Background to the BALTEEM project
The Last Interglacial (referred to in this area as the Eemian or Mikulino, and correlated with Oxygen Isotope Stage 5e) represents the most extensively studied Quaternary temperate stage beyond the Holocene, largely a reflection of its stratigraphical position. Because its climatic signature suggests warmer, and possibly more unstable conditions relative to the Holocene, its importance as a palaeo-analogue for future climatic scenarios cannot be overstressed.
It is intended that the BALTEEM project will examine a selection of the numerous Eemian sequences available in the Baltic region, along a transect from Denmark in the west to St. Petersburg in the east. The main aim of the project is to develop an interpretation of the evolution of the Baltic basin during the Eemian interglacial (c.130-115ka) by determining the nature and timing of marine transgressions and regressions in the basin following the late Saalian deglaciation and prior to the early Weichselian. A further aim is to determine the climatic effects resulting from the linking of the basin to the White Sea and through this to the Arctic Ocean. A number of specific objectives have been drawn up in order to maximise the data available, and are listed below.
Reconstructions for the Last Interglacial indicate that mean annual surface temperature in the northern hemisphere was 1-3oC higher than present (e.g. CLIMAP 1984; IPCC, 1990), while geological evidence and oxygen isotope values suggest that global sea-level was 5-7m above present (Fairbanks & Matthews, 1978; Chappell & Shackleton, 1986). Solar radiation estimates for the Last Interglacial show higher than Holocene excursions, a function of greater orbital eccentricity (Berger, 1978) and General Circulation Model results for 125ka also produce higher summer and lower winter temperatures than for 9ka (Royer et al.,1984). Atmospheric carbon dioxide levels were also higher than today, reachimg 300 ppm (IPCC, 1990).
Recent results from the GRIP ice core at Summit, central Greenland, showed a series of fluctuations between colder and warmer states during the Last Interglacial (GRIP Members, 1993), in contrast to the relative climatic stability of the Holocene. The validity of the Last Interglacial signal from the GRIP record, however, has since been called into question by results from a duplicate core GISP2 (Taylor et al.,1993; Grootes et al.,1993). Attempts to identify the GRIP events in the North Atlantic region in records of foraminiferal assemblages and ice-rafted detritus (McManus et al.,1994), as well as carbon and oxygen isotope variations of benthic foraminifera (Keigwin et al.,1994) have failed to show the presence of such oscillations. However, this may be a reflection of low resolution in the Atlantic cores and more recent studies on continental shelf sediments in North Jutland do contain distinct changes in foraminiferal assemblages (Seidenkrantz et al.,1995) and isotopic compositional changes have also been indentified in the GIN seas (Fronval & Jansen, 1996; Cortijo et al.,1996). Brief oscillations have also been reported in a number of European pollen records of the Last Interglacial, and the sensitivity of these sites relative to others that do not seem to record this oscillation has been attributed to their proximity to ecotones (de Beaulieu & Reilles, 1989). Such oscillations may be considerably spatially more extensive than hitherto thought and should be viewed within the general framework of climatic instability of the Last Interglacial (Tzedakis et al.,1994). High resolution magnetic susceptibility, organic carbon and pollen records from two sites in the Massif Central, France, have shown the presence of two cooling events during the Last Interglacial (Thouveny et al.,1994). These have been correkated with events 5e4 and 5e2 of the GRIP record, although further improvements in the time control of the terrestrial cores may be necessary before the connection is firmly established. At present, the matter remains far from settled, as different sets of data appear to be in conflict as to the presence, timing and mechanism of these oscillations. In addition to the GRIP events, there have been suggestions of a Younger Dryas type oscillation at termination 2 (e.g. Seidenkrantz & Knudsen, 1994; Seidenkrantz et al.,1996). Thus considerable attention has been focused on the Last Interglacial, as such rapid oscillations between colder and warmer states against a background of elevated CO2 content can provide an improved understanding of the sensitivity of components of the climate system, which can improve forecasts of future climate (Rind, 1993).
One fundamental point is how the abrupt climatic oscillations, apparently operating at the so-called 'sub-Milankovitch level' (i.e. they are not dependent on direct Milankovitch type forcing), are reflected in terrestrial sequences in high latitudes, and how this relates to the operation of the North Atlantic Deep Water circulation in the GIN seas. The examination of shallow marine Baltic sequences will shed considerable light on this matter because of the availability of the large number of terrestrial sequences spanning the entire interglacial in the adjacent areas. Correlation with the latter will demonstrate precisely how and in which ways such changes influenced terrestrial environments.
The Baltic Sea has an average depth of 100m, and a maximum of about 460m. It has an excess of precipitation and river runoff over evaporation and consequently low salinity, usually below 10 per mil at the surface, and up to 16 per mil in deeper parts. It does not exhibit excessive stagnation, possibly because of the moderate depths and communication with the North Sea through shallow passages with a sill depth of about 18m.
