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

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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.