Biogenic Traces Research Articles

A search for life in Palaeoproterozoic marine sediments using Zn isotopes and geochemistry

Sediments from the 2.1- to 1.9-billion-year-old Francevillian Group in southeastern Gabon include centimeter-sized pyritized structures suggestive of colonial organisms (El Albani et al., 2010), some of which may have been motile (El Albani et al., 2019). However, these interpretations were largely based on morphological and geochemical characteristics that lack metabolic clues and/or can be explained by abiotic processes. To move this work forward, we describe other centimeter-sized specimens, loosely referred to as lenticular forms (LF), from the same area and apply a more holistic approach including morphology, mineralogy, and geochemistry. The objects are 0.2–4 cm in diameter, and most of them are endowed with a regular brim that scales proportionally to external diameter reminiscent of biological order, hence rendering the LF putative biogenic traces. The LF are perfectly delineated in every direction and deflect the sedimentary layers on which they rest. X-ray microtomography further demonstrates that the LF are syn-depositional features and not concretions, while lead isotope systematics indicate that the geochemical imprint of diagenesis is inconsequential. Low sulfur content is largely concentrated in the organic matrix, and scarcity of pyrite and its persistence as micron-sized crystals show that the role of sulfate reduction is minor. Most interestingly, the fillings of the LF cavities show large and correlated excesses of organic carbon and zinc, with the latter being distinctly enriched in its light isotopes. The geochemical anomalies of the fillings relative to the host rock, notably those associated with Zn, clearly were buried with the LF, and further imply biogenicity. In this regard, a ten-fold increase in LF size towards the top of the black shale series hosting the LF might be related to increasing Zn (nutrient) availability. Although we cannot conclude with any certainty what these remnant organisms were, their features all taken together are evocative of very large agglutinate protists that grazed on bacterial biomass either in the water column or as benthic mats.

Comparison of rosette-shape traces in abyssal terrains: Environmental and faunal implications

Biogenic traces (lebensspuren) result from the activity of benthic organisms on and within the seafloor and are a common feature of deep-sea environments. Rosette-shape traces (RSTs), also referred to as spoke burrows, are a commonly observed and distinctive trace, likely created by echiuran worms during feeding and burrowing. We conduct a quantitative morphometric assessment of RSTs from two contrasting sites at the Porcupine Abyssal Plain Sustained Observatory (PAP-SO; Northeast Atlantic) with similar physico-chemical conditions except for relative sediment compaction, but substantive differences in their megabenthic communities. We quantified RST abundance (numerical density and seafloor cover) and morphological characteristics (spoke number and overlap, spoke length and width, trace area and relative completeness) for 148 individual RSTs observed in some 6800 seafloor photograph (8788 m−2 seafloor area) captured with an autonomous underwater vehicle. We derived two primary results between study sites: (a) individual spoke morphology exhibited no statistically significant differences, while (b) individual trace completeness was significantly different. The consistency in spoke morphometrics reflects our identification of a single RST morphotype, potentially representing a single functional group of echiurans. The difference in trace completeness between sites may have resulted from differences in competition for resource with other megabenthic surface deposit feeders, and not to be directly related to relative sediment compaction. By reference to literature data and observations of similar traces in other deep-sea settings, we further suggest that this connection between trace completeness and resource availability may be more generally applicable in both contemporary ecology and paleoichnology.

Rare preservation of Triassic pedorelicts with biogenic traces from a hot semi-arid upland palaeoenvironment at Portishead, SW England

