Carbon Export in the Subantarctic Zone Revealed by Multi-Year Observations From Biogeochemical-Argo Floats and Sediment Traps

The biological gravitational pump (BGP) and particle injection pumps (PIPs) are significant export pathways for particulate organic carbon from the surface ocean to the interior. Part of this exported carbon fuels remineralization in the mesopelagic ocean and part is sequestered in the deep ocean. Using observations from Biogeochemical-Argo, we characterized the seasonality and magnitude of the BGP and two PIPs: the mixed layer pump (MLP) and eddy subduction pump (ESP), in the Australian sector of the Subantarctic Zone (SAZ sector). For the first time, float-based estimates were rigorously combined with sediment trap flux (F₁₀₀₀) observations from the Southern Ocean Time Series (SOTS), to investigate these pumps' relative and cumulative contributions to carbon export. The BGP exports about 28.6 g C m⁻² year⁻¹, mostly during the productive season and dominates the F₁₀₀₀ seasonality. The MLP exports about 7.6 g C m⁻² year⁻¹, mostly while the mixing layer seasonally shoals; the ESP sporadically exports up to 100 mg C m⁻² day⁻¹, such that these two PIPs have a short but intense impact on the F₁₀₀₀. The carbon transfer efficiency is 3.6% in the SOTS region. An oxygen-based annual net community production estimate (∼50 g C m⁻² year⁻¹) further strengthens this study, and suggests the BGP and MLP make the dominant contribution to the mesopelagic carbon budget. This is representative of the broader SAZ sector in terms of the magnitude and seasonality of carbon export, the consumption of organic material in the mesopelagic, and the organic carbon sequestration in the deep sea.