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Aminochronology and Time-Averaging of Quaternary Land Snail Assemblages from Colluvial

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Aminochronology and Time-Averaging of Quaternary Land Snail Assemblages from Colluvial Aminochronology and time-averaging of Quaternary land snail assemblages from colluvial soils in the Madeira Archipelago A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology McMicken College of Arts and Sciences June 25th, 2018 By: Evan New B.A. Albion College of Michigan Advisory Committee: Yurena Yanes, Ph.D. – Committee Chair Carlton Brett, Ph.D. Andrew Cjaza, Ph.D. i Abstract Understanding the properties of time-averaging (age mixing) in a geologic stratum is essential for interpreting the paleofauna preserved in the geologic record. This work assesses the age and quantifies the scale and structure of time-averaging of land-snail-rich colluvial soils from the Deserta Islands, Madeira Archipelago (Portugal) by dating numerous individual shells using amino acid racemization (AAR) calibrated with graphite-target and carbonate-target accelerator mass spectrometry (AMS) radiocarbon methods. Snail shells were collected from seven sites on the semi-arid islands of Bugio and Deserta Grande. These islands are home to a rich indigenous malacofauna with abundant Quaternary fossils suitable for paleoenvironmental and paleoecological studies. Analyses were performed on shells of the dominant and highly abundant species Actinella nitidiuscula (Gastropoda: Hygromiidae). A radiocarbon calibration using D/L ratios of glutamic acid showed the lowest residuals relative to other amino acids and were used to estimate the ages of non-radiocarbon dated fossil snails. Results show that the fossil shells ranged in age from modern to ~48 cal ka BP, covering the last glacial and modern interglacial periods. Snail assemblages retrieved from two of these colluvial soils exhibited multi-millennial- scale time-averaging, which in turn may inform time estimates of colluvial soil formation. Different age-frequency distributions between the two deposits, despite similar taphonomic conditions, suggest variability in colluvium formation is a key driver of time averaging in such deposits. This study presents the first carbonate-target AMS radiocarbon results for land snail shells and suggests that this novel method appears to offer reliable age estimates for small land snail shells for at least ~20 cal ka BP. ii iii Table of Contents Abstract…………………………………………………………………………………………..ii List of Tables…………………………………………………………………………………….vi List of Figures…………………………………………………………………………………..vii 1. Introduction……………………………………………………………………………………1 2. Materials and Methods………………………………………………………………………..4 2.1. Geographical and environmental setting……………………………………………....4 2.2. Materials………………………………………………………………………………....5 2.3. Amino acid racemization dating………………………………………………………..6 2.4. Radiocarbon dating……………………………………………………………………..7 2.5. AAR calibration…………………………………………………………………………8 2.6. Artificial time-averaging……………………………………………………………....10 3. Results………………………………………………………………………………………...10 3.1. Graphite-target and carbonate-target AMS results…………………………………10 3.2. Establishing a calibration equation for AAR values…………………………….......11 3.3. Quantifying age-mixing of snail-rich colluvial soils………………………………….11 4. Discussion……………………………………………………………………………........….12 4.1. Scale of time-averaging in Quaternary colluvial soils………………………….........12 4.2. Structure of time-averaging in Quaternary colluvial deposits………………...........14 4.3. Implication for shell geochronology……………………………………………..........16 4.4. Implications for time of soil formation…………………………………………….....17 5. Conclusion…………………………………………………………………………................17 iv References………………………………………………………………………………….........18 Tables…………………………………………………………………………………………....26 Figures…………………………………………………………………………………………...30 v Lists of Tables Table 1. Twenty Actinella land snail shells from Deserta Grande and Bugio analyzed via carbonate-target and graphite-target AMS radiocarbon dating as well as AAR analysis. Table 2. D/L amino acid ratios of twenty land snail shells from Deserta Grande and Bugio used for calibration analysis. Table 3. Comparison of calibrated radiocarbon age derived from graphite-target and carbonate- target AMS radiocarbon dating of Actinella nitidiuscula snail shells from the Desertas islands. Table 4. D/L-glutamic acid values and numerical ages for specimens from two colluvial soils (DG4 and DG3) from Deserta Grande, Madeira, Portugal. Ages were derived from the calibration equation: Age = 507419 ∙ D/L Glu (2.357). vi List of Figures Figure 1. Location of Desertas Island chain, Madeira Archipelago. Circles indicate collection sites from Deserta Grande and Bugio dated in this study. Figure 2. Quaternary snail-rich colluvial soils from