Geochemical Evaluation of Living and Holocene Brachiopods and the Relationship With Modern Oceanography, Southern Australian Shelf
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Brachiopod shell chemistry is a commonly used proxy in paleoceanography and paleoclimate studies because their shells are composed of diagenetically-resistant low-Mg calcite and are, for the most part, in chemical equilibrium with ambient seawater. Few studies have evaluated a large database of brachiopods from a single stratigraphic interval across an extensive depositional system. The lateral isotopic variability across a carbonate shelf is, therefore, not well understood. The southern margin of Australia is a vast, latitude-parallel continental shelf that is host to the largest cool-water carbonate depositional system in the modern world. Brachiopods are continuously distributed across the shelf. The zonal orientation of the southern margin results in a continental-scale system that falls within a single climatic and oceanographic zone. This study evaluates the δ18O and δ13C values of 936 samples from 442 specimens collected from modern shelf sediments at 222 sites across ~3000 km of the southern shelf. An additional 60 samples from 11 select specimens were analyzed for Mg and Sr concentrations. A detailed evaluation of intraspecimen trace elements and stable isotopes reveals the distribution of apparent equilibrium calcite within shells of south Australian brachiopods and helps to understand the source of non-equilibrium chemistry related to vital effects. The large, laterally extensive database of brachiopod δ18O and δ13C is in excellent agreement with modern oceanographic measurements and shows the range in primary stable isotopic values that naturally occur within an individual specimen, at a single site, and across an entire continental shelf. Furthermore, brachiopod stable isotopes record the details of seasonal changes in oceanography that are known to occur on the southern shelf, such as upwelling and downwelling events. Some isotopic values occur outside the range of equilibrium calcite values and are attributed to either individual vital effects or oceanographic processes that have not yet been detected by modern oceanographic measurements. This is one of the first studies to evaluate the synsedimentary stable isotopic and trace element variability that could have been present in ancient carbonate systems, and these results now make it possible to gauge the environmental and biochemical effects recorded in a suite of fossil brachiopods.