High-resolution organic carbon isotope record across the Cretaceous–Tertiary boundary in south-central Alberta: implications for the post-impact recovery rate of terrestrial ecosystems and use of δ¹³C as a boundary marker

Abstract

A high-resolution study identified a δ 13 C excursion of -1.8‰ to -2.3‰ in terrestrial organic matter across the Cretaceous-Tertiary (K-T) boundary at two localities in the Scollard Formation of south-central Alberta, one of the northernmost occurrences of the K-T boundary in terrestrial settings. δ 13 C values are at their lowest within 6 cm above the K-T boundary claystone and return to pre-boundary levels within 10 cm above the boundary claystone. Statistical analyses reveal that the K-T isotopic shift in Alberta is related to the nature of floral changes that occurred across the K-T boundary. A radiometrically dated bentonite resting on the boundary-hosting Nevis coal at one of the localities permits us to estimate that the terrestrial carbon cycle recovered -100 000 years after the K-T boundary event, a value that supports an existing hypothesis that terrestrial ecosystems recovered more rapidly than marine ecosystems. The organic carbon isotope record of the entire Scollard Formation demonstrates that the δ 13 C excursion across the K-T boundary did not reach anomalously low values by late Maastrichtian standards in Alberta. Furthermore, the occurrence of the K-T carbon isotope shift within a restricted stratigraphic interval (<10 cm) greatly limits the probability of its preservation in the context of terrestrial sedimentary environments. These observations suggest that, on their own, δ 13 C profiles may be unreliable for locating the K-T boundary (and possibly other geologically instantaneous events) and that they should be used in combination with other approaches (e.g., palynology) to identify the K-T boundary in sections lacking the boundary claystone and iridium anomaly.Résumé : Une étude de haute résolution a identifié une excursion isotopique δ 13 C varient entre -1.8‰ et -2.3‰ dans de la matière organique d'origine continentale à travers la frontière Crétacé-Tertiaire (K-T) présent à deux localités au sein de la Formation Scollard dans le centre-sud de l'Alberta, l'une des occurrences les plus nordiques de la frontière K-T en environnements terrestres. Les valeurs δ 13 C atteignent leur plus bas niveau dans les premiers 6 cm au-dessus de l'argilite marquant la frontière K-T et retournent à un niveau typique d'avant-frontière dans les 10 cm au-dessus de l'argilite marqueur. Des analyses statistiques révèlent que l'excursion isotopique à la frontière K-T en Alberta est reliée à la nature des changements floraux. La présence d'une couche de bentonite datée radiométriquement au-dessus du lit de charbon Nevis, lequel contient l'argilite marqueur, nous permet d'estimer que le cycle terrestre du carbone se rétablit -100 000 ans après la frontière K-T, un lapse de temps supportant l'hypothèse suggérée précédemment que le rétablissement des écosystèmes terrestres fut plus rapide que celui des écosystèmes marins. La signature isotopique du carbone organique dans la Formation Scollard démontre que l'excursion δ 13 C à la frontière K-T n'a pas atteint des valeurs anormalement basses dans le contexte du Maestri...

A paleoclimatic and paleoatmospheric record from peatlands accumulating during the Cretaceous-Paleogene boundary event, Western Interior Basin, Canada

“…The results of this study are also coarsely consistent with previous studies from terrestrial sediments elsewhere in the western interior of America that show d 13 C maxima immediately at, or some centimeters below, the Cretaceous-Paleogene claystone, and a negative excursion of 1.5‰-2.5‰ culminating in d 13 C minima above the Cretaceous-Paleogene claystone (Schimmelmann and DeNiro, 1984;Beerling et al, 2001;Arens and Jahren, 2002;Gardner and Gilmour, 2002;Maruoka et al, 2007;Therrien et al, 2007;Grandpre et al, 2013). In this study, the position of the d 13 C minimum occurs within the Cretaceous-Paleogene claystone at Frenchman Valley, to 5-10 cm above the Cretaceous-Paleogene claystone at Knudsen's Coulee (Fig.…”

“…The sampling at Knudsen's Coulee and Knudsen's Farm replicates the work of Therrien et al (2007), and the d 13 C profiles at those localities largely corroborate both in terms of their shape and magnitude of excursions. The results of this study are also coarsely consistent with previous studies from terrestrial sediments elsewhere in the western interior of America that show d 13 C maxima immediately at, or some centimeters below, the Cretaceous-Paleogene claystone, and a negative excursion of 1.5‰-2.5‰ culminating in d 13 C minima above the Cretaceous-Paleogene claystone (Schimmelmann and DeNiro, 1984;Beerling et al, 2001;Arens and Jahren, 2002;Gardner and Gilmour, 2002;Maruoka et al, 2007;Therrien et al, 2007;Grandpre et al, 2013).…”

“…The fact that the shift is absent or lesser in magnitude in the contemporaneous benthos has been interpreted as evidence for the global reduction or shutdown of marine surface primary productivity (the so-called "Strangelove Ocean"; Hsü and McKenzie, 1985;Zachos et al, 1992) and/or a decrease in the flux of organic material from the surface to deep sea (D'Hondt et al, 1998;Alegret et al, 2012) at that time, resulting in the homogenization of the normal surface-to-depth positiveto-negative d 13 C gradient. A similar 1‰-3‰ negative shift in the d 13 C has also been identified across the Cretaceous-Paleogene boundary in organic carbon deposited in marine settings (e.g., Gilmour et al, 1987;Woolbach et al, 1990;Meyers and Simoneit, 1990;Hollander et al, 1993;Arinobu et al, 1999;Yamamoto et al, 2010) and fully terrestrial environments (Schimmelmann and DeNiro, 1984;Beerling et al, 2001;Gardner and Gilmour, 2002;Maruoka et al, 2007;Therrien et al, 2007;Grandpre et al, 2013). Because preserved organic carbon of terrestrial origin records the isotopic composition of the paleoatmosphere (Marino and McElroy, 1991;Jahren et al, 2008), the similarity of the terrestrial and marine d 13 C records has been used to argue for coupling of the atmospheric and shallow-marine carbon reservoirs through the mass extinction event (Beerling et al, 2001).…”

“…Because preserved organic carbon of terrestrial origin records the isotopic composition of the paleoatmosphere (Marino and McElroy, 1991;Jahren et al, 2008), the similarity of the terrestrial and marine d 13 C records has been used to argue for coupling of the atmospheric and shallow-marine carbon reservoirs through the mass extinction event (Beerling et al, 2001). Grandpre et al (2013), however, highlighted the occurrence of numerous other negative and positive shifts in d 13 C immediately before and after the Cretaceous-Paleogene boundary in their data and that of previous studies spanning the Cretaceous-Paleogene boundary in terrestrial successions in North America (i.e., Maruoka et al, 2007;Therrien et al, 2007), and some of these excursions can be greater in magnitude than the shift associated with the Cretaceous-Paleogene boundary itself. Additionally, sampling resolution has been typically biased toward an ~1-m-thick zone immediately bracketing the Cretaceous-Paleogene boundary and decreases away from it.…”

“…Altogether, the importance of the negative d 13 C excursion at the Cretaceous-Paleogene boundary in the terrestrial realm is likely to have been overstressed in the geological literature (Grandpre et al, 2013). Furthermore, Therrien et al (2007) and Grandpre et al (2013) concluded that it is difficult to correlate trends across multiple sections, and extract meaningful regional or global paleoatmospheric d 13 C time series from the terrestrial record because local, episodic deposition, incision, and reworking are characteristic of alluvial and fluvial sediments (MacLeod and Keller, 1991;Ager, 1993;Collinson, 1996).…”