George M. Staff

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 periods of years. An understanding of the rates of taphonomic loss are critical to paleoecological analyses and to paleoenvironmental studies in general. The post-depositional history of organic remains is highly dependent on the length of time the material remains at or near the sediment/water interface. In order to measure these rates, 100 gastropod and bivalve shells were scattered over a marked area of sea bottom at 21 sites in seven environments of deposition (EOD9s) in the Gulf of Mexico and at five EOD9s on the Bahamas platform edge. A total of over 2600 shells were deployed. Each site was thoroughly documented with video photography. After one year in the Bahamas and after two years in both the Gulf of Mexico and Bahamas, these sites were re-photographed and video-taped 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) within two years. Shells at these sites generally were not transported or disturbed, and disarticulation rates were low. In the Bahamas, shells on the platform shelf were completely buried within one year. On the steep platform edge from 70 to 300 m, shells on hardground ledges remained exposed, whereas shells in carbonate sands were buried by up to 3.5 cm of sediment. Transport was more common on the steep slopes of the platform edge. Net sedimentation rates for the outer continental shelf and slope of 0.01-0.06 cm yr-1 are well below our observed burial rates of 31 cm yr (super -1) . Thus, burial rate may be somewhat independent of sedimentation rate due to local reworking of sediments by storms at shallower depths and mechanisms such as deep bottom currents or bioturbation at deeper sites. Therefore, the potential for fossil preservation in offshore areas with low sedimentation rates may be much greater than previously assumed.

LOCAL VARIABILITY OF TAPHONOMIC ATTRIBUTES IN A PARAUTOCHTHONOUS ASSEMBLAGE: CAN TAPHONOMIC SIGNATURE DISTINGUISH A HETEROGENEOUS ENVIRONMENT?

ABsTRAcr-Taphofacies have been based on the likelihood that considerable variability exists in taphonomic processes between different environments and that this variability produces predictable variations in taphonomic signature between assemblages. Three stations above storm wave base that differed little in sediment texture and depth were sampled on the inner continental shelf of central Texas. Taphonomic analysis revealed subtle gradients in sediment grain size and water depth that would not be revealed by most other analyses. These gradients may exist over very small spatial scales, equivalent to those within a single extensive outcrop. Not all taphonomic attributes are equally likely to be preserved in the fossil record. Those varying with depth in our study area, such as fragmentation and articulation, are more likely to be preserved than those documenting changes in sediment texture, such as variation in the frequency of dissolution features on the shells. Nevertheless, siting and sampling protocols are important when characterizing a taphofacies because within-habitat variation is potentially as large as between-habitat variation. Description of the average taphofacies for an environment must include documentation of the variation in taphonomic attributes within the sampled area because few conservative taphonomic attributes exist. Fragments, even those that are unidentifiable, retain significant taphonomic information and should not be ignored. Careful sampling should permit the simultaneous description of general taphofacies as well as the detection of important but unsuspected gradients in the environment.

Taphonomy on the continental shelf and slope: two-year trends ^ Gulf of Mexico and Bahamas

