Robert J. Stanton

Patterns and processes of shell fragmentation in modern and ancient marine environments

Shell fragments are important components of many Recent and fossil marine benthic ecosystems and can provide crucial information on past and present environmental conditions. Interpreting such fragments requires integrated knowledge in various fields and the information potential is therefore rarely optimally utilized. This paper uses the definition of a fragment as being a piece of shell having less than 90% of its original form. It then outlines the potential characteristics, pathways, and fates that shells and their fragments can have. Fragmentation is a key factor shaping the shelly part of death assemblages, but it is difficult to interpret because it can be broadly caused by ecological, biostratinomic or diagenetic processes and also depends on shell strength. Strength, in turn, reflects multifunctionality during ontogeny and depends on a complex set of skeletal and taphonomic factors. Therefore, no particular shell parameter clearly determines strength, but thickness, microstructure type and degree of organic matrix have the strongest influence on pre- and post-mortality strength. Size measurements are usually less important for shell strength than thickness, although ecologically complex size refuges from predation do exist. Similarly, shell shape, sculptural features and specific aperture types (in gastropods) provide various defence strategies rather than increasing strength per se. Key ecological factors of fragmentation include predation due to crushing, peeling, along with mistaken predation, selfinflicted damage during predation and burrowing, and more physical aspects such as impacts by stones. Modern studies must consider damage by benthic commercial fisheries or dredging by scientific vessels. Key biostratinomic factors include transportinduced abrasion (littoral zone of surf-washed beaches), bioerosion and dissolution (mainly sublittoral environments). Diagenetically, fragmentation during compaction mainly occurs when shells are in direct contact with one another or with coarser grains; taphonomic features (e.g., drillholes) have only minor influence. A key step in interpreting fragments is to categorize breakage into repaired versus unrepaired, diagnostic versus nondiagnostic, and severe versus localized damage. Quantifying the above categories can then provide useful information on breakage patterns and underlying processes. Understanding the many characteristics of shells and their fragments is a significant interpretive tool in maximizing the information gain in palaeoecological and taphofacies analyses. D 2003 Elsevier Science B.V. All rights reserved.

Taphonomic signature as a function of environmental process: shells and shell beds in a hurricane-influenced inlet on the Texas coast

Abstract Criteria for taphonomic processes, including dissolution, breakage, abrasion, size sorting and shell orientation, were statistically correlated against environmental parameters representing shell source, depositional environment and depth of burial, using samples obtained from San Luis Pass, a microtidal inlet on the Texas coast. Core intervals were differentiated by cluster analysis into shell-poor sands, slightly shelly sands, shell-rich sands and shell gravels based on their percent carbonate and percent shell gravel content. Boundaries between these sedimentary classes represent thresholds in physical processes forming the shell deposits. We propose that this classification scheme for shelly sediments may be useful for field descriptions in a variety of depositional and tectonic settings. The death assemblages were predominantly composed of whole and fragmental molluscan shells. Most shells show the effects of physical taphonomic processes such as shell breakage, edge rounding, surface abrasion and size sorting. A significant minority of the shells show evidence of minor chemical alteration (dissolution). Patterns of dissolution, edge rounding and surface abrasion differed significantly on shells in a mixed-faunal assemblage derived from different habitats (bay, inlet/beach, inner shelf and unrestricted). Thus, the habitat where a species lived, rather than its final depositional environment, controlled most aspects of its taphonomy. Only shell orientation and size-frequency distribution were significantly affected by final environment of deposition. The hypothesis that a discrete high energy (hurricane) “event” can imprint a unique taphonomic signature is, consequently, false. Although hurricanes physically winnow shell material, a single “event” merely concentrates shells with preexisting characteristics without further significant alteration. Depth of burial was poorly correlated with most biostratinomic effects, including dissolution. Significant dissolution must occur only near the sediment-water interface. Accordingly, quantified taphonomic characteristics might be useful in identifying the original habitats of species deposited in mixed accumulations; “taphofacies” reflect not only the final depositional process, but also taphonomic processes in the habitat of origin.

