PLEISTOCENE-HOLOCENE PALEOCLIMATIC INFERENCES IN THE MADEIRA ARCHIPELAGO INFERRED FROM STABLE ISOTOPE COMPOSITION OF FOSSIL LAND SNAIL SHELLS

Terrestrial snails from the Madeira Archipelago, Portugal, were radiocarbon-dated and isotopically analyzed in order to investigate the Pleistocene-Holocene climatic transition using the oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotope values of Quaternary shells. Numerous pristine shells were retrieved from colluvial soils formed over the last ~20 cal ka BP in Deserta Grande and Bugio islands. δ¹⁸O and δ¹³C values exhibited an overall decline from the Pleistocene to recent. Shell δ¹⁸O values were ~2.9‰ during the Last Glacial Maximum (LGM: 18.6-21.3 cal ka BP). From there, values declined to ~0.7‰ during the Early Holocene (8.2-8.4 cal ka BP) and thereafter further declined to ~0.2‰ at present-day. This trend suggests that climate was significantly drier during the LGM and became wetter during the present interglacial period. Shell δ¹³C values were ~ -6.0‰ during the LGM, ~9.0‰ during the Early Holocene and ~9.8‰ at present-day. This pattern also indicates that during the LGM, the vegetation exhibited increased water stress or alternatively, C₄ plants were more abundant than today. The snail results point to a drier scenario during the LGM coinciding with other proxies developed at nearby regions and illustrate that fossil land snails recovered from volcanic, oceanic archipelagos track valuable climatic transitions in their stable isotope codes.