Late Cretaceous climatic trends and a positive carbon isotope excursion at the Santonian-Campanian boundary in British Columbia, northeastern Pacific

摘要

This study presents oxygen and carbon isotope data obtained from well-preserved ammonite and bivalve fossils of the Upper Cretaceous Nanaimo Group of southwestern British Columbia, Canada. Palaeotemperatures for the late Santonian-Campanian of British Columbia, determined on the basis of oxygen isotopic analysis, suggest a odirect relationship with basic Late Cretaceous climatic trends (e.g. temperature fall toward the cool climates of the Maastrichtian). The coolest Campanian palaeotemperatures were calculated from the ammonite Pachydiscus cf. ootacodensis (Stoliczka) (113-26.4 ℃) and the bivalve Inoceramus vancouverensis Shumard (about 19.7 ℃), from the late Campanian Occidentalis Zone (Northumberland Formation). In contrast, the highest palaeotemperatures were obtained from the shells of presumed earliest Campanian bivalves and varied between 25.1 and 33.7 ℃, which we assume to represent the regional expression of the early Campanian warming event. The Santonian-Campanian boundary in British Columbia is associated with a positive δ~(13)C excursion (to 4.2‰) which appears to be contemporaneous with the Santonian-Campanian Boundary Event reported recently from other regions (i.e., Europe, Tunisia, Japan, and Tibet). The lack of organic-rich laminated black shales (indicating strong oxygen depletion in the marine realm) through the Santonian-Campanian of the Nanaimo Group, including the Santonian-Campanian boundary interval, seems to be in agreement with the suggestion that most of the world's oceans were characterised by oxygen-rich deep waters during Coniacian-Campanian time.