Van Nieuwenhove, Nicolas (2008) Last Interglacial paleoceanography of the Nordic Seas based on dinoflagellate cyst assemblages. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 136 pp.

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Abstract

Understanding the dynamics of a warm climate is essential in order to assess possible scenarios for future climate evolution. The last interglacial, Marine Isotopic Stage (MIS) 5e, is generally believed to have been warmer than the Holocene, under a comparable orbital configuration, and is thus a good candidate to get more insight in those dynamics. Climate is steered by the rate of overturning of warm surface waters into cool deepwaters. One of these overturning cells is located in the Nordic seas, and the area is thus a key region in terms of climate regulation. Marine sediments from three locations in the Nordic seas have been studied for their dinoflagellate cyst (dinocyst) content in order to reconstruct sea surface conditions (temperature, salinity, sea ice) during MIS 5e. In combination with stable oxygen isotope, ice rafted detritus (IRD) and planktic foraminiferal assemblage data, the variations in the dinocyst assemblage composition reflect a stepwise transition from the final phase of deglaciation (Termination II) into typical interglacial conditions, and the subsequent cooling during glacial inception towards MIS 5d. The marked presence of the neritic, warm-temperate dinocyst Lingulodinium machaerophorum towards the end of Termination II tells us that quite particular water masses entered the southern Nordic seas during the latest deglacial phases of MIS 6. A shift towards the inflow of “more Atlantic” waters and a drastic decrease in both IRD input and stable oxygen isotope values mark the start of MIS 5e. The northward heat flow remained relatively weak during the first ~4-5 kyr of MIS 5e, and at no time an east to west sea surface temperature gradient, as pronounced as at present, appears to have prevailed during that period. Quantitative and qualitative analyses of the dinocyst data from the Vøring Plateau in the eastern Norwegian Sea and comparison with core-top and published data substantiate the existence of distinctly different hydrological surface conditions during MIS 5e with respect to the late Holocene. A higher number of co-dominant, subordinate species in the last interglacial samples suggests that there was a more pronounced seasonality of the surface water at this time. This is supported by the sizeable abundance of Bitectatodinium tepikiense, a species virtually absent from the area for most of the Holocene. The seasonality signal is confirmed by transfer function reconstructions, which additionally indicate a stronger stratification of the upper water column during MIS 5e. Low but persistent IRD input during the first ~4.5 kyr of MIS 5e at the Vøring Plateau suggests that prolonged deglacial meltwater input may have caused the stratification and hindered the northward protrusion of Atlantic water masses towards the Arctic. The abundance maximum of the warm-temperate dinocyst species S. mirabilis s.l. between ~117.5 and 116.5 ka reveals that most pronounced, fully marine interglacial conditions prevailed in the (north)eastern Nordic seas only late in the last interglacial. The timing of the optimum is consistent with planktic foraminiferal findings, and contrasts with the early Holocene climatic optimum. Stable oxygen isotope values from planktic foraminifera for the MIS 5e optimum are comparable with average Holocene values, but are generally ca. 0.3‰ higher than those of the earlier part of the last interglacial. These higher 18O values therefore do not reflect optimal interglacial conditions, but corroborate the existence of a differently structured sea surface, as suggested by the dinocyst data. Whereas the enhanced northward heat flow from the late MIS 5e optimum allowed interglacial conditions to finally develop in the southern Fram Strait, it also appears to have strengthened the southward flow of the East Greenland Current. As a result, cold waters and sea ice were advected towards the eastern Iceland Plateau area, and no late MIS 5e optimum could develop in the uppermost surface waters there. The work presented here also illustrates that specific ecological constraints can cause different fossil groups to express climate events in a differing way. This clearly stresses the usefulness of combining different proxies for detailed palaeoceanographic reconstructions. Furthermore, the major impact from the enhanced and prolonged influence of Saalian deglacial meltwater on the further development of the last interglacial climate implies that MIS 5e cannot simply be taken as an analogue for the Holocene solely based on insolation forcing.

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