High-resolution coccolith geochemistry through the late Pliocene Discoaster extinctions

Closely associated with the climate transition from the warmer Pliocene to the high-amplitude glacial-interglacial cyclicity of the Pliestocene, were a series of progressive species extinctions within the major Cenozoic coccolithophore clade of the Discoasteraceae. The "discoasters" form a major component of tropical and sub-tropical calcareous nannofossil assemblages from the Paleocene to the Pliocene, with their relatively heavily calcified morphologies often dominating coccolithophore carbonate production. Through the late Pliocene, however, they suffered a series of five species extinctions over a period of 1 Ma, culminating in the final extinction of the genus with the last recorded occurrence of Discoaster brouweri at 1.9 Ma. Although this series of events is clearly coupled to general trends in declining total coccoltihophore species diversity, declining coccolithophore size and the transition into a colder and more intensely glaciated climate state, there is still uncertainty as to the immediate proximal causes of extinction. To investigate these events, we have generated new coccolith-specific isotope records through this late Pliocene interval from IODP Site U1482 (Expedition 363), on the northwest Australian continental margin. Using a new size separation technique, we have been able to increase sample throughput to more rapidly generate geochemical records from large ( 7-10 µm) versus small (<4 µm) coccolith size-fractions. These show a consistent offset of 1 ‰ in carbon isotope space, but no systematic offset in oxygen isotopes. These results are consistent with previous low resolution but long-term size-fraction isotopic records, and demonstrate that these carbon isotopic offsets are retained through orbital scale variations in the climate system.