Glacial‐Interglacial Shifts in Export Productivity and Iron Flux in the Indian Sector of the Southern Ocean

Abstract

Greater ocean export productivity fueled by enhanced delivery of iron to the Sub‐Antarctic Zone (SAZ) of the Southern Ocean is hypothesized to play an important role in reducing atmospheric CO2 during Pleistocene glaciations. However, spatial variability in iron input and nutrient availability complicates quantifying the integrated impact of iron‐fertilization on carbon export. We reconstruct preserved carbon export and iron input across the central Indian sector of the Southern Ocean with a latitudinal transect of sediment cores. 230Th‐normalized fluxes of organic and inorganic productivity proxies are compared to bulk sedimentary redox‐sensitive trace metal concentrations to investigate preservation bias related to glacial‐interglacial changes in bottom water oxygenation. Across the last 28 ka, a strong relationship is observed between iron fluxes and export productivity across the SAZ, suggesting enhanced iron delivery during the Last Glacial Maximum (LGM) strengthened the biological carbon pump. Changes in bottom water oxygenation influenced the preservation of organic‐based productivity proxies, while excess barium fluxes reliably record carbon export. The southern SAZ (44.7°S) received ∼8× more iron during the LGM than the central SAZ (42.3°S). This pronounced iron flux gradient suggests terrigenous inputs from the Kerguelen Plateau entrained in the Antarctic Circumpolar Current delivered greater volumes of iron to the southern SAZ, relative to the northern SAZ dominated by aeolian input. These results, alongside previous studies, demonstrate iron fertilization increased export productivity across large swathes of the SAZ during the LGM. However, spatial heterogeneity in iron supply led to regional differences in the magnitude of glacial‐interglacial shifts in productivity.