Showing papers in "PALAIOS in 1994"
TL;DR: Foraminifera commonly dominate ocean-floor eukaryotic communities and are the most abundant benthic organisms to be preserved in the post-Paleozoic deep-sea fossil record as discussed by the authors.
Abstract: Foraminifera commonly dominate ocean-floor eukaryotic communities. They also are the most abundant benthic organisms to be preserved in the post-Paleozoic deep-sea fossil record. Living faunas contain numerous delicate, soft-bodied agglutinated forms which have virtually no fossilization potential, whereas fossil faunas usually consist of calcareous and more robust agglutinated taxa. Recent observations have emphasized the important, often active role that foraminifera play in the dynamics and structuring of deep-sea benthic ecosystems. Foraminiferal abundances are closely linked to levels of organic matter input, and to dissolved oxygen concentrations in the near-bottom water which, in part, are related inversely to the magnitude of organic fluxes
296 citations
TL;DR: The real purpose of Smith's book is not to provide a tutorial for cookbook cladistics, but instead to present an outline of the potential implications of phylogenetic studies on previous paleobiological studies and what paleontologists can do within a phylogenetic estimate.
Abstract: Describing phylogenetic relationships among fossil taxa has been an important part of paleontology since the 19th century. In the last decade, the proliferation of phylogenetic estimates generated by computer-run cladistic analyses has resulted in a reappraisal of the importance of phylogenetics in paleontological studies. Andrew Smith's book Systematics and the Fossil Record represents a powerful presentation of the cladistic agenda for paleobiological studies. Because it touches on nearly every aspect of paleontology, all paleontologists should read this book soon. However, I emphasize the word "soon." Despite the claim on the back of the book by Derek Briggs (i.e., that there will be a time when no paleontologists will use taxonomic data without consulting Smith's book), this book really is more like last year's computers. That is, Systematics and the Fossil Record is extremely useful and thought provoking, but parts of it already are antiquated and much of the rest of it will become outdated in the near future, partly because of the research that it will inspire. Readers with a background in systematic theory obviously will have a much easier time appreciating Smith's book, but Smith does include a chapter describing cladistic methods. Unfortunately, this chapter conveys the impression that systematists have universally agreed on these methods when many aspects of cladistic methodology are still hotly debated. For example, there is much debate about how workers should treat multistate characters (i.e., when, if ever, they should be ordered or unordered; Hauser and Presch, 1991; Slowinski, 1993), missing data (e.g., see Maddison, 1993), polymorphic characters (including both polymorphisms within species and variation among species within higher taxa), total evidence (i.e., whether one should include different types of data, such as different molecular and morphologic characters, in the same parsimony analysis; see Swofford, 1991; de Queiroz, 1993; Eernisse and Kluge, 1993; Bull et al., 1993), and character weighting (Neff, 1986; Bryant, 1989; Chappill, 1989; Farris, 1990). Because Smith does not discuss the issues in detail (which is understandable, given that full treatments would have doubled the length of the book), his recommendations come across as assertions. Indeed, some of Smith's counsels are contradictory. For example, in his discussion of coding inapplicable data (i.e., how to code a toothless species for characters about specific tooth types), Smith advises the multistate character state schemes such as: 1: No teeth; 2: Tooth Type A; 3: Tooth Type B. However, in a previous discussion, Smith recommends treating multistate characters as unordered rather than ordered because unordered characters (supposedly) make fewer a priori assumptions. When applying this logic to multistate solutions for inapplicable characters, Smith's recommended coding essentially assumes that the different tooth types have nothing to do with one another (i.e., that they are not necessarily homologous at the level of being teeth). Is this really preferable to ordering the characters (which would assume that teeth are homologous)? Several other difficulties render many of Smith's suggestions problematic in anything other than simple situations. Consider, for