New estimate of organic carbon export from optical measurements reveals
the role of particle size distribution and export depth
Abstract
Export of sinking particles from the surface ocean is critical for
carbon sequestration and to provide energy to the deep biosphere. The
magnitude and spatial patterns of this export have been estimated in the
past by \emph{in situ} particle flux observations,
satellite-based algorithms, and ocean biogeochemical models; however,
these estimates remain uncertain. Here, we use a recent machine learning
reconstruction of global ocean particle size distributions from
Underwater Vision Profiler 5 (UVP5) measurements to estimate carbon
fluxes by sinking particles (35 $\mu$m - 5 mm
equivalent spherical diameter) from the surface ocean. We combine global
maps of particle size distribution properties with empirical
relationships constrained against \emph{in situ} flux
observations to calculate particulate carbon export from the euphotic
zone and wintertime mixed layer depths. The new flux reconstructions
suggest a less variable seasonal cycle in the tropical ocean, and a more
persistent export in the Southern Ocean than previously recognized.
Smaller particles (less than 420 $\mu$m) contribute
most of the flux globally, while larger particles become more important
at high latitudes and in tropical upwelling regions. Export from the
wintertime mixed layer globally exceeds that from the euphotic zone,
suggesting shallow particle recycling and net heterotrophy in the deep
euphotic zone. These estimates open the way to fully three-dimensional
global reconstructions of particle fluxes in the ocean, supported by the
growing database of \emph{in situ} optical
observations.