Recent Tracer Measurements in the Antarctic Bottom Waters of the Australian-Antarctic Basin

During January to May 2018, the concentrations of the anthropogenic tracers, CFC-11, CFC-12, and SF_6, were measured during repeat occupations of the SR3 and S4P hydrographic sections from aboard the R. V. Investigator and R.V.I.B. Nathaniel B. Palmer, respectively. In addition to sampling the SR3 section along 140°E between Tasmania and the Antarctic continental shelf from aboard the R.V. Investigator, we also measured the tracers on short hydrographic sections across the continental slope at 132°E and 150°E, and reoccupied the S4I stations between these two sections. Previous studies of the bottom water tracer distributions within this basin have confirmed the inputs of Adélie Land Bottom Water, newly-formed on the Antarctic Shelf near the SR3 section, and Ross Sea Bottom Water flowing westward from its formation region. Since the last re-occupation of SR3 in 2008, the Mertz Glacial Tongue was calved. As the presence of this glacial tongue was an important factor in maintaining the polynyas on the shelf that are believed to be an important contributor to ALBW formation, the T-S properties and the formation rate of this water mass are likely to have changed. In addition, other studies have found that the RSBW freshened between the previous reoccupations of S4P (1992 and 2011). Both of these factors have resulted in an observed freshening of the bottom waters along the SR3 section. The measured changes in the CFC-11 and CFC-12 concentrations, along with the inclusion of SF₆ concentrations in this basin, provide information on the changes in the ventilation of the bottom waters of this basin. The measured tracer concentrations within newly-formed ALBW at 140°E and within the westward-flowing RSBW at 170°E can assist with estimating the time-scales involved.