Chemical and isotopic composition of modern water bodies in the Lake Kopais Basin, central Greece: analogues for the interpretation of the lacustrine sedimentary sequence

Abstract

Stable-isotope measurements and modern aquatic chemistry are used to aid the reconstruction of past hydrological changes recorded in a 126 ka-long sediment core from Lake Kopais, Greece. This is the most southeasterly European site with a long Late Quaternary sequence. Although the lake has been artificially drained, calcareous sediments from the centre of the basin, 38°26′16ʺN, 23°03′01ʺE, provide a continuous record from the early part of the last interglacial to the Early Holocene. Because the physiography and hydrology are complex, modern analogues for past environmental conditions are essential. The modern waters are of Ca–Mg–HCO3 or Mg–Ca–HCO3 type, reflecting the prevalence of limestones and metamorphic rocks in the catchment. They show a clear evolutionary trend in δ18O and δD values from the source areas to the modern lakes. However, the modern water bodies are less evolved than the palaeolake. A larger evaporating surface, higher values of δ18O in precipitation, decreased precipitation/evaporation ratios and hydrological closure increased the δ18O values of endogenic carbonates at different times in the past. However, Palaeolake Kopais never desiccated completely or became saline. Analyses of the organic fraction from the core show that: (i) it is mainly of algal origin; (ii) productivity and carbon cycling in the lake waters were relatively stable through time; and (iii) the bulk δ13C values vary between −29‰ and −25‰, indicating that the lake waters were never appreciably carbon-limited and that C4 plants did not invade the catchment. Although significant shifts in the δ18O and δ13C values of endogenic low-Mg calcite are attributable mainly to past variations in water balance, weak δ18O–δ13C covariance during marine isotope stages (MIS) 5e–5c suggests that the palaeolake leaked significantly through karstic sinkholes. Since it was effectively operating as an open system, δ18O may have retained the isotopic imprint of increased summer precipitation. Between MIS 5a and MIS 4, strong isotopic covariance suggests minimal outflow and drier conditions than today. After an increase in winter snowmelt during MIS 3, the climate again became cold and dry in the full-glacial (MIS 2). A fluctuating increase in moisture occurred during the late-glacial to early Holocene transition (MIS 2–1).