Carbon isotope compositions (δ13C) 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 δ13C 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.

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