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 Erteb0lle, 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%, change in 8180 (V-SMOW) for each salinity unit was determined. The recent Littorina periwinkles display a calcite relationship of -0.22%, change in shell 6180 per C change in temperature, as derived from the Littorina equilibrium equation (versus V-PDB): 5180(Littorina) - 8180(water) = 5.250 -0.224T (C). By using this linearity together with maximum (‘winter’) isotopic results, we extracted a Littorina growth stop isotherm equivalent to a temperature of 3.71 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 + 1C 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 2-40C warmer conditions than today.

In order to produce CO 2 for stable isotope analyses (d 18 O and d 13 C), carbonate samples are commonly digested in phosphoric acid. The acid recipe here presented is based on phase shifting crystalline orthophosphoric acid of pro-analysis quality to a liquid state through heating, followed by pre-vacuum treatment during a start-up procedure before mass analyses for common acid bath preparation, or adding a small amount of phosphoric pentoxide for single drop equipments, respectively. This methodology results in a final acid concentration of 104%.

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Coastal sea-surface temperature (SST) and sea-surface salinity (SSS), including seasonality, in northwest (NW) Europe during the early phase of the Eemian interglacial ca. 125 ka ago were reconstructed from Littorina littorea (common periwinkle) gastropods. The results were based on intra-annual δ18O analyses in recent and fossil shells, mainly originating from the sea of Kattegat (Sweden) and the English Channel (United Kingdom), and confined to intertidal settings. The Eemian L. littorea shells indicated annual SSTs in the range 8–18°C for the English Channel and 8–26°C for Kattegat. All specimens from the Eemian sites experienced summer SSTs of ca. 1–3°C above recent conditions. The estimated winter SST in the English Channel during the Eemian was comparable to modern measurements of ca. 8°C. However, the Kattegat region displayed Eemian winter SST approximately 8°C warmer than today, and similar to conditions in the western English Channel. The recent-fossil isotope analogue approach indicated high SSS above 35 practical salinity units (psu) for a channel south of England in full contact with the North Atlantic Ocean during the last interglacial. In addition, the Kattegat shells indicated a SSS of ca. 29 psu, which points out a North Sea affinity for this region during the Eemian.

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