Calibration of the oxygen isotope ratios of the gastropods Patella candei crenata and Phorcus atratus as high-resolution paleothermometers from the subtropical eastern Atlantic Ocean

The oxygen isotope composition (δ18O) recorded in shells of the gastropods Patella candei crenata and Phorcus atratus from the Canary Islands (27-29°N) potentially provide invaluable high-resolution paleoclimatic data. However, because these two species have never been studied isotopically, it is necessary to calibrate and validate this approach by using live-collected specimens that can be compared to present-day climate data. For P. candei crenata, live organisms were collected at 15-day intervals for nearly one year (between 2011 and 2012) from the rocky-intertidal coast of SE Tenerife along with sea surface temperatures (SST) and seawater δ18O values. The δ18O values of the last growth episode along the shell growth margin, representing the conditions when organisms were collected, illustrate that P. candei crenata δ18O values were 1.3 ± 0.2‰ higher than expected values from isotopic equilibrium. This finding resulted in estimated SST 5.7 ± 0.6 °C lower than observed values. This offset or “vital effect” is uniform and predictable, and therefore can be corrected by subtracting 1.3‰ from the measured shell δ18O value. Adjusted temperatures from the shell coincided with observed SST in the study area. Therefore, Patella candei crenata shells are reliable repositories of SST data, in agreement with other species of Patella from higher latitudes. For Phorcus atratus, a live specimen was analyzed isotopically along shell-growth direction. The intrashell δ18O values showed the entire range of measured SST in the region without an apparent “vital effect”, in correspondence with other Phorcus species from the Mediterranean. Both Patella candei crenata and Phorcus atratus are common components of archaeological and Pleistocene-Holocene paleontological sites across the Canary Archipelago. This study suggests that well-preserved shells of these two taxa retrieved from ancient settings have the capacity to serve as excellent high-resolution paleotemperature archives for the tropical/subtropical eastern Atlantic region.