Abstract The Shelf and Slope Experimental Taphonomy Initiative was established to measure taphonomic rates in a range of continental shelf and slope environments of deposition (EODs) over a multiyear period. We deployed experiments on the forereef slope off Lee Stocking Island, Bahamas, and on the continental shelf and slope of the Gulf of Mexico for 2 yr in 18 distinctive EODs at depths from 15 to 530 m. Overall, most shells deployed at most sites had relatively minor changes in shell condition. Most EODs generated relatively similar taphonomic signatures. A few sites did produce taphonomic signatures clearly distinguishable from the central group and these sites were characterized by one or more of the following: high rates of oxidation of reduced compounds, presence in the photic zone, and significant burial and exhumation events. Thus, unique taphonomic signatures are created by unique combinations of environmental conditions that include variables associated with regional gradients, such as depth and light, and variables associated with edaphic processes, such as the seepage of brine or petroleum or the resuspension and redeposition of sediment. Most sites, however, showed similar taphonomic signatures, despite the variety of EOD characteristics present, suggesting that insufficient time had elapsed over 2 yr to generate a more diverse array of taphonomic signatures. Discoloration and dissolution were by far the dominant processes over the 2-yr deployment period. Periostracum breakdown, loss of shell weight, and chipping and breakage was less noticeable. EODs were chosen based on the expectation that the process of burial and the influence of depth and sediment type should play the greatest roles in determining between-EOD differences in taphonomic signature. EOD-specific edaphic factors often overrode the influence of geographic-scale environmental gradients. Taphonomic alteration was greater on hardgrounds and in brine-exposed sites than on terrigenous muds. Dissolution was less effective at sites where burial was greatest. Discoloration occurred most rapidly at shallower sites and on hardgrounds. Water depth was less influential in determining taphonomic signature than burial state or sediment type. The limited influence of water depth is likely due to the presence of shallow sites that, for one reason or another, were protected from certain taphonomic processes and deeper sites that were characterized by unusually strong taphonomic signals.

Taphonomic Trends Along a Forereef Slope: Lee Stocking Island, Bahamas. I. Location and Water Depth

Abstract The Shelf and Slope Experimental Taphonomy Initiative (SSETI) Program was established to measure taphonomic rates in a range of continental shelf and slope environments. Experiments were deployed on the forereef slope off Lee Stocking Island, Bahamas, for one and two years along two transects (AA and BA) in seven distinctive environments of deposition (EODs) along each transect: in sand channels on the platform top (15 m) and the platform edge (30 m), on ledges down the wall (70–88 m), on the upper (183 m—transect BA only) and lower (210–226 m) talus slope below the wall, and on the crest (256–264 m) and in the trough (259–267 m) of large sand dunes. Discoloration was by far the dominant taphonomic process over the two-year deployment period, with dissolution or maceration of shell carbonate a close second. Periostracum breakdown was not significant, nor was loss of shell weight. Chipped edges and breakage (assayed by the edge alteration variable) were much less common, but were important in some species. The degrees of edge alteration and dissolution were correlated with discoloration more frequently than expected by chance, emphasizing that the process of discoloration progressed in a coordinated fashion with the other two over time. The degree of burial or the interaction between degree of burial and water depth explained most of the trends observed in discoloration. The deep water sites, below the photic zone, including the talus slope and dune EODs, had very similar taphonomic signatures. Shells were characterized by a low degree of discoloration, little edge alteration, and varying degrees of dissolution. Photic zone sites, including the platform top and wall locations, followed the opposite trends, with the shallowest site, on the platform top, typically attaining the most extreme degree of alteration. The wall location was most similar to the platform top despite the greater depth and less rigorous physical and sedimentological regime. The platform edge occupied an intermediate position, likely due to the greater degree of burial that resulted in shells at this site being at least as frequently under aphotic conditions as under photic conditions. The data indicate that similar taphonomic signatures can be attained in distinctly different ways over a two-year exposure period, complicating the interpretation of taphofacies and the taphonomic process.

