Holocene changes in moisture source and temperature revealed by the oxygen isotopic composition of fossil Daphnia ephippia in Sierra Nevada, southern Spain

Abstract

The study of oxygen isotopes in chitin and its use as a paleoclimatic proxy is still under development. The low oxygen content of chitin and the low sample weight of fossils have hindered this research topic, even though there is evidence of a strong correlation between the oxygen isotopic composition of chitin and the isotopic composition of the host water. In order to further this research we present a paleoclimatic reconstruction based on δ¹⁸O of cladoceran remains from the lake sediments of Laguna de Río Seco, Sierra Nevada, Spain. Here, modern water isotopic data were used as a modern analogue to establish an important influence on evaporative enrichment as the ice-free season advances. The oxygen isotope signal from Daphnia resting eggs was used as a proxy for autumn snapshots of the water isotopic composition at millennial time scales. Long-term changes were controlled by the moisture source and the summer insolation. Between deglaciation and 4.2 kyr BP, δ¹⁸O measurements exhibited generally depleted values only interrupted by a peak at ∼ 7.2 kyr BP, concurrent with a temperature maximum inferred from earlier quantitative reconstructions. A predominantly Atlantic moisture source and changes in the evaporation related to seasonality explained the isotopic variability at that time. From 4.2 kyr BP onwards, a notable enrichment in this isotopic signal occurred, which was consistent with greater influence of a Mediterranean precipitation source and higher evaporation at lower lake levels. This new isotope record provides a unique application of paleoclimates from cladocerans, which goes beyond the taxonomic and numerative methodology traditionally used in subfossil cladoceran analysis.