Coccolithophore signatures across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans

Abstract. Accumulation rates of biogenic carbonates and opal, mainly produced by marine algae such as coccolithophores and diatoms, have been widely used to understand when and why the late Miocene to early Pliocene biogenic bloom occurred and how long it lasted. Information is still limited on the synchronicity of its termination, as well as the compositional signatures of the main carbonate producers across ocean basins, although recent studies have started to address this question. A recent study demonstrated that the biogenic bloom ended at ∼ 3.3 Ma in the mid-latitude South Atlantic (ODP Site 1264), whereas a compilation of multiple deep-sea records pointed to a synchronous termination of the biogenic bloom in low latitudes at 4.6–4.4 Ma. Notably, the synchronous decrease in low-latitude paleoproductivity was accompanied by significant restructuring of the calcareous nannoplankton assemblages, at least offshore NW Australia (IODP Site U1463). Since coccolithophores are the main drivers of carbonate export production during the Neogene, such compositional shifts have the potential to be used as a first-order, biostratigraphic tool to identify the end of the biogenic bloom across ocean basins. To test this hypothesis, we compared the calcareous nannofossil compositions at ODP Site 1264 to those observed at IODP Site U1463 across the early Pliocene (5–3 Ma). This comparison reveals that although the main stepwise decrease in carbonate fluxes at Site 1264 occurred between ∼ 4.2–4.0 Ma, and therefore closer to the proposed synchronous end of the biogenic bloom in low latitudes, it was not accompanied by a change in the dominant coccolithophores. The widespread Pliocene rise of small Gephyrocapsa, as observed in the Indian Ocean, never occurred in this section of the South Atlantic. Instead, small Reticulofenestra remained dominant until ∼ 3.5 Ma. This indicates that regional contrasts in the dominant bloom-forming coccolithophores, which underpin export production, may explain the observed differences in the timing of early Pliocene declines in carbonate accumulation.