Last Interglacial conditions in southern Europe: evidence from Ioannina, northwest Greece
Introduction
The Last Interglacial has long been a major research target principally because it provides a measure for comparison with Holocene and, possibly, future climatic change. However, despite an abundance of proxy records, there still remains some confusion over the extent of climate instability during this interval, the local or regional significance of events recognized and, ultimately, the length of the interglacial, as well as the precise chronostratigraphical relation between the terrestrial Eemian and Marine Isotope sub-Stage (MIS) 5e. The extreme variability of the Last Interglacial initially suggested by the GRIP ice core data (GRIP members, 1993) is now generally considered to be an artefact of stratigraphical disturbances (e.g. Alley et al., 1995, Chappellaz et al., 1997). On the marine side, records of foraminiferal assemblages, ice-rafted detritus, oxygen and carbon isotopic variations, and other geochemical evidence from the North Atlantic have not displayed significant climatic variability (e.g. Keigwin et al., 1994, McManus et al., 1994, Adkins et al., 1997, Oppo et al., 1997). Certain marine sequences, however, revealed a short cold event at ca. 122 ka BP Cortijo et al., 1994, Maslin et al., 1998, while sea-surface temperature fluctuations at ca. 127–126, 122–121 and 117 ka BP in the Nordic seas have also been invoked (Fronval and Jansen, 1996). On the whole, if at all detected, fluctuations in marine proxies have generally been shorter and less extreme than in the GRIP ice core.
On the terrestrial side, quantitative reconstruction using climate-pollen response surfaces from Bispingen–Luhe (Lower Saxony, north Germany) suggested significant cold episodes down to cold stage values (Field et al., 1994). However, climatic analogues for these events were poor, placing some uncertainty over the reliability of the reconstruction. By contrast, on the basis of palaeoclimatic analyses utilizing the indicator species approach on central German Eemian pollen records, Litt et al. (1996) found no evidence for climatic instability and argued that the same pattern should apply to most of north-central Europe. Their reconstruction, however, considered entire pollen assemblage zones as one unit, which makes detection of rapid climatic events difficult. In France, records derived from the Massif Central showed fluctuations in tree pollen percentages associated with changes in sediment magnetic susceptibility and organic carbon content Thouveny et al., 1994, Stockhausen and Thouveny, 1999. More recently, Cheddadi et al. (1998) compared temperature and precipitation reconstructions from a number of Eemian sites in France and Poland, which suggested the presence of some low-amplitude climatic variability. Thus, despite a substantial body of evidence from central and northwest Europe (see, for example Zagwijn, 1996), results have been mixed in terms of supporting or refuting the notion of climatic instability. In addition, the relative shortage of the Last Interglacial records from southern Europe has meant that it has been difficult to evaluate the extent to which the Eemian climatic signature is a representative of terrestrial events over a wider geographical region. Finally, there appears to be a disagreement over the duration of the Last Interglacial with estimates varying significantly from ca. 10 ka Müller, 1974, Hahne et al., 1994 to 23 ka (Kukla et al., 1997).
To a certain extent, the overall uncertainty in the terrestrial record is a product of (i) differential sensitivity and resolution of records, (ii) absence of a sufficiently precise chronology to enable comparisons and (iii) a lack of definition of what constitutes instability. Here, an attempt is made to reduce these sources of uncertainty and redress the imbalance in availability of southern European sites. We thus present a new record spanning the penultimate deglaciation and the Last Interglacial that combines isotopic and palynological results from a long lacustrine sequence at Ioannina, northwest Greece. The core has the highest sediment accumulation rates of all the available long sequences from southern Europe, which has enabled us to generate results at 100–200-year resolution. The site is well-suited for the examination of the response of terrestrial ecosystems to high-frequency climate variability given that its pollen record reflects the immediate response of tree populations to climate changes without migrational lags because of the proximity of refugial populations (Tzedakis, 1993). In addition, palynological work on the last glacial section of this sequence (e.g. Galanidou et al., 2000) has revealed millennial-scale variability similar to that recognized from terrestrial sequences in southern Italy (e.g. Allen et al., 1999) and Portugal (Roucoux et al., 2001), which is linked to climate events originating in the North Atlantic. A new chronology, based on recent advances in our understanding of vegetation–orbital links, provides new estimates on the duration and timing of particular events and enables comparisons with other sequences. Finally, the continuity of the record also provides an opportunity to examine not only the full interglacial sequence, but also the intervals immediately before and after. This allows us to define a baseline of what constitutes significant change by establishing the magnitude of glacial-to-deglacial shifts, and thereby assess the extent of intra–interglacial variability.
Section snippets
Site characteristics and chronostratigraphical framework
The intermontane lake basin of Ioannina is located 480 m above sea level (a.s.l.) in the Pindus Mountains of northwestern Greece (Fig. 1). From a hydrological viewpoint, the basin is effectively closed, since it has no outflowing streams or major basin drainage. The modern lake (known as Lake Pamvotis) forms the base level of a karst aquifer underlying the adjacent Mitsikeli Ridge (altitude: 1810 m a.s.l.). Run-off from this ridge and the high ground to the east is minimal, since surface waters
Methods
Standard methods were used for the chemical preparation of the pollen samples. A total of 112 levels covering the interval between the penultimate glacial maximum and the stadial period following the Last Interglacial was examined. The basic calculation sum comprised all pollen of nonaquatic vascular plants with a mean count of 338 pollen grains (minimum: 300; maximum: 479).
Stable isotopic measurements of fine-grained (<80 μm) calcite were taken at ca. 10-cm intervals over the same section of
Pollen
The pollen signature of the Last Interglacial period at Ioannina (defined locally as the Metsovon Interglacial) in the adjacent I-249 record (Tzedakis, 1994) was found to be distinct and diagnostic, allowing differentiation from all other forest periods recognized in I-249 (Tzedakis and Bennett, 1995). The new I-284 record (Fig. 3) shows the same broad palynological features as in I-249, but with a mean sampling interval of ca. 200 years, it improves the temporal resolution by 10-fold, thereby
Character of climate variability
In order to gain some insight into the duration of the phases described above and attempt long-distance comparisons, we have developed a new age model for the interval considered. This is based on recent work suggesting that certain vegetation patterns are a result of climate changes linked to specific orbital signatures (Magri and Tzedakis, 2000). Two main patterns have been identified: (i) tree population minima corresponding to dry and/or cold episodes at times of March perihelion and (ii)
Conclusions
At Ioannina, the Last Interglacial appears to have lasted from 127.3 to 111.8 ka BP. Peak interglacial conditions, characterized by highest temperatures and maximum tree population densities, occurred between 126.8 and 120.3 ka BP. After that, there was a progressive reduction in tree biomass with forest becoming increasingly more open in character, especially after 114.2 ka BP when open vegetation expanded considerably. This was briefly interrupted by a re-expansion of oak populations ca.
Acknowledgments
We thank R.C. Preece and M.R. Chapman for their comments on the earlier version of the text and S. Boreham and H. Sloane for technical advice and assistance. In addition, we are grateful to J. Rose (Royal Holloway) and D. Magri (Rome) who provided carefully considered suggestions that improved the manuscript. The Director-General and the staff at the Department of Energy Resources (DEPY) of the Institute of Geology and Mineral Exploration in Athens (particularly Y. Broussoulis) are thanked for
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Current address: Centre for Environmental Research, School of Chemistry, Physics and Environment Science, University of Sussex, Falmer, Brighton BN1 9QJ, UK.