Karla M. Parsons-Hubbard

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.

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.

Molluscan Taphofacies in Recent Carbonate Reef/Lagoon Systems and their Application to Sub-Fossil Samples from Reef Cores

Abstract Modern taphofacies analyses have been worked out for different mixed carbonate and siliciclastic sedimentary environments using molluscan faunas. Studies of how the combined effect of the physical, chemical, and biological characteristics of the environment of deposition results in a particular taphonomic signature on fossilizable remains must be done on Recent assemblages to understand taphonomic signatures of ancient faunas. Studies have defined modern taphofacies by erecting broad damage categories to which each sample is assigned. Others graphically characterize environments based on individual taphonomic indices (e.g., fragmentation, abrasion), rely on presence-absence data, or construct ternary taphograms for each taphonomic characteristic. This study defines taphofacies based on a statistical treatment of the entire taphonomic signature of molluscs from several carbonate reef and lagoon systems in the northeastern Caribbean. The method uses non-metric multidimensional scaling ordination to test whether the combined taphonomic signature is recognizable across closely associated shallow carbonate environments. Results show that a statistical technique combining many taphonomic factors is a reliable method for deciphering taphonomic signature. It is most important, however, to test whether taphofacies defined using modern shells can be applied to the fossil record. Taphonomic traits from mollusc shells obtained from reef cores were added to the same statistical routine and assigned environments of deposition based on their taphonomic signature. Molluscs from within the cores suggest that the shelf and patch-reef environments on the SW coast of Puerto Rico have remained fairly stable up to at least 7,000 ybp. Shells were recovered that had distinct reef and open-shelf taphonomic signatures. Shells from cores from Buck Island, U.S. Virgin Islands had signatures that suggested migration of the reef over seagrass areas of the shelf, and/or landward into the lagoon behind Buck Island reef. Thus, not only does the total taphonomic signature mirror the environment of deposition, but the taphonomic signature is likely to be preserved well enough to be useful in making paleoenvironmental interpretations of fossil material.

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.

Experimental Taphonomy Of Callinectes Sapidus And Cuticular Controls On Preservation

Abstract Examination of remains of Callinectes sapidus deployed in several depth and environmental settings in the Bahamas and Gulf of Mexico as part of the Shelf and Slope Experimental Taphonomy Initiative project revealed that all specimens were uniformly and strongly degraded except those in brine-seep settings. Fragmentation and loss of cuticular material at all sites was correlated to the degree of calcification within the cuticle of different skeletal elements as observed in the undeployed specimens. Claws, tips of the last anterolateral spine, and mandibles were the most durable remains. In brine-seep areas, extraordinary preservation yielded articulated skeletal elements and some soft tissue. Examination of the cuticle in control specimens with cross-polarized light and computed tomographic scanning documents the correspondence of high degrees of calcification with portions of the exoskeleton remaining after deployment.

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.

Bioerosion or bioaccumulation? Shelf‐slope trends for EPI‐ and endobionts on experimentally deployed gastropod shells

A total of 59 taxa of epibionts and endobionts occurred on experimentally deployed gastropod shells within one year of emplacement at depths ranging from 15 m to 260 m in the Bahamas. Most of the diversity occurred within 73 m of water depth. The experimental shells at the deepest sites (210 m, 260 m) were essentially pristine. Differences in experimental treatment affected the results: shells in bags contained more bionts than tethered shells, suggesting the bags had more protective areas for biont settlement. Soft‐bodied encrusters were restricted to the upper 73 m while foraminiferans and bryozoans exhibited bathymetric trends to the deepest sites. While boring algae and cyanobacteria were ubiquitous on the shells to 73 m, other bioeroders (e.g., clionid sponges) were rare. Bioaccumulation, rather than bioerosion, is the predominant process affecting mollusc shells during the first year of taphonomic exposure in carbonate systems to depths of 260 m.

Predation on Experimentally Deployed Molluscan Shells from Shelf to Slope Depths in a Tropical Carbonate Environment

Abstract Predation was examined using experimentally deployed shells along two transects representing shallow shelf (15 to 30 m), outer shelf (70 m) and bathyal (100 m to 267 m) habitats in the Bahamas. Lethal breakage on experimentally tethered shells was not restricted to the shallow-shelf (≤ 30 m), but could occur to outer shelf (70 m) and upper slope (≥ 88 m) depths. However, significantly more shells were preyed on at shallow-shelf depths (≤30 m) than at deeper depths (≥ 70 m) for both transects. Predation was not restricted to shelf sites (≤ 70 m), and predator-induced damage, such as peeled shells or last whorl remnants, could be encountered to a depth of 195 m. However, significantly more predation occurred at shelf rather than slope (≥ 88 m) depths for both transects. Although the experimental shells were exotic species, they were preyed upon at differing frequencies depending upon transect location and depth. It appears that morphology alone may not account for the differences in predation. Long-term deployment (six years) of shells enhanced the likelihood of predation, but short term deployment (one or two years) did not. Molluscivorous predators in the Caribbean are highly diverse and are not restricted to the shallow shelf, but occur in outer shelf to slope settings. Therefore, evolutionary-ecological conclusions based on predation with depth should not be tied to a single site, to a few depths, or to a single predator, as habitat heterogeneity and patchiness in predator and prey distributions may vary.

THE TAPHONOMIC SIGNATURE OF A BRINE SEEP AND THE POTENTIAL FOR BURGESS SHALE STYLE PRESERVATION

Abstract Unusually fine preservation of soft anatomy in the fossil record, often referred to as Lagerstätte deposits, has led to great advances in understanding the evolution of life. An understanding of the potential environments of deposition that might lead to exquisite preservation may help to reconstruct the effects of the taphonomic filter and thereby better interpret the completeness of fossil Lagerstätten. Seafloor brines are potential environments leading to exceptional preservation. The Shelf and Slope Experimental Taphonomy Initiative (SSETI) placed mollusc shells, decapod crustaceans, sea urchins, and wood into a Gulf of Mexico seafloor brine pool environment to study the rates and modes of skeletal and soft tissue decay. We found that skeletons, soft tissue, and wood placed directly in the sulfidic anoxic brine were essentially not degraded or discolored over nearly a decade. Where the brine mixed with overlying seawater in a brine stream, the taphonomic signature was quite different. Calcium-carbonate shell and urchin tests underwent severe dissolution, whereas terrestrial plant remains were unaltered. Farther from the brine, shell and urchin carbonate was only slightly dissolved, wood was completely consumed by xylophagus animals, and decapods were reduced to claw parts only. From these experiments, we conclude that the taphonomic signature of a brine seep can be recognized by a unique juxtaposition of preservation styles that varies across phyla. The central area of the anoxic brine would promote exquisite preservation of carbonate, soft-animal tissue, and cellulose. The central area would be ringed by a zone of near total loss of shell carbonate, but paradoxically would promote the preservation of organic tissue such as shell periostracum and ligament, wood, nuts, and cones. Where seawater salinity is normal, the taphonomic signature would return to a seafloor assemblage appropriate to the depth and depositional environment. Brine seep systems may provide a mechanism for maintaining unaltered organism remains at the sediment-water interface long enough to become buried with soft anatomy intact and undisturbed. The very important fossil deposit known as the Burgess Shale exhibits preservation styles and patterns that might be explained by presence of brine. Our experimental work in a modern brine system may shed some light on the taphonomic conditions that led to preservation known as the “Burgess Shale type.”