Coccolithophore response to changes in surface water conditions south of Iceland (ODP Site 984) between 130 and 56 ka

Abstract

Quantitative coccolithophore analyses were performed using sediments from Ocean Drilling Program (ODP) Site 984, located south of Iceland close to the modern location of the Polar Front, to reconstruct climatically induced variations in surface water conditions from 130 to 56 ka, spanning the end of Marine Isotope Stage (MIS) 6 to the beginning of MIS 3. For this purpose, changes in the coccolith assemblages (e.g., abundances, accumulation rates, composition, diversity and preservation, as well as coccolith carbonate) and variations in sizes and morphotypes of Gephyrocapsa muellerae and Emiliania huxleyi were investigated.The coccolith assemblages were generally of low diversity and were mostly dominated by G. muellerae throughout most of the record. Two intervals of relatively warm surface water expansion (MIS 5e, 5a) were identified by increased coccolith numbers, coccolith accumulation rates (CAR), and highest coccolith carbonate content, and interpreted to be associated with the expansion of the relatively warm Irminger Current (IC). The low coccolith numbers and low CAR during the interval MIS 5d to 5b point to much lower sea-surface temperatures and the decreased influence of the warm IC. Furthermore, MIS 5c is notable due to the absence of E. huxleyi accompanied by lowest species diversity, which suggest either cooler or less productive surface water conditions during this time interval.Another intriguing result of this research is the observed significant increase in the mean length of the G. muellerae distal shield (> 15%) from the beginning of MIS 5e to MIS 4. Furthermore, E. huxleyi occurs almost exclusively as Type B/C throughout MIS 5 and is particularly characterized by a considerable proportion of strongly calcified types during MIS 5a. Emiliania huxleyi Type A is first determined in MIS 5a and became the dominant morphotype during MIS 4. In addition, E. huxleyi (> 4 μm) commonly occurred during MIS 5e and 5a, during intervals of warmer surface waters. It seems likely, that E. huxleyi as well as G. muellerae went through various adaptations during MIS 5, which may have led to a global step in the exchange in dominance between the two species at the end of MIS 5, recorded in most ocean basins.