example, what happens when one wishes to use several different types of tooth characters. Thus, although I think that paleontologists lacking a strong background in systematic theory will understand the book, the book is not really appropriate for learning cladistic methods. The real purpose of Smith's book is not to provide a tutorial for cookbook cladistics, but instead to present an outline of the potential implications of phylogenetic studies on previous paleobiological studies and what paleontologists can do within a phylogenetic estimate. There are some lapses, however, especially when Smith is detailing the inadequacies of the fossil record. For example, Smith suggests that the apparent tendency for orders to appear in nearshore habitats, as documented by Jablonski and Bottjer (1990a, b), is an artifact of preservational patterns. Counter-arguments to Smith's criticisms are presented elsewhere (Jablonski and Sepkoski, in press); what is notably absent is a description of how Jablonski & Bottjer's hypothesis might be better tested within a phylogenetic context or if there are any phylogenetic analyses that might contradict their hypothesis. Also, although Smith prefaces the book with the opinion that a phylogenetic approach better compensates for sampling than do other approaches, Smith gives little discussion to the effects of sampling on the robustness of cladistic analyses. Smith's discussion of higher taxa took me slightly by surprise. Instead of dismissing all higher taxa (as have some cladists; e.g., de Queiroz and Gauthier, 1990), Smith advocates retaining the Linnaean system if only for communication. Smith also acknowledges that paraphyletic taxa will corrupt diversity studies largely in the tallying of extinctions, and then only if the "extinction" of a paraphylum does not accompany the true extinction of a lineage within that taxon (see Fig. 1). Interestingly, Smith tacitly advocates a taxonomic scheme advocated (also tacitly) by Sepkoski and Kendrick (1993, a citation curiously absent from Smith's book): i.e., taxonomies in which both paraphyletic and monophyletic taxa always encompass monophyletic groups at any point in time and always terminate with the extinction of a lineage or subclade (as in Fig. 1A). What is not made clear by Smith is that where paraphyletic taxa are concerned, noise will not be created by arbitrary diagnoses of higher taxa, but by diagnoses systematically biased by stratigraphy (e.g., even though a Late
283 citations
TL;DR: In this article, the origin, and spatial and temporal relationships of carbonaceous sediments were investigated in fourteen sections from a range of geographic and oceanographic settings in the North and South Atlantic, Indian and Pacific Ocean Basins.
Abstract: Normal marine deposition in the early Aptian was interrupted by an episode of ocean-wide dysoxia/anoxia. This event is recorded by the occurrence of organic carbon-rich sediments in land sections from Europe and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) sites in the North and South Atlantic, Indian and Pacific Ocean Basins. To elucidate the origin, and spatial and temporal relationships of these carbonaceous sediments, we have conducted an integrated biostratigraphic, lithostratigraphic and geochemical investigation of fourteen sections from a range of geographic and oceanographic settings
264 citations
230 citations
135 citations
TL;DR: The association between reef mounds and benthic microbial communities and siliceous sponges is characteristic of early Cambrian, Early-Middle Ordovician, Late Silurian, Late Devonian, Late Mississippian and Late Permian age.
Abstract: The association between mound-building, benthic microbial communities and siliceous sponges is characteristic of some reef mounds of Early Cambrian, Early-Middle Ordovician, Late Silurian, Late Devonian, Late Mississippian, Late Permian, Late Triassic and Late Jurassic age. Significant episodes of siliceous sponge-microbe reef mound construction, each lasting 5-15 Ma, generally recurred at intervals of approximately 70-100 Ma. Each was a time when thrombolite-forming and/or stromatolite-forming calcimicrobes flourished as constructors, and associated demosponges and hexactinellid sponges diversified as bafflers and binders, and even constructors on the reef mounds
114 citations
TL;DR: Measureting the mechanical strength of drilled and undrilled valves of the Recent bivalve Mulinia lateralis suggests that drilled values may break preferentially and hence inferred patterns of predation may reflect taphonomic as well as biological processes.