Taphonomic Trends Along a Forereef Slope: Lee Stocking Island, Bahamas. II. Time

Abstract The Shelf and Slope Experimental Taphonomy Initiative (SSETI) Program was established to measure taphonomic rates in a range of continental shelf and slope environments over a long period of time. For this report, mollusk shells were deployed for one and two years at seven different environments of deposition (EODs) along two onshore-offshore transects off Lee Stocking Island in the Bahamas. The experimental sites were located: in sand channels on the platform top (15 m) and the platform edge (33 m); on ledges down the wall (70–88 m); on the upper (183 m) and lower (210–226 m) talus slope below the wall; and on the crest (256–264 m) and in the trough (259–267 m) of large sand dunes. Shell condition was assessed using a range of taphonomic attributes including dissolution, abrasion, edge alteration, discoloration, and changes in shell weight. After two years, taphonomic alteration was not particularly intense in any EOD. No species was particularly susceptible or resistant to taphonomic alteration. Taphonomic processes were unexpectedly complex. Effects of location, transect, water depth, and degree of exposure all had significant effects. On average, shells deployed in shallow sites were altered significantly from the controls more frequently than shells deployed at deeper sites. However, the number of significant interaction terms between time and the other main effects indicates a complex interaction between taphonomic processes and the local environment that, over the short term, defies any attempt at delineating taphofacies over a broader spatial area than a single deployment site. Some locations attained the same taphonomic signature in different ways making discrimination of taphonomic rules difficult. For example, deeper-water sites and shallow sites where burial rates were high yielded similar taphonomic signatures because shells were in the aphotic zone in both cases, and this limited the rate and range of taphonomic interactions. Taphonomic processes were strongly nonlinear in time for all taphonomic attributes in all species and all EODs. Nonlinear taphonomic rates hinder the interpretation of single-point-in-time studies in understanding the taphonomic process and buttress a commitment to long-term experiments.

MOLLUSCAN SHELL CONDITION AFTER EIGHT YEARS ON THE SEA FLOOR—TAPHONOMY IN THE GULF OF MEXICO AND BAHAMAS

Abstract In 1993 and 1994, the shelf and slope experimental taphonomy initiative (SSETI) deployed shells of a suite of molluscan species in a range of environments of deposition (EODs) representing a range of depths, sediment types, and environmental conditions with the goal of measuring taphonomic rates over an extended period of time. In 1999 and 2001, SSETI retrieved skeletal remains from 41 locations in the Bahamas and on the Gulf of Mexico continental shelf and upper slope that had been on the seafloor for eight years. Here, we compare taphonomic processes in two different ocean basins, across 24 environments of preservation (EOP) to evaluate the influence of species, sedimentary environment, degree of burial, and water depth on the preservational process. Taphonomic signature after eight years was almost exclusively a function of location of deployment and, frequently, taphonomically-distinctive locations of deployment were subsumed within distinctive EODs. EOD-level characteristics were insufficiently discriminative to delineate environments of preservation. EOPs and EODs are not synonymous concepts. Across all sites and species, the dominant taphonomic process was discoloration. Dissolution was of penultimate importance; nevertheless the cumulative impact over eight years was insufficient to produce a significant loss in shell weight in any EOP. Maximum dissolution intensity was normally observed on the outer shell surface; the inner and outer shell surfaces are inherently different in their time course of shell deterioration. Principal components analysis (PCA) demonstrated limited co-occurrence of discrete taphonomic processes among the 24 EOPs. Breakage and edge rounding fell on the same PCA axis, but these two processes were independent of all others. PCA divided dissolution into three independent components that discriminated the inner and outer shell surface of bivalves (and spire and body whorl of gastropods) and pitting from the development of a chalky surface. Discoloration was dissembled into five distinctive discoloring processes: fading without subsequent discoloration, the development of a brown-to-red coloration, orange/orange mottled discoloration, development of a green/green mottled color, and gray-to-black discoloration. The only concordance of ostensibly distinctive taphonomic processes was the association of small pits on the shell surface with orange discoloration on the shell. Depth did not exert a single significant effect on any of the eight primary taphonomic factors resolved by PCA, likely because of burial processes. The trends in taphonomic signature cannot be explained by any simple combination of sediment type and degree of exposure. A comparison between two-year and eight-year deployments suggests that important revelations can be gleaned from short-term experimental deployments, yet the same comparison discloses the spuriousness of other inferences. Thus, long-term experiments are essential to understand the time course of preservation. The taphonomic process is, in general, slow, and nonlinearity in rates over time constrains the subset of inferences that can be deduced accurately from short deployment periods.

