Taphonomic degradation of molluscan remains during thirteen years on the continental shelf and slope of the northwestern Gulf of Mexico
In 1993, SSETI (Shelf and Slope Experimental Taphonomy Initiative) deployed shells of a suite of molluscan species in a variety of environments of preservation (EOPs) covering a range of depths and sediment types on the Gulf of Mexico continental shelf and upper slope, with the goal of measuring taphonomic rates over an extended period of time. SSETI retrieved shells from these EOPs after 2, 8, and 13 years. The degree of shell degradation was significantly influenced by species, elapsed time-since-deployment, and EOP. A suite of ‘physical’ taphonomic processes, such as dissolution, abrasion, discoloration, and edge alteration, increased in severity over the 13 years; fragmentation occurred in some species. Dissolution proceeded apace; however the incidence of chalkiness declined from high levels observed after a few years while the more extreme levels of dissolution, such as the development of a soft or deeply-dissolved surface, rose significantly in frequency. The incidence of original color declined, while fading of original color increased. Brown-to-red and green discoloration rose rapidly in the first eight years and then declined, leaving a faded shell surface. Between-habitat differences in degradation rate were significant for most taphonomic attributes. Between-species differences were minor in comparison. Thus, taphofacies, the product of the independent actions of a suite of taphonomic processes, originate from and provide information on environmental conditions. Species composition has a lesser inherent influence on the outcome. Not uncommonly, the rates of change in shell condition differed significantly between EOPs although the direction of change was coincident. This was particularly true of the summary indices such as maximum discoloration or the average degree of dissolution. The taphonomic process is nonlinear in time. Nonlinearity is EOP-dependent, becoming a defining attribute leading to disparate taphofacies types. Some taphonomic processes cannot proceed expeditiously without prior alteration of the shell through other taphonomic means. Some taphonomic conditions such as chalkiness can be intermediate states. The presumption that similar taphonomic characteristics between EOPs indicate similar environmental processes operating at similar rates is falsified by SSETI sites in which similar taphonomic indices at Year 13 accrued from different time-varying degradational rates during the preceding years.