Early Ediacaran cap dolostones are distinctive strata deposited directly above glaciogenic rocks from the Marinoan Glaciation (650–635 Ma), one of the most dramatic ice ages in Earth's history. These cap dolostones were rapidly deposited during global synchronous deglaciation, being a superb depositional window into the immediate glaciation aftermath and suitable for investigating the icehouse-hothouse transition in this period. However, some aspects of this transition are still unclear, such as the timing of primary productivity recovery. Aiming to explore this question, we present new sedimentological, trace element, and Sr-Cr-Cd isotope data for the Puga cap dolostone in the Southern Paraguay Belt (Brazil), here termed Morraria do Sul Formation. The cap dolostone overlies diamictites, siltstones with outsized clasts, and basement rocks. It is divided into a basal dolograinstone and an upper tubestone-microbialite succession, with abundant vertical tube-like structures. The diamictites and siltstones were formed in a glaciomarine setting, within the range of ice-rafted debris. The basal dolograinstone facies of the cap dolostone was formed under high-energy post-glacial conditions, and the overlying tubestone-microbialite facies was deposited in a low-energy, well-lit environment. Rare earth elements and yttrium (REY) preserved the original seawater-like pattern, displaying true negative Ce anomalies down to 0.84, indicating oxygenated shallow waters. The cap dolostone displays relatively high 87Sr/86Sr ratios, up to 0.7123, above the assumed value for the Early Ediacaran seawater, suggesting the influence of meltwater plumes in a salinity-stratified setting. Authigenic Cr isotope values (δ53Crauth) are within the range of Bulk Silicate Earth (BSE) to slightly positive in the basal dolograinstone and negatively fractionated below BSE values in the tubestone-microbialite facies, implying that microbial-influenced Cr reduction progressively increased upwards, controlling the isotopic fractionation. A rise upwards in δ114Cd, from −0.11 to 0.17 ‰, reveals a substantial increase in Cd-uptake by biomass, and maximum estimates for seawater δ114Cd show values within the range of present-day surface seawater. The data presented herein suggest that significant primary productivity recovery occurred in the immediate aftermath of the Marinoan Glaciation. This study also shows the efficiency of combined trace elements and Sr-Cr-Cd isotope analyses on reconstructing past seawater chemistry and productivity levels.

中文翻译：

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马里诺冰川作用后生物生产力的快速恢复：来自巴西 Morraria do Sul cap 白云岩中 Sr-Cr-Cd 同位素和微量元素的证据


早期埃迪卡拉纪帽白云岩是马里诺冰川（650-635 马）冰川形成岩石正上方的独特地层，这是地球历史上最引人注目的冰河时代之一。这些顶白云岩在全球同步冰川消融过程中迅速沉积，是了解冰川消融后立即发生的极好的沉积窗口，适用于研究这一时期的冰室-温室过渡。然而，这种转变的某些方面仍不清楚，例如初级生产率恢复的时间。为了探索这个问题，我们提出了巴拉圭南部带（巴西）的 Puga 帽白云岩的新沉积学、微量元素和 Sr-Cr-Cd 同位素数据，这里称为 Morraria do Sul 组。顶白云岩覆盖在杂岩、具有超大碎屑的粉砂岩和基岩上。它分为基底白云石和上管状-微生物岩演替，具有丰富的垂直管状结构。杂岩和粉砂岩是在冰川海洋环境中形成的，在冰筏碎屑的范围内。帽白云岩的基底白云岩相是在高能后冰期条件下形成的，上覆的管状岩-微生物岩相沉积在低能量、光照充足的环境中。稀土元素和钇 （REY） 保留了原始的类似海水的模式，显示真正的负 Ce 异常低至 0.84，表明含氧浅水区。盖白云岩显示出相对较高的 87Sr/86Sr 比率，高达 0.7123，高于早期埃迪卡拉纪海水的假设值，表明融水羽流在盐度分层环境中的影响。 自生 Cr 同位素值 （δ53Crauth） 在基底白云石中处于块硅酸盐地球 （BSE） 到略为正的范围内，而在块状石-微生物岩相中低于 BSE 值呈负分馏，这意味着受微生物影响的 Cr 减少逐渐向上增加，控制同位素分馏。δ114Cd 从 -0.11 上升到 0.17 ‰，表明生物量对 Cd 的吸收大幅增加，海水 δ114Cd 的最大估计值显示在当今表层海水的范围内。此处提供的数据表明，在马里诺冰川作用之后，初级生产力立即发生了显着的恢复。这项研究还显示了痕量元素和 Sr-Cr-Cd 同位素分析在重建过去海水化学和生产力水平方面的效率。