TAPHONOMY AND TIME AVERAGING OF MOLLUSCAN DEATH ASSEMBLAGES IN FLORIDA SPRINGS AND RIVERS

Molluscan death assemblages, shells accumulating in aquatic systems, are widely used to address diverse questions ranging from historical ecology to taphonomy. In fluvial systems, harsh taphonomic conditions are expected to be common. First, in many rivers, high water energy and debris contribute to severe physical conditions resulting in elevated rates of shell fragmentation, disarticulation and abrasion. Second, typical water chemistry in freshwater settings promotes rapid dissolution of calcium carbonate. This harsh taphonomic regime results in rapid destruction of shells and consequently freshwater death assemblages forming in rivers and streams should not undergo the extensive centennial-to-millennial time averaging that is often observed in marine systems.

This project explores taphonomic processes and time averaging in freshwater spring and river systems of Florida through comparison of live (living mollusk), dead (surficial shell accumulation), and fossil (found in situ in river-bank sediments) molluscan assemblages. One hundred and eighty-one bulk samples were collected from the Silver, Wakulla, and Ocklawaha Rivers (66 fossil, 52 death, and 63 live assemblages; n=43,091 specimens). Death assemblages include specimens with two distinct preservation styles. Type 1 include shells with bleached color, chalky appearance, and chemically or physically weakened walls (i.e., comparable to shells found in fossil assemblages). Type 2 includes shells with limited physical damage, periostracum present, and original color visible. Only type 2 specimens include shells representing invasive taxa suggesting a younger provenance. These two groups suggest significant mixing of recently dead and Holocene fossil material (i.e., multi-millennial scale of time averaging).

Extensive time-averaging despite presumably rapid loss of shell material reflects a combined effect of the proximal presence of highly fossiliferous deposits, physical erosion of those materials, and intense bioturbation by invasive catfish, which produce large networks of spawning and nesting burrows. These processes have led to extensive recycling of older Holocene material into the present-day death assemblages. Consequently, time averaging of the death assemblage is inflated by extensive reworking of older Holocene fossils.