Isotope geochemistry and plankton response to the Ireviken (earliest wenlock) and Cyrtograptus lundgreni extinction events, Cape Phillips Formation, Arctic Canada

Abstract

Several Canadian Arctic Silurian stratigraphic sections from the basinal facies of Cape Phillips Formation have been sampled across the Llandovery-Wenlock and early-late Homerian (late Wenlock) boundary intervals for integration of biotal (graptolite, radiolarian, palynomorph) and geochemical (13C) data for two well-known extinction events, the Ireviken and lundgreni Extinction (LEE) events. Graptolites, abundant and well preserved, provide a refined biostratigraphic base for other paleontologic and geochemical data. They were globally affected by both extinction events: about 64% reduction for the Ireviken and 90-95% for the LEE. Recovery from the LEE event was slow and diversity low through the late Homerian. Radiolarians-diverse (28 species), abundant, and beautifully preserved through the early Homerian-are sharply reduced slightly below the LEE boundary. Data for the late Homerian are more scattered, but it appears that diversity was low; few early Homerian taxa crossed the extinction boundary and new taxa appeared. Palynological studies around the LEE interval are at a preliminary level, but it appears that chitinozoans and microflora (acanthomorph acritarchs, prasinophytes, sphaeromorphs) were impacted by the extinction event. Chitinozoans, though seldom abundant, appear to disappear briefly across the LEE boundary, as do palynomorphs. Amorphous organic matter is abundant in the upper part of the lundgreni Zone, it is much less common in the early and middle-late Homerian and common in the latest part. Stable isotope geochemistry shows well-marked, positive excursions in the δ13Corganic fraction associated with the Ireviken event and LEE. The Ireviken excursion (C1) curve has a sharp base, reaches a peak in the early Wenlock, and then tapers more slowly. The LEE excursion (C4) peaks at, or slightly below, the early-late Homerian boundary. Both are positive excursions. Considering the limits of biostratigraphic placement of the boundaries, they were close to or coincident with regressions, particularly across the LEE interval. The δ13Corganic excursions are greater for inshore sections compared with the offshore section. The most parsimonious explanation for increased carbon content is accelerated weathering of carbonates exposed during a lowstand.