Weichselian palynostratigraphy, palaeovegetation and palaeoenvironment; the record from Lago Grande di Monticchio, southern Italy

Abstract

The palynostratigraphic record from Lago Grande di Monticchio (S. Italy) is presented. An independent sedimentation-rate-based chronology provides an age of 101,530 calendar years BP for the lowermost sediments sampled. This chronology enables assessment of the rapidity of vegetation changes during the last glacial; contrary to widely held views the vegetation showed numerous large magnitude changes in periods of <150 yr. The history of vegetation and environmental changes is inferred from the palynological data. Comparison with other palynological records from southern Europe indicates that the Monticchio record reveals much greater vegetation and environmental variability than has hitherto been reported. The regionally dominant biomes have been systematically reconstructioned. Whereas forest biomes predominated during the Holocene and early-Weichselian interstadials, open `wooded steppe’ predominated during the majority of Weichselian interstadials and steppe during stadial periods including the full glacial. Quantitative palaeoclimate reconstructions indicate rapid, large magnitude and independent variations in the three climatic variables reconstructed — winter cold, seasonal warmth and moisture availability. These variations can be linked to variations in other Earth system components; comparisons upon the basis of their independent chronologies reveals contrasts between terrestrial and marine records that are potentially important indicators of underlying mechanisms and drivers of environmental change.

Introduction

Lago Grande di Monticchio (40°56′40″N, 15°36′30″E; 656 m above sea level) is a Maar lake situated in the volcanic crater on the western flank of Monte Vulture in Basilicata, southern Italy (Fig. 1). The inner flanks of the crater are forested, the dominant canopy species being Fagus sylvatica and Quercus cerris. The shrub layer is dominated by Ilex aquifolium and Corylus avellana; Hedera helix and Ruscus aculeatus are also prominent, along with a species-rich herbaceous flora that is central European in biogeographic character. Monte Vulture, which rises to 1326 m, has today an Abies woodland on its summit. This stand, however, is not natural but the result of planting earlier this century. A more detailed account of the general setting of the site has been given elsewhere (Watts et al., 1996a, Watts et al., 1996b).

The site has several important features that combine to make its sedimentary record unique amongst European terrestrial records spanning the last glacial stage. Firstly, it lies far enough south to have suffered no direct effects of the Weichselian glaciation, yet is sufficiently close to, and `downwind’ of, the North Atlantic for its climate to be influenced by weather systems that reflect conditions in that ocean basin (Conte and Colacino, 1995). Secondly, the lake has a spatially restricted hydrological catchment within the steep-sided crater of Monte Vulture and has only intermittently overflowed, apparently never having had any substantial throughput of water. The deeper part of the basin has been continuously flooded; therefore, not only have conditions been ideal for the preservation of micro-fossils, but also sediments have accumulated continuously providing a sedimentary record with no major hiatuses since the formation of the lake. Thirdly, the sediments are, for much of the stratigraphic sequence, laminated, enabling the establishment of a calendar year chronology based upon numerous measurements of sedimentation rates (Jens Mingram, pers. commun.; Zolitschka and Negendank, 1993, Zolitschka and Negendank, 1996). Furthermore, the sedimentary sequence includes numerous tephra layers; although the majority of these are thin (<10 mm), a number are >100 mm thick. Some of the latter have been directly dated by ${}^{40}\text{Ar}\text{/}{}^{39}\text{Ar}$ (Watts et al., 1996b), whereas others have been dated by tephrochronological correlations (Sabine Wulf, pers. commun.; Narcisi, 1996; Allen et al., 1999): Although in some cases there are discrepancies between the ${}^{40}\text{Ar}\text{/}{}^{39}\text{Ar}$ and sedimentation-rate-based ages, given the errors associated with the ${}^{40}\text{Ar}\text{/}{}^{39}\text{Ar}$ technique when applied to such young deposits, the results generally support the sedimentation rate chronology. Radiocarbon age measurements made upon carefully selected terrestrial macrofossils also corroborate the recent part of the chronology (Hajdas et al., 1998).

The sediments have been the subject of multidisciplinary studies for the last 18 years (Watts, 1985; Turton, 1993; Zolitschka and Negendank, 1993, Zolitschka and Negendank, 1996; Robinson, 1994; Narcisi, 1996; Watts et al., 1996a, Watts et al., 2000; Allen et al., 1999; Brandt et al., 1999; Huntley et al., 1999; Nimmergut et al., 1999); the most recent coring of the site, utilising equipment from GFZ Potsdam, Germany, took place in 1994 and extended the record to 71 m. Here we present the complete record in detail for the first time; this record is based on analyses of the two main cores (D (1990) and J (1994)). These have been securely cross-correlated using a series of thick (>100 mm) and morphologically characteristic tephra layers, enabling the composite stratigraphic sequence to be constructed that has then been used to compile a 101,530 calendar year continuous palynological record for the site.

Palynologists generally interpret the results of their pollen analyses in terms of rather subjective and qualitative inferences as to the character of the palaeovegetation represented by the pollen spectra. They often also infer aspects of the palaeoenvironment, especially the palaeoclimate, in a similar manner. In both cases the inferences are based upon the palynologists’ knowledge of the present ecological character and spatial occurrence of the plant taxa represented in the fossil assemblages. We present below such subjective interpretations of the palynological data from Monticchio. In recent years, however, attention has focussed upon the development of more systematic and/or quantitative methods for making palaeovegetation and palaeoenvironment reconstructions. These methods too are based upon knowledge of the ecological character and present patterns of occurrence of plant taxa, although now this information is expressed and utilised in a more systematic manner. We have applied such systematic palaeovegetation and quantitative palaeoclimate reconstruction techniques to our palynological data spanning the last 101,530 yr. These results are also presented below, complementing the subjective inferences and providing the basis for comparisons with other palaeoenvironmental records from marine sediments and ice-cores spanning the same interval. The conclusions that emerge from such comparisons with respect to Weichselian palaeoclimates, in particular, are discussed.