The export and fate of organicmatter in the ocean: New constraints from combining satellite and oceanographic tracer observations

The ocean's biological pump transfers carbon from the surface euphotic zone into the deep ocean, reducing the atmospheric CO2 concentration. Despite its climatic importance, there are large uncertainties in basic metrics of the biological pump. Previous estimates of the strength of the biological pump, as measured by the amount of organic carbon exported from the euphotic zone, range from about 4 to 12 Pg C yr(-1). The fate of exported carbon, in terms of how efficiently it is transferred into the deep ocean, is even more uncertain. Here we present a new model of the biological pump that assimilates satellite and oceanographic tracer observations to constrain rates and patterns of organic matter production, export, and remineralization in the ocean. The data-assimilated model predicts a global particulate organic carbon (POC) flux out of the euphotic zone of similar to 9 Pg C yr(-1). The particle export ratio (the ratio of POC export to net primary production) is highest at high latitudes and lowest at low latitudes, but low-latitude export is greater than predicted by previous models, in better agreement with observed patterns of long-term carbon export. Particle transfer efficiency (T-eff) through the mesopelagic zone is controlled by temperature and oxygen, with highest T-eff for high-latitude regions and oxygen minimum zones. In contrast, T-eff in the deep ocean (below 1000 m) is controlled by particle sinking speed, with highest deep ocean T-eff below the subtropical gyres. These results emphasize the utility of both remote sensing and oceanographic tracer observations for constraining the operation of the biological pump. Plain Language Summary Tiny marine plants known as phytoplankton absorb vast amounts of carbon, a portion of which settles into the deep ocean as the remains of dead organisms, where it is ultimately decomposed. This "biological carbon pump" is a key component of the global carbon cycle, bringing almost as much CO2 into the subsurface ocean on an annual basis as humans emit through fossil fuel burning. In this work, we present a new way to estimate how efficiently the biological pump is moving carbon from the surface to deep ocean and what factors determine the efficiency of the biological pump. By combining observations from Earth-orbiting satellites and from seagoing research cruises in a global carbon cycle model, we show that the low-latitude ocean exports more carbon out of the surface ocean than previously thought. We also show that temperature and oxygen concentrations are the most important controls on how efficiently carbon is transferred into the deep ocean. This has important implications for how the biological pump will respond to climate change, which is expected to lead to a warmer and less oxygenated ocean.