Understanding the ecological and socio-economic impacts of climatic warming requires knowledge of associated changes in moisture balance. Reconstructions of Holocene moisture-balance variation offer indispensible baseline information against which recent changes can be evaluated. We analyzed Chara-stem encrustations in the sediments of Takahula Lake, located in the south-central Brooks Range of Alaska, for oxygen and carbon-isotope composition to infer climatic change over the past 8000 years. To help constrain climatic interpretations of the sediment δ18O record, we also analyzed water samples from Takahula and other lakes in the region for oxygen and hydrogen isotope composition. Results show that winter precipitation dominates the water balance of these lakes and that post-input evaporation is a key control of lake-water isotope composition of Takahula Lake. Stratigraphic patterns in Chara-δ18O, supplemented by those in δ13C and sediment lithology, reveal distinct changes in effective moisture (precipitation minus evaporation) over the past 8000 years. Effective moisture was relatively high from 8000 to 5000 cal BP, with marked fluctuations between 6800 and 5000 cal BP. It then decreased to reach a minimum around 4000 cal BP and increased with fluctuations from 4000 to ∼2500 cal BP, followed by a decreasing trend toward the present that was interrupted by a wet Little Ice Age (centered at 400 cal BP). Aridity during the 20th century was among the highest of the entire 8000-year record. At the millennial timescale, the temporal patterns of moisture-balance shifts at Takahula Lake are broadly coherent with those inferred from previous paleoclimate records from the region. The Chara-δ18O values around 5600 cal BP and during the Little Ice Age are up to 5‰ lower than at present and 3.6‰ lower than that of the modern input-water to the lake. These exceptionally low values suggest that factors other than effective moisture must have contributed to the pronounced variations in the Takahula Lake δ18O record. Increased winter precipitation associated with a westerly Aleutian Low position may account for 1‰ of the δ18O decrease. Other factors leading to the 18O-depletion during these periods probably include decreased temperatures, as well as increased lake-ice cover and associated reductions in evaporation.
ASJC Scopus subject areas
- Global and Planetary Change
- Ecology, Evolution, Behavior and Systematics