Size and Abundance of Late Pleistocene Reticulofenestrid Coccoliths from the Eastern Indian Ocean in Relation to Temperature and Aridity

Sammanfattning: Measurements on coccolith abundance and mass can be used as a signal of primary productivity and pelagic calcification in response to environmental change. The Leeuwin Current (LC) is known to transport warm and low-salinity waters from the Indo-Pacific Warm Pool (IPWP) southwards along the coast of West Australia. Along with the onset of continental aridity during late Neogene, increased strength of the LC may have played a role in reef expansion on the Northwest Shelf. In this study the morphological variation in size and mass of reticulofenestrid coccoliths was assessed in material from IODP Site U1461 in the eastern Indian Ocean spanning the past 500 ka. Both the absolute abundance of all reticulofenstrid coccoliths (Emiliania huxleyi, Reticulofenestra spp., Gephyrocapsa spp. and Pseudoemiliania spp.) was determined, as well as the relative abundance of large versus small coccoliths. Coccolith size and mass were measured quantitatively under circularly polarized light. The data was compared to variations in sea surface temperatures (SST) of the LC, and to continental aridity of Australia. SST fluctuations could influence coccolithophore productivity by affecting their metabolic rate, whereas continental aridity may influence the influx of terrestrial matter by wind. The investigated interval is dominated by small species of Gephyrocapsa. Peak values of absolute abundance and mass were observed during Marine Isotope Stage (MIS) 11, an interglacial period of extended warmth and humidity. These results coupled with high densities of aragonite needles in the same samples indicate the sediments were diluted by material overflowing from the adjacent shallow- water carbonate platform, analogous to the whiting events observed in the modern-day Bahamas. A decrease in abundance of Gephyrocapsa caribbeanica at 240 ka can be linked to the timing of their last common occurrence (LCO), within MIS 7. The subsequent shift to Gephyrocapsa oceanica as the dominant large species may indicate an ecological replacement of G. caribbeanica, or signify warm and low-salinity waters.