EXTRACTING SEASONAL TEMPERATURE VARIABILITY FROM ARCHAEOLOGICAL LIMPET SHELLS (PATELLA VULGATA), CROIG CAVE, ISLE OF MULL, SCOTLAND

Archaeological shell deposits provide a rich source of climate proxy data preserved as oxygen isotope ratios (δ¹⁸O) in shell carbonate. Croig Cave on the Isle of Mull, Scotland, contains a nearly continuous accumulation of shells ranging from 800 BC-500 AD and possibly older. This range represents a broad chronology of human use from the late Bronze to Iron Ages and spans the Roman Warm Period (RWP) through Little Ice Age (LIA) climate episodes. Here, we present preliminary data on the seasonal temperature variability based on δ¹⁸O values from two limpet shells (Patella vulgata), one from the RWP and one from the LIA.

We selected 30 shells for AMS dating and received dates on 14 shells so far, ranging from 560-680 cal yr BP to 3330-3480 cal yr BP. Based on the AMS results, the RWP shell was 2060-1870 cal yr BP and the LIA shell was 680-560 cal yr BP. Assuming a δ¹⁸O_water value of +0.1‰, the coldest winter temperature recorded in the RWP shell was 5.6^oC and the warmest summer temperature was 13.4^oC. The coldest winter temperature recorded in the LIA shell was 3.6^oC and the warmest temperature was 16.1^oC. Both shells were microsampled using equivalent resolution (i.e. ~20 samples/yr). Therefore, the smaller amplitude recorded in the RWP shell likely reflects a lower seasonal range compared to the LIA shell, and is not a consequence of slowed ontogenetic growth rates. We compared our estimated temperatures to weekly records of modern sea surface temperature (SST) observed near the study area from 1990 to 2009. The average summer SST is 15.03±0.64^oC (n=20) and the average winter SST is 7.36±0.52^oC (n=20). Therefore, the RWP shell recorded colder summers and slightly colder winters and the LIA shell recorded slightly warmer summers and colder winters than today.