Meridional circulation across the Antarctic Circumpolar Current serves as a double 231Pa and 230Th trap

Upwelling of Circumpolar Deep Water in the Weddell Gyre and low scavenging rates south of the Antarctic Circumpolar Current (ACC) cause an accumulation of particle reactive nuclides in the Weddell Gyre. A ventilation/reversible scavenging model that successfully described the accumulation of Th-230 in this area was tested with other particle reactive nuclides and failed to adequately describe the depth-distributions of Pa-231 and Pb-210. We present here a modified model that includes a nutrient like accumulation south of the Antarctic Polar Front in an upper meridional circulation cell, as well as transport to a deep circulation cell in the Weddell Gyre by scavenging and subsequent release at depth. The model also explains depletion of 231Pa and 230Th in Weddell Sea Bottom Water (WSBW) by ventilation of newly formed deep water on a timescale of 10 years, but this water mass is too dense to leave the Weddell Gyre. In order to quantify the processes responsible for the Pa-231- and Th-230-composition of newly formed Antarctic Bottom Water (AABW) we present a mass balance of Pa-231 and Th-230 in the Atlantic sector of the Southern Ocean based on new data from the GEOTRACES program. The ACC receives 6.0 +/- 1.5 x 10(6) dpms(-1) of Th-230 from the Weddell Sea, similar in magnitude to the net input of 4.2 +/- 3.0 x 10(6) dpm s(-1) from the north. For Pa-231, the relative contribution from the Weddell Sea is much smaller, only 0.3 +/- 0.1 x 10(6), compared to 2.7 +/- 1.4 x 10(6) dpm s(-1) from the north. Weddell Sea Deep Water (WSDW) leaving the Weddell Gyre northward to form AABW is exposed in the ACC to resuspended opal-rich sediments that act as efficient scavengers with a Th/Pa fractionation factor F <= 1. Hydrothermal inputs may provide additional removal with low F. Scavenging in the full meridional circulation across the opal-rich ACC thus acts as a double Pa-231 and Th-230 trap that preconditions newly formed AABW. (C) 2016 Published by Elsevier B.V.