Taxon-specific stable carbon-isotope ratios were determined for 7 graptolite (consumer) and 4 alga (primary producer) taxa preserved as thin organic compressions within North American Silurian Konservat-Lagerstätten. Values range from −26.6‰ for a graptolite from the Goat Island Formation, New York, to −32.3‰ for an alga from the Cape Phillips Formation, Arctic Canada. Co-occurring graptolite taxa display nearly identical δ13C compositions (within 0.2‰). Algal taxa co-occurring with graptolite taxa have δ13C compositions which are 1–2‰ more negative than those of associated graptolites. The observed isotopic offset between associated graptolites and macroalgae, both of which are common sources of organic matter in Paleozoic strata, holds implications for the interpretation of time-series δ13Corg curves, and suggests that stable carbon-isotope composition could serve as a chemosystematic tool for discerning the affinities of organic macrofossil problematica.

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