The enigmatic Norian-Rhaetian boundary: Varying carbon cycle disturbances in continental and deep marine records

Abstract

The Norian-Rhaetian transition (late Sevatian through early Rhaetian) was associated with a series of faunal turnovers and a putative global carbon cycle perturbation, as evidenced by one or more negative carbon isotope excursions (NCIEs) that occur in locations around the world. Deciphering the pattern, timing and causes of these NCIEs is crucial for understanding the characteristics and causal mechanisms of environmental change across the Norian-Rhaetian boundary (NRB). Currently, however, there is no agreed position and definition of the NRB. Moreover, records of the Sevatian-early Rhaetian from terrestrial and deep marine sites are largely uninvestigated. Here we present organic carbon isotope (δ¹³C_org) and associated geochemical data across upper Norian to lower Rhaetian strata from a non-marine section at St Audrie's Bay, UK (west European lacustrine basin) and a pelagic deep-marine section at Katsuyama, Japan (central Panthalassa Ocean). Both sites show putative NCIEs, but they were likely of different ages. At St. Audrie's Bay, a cluster of NCIEs (maximum magnitude of −3.5 ‰) likely span between ∼207.6 Ma and ∼ 206.6 Ma based on paleomagnetically-calibrated astronomical time constraints, whereas smaller magnitude NCIEs (maximum magnitude of −2 ‰) at Katsuyama were likely older (∼210 Ma). Our new data underline the probable multi-phased nature of Sevatian-early Rhaetian carbon cycle changes, and also help to demonstrate that differences in NCIE magnitudes in different locations were at least in part linked to varying relative amounts of terrestrial and marine organic matter. Hg enrichment within the NCIE interval at Katsuyama supports the emerging view that volcanism was a driver of carbon cycle disturbance near the base of the Rhaetian.