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During the mid-Pleistocene Transition (MPT), 2 families, 14 genera and 41 species of benthic foraminifera (Extinction Group) declined in abundance (8-12 % of early Pleistocene benthic foraminiferal fauna) and finally disappeared at mid-bathyal depths in the Caribbean Sea (ODP Site 1000A, 916m depth), as part of the global extinction of at least 73 deep-water species at this time. At this site, the final phase of pulsed glacial decline, partial interglacial recoveries (0.8-0.67 Ma), and final extinction (0.58 Ma) was essentially the same as the youngest level so far documented elsewhere (0.57 Ma). Extinction Group specimens had smaller average sizes during periods of decline than during favorable periods with higher abundances. Census counts on different size fractions indicate that this extinction event is best recorded by studies of shells in the 150-300µm size range. Pteropod dissolution proxies indicate that intermediate waters in the Caribbean became less corrosive around the onset of the MPT (after c. 1.2 Ma). This is interpreted to be a result of increased input of northern hemisphere-sourced intermediate water at the expense of southern-sourced Antarctic Intermediate Water. Intervals of enhanced Extinction Group decline in intermediate waters in the Caribbean occurred during glacials around the start (1.15-1.05 Ma) and end (0.83-0.65 Ma) of the MPT. During these glacial periods preservation of carbonate was optimal and ä 13 C values high (in source waters at mid-depths in the North Atlantic), suggesting a causal link with enhanced inflow of a less-corrosive, colder, nutrient-depleted, well-ventilated water mass, such as Glacial North Atlantic Intermediate Water.

[1] 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 widespread pulsed declines in abundance and diversity of the extinction group species during more extreme glacials, with partial interglacial recoveries. These declines started in the late Pliocene in southern sourced deep water masses (Antarctic Bottom Water, Circumpolar Deep Water) and extending into intermediate waters (Antarctic Intermediate Water, North Atlantic Deep Water) in the MPT, with the youngest declines in sites farthest downstream from high-latitude source areas for intermediate waters. We infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source and that it was probably the demise of this microbial food that resulted in the foraminiferal extinctions. We hypothesize that it may have been 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. The food source in intermediate water was not impacted until major glacials in the MPT when there were significant expansion of polar sea ice in both hemispheres and major changes in the source areas, temperature, and oxygenation of global intermediate waters.

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