A seasonally resolved bottom-water temperature record for the period ad 1866-2002 based on shells of Arctica islandica (Mollusca, North Sea)

Existing studies on recent global warming are almost exclusively based on environmental data from the Earth's surface. Seasonal information on the effects of climate change on subsurface settings of mid to high latitudes is extremely scarce. Here, we present the first temperature proxy record from bottom (c. 50 m) water settings of the North Sea employing the oxygen isotope composition of ocean quahog shells. Results indicate that delta(18)O(aragonite) measured across shells of Arctica islandica can provide reliable estimates (+/- 0.25 to +/- 0.4 degrees C) of the ambient bottom water temperatures. Over the period AD 1880-2001, warming trends in bottom waters are of the order of 0.042 to 0.138 degrees C/decade. Apparently, the annual maximum -temperature trend shows a twofold increase over the past four decades (0.236 degrees C/decade) while the minimum-temperature trend has remained relatively stable (0.042 degrees C/decade). During the same time interval, however, annual maximum temperatures at the sea surface quadrupled. Shell oxygen-isotope-derived winter temperatures also provide a proxy for the winter North Atlantic oscillation index (WNAO). Some 28 to 50% of the variability in minimum temperatures below the thermocline can be explained by changes of the WNAO. Our new toot enables testing and verification of climate models prior to the 20th century greenhouse forcing. Copyright (c) 2005 Royal Meteorological Society.