Composite stable isotope records from zyxw a Late Weichselian lacustrine sequence at Grrenge, Lolland, Denmark: evidence of Allerod and Younger Dryas environments DAN HAMMARLUND AND BJ0RN BUCHARDT Hammarlund, D. zyxwvutsrq & Buchardt, B. 1996 (March): Composite stable isotope records from a Late Weichselian Iacustrine sequence at Grznge, Lolland. Denmark: evidence of Allertad and Younger Dryas environments. zy Boreus, Vol. 25, pp. 8 -22. Oslo. ISSN 0300-9483. Stable oxygen and carbon isotope data from a lacustrine sequence at Grznge, southeast Denmark, revealed distinct cnvironmental changes related to Late Weichselian climatic development in the region. Comparison of isotopic records obtained from sedimentary carbonates and freshwater molluscs enabled reconstruction ol' changes in the lacustrine environment. The degree of thermal and chemical stratification of the lake zy was evaluated and supported by pollen data from an earlier study of the site. During the Allertad interstadial, dimictic and stagnant conditions characterized the lake, whereas the stratification was disturbed during thc Younger Dryas stadial probably as a result of deforestation and increased wind impact. The origin of sedimentary carbonates was examined by mineralogic and morphologic studies. A considerable input of clastics and detrital carbonates, associated with pronounced enrichment of zyxwv ''0 and I3C. wds recorded in the Younger Dryas sediments indicating soil degradation and increased erosion. A climatic warming preceding the Pleis- tocene/Holocene boundary is clearly reflected in the different stable isotope records and in the lithostratigraphy. Dun Hunimcirlunct*, Department zyxwvutsr uf' Qutiternury Geology, Lund Unioersiiy, Tornaoagen 13, S-223 63 Lund. Sweden and Deparfment qf Eirrth Sciences. Uniijersity zyxwvu yf Wiirerloo. Watwloo, Onttrrio, Canada N2/. 3G I; Hjmn Buchardt, Geologicd Institute, Universiiy of Copenhagen, 0sier Voldgciile 10, zyxw DK- 1350 Kahenhnon K, Denmurk; received 2nd May 1995: urcepied 2211d November 1995. BOREAS Stable isotope studies of lake sediments have provided important contributions to our current understanding of Late Weichselian and early Holocene climatic de- velopment. Oxygen isotope records from limnic car- bonates exhibit a rapid response to climatic changes comparable in sensitivity to that of vegetation records, such as from pollen and plant macrofossil data (Lotter et ul. 1992; Goslar et (11. 1993; Wohlfarth et al. 1994). Carbon isotope variations are potential - but only partly understood - markers of local environmental changes (Buchardt & Fritz 1980; McKenzie 1985). The best-known stable isotope records obtained from late Weichselian sediments are those from the alpine regions of Central Europe (e.g. Eicher & Siegenthaler 1976, 1983). These and other records demonstrate a characteristic depletion of ''0 in limnic carbonates deposited during the Younger Dryas sta- dial as compared to the strata below and above. The depletion is interpreted to reflect a significant decrease in "0-concentration of precipitation, and thereby in mean annual air temperature. This hypothesis is sup- ported by close correspondence with vegetation data. Similar changes in oxygen isotope records from inland ice of Greenland (Siegenthaler et al. 1984; Lotter et a1. 1992) and Peru (Thompson et al. 1995) thus indicate a widespread (global?) significance of the Younger Dryas climatic anomaly. A strongly divergent oxygen isotope record from a Late Weichselian lake at Grrenge in southeast Den- mark was presented by Kolstrup & Buchardt (1982) to complement a detailed study of the vegetation history of the region. Here, the Younger Dryas stadial is represented by a distinct enrichment of I8O in sedimentary carbonates in direct contradiction to re- sults from Central Europe (e.g. Eicher 1987), Poland (Rozanski 1987; Rozanski et al. 1988; Goslar et al. 1993; Hammarlund 1994), Germany (Pachur & Riiper 1984) and the Baltic Region (Punning et al. 1984). It was suggested that a climatically induced increase in soil erosion with subsequent deposition of al- lochthonous carbonate particles in the lake was the cause of the oxygen isotope anomaly at Graenge, although no evidence of redeposited carbonates was produced. To clarify the cause of this divergence, a follow-up study was carried out on a new sequence from Grlenge including mineralogic, petrographic and isotope geo- chemical investigations of the limnic carbonates (cf. Kelts & Talbot 1990). Also included in the study were isotopic data from the original sequence at Grlenge (Kolstrup & Buchardt 1982), not formerly published. These include carbon isotope data from sedimen- tary carbonates and carbon and oxygen isotope data from shells of gastropods and pelecypods. Carbonates formed by shell-bearing aquatic organisms are inde- pendent of contamination by detrital components and have been used in several studies to separate and estimate different climatic parameters (e.g. Stuiver 1970; Fritz & Poplawski 1974; Fritz et al. 1975; Lister 1988, 1989; von Grafenstein et al. 1992, 1994). The