Abstract: The proportion of bivalve shells with drill holes in fossil assemblages is commonly used as a measure of the intensity of predation by drilling gastropods. Previous studies have assumed that drilled and undrilled bivalve shells have equal preservation potentials. We tested this assumption by measuring the mechanical strength of drilled and undrilled valves of the Recent bivalve Mulinia lateralis. Under compressive loads, drilled valves are significantly weaker than undrilled valves, a difference we attribute to local stress concentrations produced by the presence of the drill hole. Our results suggest that drilled values may break preferentially and hence inferred patterns of predation may reflect taphonomic as well as biological processes
110 citations
105 citations
TL;DR: In this paper, a set of mathematically equiualent statistics and equations were developed to measure the relative or percentage frequencies of skeletal parts, and none of these clearly accounts for differential fragmentation of skeletal elements in part because the quantitative units used in them are not explicitly defined.
Abstract: Attempts to establish relative frequencies of skeletal parts as distinctive of particular bone accumulating and depositional agents have a deep history in taphonomic research. Various authors have developed a set of mathematically equiualent statistics and equations all of which are meant to measure the relative or percentage frequencies of skeletal parts. None of these clearly accounts for differential fragmentation of skeletal elements in part because the quantitative units used in them are not explicitly defined. Comparisons of frequencies of skeletal parts deposited by different bone-accumulating agents and reported by different analysts may thus be comparisons of differential fragmentation rather than differential deposition of skeletal parts
97 citations
TL;DR: In the late Precambrian and early Cambrian, there was a concomitant increase in abundance and diversity of trace fossils that reached very high levels by the Early Cambrian as mentioned in this paper.
Abstract: Metazoa evolved in shallow water seas in the late Precambrian, and there was a concomitant increase in abundance and diversity of trace fossils that reached very high levels by the Early Cambrian. There may have been some colonization of deep water, as evidenced by the «soft-bodied» fauna at Mistaken Point, Newfoundland but it was mostly by sessile animals which left few trace fossils. Shallow water late Precambrian and Early Cambrian sequences have yielded not only shallow water trace fossils but also many examples of traces typical of later deep water deposits. Significant colonization of the deep oceans by trace making animals was, however, delayed until the Early Ordovician
80 citations
TL;DR: The deep-sea floor is traditionally perceived as a habitat where low food flux and sluggish bottom currents force life to proceed at slow, steady rates as discussed by the authors, however, a number of recent discoveries indicate that endogenous disturbances may be relatively frequent, and that pulses of food reach the seafloor from the upper ocean.
Abstract: The deep-sea floor is traditionally perceived as a habitat where low food flux and sluggish bottom currents force life to proceed at slow, steady rates. In this view, benthic community structure is controlled by equilibrium processes, such as extreme levels of habitat partitioning, made possible by remarkable ecosystem stability. A number of recent discoveries indicate, however, that endogenous disturbances may be relatively frequent, and that pulses of food reach the seafloor from the upper ocean. The biological processes driven by these events can be highly variable in space and time, exhibiting disequilibrium dynamics
TL;DR: Follmi and Grimm as discussed by the authors suggest that the persistence of oxygen-depleted environmental conditions limited the survival time of these transported infaunal dwellers and rendered them doomed pioneers.
Abstract: Laminated, organic-rich hemipelagic sediments from the Oligo-Miocene San Gregorio Formation, Baja California Sur, Mexico, contain isolated horizons (n = 41) of Thalassinoides and Gyrolithes burrows which are exclusively associated with granity-flow event deposits (phosphatic turbidites). The geological evidence suggests that some turbulent sedimentation events entrained line infaunal crustacea from neritic settings; upon deposition in anaerobic environments, these crustacea penetrated the substrate and reworked substantial volumes of sediment, in an environment which excluded successful larval recruitment. We suggest that the persistence of oxygen-depleted environmental conditions limited the survival time of these transported infaunal dwellers and rendered them doomed pioneers (Follmi and Grimm, 1990)
TL;DR: In this paper, the primary reefal cavities preserved ubiquitously within the Permian Capitan Limestone leads to a profound reappraisal of the ecology, diagenesis and construction of the Capitan reef.
