Understanding the export of biogenic particles in oceanic waters: Is there consensus?

Abstract

We examine progress towards a global view of oceanic export of particulate organic carbon (POC) and other nutrient elements (P, N, Si) from the surface (upper 100 m), through the subsurface, to the deep sea (>1000 m), focusing on syntheses published since 1999 and on the Joint Global Ocean Flux Study. Food-web structure is important, and surface and subsurface processes contribute similarly to determine the fraction of net primary production (NPP) reaching the deep sea. NPP by large cells generally favours high surface export of POC. Preferential remineralization of P and N (versus C) with depth is common, as is regional variation in subsurface POC flux attenuation. The role of mineral fluxes is complex. Annual mean fluxes of POC and minerals are correlated in global deep sediment trap records, but causality and the relative importance of different minerals depends on the assumptions made. Time-series observations at single sites can oppose the geographic trends, and their large seasonal variability in the contribution of POC to total flux is at odds with mechanistic models for POC transport by minerals. Despite generally positive correlations between biogenic carbonate and POC fluxes, the overall role of carbonate export is to decrease the transfer of carbon dioxide from the atmosphere to the ocean. Both autotrophs and heterotrophs produce minerals, and progress in separating these contributions is required for the deconvolution of mineral ballast and food-web effects. Many recent models suggest global surface POC export of 10 GTC/yr, despite widely varying biological complexity. This limits the usefulness of their prediction of ecosystem and carbon cycle responses to global change. Progress requires better observations for model validation, and more efforts to relate the models to the observed complexity, rather than to overly simplified global syntheses. We advocate more time-series stations targeting under-studied biogeochemical regions, development of automated in situ tools for study of the subsurface ocean, and increased emphasis on combining ecological and biogeochemical methods.