Reconstructing wintertime surface layer fCO2 in the western Weddell Sea using summertime observations of Weddell Sea Bottom Water

The dense waters formed on the broad continental shelves of the western Weddell Sea are the source of Weddell Sea Bottom Water (WSBW) found along the slope and at the bottom of the basin. WSBW is considered to be an effective conduit for carbon sequestration, eventually contributing to the redistribution of carbon around the global oceans. To quantify the efficiency of this carbon sequestration, it is necessary to have a good understanding of the processes that transform carbon within the ocean surface layer, as well as those occurring along the slope. Lack of biogeochemical data in the southern and western Weddell Sea is hindering progress. In situ observations are particularly necessary in the wintertime, when dense shelf waters and WSBW are formed and thus CO₂ uptake is constrained. We present a method that reconstructs the wintertime fugacity (i.e., the adjusted partial pressure) of CO₂ (fCO₂) on the southwestern continental shelves from the dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements made in the WSBW, but collected in the summertime, when most expeditions take place. The method relies on relationships between the contributions of different water masses to WSBW, and potential temperatures, as found in previous work. Results for reconstructed surface wintertime fCO₂ are comparable to the very few other wintertime observations elsewhere in the Weddell Sea. Without in situ wintertime observations, validation of the results is challenging. The results suggest a negligible role for biogeochemical processes transforming DIC between wintertime shelf water and WSBW. Assumptions in the methodology need to be tested against in situ biogeochemical measurements on the shelves and along the slope. We applied our method to recent data and relate findings to other biogeochemical variables to narrow down the uncertainty in our assumptions.