AMINOCHRONOLOGY AND TIME-AVERAGING IN QUATERNARY LAND SNAIL ASSEMBLAGES FROM THE MADEIRA ARCHIPELAGO

Understanding the properties of time-averaging (age mixing) in a geologic stratum is essential for properly interpreting the paleofauna preserved in the geologic record. This work assesses the age and quantifies the scale of time-averaging in Madeira (Portugal) land-snail-rich deposits by dating individual shells using amino acid racemization (AAR) calibrated with radiocarbon. Snail shells were collected from eleven colluvial soil sites on the semi-arid and uninhabited islands of Bugio and Deserta Grande. These islands are home to a rich indigenous malacofauna with abundant Quaternary fossils suitable for further paleoenvironmental and paleoecological studies.

Analyses were performed on 20 shells of the dominant and well-preserved genus Actinella. Amino acid D/L ratios were calibrated with rapid, low-precision AMS radiocarbon at UC Irvine, using power function regression. The D/L ratios of glutamic acid showed the least variance from the calendar-year-calibrated radiocarbon age (r-squared=0.96) and were used to estimate the ages of fossil snails from Madeira. The results suggest that shells range in age from ~9200 to ~35,700 cal yr BP and show some examples of multi-millennial-scale time averaging throughout the investigated colluvial Quaternary soils. This research illustrates that time-averaging in land snail shells can be credibly quantified using AAR, which could be invaluable in future studies on taphonomy, paleoclimate and paleodiversity using these shelly assemblages.