Antarctic Ocean Nutrient Conditions During the Last Two Glacial Cycles

Abstract:

The high concentration of the major nutrients nitrate and phosphate in the Antarctic Zone of the Southern Ocean dictates the nature of Southern Ocean ecosystems and permits these nutrients to be carried from the deep ocean into the nutrient-limited low latitudes. Incomplete nutrient consumption in the Antarctic also allows the leakage of deeply sequestered carbon dioxide (CO₂) back to the atmosphere, and changes in this leakage may have driven glacial/interglacial cycles in atmospheric CO₂. In a sediment core from the Pacific sector of the Antarctic Ocean, we report diatom-bound N isotope (δ¹⁵N_db) records for total recoverable diatoms and two assemblages of diatom species. These data indicate tight coupling between the degree of nitrate consumption and Antarctic climate across the last two glacial cycles, with δ¹⁵N_db (and thus the degree of nitrate consumption) increasing at each major Antarctic cooling event. Measurements in the same sediment core indicate that export production was reduced during ice ages, pointing to an ice age reduction in the supply of deep ocean-sourced nitrate to the Antarctic Ocean surface. The reduced export production of peak ice ages also implies a weaker winter-to-summer decline (i.e. reduced seasonality) in mixed layer nitrate concentration, providing a plausible explanation for an observed reduction in the inter-assemblage δ¹⁵N_db difference during these coldest times. Despite the weak summertime productivity, the reduction in wintertime nitrate supply from deep waters left the Antarctic mixed layer with a low nitrate concentration, and this wintertime change also would have reduced the outgassing of CO₂. Relief of light limitation fails to explain the intermediate degree of nitrate consumption that characterizes early glacial conditions, as improved light limitation coincident with reduced nitrate supply would drive nitrate consumption to completion. Thus, the data favor iron availability as the dominant control on annual Antarctic Ocean export production over glacial cycles.