Abstract: Study of the primary reefal cavities preserved ubiquitously within the Permian Capitan Limestone leads to a profound reappraisal of the ecology, diagenesis and construction of the Capitan reef. Contrary to established opinion, most calcareous sponges (such as the sphinctozoans Guadalupia spp., Amblysiphonella spp., Cystauletes spp. and Lemonia spp.) did not grow upright in thickets to form a reef framework or bafflestone. Rather they were pendant cryptobionts inhabiting cavities, often in considerable abundance. In addition to this distinctive cryptic community, the framework of the reef was constructed in part by microbially-bound sediment, with the walls and ceilings of the cavities frequently supported by fenestellid and ramose bryozoans
TL;DR: The Mimbral outcrop in northeastern Mexico represents nearly continuous sedimentation across the Cretaceous/Tertiary (K/T) transition as mentioned in this paper, and the K/T boundary is present in a 4 cm clay layer and 3 mm red layer above the top of a channel-fill deposit.
Abstract: The Mimbral outcrop in northeastern Mexico represents nearly continuous sedimentation across the Cretaceous/ Tertiary (K/T) transition. The K/T boundary is present in a 4 cm clay layer and 3 mm red layer above the top of a channel-fill deposit. The 60 m wide and 3 m high channel-fill deposit is of latest Maastrichtian age (A. mayaroensis Zone). It consists of faunally, lithologically and mineralogically distinct units that appear to represent a series of gravity flows related to the latest Maastrichtian sealevel lowstand. The biotic effects of the K/T boundary event were not catastrophic for planktic foraminiferal faunas at Mimbral, NE Mexico. Although 2/3 of the species disappeared at or below the K/T boundary, the effect on the overall foraminiferal population was small (< 17%) because only rare, already endangered taxa disappeared
TL;DR: In this article, an alternative, integrated chronostratigraphic model for the Middle Ordovician of the Mohawk Valley and the St Lawrence Lowland of Quebec was proposed.
Abstract: Published correlations of the extensive set of altered vol- canic ash beds (K-bentonites) present in the upper Middle Ordovician rocks of New York State imply that the lower Climacograptus (Diplacanthograptus) spiniferus Zone, the entire Orthograptus ruedemanni Zone, and the upper Corynoides americanus Zone are coeval. Precise new information on the ranges of graptolite and conodont species and the position of zonal boundaries within key Mohawk Valley sections together with new data on the occurrence of K-bentonites provide the foundation for an alternative, integrated chronostratigraphic model for the Middle Ordovician of the Mohawk Valley and the St. Lawrence Lowland of Quebec. This alternative model rec- onciles the K-bentonite and biostratigraphic correlations, but remains to be independently tested
TL;DR: A mixed assemblage of lucinid and mussel shells were placed in mesh bags and left at a site of autochthonous death assemblages formation in a petroleum seep community on the Louisiana upper continental slope for a period of 3 years as mentioned in this paper.
Abstract: 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
TL;DR: Xenophyophore densities are highest on sloped sediments associated with seamounts, continental slopes, canyons and trenches, and beneath productive surface waters as discussed by the authors, and they can range from simple fans, discs or mudballs, to elaborately folded or reticulated forms, and often contain specific particle types or sizes.
Abstract: Xenophyophores are large (several mm to 25 cm diameter), agglutinating protozoans, found primarily in the deep sea. Tests range from simple fans, discs or mudballs, to elaborately folded or reticulated forms, and often contain specific particle types or sizes. Xenophyophore densities are highest on sloped sediments associated with seamounts, continental slopes, canyons and trenches, and beneath productive surface waters. Most forms live as epibenthos on hard or soft substrates, feeding on surface deposits and suspended particles, but one endobenthic genus has been described. Modern xenophyophores enhance particle flux to the seabed, creating local regions of intense radiotracer and metazoan activity
TL;DR: In this article, a hiatus is found between the carbonate units in the Lower Deep Spring Formation, suggesting that the lowermost Deep Spring, Reed, and Wyman formations are Neoproterozoic in age.
