Changes in terrestrial ecosystems across the Cretaceous-Paleogene boundary in western Canada inferred from plant wax lipid distributions and isotopic measurements

Abstract

Changes in terrestrial environments across the Cretaceous-Paleogene boundary, including plant ecology and carbon and water-cycling, remain poorly defined. Fluvial sediments spanning the Cretaceous-Paleogene (K-Pg) boundary of southern Saskatchewan, Canada contain well preserved plant wax n-alkanes that provide a means of reconstructing changes to plant ecology and carbon and water cycling during this mass extinction event. We measured n-alkane carbon (δ13C) and hydrogen (δ2H) isotope ratios in two sedimentary sections and applied established fractionation factors to estimate the isotopic compositions of precipitation and bulk sedimentary organic matter sources. We also analyzed the distribution of n-alkanes as an indicator of the relative abundance of aquatic and terrestrial plants. We find a consistent shift towards a greater relative abundance of longer-chain n-alkanes across the boundary, implying a persistent increase in the relative abundance of terrestrial plants in the sedimentary basin. This is consistent with an increase in birch and elm palynomorphs immediately above the boundary. We hypothesize the extinction of all large herbivores at the boundary may have facilitated this transition to a terrestrial angiosperm dominated flora immediately after the boundary. We also find that the region was characterized by isotopically light precipitation, with δ2H values between −95‰ to −160‰, but do not observe evidence for major millennial-scale changes in regional precipitation isotopic composition spanning the boundary. n-Alkanes derived from both aquatic and terrestrial plants at one site display an upward trend in δ13C values of approximately 2‰ across the K-Pg boundary. This suggests millennial-scale local or global carbon-cycle variability altering either plant carbon isotope fractionation or the carbon isotope composition of dissolved inorganic carbon and atmospheric CO2. Overall our results suggest that carbon and water cycle changes associated with the K-Pg impact in terrestrial environments in western Canada were short-lived, but ecological shifts in plant communities were longer-lasting.