Slow Sedimentation Rates Research Articles

Transfer of rare earth elements from clay-sized fraction to phosphate in East South Pacific Ocean: Implication for REY-rich sediment related to hydrothermal influence

Pelagic sediment enriched in critical metals (e.g. rare earth elements and yttrium, REY) has attracted much attention in recent years. Extensive research has focused on identifying the specific host mineral of REY in bulk pelagic sediment, however, research on clay-sized fraction of REY-rich sediment has not been fully understood yet. In this study, we aimed to investigate the host phase and migration mechanism of REY in clay-sized fractions from two cores, GC23 and GC17, located on the western and eastern sides of the East Pacific Rise (EPR), respectively. To the best of our knowledge, this is the first comprehensive investigation of the clay-sized fraction of REY-rich sediment associated with hydrothermal activity. Results show that GC23 contains negligible clay minerals but well-crystallized Fe oxyhydroxides, while GC17 is rich in smectite and poor-crystallized Fe oxyhydroxides. REY are predominantly hosted in poorly crystallized Fe-Mn oxyhydroxides, with some phosphorus selectively scavenged by Fe oxyhydroxides from seawater. In addition, fluorapatite nanocrystals were first observed within the matrix of Fe oxyhydroxides using transmission electron microscopy (TEM), indicating the formation of fluorapatite. The post-Archean average shale (PAAS)-normalized REY patterns show similar seawater-like patterns in both the clay-sized and silt-sized fraction. The clay-sized fractions primarily derived from hydrothermal plumes plays an important role in scavenging REY from ambient seawater. This study represents a significant step towards understanding the formation of REY-rich sediment related to hydrothermal activity. A two-stage mineralization process is proposed for the formation of REY-rich sediment near the EPR fields. Firstly, REY are initially scavenged by hydrothermal Fe-Mn oxyhydroxide particles from seawater during their lateral dispersion with hydrothermal plumes under low sedimentation rate until they are buried by newly formed precipitates. With the process of early diagenesis, poor crystallized Fe oxyhydroxides will be experienced recrystallization. Subsequently, REY would be released into porewater with the process of recrystallization due to their tendency to remain in a poorly crystallized phase. Ultimately, they are captured by biogenic apatite and/or fluorapatite. The case study indicates that REY-rich sediments may primarily formed within the dispersion area of hydrothermal plumes. Simultaneously, the necessity of slow sedimentation rates, greater water depth, and deep currents all accountable for the formation of REY-rich layers.

Factors influencing mangrove carbon storage and its response to environmental stress

Mangrove forests serve as significant carbon sinks and play a crucial role in mitigating climate change. Currently, the response of mangroves to intensified climate change and human activities, and the factors that influence the magnitude of carbon storage in their sediments remain uncertain. To address these questions, two sediment cores were collected from the mangrove reserve in Pearl Bay, Guangxi, China. The activity of 210Pb in the sediment, grain size, bulk elemental composition, stable carbon isotopes, lignin, and different sediment organic matter (OM) fractions were investigated to determine the local mangrove’s response to climate change and human activities, as well as the factors influencing its carbon storage. The results showed mangrove forests with lower tidal ranges, slower sedimentation rates, and where OM predominantly originated locally tend to have larger carbon stocks. The mangrove OM (MOM) decreased progressively from the bottom to the top of the cores, indicating that the mangroves in Pearl Bay have possibly undergone degradation, which was further substantiated by the decrease in lignin content. Based on these results, the entire cores were divided into two stages: stable stage 1 (1963–2001) and degradation stage 2 (2001–2020). The cause of the mangrove degradation is likely due to the impact of human activities; however, these impacts are anticipated to gradually lessen in the future due to mangrove protection policies. Our results indicate that lignin can track and predict mangrove growth trends and provide guidance for the sustainable management of mangrove ecosystems.

