The Earth's climate shifted rapidly from a greenhouse to an icehouse state during the Eocene–Oligocene transition (~34 Ma), marked by the onset of Antarctic glaciation and global cooling. From the Late Eocene to the Early Oligocene, the stepwise opening of Drake Passage and the Tasman Gateway established the circumpolar seaway and reorganized the global circulation. This consequently changed the inter-ocean exchange and regional upwelling, thereby altering marine nutrient distributions and biological productivity. Both changes in the circulation and productivity would influence the marine barium cycle.

Marine barite forms within the mesopelagic zone during organic matter remineralization and preferentially incorporates light Ba isotopes, its abundance and isotopic composition thus provide complementary constraints on export productivity and the evolution of the marine Ba cycle. Here we present the barite accumulation rate (BAR), barite (wt%) and barium isotope of barite from DSDP 277 and ODP 744 around the Tasmanian seaway, and IODP 1333 in the East Equatorial Pacific. By comparing records in the upwelling regions through the tectonic change period (40-26 Ma), we evaluate spatial variations in the marine barium cycle associated with productivity and ocean circulation change. Our multi-site Ba isotope record suggests that productivity is the main control on the millennial time scale, while changes in ocean mixing and upwelling intensity would affect its stability. This study helps improve our understanding of the links between tectonic gateway evolution, ocean circulation, and marine biogeochemical cycling.

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