Late Pliocene and Early Pleistocene dinoflagellate cysts and sea surface temperture reconstruction for several IODP and DSDP sites in the North Atlantic

In an attempt to document the palaeoecological affinities of individual extant and extinct dinoflagellate cysts, Late Pliocene and Early Pleistocene dinoflagellate cyst assemblages have been compared with geochemical data from the same samples. Mg/Ca ratios of Globigerina bulloides were measured to estimate the spring-summer sea-surface temperatures from four North Atlantic IODP/DSDP sites. Currently, our Pliocene-Pleistocene database contains 204 dinoflagellate cyst samples calibrated to geochemical data. This palaeo-database is compared with modern North Atlantic and global datasets. The focus lies in the quantitative relationship between Mg/Ca-based (i.e. spring-summer) sea-surface temperature (SSTMg/Ca) and dinoflagellate cyst distributions. In general, extant species are shown to have comparable spring-summer SST ranges in the past and today, demonstrating that our new approach is valid for inferring spring-summer SST ranges for extinct species. For example, Habibacysta tectata represents SSTMg/Ca values between 10° and 15°C when it exceeds 30% of the assemblage, and Invertocysta lacrymosa exceeds 15% when SSTMg/Ca values are between 18.6° and 23.5°C. However, comparing Pliocene and Pleistocene SSTMg/Ca values with present day summer values for the extant Impagidinium pallidum suggests a greater tolerance of higher temperatures in the past. This species occupies more than 5% of the assemblage at SSTMg/Ca values of 11.6-17.9°C in the Pliocene and Pleistocene, whereas present day summer SSTs are around -1.7 to 6.9°C. This observation questions the value of Impagidinium pallidum as reliable indicator of cold waters in older deposits, and may explain its bipolar distribution.

Planktonic foraminifera of IODP Site 306-U1314

Surface water conditions at the Integrated Ocean Drilling Program (IODP) Site U1314 (Southern Gardar Drift, 56° 21.8' N, 27° 53.3' W, 2820 m depth) were inferred using planktic foraminifer assemblages between Marine Isotope Stage (MIS) 19 and 11 (ca. 800-400 ka). Factor analysis of the planktic foraminifer assemblages suggests that the assemblage was controlled by three factors. The first factor (which explained 49% of the variance) is dominated by transitional and subpolar species and points to warm and salty surface water conditions (Atlantic water). The second factor (37%) is dominated by Neogloboquadrina pachyderma sin and has been associated with the presence of cold and low saline surface waters (Arctic water). Finally, the third factor (9%), linked to a significant presence of Turborotalita quinqueloba, reflects the closeness of the Arctic front (the boundary between Atlantic and Arctic water). The position of the Arctic and Polar fronts has been estimated across the glacial-interglacial cycles studied according to planktic foraminifer abundances from Site U1314 (and their factor analysis) combined with a synthesis of planktic foraminifer and diatom data from other North Atlantic sites. Regarding at the migrations of the Arctic front and the surface water masses distribution across each climatic cycle we determined five phases of development. Furthermore, deep ocean circulation changes observed in glacial-interglacial cycles have been associated with each phase. The high abundance of transitional-subpolar foraminifers (above 65% at Site U1314) during the early interglacial phase indicated that the Arctic front position and surface water masses distribution were similar to present conditions. During the late interglacial phase, N. pachyderma sin and T. quinqueloba slightly increased indicating that winter sea ice slightly expanded southwestwards whereas the ice volume remained stable or was still decreasing. N. pachyderma sin increased rapidly (above 65% at Site U1314) at the first phase of glacial periods indicating the expansion of the Arctic waters in the western subpolar North Atlantic. During the second phase of glacial periods the transitional-subpolar assemblage throve again in the central subpolar North Atlantic associated with strong warming events that followed ice-rafting events. The third phase of glacial periods corresponds to full glacial conditions in which N. pachyderma sin dominated the assemblage for the whole subpolar North Atlantic. This division in phases may be applied to the last four climatic cycles.