Collaborative Research: Strontium Isotope Stratigraphy and Paleohydrology of Miocene Evaporite Sequences

随着时间的推移，海洋蒸发岩序列在某种程度上构成了海水化学成分的不连续记录。每个沉积物中不同蒸发岩矿物的沉淀顺序和相对量限制了海水成分的假设偏移。古蒸发岩的大量清查表明，一些主要的蒸发岩沉积期降低了平均海洋盐度，并可能改变了海水的主要离子比。此外，海洋蒸发岩硫酸盐矿物包含了随时间变化的海水硫和氧（硫酸盐离子）同位素组成的记录。蒸发岩地球化学记录的主要缺点是缺乏层序内的地层年龄控制。层序年龄通常由下伏和上覆海相地层的年龄插值而来，但这些和互层单元中的生物群通常是受限制的或地方性的，几乎没有地层用途。通过测量蒸发岩中石膏/硬石膏矿物组分的锶同位素比值，可以缓解这一问题。在大多数海洋蒸发盐水中，Sr的主要来源是海水，而对于海洋碳酸盐，Sr在沉淀硫酸盐矿物中掺入时没有发生明显的Sr同位素分馏。由于成岩作用（不包括深埋后石膏的脱水）导致的锶同位素重分布可能不像碳酸盐层序那样严重，因为硫酸盐矿物蚀变很容易识别和避免。苏伊士湾-红海北部中新世中上蒸发岩层序的sr同位素地层学研究，将建立一个具有较长潜在地球化学记录的矿床。通过测定蒸发岩层序的Sr同位素比值，并与根据远洋碳酸盐岩层序建立的晚新生代海水Sr同位素长期曲线进行比较，对苏伊士湾工业井中推测的Langhian（约15.5 Ma）和Messinian（约6 Ma）硬石膏进行了合理的年龄估计。改进后的地层学将在长期同位素曲线的高分辨率测年、化学质量平衡的考虑、对发育中的裂谷盆地构造过程的时间和性质的更好理解以及油气勘探方面具有重要的应用。该技术如果成功，也可应用于其他蒸发岩序列。还将考虑来自大陆径流的sr同位素信号的可能性，如果存在，将提供关于“海相”蒸发岩盆地古水文的重要资料。如果我们要把古代蒸发岩当作海洋古化学监测器，这些考虑是至关重要的。

Marine evaporite sequences constitute a somewhat discontinuous record of seawater chemical compositions through time. The sequence and relative amounts of precipitation of different evaporite minerals in each deposit constrain hypothesized excursions of seawater composition. Mass inventories of ancient evaporites suggest that some major episodes of evaporite deposition decreased mean oceanic salinity and perhaps changed major ion ratios of seawater. In addition, marine evaporite sulfate minerals contain a record of the sulfur and oxygen (in sulfate ion) isotopic composition of seawater through time. The major disadvantage in evaporite geochemical records is the lack of stratigraphic age control within sequences. Sequence ages are usually interpolated from those of underlying and overlying marine strata, but biota in these and in interbedded units are commonly restricted or endemic and have little stratigraphic utility. This problem may be alleviated by measuring strontium isotopic ratios of the gypsum/anhydrite mineral fractions of the evaporites. The dominant source of Sr in most marine evaporitic brines is seawater and, as for marine carbonates, no significant Sr isotopic fractionation takes place during incorporation of Sr into the precipitating sulfate minerals. Sr isotope redistribution due to diagenesis, excluding dewatering of gypsum upon deeper burial, is probably not as much of a problem as it is in carbonate sequences because sulfate mineral alteration is easily recognized and avoided. Sr-isotopic stratigraphy for middle to upper Miocene evaporite sequences in the Gulf of Suez-northern Red Sea, a deposit that provides a relatively long potential geochemical record will be constructed. Determination of Sr-isotopic ratios for the evaporite sequences and comparison to established Sr- isotopic secular curves for late Cenozoic seawater, based on pelagic carbonate sequences, gives reasonable age estimates for presumed Langhian (ca. 15.5 Ma) and Messinian (ca. 6 Ma) anhydrites from industry wells in the Gulf of Suez. The improved stratigraphy will have important applications for higher resolution dating of secular isotopic curves, for chemical mass balance considerations, for a better understanding of timing and nature of tectonic processes in a developing rift basin, and for hydrocarbon exploration. The technique, if successful, may also be applied to other evaporite sequences. The possibility of an Sr-isotopic signal derived from continental runoff will also be considered and, if present, will provide important information on the paleohydrology of "marine" evaporite basins. Such considerations are crucial if we are to regard ancient evaporites as marine paleochemical monitors.