Sediment Mixing Research Articles

Impact of Typhoon Talim on surface sediment records on the East China Sea continental shelf

The East China Sea (ECS) is susceptible to summer typhoons, which play an important role in sediment transport and reworking on the continental shelf. However, because observations during typhoon passage are difficult to carry out even at present, the influence of typhoons on sedimentary records in the ECS is not fully understood. In this study, we compared the elemental compositions and grain-size distributions of surface sediments collected before and after the passage of Typhoon Talim. Through end-member modeling algorithm (EMMA) analysis, the variations in sediment composition due to the typhoon passage were characterized, and the impacts of typhoons on sediment migration, redistribution and sedimentary records on the ECS continental shelf were elucidated. Comparison of the results revealed that after the passage of Typhoon Talim, the clay distributions were generally parallel to the coastline, the sand contents in the northeast and southeast of the study area decreased, and the sorting coefficient decreased in the inner shelf mud area and increased in the relict sand area. The three-end-member model was capable of adequately reproducing all variables, and the three end members were interpreted to be the fine-grained components of river terrigenous sediments (EM1), mixed sediments (EM2), and relict sands (EM3) according to the grain-size and chemical element composition characteristics of surface sediments. Summer typhoons can interrupt the cross-shelf transport of fine-grained sediments caused by cross-shelf penetrating fronts (CPFs) and can promote exchange between fine-grained sediments and relict sands on the ECS continental shelf. Thus, summer typhoons play a vital role in retaining the mud on the ECS inner shelf and redistributing the sediments on the ECS continental shelf.

A Computationally Efficient Shallow Water Model for Mixed Cohesive and Non-Cohesive Sediment Transport in the Yangtze Estuary

In this paper, a computationally efficient shallow water model is developed for sediment transport in the Yangtze Estuary by considering mixed cohesive and non-cohesive sediment transport. It is firstly shown that the model is capable of reproducing tidal-hydrodynamics in the estuarine region. Secondly, it is demonstrated that the observed temporal variation of suspended sediment concentration (SSC) for mixed cohesive and non-cohesive sediments can be well-captured by the model with calibrated parameters (i.e., critical shear stresses for erosion/deposition, erosion coefficient). Numerical comparative studies indicate that: (1) consideration of multiple sediment fraction (both cohesive and non-cohesive sediments) is important for accurate modeling of SSC in the Yangtze Estuary; (2) the critical shear stress and the erosion coefficient is shown to be site-dependent, for which intensive calibration may be required; and (3) the Deepwater Navigation Channel (DNC) project may lead to enhanced current velocity and thus reduced sediment deposition in the North Passage of the Yangtze Estuary. Finally, the implementation of the hybrid local time step/global maximum time step (LTS/GMaTS) (using LTS to update the hydro-sediment module but using GMaTS to update the morphodynamic module) can lead to a reduction of as high as 90% in the computational cost for the Yangtze Estuary. This advantage, along with its well-demonstrated quantitative accuracy, indicates that the present model should find wide applications in estuarine regions.

Mixed carbonate-siliciclastic sedimentation in a mesoproterozoic storm-dominated ramp: Depositional processes and stromatolite development

