Rafted benthic microfossils as proxies of Neogene ocean current history in the Bass Strait seaway, south-eastern Australia

Warne, Mark1, McDonald, Abbey1

1School of Life and Environmental Sciences, Deakin University, Melbourne, Australia

Cenozoic marine strata along the southeast Australian coastline contain a record of Southern Ocean evolution over geological time.  A significant aspect of this oceanic evolution, is the development and interplay of surface currents such as the East Australian Current, Leeuwin Current and Antarctic Circumpolar Current. These, and other ocean currents, have substantially shaped marine biological diversity along the southern Australian continental margin, and have left distinctive stratigraphic markers within the marine sedimentary rock record.

Traditionally, proxy records of past surface ocean currents have been derived from fossils of marine microplankton preserved in deep ocean and continental shelf sediments.  However, in life, zooplankton often exhibit variation in depth distribution, which complicates relationships with surface ocean currents.  In contrast, and paradoxically, shallow marine benthic microfossils associated with floating macroalgae, such as epiphytal ostracods and foraminifera, can provide more direct evidence of changes in global patterns of surface ocean circulation, because dispersal only occurs at the sea surface. The dispersal mechanism for these benthic micro-organisms is via attachment to seaweed, ripped up from shallow marine environments by coastal storms, and sent drifting vast distances across oceans on surface currents, until colonization occurs in new, distant shallow marine realms.

Key Neogene rafting-related ostracod migration and extinction events apparent in carbonate and siliciclastic marine strata along the southern Victorian coastline, and which defined broad phases in the oceanographic history of Bass Strait, are as follows:

Strong warm plumes of East Australian Current waters entering eastern Bass Strait (around 16.4, 5.8 and 3.2 million years ago), as evidenced by fossil occurrences of warm water, western Pacific Neohornibrookella species.  Increases in the influence of East Australian Current waters in the Bass Strait region around 3 Ma, also likely facilitated the expansion of Pacific Ambostracon spp (pumila group species) into SE Australian coastal shallow and marginal marine realms.

Sporadic incursions of Antarctic Circumpolar Current waters entering western Bass Strait between approximately 9 and 5 million years ago, as evidenced by the influx of mid to high latitude Tasmanocypris (dartnalli group) species.

The inception of warm Leeuwin Current (aka Zeehan Current) waters entering western Bass Strait (4.4 million years ago), which created a confluence with East Australian Current waters.  This is evidenced by an east-west biogeographic differentiation of shallow marine ostracod faunas across this seaway. Notable is the widespread disappearance of Neohornibrookella and Tasmanocypris (dartnalli group) species from warm shallow marine ostracod faunas west of Cape Otway, during the early Pliocene.

Widespread extinction of warm SW Pacific derived marine ostracod taxa across the entirety of Bass Strait (e.g. Neohornibrookella species) due to the inception of the cold winter Bass Cascade current (1.8 to 2.2 million years ago), which led to winter water temperature minima too cold for these taxa.  The inception of the Bass Cascade was associated with an early Quaternary northward shift in the position of the southern hemisphere, mid latitude westerly wind belt.


Biography

Mark Warne’s main work is university teaching in the disciplines of geology, physical geography, environmental science and palaeobiology. He is currently the course director for the Bachelor of Science at Deakin University. He has research interests in the fields of micropalaeontology (principally fossil Ostracoda), marine stratigraphy, palaeoecology and palaeo-oceanography.

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