Stirring Up the Biological Pump: Vertical Mixing and Carbon Export in the Southern Ocean

• Published in: Global biogeochemical cycles 2017-09, Vol.31 (9), p.1420-1434
• Main Authors: Stukel, Michael R., Ducklow, Hugh W.
• Format: Article
• Language: English
• Subjects: Advection; Approximation; Atmospheric models; Carbon; carbon cycle; Carbon dioxide; carbon export; Climate models; Cruises; Density stratification; Diffusion; Dissolved organic carbon; Dissolved organic matter; Drifters; Dye dispersion; Euphotic zone; Exports; Floats; Gradients; mixing; Modelling; Nitrates; Nitrogen; nitrogen cycle; Ocean models; Oceans; Organic carbon; Particle physics; Particulate organic matter; Physics; plankton; Primary production; Sinking; Steady state; Stratification; Subduction; Temperature (air-sea); Thermal stratification; Time series; Transport processes; Vertical distribution
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1002/2017GB005652

The biological carbon pump (BCP) transports organic carbon from the surface to the ocean's interior via sinking particles, vertically migrating organisms, and passive transport of organic matter by advection and diffusion. While many studies have quantified sinking particles, the magnitude of passive transport remains poorly constrained. In the Southern Ocean weak thermal stratification, strong vertical gradients in particulate organic matter, and weak vertical nitrate gradients suggest that passive transport from the euphotic zone may be particularly important. We compile data from seasonal time series at a coastal site near Palmer Station, annual regional cruises in the Western Antarctic Peninsula (WAP), cruises throughout the broader Southern Ocean, and SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) autonomous profiling floats to estimate spatial and temporal patterns in vertical gradients of nitrate, particulate nitrogen (PN), and dissolved organic carbon. Under a steady state approximation, the ratio of ∂PN/∂z to ∂NO3−/∂z suggests that passive transport of PN may be responsible for removing 46% (37%–58%) of the nitrate introduced into the surface ocean of the WAP (with dissolved organic matter contributing an additional 3–6%) and for 23% (19%–28%) of the BCP in the broader Southern Ocean. A simple model parameterized with in situ nitrate, PN, and primary production data suggested that passive transport was responsible for 54% of the magnitude of the BCP in the WAP. Our results highlight the potential importance of passive transport (by advection and diffusion) of organic matter in the Southern Ocean but should only be considered indicative of high passive transport (rather than conclusive evidence) due to our steady state assumptions. Plain Language Summary The biological carbon pump (BCP) transports organic carbon and nitrogen from the surface ocean to depth, leading to net uptake of CO2 from the atmosphere. It has long been assumed that sinking particles dominate the BCP, but our results show that in the Southern Ocean a substantial proportion of this transport occurs through vertical mixing of suspended or slowly sinking particles driven by ocean physics. Key Points Passive export of organic matter (OM) by subduction and mixing is an understudied component of the biological pump Regional cruises and season‐long time series near the Antarctic Peninsula show that passive transport is likely ~50% of total OM export NO3− and parti