Jurassic Paleoceanography: Oxygen Isotope Study of Lower to Middle Jurassic Carbonate Strata in the Central High Atlas Mountains of Morocco

本研究将确定摩洛哥中部高阿特拉斯山脉侏罗纪碳酸盐岩古海洋学项目的可行性。需要解决的主要问题是：1)能否克服下坡运输的复杂性，从而恢复生活在浅海和深海环境中的化石进行地球化学分析？2)这些化石的同位素组成是由沉积条件决定的（即它们是否没有改变），还是在随后的成岩事件中发生了变化？中生代腕足动物的沉积环境与形态的关系有助于区分浅海和深海环境的化石。第二个问题将通过对样品本身的各种地球化学和岩石学分析，以及对该地区碳酸盐成岩作用的广泛研究来解决，以确定区域成岩趋势。未被改变的腕足动物材料应该显示出原始腕足动物典型的纤维状、“龙骨和鞍状”微观结构，不应该有阴极发光，并且应该具有在侏罗纪腕足动物中测量到的微量元素组成范围。同位素组成本身在协变趋势中是有用的，通常可以用来区分原始特征和改变的组成。比较化石和其他岩石成分的同位素组成也有助于确定古海洋学研究中腕足动物组成的可靠性。例如，来自同一地点的浅层和深海腕足动物，如果它们保存完好，很可能具有不同的同位素组成，而相同的成分则表明成岩蚀变。同样，如果腕足动物的组成不同于其他化石、胶结物或泥晶，则表明埋藏再平衡并不广泛。通过对中高阿特拉斯山脉南部侏罗系碳酸盐岩成岩作用的区域和亚区域研究，可以更好地了解这些同位素组成。这样的研究应该能够估计穿过这些岩石的成岩流体的类型，以及这些流体的流动路径。这些方法的结合（直接观察、与其他岩石成分的比较以及与区域成岩趋势的比较）应该可以确定摩洛哥阿特拉斯山脉中部高腕足动物的同位素组成是否适合古海洋学研究。如果是这样的话，将为一项关于侏罗纪深海环流的重要结论的研究奠定基础。

This study will determine the feasibility of a paleoceanographic project on Jurassic carbonates in the Central High Atlas Mountains of Morocco. The principal questions to be addressed are: 1) Can the complexities of downslope transport be overcome, so that fossils that lived in both shallow and abyssal environments can be recovered for geochemical analysis? 2) Do the isotopic compositions of those fossils result from depositional conditions (i.e., are they unaltered), or have the isotopic compositions been changed during subsequent diagenetic events? The relationship between depositional environment and morphology in Mesozoic brachiopods should help distinguish between fossils from shallow and abyssal environments. The second question is to be addressed by a variety of geochemical and petrographic analyses of the samples themselves, as well as by a broadly-focused study of carbonate diagenesis in the area to determine regional diagenetic trends. Unaltered brachiopod material should display the fibrous, "keel and saddle" microstructure typical of pristine brachiopods, should not cathodoluminesce, and should have trace element compositions in the range of compositions previously measured in Jurassic brachiopods. Isotopic compositions themselves are of use in that covariant trends in and can often be used to distinguish between original signatures and altered compositions. Comparison of isotopic compositions of fossils and other rock components should also help determine the reliability of brachiopod compositions for paleoceanographic research. For example, shallow and abyssal brachiopods from the same locality will most likely have differing isotopic compositions if they are well-preserved, whereas identical compositions would suggest diagenetic alteration. Similarly, if brachiopod compositions differ from those of other fossils and cements or micrite, it would appear that burial reequilibration has not been extensive. These isotopic compositions will also be better understood in light of a regional to sub-regional study of diagenesis in Jurassic carbonates of the southern Central High Atlas Mountains. Such a study should allow estimation of the types of diagenetic fluids that have passed through these rocks, as well as the flow paths of those fluids. The combination of these approaches (direct observations, comparison with other rock components, and comparison with regional diagenetic trends) should allow determination of whether isotopic compositions of brachiopods from the Central High Atlas Mountains of Morocco are suitable for paleoceanographic research. If so, the stage will be set for a study that should allow significant conclusions about Jurassic deep ocean circulation.