锶同位素估算年龄与其他年代地层学方法的比较

Pengcheng Cai, Xinchun Liu, Lijun Yang, Xue Yan, Liting Wu
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引用次数: 0

摘要

锶同位素地层学通过将其87Sr/86Sr比率与全球海水87Sr/86Sr随地质时间变化的参考曲线进行比较,能够估计含碳酸盐沉积物的数值日期。该海水参考曲线是一个已发表的统计LOWESS拟合数据集(约14000),该数据集是已知生物带碳酸盐化石的87Sr/86Sr测量值,使用已发表的时间尺度将其转换为数值年龄。在本研究中,我们计算了从奥陶纪到渐新世的七个地层剖面中87Sr/86Sr比值的数值日期。塔里木盆地大湾沟剖面锶同位素地层学是一个新的数据集,为晚奥陶世基底提供了辅助GSSP;其他是已发布的数据集。这些87Sr/86Sr衍生的日期与通过其他年代地层学方法(包括根据放射性同位素日期校准的旋回地层学和标准生物地层图)为这些剖面中相同层位估计的日期进行了比较。除了已发表的关于中国安徽巢湖剖面下三叠统地层和摩洛哥中泥盆纪Jebel Mech Lrdane剖面的研究外,所有87Sr/86Sr的来源日期都在其他年代地层方法的估计不确定性范围内,从而验证了锶同位素地层学作为获取含海相碳酸盐岩矿床年龄的一种工具的有用性。然而,来自所有三个古生代剖面(两个奥陶纪,一个泥盆纪)的数据集确实显示出对较老年龄的明显偏移偏差,这可能在一定程度上是LOWESS参考曲线对生物带当前年龄模型调整不充分的假象。早三叠世牙形石带巢湖剖面的87Sr/86Sr年龄系统地年轻了约1.45Myr,令人无法接受;从而指示高于预期的87Sr/86Sr值,这在一定程度上可能是半限制性盆地和激光烧蚀(LA-ICP-MS)分析方法的结合的结果,半限制性流域被邻近地块的化学风化产生的富含87Sr的河流水略微稀释,激光烧蚀分析方法应用于那些似乎产生比溶液模式ICP-MS方法更高的87Sr/86Sr比率的样品。总之,除了极少数例外,锶同位素地层学方法和87Sr/86Sr海水统计曲线是从海洋碳酸盐沉积物中获得半精确数值日期的重要方法。
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Comparisons of age estimates from Strontium-isotope values versus other Chronostratigraphy methods

Strontium-isotope stratigraphy enables estimates of numerical dates for marine carbonate-bearing sediments by comparing their 87Sr/86Sr ratio to a reference curve of 87Sr/86Sr variations of global seawater through geologic time. That seawater reference curve is a published statistical LOWESS fit to a very large dataset (ca. 14,000) of 87Sr/86Sr measurements of carbonate fossils from known biozones which is converted to numerical ages using published timescales. In this study, we compute the numerical dates projected by this method for suites 87Sr/86Sr ratios from seven stratigraphic sections ranging from Ordovician through Oligocene. The strontium-isotope stratigraphy from the Dawangou section in Tarim Basin of NW China that hosts the Auxiliary GSSP for the base of the Late Ordovician is a new dataset; and the others are published datasets. These 87Sr/86Sr-derived dates are compared to those estimated for the same horizons in these sections by other chronostratigraphic methods, including cyclostratigraphy calibrated to radioisotopic dates and standard biochronostratigraphy. Except for a published study of Lower Triassic strata from Chaohu section in Anhui, China and Middle Devonian strata Jebel Mech Lrdane section of Morocco, all the 87Sr/86Sr-derived dates are within the estimated uncertainties of the dates from the other chronostratigraphic methods, thereby validating the usefulness of strontium-isotope stratigraphy as one tool for obtaining the ages of marine carbonate-bearing deposits. However, the datasets from all three Paleozoic sections (two Ordovician, one Devonian) did display a noticeable offset bias toward older ages, which may be partly an artifact of inadequate adjustment of the LOWESS reference curve to current age models for the biozones. The 87Sr/86Sr-derived dates from Chaohu section of Early Triassic conodont zones were unacceptably systematically younger by ca. 1.45 Myr; thereby indicating higher-than-expected 87Sr/86Sr values, which may have been partially the result of a combination of a semi-restricted basin that was slightly diluted by 87Sr-enriched fluvial waters from the chemical weathering of adjacent landmasses and of the laser ablation (LA-ICP-MS) analytical method applied to those samples that seems to yield higher 87Sr/86Sr ratios than the solution-mode ICP-MS method for 87Sr/86Sr measurements. In summary, with rare exceptions, the method of strontium-isotope stratigraphy and the statistical 87Sr/86Sr seawater curve is an important method to obtain semi-precise numerical dates from marine carbonate sediments.

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