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/δ¹³C tracers of deepwater are not significantly changed by surficial processes within the Southern Ocean. We can extend these principles to the late 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 δ¹³C differences between ocean basins. From 9.8 to 9.3 Ma, Southern Ocean δ¹³C values oscillated between high North Atlantic values and low Pacific values. The Southern Ocean recorded δ¹³C values similar to Pacific values from 9.2 to 8.9 Ma, reflecting a low contribution of Northern Component Water (NCW). The δ¹³C 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 Water (SCW) to fill the North Atlantic and causing the δ¹³C values in the North Atlantic, Pacific, and Southern oceans to converge. Deepwater δ¹³C patterns resembling the modern distributions evolved by 7.0 Ma: δ¹³C values were near 1.0‰ in the North Atlantic; 0.0‰ in the Pacific; and 0.5‰ in the Southern Ocean. Development of near‐modern δ¹³C 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 δ¹³C 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/δ¹³C differences. During the late Miocene, “interglacial” intervals prevailed during intervals of NCW production, while “glacial” intervals occurred during low NCW production.