Holocene temperatures and isotopes of precipitation in Northwest Greenland recorded in lacustrine organic materials

Reconstructions of Holocene lake water isotopic composition based upon subfossil aquatic organic material offer new insights into Arctic climate. We present quantitative estimates of warmth during the Holocene Thermal Maximum in northwest Greenland, inferred from oxygen isotopes of chironomid head capsules and aquatic moss preserved in lake sediments. δ¹⁸O values of chironomids from surface sediments of multiple Greenland lakes indicate that these subfossil remains record the δ¹⁸O values of the lake water in which they grow. Our lake water δ¹⁸O reconstruction is supported by downcore agreement with δ¹⁸O values in aquatic moss and chironomid remains. δ¹⁸O of both organic materials from Secret Lake decrease after 4 ka (ka = thousands of years ago) by 3‰ into the Neoglacial. We argue that lake water at Secret Lake primarily reflects precipitation δ¹⁸O values, which is strongly correlated with air temperature in NW Greenland, and that this signal is biased towards summer and early autumn conditions. Other factors may have influenced Secret Lake δ¹⁸O values through the Holocene, including evaporation of lake water and changing seasonality and source of precipitation. The maximum early Holocene summer and early autumn-biased temperature anomaly at Secret Lake is 2.5–4 °C warmer than present from 7.7 (the beginning of our record) to ∼6 ka. The maximum late Holocene cold anomaly (which includes the Little Ice Age) is 1.5–3 °C colder than present. These ranges of possible temperature anomalies reflect uncertainty in the δ¹⁸O – temperature relationship for precipitation at the study site through the Holocene.