Faunal transition of benthic foraminifera at the middle Eocene carbonate accumulation events in the eastern equatorial Pacific Ocean (ODP Site 1218 and IODP Site U1333)

Abstract:

High-resolution studies of ODP Leg 199 and IODP Exp 320 reported several carbonate accumulation events (CAEs) during the middle to late Eocene in the eastern equatorial Pacific (EEP), suggesting hundred meter-scale fluctuations of the carbonate compensation depth (CCD) (Lyle et al., 2005; Pälike et al., 2010). Pälike et al. (2012) proposed that partitioning of labile and refractory organic matters may account for these frequent CCD fluctuations. Depth-transect approach on benthic foraminifera with trophic condition may be useful to test this hypothesis. Additionally, deep-sea benthic foraminifera experienced small faunal transitions in the late middle Eocene and the late Eocene (e.g., Nomura, 1995). This hypothesis may be also clue to understand biotic responses of deep-sea benthic foraminifera. We analyzed faunal composition of benthic foraminifera at ODP Site 1218 (3500–3090 m paleo-depth) during ~42–38 Ma and compared it with the results (Takata et al., 2013) of IODP Site U1333 (3720–3560 m paleo-depth) during CAE-3 and CAE-4.

After common occurrence of Quadrimorphina profunda (opportunist?) in the early part of CAE-4, Virgulinopsis navarroana, morphologically infauna, increased in the late part of CAE-4 with an increase of Benthic Foraminiferal Accumulation Rate (paleo-productivity) both at Sites 1218 and U1333. This species became common in the abyssal EEP from this timing to the latest Eocene (Takata, unpublished data). Such occurrence may reflect the increased export of labile organic matter to seafloor. In contrast, Globocassidulina globosa was more common during middle part of the CAE-3 at Site U1333 than at Site 1218, suggesting some seasonal food supply at the deeper depth. Such occurrence might be related to transient diatom abundance (Lyle et al., 2002), implying the presence of the depth gradient on the export of organic matter from the surface ocean. The hypothesis about partitioning of labile and refractory organic matters may have potential to interpret the faunal transitions of deep-sea benthic foraminifera in the middle to late Eocene.