Isotope signature of the Younger Dryas and two minor oscillations at Gerzensee (Switzerland): palaeoclimatic and palaeolimnologic interpretation based on bulk and biogenic carbonates

Oxygen- and carbon-isotope ratios in the carbonate of benthic ostracodes (P seudocandona marchica) and molluscs (Pisidium ssp.) were measured across th e transitions bordering the Younger Dryas chronozone in littoral lacustrine cores from Gerzensee (Switzerland). The specific biogenic carbonate record s confirm the major shifts already visible in the continuous bulk-carbonate oxygen-isotope record (delta(18)O(Cc)). If corrected for their vital offse ts, oxygen-isotope ratios of Pisidium and juvenile P. marchica, both formed in summer, are almost identical to delta(18)O(Cc). This bulk carbonate is mainly composed of encrustations of benthic macrophythes (Chain ssp.), also mainly produced during summer. Adult P. marchica, which calcify in winter, show consistently higher delta(18)O, larger shifts across both transitions , and short positive excursions compared with the summer forms, especially during early Preboreal. Despite such complexity, the delta(18)O of adult P. marchica probably reflects more accurately the variations of the delta(18) O of former lake water because, during winter, calcification temperatures a re less variable and the water column isotopically uniform. The difference between normalised delta(18)O of calcite precipitated in winter to that for med in summer can be used to estimate the minimum difference between summer and winter water temperatures. In general, the results indicate warmer sum mers during the late Allerod and early Preboreal compared with the Younger Dryas. Altogether, the isotopic composition of lake water (delta(18)O(L)) a nd of the dissolved inorganic carbonate (delta(13)C(DIC)) reconstructed fro m adult Pseudocandona marchica, as well as the seasonal water temperature c ontrasts, indicate that the major shifts in the delta(18)O of local precipi tation at Gerzensee were augmented by changes of the lake's water balance, with relatively higher evaporative loss occurring during the Allerod compar ed with the Younger Dryas. It is possible that during the early Preboreal t he lake might even have been hydrologically closed for a short period. We s peculate that such hydrologic changes reflect a combination of varying evap otranspiration and a rearrangement of groundwater recharge during those cli matic shifts. (C) 2000 Elsevier Science B.V. All rights reserved.