Opening and Post-Rift Evolution of Alpine Tethys Passive Margins: Insights from 1D Numerical Modeling of the Jurassic Mikulov Formation in the Vienna Basin Region, Austria

This study employed 1D numerical pseudo models to examine the Upper Jurassic carbonate succession, focusing on the Mikulov Formation in the Vienna Basin region. It addresses the protracted and complex history of the Jurassic source rock play, revealing a transition from rapid syn-rift (>200 m/Ma) to slower post-rift sedimentation/subsidence of the overlying layers during extensional deformation (up to 120 m/Ma with a thickness of 1300 m). This provides valuable insights into the rift-to-drift stage of the central Alpine Tethys margin. The Mikulov marls exhibit characteristics of a post-rift passive margin with slow sedimentation rates. However, a crustal stretching analysis using syn-rift heat flow sensitivity suggested that thermal extension of the basement alone cannot fully explain the mid-Jurassic syn-rift stage in this segment of the Alpine Tethys. The sensitivity analysis showed that the mid-late Jurassic differential syn-rift sequences were exposed to slightly cooler temperatures than the crustal stretching model predicted. Heat flow values below 120 mW/m2 aligned with measurements from deeply settled Mesozoic successions, suggesting cold but short gravity-driven subsidence. This may account for the relatively low thermal maturation of the primary source rock interval identified by the time-chart analysis, despite the complex tectonic history and considerable sedimentary burial. The post-Mesozoic changes in the compaction trend are possibly linked to the compressional thrusting of the Alpine foreland and postdating listric faulting across the Vienna Basin.

Drivers of organic carbon distribution and accumulation in the northern Barents Sea

Sedimentary properties and accumulation rates on the continental shelf and in the deep sea reflect temporal oceanographic, biological and chemical processes occurring in the water column and the sediment surface. We used the radionuclides 210Pb, 226Ra, and 137Cs activities to estimate sedimentation rates during the last century at nine stations in the northern Barents Sea region. Elemental (C, N) and stable isotopic composition (δ13C, δ15N) were also analysed from the nine stations sampled in August 2018, and, for five other stations sampled in August and December 2019, and in March and May 2021. Sediment accumulation rates varied between 130 and 1 410 g m−2 y−1. The < 63 μm normalized total organic carbon (TOC63) and the total nitrogen from the sediment surface varied between 0.90–2.56 % and 0.13–0.33 %, respectively. Ice-free shelf stations had higher TOC63 and possibly fresher organic matter (high δ13C, low δ15N) than ice-covered more northern stations. The opposite trend was observed for total inorganic carbon. We found that these trends in biogeochemical parameters were spatially structured by the winter sea ice concentration and biological production differences, and exhibited a south-north separation of the Polar Front region. The low and stable organic carbon accumulation rate (1.7–13.4 g Corg m-2 y−1; ARtoc) is a function of slow sedimentation rates, and high degradation and residence time in the water column and at the sediment–water interface. Overall, the ARtoc has been stable for the past 100 years, with a slight increase from the early 1970s to the present at the shelf and slope stations. Our results highlight that spatial scales of variability of the studied sedimentary parameters are linked to spatial patterns of important environmental variables (e.g., chlorophyll-a, sea ice concentration) in the region. In contrast, no seasonal differences were observed in the sediment parameters of revisited stations, and the dated sediment geochemical profiles did not exhibit substantial longer-term variation. This means that climate-induced changes in variables that modify the sedimentary geochemistry of the environment may affect benthic community activity and structure before leaving a record in ARtoc.

Radiocarbon ages of microcrystalline authigenic carbonate in Lake Neusiedl (Austria) suggest millennial‐scale growth of Mg‐calcite and protodolomite

ABSTRACTAuthigenic Mg‐calcite and dolomite are frequently observed in restricted, evaporative environments, such as lagoon or lake systems, but their formation is difficult to capture due to slow growth rates. Lake Neusiedl, an alkaline and subhaline lake with a mean water depth of 0.7 m in Austria, offers a natural system to study the precipitation of Ca‐Mg‐carbonate phases, which occur as fine‐grained, unconsolidated and largely homogenized mud. To elucidate the timing and formation mechanisms of these authigenic carbonate phases, the mineralogical and isotopic composition and radiocarbon age of different sediment grain‐size fractions from &lt;0.2 to &gt;3.0 μm were analysed. X‐ray diffraction analyses show two broad peaks of Mg‐calcite and protodolomite (lacking ordering peaks), suggesting that the carbonates are authigenic rather than detrital in origin. Calibrated carbon‐14 ages range between 200 cal yr bp and 3700 cal yr bp. The linear correlation of age and grain size suggests a very slow growth rate of single crystals of 0.23 to 0.60 μm/ka. These rates suggest an extremely slow sedimentation rate in a shallow lake that existed during most of the Holocene. The higher abundance of protodolomite in older grain fractions, in contrast to the presence of high‐Mg calcite in the youngest fractions, suggests a growth succession where high‐Mg calcite develops first and subsequently transforms into protodolomite. Much higher ages of 6 cal ka bp to 15 cal ka bp are measured in carbonates of lake deposits exposed on land, in a section north‐west of the recent lake, suggesting a growth rate of those carbonate minerals of 0.13 μm/ka. These time constraints further suggest that some carbonate grains could already have nucleated from lake water before or during the last glacial maximum, although under slightly different hydrochemical conditions.

