Meltwater and circulation characteristics adjacent to Larsen C ice shelf: insights from seawater oxygen isotopes

The Larsen C ice shelf (LCIS) in the western Weddell Sea has recently undergone large-scale ice shelf collapse with the detachment of iceberg A68 in 2017. Cold cavity ice shelves, such as LCIS, are critical for the formation of the world’s coldest, densest waters and act to prevent the flow of land-fast ice into the ocean, which would result in sea-level rise. Their disintegration is thus of great scientific interest and growing public concern. It has been hypothesized that ice shelf breakup may result from ice shelf thinning, which can be caused by densification through surface processes, a decrease in grounded ice flow, or increased surface or basal melting. To investigate whether ice shelf melting may be contributing to the collapse of LCIS, we collected full depth profiles of seawater samples at 17 stations in the vicinity of LCIS in January 2019 during the Weddell Sea Expedition. To investigate the formation processes and distribution of water masses, as well as identify regions of ice shelf melt, the samples were measured for seawater oxygen isotopic composition (δ¹⁸O) using a Picarro Cavity Ring-Down Spectroscope (CRDS). The isotope data provide little evidence of large-scale surface or basal ice shelf melting, with basal ice shelf melt constituting a maximum of 0.5% of the Ice Shelf Water (ISW) observed in the vicinity of LCIS. One implication of this is that surface and basal melting may not be the primary factor driving the collapse of LCIS, although more data and further study are required to confirm this. In addition, the isotope data are consistent with previous work suggesting that the onshore advection of warm offshore waters occurs via the Jason Trough, a remnant depression in the seafloor caused by the flow of a palaeo-ice stream. This, in combination with the observation (based on incorporating seawater δ¹⁸O into a temperature-salinity-oxygen mass balance model) that the outflow of ISW occurs primarily to the north of the study region, supports a clockwise circulation pattern in the vicinity of LCIS.