Records of shallow-marine ramps with the mixing of carbonate and siliciclastic sediments are common throughout the geological time. All these records have pure carbonate and pure siliciclastic deposits as end members, occurring contemporaneously in distinct depositional regions along the ramp, and transitional hybrid facies between them. The two end member can mix in different scales and can alternate in time due to climatic changes and regressions and transgressions. This work presents a detailed reconstruction of a Mesoproterozoic storm-dominated mixed carbonate-siliciclastic ramp composed of hybrid sediments and whithout the presence of pure siliciclastic or carbonate deposits, a rare example in the geological record. Based on a high resolution logged section (in 1:20 scale) and qualitative thin sections, eleven lithofacies were identified and grouped into three lithofacies associations (offshore, offshore transition and shoreface), which are stacked vertically forming a transgressive–regressive cycle. This faciological distribution indicates a low relief ramp with wide microbial colonization from shallow to relatively deep waters (below storm-wave base level). In offshore low-energy distal areas, microbial mats spread laterally over large distances with little or no interference from currents, while in the offshore transition the morphology of the bioherms is shaped by currents induced by waves. In turn, the high wave energy in the shoreface inhibits the formation of stromatolites, restricting their occurrence to thin layers of microbial carpets or intraclastic lags. The mixing occurs in compositional scale and is relatively homogeneous along the whole logged interval, independent of the shifts in lithofacies or lithofacies associations. This compositional homogeneity is linked to the wide distribution and regularity in the input of siliciclastic sediments during the sedimentary accumulation. Strong currents induced by storms allow the transport and mixing of siliciclastic sediments with carbonate grains generated in the basin during fair-weather periods.

Fluid conduits formed along burrows of giant bivalves at a cold seep site, Sounthern Taiwan

At a Pliocene methane seep site in Taiwan characterized by numerous tubular carbonate bodies and bivalves, a half meter-long dolomitic concretion containing a lucinid bivalve Anodontia goliath reveals the impact of chemosymbiotic bivalves on fluid migration through shallow sediments. The concretion consists of a cortex encasing a central channel filled with sparite and varying amounts of brecciated material derived from the channel walls. The studied bivalve is preserved with the hinge upward. When restored to life position, the central channel is connected to the anterior side of the bivalve. In the lower longer segment, the channel is connected to the posterior sector of the bivalve. The channels are interpreted as the anterior tube for supply of respiration water and the posterior tube for mining sulphide generated in the sulfate-methane transition zone of the sediment. The dolomitic cortex surrounding the central channel is characterized by δ13C values as low as −29‰ VPDB, indicating that the rate of anaerobic oxidation of methane was enhanced around the conduit. At the base of the central channel, upward soft sediment deformation structures likely resulted from the dragging effect when fluid migrated out of the sediment and entered the channel. Brecciation of the channel walls indicates pulsating seepage during the incipient stages of cementation. The burrow, thus, acted as a highly permeable conduit channeling upstreaming methane-charged fluid for a substantial period of time. Therefore, burrows can be efficient in pre-determining the formation of fluid conduits at seep sites, having potentially a large impact on fluid mixing and diagenetic reaction rates in sediments, and ultimately the amount of methane released. The reported bivalve with its conserved burrow fossilized as seep carbonate concertion provides direct evidence that fluid conduits form along animal burrows.

Timescale dependent sedimentary record during the past 130 kyr from a tropical mixed siliciclastic–carbonate shelf edge and slope: Ashmore Trough (southern Gulf of Papua)

AbstractIn tropical and sub‐tropical mixed siliciclastic–carbonate depositional systems, fluvial input and in situ neritic carbonate interact over space and time. Despite being the subject of many studies, controls on partitioning of mixed sediments remains controversial. Mixed sedimentary records, from Ashmore Trough shelf edge and slopes (southern Gulf of Papua), are coupled with global sea‐level curves and anchored to Marine Isotope Stage stratigraphy to constrain models of sediment accumulation at two different timescales for the past 130 kyr: (i) 100 kyr scale for last glacial cycle; and (ii) millennial scale for last deglaciation. During the last glacial cycle, carbonate production and accumulation were primarily controlled by sea‐level fluctuations. Export of neritic carbonate to the slopes was initiated during re‐flooding of previously exposed reefs and continued during Marine Isotope Stage 5e and 1 interglacial sea‐level highs. Siliciclastic fluxes to the slope were controlled by interplay of sea level, shelf physiography and oceanic currents. Heterogeneous accumulation of siliciclastic mud on the slope, took place during Marine Isotope Stage 5d to Marine Isotope Stage 3 sea‐level fall. Siliciclastics reached adjacent depocentres during Marine Isotope Stage 2. Coralgal reef and oolitic–skeletal sand resumed at the shelf edge during the subsequent stepwise sea‐level rise of the last deglaciation. Contemporaneous, abrupt siliciclastic input from increased precipitation and fluvial discharge illustrates that climate controlled deglacial sedimentation. Siliciclastic input persisted until ca 8.5 ka. Carbonate accumulation waned at the shelf edge after ca 14 ka, whereas it increased on the slopes since ca 11.5 ka, when previously exposed reef and bank tops were re‐flooded. When comparing the last sea‐level cycle sedimentation patterns of the southern Gulf of Papua with other coeval mixed systems, sea level and shelf physiography emerge as primary controls on deposition at the 100 kyr scale. At the millennial scale, siliciclastic input was also controlled by climate change during the unstable atmospheric and oceanic conditions of the last deglaciation.