Late Neogene paleoceanography of the eastern Caribbean, the Gulf of Mexico, and the eastern Equatorial Pacific

Abstract Calcareous nannofossil census data from the late Neogene of the Caribbean, the Gulf of Mexico, and the eastern equatorial Pacific were subjected to a multifold analysis in order to extract the paleoceanographic history of the region. A detailed nannofossil biostratigraphy is the basic time framework. The paleoenvironmental signals are deduced from fluctuations of diversity, dominance, and equitability through time; by clustering of assemblages through time and examining the composition of cluster groups; by comparison of fluctuations in carbonate content and coarse-fraction with time; and by factor analysis of the assemblage. These measures variously reflect surface water characteristics such as productivity, bottom water characteristics such as corrosiveness (dissolution), and conditions of sedimentation such as dilution of the pelagic biogenic carbonate. Diversity, dominance, and equitability which are determined by surface water characteristics, show similar trends, often in considerable detail, in the Caribbean and the Gulf of Mexico from the late Miocene through Pleistocene. Broad but definite similarities exist in these measures between the Caribbean/Gulf of Mexico and the eastern equatorial Pacific in the late Miocene to mid Pliocene, but not in the late Pliocene. Clustering groups assemblages in a broad biostratigraphic pattern corresponding approximately to a major assemblage zone. The structure and composition of cluster groups indicate strong similarity between the Caribbean and the Gulf of Mexico during the late Miocene through Pleistocene, but only weak similarity of the Caribbean and Gulf of Mexico to the eastern equatorial Pacific, and only during the early Pliocene. Surface water productivity is generally similar at the three locations into the late Pliocene but seemingly diverges in the eastern equatorial Pacific after about 2.4 m.y. Variations in dissolution intensity have generally similar trends in the late Miocene and early Pliocene in the Caribbean and the Gulf of Mexico but the trends diverge in the late Pliocene and are dissimilar in the Pleistocene. In the eastern equatorial Pacific, dissolution intensity increases with the inferred thermal subsidence of the crust. Variations in the calcium carbonate content of the sediment, essentially pelagic carbonate, follow different trends at the three sites, which is attributable to different sedimentary histories at the sites (tectonic plus eustatic sea level changes for the Caribbean; eustatic sea level changes alone for the Gulf of Mexico; and dilution by biogenic silica plus dissolution for the eastern equatorial Pacific). The various measured and derived parameters indicate that as late as the mid Pliocene - approximately 3.8 m.y. - a deep water connection existed between the Pacific and the Caribbean/Gulf of Mexico. A shallower seaway, permitting exchange of surface water between the Pacific and Gulf of Mexico, seems to have existed into the late Pliocene - to between 3.0 to 2.2 m.y., and final closure probably coincided with the sea level draw-down associated with the initial buildup of a northern hemisphere ice sheet. During the late Pliocene and early Pleistocene surface productivity oscillates markedly but seemingly with different intensities in the Caribbean and the Gulf of Mexico. The most important floral change occurs in both basins at about 0.9 m.y. when the smallGephyrocapsa dominated high-productivity nannoflora of the preceeding ⋍0.3 m.y. was replaced abruptly by a moderate-productivity flora. This turnover marks a major change in ocean circulation towards a relatively warm, stably stratified tropical surface ocean, which seems to have been affected remarkably little by some of the intense late Pleistocene high latitude climatic changes.

