| Paleoenvironmental interpretation of bulk organic carbon isotopes can be complicated, because multiple organisms can contribute organic matter to sediments and their carbon-isotopic compositions could vary. In this study, compound-specific carbon isotope analyses were used to evaluate the imprint of phytoplankton taxonomic variability on organic carbon isotope distributions in modern and ancient environments.; In the Peru upwelling region, carbon-isotopic compositions of diatom and haptophyte biomarkers vary significantly. Diatoms apparently employ active uptake of carbon, and silicate-limited growth rates govern photosynthetic fractionation {dollar}(varepsilonsb{lcub}rm p{rcub}).{dollar} In contrast, haptophytes assimilate carbon via diffusion, and {dollar}varepsilonsb{lcub}rm p{rcub}{dollar} values of those organisms are governed by growth rates and CO{dollar}sb{lcub}2({lcub}rm aq{rcub}){rcub}{dollar} concentrations. Bulk organic carbon isotope abundances homogenize these taxonomic variations; consequently, bulk organic {dollar}deltasp{lcub}13{rcub}{dollar}C values do not reflect growth rates or CO{dollar}sb{lcub}2({lcub}rm aq{rcub}){rcub}{dollar} concentrations.; In the Cretaceous Western Interior Seaway (United States), variations in organic matter source subtly affected paleoenvironmental interpretation. Photoautotroph biomarker {dollar}deltasp{lcub}13{rcub}{dollar}C values are higher on the eastern margin of the basin than on the west side; this represents variation in {dollar}varepsilonsb{lcub}rm p{rcub}{dollar} values that heterotrophic processes obscured in the bulk organic carbon isotope record. In addition, both molecular abundances and compound-specific {dollar}deltasp{lcub}13{rcub}{dollar}C values suggest that bottom-waters were more reducing on the eastern margin of the basin than in the central portion. These interpretations are consistent with published predictions of seaway paleo-circulation patterns.; In the Ordovician Taconic foreland basin, organic matter sources varied and exerted a strong control on the bulk organic carbon isotope record. Although carbonate {dollar}deltasp{lcub}13{rcub}{dollar}C values exhibit similar +3{dollar}perthous{dollar} shifts among several correlative locations, the shifts in organic {dollar}deltasp{lcub}13{rcub}{dollar}C values differ significantly: an 8{dollar}perthous{dollar} shift occurs in Iowa strata, whereas a 3-4{dollar}perthous{dollar} shift occurs in coeval units from Ontario and Pennsylvania. Differences among the three locations arise from differences in organic matter sources--specifically, the presence of the {dollar}sp{lcub}13{rcub}{dollar}C-enriched microfossil Gloecapamorpha prisca in Iowa strata. The carbon-isotope excursion recorded by compounds not derived from G. prisca was similar in Iowa and Ontario, suggesting that the excursion records basin-scale processes. Moreover, {dollar}deltasp{lcub}13{rcub}{dollar}C values (the difference between inorganic and organic {dollar}deltasp{lcub}13{rcub}{dollar}C values) decrease by 2{dollar}perthous{dollar} at all three locations, suggesting that pCO{dollar}sb2{dollar} decreased during the excursion. |
