W. Russell Callender

Gulf of Mexico Hydrocarbon Seep Communities: VI. Patterns in Community Structure and Habitat

Communities of chemosynthetic fauna that depend on seeping oil and gas have been found in the Gulf of Mexico at approximately 45 sites between 88°W and 95°W and between the 350 and 2,200 m isobaths. Investigations suggest that the number of sites and the range of occurrence will increase with additional exploration. The dominant fauna consist of species within four groups: tube worms, seep mussels, epibenthic clams, and infaunal clams. These species co-occur to some degree, but tend to form assemblages dominated by a single group. Community development is closely coupled to the geological and geochemical processes of seepage.

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.

Taphonomic signature of petroleum seep assemblages on the Louisiana upper continental slope; recognition of autochthonous shell beds in the fossil record

Chemoautotrophically-based benthic communities on the Louisiana continental slope are currently producing the only significant localized, autochthonous shell accumulation in the northern Gulf of Mexico shelf and slope region. These sites are well below storm wave base and are not subject to anthropogenic disturbance. Five distinctive biofacies are associated with petroleum seepage, dominated respectively by vestimentiferan tubeworms, lucinid, thyasirid and vesicomyid clams and mytilid mussels. The taphonomy of petroleum seeps includes dissolution as the most pervasive mode of shell alteration throughout all the biofacies

Fine-scale distribution of methanotrophic mussels at a Louisiana cold seep

Abstract Extensive aggregations of methanotrophic mussels (Mytilidae: Bathymodiolus -like) were found in a 60×300m zone on the 640m isobath of the Louisiana slope. Within the aggregations, living mussels occurred in dense curvilinear clusters up to 5m in length. Defunct clusters, consisting of gaping and disarticulated valves, were also common. Comparison of length frequency distributions and mean densities of mussel clusters demonstrated that recruitment of juvenile mussels was ongoing in certain clusters and completely lacking in others. Surface sediments within the zone were characterized by dark patches and linear depressions, apparently associated with seeping hypersaline fluids. Pore fluids in surface sediments contained elevated salt concentrations (289% of ambient sea water), concentrations of ammonia up to 1.6mM, and up to 3mM H 2 S.

Taphonomic rates of molluscan shells placed in autochthonous assemblages on the Louisiana continental slope

A mixed assemblage of lucinid and mussel shells were placed in mesh bags and left at a site of autochthonous death assemblage formation in a petroleum seep community on the Louisiana upper continental slope for a period of 3 yr. Upon recovery, the shells were assessed for taphonomic alteration and compared to a control assemblage of unaltered shells. The data verify a basic assumption of taphofacies analysis; that evidence of taphonomic processes preserved with the assemblage does in fact document the primary taphonomic processes that biased the assemblage from the original assemblage of living preservable organisms. Significant variability in taphonomic rates existed between shells from locations 10 m apart, as is typical of autochthonous assemblages, so that small-scale variability in the taphonomic process was important

Distinguishing autochthony, parautochthony and allochthony using taphofacies analysis; can cold seep assemblages be discriminated from assemblages of the nearshore and continental shelf?

Identifying ancient cold seep faunas is intertwined with the problem of distinguishing autochthony from parautochthony. Cold seep faunas are autochthonous assemblages. Taphofacies from an allochthonous microtidal inlet, a parautochthonous inner shelf and an autochthonous upper slope (representing cold seep faunas) were compared to identify their distinguishing characteristics and to facilitate cold seep recognition. The taphonomic signature of the allochthonous inlet assemblage is dominated by abrasion and edge alteration. Bivalves are completely disarticulated

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.

Autochthonous death assemblages from chemoautotrophic communities at petroleum seeps: Palaeoproduction, energy flow, and implications for the fossil record

Death assemblages produced by chemoautotrophic communities at cold seeps represent a type of autochthonous accumulation that is difficult to differentiate from other heterotrophic autochthonous communities using taphonomic characteristics. We test the hypothesis that cold‐seep assemblages can be discriminated by unique biological or community attributes rather than taphonomic attributes. To test this hypothesis, we compared several cold seeps on the Louisiana upper continental slope to heterotrophic sites on the Louisiana slope and to a putative seep site in the middle‐late Campanian Pierre Shale near Pueblo, Colorado. Seep assemblages are characterized by a unique tier and guild structure, size‐frequency composition, and animal density that together identify the palaeoenergetics structure of these communities and distinguish them from the other assemblages of the shelf and slope. All seep assemblages were dominated by primary consumers, whereas the heterotrophic assemblage was dominated by carnivores. Ca...