Bioturbation feedbacks on the phosphorus cycle

Bioturbation—sediment mixing and ventilation by burrowing animals—provides one of the most prominent examples of how animals shape their surroundings. A critical but longstanding question is when and how in Earth's history bioturbators began to similarly influence marine biogeochemistry. Recent work has proposed that even though the development of well-mixed sediments was a protracted process, the initial rise of bioturbators led to a decrease in marine phosphate levels, global productivity crises and ultimately deoxygenation events in the early Cambrian oceans. Herein, using a diagenetic model, we provide a new view of how bioturbators impact the global phosphorus cycle. Our work focuses on incorporating more realistic representation of bioturbation with a more complete and mechanistic parameterization of benthic phosphorus cycling than has previously been used to explore the early Paleozoic. We find, in contrast to previous modeling studies, that bioturbation does not uniformly or unidirectionally mediate increased phosphorus burial. Enhanced biodiffusion of sedimentary particles can mediate enhanced phosphorus burial. In contrast, bioirrigation—nonlocal transport of solutes via burrows—enhances phosphorus recycling and may therefore stimulate rather than stymy productivity. Given evidence from the geologic record that bioirrigation rather than biodiffusion was predominant during the ramping up of bioturbation (through the early Paleozoic), the emergence of bioturbation is unlikely to have driven deoxygenation events.

Field observations of turbulence, sediment suspension, and transport under breaking tidal bores

Turbulence dynamics and sediment suspension, under intense tidal bore conditions are investigated using field observations from the Qiantang Estuary, China. During the passage of the bore the tidal current bottom generated turbulence is augmented by turbulence injection from the surface as part of the breaking processes of both bore front and the bore associated secondary waves. These processes lead to enhanced vertical mixing and vertical distribution of the advected sediments. The latter contributes to increased sediment transport in the direction of the bore propagation that potentially modifies the normal tidally-driven estuarine sediment dynamics and morphology. The results show that turbulence intensity increases with increased tidal currents and oscillating motions associated with the breaking processes. At the early stage of the tidal bore, the vertical mixing of suspended sediment is more convective than diffusive due to the elevated length scales associated with bore and breaking secondary wave generated turbulence. Turbulent momentum and vertical sediment flux are determined by intermittent, large magnitude turbulent events (i.e., turbulent bursting). At early period of the tidal bore (~ 40s), inward and outward interactions contribute most to the Reynolds stress and vertical sediment flux, in contrast to later periods when sweep and ejection become more important. Negative vertical sediment flux (i.e., toward the bed) is observed, indicating that the sediment buoyancy acts as a source of turbulent kinetic energy, possibly due to the modification of vertical distribution of suspended sediment by the bore. These observations, in flow conditions under intense tidal bore breaking, reveal different sediment dynamics from traditional boundary process by injecting breaking induced turbulence which modifies the distribution and transport of the suspended sediment.

