Larsen C Ice Shelf in the Western Weddell Sea is a Hotspot for Export Production in the Antarctic Ocean

Abstract The Southern Ocean accounts for >30% of the global ocean CO 2 sink as a result of its solubility and biological pumps. To investigate the biological pump near Larsen C Ice Shelf (LCIS) in the Southern Ocean's western Weddell Sea, we measured the concentrations and nitrogen isotope ratios of nitrate, ammonium, and particulate organic nitrogen sampled in mid‐summer 2019. From the isotope data, we estimate a mean f ‐ratio (i.e., fraction of primary production available for export from the mixed layer, equivalent to the extent of phytoplankton reliance on subsurface nitrate) of 0.81 ± 0.18. This value is higher than parallel incubation‐based f‐ ratio estimates (≤0.5), a divergence that we ascribe to the different integration timescales of the two methods (weeks for the isotope method and hours to days for the incubations) and to a tendency for increased phytoplankton reliance on regenerated nitrogen later in the growth season. The carbon export flux estimated from the seasonal decline in nitrate concentration was >4.3 ± 1.7 mol C m −2 year −1 , as much as nine times that estimated for the open Antarctic Zone. Our data reveal that the waters adjacent to LCIS contribute disproportionately to the Weddell Sea's biological pump, which we attribute mainly to the effect of ice‐melt on stratification and iron supply. The CO 2 transferred by the biological pump into newly forming bottom waters will remain sequestered from the atmosphere for centuries.