Tracing winter temperatures over the last two millennia using a north-east Atlantic coastal record

Abstract

We present 2500 years of reconstructed bottom water temperatures (BWT) using a fjord sediment archive from the north-east Atlantic region. The BWT represent winter conditions due to the fjord hydrography and the associated timing and frequency of bottom water renewals. The study is based on a ca. 8m long sediment core from Gullmar Fjord (Sweden), which was dated by 210Pb and AMS 14C and analysed for stable oxygen isotopes (δ18O) measured on shallow infaunal benthic foraminiferal species Cassidulina laevigata d'Orbigny 1826. The BWT, calculated using the palaeotemperature equation from McCorkle et al. (1997), range between 2.7 and 7.8°C and are within the annual temperature variability that has been instrumentally recorded in the deep fjord basin since the 1890s. The record demonstrates a warming during the Roman Warm Period ( ∼ 350BCE-450CE), variable BWT during the Dark Ages ( ∼ 450-850CE), positive BWT anomalies during the Viking Age/Medieval Climate Anomaly ( ∼ 850-1350CE) and a long-term cooling with distinct multidecadal variability during the Little Ice Age ( ∼ 1350-1850CE). The fjord BWT record also picks up the contemporary warming of the 20th century (presented here until 1996), which does not stand out in the 2500-year perspective and is of the same magnitude as the Roman Warm Period and the Medieval Climate Anomaly.