Carbonate-Associated Sulfate in Modern Sediments of South Florida: Diagenetic Relationships

南佛罗里达现代沉积物中的碳酸盐伴生硫酸盐：成岩作用关系。地表及其附近硫的生物地球化学循环通过一系列复杂的反馈与环境氧化还原条件，包括大气氧气的可获得性密切相关。我们描述和量化这些循环和相关生物圈影响的能力取决于在细菌硫酸盐还原过程中32S相对于34S的强烈优先利用。从历史上看，关键氧化还原路径相对大小的时间趋势是通过监测海洋的34S来估计的，这在有限的石膏沉积记录中得到了体现。因此，模型只能和数据一样好，这些数据甚至不能为大多数前寒武纪提供最小的约束。在地球早期的历史中，石膏沉积不那么丰富，由于其在风化过程中的脆弱性，保存很差。鉴于这些复杂性，碳酸盐伴生硫酸盐或CAS作为一种有前景的新重晶石方法的补充但通常不会重叠，已成为一种可行的替代方法。CAS通常出现在数百到数千ppm的浓度，是大多数石灰岩和白云岩的成分，对连续、高分辨率记录古代海水硫酸盐显示出巨大的希望。人们对这种方法的兴趣正在达到顶峰，但应用程序的数量远远超过了追踪CAS的基本系统学和成岩作用可能造成的海水信号损失的研究。尽管CAS在古代沉积物中的初步结果令人鼓舞，但在控制因素更容易和更独立地受到约束的现代环境中，还没有进行系统的尝试来测试该方法。此外，任何现代校正的尝试都忽略了地质记录中最丰富的碳酸盐组分的先驱--碳酸盐泥浆和成岩作用的影响。这项拟议研究的主要目标是了解CAS如何被并入、分布和保存在佛罗里达州南部的现代碳酸盐沉积物中。具体地说，我们将在佛罗里达州海湾和西沙群岛沿海的泻湖遗址仔细选择一组具有良好特征的沉积环境，观察同位素和浓度趋势。由此产生的多样性转化为控制硫循环的参数的空间和时间梯度，例如不稳定有机化合物的可用性、盐度以及沉积物的物理和生物特性。在演化的孔隙水硫酸盐储集层范围内，在早期成岩时间尺度上经历碳酸盐溶解、沉淀和矿物转化的大量泥浆样品中记录的海水同位素信号的总体保真度是多少？尽管我们认识到现代和古代环境之间的深刻差异，但正是这些大量的泥浆样本将阐明原始信号可能被保存或可能不被保存的条件。各种更粗糙的素描部件也将被追踪到从生前到埋葬到深达几米的地方。这些目标都是由里昂促成的。在现代环境中工作的经验，包括佛罗里达州南部，尽管提出的许多问题将源于测试古环境指标的稳健性的最终目标。尤其是里昂。过去在沉积物成岩作用、碳酸盐沉积学/地球化学和硫地球化学方面的工作将是最有帮助的。根据专家小组在审查该提案的早期版本后提出的要求，这项研究的紧密重点将使我们能够在严格的有机和无机地球化学背景下追踪CAS。多组分方法，包括对广泛相关物种的速率测量和浓度和同位素测定，旨在提供了解活性碳酸盐颗粒中硫酸盐的ppm水平所需的生物地球化学硫质量平衡。最终，CAS可能有助于填补.Claypool曲线上的许多空白。通过使用连续的浅水碳酸盐切片，即使是最古老的岩石也是如此。但就像所有代表古代海水的碳酸盐一样，我们的解释只取决于我们对多种成岩途径的理解。

Carbonate-Associated Sulfate in Modern Sediments of South Florida: Diagenetic Relationships Biogeochemical cycling of sulfur at and near the earth.s surface is intimately linked.through a complex set of feedbacks.to ambient redox conditions, including the availability of atmospheric oxygen. Our ability to delineate and quantify these cycles and the associated biospheric impacts hinges on the strong preferential utilization of 32S relative to 34S during bacterial sulfate reduction. Historically, temporal trends in the relative magnitudes of critical redox pathways have been estimated by monitoring the 34S of the ocean as manifested in a limited sedimentary record of gypsum. Consequently, the models are only as good as the data, which fail to provide even minimal constraints for most of the Precambrian. During the earth.s early history, gypsum deposition was less abundant, and preservation is poor due to its vulnerability during weathering. In light of these complications, carbonate-associated sulfate or CAS, which is complementary to but generally not overlapping with a promising new barite approach, has emerged as a viable alternative. CAS, which often occurs at concentrations of hundreds to thousands of ppm, is a component of most limestones and dolostones and shows great promise for continuous, high-resolution records of ancient seawater sulfate. Interest in the method is peaking, yet applications greatly outnumber studies tracking even the basic systematics of CAS and the potential loss of seawater signals through diagenesis. Despite encouraging initial results for CAS in ancient sediments, no systematic attempt has been made to test the method in a modern setting where the controlling factors are more easily and independently constrained. Furthermore, any attempts at modern calibration have neglected the precursor of the most abundant carbonate component of the geologic record.carbonate mud.and the effects of diagenesis. The primary objective of the proposed study is to understand how CAS is incorporated, distributed and preserved in modern carbonate sediments in south Florida. Specifically, we will observe isotopic and concentration trends across a carefully chosen set of well-characterized depositional settings in Florida Bay and at lagoonal sites seaward of the Keys. The resulting diversity translates into spatial and temporal gradients in the parameters that control sulfur cycling, such as the availability of labile organic compounds, salinity and the physical and biological properties of the sediment. What is the overall fidelity of the seawater isotopic signal recorded in bulk mud samples that are undergoing carbonate dissolution, precipitation and mineral transformations on early diagenetic time scales within the confines of an evolving pore-water sulfate reservoir? Although we recognize the profound differences between modern and ancient settings, it is these bulk mud samples that will illuminate the conditions under which primary signals may or may not be preserved. Various coarser sketetal components will also be tracked from life through burial to depths up to several meters. These goals are facilitated by Lyons. experience working in modern settings, including south Florida, although many of the questions asked will stem from the ultimate goal of testing the robustness of a paleoenvironmental proxy. In particular, Lyons. past work with sediment diagenesis, carbonate sedimentology/geochemistry and sulfur geochemistry will be most helpful. The tight focus of this study.as requested by the panel following review of an earlier version of this proposalwill allow us to track CAS within a rigorous organic and inorganic geochemical context. The multi-component approach, including rate measurements and concentration and isotopic determinations for a wide range of relevant species, is designed to provide the biogeochemical sulfur mass balance necessary for understanding ppm levels of sulfate within reactive carbonate grains. In the end, CAS may help fill the many gaps in the .Claypool curve. through use of continuous sections of shallow-water carbonates from even the oldest rocks. But as with all carbonate proxies for ancient seawater, our interpretations are only as good as our understanding of the multiple diagenetic pathways.