Abstract:
We documented new data on the late early Oligocene benthic foraminifera and sediment geochemistry in the eastern Equatorial Pacific Ocean (EEP) from Integrated Ocean Drilling Program (Exp. 320) Site U1334 (paleo-water depth: approximately 3650 to approximately 3850 m) with some published data on those from Ocean Drilling Program (Leg 199) Sites 1218 ( approximately 3850 to approximately 4000 m) and 1219 ( approximately 4270 to approximately 4370 m) in order to understand the response of abyssal benthic foraminifera to paleoceanographic conditions during the interval approximately 32-28 Ma. The relative abundances of Nuttallides umbonifer were distinctly different between Sites 1218 and 1219 during 31.0 to 29.9 Ma, although other major taxa (e.g., Globocassidulina subglobosa, Cibicidoides spp. and Oridorsalis umbonatus) showed no marked changes. The characteristics (e.g., Shannon-Wiener (H')) of the benthic foraminiferal fauna are affected by the abundance of a few dominant species rather than by the change in the number of species in the EEP. Based on multi-dimensional scaling (MDS), two MDS axes were recognized: MDS axis 1 (characteristic taxa: N. umbonifer and Cibicidoides sp. A) and MDS axis 2 (characteristic taxa: G. subglobosa and Bolivina huneri). The tendency of both MDS axes (1 and 2) showed a distinct depth gradient among the three sites after 28.6 Ma, whereas the tendency was different from 31.0 to 28.6 Ma, especially at Site 1218. Increased abundance of N. umbonifer in the EEP after approximately 29 Ma was similar to that in the Atlantic Ocean after the expanded distribution of this species to shallower depths was finished. The faunal change at 28.6 Ma might be related to the change of deep water circulation, possibly with the influences of the Southern Component Water (SCW) and/or Northern Component Water in the South Atlantic Ocean. In contrast, the faunal change at 31.0 Ma might be related to the formation of the SCW and/or meridional overturning circulation in the Southern Ocean. In particular, MDS axis 1 exhibited the opposite tendency between Sites 1218 and 1219 around the Oi-2 event, whereas the depth gradient was clearly observed among the sites around the Oi-2* event. The opposite tendency of MDS axis 1 around the Oi-2 event might be explained by the varying influences of the SCW on different depths of the EEP, especially in terms of carbonate undersaturation. The influence of the SCW on benthic foraminifera in the EEP was different among the depths of the three sites in the Oi-2 event, however, the depth gradient of the SCW influence was gradually intensified in the Oi-2* event and the 28.6-28.0 Ma interval. Abstract Copyright (2012) Elsevier, B.V.