Carbon Cycle Recovery After the Cretaceous-Paleogene Mass Extinction: a Terrestrial Perspective

Abstract:

The Cretaceous-Paleogene boundary (KPB, ca. 66 Ma) witnessed a mass extinction roughly coincident with a bolide impact. Deccan Traps (DT) volcanism (~ 66.4-65.5 Ma) spanned the KPB, with large eruptions in the Paleocene potentially affecting the complex carbon cycle dynamics during the post-extinction interval. The KPB coincides with a decrease in marine carbon isotope values lasting ≥ 0.5 Ma: a sharp excursion in open-ocean bulk marine carbonate (δ¹³C_bulk) and a gradual decrease in benthic carbonate (δ¹³C_benth). In contrast, the proposed excursion in existing terrestrial carbon isotope records is short, returning to pre-boundary values within 5 ka. Because the marine records are complicated by changing export productivity, terrestrial records may reflect changes in atmospheric composition more clearly, leading to the interpretation that global primary productivity also recovered rapidly. However, existing high-resolution terrestrial KPB carbon isotope records extend only ≤100 ka post-KPB, thus terrestrial, and hence atmosphere, δ¹³C dynamics during the entire marine δ¹³C excursion are unclear.

We interrogate a new terrestrial δ¹³C_org record which spans 300 ka pre- to 1.2 Ma post-KPB, from sections within the Williston Basin in Montana. We find δ¹³C_org gradually decreased for 100s of ka starting ~ 50 ka after the KPB, then increased back to pre-boundary values between 65.3 - 64.8 Ma, post-dating the beginning of increased marine δ¹³C_bulk at ~ 65.9 Ma. The KPB coincides with a brief increase in variability within the δ¹³C_org record, and the proposed 5 ka negative excursion at the KPB is not a consistent feature. While the terrestrial δ¹³C_org record is more like marine δ¹³C than previously thought, the shape of the excursion is markedly different than δ¹³C_bulk. Instead of a sharp drop, our terrestrial δ¹³C_org record has a more gradual decrease similar to δ¹³C_benth and some shelf marine records. We examine the differences between terrestrial and marine δ¹³C records around the KPB, and by comparing model results with existing paleoproxy records we find they cannot be explained by pCO₂ or precipitation changes altering plant isotope discrimination. Thus, contrary to previous studies, our results indicate that global primary productivity recovered slowly, and likely did not return to pre-KPB levels until ca. 65 Ma.