Periwinkle (Littorina littorea) intrashell delta O-18 and delta C-13 records from the mid-Holocene Limfjord region, Denmark: a new high-resolution palaeoenvironmental proxy approach
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Shells of the common intertidal gastropod Littorina littorea both of recent and mid-Holocene origin were analysed along their growth spirals for seasonally induced variations in stable oxygen and carbon isotope compositions. The subfossil specimens are from the Stone Age kitchen midden in Ertebolle, northern Denmark, dated between 5970 +/- 95 and 5070 +/- 90 BP The recent specimens were collected along a west-east salinity gradient within the Limfjord. A Limfjord mixing line of 0.25 parts per thousand. change in delta(18)O (V-SMOW) for each salinity unit was determined. The recent Littorina periwinkles display a calcite relationship of - 0.22 parts per thousand change in shell delta(18)O per degrees C change in temperature, as derived from the Littorina equilibrium equation (versus V-PDB): delta(18)O((Littorina)) - delta(18)O((water)) = 5.250 - 0.224T (degrees C). By using this linearity together with maximum ('winter') isotopic results, we extracted a Littorina growth stop isotherm equivalent to a temperature of 3.7 +/- 1 degrees C. In order to constrain the climate and salinity in the Limfjord region during the mid-Holocene, we used this regression-derived growth stop isotherm as a boundary condition. The subfossil shells indicate a summer surface-water temperature close to 22 +/- 1 degrees C together with a salinity of 31 +/- 1 PSU. These results suggest a more pronounced water exchange between the central Limfjord and the North Sea during the late Atlantic time, and in terms of temperature they indicate 2-4 degrees C warmer conditions than today.