Mixed siliciclastic-carbonate sedimentation on the Miocene Pearl River continental shelf, northeastern South China Sea: Implications for exploring lithologic traps

During the Miocene, when the Zhujiang Formation was deposited, the eastern part of the Pearl River Mouth Basin, northern South China Sea, underwent intense mixed siliciclastic and carbonate sedimentation, particularly on the platform of the marginal ramp of the Dongsha Rise, northeastern margin of the South China Sea. Cores and well logs from industrial boreholes were collected and systematically investigated to reveal the sedimentary-petrographic characteristics of the mixed deposits and their formative processes. Four lithotypes were differentiated in the deposits, that is, (1) sandstone, (2) dolomitic sandstone and calcitic sandstone, (3) sandy dolomite and sandy limestone, and (4) dolomite, bioclastic limestone, and micrite limestone, which were formed by three major types of mixing processes: punctuated, facies, and in situ mixing processes. The porosity and permeability of the mixed deposits indicated that the siliciclastic component of the mixed deposits that experienced punctuated mixing formed the most high-quality hydrocarbon reservoirs. Exploration revealed that the lithologic reserves in the study area were mainly developed in mixed sedimentation areas. Our results suggest that the mixed sedimentation area, particularly with pure sandstone sedimentation or bioclastic sandstone, that underwent compaction or structural fracture activities, may be the next preference for lithologic exploration.

The relationship between source supply and mixed deposition of siliciclastic and carbonate: First to second member of the Shahejie Formation, Paleogene, Bohai Sea area, China

There are various types of mixed siliciclastic-carbonate sediments developed in the Bohai Sea area during the period of the first to second member of the Shahejie Formation (E2s1-2) of the Paleocene. We have concluded that the period of E2s1-2 was very suitable for the development of carbonate minerals and organisms because of the stable tectonic background, the weak siliciclastic influence of large source systems outside the basin, and the high salinity of the water. There were many local uplifts inside the basin during E2s1-2, and the source area, supply direction, and quantity of the local provenance varied greatly. We summarized that the mixed sediments generally developed in the intermittent and stagnant periods of the source supply, or on the flank or distal end of the source supply direction due to the absence of direct interference of terrigenous clasts. To a large extent, the formation of different types of mixed deposits is controlled by the different spatiotemporal relationship with siliciclastic supply. The background of strong source supply led to the formation of large-scale mixed deposits that were mainly composed of terrigenous clasts. Mixed deposits are mainly composed of organisms and carbonate with relatively large depositional thickness formed on the flank of source supply in the steep slope area. On the flank of source supply in the gentle slope belt, thinner mixed deposits with terrigenous clasts mainly formed and thin-layer carbonate clastic-dominated deposits formed on abandoned deltas. On the uplift of the buried hill far away from the provenance, thick mixed deposits mainly composed of bioclastic were formed whereas fine-grained mixed deposits formed under the low-energy argillaceous background.

Detrital zircon geochronology and heavy mineral analysis as complementary provenance tools in the presence of extensive weathering, reworking and recycling: the Neogene of the southern North Sea Basin

AbstractHeavy mineral analysis is a long-standing and valuable tool for sedimentary provenance analysis. Many studies have indicated that heavy mineral data can also be significantly affected by hydraulic sorting, weathering and reworking or recycling, leading to incomplete or erroneous provenance interpretations if they are used in isolation. By combining zircon U–Pb geochronology with heavy mineral data for the southern North Sea Basin, this study shows that the classic model of sediment mixing between a northern and a southern source throughout the Neogene is more complex. In contrast to the strongly variable heavy mineral composition, the zircon U–Pb age spectra are mostly constant for the studied samples. This provides a strong indication that most zircons had an initial similar northern source, yet the sediment has undergone intense chemical weathering on top of the Brabant Massif and Ardennes in the south. This weathered sediment was later recycled into the southern North Sea Basin through local rivers and the Meuse, leading to a weathered southern heavy mineral signature and a fresh northern heavy mineral signature, yet exhibiting a constant zircon U–Pb age signature. Thus, this study highlights the necessity of combining multiple provenance proxies to correctly account for weathering, reworking and recycling.