Madeira. (A) Deserta Grande slopes including site DG-4 (Indicated by arrow). (B) Site DG-4, showing collection of colluvium along the slope. (C) Slope peaks of Bugio including site Bugio-Y (Indicated by arrow). (D) Colluvium soil at site Bugio-Y. (E) Detailed view of colluvium (unconsolidated sediment) rich in fossil snail shells at site Bugio X. (F) Actinella nitidiuscula shell in colluvium soil at site Bugio X). Figure 3. Land snail species selected for AAR and 14C AMS for analysis. (A) A modern example of Actinella nitidiuscula nitidiuscula, and (B) a fossil sample of Actinella nitidiuscula saxipotens. Figure 4. Comparison of carbonate-target to graphite-target AMS results of four Pleistocene and two early Holocene Actinella nitidiuscula shells (R2=0.9852). Dashed line depicts 1:1 line. Error bars represent standard deviation. Figure 5. Glutamic acid (Glu) D/L ratio against calibrated AMS radiocarbon age in calendar years, utilizing both carbonate and graphite-target radiocarbon dating methods. Line represents best fit obtained with power regression function. Figure 6. Age-frequency distributions of individual shell ages from Madeira Archipelago site vii DG3 (A) and DG4 (B). The bin size was chosen based on an age range that would eliminate the most gaps while also preserving structure of age distribution. viii 1. Introduction Time-averaging, the age mixing of non-contemporaneous fossils, is an inherent quality of most geologic strata and is important for understanding sedimentary and taphonomic processes (Kowalewski, 1996). When unknown, time-averaging on significant scales may generate difficulties in examining geologic horizons, such as condensing accumulations of fossils from a broad period of time into a relatively thin stratum to give the appearance of contemporaneity (Kidwell and Bosence, 1991). As a result, fossils collected from a single bed may have been deposited hundreds or thousands of years apart. Age-mixing can also generate false patterns of fossil distribution through time, such as giving an assemblage that spans a broad period of time the appearance of comparative contemporaneity between taxa. Quantifying time-averaging is therefore an essential step during any interpretation of deposits in which time plays a factor such as ecological, taxonomical, or climatological analysis (Bush et al., 2002; Kowalewski, 1996; Kowalewski et al., 1998). More importantly, not all taxonomic groups preserved within a deposit will necessarily exhibit equivalent degrees of age-mixing. Thus, specimens with durable hard parts (e.g., molluscs) are expected to exhibit significant (multi-millennial) time averaging, while less durable skeletons (e.g., echinoderms) are likely accumulated across more limited timespans due to higher taphonomic decay rates (Kowalewski et al., 2017). Similarly, the remains of short- lived organisms (such as those with a sub-decadal lifespan), are more likely to be significantly time-averaged than long-lived organisms due to the differences in lifespan compared to deposit formation. When examining fossils of short-lived organisms, as is the case with the subjects of many Holocene studies, the likelihood of significant age mixing within a deposit increases (Goodwin et al., 2004; Kidwell and Bosence, 1991). Consequently, understanding the extent of time-averaging is necessary for paleoecological and paleoenvironmental reconstructions. 1 Time-averaging can be defined by scale, i.e., the range of years across which age-mixing has occurred, and by structure, i.e. the distribution of specimen frequency through the observed age range of the sample (Kowalewski et al., 1998; Meldahl et al., 1997). The standard method to evaluate the scale and structure of time-averaging is by directly age-dating individual fossils within a deposit and using the results to infer the extent and shape of age mixing (Flessa et al., 1993; Kowalewski and Bambach, 2008). Much of our knowledge of scale and structure of time- averaging of fossils is based on Holocene (last 11,700 yr) remains of marine bivalves (e.g. Dominguez et al., 2016; Kidwell, 1998; Kidwell et al., 2005), brachiopods ( Carroll et al., 2003), and more recently echinoderms (Kowalewski et al., 2017 ). Estimates of time-averaging in continental fossil assemblages have received less focus. Some published studies of time- averaging in terrestrial fossils from both the Holocene and the Mid-Late Pleistocene include small mammal assemblages (Terry, 2015), and land snails (Yanes et al., 2007). Land snail shells are abundant in many Quaternary sedimentary settings. The usefulness of land snail shells as paleoecological and paleoclimatic proxies are due to their abundance, ubiquity in numerous terrestrial environments, and their sensitivity to environmental and climatic factors (Goodfriend, 1989). However, to prevent false
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