Late Pleistocene-Holocene environmental conditions in Lanzarote (Canary Islands) inferred from calcitic and aragonitic land snail shells and bird bones

摘要

Aragonitic and calcitic land snails from carbonate-rich paleosols in northwestern Lanzarote (Canary Islands) were analyzed for ~(13)C/~(12)C and ~(18)O/~(16)O ratios to deduce the Pleistocene-Holocene transition in the westernmost Sahara zone. Modern, mid-late Holocene (~2.1-5.5calka BP) and late Pleistocene (~23.3-24.0calka BP) aragonitic shells exhibited respective values of -9.5±1.6‰, -7.7±1.5‰, and -2.3±2.8‰ for δ~(13)C; and +0.3±0.3‰, +0.1±0.7‰, and +2.5±0.4‰ for δ~(18)O. Holocene and Pleistocene calcitic shells of the endemic slug Cryptella canariensis showed respective values of -0.7±2.6‰ and -8.5±2.5‰ for δ~(13)C; and +0.8±1.5 and +3.6±0.4‰ for δ~(18)O. Both aragonitic and calcitic shells showed equivalent temporal isotopic trends. Higher δ~(13)C values during ~23.3-24.0calka BP suggest higher abundance of C_4 and/or CAM plants, likely associated with drier conditions and/or lower atmospheric CO_2 concentration. Maximum shell δ~(18)O values during ~23.3-24.0calka BP opposes minimal values of Greenland ice cores and probably reflect the combined effects of (1) higher rain δ~(18)O values linked to higher glacial seawater δ~(18)O values and/or larger snail activity during summer seasons; (2) relative humidity values similar or slightly lower than at present; (3) higher evaporation rates; and (4) cooler temperatures. Bone remains of the extinct Dune Shearwater Puffinus holeae were only recovered from the Holocene bed. Collagen δ~(13)C and δ~(15)N values (-13.5±0.2‰[PDB] and +13.7±1.0‰[air], respectively) match with the signature of a low trophic level Macaronesian seabird that fed upon local fish. Bone carbonate δ~(13)C (-7.4±1.0‰[PDB]) and phosphate δ~(18)O (+18.2±0.4‰[SMOW]) values exhibited pristine signals denoting their potential value in future paleoenvironmental studies in the region. The age of P. holeae (~2.1-2.7calka BP) supports that the aboriginal population possibly caused its extinction. In contrast, the extinction of the endemic helicid Theba sp. (~23.3-24.0calka BP) was likely caused by environmental change.