Effects of volcanic ashfall recorded in ancient marine benthic communities: comparison of a nearshore and an offshore environment

Abstract

Ecologic and taphonomic impacts of volcanic ashfall on a Pleistocene nearshore hard-bottom community and a Cretaceous offshore soft-bottom community are compared and contrasted. The hard-bottom (reef) community, dominated by sessile, epifaunal organisms, was devastated by rapid deposition of volcanic sediment. In contrast, the offshore community, dominated by motile, burrowing organisms, was considerably less affected, since many of these organisms could escape burial. These examples emphasise the potential bias that volcanism may introduce into the fossil record, particularly the over-representation of sessile fauna. The new substrate provided by volcanic sediment inhibited the re-establishment of the reef community, but was not sufficiently different from terrigenous mud to alter the structure of the offshore community. Unlike other agents of rapid burial (e.g. storms, mass wastage), volcanic sedimentation in these settings was not accompanied by a significant increase in hydraulic energy. Consequently, faunal remains were buried in situ and rapidly isolated from the destructive effects of subsequent hydraulic disturbance, scavenging, bioerosion and bioturbation. In both of the examined settings, this contributed to exceptional preservation of skeletal remains. Preservation was also promoted by early diagenetic cementation. In the reef setting, cementation of ash was laterally continuous, whereas in the offshore setting, early diagenetic minerals were precipitated as small concretionary bodies. Both styles of early diagenetic cementation enhanced preservation by effectively armouring buried fauna from destructive taphonomic processes. OTaphonomy, coral reef, fossil preservation, fossil lagerstiftten, volcanic ash, benthic communities, fossil diagenesis, concretions, Bearpaw Formation, Cretaceous, Pleistocene, Alberta, Indonesia.