Onshore–offshore trends in community structural attributes: death assemblages from the shallow continental shelf of Texas

Abstract Death assemblages were compared at three sites on the inner continental shelf of Texas using the community attributes of taxon richness, taxonomic composition, habitat tiers, and feeding guilds, by means of three descriptor variables, numerical abundance, paleoproduction (biomass-at-death), and paleoingestion (lifetime ingestion, a measure of energy flow). Each death assemblage was compared to the co-occurring life assemblage and to six other death assemblages covering a transect from the estuary to the continental slope. Analysis of death assemblage composition is increased by at least a factor of two if fragments are included. In each of these nine assemblages on the bay-to-slope transect, the whole-shell component of the assemblage was adequate for a thorough analysis of community guild, tier, and taxon structure. The assemblage types were each unique in a combination of key abundance, paleoproduction and paleoingestion-derived community attributes. At least as important, however, were the resemblances between certain assemblages. All shelf and heterotrophic slope assemblages were characterized by predator dominance of paleoingestion. Deposit feeders and chemoautotrophs increased in importance numerically offshore, but not when evaluated by energy flow. All offshore assemblages were characterized by 40% or more of the individuals being infaunal. On the whole, tier structure was more variable than guild structure within habitat. On the whole, paleoingestion was more variable than numerical abundance or paleoproduction within habitat, probably because of the reliance of paleoingestion on long-lived taxa that are normally relatively rare. Greatest similarity was seen in paleoguild structure (a simplified guild structure) within and between habitat. Using paleoguild structure significantly limited the discrimination of assemblage types. The relative proportion of predators in the shelf and slope heterotrophic assemblages was striking. Comparing predator lifetime ingestion with prey lifetime production reveals that the slope cold-seep assemblages and bay assemblages have a large surplus of primary consumers, whereas the normal slope assemblage and the three inner shelf assemblages are overrepresented by predators. Assuming that the proportions of predator and prey indicate the relative importance of non-preservable prey, non-preservable prey were relatively more abundant on the shelf and slope, and, in fact, nonpreservable species contributed about 90% of the individuals to the life assemblage at the three inner shelf sites.

Application of trophic transfer efficiency and age structure in the trophic analysis of fossil assemblages

We evaluate onshore-offshore trends in age-frequency distributions and trophic transfer efficiencies using 11 modern death assemblages off the Texas coast. Trophic transfer efficiencies within trophic levels offer little insight over that achieved by a size-frequency distribution. Production/biomass ratios will always be 1 in the fossil record. Within trophic-level estimates of paleogrowth efficiency, the ratio of paleoproduction to paleoingestion (P i glt / I i lt where i indicates the i th trophic level and lt indicates the time-averaged value) follow the expected ecological trend precisely in that paleogrowth efficiency is consistently higher in primary consumers than in predators in all 11 death assemblages. Paleoutilization efficiency, the ratio of predator paleoingestion to prey paleoproduction, I 2 lt °/ P 1 glt °, may provide information on the degree of bias in the preservation of primary (1 °) and secondary (2 °) consumer trophic groups. I 2 lt °/ P 1 glt ° fell below 0.1 in most cold-seep and bay assemblages, indicating a large surplus of primary consumers. In sharp contrast, I 2 lt °/ P 1 glt ° was above 10.0 in most heterotrophic continental slope and inner continental shelf assemblages, indicating an overrepresentation of predators. In the classic food web, predator life span should exceed prey life span (Δ L lt: L 2 lt ° > L 1 lt °). Predators should be more insulated from fluctuations in food supply than their prey and the degree of this insulation should control Δ L lt. Of the 11 death assemblages covering the bay-to-continental slope transect, predators’ life spans were longer than their prey only on the continental shelf and slope, indicating a requirement for a stable enough environmental regime to permit biological processes rather than physical factors to control life spans. Paleoproduction transfer efficiency, the ratio of paleoproduction between two consecutive trophic levels, P 2 glt °/ P 1 glt °, is the most accurately estimated trophic transfer efficiency in paleoecology because it does not require an estimate of age-at-death. The relationships of paleoutilization efficiency and paleoproduction transfer efficiency are nearly identical among the 11 death assemblages. Although theoretically this need not be the case, these assemblages suggest that predator overrepresentation might be identified from a ratio dependent solely upon size-at-death.