Pedorelicts occur sporadically in coastal exposures of Middle to Late Triassic continental sediments, at Portishead, SW England. The clasts include aeolian and bedrock constituents, calcrete textures, vesicles and other pedogenic features reflecting a hot semi-arid palaeoenvironment. Interpreted as being derived from an upland soil, they are classified in palaeosol terms as Calcic Protosol. In the most significant exposure, cryptic tubular trace fossils in branching networks occur within these pedorelict clasts in a localised fluvial deposit. This extremely rare palaeoecological archive offers a glimpse into continental upland life in a Pangaean desert landscape. It is documented for its own merit, to support continuing studies of its palaeobiology and to prompt investigation for comparable deposits.The pedorelicts occur within the Mercia Mudstone Marginal Facies (MMMF), close to its basal unconformity with underlying Paleozoic strata, at the margin of the Somerset Basin. The soil from which they are derived is interpreted to have developed in an upland regolith over Tournaisian bedrock of interbedded limestone and siltstone. These weathered and, with aeolian dust containing calcite and iron minerals and siliciclastic sand, formed a structured soil. This was aided by intermittent light and moderate rainfall that promoted mainly vadose pedogenic calcretization with displacive calcite crystallisation and siliciclastic grain breakage. In places, the soil was vesicular, as an ‘Av’ horizon, and in others, it was bioturbated with the development of calcite-lined tubules and unlined tunnels. At intervals of perhaps 104 to 105 years, catastrophic deluges eroded the regolith and transported clasts downslope towards the basin to form onlapping coarse clastic beds typical of the MMMF. The soil structure disintegrated but fragments that became pedorelicts were segregated in a fault-controlled palaeo-valley, possibly partly as a debris-flow, to be deposited as localised conglomeratic lenses with fluvial sand. After burial by further sedimentation, this deposit underwent diagenetic calcite cementation and baryte mineralisation before its present exposure by coastal erosion.

Mammal tooth traces in a ferruginous cave in southeastern Brazil and their relevance to cave legal protection.

We describe the first occurrence of biogenic traces made by mammals within an iron formation cave located in the Serra da Ferrugem Ridge, in Southeastern Brazil. These bioerosions are tooth traces produced in boulders, walls and floor within the cave. The traces occur as sets of two or more grooves, which are highly variable in size. The grooves were compared to tooth traces artificially produced by imprinting the incisors of different mammal species collected in the cave region on soft clay. Among those, the following taxa are potential tracemakers: Akodon sp., Oligoryzomys sp., Necromys lasiurus, Rhipidomys mastacalis, Oecomys gr. concolor, Trinomys moojeni, and Hydrochoerus hydrochaeris. The age of the traces is unknown; therefore, any discussion on its fossil nature is circumstantial. Regardless of its relevance to paleontology, the presence of ichnological features should be considered as an additional cave value, according to the current Brazilian legislation regarding cave protection.

Recent biogenic traces from the coastal environments of the southern Red Sea coast of Saudi Arabia

A neoichnological study was done at a site west of Abha on the southern Red Sea coast of Saudi Arabia. This area covers Wadi Hali and its surroundings (180 49′35.27″N: 410 22′44.23″E). The objective of the study is to document biogenic traces that occur in the upper intertidal ripple-marked sand flats and supratidal flats that include microbial mats, mud flats, and scattered patches of mangrove stands and their muddy environments. Based on morphological descriptions, four different kinds of burrows are identified and their fossil equivalents are Psilonichnus upsilon (burrow type 1), Cylindricum (burrow types 2 and 3), and Arenicolites (burrow type 4). Tracks of land hermit crabs Coenobita clypeatus are described as ichnospecies Coenobichnus currani. Gastropod grazing trails and their fossil equivalent Planolites is identified as well. Supratidal algal mats and their microscopic contents are presented. Fossil plant roots (rhizomes) are reported from a localized fluvial deposit from a dry channel (wadi). The morphology of the traces, trace maker, and ethology and the environmental setting indicate that the ichnofauna and botanical traces belong to the Psilonichnus ichnofacies.

Biogenic control on the origin of a vertebrate monotypic accumulation from the Late Triassic of southern Brazil

This study is a taphonomic investigation of a new Late Triassic monotypic association from the Hyperodapedon Assemblage Zone (Santa Maria Formation, southern Brazil). All skeletal elements belong to the traversodontid cynodont Exaeretodon, representing a minimum of four individuals from different ontogenetic stages. Biogenic traces are identifiable on some elements, such as invertebrate scavenging traces, vertebrate bite marks, and evidence of trampling which was probably responsible for accelerating the disarticulation of upper postcanine teeth and for fracturing a skull and a lower jaw. We classify this accumulation as generated by extrinsic biogenic action (predation/necrophagy and possibly trampling) due to the following reasons: (i) random spatial orientation of the elements enclosed into the matrix; (ii) absence of hydraulic equivalence among the specimens and presence of mudstone lenses in close association with the fossils; (iii) presence of nearly all Voorhies’ groups; (iv) association of a large number of cranial elements presenting different stages of disarticulation; and (v) presence of biogenic traces. Both disarticulation patterns and invertebrate scavenging traces indicate that the bones remained exposed for some time before burial. During this time of exposure, carnivore vertebrates also scavenged on some nutritive and transportable postcranial elements. We suggest an ecteniniid cynodont as a potential modifier agent (predation/necrophagy) of this particular monotypic accumulation of synapsids.