Controlling Factors on REY Enrichments in Basins From the Pacific Ocean: Early Diagenesis and Local Constraints

AbstractIncreasing interest in high‐tech metals (e.g., rare earth elements and yttrium, REY) has triggered extensive research on the enrichment of these metals in pelagic sediments. In the sediments collected worldwide, Ca‐phosphate and Fe‐Mn (oxyhydr)oxides are targeted as the two most important REY carriers based on multiple geochemical and mineralogical analyses. However, the formation of the REY‐rich stratum remains enigmatic as the influences of diagenesis and the local restrictions of different oceanic basins are complex, which require further investigation. In our study, we analyzed four sediment cores from the Western, Central North, and Eastern South Pacific (WP, CNP, and ESP), which are three of the most promising REY‐rich regions in global oceans. Sequential leaching quantifies the dominance of Ca‐phosphate by controlling ∼73.9%–93.3% of ΣREY and the secondary role of Fe‐Mn (oxyhydr)oxides by holding up to ∼97.8% of Ce and ∼4.82%–13.9% of ΣREY. The shallowly and deeply buried sediments show similar features regarding the REY proportions hosted by different phases, which indicates that the REY accumulation by Ca‐phosphate might majorly occur on the seawater‐sediment interface. The geochemistry of the studied sediment cores reveals that (a) REY enrichment in ESP is affected by hydrothermal Fe‐Mn (oxyhydr)oxides which can promote adsorptions of P and REY. (b) The Fe and Mn contents in WP sediments show an inverse relationship against REY enrichment which might be explained by the dust input from Central Asia. (c) The slow sedimentation rate of CNP enables the extraordinary REY‐uptake by Ca‐phosphate, but the low flux of phosphorus constrains the formation of high‐grade REY deposits.

Taphonomic damage of molluscan shells in the Nile Delta under natural and anthropogenic sources of environmental variability

Environmental reconstruction can benefit from the interpretation of the postmortem history of shell remains. The molluscan death assemblages obtained from grab samples at forty-nine sites in the Manzala lagoon, the largest lagoon in the Nile Delta, were analyzed to infer the taphonomic alteration. The preservation of shells at each site has been semi-quantitatively ranked based on the taphonomic features formed by encrustation, bioerosion, abrasion, dissolution, precipitation, fragmentation, discoloration, and loss of sheen (low, moderate, high). A significant difference has been observed between sites of low and high environmental stress. Water depth and substrate type (hydrodynamics) correlate significantly to the percent number of altered shells. The southern parts of the lagoon, which are characterized by lower water energy and higher rates of sedimentation, are highly polluted by heavy metals (Cd, Pb, Zn) and organic pollutants. The southern parts receive much sediment from the agricultural drains, which increases burial of shelly material and decreases their time on the TAZ. The northern parts of the lagoon have slower rates of sedimentation, so that the molluscan shells remain longer at the sediment-water interface (Taphonomic Active zone; TAZ), where fragmentation is enhanced by winnowing under higher water-energy. The long residence in the TAZ also favored both bioerosion (Trypanites and Oichnus) and encrustation by bryozoan colonies, polychaetes, and oysters. Therefore, the taphonomic processes have the potential to cause severe damage and taphonomic loss of shell materials. Chemical dissolution and precipitation of shell remains may be related to acidic pore water. Moreover, high alkalinities in the southern polluted parts may also reduce the dissolution in contrast to the acidic water in the northern parts of the lagoon.

First occurrence of the genus Sphenothallus Hall, 1847 (Cnidaria) in the Carboniferous of the Dnipro-Donets Depression, Ukraine

The first occurrence of the problematic cnidarians Sphenothallus Hall (S. sp. 1 and S. sp. 2) is described from upper Viséan and ?Bashkirian (Carboniferous) black shales of the Dnipro-Donets Depression in NE Ukraine. Carboniferous Sphenothallus spp. from the Dnipro-Donets Depression apparently led an epifaunal gregarious lifestyle, preferring paleobasin areas with slow sedimentation rates, low current energy, and dysaerobic environments, as already suggested by literature data. The discovery of representatives of the genus Sphenothallus in Carboniferous strata of Ukraine significantly extends our knowledge about the geographic distribution of these animals. Further, it confirms their preference for environments with slow sedimentation. Keywords: cnidarians, Sphenothallus, Carboniferous, Dnipro-Donets Depression, Ukraine.

