Preservation of stable isotope signatures of amino acids in diagenetically altered Middle to Late Holocene archaeological mollusc shells
Preservation of stable isotope signatures of amino acids in diagenetically altered Middle to Late Holocene archaeological mollusc shells
复制标题
成岩改变的中晚期全新世考古软体动物壳中氨基酸稳定同位素特征的保存
DOI：
10.1016/j.gca.2023.05.005
复制
发表时间：
2023
影响因子：
5
通讯作者：
McCarthy, M.D.
中科院分区：
文献类型：
--
作者：
Vokhshoori, N.L.;Rick, T.C.;Braje, T.J.;McCarthy, M.D.
在考古软体动物壳的蛋白质部分中测量的稳定同位素代理代表了重建近岸环境过去的生态和生物地球化学条件的日益重要的档案。然而，一个主要问题是了解成岩作用对亚化石贝壳同位素值的影响。氮 (δ15N)，尤其是碳 (δ13C) 的“大量”稳定同位素值通常会随着退化壳中 C/N 比的增加而发生剧烈变化，从而导致数据不可靠。在这里，我们检查了一组全新的贝壳古代理、氨基酸化合物特异性同位素 (CSI-AA) 的保存情况。我们研究了来自加州海峡群岛的加州贻贝 (Mytilus californianus) 壳的有机部分的碳 (δ13CAA) 和氮 (δ15NAA) 模式和值。考古贝壳样本的年龄从约从不同的沉积环境（例如暴露的沿海断崖、埋藏地层等）收集了表现出多种降解状态的 6,100 至 250 cal BP，并直接与相同物种和地区的现代贝壳进行比较。我们的结果表明，有机质 C/N 比是贝壳有机部分相对降解状态的最佳整体诊断指标，包括分子水平的变化。现代贝壳的有机碳氮比范围为 2.8 至 3.5，而考古贝壳中的有机碳氮比大幅升高（3.4–9.5），与体积 δ13C 值、重量％C 和重量％N 呈现出强烈且显着的负相关性，与 δ15N 值呈现显着但较弱的相关性。使用弱 NaOH 的额外“清洁”步骤有助于去除可能的外源污染物并提高某些样品的体积值。然而，AA 的相对摩尔丰度表明，一些 AA，尤其是最丰富的两种氨基酸，甘氨酸和丙氨酸，随着 C/N 比的增加而逐渐减少。无论是否去除污染物，这些氨基酸的损失都会永久改变大量同位素值。对由于氨基酸摩尔组成而预期的大量同位素变化进行建模显示，由于选定的 AA 损失，大量 δ13C 值发生了重大且可预测的变化，以及来自外源污染物的同样大但更加可变的影响。与大量数据相比，关键的 CSI-AA 值和模式几乎完全保持不变，即使在最退化的贝壳样本中，也与现代贻贝壳中预期的生物合成同位素模式密切匹配。 “基线”（δ15N-苯丙氨酸和平均 δ13C-必需 AA）和浮游营养结构（δ15N-谷氨酸和 δ15N-苯丙氨酸）的 AA 同位素代理在任何样品中均未随降解而发生统计变化。总体而言，我们的结论是，虽然大量同位素，特别是 δ13C，在 C/N 比高于~4.0 的考古或亚化石壳中很可能不可靠，但 CSI-AA 代理仍然可以用于重建近岸海洋环境过去的气候和生态条件。
Stable isotope proxies measured in the proteinaceous fraction of archaeological mollusc shell represents an increasingly important archive for reconstructing past ecological and biogeochemical conditions of nearshore environments. A major issue, however, is understanding the impact of diagenetic alteration in sub-fossil shell isotope values. “Bulk” stable isotope values of nitrogen (δ15N), and especially carbon (δ13C) often shift strongly with increasing C/N ratios in degraded shell, resulting in unreliable data. Here, we examine preservation of an entirely new set of shell paleo-proxies, compound-specific isotopes of amino acids (CSI-AA). We examine carbon (δ13CAA) and nitrogen (δ15NAA) patterns and values from the organic fraction of California mussel (Mytilus californianus) shells from the California Channel Islands. Archaeological shell samples ranging in age from ca. 6,100 to 250 cal BP exhibiting a wide range of degradation states were collected from varied depositional environments (e.g., exposed coastal bluff, buried strata, etc.), and were directly compared to modern shells of the same species and region.Our results indicate organic matter C/N ratios as the best bulk diagnostic indicator of the relative degradation state of shell organic fraction, including changes at the molecular level. Modern shell organic C/N ratios ranged from 2.8 to 3.5, while those in archaeological shell were substantially elevated (3.4–9.5), exhibiting strong and significant negative correlations with bulk δ13C values, weight %C, and weight %N, and a significant but weaker correlation with δ15N values. An additional “cleaning” step using weak NaOH helped to remove possible exogenous contaminants and improved bulk values of some samples. However, relative molar AA abundances revealed that some AAs, especially the two most abundant, Glycine and Alanine, progressively decreased with increasing C/N ratio. The loss of these amino acids permanently alters bulk isotope values regardless of removal of contaminants. Modeling the bulk isotope change expected due to amino acid molar composition showed major and predictable shifts in bulk δ13C values from selected AA loss, and similarly large but far more variable impacts from exogenous contaminants.In contrast to bulk data, key CSI-AA values and patterns remained almost entirely unaltered, even in the most degraded shell samples, closely matching expected biosynthetic isotope patterns in modern mussel shell. AA isotope proxies for “baseline” (δ15N-Phenylalanine and average δ13C-Essential AAs) and planktonic trophic structure (δ15N-Glutamic Acid and δ15N-Phenylalanine) were not statistically altered with degradation in any sample. Overall, we conclude that while bulk isotopes, particularly δ13C, are very likely to be unreliable in archaeological or subfossil shell with C/N ratios higher than ∼4.0, CSI-AA proxies can still be used to reconstruct past climatic and ecological conditions of the nearshore marine environment.