Clinton A. Cowan, David L. Fox, Anthony C. Runkel, Matthew R. Saltzman; Terrestrial-marine carbon cycle coupling in ∼500-m.y.-old phosphatic brachiopods. Geology 2005; 33 (8): 661-664. doi: 10.1130/G21434AR.1
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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.