Get access

The influence of denitrifying seawater on graptolite extinction and diversification during the Hirnantian (latest Ordovician) mass extinction event

Authors

  • STANLEY C. FINNEY,

  • WILLIAM B. N. BERRY,

  • JOHN D. COOPER


Stanley C. Finney [scfinney@csulb.edu], Department of Geological Sciences, California State University, Long Beach, CA 90840, USA; William B.N. Berry [bberry@uclink4.berkeley.edu], Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA; John D. Cooper [jcooper@fullerton.edu], Department of Geological Sciences, California State University, Fullerton, CA 92834, USA.

Abstract

A continuous trench exposure within the uppermost type Vinini Formation at Vinini Creek, Roberts Mountains, Nevada, provides an unparalleled opportunity to examine the fate of graptolites, prominent Paleozoic zooplankton, during most of the Hirnantian mass extinction event. On the basis of a detailed biostratigraphic and sedimentological dataset, the relatively complete extinction record is examined in the context of ecological constraints, and it is found to reflect an ecological collapse driven by glacio-eustatic sea-level fall and associated changes in oceanic circulation. Diverse graptolite populations of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which flourished in denitrifying waters within the oceanic oxygen-minimum zone (OMZ) during sea-level highstand, largely vanished with the loss of these conditions during glacio-eustatic sea-level fall. However, populations of one clade, the normalograptids, which inhabited the oxygenated waters of the photic zone, not only survived but diversified. These survivors gave rise to rapid recolonization and diversification with re-establishment of the oxygen-minimum and denitrifying conditions during post-Hirnantian sea-level rise. This ecological model also applies globally to other well-documented coeval stratigraphic intervals, representing both oceanic and platform sea settings.

Get access to the full text of this article

Ancillary