NOAA/WDS Paleoclimatology - Takahula Lake, Alaska 8,000 Year Stable Isotope Data

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.

要理解气候变暖带来的生态与社会经济影响，需掌握水分平衡的相关变化。全新世（Holocene）水分平衡变化的重建研究，可为评估近期气候变化提供不可或缺的基准参照。我们对位于阿拉斯加布鲁克斯山脉（Brooks Range）中南部的塔哈乌拉湖（Takahula Lake）沉积物中的轮藻茎部结壳（Chara-stem encrustations）开展了氧碳同位素组成分析，以此反演过去8000年的气候演变。为辅助限定沉积物氧同位素（δ¹⁸O）记录的气候解译，我们还对塔哈乌拉湖及该区域其他湖泊的水样开展了氢氧同位素组成分析。研究结果显示，冬季降水主导了这些湖泊的水分平衡，而入湖后的蒸发作用是控制塔哈乌拉湖湖水同位素组成的关键因素。轮藻氧同位素（Chara-δ¹⁸O）的地层序列特征，结合碳同位素（δ¹³C）与沉积物岩性数据，揭示了过去8000年有效湿度（即降水减蒸发）的显著变化。8000至5000校准年前（cal BP）期间，有效湿度整体偏高，且在6800至5000 cal BP间存在显著波动；随后有效湿度下降，在约4000 cal BP时降至最低；4000至约2500 cal BP期间，湿度随波动有所回升，之后整体呈下降趋势直至现代，期间被以400 cal BP为中心的湿润小冰期（Little Ice Age）打断。20世纪的干旱程度位列整个8000年记录的最高水平之一。在千年尺度上，塔哈乌拉湖水分平衡变化的时间模式，与该区域此前基于古气候记录（paleoclimate records）推断的结果大体一致。约5600 cal BP以及小冰期期间的轮藻氧同位素值，较现代值低最多5‰，较塔哈乌拉湖现代入湖水样的同位素值低3.6‰。这些异常偏低的同位素值表明，除有效湿度外，其他因素也对塔哈乌拉湖沉积物δ¹⁸O记录的显著波动产生了影响。与阿留申低压（Aleutian Low）西风位相相关的冬季降水增加，可能解释了δ¹⁸O下降中的1‰幅度。这两个时期导致氧18贫化的其他因素，可能包括气温降低、湖冰覆盖范围扩大以及由此引发的蒸发量下降。