A record of climate over the last millennium based on varved lake sediments from the Canadian High Arctic

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.