Evaluation of paleoclimatic conditions east and west of the southern Canadian Cordillera in the mid-late Paleocene using bulk organic δ13C records

Carbon isotope records from organic matter provide a key data set for constraining past fluctuations in the isotopic composition of atmospheric carbon dioxide (δ13Catm), hydrologic conditions, and plant community composition. Here we evaluate spatiotemporal shifts in these three conditions using δ13C values of bulk organic carbon (δ13Cbulk) from Paleocene strata in northwest Washington, U.S.A. and west-central Alberta, Canada. Our data set contains 352 analyses from the Chuckanut and Paskapoo formations. Mean δ13Cbulk values in the two areas are statistically indistinguishable (− 24.5‰ and − 24.4‰ (VPDB), respectively), and exhibit a wide range of values (~ 5–7‰). In both areas sequential vertical samples show large fluctuations in δ13Cbulk values, on average of ~ 1.0‰ ± 1.0 (1σ). There are no distinct up-section trends or structured excursions in the ~ 850 meter stratigraphic section in the Chuckanut Formation nor the ~ 125 meter section in the Paskapoo Formation. Furthermore, δ13Cbulk values are independent of the weight percent organic carbon (%C) and lithology of the sample. ck of a systematic relationship and correlation between δ13Cbulk and %C values indicates isotopic fractionation during post-mortem processes, such as microbial respiration, exerted minimal influence on the isotopic patterns. The independence of δ13Cbulk values from lithology suggests variability is unrelated to local spatial gradients in vegetation community and water availability across the paleo-landscape. However, the wide range and large fluctuations in δ13Cbulk values is similar to stochastic intra-site variability of plant δ13C values within modern settings. The absence of structured, up-section trends indicates minimal secular variation in the carbon isotopic composition of atmospheric CO2, rainfall, and plant community composition. Moreover, we show that a previous carbon isotope excursion ascribed to the Paleocene–Eocene Thermal Maximum in the Paskapoo Formation is actually a result of aliasing related to sampling density. Finally, the statistically indistinguishable mean δ13Cbulk values in the two regions imply similarly wet and humid climatic conditions when combined with sedimentologic and paleobotanical data. The records support early Paleogene circulation models that show strong orographic precipitation patterns in the Pacific Northwest associated with incipient extensional tectonics in the region and enhanced transport of moisture-laden polar air masses southward into the Canadian continental interior.