Last Interglacial Ocean of the Nordic Seas
Zusammenfassung der Projektergebnisse
Four sediment cores from areas in the eastern and northern Nordic seas were studied on their foraminiferal and dinoflagellate cyst (dinocyst) content. Supported by records of carbon and oxygen stable isotope and ice-rafted debris (IRD), the microfossil data was used to reconstruct the last interglacial (Marine Isotope Stage or MIS 5) climate evolution in the Nordic seas, with main emphasis on its warmest phase, MIS 5e. The importance of the Nordic seas in terms of climate evolution lies in the fact that the stability of the Meridional Overturning Circulation (MOC) in the northern Atlantic, one of the main drivers of climate (change), is largely dependent on sea surface properties in the subarctic region. One of the first points on which was focused, was Termination II, i.e. the deglacial transition from the Saalian glacial (MIS 6) towards the MIS 5e interglacial. The deglacial phase shows the typical two-step decrease of the δ18O values known from other areas in the North Atlantic. However, this general trend is interrupted by distinct reversals in planktic δ18O values, often associated with marked changes in the benthic δ18O and δ13C records, as well. The occurrences of the subtropical planktic foraminifer Beella megastoma and dinocyst Lingulodinium machaerophorum around these events seem to suggest that this isotopic behaviour was steered by (first) pulses of warm water entering the Nordic seas from the south. The abundance of B. megastoma is higher in the central Nordic seas than in the east, suggesting that a large body of meltwater in the eastern Nordic seas caused the northward moving warm water to be deflected towards the west. Foraminiferal analysis of the main interglacial interval, MIS 5e, revealed that favourable climate conditions were interrupted by one reversal back to near-glacial conditions in the southern Nordic seas. A strong zonal, east-west sea surface temperature (SST) gradient apparently existed during the first warm interval of MIS 5e. This time interval also appears to have been characterised by pronounced stratification – due to meltwater input – with large seasonal differences in temperature and salinity, as suggested by the dinocyst assemblages. The intra-interglacial cooling mentioned above was more pronounced in the east, where the influence of warm water advection is strongest, than in the west. The planktic δ18O values do not show the cooling very clearly, probably because they were “overprinted” by the meltwater input inferable from the IRD content. Nutrients that came with this terrestrial input, in combination with different habitat depths, can explain the contradictive warming signal shown by the dinocysts with respect to the foraminifera. The high abundances of subpolar planktic foraminifera and of the warm-temperate dinocyst S. mirabilis s.l. clearly indicate that optimal, fully marine interglacial conditions with the most intensive Atlantic water advection prevailed during the second warm phase of MIS 5e. The timing of this optimum contrasts with that of the Holocene, where warmest conditions developed immediately after Termination I. Our data furthermore suggest that in the northern Nordic seas, real interglacial conditions could only develop during this second warm interval. Our data therefore allow to conclude that MIS 5e experienced the consequences of the preceding deglaciation much longer than the Holocene. A severe cooling followed the late MIS 5e marine optimum. Warmer conditions returned in the Nordic seas during MIS 5c and MIS 5a, although the interglacial values characterising the MIS 5e optimum were never reached. The MIS 5a interval appears to have undergone stronger warm water advection compared to MIS 5c, but was also more unstable, showing a distinct reversal towards heavier δ18O values and thus colder conditions. Our data furthermore showed that ocean systems at times of intermediate climate conditions, such as those of MIS 5d-a, are very different from those characterising fully developed interglacials like the Holocene and MIS 5e.
