Dactyloidites ottoi (Geinitz, 1849) in Bahamian Pleistocene Carbonates: A Shallowest-marine Indicator

The rosetted trace fossil Dactyloidites ottoi (Geinitz, 1849) is reported and described for the first time from late Pleistocene (MIS 5e) carbonates of the Bahama Archipelago in shallowing-upward, shelly calcarenites from Great Inagua and Great and Little Exuma islands. The distinctive, fan-shaped D. ottoi specimens from the Bahamas, while not preserved in fine detail and not revealing a shaft, compare favourably in shape and size with specimens from other localities around the world, including the oldest well-documented specimens from the Jurassic of Argentina. D. ottoi is interpreted as a fodinichnion formed by the activity of a deposit-feeding worm, probably a polychaete, consuming marine-plant remains within host sediment. The late Pleistocene palaeodepositional environment of these carbonate sediments is interpreted as within the lower foreshore-upper shoreface zone in full marine, tropical waters. This discovery of D. ottoi marks an addition to the Bahamian shallow-marine ichnocoenose within the Skolithos ichnofacies.

Deposit feeding of a foraminifera from an Arctic methane seep site and possible association with a methanotroph revealed by transmission electron microscopy

Several foraminifera are deposit feeders that consume organic detritus (dead particulate organic material along with entrained bacteria). However, the role of such foraminifera in the benthic food-web remains understudied. As foraminifera may associate with methanotrophic bacteria, which are 13C-depleted, feeding on them has been suggested to cause negative δ13C values in the foraminiferal cytoplasm and/or calcite. To test whether the foraminiferal diet includes methanotrophs, we performed a short-term (1 d) feeding experiment with Nonionellina labradorica from an active Arctic methane-emission site (Storfjordrenna, Barents Sea) using the marine methanotroph Methyloprofundus sedimenti, and analyzed N. labradorica cytology via Transmission Electron microscopy (TEM). We hypothesized that M. sedimenti would be visible, as evidenced by their ultrastructure, in degradation vacuoles after this feeding experiment. Sediment grains (mostly clay) occurred inside one or several degradation vacuoles in all foraminifers. In 24 % of the specimens from the feeding experiment degradation vacuoles also contained bacteria, although none could be confirmed to be the offered M. sedimenti. Observations of the area adjacent to the aperture after 20 h incubation revealed three putative methanotrophs, close to clay particles. These methanotrophs were identified based on internal characteristics such as a type I stacked intracytoplasmic membranes (ICM), storage granules (SG) and gram-negative cell walls (GNCW). Furthermore, N. labradorica specimens were examined for specific adaptations to this active Arctic methane-emission site; we noted the absence of bacterial endobionts in all specimens examined but confirmed the presence of kleptoplasts, which were often partially degraded. Based on these observations, we suggest that M. sedimenti can be consumed by N. labradorica via untargeted grazing in seeps and that N. labradorica can be generally classified as a deposit feeder at this Arctic site. These results suggest that if methanothrophs are available to the foraminifera in their habitat, their non-selective uptake could make a substantial contribution to altering δ13Ctest values. This in turn may impact metazoans grazing on benthic foraminifera by altering their δ13C signature.

Marine record of late-glacial readvance and last recession of Laurentide ice, inner Frobisher Bay, Baffin Island, Arctic Canada

The Cockburn Substage readvance marks the last major late-glacial advance of the northeast sector of the Laurentide Ice Sheet on Baffin Island. The causes of this abrupt, late reversal of retreat are still unclear, but greater chronological control may provide some insight. To date, the literature has focused on the large terminal moraines in the region, providing a date of readvance (c. 9.5-8.5 ka cal BP). In Frobisher Bay, the Cockburn Substage readvance and recession onshore are marked by a series of moraines spread over ~20 km along the inner bay. Acoustic marine mapping reveals five distinct transverse ridges, morphologically suggestive of grounding-zone wedges, and two later fields of DeGeer moraines on the floor of the inner bay. These indicate that the style of ice retreat (beginning no later than 8.5 ka cal BP) changed over time from punctuated recession of a floating ice-front (20 km over >680 years, with four pauses) to more regular tidewater ice-front retreat, reaching the head of the bay 900 years or more after withdrawal from the outer Cockburn limit. The established chronology for final recession in the region is based largely on radiocarbon dating of bulk shell samples and single shells of deposit-feeding molluscs, notably <i>Portlandia arctica</i>, affected by old carbon from carbonate-rich sediments. Sedimentary analysis and judicious sampling for ¹⁴C dating of glaciomarine and marine facies in seabed sediment cores enables development of a late- and postglacial lithostratigraphy that indicates final withdrawal of ice from the drainage basin by 7 ka cal BP.

Early Invasion of Common Cordgrass (Spartina Anglica) Increases Belowground Biomass and Decreases Macrofaunal Density and Diversity in a Tidal Flat Marsh

Spartina anglica is an invasive perennial marsh grass causing significant negative impacts on tidal flat ecosystem. There have been only a few studies focusing on the patch structure of S. anglica according to size and its effects on macrofauna. A field experiment was conducted to identify effects of S. anglica patches where they have been introduced no later than 5 years after invasion occurred on macrofauna assemblages. The survey area was divided into two sections according to vegetation: (1) Suaeda japonica vegetation from 0 to 60 m away from the embankment, and (2) bare mudflat from 60 to 90 m away from the embankment. The patch sizes of S. anglica were categorized into small (1–4 m2), medium (5–11 m2), and large (13–40 m2) in area with four replicates for each section. The biomass ratio of the belowground and aboveground in the small size patch of S. anglica was significantly higher than those in the medium and large size patch of S. anglica. It indicated that more resource was allocated to rhizomes in small size patch with short invasion history (1 ~ 2 years). After S. anglica invaded, macrofauna richness (70%), Shannon-Wiener diversity (80%), and density (67%) were decreased. However, infaunal deposit-feeding polychaete Perinereis linea and epifaunal gastropods Batillaria cumingi and Lactiforis takii increased by S. anglica. Ordination of macroinvertebrate assemblages separated the habitat with S. anglica invasion from the adjacent uninvaded tidal flat and Suaeda japonica habitats. This study offers a significant insight into early invasion strategies of an aggressive plant invader, S. anglica for management of coastal wetlands and its impacts on macrofaunal assemblages.

