Rates of burial and disturbance of experimentally-deployed molluscs: Implications for preservation potential

Rates of burial and transport of molluscan remains are essentially unknown for deeper continental shelf and slope environments, especially over period s of years. An understanding of the rates of taphonomic loss are critical t o paleoecological analyses and to paleoenvironmental studies in general. Th e post-depositional history of organic remains is highly dependent on the l ength of time the material remains at or near the sediment/water interface. In order to measure these rates, 100 gastropod and bivalve shells were sca ttered over a marked area of sea bottom at 21 sites in seven environments o f deposition (EOD's) in the Gulf of Mexico and at five EOD's on the Bahamas platform edge. A total of over 2600 shells were deployed. Each site was th oroughly documented with video photography. After one year in the Bahamas a nd after two years in both the Gulf of Mexico and Bahamas, these sites were re-photographed and videotaped to measure rates of burial and movement of shells. Shell condition (e.g., articulation, encrustation, and color loss) for those shells that remained exposed was also determined. Shells deployed in Gulf of Mexico petroleum seep sites, on the Open continental shelf; and on the continental slope experienced high rates of burial (0.5-3.0 cm) wit hin two years. Shells at these sites generally were not transported or dist urbed, and disarticulation rates were lour. ln. the Bahamas, shells on the platform shelf were completely buried within one year. On the steep platfor m edge from 70 to 300 m, shells on hardground ledges remained exposed, wher eas shells in carbonate sands were buried by up to 3.5 cm of sediment. Tran sport was more common on the steep slopes of the platform edge. Net sedimen tation rates for the outer continental shelf and slope of 0.01-0.06 cm yr-l are well below our observed burial rates of (3)1 cm yr-1. Thus, burial rat e may be somewhat independent of sedimentation rate due to local reworking of sediments by storms at shallower depths and mechanisms such as deep bott om currents or bioturbation at deeper sites. Therefore, the potential for f ossil preservation in offshore areas with low sedimentation rates may be mu ch greater than previously assumed.