Stability of hydrate-bearing sediment during methane hydrate production by depressurization or intermittent CO2/N2 injection

Ensuring reservoir stability during natural gas hydrates production is the key to balance the geology-engineering-environment nexus. Under the overlying stress of 10 MPa, a series of experiments were conducted to investigate the variations of P-wave velocity, subsidence, and stratum stiffness during hydrate depressurization mining. The influence of hydrate saturations, gas production pressure, and sediment type were examined. The stratum stiffness of hydrate-bearing sediment gradually decreased with the decomposition of hydrate, despite the continuous compaction of overlying stress. Higher hydrate saturation led to larger subsidence and slower sedimentation rate. Lower gas production pressure resulted in greater subsidence and faster sedimentation rate. In sandy sediments, a larger particle size generated a larger sedimentation rate due to the strong heterogeneity of hydrate. However, the subsidence of silty clay is smaller with slower sedimentation rate. Hence, a novel method of intermittent CO2/N2 injection below the freezing point was proposed, which kept the stratum stiffness, reduced the sedimentation rate and subsidence of the reservoir, but also realized the sequestration of CO2 simultaneously. The method greatly enhanced the recovery ratio of methane hydrate below the freezing point. These findings provide guidance for the safe and efficient exploitation of natural gas hydrate in future.

Taphonomy of bivalve skeletal remains as a means of detecting changes in oxygen depletion and recognizing ancient upwelling environments

Abstract Bivalve shells from shelf and slope environments of the Benguela upwelling system were examined for their state of preservation for the first time, and were used to test whether their preservation is a proxy for spatially variable seafloor taphonomic conditions that lead to different degrees of time averaging. Distinctive shell damage-profiles show increasing shell damage further from the centre of the upwelling cells affected by anoxic conditions (opal and organic-rich) and towards the carbonate- and phosphate-rich facies (with slower sedimentation rates at the margins of the cells. Discrete taphofacies range from a very low damage profile (opal and organic-rich, facies 1) to very high damage (phosphate, facies 3), with elevated numbers of rounded, fragmented and bored shells as well as very little associated sedimentary matrix. This gradient in taphofacies is associated with a gradient from census assemblages with low damage to (within-habitat time-averaged) assemblages in carbonate and (environmentally-condensed) phosphate facies of moderate and higher species richness that are associated with increasing time averaging. This pattern follows the upwelling facies gradient of high opal and organic matter in areas of most intense upwelling cells and outward to aerated but food-abundant carbonate-rich seafloors and spatially coexisting phosphorite-rich concentrations. Thus, the taphonomic gradient ranges from stressful, oxygen-depleted seafloor environment under upwelling regimes that limit metazoan predators and borers (and thus minimize destruction), to well-oxygenated seafloors with increased predators and micro-boring animals that all foster post-mortem shell destruction. The rarity of encrusters is a conspicuous feature of these facies across the whole gradient that may reflect temporary recurrence of oxygen-depleting conditions even at the carbonatic and phosphatic facies. The state of preservation of these shells can serve as a modern analogue for identifying similar settings in the geological record but also for Anthropocene settings affected by eutrophication and expanding oxygen minimum zones. Molluscan taphofacies with unusually low levels of skeletal damage from borers and encrusters can thus represent sensitive proxies of conditions affected by Anthropocene deoxygenation and eutrophication.

Diachronous basin evolution along northern South China Sea: Result of a migrating Hainan plume?

The diachronous basin evolution from west to east along the northern margin of the South China Sea (SCS) is closely related to the formation of the SCS, but in details remains ambiguous. We focused our study on the crustal structure and Cenozoic subsidence and sedimentary evolution of the Pearl River Mouth Basin (PRMB) and the Qiangdongnan Basin (QDNB) to reveal the divergences between the western and eastern segments of this margin. The back-stripping method and gravity modeling were employed based on drilling wells and geophysical data. We revealed that the crustal structure of the PRMB has a wedge shape, thinning monotonously from the coastline to the continent ocean transition (COT), while that of the QDNB has a symmetrical shape, thinning intensively in the Central Canyon. The characteristics of multi-episodic sedimentation and subsidence evolution were revealed in the PRMB, and its rapid subsidence rates were roughly synchronized with the decrease in the seafloor spreading rate of the SCS basin. However, the sedimentary and subsidence rate curves mainly featured accelerated sedimentation and subsidence in the western QDNB and an accelerated subsidence rate curve but a slow sedimentation rate curve in the eastern QDNB after 11.6 Ma. Some low-velocity conduits, which might be the crustal magmatic footprints of the Hainan mantle plume, were revealed in the middle and eastern QDNB. One possible dynamic model that can cause accelerated subsidence after 11.6 Ma in the eastern QDNB is that the Hainan mantle plume has migrated northwestward which may be derived from the contemporaneous southeastward movement of the South China Block. This conjecture coincides with magmatism and subsidence rate variations in the northwestern margin and is evidenced by the low-velocity conduits in the eastern QDNB.

