Chironomid δ18O as a proxy for past lake water δ18O: a Lateglacial record from Rotsee (Switzerland)

We explored whether the stable oxygen isotope composition (delta O-18) of fossil chironomid remains can be used to reconstruct past variations in lake water delta O-18 from Lateglacial and early Holocene sediments from Rotsee (Switzerland). A sediment core from the former littoral zone of the lake was examined since it contained both high concentrations of chironomid remains and abundant authigenic carbonates and therefore allowed a direct comparison of chironomid delta O-18 with values measured on bulk carbonates. Since carbonate particles adhering to chironomid remains potentially affect O-18 measurements we tested two methods to chemically remove residual carbonates. Trials with isotopically heavy and light acid solutions indicated that treatment with hydrochloric acid promoted oxygen exchange between chironomid remains and the water used during pretreatment. In contrast, a buffered 2 M ammonium chloride (NH4Cl) solution did not seem to affect chironomid delta O-18 to a significant extent. Fossil chironomid delta O-18 was analyzed for the Rotsee record both using standard palaeoecological methods and after pretreatment with NH4Cl. Samples prepared using standard techniques showed a poor correlation with delta O-18 of bulk carbonate (r(2) = 0.14) suggesting that carbonate contamination of the chironomid samples obscured the chironomid delta O-18 signature. Samples pretreated with NH4Cl correlated well with bulk carbonate delta O-18 (r(2) = 0.67) and successfully tracked the well-known Lateglacial changes in delta O-18. Chironomid delta O-18 indicated depleted lake water delta O-18 during the Oldest Dryas period, the Aegelsee and Gerzensee Oscillations, and the Younger Dryas, whereas enriched delta O-18 values were associated with sediments deposited during the Lateglacial interstadial and the early Holocene. Differences in the amplitude of variations in bulk carbonate and chironomid delta O-18 are attributed to differential temperature effects on oxygen isotope fractionation during the formation of carbonates and chironomid head capsules or seasonal changes of lake water delta O-18, potentially affecting delta O-18 of these two substances to a different extent. Our results indicate that chironomid delta O-18 can successfully reconstruct centennial to millennial-scale changes in lake water delta O-18 and that the method can be applied to carbonate-rich records provided that care is taken to eliminate carbonate contamination from the samples.