Paleocommunity Reconstruction from Shell Beds: A Case Study from the Main Glauconite Bed, Eocene, Texas

Abstract The Main Glauconite Bed (MGB), near the top of the Eocene Stone City Member (Crocket Formation), Texas, has been considered to contain a typical local paleocommunity (parautochthonous assemblage formed within a stable habitat). Microstratigraphic analysis, however, reveals a complex sedimentologic and taphonomic history for the MGB, a unit that is 1.7–1.9 m thick and consists of three intercalated small-scale facies interpreted to represent differing modes of deposition. The primary autochthonous inner-shelf sediments are dark glauconitic clay-silts with a matrix-supported polytaxic fossil assemblage. Recurrent storms produced thin (few mm to cm) layers of mostly simple, bioclast-supported, polytaxic shell concentrations. These distal tempestites occur mainly as small-scale lenses and as a few beds and pods, associated with glauconite-pellets, terrigenous sands, and scarce sedimentary structures. Subsequent burrowing destroyed most skeletal concentrations and formed patches of fossils, glauconite-pellet sand, and terrigenous, very fine sand. The assemblages in the three facies are dominated by corbulids, naticids, turrids, noetiids, and the solitary coral Turbinolia sp., and are indistinguishable based on their taxonomic composition and most of the taphonomic features (disarticulation, fragmentation, incrustation, corrasion, shell repair, and predatory drill holes). Only drilled shells are significantly more abundant in the bioturbated patches than in the two other facies. The only strong evidence for the presence of allochthonous faunal elements is the lack of right valves of anomiid bivalves. The scarcity of significant differences between facies indicates the presence of one basic paleocommunity that was modified by small-scale and short-term depositional events and bioturbation, but which can still be recognized in spite of having been preserved by three different suites of depositional processes. Microstratigraphic analysis of bioclastic deposits can recognize small-scale sedimentologic and biostratinomic processes that otherwise frequently are overlooked in paleoecological studies. Such processes have only minor influence on taxonomic composition and taphonomic features, which are therefore robust characteristics of a fossil assemblage.

An accretionary distally steepened ramp at an intrashelf basin margin: an alternative explanation for the Upper Triassic Steinplatte “reef” (Northern Calcareous Alps, Austria)

Abstract The Upper Triassic massive limestone (the Oberrhdtkalk) at the Steinplatte, Austria, formed along the edge of an intrashelf basin that was situated within a shallow-marine to supratidal shelf approximately 40 km wide. This transition from shelf to intrashelf basin is marked by a distinct slope break that has been widely cited as an example of a reef-rimmed shelf. It is, instead, a distally steepened ramp. The Oberrhdtkalk consists of the ramp sediments lying between the Dachsteinkalk of the shelf and the Kossen Formation of the intrashelf basin. The ramp interpretation is based on the geometry and lateral facies progression in the profile from the shallow-marine to peritidal shelf, across the ramp, and into the basin, and on the absence of reef or grainstone deposits at the slope break typical of a rimmed shelf. The Oberrhdtkalk at the Steinplatte is an important example of shelf-margin deposition because: 1. (1) It formed in a protected and low-energy intrashelf basin setting, thus expanding the range of distally steepened ramps that have been used to develop a general model. 2. (2) It differs from previously described examples of distally steepened ramps, largely from open-ocean shelf-margin settings, in that: (a) it evolved constructionally from a homoclinal ramp rather than forming at a drowned shelf edge or an antecedent tectonic structure; (b) the sediments of the distal slope are largely autochthonous; (c) slumping and breccia deposits on the distal slope are of minor importance; turbidites on the basin floor are absent; and (d) skeletal packstone and grainstone characterize the outer ramp and slope rather than mudstone and wackestone. 3. (3) It provides an alternative ramp model for other Upper Triassic shelf and platform margins in the Northern Calcareous Alps, which have commonly been interpreted as reef or sediment rimmed.