Abstract: Comparison of secular variations in carbon-isotopic compositions of carbonates from the NeoproterozoicCambrian succession in the Whiteinyo region of eastern California with similarly-aged sequences worldwide suggests that, in California, a hiatus potentially encompassing the latest Neoproterozoic and much of the Tommotian may be present. The hiatus is found between the carbonate units in the Lower Deep Spring Formation, suggesting that the lowermost Deep Spring, Reed, and Wyman formations are Neoproterozoic in age, and the remaining Deep Spring Formation is uppermost Tommotian or Atdabanian in age. It is unclear where the Neoproterozoic-Cambrian boundary may lie, since chemostratigraphic data is not available for the boundary in New foundland type section, but it is most-likely encompassed within the inferred hiatus
TL;DR: In Texas coastal bays and in many other low energy environments a high degree off shell,fragmentation (generally,from 75 to 95 weight percent) is characteristic of shell material in sediment samples as mentioned in this paper.
Abstract: In Texas coastal bays and in many other low energy environments a high degree off shell,fragmentation (generally,from 75 to 95 weight percent) is characteristic of shell material in sediment samples. The feeding activity of predators such as the euryhaline sciaenid Pogonias cromis (Black drum), which is a common inhabitant off Atlantic and Gulf of Mexico shallow marine waters, is responsible ,for,fragmenting large quantities of live molluscan shell material in environments in which the natural energy gradient precludes extensive mechanical,fragmentation. P. cromis locates potential,food items with sensory barbels located on its lower jaw. Prey are ingested and crushed with pharyngeal teeth. Shell material may be rejected orally or may pass through the digestive system
TL;DR: The Shiramazu Zoophycos represent the product of an inverted conveyor activity of the tracemaker throughout its growth as mentioned in this paper, and are found abundantly in upper Pliocene middle bathyal deposits of the southernmost Boso Peninsula, central Japan.
Abstract: Well-preserved specimens of the trace fossil Zoophycos are found abundantly in upper Pliocene middle bathyal deposits (Shiramazu Formation) of the southernmost Boso Peninsula, central Japan. The Shiramazu Zoophycos represents the product of an inverted conveyor activity of the tracemaker throughout its growth. Some of the Zoophycos occur in mudstone beneath a volcanic ash layer. These specimens contain tuffaceous pellets composed of pyroclastic material alone or of mud mixed with pyroclastic sand grains. Such specimens allow the recognition of synchronous individuals within the complex ichnofabric. This is because the accumulation of pyroclastic material on the seafloor, and its subsequent stuffing into the burrows via the gut by the burrowproducer, are thought to be a short-term event
TL;DR: A sequence of strata containing 24 or more ichnogenera was investigated along the western outcrop margin of the Breathitt Formation in eastern Kentucky as discussed by the authors, where tour facies were delineated within the study interval.
Abstract: A sequence of strata containing 24 or more ichnogenera was investigated along the western outcrop margin of the Breathitt Formation in eastern Kentucky. Tour facies were delineated within the study interval. Dark shales of the Carbonaceous Shale Facies (CSF) exhibit a low-diversity/low-abundance ichnofossil assemblage dominated by small Conostichus, Planolites, and Thalassinoides. Some interbedded siltstones exhibit greater diversity with Lockeia, Paleophycus, Monocraterion, and Rhabdoglyphus. The CST is truncated by the Bioturbated Heterolithic Facies (BHF), consisting of bioturbated sandstones with robust Asterosoma, Conostichus, and Rosselia, soltstones and gray shales with abundant Planolites and Teichichnus, and non- bioturbated sideritic claystones
TL;DR: Sediment cores from the north and south slope of the Iceland-Faeroe Ridge taken at water depths from 500 m on the crest and 1500 m from the south slope show the influence of different hydrographic and sedimentary regimes on biogenic structures as mentioned in this paper.
Abstract: Sediment cores from the north and south slope of the Iceland-Faeroe Ridge taken at water depths from 500 m on the crest and 1500 m from the south slope show the influence of different hydrographic and sedimentary regimes on biogenic structures. Bottoms on the north slope contain an almost monospecific community of Chondrites-like burrows, whereas ichnocoenoses on the south slope are more diverse and show partly complex ichnofabrics with the ichnogenera Scolicia, Zoophycos, Trichichnus, Anconichnus-like, large Planolites-like, Teichichnus-like burrows, Palaeophycus, Gyrolithes and Helminthopsis. Grain size, controlled by bottom currents and ice-rafting, as well as redox conditions of the sediments seem to be the main factors controlling this ichnological pattern
TL;DR: In this paper, the nature and distribution of trace-fossil and microbenthic assemblages in Tethyan hemipelagic carbonates and marks of the Podzamcze Formation (Bajocian, Pieniny Klippen Belt) are studied.
