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ABSTRACT

During the Last Global Extinction (LGE) c. 20% (30 genera, 105 species) of cosmopolitan, mainly deep-sea (600–4000 m), benthic foraminiferal species (excluding unilocular taxa), belonging to seven families, became extinct. During this late Pliocene–middle Pleistocene interval (3.6–0.13 Ma), five families (Chrysalogoniidae, Glandulonodosariidae, Stilostomellidae, Ellipsoidinidae, Pleurostomellidae) were wiped out and one more (Plectofrondiculariidae) was almost wiped out with just one species surviving to the present. Most (76 of 105 species) of these extinctions occurred during the mid-Pleistocene Climate Transition (MPT, 1.2–0.55 Ma) at an extinction rate of 25% myr-1 of the deep-sea benthic foraminifera, compared with a background rate through the Cenozoic of c. 2% myr-1.

Most species in the families Chrysalogoniidae, Stilostomellidae, Ellipsoidinidae and Pleurostomellidae had equal levels of abundance throughout their middle bathyal–middle abyssal depth ranges. The Glandulonodosariidae mostly lived at middle bathyal to uppermost abyssal depths and the Plectofrondiculariidae at bathyal to outer shelf depths. These Extinction Group (Ext. Gp) families comprised 30–70% of the deep-sea benthic foraminiferal fauna in the middle to late Eocene. Major declines in their relative abundance and species richness at abyssal depths began in the late Oligocene–Miocene in the Southern Ocean, in the late Miocene in the deep Indian Ocean, in the early Pliocene in the West Pacific, then globally in the late Pliocene at upper abyssal (2300–3000 m) depths and all depths in the Mediterranean Sea. At bathyal depths (900–2200 m) declines and extinctions were largely confined to the Pleistocene. These declines occurred in pulses mostly coinciding with glacial episodes of expansion of polar ice sheets, initially in Antarctica but during the MPT in the Arctic.

The LGE preferentially impacted species with specific morphologies (elongate, cylindrical, often uniserial tests) and apertural types (e.g., small rounded, dentate, cribrate, or lunate slit). The precise functions of these are not known but the apertural modifications could be related to having a specific food source whose pulsed decline in abundance in the plankton resulted in the LGE. Data on δ13C analyses suggest that Ext. Gp species lived infaunally. Strong positive correlation of Ext. Gp abundance in the Pliocene–MPT with foraminiferal proxies for sustained and pulsed organic carbon flux supports the hypothesis that the Ext. Gp favoured enhanced food supply with consequent lower oxygen concentrations.

Decreased bottom temperature, increased bottom water ventilation or carbonate corrosiveness, increased interspecific competition and predation, or increased or more wildly fluctuating food supply are all rejected as unlikely to be the causes of the LGE. We hypothesise that the cause may have been the progressive decline or demise of the specific phytoplankton source of the detritus that the Ext. Gp fed upon, during global cooling and later increasingly cold glacials of the MPT with lowered atmospheric CO2.

The LGE and regional highest occurrence levels of Ext. Gp species have considerable biostratigraphic value in providing rapid age assessments of Quaternary oceanic sediment where planktic foraminiferal age datums are rare.

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