Inhibition mechanism of Penicillium chrysogenum on Microcystis aeruginosa in aquaculture water

Excessive Microcystis aeruginosa in aquaculture water can affect the quality and safety of aquatic products. An algicidal strain Penicillium chrysogenum (CHS) was isolated from a mix of sediment and water samples from an aquaculture pond. Subsequently, the inhibition mechanism of CHS on M. aeruginosa growth and physiological-biochemical index were investigated. The inhibition rate of M. aeruginosa growth was found to be 69.56% with CHS inoculum density of 3.85% after 6 days. The mycelium or spores of CHS did not inhibit the growth of M. aeruginosa. The CHS inhibited the growth of M. aeruginosa indirectly by secreting extracellular substances. The results showed that CHS secretions destroyed the cell wall and organelles of M. aeruginosa as well as the antioxidant system of M. aeruginosa. The super oxide dismutase (SOD), catalase (CAT), peroxidase (POD) and glutathione S-transferase (GST) activities of M. aeruginosa were significantly different from the control group (p < 0.05) with CHS inoculum density higher than 1.32%. CHS secretions also affected the expression of photosynthesis genes and microcystin synthesis genes of M. aeruginosa. The expression of genes psaB and rbcL involved in photosynthesis were inhibited, while the expression of gene psbD1 was promoted. The relative transcriptional abundance of genes mcyA, mcyB and mcyD involved in microcystins synthesis was significantly reduced. On the 6th day, the microcystin-LR (MC-LR) concentration was reduced by 75.4% and 32.0% in the groups with CHS inoculum density of 1.32% and 2.60%, respectively. The concentration of penicillin V and penicillin G, the main components believed to be inhibiting M. aeruginosa, was 15.23 mg ml−1 and 18.97 mg ml−1, respectively in the CHS solution.

Factors that controlled deposition of lacustrine, mixed siliciclastic-carbonate sediments in the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, East China

The Zhanhua Sag is located in the east-central Jiyang Depression, southern Bohai Bay, northeast China, and is a half-graben, lacustrine basin. Mixed siliciclastic-carbonate sediments are quite common in Eocene deposits of the Zhanhua Sag. We established characteristics of the lithofacies, facies associations, and depositional environments of the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, using core descriptions, thin sections and well logging data. Mixed grainstones and mixed wackestones are two lithofacies of mixed sediments identified in the study area. Paleogeomorphology, sediment supply and gravity flows jointly controlled development of the mixed sediments in this study. Subaqueous highs, formed by terrigenous clastic sediments, provided substrates for carbonate production. The Ordovician carbonate strata exposed on both sides of the rise provided the material for carbonate production on the north and south sides. The clastic sediment supply, however, developed mainly on the south side. Less siliciclastic influx in the north was probably associated with weakening of tectonic activity and less topographic difference. Massive mixed sediments were the product of gravity flows developed under low-energy hydrodynamic conditions between the carbonate shoals and a deltaic system. Most of the siliciclastic material in the mixed sediments was transported by gravity flows from the fan delta to lows in a shallow lake.

Distribution and habitat of Scalpellidae (Cirripedia: Thoracica) in the Norwegian and Barents Seas

