A23 section: JR15006: RRS James Clark Ross, 2016-03-31 — 2016-04-26

The voyage of the RRS James Clark Ross JR15006 consisted of the semi-annually repeated A23 hydrographic CTD section between the Weddell Sea and South Georgia, combined with the opportunistic request for 6 extra science days to take advantage of the newly installed oxygen isotope auto-analyser connected to the ship underway system. The goal of this extension was to collect ‘end member’ samples of oxygen isotopes from freshwater input to the region in order to attribute the freshwater component of underway observations to various percentage contributions from (for e.g.) sea ice melt, precipitation, glacial outflow etc. In addition oxygen isotopes were collected from CTDs on both A23 and in the vicinity of strong freshwater input, to establish the mixing between deep ocean interior values and local surface sources.

The voyage proceeded from Mare Harbour, Falkland Islands, immediately to Signy Island in the South Orkneys group. A day of ice/meltwater/shore water sample collection from and around MacLeod Glacier was undertaken, both by shore parties and small boats in front of the glacial terminus, before three CTDs were conducted away from the island. We then headed to the southern end of the A23 section and entered the advancing pack ice edge. Here we found a good example of multi-year sea ice and took ice core sections from it, as well as adjacent new ice and surface water samples, along with two CTDs, both within the pack and on its margins. From here we conducted six days of 24 hour CTD operations north along A23, occupying almost 30 full depth stations, with d18O sampled on all stations.

At the northern end of A23 we steamed to Cumberland Bay, South Georgia and repeated shore sampling, small boat surface sampling and CTDs along and across Cumberland Bay East and the entrance to Cumberland Bay West. This included shore parties taking glacial cores and water samples from Glacier Col and Gul Lake.

Following this work we proceeded back to Stanley, the Falkland Islands. During all ship operations underway instruments were left on. These included underway water samples taking pCO2, temperature, salinity and d18O, VMADCP velocity measurement, EM122 swath and EA600 acoustics and a full suite of meteorological readings. The only exception to this was within dense pack ice where underway water supplied had to be turned off to avoid ice damage to the intake/pumps.

Oceanographic data for this cruise are with BODC, freshwater with PDC.