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Terrestrial-marine carbon cycle coupling in 500-m.y.-old phosphatic brachiopods

¹ Geology Department, Carleton College, One North College Street, Northfield, Minnesota 55057, USA
² Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455-0129, USA
³ Minnesota Geological Survey, 2642 University Avenue W, St. Paul, Minnesota 55114-1057, USA
⁴ Department of Geological Sciences, The Ohio State University, 275 Mendenhall Laboratory, Columbus, Ohio 43210-1398, USA

Carbon isotope compositions (¹³C) of inarticulate brachiopod shells from Upper Cambrian sandstone in the cratonic interior of Laurentia record a 5 positive excursion that correlates biostratigraphically with the global Steptoean positive isotopic carbon excursion. A consistent 6 negative displacement in brachiopod ¹³C relative to carbonate values is interpreted to represent an onshore-offshore gradient in the isotopic composition of dissolved inorganic carbon in Cambrian seawater. Thus, these 500-m.y.-old chitinophosphatic brachiopod shells preserve evidence for carbon cycle coupling between the ancient atmospheric, oceanic, and terrestrial reservoirs in the time before embryophytic land plants.

Key Words: ¹³C • Cambrian • inarticulate brachiopod • chitinophosphatic • brachiopod