Stable carbon isotope record of middle to late Holocene climate changes from land snail shells at Hinds Cave, Texas
Stable carbon isotope record of middle to late Holocene climate changes from land snail shells at Hinds Cave, Texas
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DOI：
10.1016/s1040-6182(00)00008-2
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发表时间：
2000-05
影响因子：
2.2
通讯作者：
G. Goodfriend;G. Ellis
中科院分区：
文献类型：
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作者：
G. Goodfriend;G. Ellis
利用得克萨斯州Hinds Cave陆生蜗牛(Rabdotus Ternatus)贝壳碳酸盐和贝壳有机质的稳定碳同位素比值，重建了美国大平原南部较干燥的西部地区(现在的年平均降雨量为400 mm)5000年来的气候历史。AMS对两个地层中每一层的四个单独贝壳进行的放射性碳分析表明，地层中不存在返工或年龄混合的问题。然而，在许多炮弹中，燃烧是显而易见的。氨基酸组成和消旋作用被用来识别已经被显著加热的贝壳，这些贝壳被拒绝进行同位素分析，因为可能改变了同位素信号。对采自Hinds Cave的14个蜗牛样品进行了AMS测年和稳定碳同位素分析，每个样品由2-6个来自同一种源的贝壳组成。此外，对从附近遗址收集的约公元1890年活着的样本进行了分析，以代表现代条件。贝壳有机质含量不随年代变化而下降，说明贝壳有机质保存良好。有机和碳酸盐同位素值显示出相似的时间趋势。最古老的样品(4100-4600 calyr BP)的13C相对贫化。最大δ13C值在约3500 calyr BP时达到。此后，13C有规律的消耗，最近的样品显示δ13C值最低。同位素值随时间的变化相当大(有机质的变化范围为4.5‰)，表明气候发生了实质性的变化。同位素值的变化被解释为主要是由于水分条件的变化，影响了适应干旱的景天酸代谢植物物种(富含13C)的丰度。因此，相对湿润的中全新世条件是指示的，日益干燥的条件在大约3500 calyr BP时达到最大值，随后是整个全新世晚期逐渐潮湿的条件。
Stable carbon isotope ratios of shell carbonate and shell organic matter from land snails (Rabdotus alternatus) from Hinds Cave, Texas, are used to reconstruct the climate history of the drier western part of the southern Great Plains (present mean annual rainfall of 400mm) over the last ca. 5000yr. AMS radiocarbon analyses of four individual shells from each of two strata indicate that there are no problems of reworking or age mixtures within strata. However, burning is evident in many shells. Amino acid composition and racemization were used to identify shells that had been significantly heated and these were rejected for isotopic analysis because of possible alteration of the isotopic signal. Fourteen samples of snails from Hinds Cave, each consisting of 2–6 individual shells from the same provenience, were radiocarbon dated by AMS and analyzed for stable carbon isotopes. In addition, a sample collected alive ca. AD 1890 from a nearby site, was analyzed to represent modern conditions. The shells show no decrease in organic content with age, indicating good preservation of shell organic matter. Both the organic and carbonate isotope values show similar temporal trends. The oldest samples (4100–4600calyr BP) are relatively depleted in13C . Maximum δ13C values are reached at ca. 3500calyr BP. Thereafter there is a regular depletion in13C , with the recent sample showing the lowest δ13C value. The change in the isotopic values over time is quite large (range of 4.5‰ for organic matter) and indicates substantial climatic changes. The variations in isotopic values are interpreted as resulting primarily from changes in moisture conditions, affecting the abundance of drought-adapted CAM (Crassulacean acid metabolism) plant species (enriched in13C ). Thus relatively moist mid-Holocene conditions are indicated, with increasingly dry conditions reaching a maximum at ca. 3500calyr BP, followed by progressively moister conditions through the late Holocene.