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Bruce Hayward, Geomarine Research (New Zealand)
Shungo Kawagata, Yokohama National University (Japan)
Hugh Grenfell, Geomarine Research (New Zealand)
Ashwaq Sabaa, Geomarine Research (New Zealand)
Tanya O'Neill, Massey University (New Zealand)
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Twenty percent (19 genera, 95 species) of cosmopolitan, deep-sea (500-4000 m), benthic foraminiferal species became extinct during the late Pliocene-Middle Pleistocene (3-0.12 Ma), with the peak of extinctions (76 species) occurring during the mid-Pleistocene Climate Transition (MPT, 1.2-0.55 Ma). One whole family (Stilostomellidae, 30 species) was wiped out and a second (Pleurostomellidae, 29 species) was decimated with just one species possibly surviving through to the present.
Our studies at 21 deep-sea core sites show pulsed declines in abundance and diversity of the extinction group species during more extreme glacials, with partial interglacial recoveries. These declines started in late Pliocene in southern-sourced deep water masses (AABW, CPDW) and extending into intermediate waters (AAIW, NADW, GNAIW) in the MPT, with the youngest declines in sites furthest downstream from high-latitude intermediate waters source areas. We infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source, and it was probably the demise of this microbial food that resulted in the foraminiferal extinctions. We hypothesise that it was increased cold and oxygenation of the southern-sourced deep water masses that impacted on this deep-water microbial food source during major late Pliocene and Early Pleistocene glacials when Antarctic ice was substantially expanded. In intermediate, water the food source was not impacted until major glacials in the MPT when there was significant expansion of polar sea ice in both hemispheres and major changes in global intermediate waters.
How were cosmopolitan deep-sea benthic foraminifera progressively wiped out over a long period, when it is known that they are capable of rapid bottom-current dispersal and repopulation of the sea-floor after a die-off? Full or partial interglacial recoveries of the abundance and diversity occurred when favourable conditions returned to these sites, presumably resulted from down-current dispersal from surviving populations. Where there were no survivors or refugia upstream, recolonisation did not occur or was extremely slow. This could explain the permanent loss of diversity beneath deep-water masses earlier than intermediate waters, and why species disappearing often differed between ocean basins.
Significant declines beneath intermediate and northern-sourced deep water did not occur until MPT glacials. As these waters circulated they gradually warmed and become less oxygenated and thus sites furthest downstream (e.g. Caribbean) were impacted last. Again, recolonisation during interglacials preferentially occurred in a downstream direction, contributing to the partial interglacial recoveries and the timing differences in the declines between different sites and basins.
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