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The abundant epifaunal scallop Adamussium colbecki and ophiuroid Ophionotus victoriae bioturbate the seafloor under the multi-year sea ice in Explorers Cove, western McMurdo Sound, Antarctica by churning the upper few centimeters of the sandy substrate. Laboratory investigation of these animals' activities demonstrates that the scallops on average resuspend 45 cm3 d−1 ind−1 via water jets ejected during swimming and clapping movement and that the ophiuroids on average disrupt 861 cm3 d−1 ind−1 via rowing motion and self-burial in the upper centimeters of the substrate. At observed densities in Explorers Cove, bioturbation rates greatly exceed sediment accumulation rates and physical reworking processes, resulting in homogenized sediment lacking both lamination and discrete traces that would signal its biogenic modification. Adamussium colbecki employs water jets to resuspend the limited phytodetritus needed by the suspension-feeding scallop in the quiet sub-sea ice conditions and produces depressions ∼ 3 cm deep that increase substrate surface area and microtopography in Explorers Cove. The scallop thus acts as an ecosystem engineer, modifying the sediment and controlling the flux of materials between the substrate and water as it claps for food gathering rather than for predator evasion like other scallops. The resulting rapid, shallow diffuse bioturbation differs from the more commonly reported vertical bioturbation by infaunal animals recorded in cores, yet it exerts strong control on the sedimentary record and may be characteristic of low-energy, oligotrophic environments beneath multi-year sea ice around Antarctica where a lack of fast predators and low sedimentation rate allow proliferation of epifaunal animals.
Study of ancient cold-methane seep deposits provides insight into the changes in seep communities over the lifetime of a seep, which are otherwise difficult to observe in modern settings. We studied 24 cold-methane seep deposits in the Upper Cretaceous (Campanian) Pierre Shale of southwestern South Dakota. These deposits were subdivided into three categories depending on their physical characteristics: (1) those with a single main conduit, few secondary pipes and concretionary bodies, and no carbonate cap, implying strong advective flow to the sediment-water interface; (2) those with a single main conduit, a moderate number of secondary pipes and concretionary bodies, and a small carbonate cap, implying both advective and diffusive flow to the sediment-water interface; and (3) those with a single main conduit, a high number of secondary pipes and concretionary bodies, and a broad carbonate cap, implying extensive flow, but dampening at the sediment-water interface due to the presence of the large carbonate cap. We analyzed the faunal composition at all 24 seeps. The number of species ranges from five to 20. All of the seeps are dominated by baculitid ammonites, inoceramids, and lucinids (“foundation” organisms). These species are the same as those in time-equivalent non-seep sites in the Pierre Shale and are not seep-obligate. However, in seep categories 2 and 3, the number and kind of secondary organisms increases in association with the development of the large carbonate cap. These organisms include oysters, gastropods, echinoids, sponges, crinoids, and scaphitid ammonites. We infer that these organisms appear because (1) the carbonate hardground provides a more diverse habitat allowing attachment and encrustation and (2) the bottom waters are better oxygenated and/or the level of hydrogen sulfide is reduced because the methane rich fluids are diverted away from the carbonate cap, thus providing a more suitable habitat for organisms such as scaphitid ammonites that require a well-oxygenated environment. However, even at these seeps, the number of secondary organisms usually does not exceed that of foundation organisms.
Skeletal concentrations, defined here as deposits ≥ 10% by volume invertebrate bioclasts (> 2 mm), are very common targets of paleobiological investigations. The complex interactions among biological, taphonomic, and physical environmental processes influence the type and quality of information that can be drawn from these paleobiological repositories. This study examines the relative roles of bioclast input and burial on the formation of Middle–Upper Devonian skeletal concentrations from tropical carbonate-dominated settings. Based on original field observations of skeletal concentrations and host lithologies, skeletal concentrations are compared (1) between a thermally subsiding passive margin (Nevada) and a relatively stable cratonic interior (Iowa) and (2) across a range of comparable subtidal depositional environments. The majority of skeletal concentrations are thin (≤ 30 cm), monotypic, have simple internal stratigraphy, and exhibit moderate to high degrees of skeletal fragmentation and disarticulation. Compared to the subsiding margin of Nevada, the cratonic interior of Iowa preserves a higher proportion of polytaxic skeletal concentrations with complex internal stratigraphy and higher taphonomic damage, consistent with control by delayed or slow burial. From shallow to deep subtidal environments, stratigraphic thickness and internal complexity decrease and fragmentation increases, consistent with strong control of these attributes by the frequency of physical and biogenic reworking. While low bioclast production rates may exert a dominant control on the failure to accumulate thick, dense skeletal concentrations in these deposits, taphonomic and physical environmental processes, such as low net sediment accumulation, sediment winnowing and starvation, and bioturbation can explain much of the variation in Middle–Upper Devonian skeletal concentrations.
The rich record of vertebrate, hominin and archaeological remains recovered from Olduvai Gorge in northern Tanzania stands in stark contrast to the largely unexplored macroinvertebrate record from the region. Here we examine fossil malacofauna from Olduvai Gorge, inclusive of new discoveries and previous reports, and survey their potential as paleoecologic indicators. Recorded for the first time from Olduvai, an assemblage of fossil bivalve shells is attributed by character comparison to modern Chambardia wahlbergi, a freshwater unionid species widespread across Africa. The fossilized shells were localized in Bed III conglomerate channel deposits, with channel geometry exhibiting scour bases and superimposed fill structures with fining upward sequences. The ecology of recent C. wahlbergi combined with sedimentological data indicate the aquatic environment in this region during Olduvai Bed III times can be reconstructed as a periodically desiccated floodplain bordering a river channel or channels with permanent running water and marked seasonal fluctuations. This paleo-environmental setting presents drastic change compared with that of the lower Bed I and Bed II deposits, when an alkaline/saline lake extended over the site and fresh water was restricted to standing groundwater-fed pools with snail species known today to be intermediate hosts for the trematode genera Schistosoma (schistosomiasis) and Fasciola (fascioliasis). This research enhances details of landscape evolution at Olduvai basin and furthers paleoenvironmental interpretations during the time of Bed III deposition.