Global patterns in efficiency of particulate organic carbon export and transfer to the deep ocean

The ocean's biological carbon pump is a key component of the global carbon cycle. Only a small fraction of the carbon fixed by primary production is exported to the deep ocean, yet this flux sets to first order the efficiency with which carbon is sequestered out of further contact with the atmosphere on long time scales. Here we examine global patterns in particle export efficiency (PEeff), the proportion of primary production that is exported from the surface ocean, and transfer efficiency (T-eff), the fraction of exported organic matter that reaches the deep ocean. Previous studies have found a positive correlation between T-eff and deep ocean calcite fluxes recovered from sediment traps, implying that ballasting by calcium carbonate may play an important role in regulating T-eff. An alternative explanation is that this correlation is not causative, as regions where the dominant biomineral phase is calcite tend to be subtropical systems, which are hypothesized to produce sinking aggregates highly resistant to degradation. We attempt to distinguish between these alternative hypotheses on the control of T-eff by examining the relationship between T-eff and biomineral phases exported from the upper ocean, rather than those collected in deep traps. Global scale estimates derived from satellite data show, in keeping with earlier studies, that PEeff is high at high latitudes and low at low latitudes, but that T-eff is low at high latitudes and high at low latitudes. However, in contrast to the relationship observed for deep biomineral fluxes in previous studies, we find that T-eff is strongly negatively correlated with opal export flux from the upper ocean, but uncorrelated with calcium carbonate export flux. We hypothesize that the underlying factor governing the spatial patterns observed in T-eff is ecosystem function, specifically the degree of recycling occurring in the upper ocean, rather than the availability of calcium carbonate for ballasting.