ARE STABLE ISOTOPES OF LACUSTRINE CARBONATE A GOOD TRACER OF LAKE HYDROLOGY AND LAKE LEVEL VARIABILITY? THE LAKE LEDRO CASE (NORTHERN ITALY) (STABLE ISOTOPES OF CARBONATE AS TRACERS OF LAKE HYDROLOGY)

Abstract

Oxygen and carbon isotopes were analysed on bulk samples and carbonate concretions from Lake Ledro, northern Italy, with the aim to assess their suitability as reliable proxy of the hydrological changes in the lake. For this purpose, isotopic composition of bulk marls and concretions were compared to lake level reconstruction obtained with sedimentological approach. The δ13Cbulk record mostly mirrors the δ18Obulk record, while carbon and oxygen isotopes from concretions are not correlated. We found that there is a statistically significant correlation between the oxygen and carbon isotopic record of the bulk carbonate and lake level changes, with the positive changes in lake level corresponding to lower values on δ18Obulk and δ13Cbulk and vice versa. Assuming that most of the bulk carbonate is formed by bio-induced calcite precipitation, this means that this kind of carbonate reflects the local hydrological condition of the lake water better than lacustrine concretions, which instead are more affected by local conditions of precipitation like biological plant activity.