Palaeogeography, Palaeoclimatology, Palaeoecology
Response of nannoplankton to early Eocene ocean destratification
Highlights
▶ We use calcareous nannofossil counts to track assemblage change during the Eocene. ▶ The early Eocene was marked by a large shift in the nannoplankton community. ▶ The genus Reticulofenestra rapidly expands during the early Eocene. ▶ Isotopes show a parallel decline in biological pumping efficiency and stratification.
Introduction
Calcareous nannoplankton (including coccolithophores) are among the most prolific phytoplankton groups in the world ocean (Westbroek et al., 1993). These organisms have played a vital role in the global carbon cycle since the Mesozoic Era by supplying organic carbon and calcium carbonate to the deep ocean (Hay, 2004). The distribution of nannoplankton in the ocean is regulated by properties such as temperature, nutrients, and water stratification. In the modern ocean, nannoplankton are primarily found in higher abundances in low nutrient, warm water settings, while diatoms, their siliceous counterparts, have a peak abundance in cool water, high nutrient zones (e.g. Okada and Honjo, 1973). Predictions for modern global warming include a reduction in diatom productivity and wider ranges of nannoplankton species with increasingly warm, oligotrophic conditions (Cermeño et al., 2008), making studies of nannoplankton in ancient greenhouse intervals highly relevant for the future.
The warmest temperatures of the last 90 m.y. occurred during the early Eocene Climatic Optimum (EECO; 53–51 Ma) (e.g., Zachos et al., 2001). Sea surface temperatures (SSTs), calculated from planktonic foraminifera δ18O values, changed little from the Paleocene–Eocene boundary through the EECO (Barrera and Huber, 1991, Dutton et al., 2005). SST proxies, such as TEX86 and Uk′37, which are based upon glycerol dialkyl glycerol tetraether (GDGT) lipids and alkenones respectively, demonstrated greatly reduced latitudinal thermal gradients (Bijl et al., 2009). The termination of the EECO initiated a long benthic and surface cooling trend into the early Oligocene “Oi-1” glaciations as indicated by oxygen isotopes (e.g. Zachos et al., 2001, Dutton et al., 2005). In contrast to these trends, TEX86 and δ18O extracted from “glassy” foraminifera indicate a warm, stable environment in an outer shelf region for the entire epoch (Pearson et al., 2007).
The conditions during the early Eocene greenhouse supported a widespread and diverse nannoplankton population (Bown et al., 2004). Changes in ocean circulation, water column structure, or climate during this time would affect nannoplankton populations and, therefore, primary productivity and the carbon cycle. To this end, understanding the physical mechanisms and responses of nannofossil assemblages to changes in oceanography are of particular interest.
Calcareous nannofossils are useful paleoenvironmental indicators primarily based upon the ecologic associations of individual taxa. Haq and Lohmann, 1976, Haq et al., 1977 first mapped out Paleogene nannofossil distributions and used them to propose species groups as ecological indicators. Since that time, several studies have expanded on this work to determine the ecological significance of nannofossil taxa. However, the majority of these studies are qualitative and based upon inferences of environment, for instance temperature and nutrients are often assumed to co-vary. In Hallock's (1987) trophic resource continuum (TRC) concept, cool temperatures and greater nutrient availability coincide, whereas warm temperatures and limited nutrients correspond. Because nannofossil paleoecology has the potential to help shed light on paleoceanographic events, it would be beneficial to have a more precise grasp on the paleoecology of individual taxa in terms of their temperature and nutrient tolerances, as well as their correlation with chemical environmental proxies, such as stable isotopes.
In this investigation we explore how global nannofossil assemblages change over the Eocene during a time of minimal or low extinction and origination rates (Bown et al., 2004). Our results indicate a dramatic nannofossil assemblage turnover event at the height of the EECO that was first noted by Haq et al. (1977), involving the expansion of the genus Reticulofenestra, the ancestor of the modern genera Geophyrocapsa and Emiliania. This event is correlated to the significant changes in the stable isotope values that indicate a decrease in water column stratification and in the efficiency of the oceanic biological pump. These changes could ultimately release CO2 into the atmosphere, elevating greenhouse conditions. The global shifts in nannofossil assemblages allow us to make new inferences on Eocene nannofossil paleoecology.
Section snippets
Methods
The 192 samples for nannofossil study (Table S1), were collected from cores at eight DSDP and ODP sites from a range of paleolatitudes (Fig. 1a) through the Eocene (55–33.9 Ma). The average sampling resolution is lower than many micropaleontological studies (660 kyr), but because we are looking at the long-term assemblage evolution on a global scale, it is not feasible to have a high sampling resolution. However, to test whether a higher resolution sample set would alter the trends observed, we
Results
Nannofossil abundances show a very similar assemblage structure among sites located in a similar latitudinal zone (Fig. 2). For each latitudinal division, the abundances of each genera are remarkably similar with a few exceptions in the late Eocene: a high Zygrhablithus population at Walvis Ridge, not found at Shatsky Rise; an increase in Cyclicargolithus at Exmouth Plateau, not found at Goban Spur; and a dramatic increase in Coccolithus at Maud Rise, not found at Kerguelen Plateau. Moreover,
Discussion
The expansion of Reticulofenestra in the Eocene is one of the most significant nannofossil assemblage changes of the entire Cenozoic, with this genus rapidly assuming a large proportion of the global nannoplankton population. Paleoceanographic and climatic events often lead to the initiation or demise of a population by changing hydrography and other physical or biological variables (Norris, 2000). For example, during Oceanic Anoxic Event (OAE) 1a (Erba, 1994, Premoli-Silva et al., 1999), OAE 2
Conclusions
Nannofossil counts show that the expansion of Reticulofenestra is one of the largest assemblage changes of the Cenozoic, with this genus comprising approximately 50% of the global nannoplankton population by the late Eocene. The initial rise of Reticulofenestra in the Southern Ocean corresponds to a major decline in the abundance of Toweius, which dwindles to extinction in the middle Eocene. The oxygen and carbon isotope records for the Southern Ocean and tropical Pacific indicate progressive
Acknowledgments
We thank Anna Hilting for sharing her oxygen and carbon isotope dataset. We also thank Giuliana Villa and two anonymous reviewers for detailed and constructive comments that improved the quality of this manuscript. This research used samples provided by the Ocean Drilling Program (ODP) and the Deep Sea Drilling Program (DSDP). Research was supported by the Schlanger Ocean Drilling Fellowship from the Consortium for Ocean Leadership awarded to Schneider and by NSF Grant EAR06-28394 to Bralower
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