Incipient reddening of Ordovician carbonates: The origin and geochemistry of yellow and pink colouration in limestones

Red colouring in marine red beds (MRB) is commonly attributed to deposition and early diagenesis under specific redox conditions. Therefore, the MRB can be considered time-specific facies. However, since red colouring is a subjective criterion, it is difficult to establish a colour limit for the MRB in the scale from grey to yellow, orange, pink to red. Using spectral reflectance, carbonate petrology, bulk-rock and in-situ geochemistry data from three sections of Ordovician orthoceratite carbonates of South China, we addressed the question whether the incipient reddening in the pink carbonates was associated with similar redox changes and palaeoceanographic conditions like in the MRB. The yellowish grey to greyish orange pink (Munsell Rock Colour Chart) carbonates with low concentrations of hematite (< 0.01 %) are transitional from goethite-bearing grey to hematite-enriched true MRB. The red-coloured skeletal interiors, microstromatolites, nodules and filamentous microborings suggest an extensive microbial activity which was accompanied by precipitation of authigenic aluminosilicates (clays). We hypothesize that the microbial clay precipitation is an important intermediate step in Fe transformation from its primary sources to hematite in the MRB. The carbonate deposition was followed by early diagenetic, shallow-subsurface REE fractionation, and FeMn (+Mo, U and V) redox cycling along microbially controlled redox microgradients. The geochemical redox signature of the pink carbonates is very similar to the MRBs of Devonian and Ordovician age. They were deposited under similar palaeoenvironmental conditions on a deeper shelf inhabited by skeletal heterotrophs, with reduced rates of organic matter burial and slow sedimentation rates. The sedimentation of the pink carbonates and MRBs seem to randomly coincide with the coeval global sea-level changes and δ13Ccarb fluctuations suggesting that the local controls of sediment colour override the global ones.

Provenance Studies of Alluvial Tin Deposits in Parts of Ropp Younger Granite Complex, Northcentral Nigeria

The study area in this research is within the Ropp Complex, part of the Nigerian Mesozoic Younger Granite province. The study aims to interpret the depositional environment and establish the provenance of the alluvial cassiterite deposits in the study area. Boreholes/mining pits were logged for this study, and stanniferous sandstone samples were collected, which were used for textural and mineralogical studies. The mineral assemblages documented in the samples include ilmenite (3% to 27%). Cassiterite (2 to 14%), Zircon (2 to 16%), magnetite (0 to 17%), tourmaline (5 to 11%), rutile (2 to 8%) and monazite (2 to 7%).The ZTR Index calculated from the result of heavy minerals analysis for the selected pieces is 59%. Mineralogical studies revealed that quartz is the most dominant detrital mineral averaging about 93-99%, indicating that the stanniferous sandstones are compositionally matured and have experienced a high degree of chemical weathering. The quartz grains have grain sizes ranging from coarse to very coarse. They are poorly sorted, sub-angular to sub-rounded, with low sphericity. This also indicates a closeness to the source and textural immaturity. The occurrence of relatively very few feldspar grains suggests a slow sedimentation rate, very high rate of chemical weathering and composition maturity. The bivariate plots, univariate grain size parameters and probability plots, and the absence of fossils and trace fossils suggest deposition in a fluvial environment. The results of the granulometric analysis indicate that the study area's stanniferous sandstone was deposited in a fluvial environment by a low-energy fluvial (river) depositional system and the deposition in proximal (close to the source). This study suggests that the Basement complex and Younger Granite are the sources of the stanniferous placer deposits.

