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doi:10.2204/iodp.proc.320321.212.2013

Early and middle Eocene benthic foraminiferal assemblages

Because of the poor preservation of foraminifers in Hole U1331C and the fact that we found foraminifers in four samples only, we could not observe evidence of any foraminiferal response to the EECO. Carbonate-free agglutinated foraminifers such as Thalmannammina and Cyclammina occurred only in the lower Eocene, but they are not diagnostic of this climatic optimum (Kaminski and Gradstein, 2005). Rather, they indicate the presence of carbonate-corrosive deep water, as also suggested by the occurrence at other sites of Eocene radiolarian ooze and low-carbonate sediments (Shipboard Scientific Party, 2002). In the early Eocene, Hole U1331C was above the CCD (Pälike et al., 2009), but the common occurrence of the agglutinated taxa indicates that this site did not present a good habitat for calcifying foraminifers or these forms did live there but were not preserved, possibly due to circulation of hydrothermal fluids through the underlying basalts. The lowermost Eocene calcareous sediments directly above the basement basalt contain zeolitic clays formed by such hydrothermal circulation (Pälike et al., 2009). However, sediments further away from the basement basalt are less influenced by hydrothermal activity; thus, the calcareous assemblage could be preserved as seen in Sample 320-U1331C-17H-3, 78–80 cm, with N. truempyi, O. umbonatus, A. spissiformis, and A. quadrata. These species are also major constituents of assemblages occurring after the Paleocene/Eocene benthic extinction at Site 1220, near Hole U1331C (Nomura and Takata, 2005). However, the assemblage in Hole U1331C differs from that of Site 1220 in having lower species diversity. The most common species at Site 1220 include Bulimina bradburyi, Bulimina trihedra, Globocassidulina globosa, Pleurostomella paleocenica, Pullenia subcarinata, Quadrimorphina profunda, Tappanina selmensis, and small-sized Valvalabamina, but these are not as common in the assemblage in Hole U1331C.

Except for the low-diversity Samples 320-U1333A-16X-1, 25–27 cm, to 16X-5, 75–77 cm, foraminifers in both holes at Site U1333 show almost the same assemblage, characterized by common cosmopolitan species such as N. truempyi, C. grimsdalei, C. eocaenus, and O. umbonatus. Their distribution in core sections shows a prominent foraminiferal occurrence at ~200 revised meters composite depth (rmcd) (Fig. F2), which may indicate correlation to either the CAE 2 (47.9–46.9 Ma) or CAE 1 (45.9–44.2 Ma) event. However, the age of the highest foraminiferal samples and correlation to CAE 2 or CAE 1 is uncertain, with nannofossil datums indicating the age of ~200.0–200.4 rmcd at ~45–47 Ma (Westerhold et al., 2012), with both CAE 1 and CAE 2 occurring within this time interval. A distinct decrease in foraminiferal abundance between Sections 320-U1333A-16X-5 and 16X-6 (174.1–175.5 rmcd) occurring in Magnetochron C18r is correlated with CAE 3, which occurred also in Magnetochron C18r. CAE 3 is associated with shallowing of the CCD at the end of the event (Lyle et al., 2005).

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