Fossils and astrobiology: new protocols for cell evolution in deep time

AbstractThe study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called ‘fossils’, here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and ‘mats’; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding ‘waste products’; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.

Epi-benthic megafaunal zonation across an oxygen minimum zone at the Indian continental margin

The Arabian Sea oxygen minimum zone (OMZ) impinges upon the Indian continental margin at bathyal depths (150–1500 m) producing changes in ambient oxygen availability and sediment geochemistry across the seafloor. The influence of these environmental changes upon the epi-benthic megafaunal assemblage was investigated by video survey at six stations spanning the OMZ core (540 m), lower boundary (800–1100 m) and below the OMZ (2000 m), between September and November 2008. Structural changes in the megafaunal assemblage were observed across the six stations, through changes in both megafaunal abundance and lebensspuren (biogenic traces). Most megafauna were absent in the OMZ core (540 m), where the assemblage was characterised by low densities of fishes (0.02–0.03 m −2). In the lower OMZ boundary, megafaunal abundance peaked at 800 m, where higher densities of ophiuroids (0.20–0.44 m −2) and decapods (0.11–0.15 m −2) were present. Total abundance declined with depth between 800 and 2000 m, as the number of taxa increased. Changes in the megafaunal assemblage were predicted by changes in abundance of seven taxonomic groups, correlated to both oxygen availability and sediment organic matter quality. Lebensspuren densities were highest in the OMZ boundary (800–1100 m) but traces of large infauna (e.g., echiurans and enteropneusts) were only observed between 1100 and 2000 m station, where the influence of the OMZ was reduced. Thus, changes in the megafaunal assemblage across the Indian margin OMZ reflect the responses of specific taxa to food availability and oxygen limitation.

Spatiotemporal distributions and controlling factors of Devonian reefs in south china

Wu Yibu* (吴义布) Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China Gong Yiming (龚一鸣) Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China; Institute of Resources and Environment, Key Laboratory of Biogenic Traces & Sedimentary Minerals of Henan Province, Henan Polytechnic University, Jiaozuo 454003, China Zhang Lijun (张立军), Feng Qi (冯启) Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China

Modern Subsurface Bacteria in Pristine 2.7 Ga-Old Fossil Stromatolite Drillcore Samples from the Fortescue Group, Western Australia

BackgroundSeveral abiotic processes leading to the formation of life-like signatures or later contamination with actual biogenic traces can blur the interpretation of the earliest fossil record. In recent years, a large body of evidence showing the occurrence of diverse and active microbial communities in the terrestrial subsurface has accumulated. Considering the time elapsed since Archaean sedimentation, the contribution of subsurface microbial communities postdating the rock formation to the fossil biomarker pool and other biogenic remains in Archaean rocks may be far from negligible.Methodology/Principal FindingsIn order to evaluate the degree of potential contamination of Archean rocks by modern microorganisms, we looked for the presence of living indigenous bacteria in fresh diamond drillcores through 2,724 Myr-old stromatolites (Tumbiana Formation, Fortescue Group, Western Australia) using molecular methods based on the amplification of small subunit ribosomal RNA genes (SSU rDNAs). We analyzed drillcore samples from 4.3 m and 66.2 m depth, showing signs of meteoritic alteration, and also from deeper “fresh” samples showing no apparent evidence for late stage alteration (68 m, 78.8 m, and 99.3 m). We also analyzed control samples from drilling and sawing fluids and a series of laboratory controls to establish a list of potential contaminants introduced during sample manipulation and PCR experiments. We identified in this way the presence of indigenous bacteria belonging to Firmicutes, Actinobacteria, and Alpha-, Beta-, and Gammaproteobacteria in aseptically-sawed inner parts of drillcores down to at least 78.8 m depth.Conclusions/SignificanceThe presence of modern bacterial communities in subsurface fossil stromatolite layers opens the possibility that a continuous microbial colonization had existed in the past and contributed to the accumulation of biogenic traces over geological timescales. This finding casts shadow on bulk analyses of early life remains and makes claims for morphological, chemical, isotopic, and biomarker traces syngenetic with the rock unreliable in the absence of detailed contextual analyses at microscale.

