The South Indian Ocean is a key junction linking the Atlantic, Southern, and Pacific oceans through the global overturning circulation while receiving diverse lithogenic inputs from surrounding continental margins. Disentangling the relative influence of interbasin water mass exchange and continental boundary processes on trace element and isotope (TEI) distributions therefore remains a major challenge. Radiogenic neodymium isotopes (εNd), a quasi-conservative tracer of water mass provenance that remains sensitive to lithogenic inputs, provide an ideal tool to distinguish these processes. Here we present full-depth εNd data from the GEOTRACES GI07 transect (Nov. to Dec. 2024) along ~23°S from South Africa to Western Australia. A pronounced, vertically coherent west-to-east εNd gradient throughout the water column highlights the strong contrast between African/Atlantic and Pacific influences.
Surface water εNd values range from highly unradiogenic compositions (−10.6 to −15.5) along the Mozambique shelf to ~−8 east of Madagascar and become progressively more radiogenic (−4 to −5) toward Western Australia, reflecting African continental inputs in the west and the Indonesian Throughflow in the east. In the thermocline, Subantarctic Mode Water (350–500 m) exhibits highly unradiogenic εNd values (−13.6 to −10.2), with the lowest signatures above the Madagascar Ridge and increasingly radiogenic values eastward. Antarctic Intermediate Water shoals eastward from ~1200 to ~700 m and exhibits εNd values between −8.7 and −7.2, whereas Indonesian Intermediate Water is characterized by more radiogenic signatures (−5 to −6). Below 2000 m, εNd values increase eastward from ~−11 to ~−7, reflecting decreasing North Atlantic Deep Water influence and increasing Pacific contributions to Indian Deep Water.
Authigenic and detrital εNd signatures in surface sediments are compared with overlying bottom waters to evaluate the reliability of sedimentary εNd as a paleoceanographic proxy. Across most of the transect, authigenic εNd closely matches bottom water compositions (within 1 epsilon unit). A notable exception occurs in the Mozambique Channel, where authigenic εNd shifts toward unradiogenic lithogenic values, indicating strong boundary exchange. In contrast, authigenic εNd along the Western Australian margin remains consistent with bottom water despite unradiogenic detrital signatures. Together, these results establish a basin-scale εNd framework for the South Indian Ocean and improve the interpretation of sedimentary authigenic εNd records for paleoceanographic reconstructions.