NOAA/WDS Paleoclimatology - Ongoke Lake, Alaska 2,000 Year Diatom, Chironomid, and Geochemical Data

We analyzed sediments of the past 2000 years from Ongoke Lake, southwest Alaska, for organic carbon, organic nitrogen, biogenic silica (BSi), and diatom assemblages at decadal to centennial resolution to infer limnological changes that may be related to climatic variation in southwestern Alaska. The chronology is based on a 210Pb profile from bulk sediments and nine AMS 14C ages from terrestrial plant macrofossils. Four of the 14C ages span a core depth interval of 60.5 cm but are statistically indistinguishable from one another with a mean of ~1300 AD, which compromises the determination of temporal trends at Ongoke Lake and comparison with other paleoclimate records. The diatom record suggests changes in the duration of ice cover and strength of thermal stratification that are probably related to temperature variation. This variation includes a cold interval around the first millennium cooling (FMC) and a warm interval spanning the medieval climate anomaly (MCA). However, the lake-sediment record shows no clear signals of temperature variation for the period of the Little Ice Age (LIA) or the twentieth century. Climatic changes during these periods may have been manifested through effective-moisture (precipitation minus evapotranspiration) variation in the Ongoke Lake area. We estimate water depths and infer effective-moisture fluctuations by applying a regional transfer function to our diatom record. Together with inferences from diatom autecologies, this water-depth reconstruction suggests that effective moisture increased steadily from 50 BC to 350 AD, which was followed by relatively dry conditions between 550 and 750 AD and relatively wet conditions between 750 and 1450 AD. Effective moisture was low from ~1450 to 1850 AD, coinciding with the LIA; an alternative age model places this interval between ~1315 and 1850 AD. During the past 150 years, effective moisture increased, with estimated water depths reaching peak values in the second half of the twentieth century. This study offers the first paleolimnological record for inferring centennial-scale climatic variation over the past two millennia from southwestern Alaska.

本研究针对阿拉斯加西南部翁戈克湖（Ongoke Lake）2000年以来的沉积物，以十年至百年分辨率开展了有机碳、有机氮、生物成因二氧化硅（biogenic silica, BSi）及硅藻组合（diatom assemblages）分析，旨在反演可能与阿拉斯加西南部气候变率相关的湖沼学的（limnological）变化。本研究的年代序列基于全岩沉积物的铅210（210Pb）剖面，以及9件陆生植物大化石（terrestrial plant macrofossils）的加速器质谱碳14（AMS 14C）测年结果。其中4件14C测年结果对应岩芯60.5厘米的深度区间，但统计上彼此无显著差异，平均年代约为公元1300年，这削弱了翁戈克湖时间序列趋势的确定，以及与其他古气候记录（paleoclimate records）的对比工作。硅藻记录显示，冰盖持续时长与热力分层（thermal stratification）强度的变化可能与温度变率相关。这类变化包括千年初期冷期（First Millennium Cooling, FMC）前后的冷期，以及覆盖中世纪气候异常期（Medieval Climate Anomaly, MCA）的暖期。然而，湖相沉积物记录并未呈现小冰期（Little Ice Age, LIA）及20世纪的明确温度变率信号。该时期的气候变化可能通过翁戈克湖区域的有效湿度（effective-moisture，即降水量减去蒸散量）变化体现。本研究通过将区域转换函数（regional transfer function）应用于硅藻记录，估算了湖水水深并反演了有效湿度波动。结合硅藻生态习性（diatom autecologies）的推断，该水深重建结果显示，有效湿度自公元前50年至公元350年持续升高，随后在公元550年至750年处于相对干旱状态，而公元750年至1450年则相对湿润。约公元1450年至1850年，有效湿度处于低位，这与小冰期（LIA）的时间范围重合；另一种年代模型则将该区间定为公元1315年至1850年。在过去150年中，有效湿度有所升高，估算的湖水水深在20世纪下半叶达到峰值。本研究提供了阿拉斯加西南部首个用于反演过去两千年百年尺度气候变率的湖沼学记录。