Abstract: The nature and distribution of trace-fossil and microbenthic assemblages in Tethyan hemipelagic carbonates and marks of the Podzamcze Formation (Bajocian, Pieniny Klippen Belt) are studied. These analyses allow the identification of (1) recurring Planolites-Chondrites, Planolites-Chondrites-Zoophycos/Teichichnus and Chondrites ichno-assemblages, and (2) poor microbenthic assemblages dominated by agglutinated foraminifers and rich microbenthic assemblages dominated by diversified calcareous foraminifers as well as ostracods. A cross correlation of trace-fossil and microbenthic assemblages with the lithology and petrography of their host sediments suggests the presence of three different paleo-oxygen levels ranging from strongly dysaerobic, moderately dysaerobic to slightly dysaerobic (or aerobic)
TL;DR: The maceral and palynological composition of the C coal bed (Upper Cretaceous), central Utah, was significantly affected by the periodic deposition of volcanic ash in the precursor peat mire as discussed by the authors.
Abstract: The maceral and palynological composition of the C coal bed (Upper Cretaceous), central Utah, was significantly affected by the periodic deposition of volcanic ash in the precursor peat mire. The coal bed contains four altered volcanic ash partings (tonsteins). Sixty-seven coal and rock samples from 10 cores of the coal bed were examined for maceral composition, and 163 coal and rock samples from 10 cores were analysed for palynological composition. Abundant semifusinite (an inertinite maceral with a poorly preserved cell structure and a hight gray reflectance) is found in coal samples directly below the upper tonstein, the thickest (30-40 cm thick) of the tonstein partings
TL;DR: This article found that brachiopod shells often contain skeletal fragments embedded in their exterior surfaces that cleanly pierce one or both valves of the host specimen, and that the fragments were emplaced via pressure-dissolution during post-burial sediment compaction.
Abstract: Well-preserved brachiopods from two Paleozoic deposits (Ordovician Richmond Group, Indiana; Devonian Silica Formation, Ohio and Michigan) occasionally contain skeletal fragments embedded in their exterior surfaces. Cross-sections of the shells reveal that the fragments are the remains of brachiopods that cleanly pierce one or both valves of the host specimen. A sharp contact between fragment and host, a coincident film of insoluble residue, and, in some samples, a thin zone of recrystallization, all suggest that the fragments were emplaced via pressure-dissolution during post-burial sediment compaction. Diagenetically emplaced fragments may disintegrate during weathering and leave grooves (pseudo-borings) that resemble the traces of shell-boring organisms. Apparent teeth or rhyncholites found embedded in brachiopod shells may arise from pressure-dissolution between fortuitously associated particles. If pressure-dissolution artifacts, like these, are not distinguished from true biological traces, an overestimation of boring and predation intensities in paleoecological studies may result. Several simple criteria for distinguishing artifacts from traces are proposed, including regular plicate or linear shape of groove, random orientation and lack of host response. This study documents that even in exceptionally well-preserved fossil deposits, pressure-dissolution can be an important taphonomic process.
TL;DR: The Abrakurrie Limestone is cyclic at the meter-scale as mentioned in this paper and the best developed cycles are asymmetric and comprise three distinct parts with a capping hardground, the basal part is a thin, coarse, grainstone or rudstone that is rich in robust bryosoan and epifaunal echinoid fragments and pecten bivalves.
