Collaborative Research: Radiocarbon Dating of North American Terrestrial Gastropod Shells

Jeffrey S. Pigata，亚利桑那大学（EAR-0614840）Jason A. Rech，迈阿密大学（EAR-0614647）Jeffrey C. Nekola，新墨西哥大学（EAR-0614903）摘要陆栖腹足类动物是地球上最成功的动物群体之一。它们在各种各样的栖息地繁衍生息，从沼泽和草原到高地森林和北极苔原。它们在化石记录中的分布也同样多样化。腹足类贝壳常见于湖泊、湿地、冲积、黄土和冰川沉积物以及世界各地考古遗址的沉积物中。然而，尽管腹足类动物的壳在化石记录中广泛存在，而且它们含有约12%重量的碳，但通常避免对其进行14C测年，因为许多分类群在建造壳时从石灰石中吸收了缺乏14C（或“死亡”）的碳。地质年代学家将此称为“石灰岩问题”，因为腹足类动物壳中的死碳含量在所有分类水平上都是可变的，这可能导致14C年龄超过3000年。然而，PIs在美国西南部的初步结果表明，一些小型（最大体长10毫米）腹足类动物即使在石灰岩很容易获得的情况下也会避开石灰岩。此外，在干旱环境中，这些相同的分类群在地质时间尺度上似乎保持着封闭的碳系统。因此，至少在干燥的条件下，一些小陆生腹足类动物的壳实际上可能提供可靠的14C年代。本研究的主要研究目的是明确识别能够持续产生可靠的14C年龄的腹足类动物，而不受当地岩性、环境条件、气候或生态栖息地的影响。为此，pi将对一套北美特有的现代和化石陆生腹足类动物的14C含量进行系统分析。他们以现存的30多个物种为目标，将用于回答以下研究问题：(1)特定分类单元的个体在建造外壳时是否总是避开石灰岩；(2)如果是这样，它们的外壳是否长期保持封闭系统，而不管当地的环境条件如何？这项研究广泛的分类学、生态学和地貌学范围将使pi能够确定哪些类型的腹足类贝壳可以用来可靠地确定第四纪沉积物的年代。此外，这种方法将帮助其他研究人员确定更多的目标物种，包括北美以外的物种，通过为鉴定仅使用代谢产生的碳构建外壳的分类群提供先验指导，从而提供准确的14C年龄。

Collaborative Research: Radiocarbon Dating of North American Terrestrial Gastropods ShellsJeffrey S. Pigata, University of Arizona (EAR-0614840)Jason A. Rech, Miami University (EAR-0614647)Jeffrey C. Nekola, University of New Mexico (EAR-0614903) ABSTRACTTerrestrial gastropods are one of the most successful animal groups on Earth. They occupy and thrive in incredibly diverse habitats, ranging from marshes and grasslands to upland forests and arctic tundra. Their distribution in the fossil record is equally diverse. Gastropod shells are commonly found in lake, wetland, alluvial, loess, and glacial deposits, as well as in sediments at archeological sites worldwide. Yet despite their widespread occurrence in the fossil record and the fact that they contain ~12% by weight carbon, gastropod shells are usually avoided for 14C dating because many taxa incorporate 14C-deficient (or "dead") carbon from limestone when building their shells. Geochronologists refer to this as the "Limestone Problem" because the amount of dead carbon incorporated in gastropod shells is variable at all taxonomic levels and can cause 14C ages to be up to 3,000 years too old. However, PIs preliminary results from the American Southwest suggest that some small (maximum body dimension 10 mm) gastropods avoid limestone even when it is readily available. Moreover, these same taxa appear to remain closed systems with respect to carbon over geologic timescales in arid environments. Thus, at least in dry conditions, some small terrestrial gastropod shells may in fact provide reliable 14C ages. The primary research objective of this study is to clearly identify gastropod taxa that are capable of consistently yielding reliable 14C ages, regardless of local lithology, environmental conditions, climate, or ecological habitat. To this end, PIs will conduct a systematic analysis of the 14C content of a suite of modern and fossil terrestrial gastropods that are endemic to North America. They have targeted more than 30 extant species that will be used to answer the following research questions: (1) do individuals of a specific taxon always avoid limestone when building their shells, and (2) if so, do their shells remain closed systems over time regardless of local environmental conditions? The wide taxonomic, ecological, and taphonomic scope of this study will allow PIs to determine what types of gastropod shells can be used to reliably date Quaternary sediments. Moreover, this approach will help other investigators identify additional target species, including those outside North America, by providing a priori guidelines for the identification of taxa that use only metabolically-generated carbon in shell construction and that will hence provide accurate 14C ages.