Late Eocene Ostracoda of the Blanche Point Formation, South Australia

The Blanche Point Formation (late Eocene) was deposited in the geographically restricted St Vincent Basin, South Australia. The Ostracoda of the Gull Rock and the Perkana Members (of this formation) are characterized by many cytherellids, krithiids, pontocypridids, bythocypridids and many sighted trachyleberidids. These taxa indicate outer shelf or epibathyal depths. Most specimens are represented by single valves, probably mainly as a consequence of the relatively slow sedimentation rate. The basal part of the Gull Rock Member yields a striking dominance of autochthonous cytherellids (synchronous with a peak in dominance of uvigerinid foraminifera) and might perhaps reflect intensification of oxygen minimum conditions. Six new species are proposed: Bairdoppilata willungaensis sp. nov., Trachyleberis maslinensis sp. nov., Trachyleberis reticulopustulosa sp. nov., Actinocythereis? onkaparingaensis sp. nov., Werribeeleberis? mediacosta sp. nov. and Bradleya mypongaensis sp. nov.

Kilometre-scale coral carpets on mixed carbonate-siliciclastic platforms; a sedimentological study from the Lower Cretaceous of northwestern Africa

Coral carpets are biostromes that form laterally continuous but low-relief (typically less than 1-metre-high) coral communities with a lack of clear internal zonation. There are few locations that allow analysis of such systems on a regional (tens of kilometre) scale in the rock record. This study describes extensive, well-exposed, coral-rich bodies (coral carpets), deposited on the Moroccan northwest Atlantic shallow-marine margin during the Cretaceous (early Hauterivian). The coral-rich sequence reaches 50 m in thickness, with individual carpets between 10 and 100 cm thick, extending over an area of more than 150 km 2 . Logging and regional correlation, integrated with thin-section analysis, has allowed the evaluation of facies distribution, geometries and temporal evolution. The results provide information on allogenic and/or autogenic driving processes for coral development, and allows the generation of conceptual models for their growth. The Tamanar Fm. corals reflects a shallowing up sequence and the recovery of carbonate productivity following stressed environmental conditions in the late Valanginian. Temporally, coral morphological response relates to high-frequency relative sea level changes that affected water energy and turbidity; thick-branching and massive forms grew in shallow high energy environments whilst platy/flat forms grew in poorly lit and turbid environments. However, local-scale (over hundreds of metres) lateral changes in morphology and facies can be explained by autogenic factors independent of sea level change; varying sedimentation, hydrodynamism and turbidity across the extensive open marine platform. The termination of coral deposition is marked by a hardground surface, indicating slow rates of sedimentation and a transgression, which is followed by an influx of clastic sediments onto the platform, resulting in the deterioration of carbonate platform health, as conditions became intolerable for coral growth. These coral-rich outcrops contribute to the relatively sparse documentation of Hauterivian corals globally and suggest niche palaeoecological and palaeoclimatic relationships that allowed corals to develop despite stressed platform conditions. The low diversity, low-relief coral carpets may provide a proxy for the recovery and demise of corals in response to environmental stress on modern carbonate platforms and future climate change projections. • Kilometre-scale coral-rich outcrops onshore the Moroccan Atlantic Margin. • Non-framework coral carpets show no internal zonation. • Coral morphology and abundance controlled chiefly by water energy and turbidity. • Establishment and demise of corals linked to sea level and climatic change. • Low diversity corals possible proxy for recovery/loss in response to environmental stress.

Pyrite-lined shells as indicators of inefficient bioirrigation in the Holocene–Anthropocene stratigraphic record