The Phycosiphon Ichnofacies and the Rosselia Ichnofacies: Two new ichnofacies for marine deltaic environments

ABSTRACT Seilacherian Ichnofacies have been established, to date, for characterizing relatively stable depositional settings. Environments characterized by temporally and spatially varying physico-chemical stresses, however, have languished and been described ichnologically in the context of their “departures” from the archetypal expressions of otherwise ambient environments. Correspondingly, discrimination between shoreface and marine deltaic deposits have been addressed mainly by identifying variations in the individual trace-fossil suites without an over-arching ichnological model. Based on trace-fossil suites reported from globally distributed strata throughout the Phanerozoic, the case can now be made for erecting two new temporally and geographically recurring Seilacherian Ichnofacies for marine deltaic successions—the Phycosiphon Ichnofacies for muddy prodelta environments and the Rosselia Ichnofacies for sandy delta-front settings. The Phycosiphon Ichnofacies is characterized by variable bioturbation intensities (BI 0–5), although many sandstone and mudstone beds may have very low bioturbation intensities (BI 0–1), pointing to their rapid, event-style deposition. Most biogenic structures record grazing or deposit-feeding behaviors, with subordinate horizontal dwellings that reflect deposit feeding and/or carnivory. Meiofaunal cryptic bioturbation is locally present in tempestites. Episodic deposition is accompanied by common escape structures and locally, sediment-swimming structures. The Phycosiphon Ichnofacies typically shows beds characterized by diverse, fully marine trace-fossil suites intercalated with discrete beds dominated by low-diversity, facies-crossing traces. Such bed-scale juxtaposition points to short-term but recurring alternations between stable marine conditions and physico-chemically stressed conditions. The Rosselia Ichnofacies is also characterized by variable (BI 0–5; though typically BI 0–3) bioturbation intensities and sporadically distributed burrows. Most ichnogenera record deposit-feeding behaviors, many associated with vertically and horizontally oriented permanent dwelling structures. Dwelling structures commonly display re-equilibration and escape structures are typically abundant, both characteristic of elevated sedimentation rates and sporadic deposition. Most spreitenated structures are markedly retrusive, also attesting to elevated rates of sedimentation. Cryptic bioturbation is only locally abundant, particularly associated with erosionally amalgamated tempestites lacking mudstone drapes. While many of the trace fossils constitute facies-crossing elements, fully marine (ambient) ichnogenera also occur in some suites. Associated mudstone interbeds display low BI values, sediment-swimming structures, and top-down bioturbation, consistent with their rapid deposition as fluid mud. The two ichnofacies may pass gradationally into one another vertically, associated with lobe progradation or autogenic lobe abandonment. Further, the two ichnofacies may pass along depositional strike into their non-deltaic counterparts—the Phycosiphon Ichnofacies grading into the Cruziana Ichnofacies in distal positions, and the Rosselia Ichnofacies passing into the Skolithos Ichnofacies in shallow-water locales. The erection of these new ichnofacies will enhance the reliable identification of deltaic successions, particularly in wave-dominated settings, and their differentiation from classical strandplain shoreface deposits. As well, the two ichnofacies better explain animal–substrate relations in the context of the detailed sedimentological characteristics of delta deposits, refining the overall facies characterization of shallow-marine environments.

The influence of deposit-feeding organisms on sediment stability and community trophic structure

Deposit-feeding and suspension-feeding benthos in Buzzards Bay, Massachusetts, show marked spatial separation; suspension feeders are largely confined to sandy or firm mud bottoms while deposit feeders attain high densities on soft muddy substrata. Food source and bottom stability have been investigated as potential factors effecting this trophic-group separation. Between October 4, 1967 and August 22, 1969, observations were made at 11 stations in Buzzards Bay, Massachusetts, along two widely separated transects over bottoms ranging in texture from silt to fine and medium sand. Water depths at these stations ranged from 3 m to 20 m. Scuba divers made many of the field observations and collected most of the samples. This study included sampling of benthic macrofauna, taking bottom photographs, analyzing sedimentary structures, texture, organic content and water content of the sediments, and measuring both water currents and suspended sediment above the bottom. Laboratory experiments were also carried out to determine differential resuspension between burrowed and unburrowed muds. Intensive reworking of the upper few centimeters of a mud bottom by deposit feeders produces a fluid fecal-rich surface that is easily resuspended by low-velocity tidal currents. We suggest that the physical instability of this fecal surface tends to: (i) clog the filtering structures of suspension-feeding organisms, (ii) bury newly settled larvae or discourage the settling of suspension-feeding larvae, and (iii) prevent sessile epifauna from attaching to an unstable mud bottom. Thus suspension feeders are unable to successfully populate all areas of the bottom where a suspended food source is available, especially in areas where mud bottoms are intensively reworked by deposit feeders. Modification of the benthic environment by deposit feeders, resulting in the exclusion of many suspension feeders and sessile epifauna, is an example of trophic group amensalism. This biotic relationship appears to be important in shaping trophic-group distributions in embayments and basins on continental shelves.