The limited number of accurate and precise radiometric ages through the ∼100 Myr span of the Cambrian and Ordovician impedes reliable age determinations for stage boundaries in these periods. Here, we fill significant gaps in the early Paleozoic chronostratigraphy by providing precise Re-Os time-pins. Sample selection is linked to a firm biostratigraphic framework built on the appearance and distribution of trilobites, graptolites, and conodonts. A Furongian (upper Cambrian) Alum Shale section (Andrarum-3 drill core, Scania, Sweden) at the onset of the Steptoean Positive Carbon Isotopic Excursion (SPICE) yields highly non-isochronous Re-Os isotopic data from a section with wildly fluctuating δ13Corg; however, selected data from a narrow sediment band with steady carbon isotope stratigraphy provides an imprecise Re-Os age of 497 ± 28 Ma (2σ; Model 3; n = 3), with an initial 187Os/188Os ratio (Osi) of 0.74 ± 0.05. Organic-rich Alum Shale (Tomten-1 drill core, Västergötland, Sweden) from ∼120 cm below the Cambrian-Ordovician boundary yields a Model 1 age of 488.6 ± 5.1 Ma (2σ; MSWD = 1.5; n = 25) and an Osi of 0.82 ± 0.04 for Stage 10, uppermost Cambrian. Biostratigraphic data indicate the dated Alum Shale is from an interval slightly below the Top Of Cambrian Excursion (TOCE) and slightly above the First Appearance Datum (FAD) of the agnostoid Lotagnostus americanus. Organic-rich Tøyen Shale (Lerhamn drill core, Scania, Sweden) yields a precise Model 1 Re-Os age of 469.7 ± 1.4 Ma (2σ; MSWD = 1.0; n = 10) and Osi of 0.802 ± 0.002 for the maximum age of the Floian–Dapingian stage boundary (Lower–Middle Ordovician boundary). The Os isotopic composition of seawater from the latest Ediacaran through the Cambrian to Early-Middle Ordovician hovers around 0.8 but falls to 0.54 by early Silurian. This significant decrease in seawater 187Os/188Os is consistent with reduced chemical weathering and cooler seawater temperatures through the Middle–Late Ordovician. Overall, Redox Sensitive Element (RSE; Re, Os, Mo, U) abundances correlate positively with Total Organic Carbon (TOC), suggesting efficient removal of these elements from an anoxic water column by organic matter. However, these relationships break down for high TOC (>10%) shales depositing under euxinic conditions. The RSE-TOC relationship breakdown supports enhanced metal drawdown from the water column with local pyrite accumulation. Geochemical data suggest the deposition of Alum and Tøyen shales under hydrographically restricted settings with increased primary productivity along the Baltica's margin during the latest Cambrian to Early-Middle Ordovician.

中文翻译：

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瑞典寒武世晚期至中奥陶世明矾和 Tøyen 页岩的 Re-Os 地质年代学和地球化学演化


寒武纪和奥陶纪 ∼100 Myr 跨度的准确和精密辐射年龄数量有限，阻碍了这些时期阶段边界的可靠年龄测定。在这里，我们通过提供精确的 Re-Os 时间别针填补了早期古生代年代地层学中的重要空白。样品选择与建立在三叶虫、镰石和齿形动物的外观和分布基础上的坚实生物地层学框架相关联。在 Steptoean 正碳同位素偏移 （SPICE） 开始时，Furongian（上寒武统）明矾页岩剖面（Andrarum-3 钻芯，瑞典斯堪尼亚）从具有剧烈波动的 δ13Corg 的剖面中产生了高度非同步的 Re-Os 同位素数据;然而，从具有稳定碳同位素地层学的狭窄沉积物带中选择的数据提供了 497 ± 28 马 （2σ;型号 3;n = 3），初始 187Os/188Os 比值 （Osi） 为 0.74 ± 0.05。在寒武纪-奥陶纪边界以下约 120 cm 处发现富含有机物的明矾页岩（Tomten-1 钻芯，瑞典西约特兰）的模型 1 年龄为 488.6 ± 5.1 马 （2σ;MSWD = 1.5;n = 25），第 10 阶段的 Osi 为 0.82 ± 0.04，最上寒武纪。生物地层学数据表明，测年的明矾页岩来自略低于寒武纪游览顶部 （TOCE） 的区间，略高于不可知论 Lotagnostus americanus 的首次出现基准面 （FAD）。富含有机物的 Tøyen 页岩（瑞典斯堪尼亚的 Lerhamn 钻芯）得出的精确 Model 1 Re-Os 年龄为 469.7 ± 1.4 马 （2σ;MSWD = 1.0;n = 10），Osi 为 0.802 ± 0.002，用于 Floian-Dapingian 阶段边界（下-中奥陶世边界）的最大年龄。从最新的埃迪卡拉纪到寒武纪到早中奥陶世的海水的 Os 同位素组成徘徊在 0.8 左右，但下降到 0。54 由志留纪早期。海水 187Os/188Os 的这种显着减少与中晚奥陶世化学风化作用的减少和较低的海水温度一致。总体而言，氧化还原敏感元件 （RSE;Re、Os、Mo、U） 的丰度与总有机碳 （TOC） 呈正相关，表明有机物可有效去除缺氧水柱中的这些元素。然而，在真物质条件下沉积的高 TOC （>10%） 页岩，这些关系被打破了。RSE-TOC 关系细分支持通过局部黄铁矿积累从水柱中增强金属抽取。地球化学数据表明，在最新的寒武纪至早中奥陶世期间，明矾和 Tøyen 页岩在水文限制环境中沉积，波罗的海边缘的初级生产力增加。