A 50,000-YEAR LAND SNAIL ISOTOPIC RECORD OF PLEISTOCENE-HOLOCENE ENVIRONMENTAL CHANGE IN THE CANARY ARCHIPELAGO

The oxygen isotopic compositions of Late Quaternary snail shells from the eastern Canary Islands (28-29ºN; elevation 2-360 m) were measured to estimate environmental shifts over the last 50,000 years. The fossil shell δ¹⁸O values range from -1.9 to 4.5‰, which exceeds the range of modern shells (-0.3 and 2.5‰) from that locale. Average shell δ¹⁸O values of various age intervals were as follows: 0.3-0.5‰ before the Last Glacial Maximum (LGM), 1.3-1.8‰ during the LGM (15-22 kyr BP in this study), and 0.4-0.8‰ in the Holocene. The associated sediment matrix carbonate shows a similar, but subdued, pattern of oxygen isotope variation. Calculations performed using a published land snail evaporative steady-state flux balance model suggest that the higher shell δ¹⁸O values (individual shell values up to 4.5‰) of the LGM can be explained by lower relative humidity, together with higher δ¹⁸O values of rain and the effect of lower temperature on the shell-water oxygen isotope fractionation factor. The inference that the Canary Islands were drier (and cooler) during the LGM is in agreement with published studies of North African LGM climate. The higher δ¹⁸O values reported here for the LGM land snail shells in the Canary Islands echo published observations of higher δD or δ¹⁸O values measured for LGM groundwater or proxies (tree cellulose and paleosol carbonate) of precipitation from ice-free North America and southwestern Africa. However, the relative δ¹⁸O maximum in the land snail LGM data contrasts with a relative minimum for published LGM δ¹⁸O values from Greenland ice cores. Seasonal specificity in the ice-free records, differing responses to environmental variables other than temperature and meteoric water isotope ratios, and/or differences in LGM to Holocene climatic transitions between low/mid-latitudes and high latitudes may play a role in these contrasting patterns of glacier and “ice-free” measured isotopic variations.