Tasman Leakage: a different kind of ocean gateway

Abstract:

Recent compilations of stable isotope records in the southern Indian Ocean region (Tasman Sea, and on Broken Ridge (BR), 90E Ridge and Kerguelen Plateau) indicates the onset of Tasman Leakage (TL) at 7 Ma. TL is an intermediate water mass that moves dominantly EAC waters out of the Tasman Sea and ultimately towards the Mascarene Plateau region at intermediate depths (van Sebille et al., 2014). It is part of the Southern Hemisphere Supergyre (Speich et al., 2007; Ridgway and Dunn 2007), or linkage of South Atlantic, South Pacific and Indian Oceans at intermediate depths. The onset of TL is coincident with global late Miocene cooling and OMZ expansion in the Indian Ocean, and results from the adequate space for current to flow north of the ACC. Expansion to the BR suggests either 1) Indian Intermediate Water advected low oxygen waters to the BR at the expense of TL; 2) productivity over the BR was elevated; or 3) some combination of surface and water column changes.

Benthic foraminiferal faunal analyses at Site 752 (Singh et al., 2012; Ridha et al., 2019) and Site 1141 (this study) suggest OMZ waters at the BR from middle Miocene to the early Pliocene supported by high productivity indicators. This supports hypothesis #2 and is in agreement with reconstructions of a northward migration of the Subtropical Front (Nelson and Cooke, 2010). However, these OMZ indicators are reduced at ~ 7Ma in association with greater species diversity and a pulse of abundance of the epifaunal species Gavelinopsis lobatulus. This species is associated with oligotrophic conditions and higher oxygen (Singh et al., 2012). It is in accord with the onset of TL, and ushers in a period of reduced species diversity through the early Pliocene. This interval of reduced diversity is dominated by a different assemblage of OMZ and coincides with the Indo- Pacific Biogenic Bloom (Dickens and Owen, 1994). One explanation for the assemblage change at 7 Ma as well as the Biogenic Bloom is the upwelling of nutrient- rich Indian Deep Water in the northerly part of the ACC. This water mass is increasingly recognized as a dominant source of upper ocean waters of the Pacific and Indian Oceans (Talley 2013).