CAMBRIAN CHITINOPHOSPHATIC BRACHIOPOD SHELLS YIELD A PRIMARY MARINE d¹³C_CARB SIGNATURE AND REVEAL TERRESTRIAL-MARINE PALEOCEANOGRAPHIC INTERACTION

Fossil lingulid brachiopod shells of the genera Dicellomus and Lingulepis were extracted from biostratigraphically tightly-constrained intervals of Upper Cambrian (Sunwaptan) siliciclastics in the cratonic interior of Laurentia (Minnesota and Iowa U.S.A.). Brachiopod valves were powdered and analyzed for d¹³C_carb of CO₃^2- that substitutes in the apatite structure. The resulting stratigraphic d¹³C_carb curve corresponds unambiguously with the global d¹³C_carb positive excursion (the + ~5‰ SPICE event) well-known from whole-rock isotopic analyses of coeval carbonate sections that span the Crepicephalus through Elvinia biozones (Sauk II-III subsequence boundary interval). The d¹³C SPICE trend recorded in lingulids at the Cambrian shoreline in Minnesota, however, is systematically shifted lighter by ~6‰ compared to values from micritic and lingulid components in contemporaneous carbonate strata farther from the paleo-shoreline (e.g., in Iowa). The preservation of the global d¹³C_carb curve rules out diagenetic effects for this nearshore negative shift. Likewise, lingulid vital effects are unlikely to vary geographically and be uniform between different genera. Instead, we are documenting a paleoceanographic gradient in Cambrian ocean DIC: isotopically lighter carbon in relatively eutrophic inshore waters, as distinct from marine DIC in oligotrophic waters of the outer carbonate bank. Thus, Cambrian onshore-offshore gradients in DIC were similar in magnitude to d¹³C_carb trends observed in many modern inshore areas where freshwater brings isotopically light DIC from land.