NOAA/WDS Paleoclimatology - Lower Murray Lake, Ellesmere Island 1000 Year Varved Sediment Record

A varved sediment record that extends back over the last millennium was recovered from Lower Murray Lake, northern Ellesmere Island, Nunavut, Canada (81°20'N, 69°30'W). Flatbed scan images and backscattered electron images were analysed to provide varve thickness and other quantitative sedimentary indices on an annual basis. In many studies of lakes from the High Arctic, varve thickness is a good proxy for summer temperature and we interpret the Lower Murray Lake varves in this way. On that basis, the Lower Murray Lake varve thickness record suggests that summer temperatures in recent decades were among the warmest of the last millennium, comparable with conditions that last occurred in the early twelfth and late thirteenth centuries, but estimates based on the sediment accumulation rate do not show such a recent increase. The coldest conditions of the 'Little Ice Age' were experienced from ~AD 1700 to the mid-nineteenth century, when extensive ice cover on the lake led to widespread anoxic conditions in the deepest parts of the lake basin. An overall decline in median grain size over the last 1000 years indicates a reduction in the energy available to transport sediment to the lake. Many of these features of the record are also observed in other palaeoclimatic records from the North American Arctic. The very recent appearance of the diatom Campylodiscus, which was not observed throughout the record of the last millennium, suggests that a new threshold in the ontogenetic development of the lake has now been passed.

从加拿大努纳武特地区埃尔斯米尔岛北部的下默里湖（Lower Murray Lake，坐标81°20'N, 69°30'W）获取了一段可追溯至近千年前的纹层沉积物记录（varved sediment record）。研究通过对平板扫描图像与背散射电子图像（backscattered electron images）的分析，获取了年分辨率的纹层厚度（varve thickness）及其他定量沉积学指标。在高北极地区的诸多湖泊研究中，纹层厚度可作为夏季气温的良好代用指标（proxy），本研究亦采用该思路对下默里湖的纹层进行解译。基于该纹层厚度记录，下默里湖所在区域近数十年的夏季气温处于近千年来最高水平之列，与12世纪早期及13世纪晚期的气候状况相当；但基于沉积物沉积速率（sediment accumulation rate）得到的估算结果，并未呈现出此类近期升温趋势。“小冰期（Little Ice Age）”的最冷时段大致为公元1700年至19世纪中期，彼时湖泊被大面积冰盖覆盖，导致湖盆最深区域广泛出现缺氧（anoxic）环境。近千年来沉积物中值粒径（median grain size）的整体下降，表明输入湖泊的沉积物搬运所需的水动力条件有所减弱。该记录中的诸多特征，也在北美北极地区的其他古气候（palaeoclimatic）记录中有所体现。近段时间首次出现的弯楔藻属（Campylodiscus）硅藻（diatom）在整个近千年的沉积记录中从未被观测到，这表明该湖泊的个体发育（ontogenetic development）现已越过全新阈值。