Scale and Structure of Time-Averaging (Age Mixing) in Terrestrial Gastropod Assemblages from Quaternary Eolian Deposits of the Eastern Canary Islands
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Scale and structure of time-averaging (age mixing) in terrestrial gastropod assemblages from Quaternary eolian deposits of the eastern Canary Islands Yurena Yanesa'*, Michal Kowalewskia, José Eugenio Ortiz , Carolina Castillo0, Trinidad de Torres b, Julio de la Nuezd a Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA, 24061, US Biomolecular Stratigraphy Laboratory, Escuela Técnica Superior de Ingenieros de Minas de Madrid, C/ Ríos Rosas 21, 28003, Madrid, Spain c Departamento de Biología Animal, Facultad de Biología, Universidad de La Laguna, Avda. Astrofísico Feo. Sánchez, s/n. 38206, La Laguna, Tenerife, Canary Islands, Spain Departamento de Edafología y Geología, Facultad de Biología, Universidad de La Laguna, Avda. Astrofísico Feo. Sánchez, s/n. 38206, La Laguna, Tenerife, Canary Islands, Spain 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 left- skewed distributions, suggestive of different dynamics of age mixing in marine versus terrestrial shell accumulations. 1. Introduction mates calibrated against radiocarbon (14C) have been established in recent years as a successful strategy for Due to time-averaging (age mixing), fossils collect quantifying time-averaging by allowing for affordable, ed from a single stratigraphic layer tend to vary in age precise and accurate dating of large numbers of speci over multi-centennial to multi-millennial timescales, mens from late Quaternary deposits (e.g., Kowalewski and many individual specimens may be hundreds et al., 1998; Carroll et al., 2003; Kidwell et al., 2005; or even thousands of years older than the deposits Barbour Wood et al., 2006). in which they are contained (for recent reviews see To estimate the magnitude of time-averaging, we Kidwell, 1998; Behrensmeyer et al., 2000; Kowalewski have used a combination of amino acid and radiocarbon and Bambach, 2003). In late Quaternary deposits, dating techniques. Radiocarbon-calibrated amino acid where individual fossils can be dated with a sufficient racemization/epimerization rates are among the most precision, multiple dating projects conducted recently efficient techniques for dating individual Quaternary suggest that time-averaging is a prevailing phenome shells (e.g., Goodfriend, 1987; Kowalewski et al., 1998; non for shelly fossil (e.g., gastropods, bivalves, Carroll et al., 2003; Kidwell et al., 2005; Ortiz et al., brachiopods, etc.) from diverse depositional settings, 2006; and numerous references therein). The shell age including marine (e.g., Flessa et al., 1993; Wehmiller estimates reported in this study are thus methodologi et al., 1995; Meldahl et al., 1997; Kowalewski et al., cally comparable to many other geochronological 1998; Carroll et al., 2003; Fujiwara et al., 2004; datasets published previously (e.g., Goodfriend, 1987, Kidwell et al., 2005; but see Powell et al., 2006), 1989, 1992a,b; Hearty et al., 1992; Hearty, 2003; Cook lacustrine (Cohen, 1989), and terrestrial (e.g., Good- et al., 1993; Engel et al., 1994; Preece and Day, 1994; friend, 1989; Goodfriend and Mitterer, 1993; Good- Goodfriend et al., 1994, 1996; Hearty, 1997; Meyrick friend et al., 1996) systems. The pervasive presence of and Preece, 2001; Brooke et al., 2003a,b; Pigati et al., time-averaging is not surprising given that physical 2004; Hearty et al., 2004). and/or biological processes that mix non-contempora The method is based on the fact that organisms neous shells (e.g., erosional reworking, lateral sediment produce only L-amino acids, which subsequent transport, bioturbation, etc.) affect nearly all deposi ly racemize (epimerize) into D-amino acids. Conse tional settings. quently, the D/L ratio increases with time until an Time-averaging may limit our ability to study ancient equilibrium (D/L=1, but D/L=1.3 for isoleucine) is ecological and environmental patterns (e.g., Kowa reached. The racemization/epimerization process is a lewski, 1996; Goodwin et al., 2004), not only because it first-order-reversible chemical reaction, which is taxon reduces the resolution of the fossil record, but also and temperature-dependent. because averaging can generate false patterns (e.g., The land snail shells are one of the better targets for Kowalewski, 1996). As important, because of age amino acid dating because their non-porous shells are mixing, geochronologic estimates derived by numerical unlikely to suffer notable contamination by soil-derived dating of shells may overestimate the actual age of organic matter (Hare and Mitterer, 1968). In addition, stratigraphic units that are being dated (e.g., Goodfriend, their subaerial habitat makes them less vulnerable to 1989). Consequently, evaluating the magnitude of time- protein hydrolysis when comparing with aquatic shell averaging is essential for both reconstructing past envi fish (Goodfriend, 1987). ronments and ecological conditions as well as assessing Here, we present quantitative estimates of time-av geochronological inaccuracies inherent to dated speci eraging in land snail assemblages found in Quaternary mens from time-averaged accumulations. eolian deposits from the Canary Islands, Spain. We have The scale of time-averaging is not easily estimated applied a combination of AAR and radiocarbon to date in the fossil record and may vary across depositional numerous shells of helicid land snails from multiple dune environments (e.g., Kidwell et al., 2005) or taxa (e.g., and paleosol levels. The approach provides an opportu Kowalewski, 1996). While several indirect techniques nity to estimate directly the scale and structure of time may be attempted to estimate the scale of time-averaging averaging for numerous horizons and multiple section (e.g., Cummins et al., 1986; Flessa, 1993; Cummins, representing Late Quaternary terrestrial environments 1994; Goodwin et al., 2004), direct estimates based that span the last 50,000 years. We augment the analysis on numerical dating of individual fossils appear to be by applying computer-intensive statistical approaches, the most reliable quantitative strategy (see Flessa, 1993; which serve to evaluate the significance of time-av Kowalewski and Bambach, 2003). In particular, the eraging in the context of age imprecision inherent to the amino acid racemization/epimerization (AAR) esti applied dating techniques. 2. Study area, materials, and methods and sampled (Fig. 1; Table 1). Each section is named using a three-letter acronym combining the name of 2.1. Geographical setting island-islet on which that section is located and the name of site where the section was sampled. For example, Thick sequences of Quaternary sediments, which Barranco de los Encantados section from Fuerteventura consist of alterations of paleosols and eolian sand Island is named FBE. Additional details about each deposits, crop out on the eastern islands of the Canary island and stratigraphic section figures are provided in Archipelago (Fig. 1). These sediments have been Ortiz et al. (2006). attributed to alternating humid and arid periods during the Quaternary (Meco et al., 1997; De La Nuez et al., 2.2. Materials and methods 1997; Castillo et al., 2002) and preserve a rich fossil record of vertebrates and invertebrates. Among the We have focused primarily on shells of the helicid invertebrates, the great abundance of land snail shells gastropod Theba geminata (Mousson) (Fig. 2A). How (e.g., Yanes etal., 2004), frequent brood cells of solitary ever, in one locality (MCG-1), analyzed specimens also bees (Ellis and Ellis-Adam, 1993; Edwards and Meco, included the second congeneric species Theba arinagae 2000; Alonso-Zarza and Silva, 2002), and nests