2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 6
Presentation Time: 2:45 PM

CAMBRIAN CHITINOPHOSPHATIC BRACHIOPOD SHELLS YIELD A PRIMARY MARINE d13CCARB SIGNATURE AND REVEAL TERRESTRIAL-MARINE PALEOCEANOGRAPHIC INTERACTION


COWAN, Clinton A.1, FOX, David L.2, RUNKEL, Anthony C.3, SALTZMAN, Matthew R.4 and AUERBACH, David J.1, (1)Geology Department, Carleton College, One North College Street, Northfield, MN 55057, (2)Department of Geology and Geophysics, Univ of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, (3)Minnesota Geol Survey, 2642 University Ave W, Minneapolis, MN 55114, (4)Geological Sciences, Ohio State Univ, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, ccowan@carleton.edu

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 d13Ccarb of CO32- that substitutes in the apatite structure. The resulting stratigraphic d13Ccarb curve corresponds unambiguously with the global d13Ccarb 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 d13Ccarb 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 d13Ccarb trends observed in many modern inshore areas where freshwater brings isotopically light DIC from land.