Isotope palaeotemperatures from the Tjِrnes beds in Iceland: evidence of Pliocene cooling

The mid-Pliocene warming episode is well established along the margins of the North Atlantic and probably affected areas as far north as Iceland. Rich marine mollusc faunas have been described from the 500 m of Pliocene siliciclastic deposits on the Tjörnes Peninsula in northeast Iceland. The faunal compositions indicate a development from warm-water to arcto–boreal conditions. We have analysed the oxygen isotope composition of 96 well-preserved specimens of the bivalves Arctica islandica and Pygocardia rustica and interpreted the data as palaeotemperatures. After corrections for the isotopic composition of Pliocene seawater, the temperatures are in accordance with other palaeotemperature proxies. We interpret our data as reflecting a gradual change from warm-water conditions (summer temperatures between 10 and 15°C) during the deposition of the lower part of the Tjörnes beds (Tapes Zone and lower part of Mactra Zone) to cold-water conditions (summer temperatures between 5 and 10°C) at the top (top of Mactra Zone and Serripes Zone). The coldest interval is found in the middle of the Serripes Zone, where summer temperatures (5–8°C) are comparable to those of the present environment off northern Iceland. The arrival to Tjörnes of cold-water molluscs of Pacific origin postdates the cooling of the seas around Iceland. We suggest that the cold interval in the Serripes Zone represents a cooling event prior to the final warm period (ca. 3.3–3.0 Ma [Dowsett et al. (1999) Middle Pliocene paleoenvironmental reconstruction: PRISM2. US Geol. Survey Open File Rep. 99–53]) of the mid-Pliocene warming.