Misarti Nicole, Gier Elizabeth, Finney Bruce, Barnes Kelli, McCarthy Matthew
Water and Environmental Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, USA.
Physical and Biological Sciences, Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, USA.
Rapid Commun Mass Spectrom. 2017 Nov 30;31(22):1881-1891. doi: 10.1002/rcm.7963.
Reconstructing stable isotope (SI) ratios at the base of paleo-food webs is often challenging. For coastal systems, the SI ratios of organic matter in archeological shell represents a possible solution, providing a direct record of primary consumer SI ratios in the littoral zone. However, shell is often porous, with organic compounds susceptible to diagenetic alteration or contamination. If molecular isotopic information is well preserved, compound-specific amino acid isotope analysis (CSI-AA) has the potential to provide direct proxies for baseline SI ratios, bypassing many contamination issues, and to allow assessment of the diagenetic state.
We collected shell from both archeological middens and nearby littoral zones in coastal Alaska, and used a simple organic extraction approach based on decalcification with sequential weak HCl additions to liberate organic material. We measured CSI-AA patterns, molar AA distributions, and the CSI-AA degradation parameter (ΣV), in the context of bulk SI ratios in fossil shell, modern shell, and soft tissue from five common taxa (urchin, limpet, mussel, periwinkle, chiton).
CSI-AA patterns in both soft tissue and shell were consistent with primary consumers, and were indistinguishable in most modern and fossil shell pairs, showing that amino acid δ N values can be well preserved in archeological shell. AA molar distributions were also similar, although most fossil shell was enriched in Asx and Gly. Comparison between CSI-AA results from modern specimens confirmed that the source AA group (tracking isotopic baselines) are transferred without substantial modification into the shell record. In contrast, the Trophic AA group had elevated δ N values in shell versus soft tissue for all taxa examined, suggesting that a correction factor will be required for any CSI-AA proxies using these AAs.
Overall, this new data indicates that the CSI-AA analysis of fossil shell represents a promising new approach to determining isotopic baselines in coastal paleo-ecosystems.
重建古食物网底部的稳定同位素（SI）比值通常具有挑战性。对于沿海系统而言，考古贝壳中有机物的SI比值是一种可能的解决方案，可提供滨海带初级消费者SI比值的直接记录。然而，贝壳通常具有多孔性，其有机化合物易受成岩作用改变或污染。如果分子同位素信息保存良好，化合物特异性氨基酸同位素分析（CSI - AA）有可能提供基线SI比值的直接代理指标，绕过许多污染问题，并能够评估成岩状态。
我们从阿拉斯加沿海的考古垃圾堆和附近的滨海带收集了贝壳，并采用一种基于依次添加稀盐酸脱钙的简单有机提取方法来释放有机物质。我们在来自五个常见分类群（海胆、帽贝、贻贝、滨螺、石鳖）的化石贝壳、现代贝壳和软组织的总体SI比值背景下，测量了CSI - AA模式、氨基酸摩尔分布以及CSI - AA降解参数（ΣV）。
软组织和贝壳中的CSI - AA模式均与初级消费者一致，并且在大多数现代和化石贝壳对中无法区分，这表明氨基酸δN值在考古贝壳中能够得到很好的保存。氨基酸摩尔分布也相似，尽管大多数化石贝壳中的天冬氨酸和甘氨酸含量有所增加。现代样本的CSI - AA结果之间的比较证实，源氨基酸组（追踪同位素基线）在转移到贝壳记录中时没有实质性改变。相比之下，在所研究的所有分类群中，贝壳中的营养氨基酸组的δN值相对于软组织有所升高，这表明对于使用这些氨基酸的任何CSI - AA代理指标都需要一个校正因子。
总体而言，这些新数据表明，对化石贝壳进行CSI - AA分析是确定沿海古生态系统同位素基线的一种有前景的新方法。
