LATE HOLOCENE MARINE CLIMATE ARCHIVES FROM VIKING AND MEDIEVAL AGE SHELLS, ORKNEY ISLANDS, SCOTLAND

Proxy records reconstructing marine climatic conditions during the Medieval Climate Anomaly (MCA; ~900-1350 AD) and Little Ice Age (LIA; ~1350-1850 AD) are strongly biased towards decadal to annual resolution and summer/growing seasons. Here we present new archives of seasonal variability in North Atlantic sea surface temperature (SST) from shells of the European limpet, Patella vulgata, which accumulated in Viking and medieval shell and fish middens at Quoygrew on Westray, Orkney. SST was reconstructed at submonthly resolution using oxygen isotope ratios preserved in shells from the 12^th and mid 15^th centuries (MCA and LIA, respectively). MCA shells recorded warmer summers and colder winters by ~2ºC relative to the late 20th Century (1961-1990). Therefore, seasonality was higher during the MCA relative to the late 20th century. Without the benefit of seasonal resolution, SST averaged from shell time series would be weighted toward the fast-growing summer season, resulting in the conclusion that the early MCA was warmer than the late 20th century by ~1°C. This conclusion is broadly true for the summer season, but not true for the winter season. Higher seasonality and cooler winters during early medieval times may result from a weakened North Atlantic Oscillation index. We will test the hypothesis that LIA shells have cooler summers and winters relative to the MCA shells and late 20^th century. Our findings provide a new test for the accuracy of seasonal amplitudes resulting from paleoclimate model experiments.