Jonathan P. Warnock

A revised method for determining the absolute abundance of diatoms

Diatoms and other microfossils are used extensively in reconstructions of past climates and environments, in both terrestrial and marine settings. Both relative and absolute abundances of diatom taxa in sediment are important tools in these reconstructions. However, acquiring these data is a labor-intensive process. Settling-based diatom slide preparation techniques often bias samples through aliquot subsampling and sediment washing. Other techniques involve the use of added markers, which might obscure diatoms on the slide. This paper presents a revision to the widely adopted settling-based diatom slide preparation method presented by Scherer (J Paleolimnol 12:171–179, 1994) and provides a direct comparison to another widely used method. Evenly distributed diatom slides can be created by a settling process, which yields multiple statistically similar diatom slides without needing to clean sediment of salts or do aqueous subsampling, which may impart a bias in the sample when there is a wide range of particle shapes and sizes in the assemblage. Two samples originally utilized by Scherer (J Paleolimnol 12:171–179, 1994) were prepared via the updated method through a series of replicates. These results were compared to the same samples, processed with the method of Schrader and Gersonde (Utrecht Micropaleontol Bull 17:129–176, 1978), utilizing Petri dishes and the original results of Scherer (J Paleolimnol 12:171–179, 1994). The new modification presented here produces smaller standard deviations than the original Scherer method, and order of magnitude better statistics than the Schrader and Gersonde (Utrecht Micropaleontol Bull 17:129–176, 1978) method.

Abundance and richness of key Antarctic seafloor fauna correlates with modelled food availability

Most seafloor communities at depths below the photosynthesis zone rely on food that sinks through the water column. However, the nature and strength of this pelagic–benthic coupling and its influence on the structure and diversity of seafloor communities is unclear, especially around Antarctica where ecological data are sparse. Here we show that the strength of pelagic–benthic coupling along the East Antarctic shelf depends on both physical processes and the types of benthic organisms considered. In an approach based on modelling food availability, we combine remotely sensed sea-surface chlorophyll-a, a regional ocean model and diatom abundances from sediment grabs with particle tracking and show that fluctuating seabed currents are crucial in the redistribution of surface productivity at the seafloor. The estimated availability of suspended food near the seafloor correlates strongly with the abundance of benthic suspension feeders, while the deposition of food particles correlates with decreasing suspension feeder richness and more abundant deposit feeders. The modelling framework, which can be modified for other regions, has broad applications in conservation and management, as it enables spatial predictions of key components of seafloor biodiversity over vast regions around Antarctica.Combining data on sea-surface chlorophyll-a with a regional ocean model and diatom abundance from sediment grabs, the authors determine the strength of pelagic–benthic coupling across the George V region in East Antarctica.

New data towards the development of a comprehensive taphonomic framework for the Late Jurassic Cleveland-Lloyd Dinosaur Quarry, Central Utah

The Cleveland-Lloyd Dinosaur Quarry (CLDQ) is the densest deposit of Jurassic theropod dinosaurs discovered to date. Unlike typical Jurassic bone deposits, it is dominated by the presence of Allosaurus fragilis. Since excavation began in the 1920s, numerous hypotheses have been put forward to explain the taphonomy of CLDQ, including a predator trap, a drought assemblage, and a poison spring. In an effort to reconcile the various interpretations of the quarry and reach a consensus on the depositional history of CLDQ, new data is required to develop a robust taphonomic framework congruent with all available data. Here we present two new data sets that aid in the development of such a robust taphonomic framework for CLDQ. First, x-ray fluorescence of CLDQ sediments indicate elevated barite and sulfide minerals relative to other sediments from the Morrison Formation in the region, suggesting an ephemeral environment dominated by periods of hypereutrophic conditions during bone accumulation. Second, the degree of abrasion and hydraulic equivalency of small bone fragments dispersed throughout the matrix were analyzed from CLDQ. Results of these analyses suggest that bone fragments are autochthonous or parautochthonous and are derived from bones deposited in the assemblage rather than transported. The variability in abrasion exhibited by the fragments is most parsimoniously explained by local periodic re-working and re-deposition during seasonal fluctuations throughout the duration of the quarry assemblage. Collectively, these data support previous interpretations that the CLDQ represents an attritional assemblage in a poorly-drained overbank deposit where vertebrate remains were introduced post-mortem to an ephemeral pond during flood conditions. Furthermore, while the elevated heavy metals detected at the Cleveland-Lloyd Dinosaur Quarry are not likely the primary driver for the accumulation of carcasses, they are likely the result of multiple sources; some metals may be derived from post-depositional and diagenetic processes, and others are potentially produced from an abundance of decomposing vertebrate carcasses. These new data help to support the inferred depositional environment of the quarry as an ephemeral pond, and represent a significant step in understanding the taphonomy of the bonebed and Late Jurassic paleoecology in this region.

