GEOCHEMISTRY OF CHITINOPHOSPHATIC BRACHIOPOD SHELLS FROM CAMBRIAN NEARSHORE DEPOSITS OF THE CRATONIC INTERIOR OF LAURENTIA
Fossil lingulid brachiopod shells of the genera Dicellomus and Lingulepis were extracted from biostratigraphically constrained intervals of Upper Cambrian (“Cedaria” through Elvina biozones) siliciclastics in the cratonic interior of Laurentia (Minnesota and Iowa U.S.A.). Brachiopod valves were powdered and analyzed for REE, major and trace element abundances and C and Sr stable isotope compositions. Stratigraphic plots of 87Sr/86Sr and d13C values show unambiguous correlation with their respective global curves, suggesting that pristine biogenic chemical signatures are retained in these remarkably well-preserved shells. Stratigraphic plots also show that some major and trace metal abundances vary in synch with the presumed sea-level curve for the Steptoean stage. REE abundances are enriched compared to typical values from modern inarticulate brachiopods, which is a common characteristic of Lower Paleozoic chitinophosphatic shells. These data allow us to piece together a chemical history of epeiric seawater for the cratonic interior of Late Cambrian Laurentia. For example, the the global SPICE excursion in d13C values is recorded in lingulids proximal to the paleoshoreline in Minnesota but is systematically shifted negatively by ~6‰ compared to values from micrite and lingulids in contemporaneous carbonate strata farther from the paleoshoreline (e.g., in Iowa). The negative shift in lingulid d13C values suggests a paleoceanographic gradient in Cambrian ocean DIC along which relatively eutrophic inshore waters had isotopically lighter carbon compared to oligotrophic waters of the outer carbonate bank. The retrieval of demonstrably pristine marine C and Sr isotopic compositions from Lower Paleozoic chitinophosphatic inarticulate brachiopod shells suggests that lingulids represent a potential bonanza of near-shore, paleoceanographic information and expands the chemostratigraphic application of d13C values. Geochemical study of the Early Paleozoic can now be broadened to include sandstone-dominated strata of cratonic interiors, where interactions between the ancient terrestrial and marine biospheres are best explored.