The Nd isotopic composition of Adélie Coast Bottom Water – insights from GIPY6 cruise along 140°E

Tuesday, 16 December 2014: 9:30 AM
Tina van de Flierdt1, Myriam Lambelet2, Edward C.V. Butler3,4, Andrew R Bowie3,5, Stephen R Rintoul3,6, Roslyn J. Watson7, Tomas Remenyi3 and Delphine Lannuzel3,5, (1)Imperial College London, London, SW7, United Kingdom, (2)Imperial College London, London, United Kingdom, (3)Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC), Hobart, Australia, (4)Australian Institute of Marine Sciences, Darwin, Australia, (5)Institute for Marine and Antarctic studies (IMAS), Hobart, Australia, (6)CSIRO Wealth from Oceans National Research Flagship, Hobart, Australia, (7)CSIRO National Research Collections Australia, Hobart, Australia
Cold and dense Antarctic Bottom Water (AABW) is an important component of the global overturning circulation. The majority of AABW forms in three discrete locations around the Antarctic continent, the Weddell Sea (Atlantic sector of the Southern Ocean), the Ross Sea (Pacific sector), and the Adélie Depression (~142-145°E, ~66-67°S; Indian sector). Adélie Land Bottom Water (ALBW) is formed as dense shelf waters beneath coastal polynyas, and periodically overspills and is exported to the abyssal ocean. Previous work suggested that this particular water mass not only carries characteristic hydrographic properties, but that it may also carry a distinct radiogenic isotope composition, which would allow tracing local bottom water formation back in time.

We here present new results for the Nd isotope composition and concentration of seawater collected during the GIPY6 voyage from Tasmania (Australia) to the Adélie Depression (Antarctica) (SR3 CASO-GEOTRACES transect; March-April 2008). A total of four seawater profiles were sampled at four depths each across the Southern Ocean (48 to 65.5°S) along the 140°E meridian. Seawater was filtered and acidified aboard the Aurora Australis, and analysed back in the home laboratory using pre-concentration on C18 cartridges, 2 stage ion chromatography and NdO+ analysis by TIMS.

Results confirm a rather homogenous isotopic composition of Circumpolar Deep Waters (eNd = -8.2 to -9.0), but a distinct isotopic fingerprint of ALBW (eNd = -9.3 to -10.5). ALBW contributes up to 2 Sverdrup of AABW annually, and cold temperatures and relatively high oxygen and CFC-rich contents of this water mass correlate with rather negative Nd isotopic compositions. Absolute values are more than two epsilon units lower than the ones observed in Ross Sea bottom water, supporting the idea that the proximity of the old Adélie craton facilitates the formation of a particularly unradiogenic variety of AABW.