Abstract: The open-shelf, subtidal, bryozoan-rich Abrakurrie Limestone beneath the Nullarbor Plain (Eucla Platform) is cyclic at the meter-scale. Best developed cycles are asymmetric and comprise three distinct parts with a capping hardground. The basal part (A) is a thin, coarse, grainstone or rudstone that is rich in robust bryosoan and epifaunal echinoid fragments and pecten bivalves, reflecting growth and accumulation in generally high energy, hard bottom environments. The middle part (B), intepreted as a low-energy, sub-swellbase accumulation, is a burrowed to planar cross-laminated fine grainstone or packstone with a low diversity, delicate-branching bryozoan assemblage and little else except scattered infaunal echinoids and pectens. The upper part (C) is a burrowed, abundantly fossilferous (bryozoan, bivalve, echinoid) rudstone or floatstone. Upward increases in the numbers and diversity of Mg-calcite and aragonitic cheilostome bryozoans (especially erect rigid, flat robust branching and nodular/arborescent types), gastropods, infaunal bivalves, and infaunal echinoids points to a high-energy environment. Sediments at the top of C are variably cemented by inclusion-rich marine cement (now calcite) that formed a hardground which was subsequently physically and biologically eroded and stained by iron oxides during a period of non-deposition. Sediments from the next overlying cycle succeed cements in uppermost intergranular pores and fillmore » open crustacean burrows. Variably developed cyclicity is interpreted, on the basis of comparable Holocene cool-water shelf sediments, to reflect deposition in generally sub-photic environments that ranged from just below swell base (B) upwards towards the zone of wave abrasion (A and C). Hardgrounds (H) formed when the seafloor was within the zone of wave abrasion. Shallowing and deepening of these critical interfaces was controlled by fluctuating sea level and/or climatic change. 55 refs., 12 figs.« less
TL;DR: In this article, an alternative method of producing a frozen profile within event beds as a result of the nature of the depositional regime is proposed, based on the crosscutting relationships displayed by ichnofauna, or the maximum or characteristic depths of their occurrence.
Abstract: The recognition of habitat partitioning in ancient marine sedimentary sequences, via the development of a tiering profile, is an important part of the reconstruction of paleocommunities. Such tiering profiles are often establisheh on the basis of either the cross-cutting relationships displayed by ichnofauna, or, the maximum, or characteristic, depths of their occurrence. However, a complete tiering profile may be preserved as a frozen profile. A change in the environmental conditions, for example upon rapid deoxygenation of the sediment profile, may result in the evacuation of the profile by the infauna and thus the production of a frozen profile. An alternative method of producing a frozen profile within event beds as a result of the nature of the depositional regime is proposed
TL;DR: In this article, an adult Laternula elliptica, an infaunal, aragonitic, Antarctic bivalve, was obtained from microsamples of an adult.
Abstract: Oxygen and carbon isotopic measurements were obtained from microsamples of an adult Laternula elliptica, an infaunal, aragonitic, Antarctic bivalve. The δ 18 O values of samples from the exterior surface are, on average (∼ 4.5‰), similar to calculated values inferred to represent precipitation in equilibrium with ambient environmental conditions. This indicates that bulk samples as well as many microsamples from the exterior surface would provide reliable isotopic estimates of paleotemperatures and paleosalinities. Nevertheless, both oxygen and carbon isotopic values from discrete shell areas may be influenced by vital effects
TL;DR: In this paper, a detailed serial sectioning of cores by computed tomography (CT) has been proposed to enable the comparison of traces in modern and ancient deep-sea deposits, which is limited by the fact that horizontal traces, which are dominant in deep-water deposits, are poorly expressed on conventional vertically oriented X-radiographs of unconsolidated sediments.
Abstract: The study of biogenic structures in cores is hampered by the facts that the structures are generally exposed only on vertical surfaces, and that the area available for study is limited by typically small core diameter. Computed tomography (CT) gives a three-dimensional picture by producing a series of bedding-parallel sections. It thus provides a technique that gleans the maximum information possible from the limited volume of sediment without destroying the core. Enhanced images of beddingparallel traces in cores of unconsolidated sediments in CT images will facilitate the comparison of traces in modern and ancient deep-sea deposits. This comparison has been limited previously by the fact that horizontal traces, which are dominant in deep-water deposits, are poorly expressed on conventional vertically oriented X-radiographs of unconsolidated sediments. The detailed serial sectioning of cores by CT will also allow reconstruction of the tiering relationships of deep-water trace producers.