Research paperLate early Oligocene deep-sea benthic foraminifera and their faunal response to paleoceanographic changes in the eastern Equatorial Pacific
Highlights
► Benthic foraminifera in the eastern Equatorial Pacific Ocean ► Documentation of abyssal benthic foraminifera during ~ 32 to ~ 28 Ma ► Characteristics of abundant common taxa of benthic foraminifera ► Influence of Southern Component Water at the Oi-2 event.
Introduction
In the Cenozoic, four faunal turnovers have been recognized in deep-sea foraminifera (e.g., Woodruff and Douglas, 1981, Miller et al., 1992, Thomas, 1992, Nomura, 1995, Hayward, 2001, Hayward, 2002, Thomas, 2007, Cramer et al., 2009): (1) an abrupt extinction of “Velasco-type” fauna in the Paleocene–Eocene transition, (2) a gradual turnover in the late Eocene to the early Oligocene, e.g., gradual decrease in relative abundance and subsequent extinction of Nuttallides truempyi, (3) a gradual turnover in the middle Miocene, e.g., decrease in relative abundance of several “cylindrical” taxa (e.g., stilostomellids and pleurostomellids) and increase in relative abundance of miliolids, and (4) a gradual turnover in the middle Pleistocene with extinction of several “cylindrical” taxa. Oligocene deep-sea foraminifera have been regarded as the “Transitional Fauna” between “Paleogene Fauna” and “Modern Fauna” (e.g., Miller et al., 1992, Thomas, 1992, Thomas, 2007). It is believed that the gradual faunal turnover occurred from the late Eocene to the early Oligocene with global cooling (Miller et al., 1992, Thomas, 1992, Thomas, 2007). Thomas and Gooday (1996) pointed out that the gradient in species richness of deep-sea benthic foraminifera between high-latitude and tropical sites increased after the Eocene–Oligocene transition. They also noted that species richness of benthic foraminifera decreased at the high-latitude sites after the Eocene–Oligocene transition, but such a significant decline was not observed in the tropical region. In general, deep-sea foraminifera are mainly affected by mode of food supply from the ocean surface and by deep water properties (e.g., Loubere, 1991, Schmiedl et al., 1997, Gooday, 2003, Jorissen et al., 2007). Thomas and Gooday (1996) suggested the enhanced seasonal food supply from the ocean surface as a plausible explanation for the gradient of deep-sea benthic foraminifera at both high latitudes and tropical regions, based on the occurrence of phytodetritus species. High primary production was also reported in tropical regions during the earliest Oligocene (Coxall and Wilson, 2011). Thus, changes in trophic conditions seem to give important insights into understanding the gradual faunal turnover around the Eocene–Oligocene transition (Thomas, 2007).
The early Oligocene is the period when the intensity of deep water formation changed in the Southern Ocean (Diester-Haass et al., 1996, Cramer et al., 2009, Katz et al., 2011). Nd isotope compositions diverged between Walvis Ridge and Maud Rise in the early Oligocene (Scher and Martin, 2004, Via and Thomas, 2006, Scher and Martin, 2008), which suggests that meridional overturning circulation changed in the South Atlantic–Southern Ocean, including the Northern Component Water (NCW) that originated from the North Atlantic. Katz et al. (2003) reported that some benthic foraminifera shifted their depth habitats in the Atlantic Ocean during ~ 32 to ~ 29 Ma. For example, Nuttallides umbonifer, one of the common species in the Southern Component Water (SCW), occurred at a shallower depth in the South Atlantic during this period. Thus, changes in deep water circulation appear to be an important oceanographic condition that induces an ecological response from the deep-sea benthic foraminifera in the early Oligocene.
Studies of Ocean Drilling Program (ODP) Leg 199 in the eastern Equatorial Pacific (EEP) documented the episodes of Oligocene Antarctic glaciation and their relationship to deep-sea paleoceanographic events (Lear et al., 2004, Coxall et al., 2005, Pälike et al., 2006, Coxall and Wilson, 2011), based on excellent age control using astronomical tuning. From faunal analyses at Sites 1218 and 1219 (ODP Leg 199), Takata et al. (2010) reported on the Oligocene abyssal benthic foraminiferal fauna in the EEP. The alternations of the two dominant faunas were associated with SCW possibly with carbonate undersaturation or less seasonal food input (Factor 1 assemblage; characteristic taxon: Nuttallides umbonifer) and various trophic conditions with some seasonal food input (Factor 2 assemblage; characteristic taxa: Oridorsalis umbonatus, Cibicidoides sp.A, Cibicidoides mundulus, Globocassidulina subglobosa and Gyroidinoides spp.). Factor 1 assemblage decreased at ~ 30 Ma, and then increased continuously after ~ 29 Ma. Thus, temporal changes in deep-sea benthic foraminiferal faunas appeared to occur in the Pacific and Atlantic Ocean simultaneously during the late early Oligocene, but knowledge about the benthic foraminifera in the Pacific Ocean, especially their depth habitat in the abyssal areas, is still limited. Takata et al. (2010) also suggested that the Factor 1 assemblage was common in the mid-Oligocene glacial events, based on the results of Site 1218. The two glacial events, Oi-2 and Oi-2*, occurred during ~ 32–28 Ma (Pekar et al., 2002, Wade and Pälike, 2004). The occurrences of benthic foraminifera around glacial events provide further critical clues to understanding the faunal response of benthic foraminifera in the late early Oligocene.
