Stable isotope (δ 13C carb and org, δ 15N org) and trace element anomalies during the Late Devonian ‘ punctata Event’ in the Western Canada Sedimentary Basin

Abstract

Late Devonian marine deposits in the Western Canada Sedimentary Basin were analyzed for (1) accumulations of bioproductivity and paleoredox trace element proxies and for (2) variations of δ 13C (carb and org) and δ 15N org in order to understand and interpret the regional history of the global yet short-term ‘ punctata Event’ geochemical perturbation. Statistical correlations suggest that changes in detrital input and associated micronutrient delivery were the main driver of a eutrophication event noted early and possibly also late in the punctata zone around the isolated Miette platform, manifested by increased TOC, positive δ 13C excursions and concurrent enrichments of all elemental proxies. Evaluation of data within a regional sequence stratigraphic perspective revealed a eustatic sea level influence, with proxy accumulations associated mostly with early transgression (IIc1). Deposition and preservation of organic matter-rich facies occurred during conditions of enhanced primary production and facilitated bottom water suboxia–anoxia. Following eutrophication, lower δ 15N org and δ 13C org values predominated and persisted throughout much of the punctata zone before rebounding toward its close, suggesting a time interval of environmental stagnation and lower overall productivity during which N 2-fixing autotrophs may have had an ecological advantage under nitrate-limited conditions. The punctata Event approximately coincided with the advent of archaeopterid forest expansion beginning around mid-Frasnian time, which fundamentally altered the nature of continental weathering through extensive soil formation and increased nutrient delivery to the oceans. This evolutionary event may have amplified the detrital influx, already elevated by sea level lowstand and early transgression and increased denudation of rising mountain ranges in near-equatorial regions.