Relative contributions of sinking and non-sinking carbon to the downward
carbon flux
Abstract
The downward flux of organic carbon exported from the surface ocean is
of great importance to the Earth’s climate because it represents the
major pathway for transporting CO from the surface ocean and atmosphere
into the deep ocean and sediments where it can be sequestered for a long
time. Here we present global-scale estimates for the export fluxes of
total, dissolved, and particulate organic carbon (TOC, DOC, and POC,
respectively) constrained by observed thorium-234 (Th) activity and
dissolved phosphorus (DIP) concentration in a global inverse
biogeochemical model for the cycling of phosphorus and Th. We find that
POC export flux is low in the subtropical oceans, indicating that a
projected expansion of the subtropical gyres due to global warming will
weaken the gravitational biological carbon pump. We also find that DOC
export flux is low in the tropical oceans, intermediate in the upwelling
Antarctic zone and subtropical south Pacific, and high in the
subtropical Atlantic, subtropical north Pacific, and productive
subantarctic zone (SAZ). The horizontal distribution of DOC export ratio
(F/F) increases from tropical to polar regions, possibly due to the
detrainment of DOC rich surface water during mixing events into
subsurface waters (increasing the strength of the mixed layer pump
poleward due to stronger seasonality). Large contribution to the export
flux from DOC implies that the efficiency with which photosynthetically
fixed carbon is exported as particles may not be as large as currently
assumed by widely used global export algorithms.