Articles | Volume 12, issue 13
Research article
02 Jul 2015
Research article |  | 02 Jul 2015

Carbon export and transfer to depth across the Southern Ocean Great Calcite Belt

S. Z. Rosengard, P. J. Lam, W. M. Balch, M. E. Auro, S. Pike, D. Drapeau, and B. Bowler

Abstract. Sequestration of carbon by the marine biological pump depends on the processes that alter, remineralize, and preserve particulate organic carbon (POC) during transit to the deep ocean. Here, we present data collected from the Great Calcite Belt, a calcite-rich band across the Southern Ocean surface, to compare the transformation of POC in the euphotic and mesopelagic zones of the water column. The 234Th-derived export fluxes and size-fractionated concentrations of POC, particulate inorganic carbon (PIC), and biogenic silica (BSi) were measured from the upper 1000 m of 27 stations across the Atlantic and Indian sectors of the Great Calcite Belt. POC export out of the euphotic zone was correlated with BSi export. PIC export was not, but did correlate positively with POC flux transfer efficiency. Moreover, regions of high BSi concentrations, which corresponded to regions with proportionally larger particles, exhibited higher attenuation of > 51 μm POC concentrations in the mesopelagic zone. The interplay among POC size partitioning, mineral composition, and POC attenuation suggests a more fundamental driver of POC transfer through both depth regimes in the Great Calcite Belt. In particular, we argue that diatom-rich communities produce large and labile POC aggregates, which not only generate high export fluxes but also drive more remineralization in the mesopelagic zone. We observe the opposite in communities with smaller calcifying phytoplankton, such as coccolithophores. We hypothesize that these differences are influenced by inherent differences in the lability of POC exported by different phytoplankton communities.

Short summary
The transfer of particulate organic carbon (POC) into the deep ocean is an important atmospheric carbon dioxide sink. Observations from the Southern Ocean Great Calcite Belt region show that the relationship between POC and biogenic mineral fluxes varies with depth, between the surface and 1000m below. The results suggest that the transfer of POC into the deep ocean is more closely related to phytoplankton community structure than to mineral composition alone.
Final-revised paper