Collaborative Research: Radiocarbon Dating of North American Terrestrial Gastropod Shells

合作研究：北美陆栖腹足类贝壳的放射性碳测年亚利桑那大学杰弗里·S·皮加塔(EAR-0614840)迈阿密大学杰森·A·雷赫(EAR-0614647)新墨西哥大学杰弗里·C·内科拉(EAR-0614903)ABSTRACT陆栖腹足类是地球上最成功的动物群体之一。它们在令人难以置信的多样化的栖息地中生存和繁衍，从沼泽和草原到高地森林和北极冻土带。它们在化石记录中的分布也同样多样化。腹足类贝壳通常存在于湖泊、湿地、冲积物、黄土和冰川沉积物中，以及世界各地考古遗址的沉积物中。然而，尽管腹足类贝壳在化石记录中广泛存在，而且它们含有约12%的重量碳，但腹足类贝壳通常在14C测年时被避免，因为许多分类群在建造贝壳时包含了来自石灰岩的缺乏14C的碳(或“死”碳)。地质年代学家将此称为“石灰岩问题”，因为腹足类贝壳中的死碳含量在所有的分类水平上都是不同的，可能导致14C年龄比14C年龄高出3000年。然而，来自美国西南部的PI初步结果表明，一些小型(最大身体尺寸为10毫米)腹足类避免使用石灰石，即使它很容易获得。此外，就干旱环境中的碳在地质时间尺度上而言，这些相同的类群似乎仍然是封闭的系统。因此，至少在干燥的条件下，一些小型陆生腹足类贝壳实际上可能提供可靠的14C年龄。这项研究的主要研究目标是清楚地识别腹足类分类群，这些类群能够一致地得出可靠的14C年龄，无论当地的岩性、环境条件、气候或生态生境如何。为此，PIS将对北美特有的一套现代和化石陆栖腹足类的14C含量进行系统分析。他们已经瞄准了30多个现存物种，这些物种将被用来回答以下研究问题：(1)特定分类群的个体在建造贝壳时是否总是避开石灰石，以及(2)如果是这样的话，它们的贝壳是否会随着时间的推移保持封闭系统，而不考虑当地的环境条件？这项研究的广泛的分类学、生态学和挖掘学的范围将使PI能够确定哪些类型的腹足类贝壳可以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识别更多的目标物种，包括北美以外的物种，因为它提供了一个先验指南，用于识别在贝壳结构中只使用代谢生成的碳的分类群，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.