Abstract. Although the depth of bioturbation can be estimated on the basis of ichnofabric, the timescale of sediment mixing (reworking) and irrigation (ventilation) by burrowers that affects carbonate preservation and biogeochemical cycles is difficult to estimate in the stratigraphic record. However, pyrite linings on the interior of shells can be a signature of slow and shallow irrigation. They indicate that shells of molluscs initially inhabiting oxic sediment pockets were immediately and permanently sequestered in reduced, iron-rich microenvironments within the mixed layer. Molluscan biomass-stimulated sulfate reduction and pyrite precipitation was confined to the location of decay under such conditions. A high abundance of pyrite-lined shells in the stratigraphic record can thus be diagnostic of limited exposure of organic tissues to O2 even when the seafloor is inhabited by abundant infauna disrupting and age-homogenizing sedimentary fabric as in the present-day northern Adriatic Sea. Here, we reconstruct this sequestration pathway characterized by slow irrigation (1) by assessing preservation and postmortem ages of pyrite-lined shells of the shallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) by evaluating whether an independently documented decline in the depth of mixing, driven by high frequency of seasonal hypoxia during the 20th century, affected the frequency of pyrite-lined shells in the stratigraphic record of the northern Adriatic Sea. First, at prodelta sites with a high sedimentation rate, linings of pyrite framboids form rapidly in the upper 5–10 cm as they already appear in the interiors of shells younger than 10 years and occur preferentially in well-preserved and articulated shells with periostracum. Second, increments deposited in the early 20th century contain &lt; 20 % of shells lined with pyrite at the Po prodelta and 30 %–40 % at the Isonzo prodelta, whereas the late 20th century increments possess 50 %–80 % of shells lined with pyrite at both locations. At sites with slow sedimentation rate, the frequency of pyrite linings is low (&lt; 10 %–20 %). Surface sediments remained well mixed by deposit and detritus feeders even in the late 20th century, thus maintaining the suboxic zone with dissolved iron. The upcore increase in the frequency of pyrite-lined shells thus indicates that the oxycline depth was reduced and bioirrigation rates declined during the 20th century. We hypothesize that the permanent preservation of pyrite linings within the shells of V. gibba in the subsurface stratigraphic record was enabled by slow recovery of infaunal communities from seasonal hypoxic events, leading to the dominance of surficial sediment modifiers with low irrigation potential. The presence of very young and well-preserved pyrite-lined valves in the uppermost zones of the mixed layer indicates that rapid obrution by episodic sediment deposition is not needed for preservation of pyrite linings when sediment irrigation is transient and background sedimentation rates are not low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organisms die at their living position within the sediment. Abundance of well-preserved shells lined by pyrite exceeding ∼ 10 % per assemblage in apparently well-mixed sediments in the deep-time stratigraphic record can be an indicator of inefficient bioirrigation. Fine-grained prodelta sediments in the northern Adriatic Sea deposited since the mid-20th century, with high preservation potential of reduced microenvironments formed within a mixed layer, can represent taphonomic and early diagenetic analogues of deep-time skeletal assemblages with pyrite linings.

Genesis of REY-rich deep-sea sediments in the Tiki Basin, eastern South Pacific Ocean: Evidence from geochemistry, mineralogy and isotope systematics

Deep-sea sediments in the eastern South Pacific Ocean were recently found to contain high concentrations of rare-earth elements and yttrium (REY). To understand the genesis of REY enrichment in the sediments, we performed detailed element and isotope geochemical analyses on samples from gravity core S028GC23 recovered from the Tiki Basin in the eastern South Pacific Ocean.The REY-rich sediments in core S028GC23 are dark-brown to black zeolite clays, with REY (∑REY) contents of 1136–2213 ppm (average of 1857 ppm). The ∑REY contents in the sediments are positively correlated with P2O5 and negatively correlated with Ce anomalies. The REY-rich sediments display obvious Ce depletion and positive Y anomalies in the post-Archean Australian shale (PAAS)-normalized REY diagrams. Bioapatite fossils contain the highest REY concentrations (average of 8921 ppm) among the constituents, indicating that they are the primary hosts of REY. The REY enter through the roots and diffuse to the tops in the bioapatite fossils. Fe–Mn micronodules are divided into two different types based on Ce and Y anomalies in the PAAS-normalized REY patterns, and they have average REY concentrations of 781 ppm and 937 ppm, respectively. The phillipsite has low REY contents (average of 106 ppm) and is associated with a low sedimentation rate. REY patterns and Sr-Nd isotopic signatures show that seawater was the main ore-forming fluids during the formation of REY-rich sediments. The sedimentation rate of the sediments calculated by the 230Thex activities was 0.8 mm/Ka. The REY accumulation process can be explained by two-stage fluid-bioapatite fossil interactions and small contributions of Fe-Mn micronodules in an oxidized environment with slow sedimentation rate.

Origin of Phosphate‐bearing Carbonate Concretions in the Upper Triassic Lacustrine Black Shales of the Southern Ordos Basin, China

