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

We explored whether the stable oxygen isotope composition (δ18O) of fossil chironomid remains can be used to reconstruct past variations in lake water δ18O 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 δ18O with values measured on bulk carbonates. Since carbonate particles adhering to chironomid remains potentially affect 18O 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 δ18O to a significant extent. Fossil chironomid δ18O 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 δ18O of bulk carbonate (r2 = 0.14) suggesting that carbonate contamination of the chironomid samples obscured the chironomid δ18O signature. Samples pretreated with NH4Cl correlated well with bulk carbonate δ18O (r2 = 0.67) and successfully tracked the well-known Lateglacial changes in δ18O. Chironomid δ18O indicated depleted lake water δ18O during the Oldest Dryas period, the Aegelsee and Gerzensee Oscillations, and the Younger Dryas, whereas enriched δ18O 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 δ18O 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 δ18O, potentially affecting δ18O of these two substances to a different extent. Our results indicate that chironomid δ18O can successfully reconstruct centennial to millennial-scale changes in lake water δ18O and that the method can be applied to carbonate-rich records provided that care is taken to eliminate carbonate contamination from the samples.