Citation
Torres Pérez-Hidalgo, Trinidad José
(2007).
Scale and structure of time-averaging (age mixing) in terrestrial gastropod assemblages from Quaternary eolian deposits of the eastern Canary Islands.
"Palaeogeography, Palaeoclimatology, Palaeoecology", v. 251
(n. 2);
pp. 283-299.
ISSN 0031-0182.
https://doi.org/10.1016/j.palaeo.2007.04.002.
Abstract
Quantitative estimates of time-averaging (age mixing) in gastropod shell accumulations from Quaternary (the late Pleistocene
and Holocene) eolian deposits of Canary Islands were obtained by direct dating of individual gastropods obtained from
exceptionally well-preserved dune and paleosol shell assemblages. A total of 203 shells of the gastropods Theba geminata and
T. arinagae, representing 44 samples (= strati graphic horizons) from 14 sections, were dated using amino acid (isoleucine)
epimerization ratios calibrated with 12 radiocarbon dates. Most samples reveal a substantial variation in shell age that exceeds the
error that could be generated by dating imprecision, with the mean within-sample shell age range of 6670 years and the mean
standard deviation of 2920 years. Even the most conservative approach (Monte Carlo simulations with a non-sequential Bonferroni
correction) indicates that at least 25% of samples must have undergone substantial time-averaging (e.g., age variations within those
samples cannot be explained by dating imprecision alone). Samples vary in shell age structure, including both left-skewed (17 out
of 44) and right-skewed distributions (26 out of 44) as well as age distributions with a highly variable kurtosis. Dispersion and
shape of age distributions of samples do not show any notable correlation with the stratigraphic age of samples, suggesting that the
structure and scale of temporal mixing is time invariant. The statistically significant multi-millennial time-averaging observed here
is consistent with previous studies of shell accumulations from various depositional settings and reinforces the importance of dating
numerous specimens per horizon in geochrono logical studies. Unlike in the case of marine samples, typified by right-skewed age
distributions (attributed to an exponential-like shell loss from older age classes), many of the samples analyzed here displayed leftskewed
distributions, suggestive of different dynamics of age mixing in marine versus terrestrial shell accumulations.