Aeolian sand sea development along the mid‐Cretaceous western Tethyan margin (Spain): erg sedimentology and palaeoclimate implications

AbstractThe existence of a mid‐Cretaceous erg system along the western Tethyan margin (Iberian Basin, Spain) was recently demonstrated based on the occurrence of wind‐blown desert sands in coeval shallow marine deposits. Here, the first direct evidence of this mid‐Cretaceous erg in Europe is presented and the palaeoclimate and palaeoceanographic implications are discussed. The aeolian sand sea extended over an area of 4600 km2. Compound crescentic dunes, linear draa and complex aeolian dunes, sand sheets, wet, dry and evaporitic interdunes, sabkha deposits and coeval extradune lagoonal deposits form the main architectural elements of this desert system that was located in a sub‐tropical arid belt along the western Tethyan margin. Sub‐critically climbing translatent strata, grain flow and grain fall deposits, pin‐stripe lamination, lee side dune wind ripples, soft‐sediment deformations, vertebrate tracks, biogenic traces, tubes and wood fragments are some of the small‐scale structures and components observed in the aeolian dune sandstones. At the boundary between the aeolian sand sea and the marine realm, intertonguing of aeolian deposits and marine facies occurs. Massive sandstone units were laid down by mass flow events that reworked aeolian dune sands during flooding events. The cyclic occurrence of soft sediment deformation is ascribed to intermittent (marine) flooding of aeolian dunes and associated rise in the water table. The aeolian erg system developed in an active extensional tectonic setting that favoured its preservation. Because of the close proximity of the marine realm, the water table was high and contributed to the preservation of the aeolian facies.A sand‐drift surface marks the onset of aeolian dune construction and accumulation, whereby aeolian deposits cover an earlier succession of coastal coal deposits formed in a more humid period. A prominent aeolian super‐surface forms an angular unconformity that divides the aeolian succession into two erg sequences. This super‐surface formed in response to a major tectonic reactivation in the basin, and also marks the change in style of aeolian sedimentation from compound climbing crescentic dunes to aeolian draas.The location of the mid‐Cretaceous palaeoerg fits well to both the global distribution of other known Cretaceous erg systems and with current palaeoclimate data that suggest a global cooling period and a sea‐level lowstand during early mid‐Cretaceous times. The occurrence of a sub‐tropical coastal erg in the mid‐Cretaceous of Spain correlates with the exposure of carbonate platforms on the Arabian platform during much of the Late Aptian to Middle Albian, and is related to this eustatic sea‐level lowstand.

Footprints as Inquiry-Based Learning Tools

Three high-interest activities using biogenic and physical traces in the recreation of past events have been created to help develop the student understanding of the process of science without sacrificing substantive geoscience content. Adaptations of human footprints, dinosaur trackways, and slidingrock trails investigations presented in this paper can be utilized in a variety of K-12 grade levels, with multi-disciplinary scope. In each module, students are encouraged to act as detectives by gathering information, collaborating with peers, and reaching logical conclusions. Students are therefore charged with observing, assimilating, communicating, and formulating hypotheses. Hence, the process of science is experienced while students explore archaeology, paleontology, and a terrestrial surface process.

Patterns in deep-sea macrobenthos at the continental margin: standing crop, diversity and faunal change on the continental slope off Scotland

Depth-related patterns of macrobenthic community structure and composition have been studied from box-core samples from the Scottish continental slope where deep-sea trawling and oil exploration are becoming increasingly important. There is a strong pattern of declining biomass and faunal abundance with increasing depth, but results also indicate reduced biomass and numbers of macrobenthos in the shallowest samples from just below the shelf edge where there are coarse sediments and a regime of strong bottom currents. There is also reduced species diversity at the shallowest stations, probably caused by hydrodynamic disturbance, but no clear mid-slope peak in species diversity as described from the northwest Atlantic. Taxonomic composition of the macrobenthic community shows most change between about 1000 and 1200 m, expressed as a major dichotomy in multivariate analysis by cluster analysis and ordination. It also shows up as a step-like increase in the rate of accumulation of new macrofaunal species. This corresponds to a change in hydrodynamic regime, from a seabed rich in suspension- and interface-feeding epifauna, to one where biogenic traces from large, burrowing deposit feeders are well developed, and visible epifauna rare in seabed photographs. It also corresponds to the depth zone where earlier study of megafaunal echinoderms in trawl and epibenthic sled samples also shows a clear peak in across-slope rate of change in faunal composition.