Role of Sediment Size and Biostratinomy on the Development of Biofilms in Recent Avian Vertebrate Remains

Microscopic soft tissues have been identified in fossil vertebrate remains collected from various lithologies. However, the diagenetic mechanisms to preserve such tissues have remained elusive. While previous studies have described infiltration of biofilms in Haversian and Volkmann’s canals, biostratinomic alteration (e.g., trampling), and iron derived from hemoglobin as playing roles in the preservation processes, the influence of sediment texture has not previously been investigated. This study uses a Kolmogorov Smirnov Goodness-of-Fit test to explore the influence of biostratinomic variability and burial media against the infiltration of biofilms in bone samples. Controlled columns of sediment with bone samples were used to simulate burial and subsequent groundwater flow. Sediments used in this study include clay-, silt-, and sand-sized particles modeled after various fluvial facies commonly associated with fossil vertebrates. Extant limb bone samples obtained from Gallus gallus domesticus (Domestic Chicken) buried in clay-rich sediment exhibit heavy biofilm infiltration, while bones buried in sands and silts exhibit moderate levels. Crushed bones exhibit significantly lower biofilm infiltration than whole bone samples. Strong interactions between biostratinomic alteration and sediment size are also identified with respect to biofilm development. Sediments modeling crevasse splay deposits exhibit considerable variability; whole-bone crevasse splay samples exhibit higher frequencies of high-level biofilm infiltration, and crushed-bone samples in modeled crevasse splay deposits display relatively high frequencies of low-level biofilm infiltration. These results suggest that sediment size, depositional setting, and biostratinomic condition play key roles in biofilm infiltration in vertebrate remains, and may influence soft tissue preservation in fossil vertebrates.

Increased diatom dissolution in Prydz Bay, East Antarctica linked to inception of the Prydz Bay gyre

Diatom assemblages and abundance in marine sediment cores are utilised as proxies of paleoceanographic conditions. However, diatoms are altered by taphonomic processes, which cause diatom accumulation rates to vary while simultaneously altering assemblages by removing some species from the sedimentary record. Furthermore, these taphonomic processes vary with space and time. Additionally, significant insights into the marine carbon and silicon cycles can be gained by assessing variability in diatom dissolution. Sediment core NBP0101-JPC24 from Prydz Bay, East Antarctica, provides a detailed Holocene record of diatom community shifts in an ecologically significant section of the Antarctic coast. The sedimentary and paleontologic record in JPC24 records the inception of the Prydz Bay gyre during deglaciation, as well as an increase in sea ice and in wind-related turbulence over the last 7000 years. A record of diatom preservation for JPC24 has been established by examining dissolution-induced changes in the morphology of Fragilariopsis curta, a sea-ice-related diatom common in JPC24. Diatom dissolution in NBP0101-JPC24 does not significantly correlate with the relative abundance of any diatom species or ecological species group. However, dissolution is low during the deglacial, prior to the inception of the Prydz Bay gyre and resultant polynya, and increases as the gyre increases in size and strength to present conditions. Circulation in the upper water column associated with the modern persistent gyre causes diatoms and other silt-sized particles to remain in suspension. Prior studies have shown that diatoms exposed to caustic marine water continue to dissolve until they are sequestered in sediments. Once sequestered in the sediments, diatoms will stop dissolving when a chemical equilibrium has been achieved between silicic acid in solution and solid opal. Diatom dissolution, therefore, is seen to be controlled by physical oceanographic factors rather than diatom ecology.