Global shifts in Noelaerhabdaceae assemblages during the late Oligocene–early Miocene

Highlights

• We quantified Noelaerhabdaceae assemblages during the late Oligocene–early Miocene.

• A successive dominance of Cyclicargolithus and reticulofenestrids is highlighted.

• Similar succession is observed in three sites from the Atlantic and Pacific oceans.

• This change is linked to global paleoceanographic changes or evolutionary process.

Abstract

This study investigates abundance variations in Noelaerhabdaceae assemblages during the late Oligocene–early Miocene at three subtropical sites in the Atlantic and Pacific oceans (DSDP Sites 516, 608 and 588). At these three sites, nannofossil assemblages were characterized by the successive high proportion of Cyclicargolithus, Dictyococcites and Reticulofenestra. Local paleoceanographic changes, such as the input of nutrient-poor water masses, might explain shifts in ecological prominence within the Noelaerhabdaceae at DSDP Site 516 (South Atlantic). But the similar timing of a decline in Cyclicargolithus at the three studied sites more likely corresponds to a global process. Here, we explore possible causes for this long-term taxonomic turnover. A global change in climate, associated with early Miocene glaciations, could have triggered a decline in fitness of the taxon Cyclicargolithus. The ecological niche made vacant because of the decrease in Cyclicargolithus could then have been exploited by Dictyococcites and Reticulofenestra that became prominent in the assemblages after 20.5 Ma. Alternatively, this global turnover might reflect a gradual evolutionary succession and be the result of other selection pressures, such as increased competition between Cyclicargolithus and Dictyococcites/Reticulofenestra. A diversification within Dictyococcites/Reticulofenestra, indicated by an expansion in the size variation within this group since ~ 20.5 Ma, may have contributed to the decreased fitness of Cyclicargolithus.

Introduction

The coccolithophore family Noelaerhabdaceae is dominant in most Neogene calcareous nannofossil assemblages and include the two most prominent modern coccolithophores and alkenone producers, namely Emiliania huxleyi and Gephyrocapsa oceanica, known to produce extensive blooms in today's oceans. Those large scale blooms have important implications for the global carbon cycle through processes of photosynthesis, calcification and respiration (Rost and Riebesell, 2004). During the Pleistocene, distinct intervals of dominance are recorded within the gephyrocapsids (e.g., Matsuoka and Okada, 1990, Bollmann et al., 1998, Flores and Marino, 2002, Flores et al., 2003, Baumann and Freitag, 2004, Barker et al., 2006) while E. huxleyi rose to global dominance ~ 270 kyr ago (Thierstein et al., 1977). The cosmopolitan distribution of the Noelaerhabdaceae family, which is believed to represent the main lineage of ancient alkenone producers (Marlowe et al., 1990), was sustained throughout most of the Cenozoic. However, our knowledge about the ecological preference or assemblage dynamics of the Oligocene and Miocene representatives of the Noelaerhabdaceae is relatively poor (Haq, 1980; Rio et al., 1990, Young, 1990, Kameo and Sato, 2000).

Several studies have shown that the late Oligocene–early Miocene period is marked by an important turnover of calcareous nannofossils, with first and last occurrences that are of biostratigraphic interest (Pujos, 1985, Olafsson, 1989, Young, 1998). For example, the early definition of the Oligocene–Miocene boundary relied on the last occurrence of Dictyococcites bisectus (Reticulofenestra bisecta of some authors; Berggren et al., 1985). However, only a few studies have estimated absolute abundances of the different genera of Noelaerhabdaceae during this period (e.g., Olafsson, 1989, Henderiks and Pagani, 2007, Plancq et al., 2012) and the paleoecological affinities of most species are still poorly known. The late Oligocene to early Miocene was a period of relative global warmth interrupted by large, transient Antarctic glaciations (named “Oi-2” and “Mi-1” events) as attested by significant oxygen isotopic (δ¹⁸O) increases (0.50 to > 1.0‰) in benthic foraminiferal records (e.g., Miller et al., 1991, Miller et al., 1996, Zachos et al., 2001a, Zachos et al., 2001b, Billups et al., 2002). The deepening of the Drake Passage and the subsequent intensification of the Antarctic Circumpolar Current (ACC) may have been coincident with the Oligocene–Miocene boundary and are believed to constitute important events in the Antarctic ice sheet expansion (e.g., Barker and Thomas, 2004, von der Heydt and Dijkstra, 2006), although the causes of the inception of the glacial history of Antarctica are still being debated (Scher and Martin, 2006, Livermore et al., 2007, Cramer et al., 2009). The paleoceanographic changes associated with these cooling events could have influenced the composition of the nannofossil assemblages, which are largely controlled by variations in the temperature and nutrient characteristics of the upper water column (e.g., McIntyre and Bé, 1967, Okada and Honjo, 1973, Winter and Siesser, 1994).

In the present study, we investigate the relative and absolute abundances of three Noelaerhabdaceae genera – Cyclicargolithus, Reticulofenestra and Dictyococcites – in sediments from three Deep Sea Drilling Project (DSDP) sites covering the late Oligocene–early Miocene. These three sites are located in the North and South Atlantic and South-West Pacific, in areas corresponding to subtropical gyres. Results highlight similar variations in Noelaerhabdaceae assemblages at all studied sites, with an apparent global decline in Cyclicargolithus, a dominant species in Oligocene sediments, at ~ 20.5 million years ago (Ma). This overall turnover cannot be explained by local paleoceanographic changes alone, and suggests the influence of global abiotic and/or biotic selection pressures.