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.

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

Arumberia is an enigmatic sedimentary surface texture that consists of parallel, sub-parallel or radiating ridges and grooves, most commonly reported from upper Neoproterozoic – lower Palaeozoic strata. It has variably been interpreted as the impression of a small metazoan, a ‘vendobiont’, a physical sedimentary structure formed on a substrate with or without a microbial mat covering, or a non-actualistic microbial community. In this paper we contribute new insights into the origin of Arumberia, resulting from the discovery of the largest contiguous bedding plane occurrence of the texture reported to date: a 300 m2 surface in the lower Cambrian Port Lazo Formation of Brittany, NW France. We compare the characteristic features of Arumberia at this locality with 38 other global records, revealing four defining characteristics: (1) the three-dimensional (3D) morphology of exposed Arumberia lines (either positive relief ‘ridges’ or negative relief ‘grooves’) records fully preserved cords within clay laminae; (2) lines may transition laterally into reticulated patterns; (3) characteristic parallel and sub-parallel Arumberia lines can become modified by desiccation on emergent substrates prior to interment; and (4) Arumberia are streamlined with palaeoflow in successions showing evidence of unidirectional currents, but are organized parallel to ripple crests where strata were sculpted by oscillatory flows. These characteristics indicate that Arumberia records a 3D entity, distinct in material properties from its host sediment, which occurred in very shallow water settings where it was prone to passive reorganization in moving water, and desiccation when water drained. A literature survey of all known Arumberia occurrences reveals that the most reliable examples of the form are stratigraphically restricted to a 40 Ma interval straddling the Ediacaran–Cambrian boundary (560–520 Ma). Together these characteristics suggest that Arumberia records the remains of extinct, sessile filamentous organisms (microbial or algal?) that occupied very shallow water and emergent environments across the globe at the dawn of the Phanerozoic Eon.

Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism

Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major- and trace-element analyses. Deformation bands are not only more compacted than the host sediment but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile-element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, one shear zone sample displays a destabilization of smectite or illite–smectite mixed layers and a slight crystallization of illite relative to its sediment matrix, and another sample shows correlated increases in B and Li in shear zones and veins compared to the host sediment, both effects suggesting a transformation of smectite into illite in deformation bands. The two diagenetic reactions of sulfide precipitation and smectite-to-illite transformation are explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may contribute to the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.

Physio-Chemical and Mineralogical Characteristics of Gas Hydrate-Bearing Sediments of the Kerala-Konkan, Krishna-Godavari, and Mahanadi Basins

The characteristics of the hydrate-bearing sediments affect the formation and dissociation of gas hydrate in sediments. The mineral composition, their dispersion, and chemical composition of hydrate-bearing sediment samples plays a dominant role in the hydrate stability condition and its economic development. In this paper, the physical properties of hydrate-bearing sediment of India are compared with each other. The sediment samples are taken from the Krishan-Godavari basin (Depth—127.5 and 203.2 mbsf), Mahanadi basin (Depth—217.4 mbsf), and Kerala-Konkan basin (Depth—217.4 mbsf). The saturation of the gas hydrate observed at these sites is between 3 and 50%. Particle size is an important parameter of the sediments because it provides information on the transportation and deposition of sediment and the deposition history. In the present study, we investigated the mineralogy of hydrate-bearing sediments by chemical analysis and X-ray Diffraction. XRD, FTIR, and Raman Spectroscopy distinguished the mineralogical behavior of sediments. Quartz is the main mineral (66.8% approx.) observed in the gas hydrate-bearing sediments. The specific surface area was higher for the sediment sample from the Mahanadi basin, representing the sediments’ dissipation degree. This characterization will give important information for the possible recovery of gas from Indian hydrate reservoirs by controlling the behavior of host sediment. SEM analysis shows the morphology of the sediments, which can affect the mechanical properties of the hydrate-bearing sediments. These properties can become the main parameters to consider for the design of suitable and economic dissociation techniques for gas hydrates formed in sediments.

Diagenetic Controls on the Formation of the Anarraaq Clastic-Dominated Zn-Pb-Ag Deposit, Red Dog District, Alaska

The Anarraaq clastic-dominated (CD) Zn-Pb-Ag deposit (Red Dog district, Alaska, USA) has an inferred mineral resource of 19.4 Mt at 14.4% Zn, 4.2% Pb, and 73 g/t Ag and is spatially associated with a separate ~1 Gt barite body. This study presents new cross sections and petrographic evidence from the Anarraaq area. The barite body, previously shown to have formed in a shallow subsurface environment akin to a methane cold seep, contains multiple generations of barite with locally abundant calcite masses, which are discordant to sedimentary laminae, and is underlain by an interval of massive pyrite containing abundant framboids and radiolarians. Calcite and pyrite are interpreted to have formed by methane-driven diagenetic alteration of host sediment at the sulfate-methane transition (SMT). The sulfide deposit contains two zones of Zn-Pb mineralization bounded by faults of unknown displacement. The dominant hydrothermal minerals are marcasite, pyrite, sphalerite, quartz, and galena. The presence of hydrothermal pseudomorphs after barite, early pyrite resembling diagenetic pyrite associated with the barite body, and hydrothermal quartz and sphalerite filling voids formed by dissolution of carbonate all suggest that host sediment composition and origin was similar to that of the barite body prior to hydrothermal mineralization. Rhenium-osmium isochron ages of Ikalukrok mudstone (339.1 ± 8.3 Ma), diagenetic pyrite (333.0 ± 7.4 Ma), and hydrothermal pyrite (334.4 ± 5.3 Ma) at Anarraaq are all within uncertainty of one another and of an existing isochron age (~338 Ma) for the Main deposit in the Red Dog district. This indicates that the Anarraaq deposit formed soon after sedimentation and that hydrothermal activity was approximately synchronous in the district. The initial Os composition of the Anarraaq isochrons (0.375 ± 0.019–0.432 ± 0.025) is consistent with contemporaneous seawater, indicating that a mantle source was not involved in the hydrothermal system. This study highlights the underappreciated but important role of early, methane-driven diagenetic processes in the paragenesis of some CD deposits and has important implications for mineral exploration.

Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism

Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major-trace element analyses. Deformation bands are not only more compacted than the host sediment, but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, clays in deformation bands undergo a destabilization of smectite or illite/smectite mixed layers, and/or a slight crystallization of illite, which is matched by a correlated increase in B and Li compared to the host sediment. The two diagenetic reactions of sulfide precipitation and clay transformation are both explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may participate in the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.

Ediacaran metazoan fossils with siliceous skeletons from the Digermulen Peninsula of Arctic Norway

In this study, a new assemblage of Ediacaran metazoan fossils is reported from the basal Stáhpogieddi Formation on the Digermulen Peninsula of Arctic Norway, including Anulitubus n. gen. Moczydłowska in Moczydłowska et al., Anulitubus formosus n. gen. n. sp. Moczydłowska in Moczydłowska et al., Coniculus n. gen. Moczydłowska in Moczydłowska et al., Coniculus elegantis n. gen. n. sp. Moczydłowska in Moczydłowska et al., Fistula n. gen. Moczydłowska in Moczydłowska et al., and Fistula crenulata n. gen. n. sp. Moczydłowska in Moczydłowska et al. The specimens are three-dimensionally preserved and include tubular and conical skeletons that are morphologically distinguished by their body-wall constructions, radial symmetry, polarity, segmentation, and annulation. The skeletons are interpreted to be biomineralized by primary silica based on computed micro-tomographic, petrographic, geochemical, and spectroscopic evidence of originally rigid body wall with layers of constant thicknesses, composed of opal, microcrystalline quartz, and an admixture of carbonaceous material, which differ from the host sediment mineralogy and do not show replacement or encrustation. The fossil-bearing interval immediately overlies strata of Gaskiers age and can be bracketed within 580–541 Ma, but it is estimated to be ca. 575 Ma on the basis of averaged sedimentation rates and biostratigraphic correlations with Ediacaran biota found in up-section deposits of ca. 558–555 Ma. Future new findings of such fossils in different preservation modes and further multi-collector inductively coupled plasma mass spectrometry, which shows the silicon fractionation and traces its biogenic origin versus inorganic mineralization, may corroborate the interpretation of biogenic silicification of these earliest skeletal fossils.UUID: https://zoobank.org/6bccada1-870e-47b0-b819-82685152ea54

Pore-water chemistry: A proxy for tracking the signature of ongoing silica diagenesis

ABSTRACT Silica diagenesis leads to dramatic petrophysical variations in the host sediment across the depth of an opal-A to opal-CT transition zone. Predicting the present-day diagenetic status of opal-A to opal-CT transition zones, i.e., active versus fossilized fronts, is essential to constraining the drivers that control abrupt changes in the physical state of sediment. This study assesses whether there are modern signatures of ongoing silica diagenesis in the sediment pore water, and demonstrates the potential for pore-water-chemistry profiles for distinguishing between active opal-CT precipitation and fossil transition zones. Pore-water chemistry, mineralogy, and thermodynamic analyses of the Ocean Drilling Program Wells 794 and 795 indicate that solubility equilibrium has been reached with respect to opal-CT in the transition zones captured by the Neogene biosilica in the Sea of Japan. Even though silica dissolution might be triggering a reverse-weathering process, the equilibrium reached with respect to diagenetic opal strongly suggests that the silica drop across the transition zones is mainly influenced by active opal-A to opal-CT transformation. Owing to abrupt petrophysical variations associated with opal-CT formation, other interstitial profiles—major ions and primary parameters—have been influenced by silica diagenesis. The extremely low silica diffusion fluxes in the sediment, the low permeability of host sediment, and the occurrence of considerable pore-water loss at the depth of the transition zone all support this conclusion that the dissolved species have not been diffused in the sediment at rates comparable to those by pore-water advection. Advection and diffusion, however, appear to have ceased recently because they have failed to smooth the signature of ongoing silica diagenesis. The porosity drop during opal-A to opal-CT diagenesis at Sites 794 and 795 is principally attributed to chemically induced anomalous compaction, causing the sediment framework to lose its strength under fragmentation and extensive opal-A dissolution.

The glaciogenic origin of the Pleistocene calcareous dust in Argentina on the basis of field, mineralogical, textural, and geochemical analyses

Calcareous dust occurs in Argentina as layers and pockets closely associated with Pleistocene deposits and periglacial features from southernmost Patagonia to at least the Mendoza Precordillera and has been traditionally interpreted as a soil horizon resulting from postdepositional pedogenesis during interglacials. Detailed field and microscopic observations and sedimentological and geochemical analyses of more than 100 samples collected from lower to upper Pleistocene deposits between 51°S and 33°S and from near sea level to 2800 m asl allow us to interpret the dust as synchronous with the host sediment. All observations and analyses lead us to conclude that: (1) the cryogenic morphology and the chemical signatures of the calcite component show that the dust is glaciogenic, (2) the dust was carried by southeasterly Antarctic winds, and (3) it was deposited over most of southern and central Argentina. Field observations, geomorphic evidence, and radiocarbon dates suggest that the dust was deposited during several Pleistocene glacial episodes.