Preservation of Mollusca in Copano Bay, Texas. The long-term record

Abstract The stratigraphic, taphonomic and biologic records from two cores in Copano Bay, Texas were analyzed to determine (1) whether variations in shell content with depth were caused by variations in carbonate preservation, carbonate production or sedimentation rate and (2) the extent to which characteristics of fossil assemblages, such as species composition, numerical abundance, biomass, and trophic and habitat structure, identified similar or different trends. Below the top few cm of the sedimentary column, variations in carbonate content with depth could be attributed to variations in carbonate production. Most biological attributes varied similarly with depth and, hence, time on both long (⪖100 yr) and short (∼10 yr) temporal scales. These variations could not be explained by any taphonomic process, sediment reworking and burial, or sedimentation rate. Despite a vigorous taphonomic milieu, to obtain the shell content of Copano Bay sediments requires the preservation of nearly all carbonate produced. Preservation of a large fraction of the shell carbonate added requires the preservation of most of the relatively large-shelled biota (large species and adults) which retain important evidence of changes in the communitys history in this area. The results reemphasize the importance of large individuals and biomass in paleontologic reconstruction and suggest that changes in community productivity, which in paleontologic usage, must be carbonate productivity, are preserved in the fossil record.

Paleomagnetic stratigraphy of Pliocene Centerville Beach section, northern California

Abstract Samples from the upper two-thirds of the approximately 1900 m thick Neogene section exposed south of Centerville Beach on the northern California coast have predominantly reversed detrital remanent magnetism. Fossil evidence suggests a lower Pliocene through lower Pleistocene age for the section. The combined paleomagnetic and fossil data indicate that a large part of the section was deposited during the Matayama reversed epoch (2.4-0.7 million years ago). Samples from correlative sections exposed a few kilometers inland from the Centerville Beach section have a predominantly normal polarity and appear to have been remagnetized. The Centerville Beach section is important because it may serve as a standard with which to compare both other on-land Pliocene sections from western North America and nearby Deep Sea Drilling Project cores.

Population dynamics in Dendraster, Merriamaster, and Anadara from the neogene of the Kettleman Hills, California

Abstract Samples of the echinoid Dendraster and the bivalve Anadara from the Neogene sedimentary rocks exposed in the Kettleman Hills of California contain few small specimens. The echinoid Merriamaster shows a more complete range of specimen sizes. Factors such as current winnowing, migration of adults to an area separate from juveniles, and mechanical, chemical, or biological destruction perhaps contributed to the lack of small specimens. However, rapid growth rate in juveniles and low juvenile mortality are probably the major reasons for the lack of small specimens. The adult portion of survivorship curves should be little affected by taphonomic processes and should thus contain useful information about population dynamics. The convex survivorship curves of Dendraster show low juvenile mortality followed by increasing mortality with age in adults. Dendraster specimens are always found in sediments indicating firm substrates and turbulent conditions. Convex survivorship curves for populations from this type of environment have been previously noted in other species. Merriamaster populations have linear to slightly convex survivorship curves. They probably lived in slightly less turbulent conditions than most Dendraster populations. Anadara populations lack small individuals, suggesting low juvenile mortality or taphonomic processes, but the post-juvenile portion of their survivorship curves range from linear to strongly convex. The linear pattern is frequently found in populations in sediments with a high proportion of mud, suggesting lower energy conditions.

The application of guild and tier structure and energy flow in paleoecologic analysis: An example using parautochthonous death assemblages from a variable salinity bay

We examine the use of paleoproduction (the net production of somatic tissue over an individuals lifetime) and paleoingestion (the energy required to sustain the animal over its life span) in habitat tier and feeding guild analysis of paleocommunities, using modern death assemblages from a variable salinity bay. Community attributes evaluated by the abundance of the constituent species differed markedly from the same attributes described by paleoproduction or paleoingestion. Examination of the guild and tier structure from the perspective of paleoproduction and paleoingestion identified significant temporal changes in environmental optimality, probably related to changes in food supply, and in the division of resources amongst small and large species. Significant downcore changes in habitat optimality occurred only for the large species in the assemblage; those potentially most affected by changes in food supply. Tier structure was less variable than guild structure because species replacement always occu...