Winter-spring Warming Over the Past 2,000 Years: Alkenone Temperature Reconstruction from Southwest Iceland

Temperature reconstructions from the northern high latitudes commonly use proxies that record summer or mean annual temperature changes, biasing our view of past temperature variations. In this study, we use a robustly-dated lake sediment record from southwest Iceland to develop a high-resolution winter-spring temperature reconstruction for the past 2,000 years. We employ alkenones, long-chain ketones, produced by Group I and II Isochrysidales in lakes. To obtain a temperature record solely based on Group I alkenones, we use an alkenone mixing ratio, the RIK₃₇ index, to identify periods with only Group I alkenones. Group I Isochrysidales produces alkenones in the spring as lake ice starts to melt, thus recording spring water temperatures. Tests with a lake energy balance model indicate that spring water temperatures are sensitive to temperature perturbations during the winter and early spring seasons, and thus provide insight to cold season temperatures. Over the past 2,000 years, our cold season temperature record shows a long-term warming trend as well as centennial variations in temperature. The warming trend is contrary to the winter-spring trends observed in the Last Millennium Ensemble and to many compilations of Northern Hemisphere temperature. However, there is some evidence of wintertime warming over the past 2,000 years in parts of the North Atlantic region. In addition, rising greenhouse gases and increasing winter-spring solar insolation throughout the Holocene would support warming during winter and spring.