Pacific-Indian Ocean Intermediate Water Exchange Since the Middle Miocene: From Indonesian Throughflow to Tasman Leakage

Surface and intermediate waters flow from the Pacific to the Indian Ocean as part of the global thermohaline circulation, connected through the Indonesian Throughflow (ITF) and Tasman Leakage (TL). Modern TL likely began in the Late Miocene, but its dynamics in response to astronomical climate forcing remain unclear. To reconstruct TL history, we present new fish-teeth neodymium (Nd) and benthic-foraminifera δ¹³C and δ¹⁸O data from Ocean Drilling Program Site 752 (Broken Ridge, Indian Ocean). During the Middle Miocene, Nd-isotopes indicate a Pacific influence on intermediate waters at Broken Ridge, likely via Indonesian Intermediate Waters from the ITF. In the Late Miocene, Antarctic Intermediate Waters (AAIW) invigorated, shifting Nd-isotopes toward less radiogenic values. At 5.5 Ma, TL became established, restoring Pacific control over Broken Ridge waters, now sourced south of Australia. Consequently, Site 752 δ¹³C diverges from AAIW-influenced sites at this time. An East-West δ¹³C gradient between Sites 752 and U1506 (Lord Howe Rise) reveals ephemeral proto-TL before 5.5 Ma, solely active during eccentricity maxima: Warmer climates and a southward-shifted Subtropical Front (STF) allowed proto-TL to flow westward through an open-bottleneck configuration, with Australia in the north and the STF in the south. During eccentricity minima, the bottleneck closed, leaving Broken Ridge influenced by AAIW. After 5.5 Ma, the δ¹³C gradient loses its eccentricity modulation as the bottleneck remains constantly open, as Australia moved tectonically further north. Our findings highlight the role of orbital eccentricity in shaping Indian Ocean water-mass exchange and reveal the dynamics of TL over astronomical timescales.