IODP Exp. 320 collected continuous Paleogene sediments successfully at five sites near ODP Leg 199 Site 1218 in the EEP (Pälike et al., 2010). At Site U1334 (Fig. 1), almost continuous Oligocene sediments were recovered, which were deposited at shallower paleo-water depths than Sites 1218 and 1219. By combining the new faunal data of Site U1334 with the previous results of Sites 1218 and 1219 (Takata et al., 2010), we can address more precisely the occurrences of benthic foraminifera in the EEP during the late early Oligocene with respect to a depth transect. We investigated early Oligocene abyssal benthic foraminifera in the EEP to describe firstly the depth habitat of benthic foraminifera in the ~ 32–28 Ma interval, and secondly to examine the relationship between benthic foraminifera and paleoceanographic events (e.g., changes in meridional overturning circulation and early Oligocene glacial events), based on faunal and sediment geochemical analyses of Site U1334 (and some samples at Site 1219) and the published results (Takata et al., 2010) of Sites 1218 and 1219 for the ~ 32–28 Ma interval.
Section snippets
Study area
The EEP is well known for its high primary productivity (Chavez and Barber, 1987), which is attributed to the upwelling of nutrient-rich subsurface water by the trade winds. Abundant biogenic carbonate scales and shells are produced by calcareous nannoplankton and planktonic foraminifera, respectively. The large export flux of biogenic carbonate to the seafloor leads to a deeper calcium carbonate compensation depth by about 1 km in the EEP than in any other Pacific region (Honjo et al., 1995,
General features
Benthic foraminifera were present in 33 analyzed samples and the preservation of both calcareous and agglutinated forms was good to moderate. BFAR decreased with increasing paleo-water depth at the three sites (Fig. 2) in the following order: Site U1334, Site 1218, and Site 1219. However, the BFAR at Site U1334 temporally decreased from 29.8 to 29.4 Ma to a similar level to that at Sites 1218 and 1219. Nuttallides umbonifer (Cushman), Oridorsalis umbonatus (Reuss), Gyroidinoides spp. and
Benthic foraminifera at Sites U1334, 1218 and 1219 during ~ 32–28 Ma
The preservation of calcareous foraminiferal tests is important for the faunal study of deep-sea benthic foraminifera, particularly in abyssal depths (Coxall and Wilson, 2011). The preservation of calcareous benthic foraminifera in this study was good to moderate in the interval from ~ 32 to 28 Ma, which is generally characterized by high carbonate content; Site U1334: mean 88.3% with a few < 75% samples; Site 1218: > 82% (Lyle et al., 2002, Wade and Pälike, 2004); Site 1219: > 75%, except for one
Conclusions
Our investigation of early Oligocene benthic foraminifera at Sites U1334 (IODP Exp. 320), 1218 and 1219 (ODP Leg 199) in the eastern Equatorial Pacific Ocean has led to the following conclusions:
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The relative abundance of Nuttallides umbonifer showed several depth changes between Sites 1218 and 1219 during 31.0 Ma and 29.9 Ma, although other major taxa did not show a marked change. It is suggested that the characteristics of the benthic foraminiferal fauna (e.g., Shannon–Wiener (H′)) in the EEP
Acknowledgements
We thank Dr. Thomas Westerhold (University of Bremen) for providing composite depth data at the studied sites and Dr. Heiko Pälike (Southampton University/University of Bremen) for providing age model and paleo-depth data. We appreciate Mr. Jong-Min Lee (Pusan National University) for the laboratory experiments of geochemical analyses of calcium carbonate and biogenic opal. Thanks are also due to Dr. Moriaki Yasuhara (University of Hong Kong) for constructive comments on the earlier manuscript.
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