The evolution of the present Baltic Sea has been studied extensively, its Holocene history showing a complex interaction of ice retreat, eustatic sea-level changes and isostatic rebound (e.g. Donner, 1995; Eronen, 1987). This is mainly manifest as a broadly four stage development beginning in the Late Weichselian when a substantial freshwater glacially dammed Baltic Ice Lake was already in existence. Ice retreat from the Younger Dryas end moraine complexes and, in particular, the Billingen area in central Sweden led to a very rapid, possibly catastrophicdrainage of the lake by 28m to the Yoldia Sea level. This brackish waterbody, linked via the Närke Strait in central Sweden to the North Sea, was also linked to Lake Ladoga in the east. However, through the combined effect of rapid deglaciation, isostatic crustal rebound and rapid global eustatic sea-level rise, the Baltic evolved rapidly. By 9ka, the Swedish outlet was closed, and a new outlet through the Great Belt began developing, giving rise to the Ancyluslake stage. This freshwater event lasted until 8.5ka when renewed influx of saline water gave rise to the MastigloiaSea. Continued global sea-level recovery, accompanied by slowing isostatic rebound, led to a greater influx of saline water, culminating in the modern LitorinaSea phase. These events were accompanied by water level changes that can be recognised throughout the basin and provide a potentially vital analogue for older pre-Weichselian events.
By comparison with the Holocene, the evolution of the Baltic basin is very poorly understood, especially in relation to the higher sea-levels of that period. There is a substantial body of evidence suggesting that, during the Eemian, the Baltic Sea was considerably more extensive than the present day, with a possible connection to the White Sea (Zans, 1936; Gross, 1967; Forsström et al.1988; Raukas, 1991; Donner, 1995; Zagwijn, 1996; van Andel & Tzedakis, 1996). The nature, timing and duration of this connection, however, remain unclear, and it is also not known whether this was a single or multiple event. As Donner (1991) emphasises, considering that the Eemian sea-level was higher than that in the Holocene and that the crust in the central region of the Saalian glaciation was more depressed than during the Weichselian, it is likely that larger areas were submerged during the Eemian. This will have been particularly important in northern Sweden and Finland. Numerous sequences across the region (e.g. Liivrand, 1991; Sokolova et al.,1972; Donner, 1995; Grönlund, 1991; Nilsson, 1983; Gross, 1967; Lukashov, 1982; Makowska, 1979; 1986) suggest that the Baltic during the Eemian had a greater salinity but with cold water influxes, and was thus more oceanic than the present brackish water body.
Although the pattern of isostatic rebound appears to be similar to that during the Holocene (Forsström et al.,1987; 1988), current evidence is very limited, and it is important to develop an understanding of the interaction of crustal rebound and water level changes, since they control the evolution of the basin through the period.
The land-sea distribution during the Eemian, in comparison to that in the Holocene, had a profound effect on climate in the highly sensitive Baltic Sea region. Not only was there current inflow of cold water from the White Sea, but there was a significant influx of warm water from the Atlantic. For example, Zagwijn (1996) considers that the presence of free interchange of sea-water would have been responsible for the pronounced maritime climate in western and central Europe during the Eemian, and that the increase in winter temperature may thus be related to the rise of sea-level through the interglacial. Furthermore, as Donner (1995) points out, there was a greater uniformity in vegetation around the Baltic region than in the Holocene. This is a considerable advantage for pollen biostratigraphical identification of fragmentary sequences.
By way of conclusion, the importance of the interaction of marine transgression and regression as a consequence of the interaction of global sea-level, isostasy, deglaciation and topography to both the terrestrial and marine biota (including human colonists in the Holocene) cannot be overstressed. Clearly the presence or absence of a waterbody, coupled with its form, can have implications for the whole of northern Europe bordering the basin.
The specific objectives of the project are:
- To determine the relative timing, rate and direction of marine transgressions and regressions and produce a generalised sea-level curve for the southern Baltic region during the Last (Eemian, Mikulino) Interglacial.
- To provide an integrated palaeo-oceanographic reconstruction of the impact of marine transgressions and large-scale water circulation in the Baltic basin during the Eemian interglacial Stage (including the transitions from latest Saalian to earliest Weichselian.
- To reconstruct the patterns of salinity changes, the changing tidal amplitudes, changes in tide-dependent variables such as sediment transport path and shelf sea fronts of the North Baltic-White Sea through the basin during the interglacial cycle. To calibrate the models with observational data for well-defined timeslices.
- To determine the impact of the changes of water body circulation on the surrounding terrestrial environments (vegetational changes).
- To compare with northern hemisphere climate changes as described from Greenland ice cores and particularly with the deeper water eastern North Sea Danish sequences to determine the impact of climate changes in the northern European region.
- To reconstruct the detailed environmental evolution of the basin through the interglacial and compare this with that during the Holocene (the last 10 000 years). This is a necessary pre-requisite to the detailed modelling of the isostatic history of the Baltic during the Eemian.
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