Miocene evolution of Southern Ocean circulation by evaluating stable isotopic trends of shallow- and deep-dwelling planktonic foraminifera, as well as epsilon(p) records reconstructed from the carbon isotopic composition of diunsaturated alkenones in the southwestern Altantic Ocean (Deep Sea Drilling Project site 516). Changes in epsilon(p) at site 516 closely paralleled the opening and deepening of the Drake Passage as inferred from seafloor magnetic anomalies. A large negative shift in epsilon(p) at similar to 20.3 Ma is interpreted to reflect an increase in upper water column nutrient concentrations, caused by the onset or strengthening of the Antarctic Circumpolar Current (ACC). Measurable alkenone concentrations disappear by similar to 17 Ma, prior to a collapse in surface-to-thermocline 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 reestablished by 14.5 Ma, coincident with the hypothesized East Antarctic ice sheet expansion, suggesting a direct relationship between increased strength of the ACC and the largest climate shift of the middle Miocene.