The Roles of Iron and Vertical Mixing in Regulating Nitrogen and Silicon Cycling in the Southern Ocean over the Last Glacial Cycle

Abstract:

The air-sea CO₂ balance is regulated by both biological and physical mechanisms in the Southern Ocean. Changes in the vertical supply of the major nutrients, nitrogen, phosphorus, and silicon, and in the availability of the micronutrient Fe are all thought to play a role in the glacial-interglacial variation in atmospheric CO₂ concentrations. Here we present the most spatially extensive dataset of silicon and nitrogen isotope measurements from diatom frustules to examine the controls on nutrient drawdown during the last glacial period and across the glacial termination. The data confirm existing views that differing silicon and nitrate consumption patterns are likely the result, at least in part, of iron addition during the last glacial maximum. However, earlier in the glacial, a more coordinated response between the two proxy records, with both reflecting enhanced consumption during episodes of increased iron accumulation and export production, implies a different response to iron than observed for the LGM. Extended spatial coverage in the Subantarctic provides evidence for vertical mixing of nutrients directly in this zone. The collapse of the expected equatorward gradient in silicon isotope values and contraction of the nitrogen isotope gradient during the deglaciation suggests that nutrient supply increased not only in the Antarctic Zone, but also in the Subantarctic, perhaps due to enhanced deep mixing locally. Increased vertical supply in both zones of the Southern Ocean may better explain the enhanced nutrient supply to the low latitude thermocline than Antarctic Zone dynamics alone.