AbstractCarbonate concretions are conspicuous in organic‐rich shales and are generally related to decomposition of organic matter. The black shales from the Chang 7 Member of the Upper Triassic Yanchang Formation of the southern Ordos Basin host abundant carbonate concretions, which provide a unique record of depositional and early diagenetic conditions of the paleo‐lake sediments. However, little attention has been given to the genesis and growth processes of the concretions in these lacustrine petroleum source rocks. New petrographic observations and geochemical analysis show that the concretions are composed of calcite, phosphate fossil fragments, K‐NH4‐feldspar, quartz, bitumen, and minor Fedolomite. Phosphate minerals, mainly carbonate fluorapatite (CFA), show pervasive replacement by calcite, most of which contains phosphorus, ranging in concentration from 0.26 to 2.35 wt%. This suggests that the phosphate minerals are the precursors for concretion growth. Positive δ13C (+5.6 to +12.4 &amp;#x2030; V‐PDB) signatures and the absence of pyrite indicate that microbial methanogenesis was the dominant driver for concretion growth, rather than bacterial sulfate reduction. Quartz, bitumen, and Fe‐dolomite are the last cements that occurred, at deep burial depths and high temperatures. The formation of phosphate minerals might have been induced by upwelling of phosphate‐enriched deep water in the Late Triassic paleolake, which promoted phytoplankton blooms and further enrichment of organic matter. Extremely slow sedimentation rates of fine‐grained detrital minerals, relative to dead organism accumulation, led to the high permeabilities of the organic‐rich sediments and rapid concretion growth during shallow burial. The close association of phosphate‐bearing carbonate concretions and organic‐rich shales reflects that upwelling played a critical role in the formation of the high‐quality petroleum source rocks in the Triassic paleo‐Ordos lake.

Palaeoenvironment evolution and organic matter accumulation of the Upper Triassic mudstones from the eastern Qiangtang Basin (Tibet), eastern Tethys

The Upper Triassic successions in the Qiangtang Basin are important targets for petroleum exploration, although they are poorly understood at present. The black mudstones collected from well QZ-16 are the deepest Upper Triassic source rocks to date and have over-mature organic matter (OM) (Ro > 2). In this study, the total organic carbon (TOC) and systematic inorganic geochemistry were determined to reconstruct depositional conditions (palaeoclimate, palaeosalinity, primary productivity, redox conditions, sedimentation rate, and detrital influx) and to assess the OM accumulation mechanism. The TOC contents of the Bagong Fm. mudstones first decrease and then increase from bottom to top (Unit A to Unit D), and Unit C has the lowest TOC contents. These mudstones have relatively obvious correlations between TOC content and the palaeosalinity proxy (Sr/Ba), redox indexes (U/Al, V/Al, Mo/Al, U/Th, and V/Cr) and sedimentation rate indicator ((La/Yb)N), which indicate that OM accumulation was dominated by both high salinity and a slow sedimentation rate in a dysoxic environment. The high Al2O3/TiO2 ratios indicate that the provenance of these mudstones was primarily from felsic igneous rocks. High detrital influx of felsic igneous rock provenance would have diluted the OM and inhibited OM preservation, as evidenced by the negative correlations between TOC content and detrital influx parameters, including SiO2, Al2O3, TiO2, and K2O. The primary productivity (P/Al and Ba/Al) is poorly correlated with TOC for these mudstones, suggesting that its effect on OM accumulation was negligible or limited. The relatively high TOC contents of mudstones from Units A and B are mainly related to the interaction of dysoxic conditions, a high-salinity water column, slow sedimentation rates, and a low detrital influx. An oxidizing water column with low salinity and a relatively high detrital influx and sedimentation rate upward in Unit C led to greater OM degradation and dilution and the lowest TOC contents. The significant TOC content fluctuations in Unit D may be related to vertical variations in the depositional conditions.

Removal process of phosphorus during the settlement of particulates with runoff and its implication for reservoir management.

Substantial particulates and phosphorus (P) loads are carried into the reservoir by flood runoff and the P exchange between water and settling particulates under variable water conditions in the reservoir after flooding is critical to the removal of active P from water. To investigate the impact of particulate sedimentation on P changes in reservoir water, runoff samples were collected at four locations in the upstream channels of two reservoirs after a rainstorm. Two batches of particulate sedimentation simulations were conducted separately in four plexiglass columns to analyze the changes of water P and environmental factors during particulate sedimentation. The results showed that the contents of total P (TP), total particulates P (TPP), and phosphate (PO43-) decreased with the settlement of particulates. The correlation between the environmental factors and the amount of PO43- in water changed from uncorrelated to correlated with particulate settlement, implying that the rapid settling of particulates might weaken the effect of environmental factors on P exchange between water and particulates. Particulates firstly release PO43- rapidly in river or reservoir and then adsorb PO43- slowly during settlement in reservoirs. Coarse particulates release more and adsorb less PO43- during settlement, and fine particulates play an important role in the removal of water PO43- due to the slow sedimentation rates. Therefore, to mitigate the reactive P content of reservoir water, the loss of coarse particulates from the watershed should be controlled, and the discharge of water with particulates downstream should also be avoided during the flood season.