| Summary: | The Southern Ocean hosts complex connections between ocean physics, chemistry, and biology. Changes in these connections are hypothesized to be responsible for significant alterations of ocean biogeochemistry and carbon storage both on glacialâ€interglacial timescales and in the future due to anthropogenic forcing. Isotopes of thorium (²³â°Th and ²³²Th) and protactinium (²³¹Pa) have been widely applied as tools to study paleoceanographic conditions in the Southern Ocean. However, understanding of the chemical behavior of these isotopes in the modern Southern Ocean has been limited by a paucity of highâ€resolution observations. In this study, we present measurements of dissolved ²³â°Th, ²³¹Pa, and ²³²Th on a meridional transect along 170°W from 67°S to 54°S in the Pacific sector of the Southern Ocean, with high vertical and meridional sampling resolution. We find Th/Pa fractionation factors below 1, highlighting the preferential removal of Pa relative to Th in a region with low lithogenic inputs where the particle flux is dominated by biogenic opal. We also find steep gradients in all three of these isotopes along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Our results suggest that ²³¹Pa and ²³â°Th in the Southern Ocean are highly sensitive tracers of physical transport that may find use in studies of Southern Ocean biogeochemicalâ€physical connections in the past, present, and future. © 2020 American Geophysical Union. Issue Online: 25 November 2020; Version of Record online: 25 November 2020; Accepted manuscript online: 12 November 2020; Manuscript accepted: 02 November 2020; Manuscript revised: 19 October 2020; Manuscript received: 21 July 2020. This work was supported by U.S. National Science Foundation grant OPPâ€1542962 to LDEO and an NSF Graduate Research Fellowship to F. J. P. (DGEâ€1644869). We thank the captain and crew of the R/V Nathaniel B. Palmer for their support in ...
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