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

Abstract

Oxygen- and carbon-isotope ratios in the carbonate of benthic ostracodes ( Pseudocandona marchica) and molluscs ( Pisidium ssp.) were measured across the transitions bordering the Younger Dryas chronozone in littoral lacustrine cores from Gerzensee (Switzerland). The specific biogenic carbonate records confirm the major shifts already visible in the continuous bulk-carbonate oxygen-isotope record ( δ 18O Cc). If corrected for their vital offsets, oxygen-isotope ratios of Pisidium and juvenile P. marchica, both formed in summer, are almost identical to δ 18O Cc. This bulk carbonate is mainly composed of encrustations of benthic macrophythes ( Chara ssp.), also mainly produced during summer. Adult P. marchica, which calcify in winter, show consistently higher δ 18O, larger shifts across both transitions, and short positive excursions compared with the summer forms, especially during early Preboreal. Despite such complexity, the δ 18O of adult P. marchica probably reflects more accurately the variations of the δ 18O of former lake water because, during winter, calcification temperatures are less variable and the water column isotopically uniform. The difference between normalised δ 18O of calcite precipitated in winter to that formed in summer can be used to estimate the minimum difference between summer and winter water temperatures. In general, the results indicate warmer summers during the late Allerød and early Preboreal compared with the Younger Dryas. Altogether, the isotopic composition of lake water ( δ 18O L) and of the dissolved inorganic carbonate ( δ 13C DIC) reconstructed from adult Pseudocandona marchica, as well as the seasonal water temperature contrasts, indicate that the major shifts in the δ 18O of local precipitation at Gerzensee were augmented by changes of the lake's water balance, with relatively higher evaporative loss occurring during the Allerød compared with the Younger Dryas. It is possible that during the early Preboreal the lake might even have been hydrologically closed for a short period. We speculate that such hydrologic changes reflect a combination of varying evapotranspiration and a rearrangement of groundwater recharge during those climatic shifts.