VOLCANIC ASH AND EXCEPTIONAL FOSSIL PRESERVATION: INSIGHTS INTO TAPHONOMY AND EARLY DIAGENESIS

Several globally distributed field localities yield exceptionally well preserved marine invertebrate fossil assemblages in co-occurrence with fine-grained volcanic ash. Field locales include several sites in Argentina and the Pacific Northwestern USA. Additional sites in New Zealand, Italy, and the UK will also be investigated. In this study, we hypothesize that the unique chemical and physical properties of volcanic ash, coupled with its rapid deposition into marine settings, causes mass mortality of marine invertebrates, and facilitates exceptional fossil preservation. Factors controlling the style and quality of preservation, include grain size, shape, and geochemistry of the ash. Decay of organic material begins immediately after death and burial; thus, exceptional preservation relies on the stabilization of labile tissues in the very early stages of diagenesis, immediately following burial. Tissue stabilization usually occurs via the formation of geologically stable minerals that faithfully replicate biologic structures, and remain unaltered through geologic time. Volcanic ash is a highly reactive material which, under certain conditions, is subject to rapid diagenesis. For example, it was historically used as a binder in early concrete due to its pozzolanic properties, which facilitate the formation of cementitious compounds in the presence of water and calcium hydroxide. Its ability to set under-water makes volcanic pozzolan desirable for the construction of seawalls, which were built by ancient Romans, some of which still stand today. These same properties may play a role in the occurrence of exceptional fossil preservation in volcanic ash. This study combines field observations with taphonomic experiments to explain the role of volcanic ash in early diagenetic processes resulting in exceptional fossil preservation.