Sedimentology and depositional history of the Claret Formation (‘carse clay’) at Bothkennar, near Grangemouth

Synopsis The estuarine sediments of Flandrian age which comprise the Claret Formation in the Bothkennar area can be divided into bedded, mottled and laminated facies on the basis of primary sedimentary structures, frequency of silty laminae and the nature and extent of bioturbation. The mottled facies can be further subdivided by the style of bioturbation. The facies are very similar in overall particle size distribution and mineralogy. Biogenic traces occur in all the facies and have been tentatively matched to organisms present in the modern infauna Equation ()of the Firth of Forth. The succession of facies has been controlled by the fall of relative sea-level since the Flandrian maximum and records an emergent sequence from sub-tidal (probable water depth around 20 m) to inter-tidal. In the Bothkennar area four local subdivisions can be recognized in the Claret Formation, which reflect changes in sedimentary environment consequent on the reduction in water depth.

Topologic configuration of a graphoglyptid and its functional morphologic analysis

1. Topologic analysis of representative Devonian graphoglyptids of China shows that a huge graphoglyptid clan can be grouped into three kinds of topologic configurations homeomorphic to line, tree and network, which correspond to foraging, probing feeding and the catching and feeding of constructing and inhabiting in anoxic, suboxic and dysoxic2) deep-water environments respectively. 2. It is well known that, in paleoecology and paleontology, morphologic features are bases of functional morphologic analysis of body and trace fossils, but, the features are not only Euclidean geometric, but also topologic and fractal geometric. In terms of the topologic graphoglyptids homeomorphic to line, tree and network, the functional morphologic analysis based on topologic configuration can reveal trace producer’ s ethology and ecological function mysteries contained in trace fossils from new aspects. 3. A functional topologic morphologic analysis first can get rid of interference of original biogenic traces to be subjected to compacting, diagenesis and deformation. In addition, it can simplify existing classification and naming system of ichnology, enrich and develop the theory and methodology of a functional morphologic analysis.

Comparison of the bedform distribution and dynamics of modern and ancient sandy ephemeral flood deposits in the southwestern Karoo region, South Africa

During January 1981, unusually high rainfall patterns in the semi-arid southwestern Karoo region of South Africa gave rise to catastrophic floods in the area, the most devastating of which was the ephemeral flash-flood that partially destroyed the town of Laingsburg. Flood waters at Laingsburg spread from a dry river bed, rising to a depth of 12 m within 7 h, washing away houses and burying the town with up to 3.5 m of sand and silt. Facies sequences and bedforms in the Laingsburg flood deposits are related to specific subenvironments and flow conditions during the flood event. Two large-scale sedimentary units in erosive contact with each other were deposited by two major discharge pulses within the same flood event. Vertical and lateral variations in the bedform distributions reflect fluctuations in flow velocity and turbulence. Comparison is drawn between the Laingsburg flood deposits and fluvial sandstones of the Permian Lower Beaufort in the southwestern Karoo. The multistoried sandstone bodies, composed mainly of a horizontally laminated lithofacies, are interpreted as the products of a complex series of ephemeral flooding events. Individual storey units in the palaeochannel sequences are generally envisaged as the deposits of different floods. However, evidence from the modern flood shows that separate pulses of a single flooding episode can also produce a sequence of storey units bounded by erosive scours. Spectacular palaeosurfaces in the Lower Beaufort also have modern analogues in the Laingsburg overbank deposits. Discussion on the preservation potential of delicate sedimentary structures and biogenic traces on these surfaces emphasizes the necessity for pulsatory deposition of bedload and suspended-load material over short intervals of time. By comparing facies profiles and surface bedforms the ephemeral fluvial facies model for the Lower Beaufort is critically appraised.

