EVOLUTION OF MODERN DEEPWATER CIRCULATION: EVIDENCE FROM THE LATE MIOCENE SOUTHERN OCEAN

Deepwater circulation plays an important role in climate modulation through its redistribution of heat and salt and its control of atmospheric CO2. Oppo and Fairbanks (1987) showed that the Southern Ocean is an excellent monitor of deepwater circulation changes for two reasons: (1) the Southern Ocean is a mixing reservoir for incoming North Atlantic Deep Water and recirculated water from the Pacific and Indian oceans; and (2) the nutrient/delta C-13 tracers of deepwater are not significantly changed by surficial processes within the Southern Ocean. We can extend these principles to the l ate Miocene because tectonic changes in the Oligocene and early and middle Miocene developed near-modern basinal configurations. However, on these time scales, changes in the oceanic carbon reservoir and mean ocean nutrient levels also affect the delta C-13 differences between ocean basins. From 9.8 to 9.3 Ma, Southern Ocean delta C-13 values oscillated between high North Atlantic values and low Pacific values. The Southern Ocean recorded delta C-13 values similar to Pacific values from 9.2 to 8.9 Ma, reflecting a low contribution of Northern Component Water (NCW). The delta C-13 differences between the NCW and Pacific Outflow Water (POW) end-members were low from 8.9 to 8.0 Ma, making it difficult to discern circulation patterns. NCW production may have completely shutdown at 8.6 Ma, allowing Southern Component ater (SCW) to fill the North Atlantic and causing the delta C-13 values in the North Atlantic, Pacific, and Southern oceans to converge. Deepwater delta C-13 patterns resembling the modern distributions evolved by 7.0 Ma: delta C-13 values were near 1.0 parts per thousand in the North Atlantic; 0.0 parts per thousand in the Pacific; and 0.5 parts per thousand in the Southern Ocean. Development of near-modern delta C-13 distributions by 7.0 Ma resulted not only from an increase in NCW flux but also from an increase in deepwater nutrient levels. Both of these processes increased the delta C-13 difference between the North Atlantic and Pacific oceans. Deepwater circulation patterns similar to today's operated as early as 9.8 Ma, but were masked by the lower nutrient/delta C-13 differences. During the late Miocene, "interglacial" intervals prevailed during intervals of NCW production, while "glacial" intervals occurred during low NCW production.