Variability of Antarctic Bottom Water formation and export from the Weddell Sea
Final Report Abstract
In the project „Variability of Antarctic Bottom Water formation and export from the Weddell Sea” we obtained successfully two new data sets with high spatial resolution and excellent data quality of chlorofluorocarbons (CFCs) and noble gases (3He, 4He, Ne), from Polarstern expedition ANT XXIV/3 (2008) along the Prime Meridian, across the Weddell Sea and in the Drake Passage and from Polarstern expedition ANT XXV/4 (2009) in the Drake Passage. Furthermore we published three previously obtained CFC and noble gas data sets (ANT XXII/3, 2005; ANT XXIII/3, 2006; and ANT XXIII/7, 2006) on the PANGAEA data base to protect them and to make them available for the scientific community. Our works were assessing the following issues: • Water mass formation and its variability due to changing environmental conditions in the western Weddell Sea: We interpreted the observed freshening of shelf water in the western Weddell Sea as being caused by a combination of reduced salt input due to a southward sea ice retreat and higher precipitation. However, that freshening might also result from a delicate interplay between enhanced salt input due to sea ice formation in coastal areas formerly occupied by Larsen A and B ice shelves and increased basal melting at the remaining Larsen C Ice Shelf. Its impact on the deep and bottom water formation must be subject to further investigations and requires new observations close to the areas of deep and bottom water formation. • Interpretation of the iodine distributions in the Weddell Sea: Our CFC measurements helped to interpret the recently observed iodine distribution in the Weddell Sea and to identify the surface water to bottom water transformation as its source. • The leakage of Weddell Sea Deep Water (WSDW) into Drake Passage towards the west : We derived fractions of WSDW in the southern Drake Passage that reach values up to 80%, and a flow of 2.2 Sv WSDW into Drake Passage was estimated. • The transfer of water masses and their ventilation trough Drake Passage into the Atlantic Ocean: A maximum fraction of water with hydrothermal vent origin in the deep Pacific (South Pacific Deep Slope Water, SPDSW) of 60% was determined. That water mass was located off the South American continental slope, in contrast to previous observations. The more ragged structure of the 2006 section demonstrates a highly variable transport, possibly interacting with the mesoscale features of the ACC and the complex topography. Considerable differences in the water mass properties between two short-term occupations of the section were observed throughout the whole water column. We could only attributable part of the differences to frontal or eddy displacements. The other part results from the spatial heterogeneity of water properties upstream the section and topographic constraints. The Shackleton Fracture Zone constitutes a barrier, promoting an active recirculation region in the southern Drake Passage, and causing effective ventilation of the whole Circumpolar Deep Water density range. • Interpretation of the manganese distribution in the Drake Passage: Our 3He/4He measurements helped to interpret the observed manganese distribution in the Drake Passage and to identify water with contact to hydrothermal vents in the deep Pacific as its source. • The import of deep water from external sources into the Weddell Sea and the export of Weddell Sea Bottom Water across the Prime Meridian and the related content of anthropogenic carbon: A novel approach using CFC time series based Transit Time Distributions (TTDs) was used to determine the age spectra of two well ventilated deep boundary currents flowing across the Prime Meridian. Extending the common TTD approaches we accounted for a more realistic exchange between the well ventilated boundary currents and the ocean interior and a variable oceanic uptake of CFCs and anthropogenic carbon (Cant) at the sources of the deep boundary currents. We could determine realistic TTDs and values for Cant (11 μmol/kg flowing into the Weddell Sea and 12 μmol/kg leaving the Weddell Sea across the Prime Meridian). Accounting for a reduced oceanic uptake of Cant than of the CFCs due to their different atmospheric evolutions and ocean-atmosphere gradients, we determined even smaller values for Cant (5.3 μmol/kg into the Weddell Sea and 5.7 μmol/kg leaving the Weddell Sea). These values are on the lower side or even smaller then previous Cant estimates, using other methods, but agree well with the findings of more recent studies. • Exploiting a 35 year long time series of directly observed total carbon confirm the weak increase of Cant in the well ventilated deep and bottom water along the Prime Meridian. • Evidence of volcanic hydrothermal activity in the Bransfield Strait (Rodrigo et al., to be submitted): A unique and unexpected opportunity to analyse samples for noble gas and CFCs from one station at the Orca Seamount gave clear evidence that there exists volcanic hydrothermal activity at the Orca Sea Mount in the Bransfield Strait.
Publications
- Direct observation of increasing CO2 in the Weddell Gyre along the Prime Meridian during 1973-2008. Deep-Sea Research II, 58, 25-26
Heuven, S. v., M. Hoppema, O. Huhn, H. Slagter, H. d. Baar
(See online at https://doi.org/10.1016/j.dsr2.2011.08.007) - (2008). Evidence of deep and bottom water formation in the western Weddell Sea. Deep-Sea Research II, 55/8-9, pp. 1098-1116
Huhn, O., H. H. Hellmer, M. Rhein, W. Roether, C. Rodehacke, M. Schodlok, and M. Schröder
(See online at https://doi.org/10.1016/j.dsr2.2007.12.015) - 2011. Distribution of iodide and iodate in the Atlantic sector of the southern ocean during austral summer. Deep Sea Research Part II: Topical Studies in Oceanography
Bluhm, K, P. Croot, O. Huhn, G. Rohardt, K. Lochte
(See online at https://doi.org/10.1016/j.dsr2.2011.02.002) - 2011. On the freshening of the northwestern Weddell Sea continental shelf. Ocean Science, 7, 305-316
Hellmer, H. H., O. Huhn, D. Gomis, and R. Timmermann
(See online at https://doi.org/10.5194/os-7-305-2011) - 2011. Short-term variations of deep water masses in Drake Passage revealed by a multiparametric analysis of the ANT-XXIII/3 bottle data. Deep Sea Research Part II: Topical Studies in Oceanography
Sudre, J, V. Garçon, C. Provost, N. Sennechael, O. Huhn, M. Lacombe
(See online at https://doi.org/10.1016/j.dsr2.2011.01.005)