Early neurological predictors of one-year outcome in birth asphyxiated infants

This study is a taphonomic investigation of a new Late Triassic monotypic association from the Hyperodapedon Assemblage Zone (Santa Maria Formation, southern Brazil). All skeletal elements belong to the traversodontid cynodont Exaeretodon, representing a minimum of four individuals from different ontogenetic stages. Biogenic traces are identifiable on some elements, such as invertebrate scavenging traces, vertebrate bite marks, and evidence of trampling which was probably responsible for accelerating the disarticulation of upper postcanine teeth and for fracturing a skull and a lower jaw. We classify this accumulation as generated by extrinsic biogenic action (predation/necrophagy and possibly trampling) due to the following reasons: (i) random spatial orientation of the elements enclosed into the matrix; (ii) absence of hydraulic equivalence among the specimens and presence of mudstone lenses in close association with the fossils; (iii) presence of nearly all Voorhies’ groups; (iv) association of a large number of cranial elements presenting different stages of disarticulation; and (v) presence of biogenic traces. Both disarticulation patterns and invertebrate scavenging traces indicate that the bones remained exposed for some time before burial. During this time of exposure, carnivore vertebrates also scavenged on some nutritive and transportable postcranial elements. We suggest an ecteniniid cynodont as a potential modifier agent (predation/necrophagy) of this particular monotypic accumulation of synapsids.

Biogenic structures in modern slope to deep-sea sediments in the sulu sea basin (Philippines)

Slowly accumulated slope and rise sediments (5–20 cm/1000 yr) are totally bioturbated as compared to only 10–30% in rapidly deposited abyssal plain sediments (100–200 cm/1000 yr). In contrast to a decrease in degree of bioturbation, biogenic trace diversity increases with water depth. All ichnocoenoses are clearly influenced by the low oxygen content of the bottom water caused by minimal circulation and restricted water exchange. Oxygen deficiency and relatively high sedimentation rates have created nutrient-rich sediments. Thus, there is no need for optimal fractionation into individual ecological niches and behavioral programs are not highly developed: (1) in slope and rise sediments, animal traces are associated with biodeformational structures; (2) in slope to abyssal plain sediments, trace assemblages are dominated by burrows with open tubes for circulation of respiration water; (3) in abyssal plain sediments, small-diameter biogenic traces common in turbidite sequences (graphoglyptids, e.g., Paleodictyon) are absent. This may be due to rapid sedimentation leading to inefficiency of graphoglyptid behavior. Where graphoglyptids are lacking, ichnocoenoses are dominated by forms like Phycosiphon. In slope and rise sediments, graphoglyptids may have been destroyed by deeper burrowing organisms.

Lebensspuren of Dysaerobic, Bathyal Basin, California Borderland: ABSTRACT

The low oxygen waters and bathyal depths of Santa Cruz basin provide harsh conditions for marine life, yet the basin supports a surprisingly high density and diversity of macrofauna. In contrast, biogenic sedimentary structures are low in density. Bottom photographs from 117 stations show a biogenically produced micro-hummocky topography with few resolvable biogenic traces. Recognizable lebensspuren are of three main classes: tracks and trails, depressions, and fecal matter. Echinoderms, the most abundant epifauna on the slope and adjacent basin floor, produce most of the large tracks and trails. Significantly, many tracks and trails are less distinct than similar markings of the abyss owing to the soupy, thixotropic nature of the basinal sediment. Depressions made by ast roids, regular urchins, and bottom-dwelling fish are most common at relatively shallow depths. A characteristic, circular depression made by a feeding, tubicolous polychaete is restricted to the lowermost slope and adjacent basin floor. Holothurian fecal strands dominate the photographically resolvable feces of this bathyal environment. These holothurian feces take the clothes-line form common to the abyss. X-ray radiography and photography of box-core sediment slabs reveal an indistinct burrow mottling resulting from the thixotropic response of the sediment to biogenic disturbance and lack of density contrast for radiography. Although open burrows are common in bottom photographs, few are recognizable in box-core slabs. Similarly, surface lebensspuren have poor preservation potential. When lithified, this bathyal sediment will probably be bioturbate-textured. End_of_Article - Last_Page 703------------

Deep-sea traces from the central Arctic: an analysis of diversity

Measures of diversity of deep-sea epifaunal biogenic traces are highest on the abyssal plains, intermediate on the ridges and continental rise, and lowest on the Alpha Cordillera. Measures of species dominance are inversely related to diversity measures. Arctic epifaunal trace diversity is significantly greater than reported world-wide burrow diversity, despite limited productivity and low benthic biomass. In contrast with reported deep-sea burrows, epifaunal traces are non-cosmopolitan among seven analyzed provinces, suggestive of ecologic preferences. Physical homogeneity is correlated with increased diversity. The results suggest that species interactions exert a greater influence on community structure than do observed gradients of heterogeneity, disturbance, productivity, or geologic age.