Research paperMiddle Eocene large coccolithaceans: Biostratigraphic implications and paleoclimatic clues
Introduction
The Coccolithaceae is an extant coccolithophore family with a long geological record that extends back to the earliest Paleogene. This family represents a significant component of Paleogene assemblages, both in terms of abundances and species richness, and several representatives of this family are used as biostratigraphic markers in calcareous nannofossil biozonations (Martini, 1971; Okada and Bukry, 1980; Agnini et al., 2014).
Although the Cenozoic Family Coccolithaceae is a well-established taxonomic group (Young and Bown, 1997), the genus and species-level taxonomy of different coccolithaceans have been a matter of discussion for many authors (e.g., Gartner, 1970; Perch-Nielsen, 1985; Varol, 1992). One of the main taxonomic controversies concerns the generic classification of middle-Eocene Coccolithus-like coccoliths that possess robust crosses in the central-area (Bown and Newsam, 2017). A clear understanding of the evolution of these forms and an improved clarity of their taxonomy is of particular interest because their evolution preceded the appearance of the biozonal marker species Coccolithus gigas. The taxonomic definition of this biostratigraphically-significant species has become less clear in recent years, as a plexus of morphologically-related middle Eocene coccolithaceans has come to light, including transitional forms. This has led to uncertainties in the identification of C. gigas and undermined reliability of the stratigraphic placement of its origination and extinction levels, which are both widely applied in standard nannofossil zonations (e.g., CP13b total range zone of Okada and Bukry, 1980 and zones CNE10 and 11 of Agnini et al., 2014).
The time interval during which these morphotypes evolved, the middle Eocene, is a relatively little known and poorly studied interval, that lies between the extreme greenhouse conditions that peaked during the Early Eocene Climatic Optimum (EECO; c. 52–50 Ma) and the onset of Antarctic glaciation in the earliest Oligocene (e.g., Zachos et al., 2001, Zachos et al., 2008). The global cooling trend that characterizes most of the middle Eocene, was accompanied by a remarkable decline in the species richness of calcareous nannofossils, suggesting a causal link between the climate deterioration and diversity loss (Aubry, 1992, Aubry, 1998; Bown et al., 2004; Bown, 2005a). Calcareous nannoplankton are particularly important when trying to understand marine ecosystem dynamics because this group was the main primary producer of the Paleogene lying at the base of the ocean food chain and plays a fundamental role in the organic (biomass) and inorganic (CaCO3 test) carbon cycle. Their abundance, wide biogeographic distribution and rapid evolutionary rates make them the perfect tool with which to investigate short-and long-term modifications of plankton communities and particularly the influence of climate-ocean system change through time.
The early to middle Eocene time interval is well represented at Integrated Ocean Drilling Program (IODP) Site U1410 (northwest Atlantic Ocean, Norris et al., 2014) and the section shows a major change in sedimentary regime related to the onset of drift sedimentation (Norris et al., 2014; Bornemann et al., 2016; Boyle et al., 2017; Coxall et al., 2018). At this site, the middle Eocene deposits occur as expanded packages of clay-rich sediments that contain nannofossils with an exquisite quality of preservation (Norris et al., 2014).
Here we document the evolution of the Coccolithus gigas group using the exceptionally well-preserved nannofossil assemblages from Site U1410. Our reassessment of these large middle Eocene coccolithaceans aims to provide a more rigorous and practical taxonomy for this group, and, in turn, to improve the use of C. gigas in the current biozonation schemes. In addition, we compare the paleontological record with paleoenvironmental conditions at the early-middle Eocene transition in the Northwest Atlantic, shedding new light on the abiotic background that might have favored the radiation of these distinctive forms.
Section snippets
Geological setting
IODP Site U1410 is located in the Northwest Atlantic on the Southeast Newfoundland Ridge (Latitude: 41°19.6987 N; Longitude: 49°10.1995 W) (Fig. 1). During the Eocene, sediments were deposited at an estimated paleo-water depth of 2950 m, well above the calcite compensation depth (CCD) for this time interval. The lower Eocene sediments are pinkish-white nannofossil chalk (Unit IV, Norris et al., 2014) overlain by middle Eocene greenish-gray nannofossil ooze with some bands (10–25 cm) of light
Taxonomic background
The Family Coccolithaceae is characterized by the possession of a Coccolithus-type placolith rim structure as described by Young (1992). These placoliths have V-units forming both the distal shield and proximal layer of the inner tube cycle, and R-units that form a bicyclic proximal shield and the distal layer of the inner tube (Young, 1992). Classification within the Coccolithaceae at the genus level is based on three morphologic features: i) the tube-cycle width, ii) the central-area width
Light microscope observations and morphometric results
Calcareous nannofossils are abundant throughout the study interval and their preservation ranges from good to excellent. The extraordinary quality of preservation observed in the middle Eocene study interval is explained by the clay-rich nature of the sediments, with the clay acting as a protection that prevents recrystallization and overgrowth (Bown and Newsam, 2017).
At Site U1410, Pletolithus mutatus (see taxonomic notes) is observed from Zone CNE8 to the top of the section and is rare but
Biometric parameters and the differentiation of Pletolithus gigas
The P. giganteus and P. gigas morphometric data indicates a continuous morphological gradient between the two taxa with forms showing progressive rotation of the central-area cross-bars from near-axial to near-diagonal prior to the establishment of the separate P. gigas species. This gradual transition makes differentiation of these taxa difficult, especially using subjective visual assessment. To minimize this subjectivity, Bown and Newsam (2017) proposed that the angle formed between the long
Conclusion
The sedimentary succession recovered at Site U1410 has allowed us to document the emergence of a new lineage of large coccolithaceans. We have documented a series of morphological changes that have enabled us to reconstruct the evolutionary history of the biostratigraphically important species Pletolithus gigas. Based on detailed observations in both LM and SEM we propose a new genus, (Pletolithus), a new species (Pletolithus giganteus) and four new combinations (Pletolithus opdykei,
Systematic paleontology
Order COCCOLITHALES Haeckel, 1894 emend. Young and Bown, 1997
Family COCCOLITHACEAE Poche, 1913 emend. Young and Bown, 1997
Genus Pletolithus gen. nov.
Type species. Coccolithus gigas Bramlette and Sullivan, 1961
Derivation of name. From latin pletum, meaning filled, and referring to the cross-bars that fill or near fill the central-area, and from greek lithos meaning rock.
Type locality. IODP Exp. 342 Site U1410, Southeast Newfoundland Ridge, northwest Atlantic Ocean.
Type level. Lutetian, Subzones
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by University of Padova SID project [prot. BIRD 161002]. The SEM work was supported by the Department of Earth Sciences, University College London. This research used samples and data provided by the International Ocean Discovery Program (IODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of the Joint Oceanographic Institution (JOI) Inc. We are thankful to one anonymous reviewer for the constructive
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