ABSTRACT We describe the distribution and habitats of Amigdoscalpellum hispidum, Weltnerium stroemii, Scalpellum scalpellum, Weltnerium cornutum, Catherinum striolatum, Weltnerium nymphocola and Hamatoscalpellum hamatum in the Norwegian and Barents Seas, and their morphology. These are seven of the eight scalpellid species recorded in the area. Expeditions from 2006 to 2018 of the MAREANO (Marine areal database for Norwegian waters) programme have provided a comprehensive collection of Scalpellidae that can contribute to our knowledge of their distribution. The collections were from 335 localities in 40–2300 m depth. Of the 1406 specimens collected, 1396 were collected by beam trawl and 37 were from grabs. Patchy distributions and the association of species with other organisms (e.g. hydroids and gorgonian corals) can explain the low abundance in grabs compared with beam trawls, the latter covering larger areas and mixed sediments better. Weltnerium stroemii was the most abundant species and had the largest depth and temperature range, followed by A. hispidum and W. nymphocola. Species were assigned into three geographic groups based on their depth distribution. Shelf species: W. stroemii, S. scalpellum and A. hispidum occurring mainly shallower than 400 m in water temperatures >4°C; Upper slope species: W. nymphocola, W. cornutum and H. hamatum, found mainly between 600–1000 m in temperatures from 4 to 0°C; Lower slope species: C. striolatum occurring below 1500 m in temperatures <0°C. The species C. striolatum and W. cornutum are new to the Norwegian fauna and future deep-sea exploration will probably discover further scalpellid species.

Lacustrine mixed siliciclastic-carbonate sedimentary rocks in the Paleogene Funing Formation of the Subei Basin, eastern China: Characteristics and origin

The study of sedimentary characteristics and genetic mechanism of mixed siliciclastic-carbonate sediments (MSCSs) is not only the focus of sedimentary geology, but it is also an important issue in petroleum exploration and development. We have discovered the lacustrine MSCSs, the interstratified siliciclastic-carbonate sediments and the textural mixture of siliciclastic and carbonate materials (s-MSCSs) in the second member of the Paleogene Funing Formation ([Formula: see text]) in the Gaoyou and Jinhu sags, Subei Basin, eastern China. We use core, mud-log and thin-section data to clarify the [Formula: see text] s-MSCSs into five types, and we also use geochemical data of the [Formula: see text] mudstone including X-ray diffraction (XRD) whole rock, XRD clay, and trace elements to quantitatively analyze lake paleosalinity. The [Formula: see text] s-MSCSs mainly consist of siliciclasts, ooids, intraclasts, and mud, and they could be siliciclast-dominated, allochem-dominated, or without dominant components. We analyze that mixed sediments occur under either traction flow or gravity flow. The s-MSCSs of the traction flow origin develop small-scale, wavy cross-bedding and consist of siliciclasts and ooids of similar size; the s-MSCSs of the gravity flow origin develop massive bedding and graded bedding with complex lithology, and they mainly consist of siliciclasts and allochems of different size. We hypothesize that the increase of lake paleosalinity controlled by the climate and the moderate siliciclast influx under stable tectonic setting provide the material source for s-MSCSs in the lacustrine basin. Multidirectional flow is common during the deposition of s-MSCSs of traction flow origin, and seasonal flooding produces s-MSCSs of the gravity flow origin. Lacustrine s-MSCSs in the Gaoyou and Jinhu sags have significantly smaller scale and quantity, and ooids rather than bioclasts occupy most of the carbonate components.

Effects of environmental history and post-depositional processes on the organic matter record of Lake Łebsko, Poland

This study explores biomarker composition in the sediment record from Lake Łebsko (LŁ) acting as a representative case of a coastal lake system. The broad context for the research was to provide insights into carbon cycling as well as the origin and transformations of organic C in this unique depositional environment over a long-term perspective. We studied 25 samples from a ca. 9 m long sediment sequence covering the entire Holocene. The samples were analysed with regard to inorganic (TIC, Fe/Mn, Mg/Ca, Al) and organic matter composition (TOC, hydrocarbons, hopanoids, n-alkanols, fatty acids, stanols, sterols etc.) as well as stable C and N isotopes. Our major finding was that the sedimentary organic matter (SOM) in the LŁ core was predominantly autochthonous albeit highly degraded. The degradation occurred during sedimentation in the water column and/or shortly after deposition and, based on earlier research of coastal lakes, it was related to high water turbulence and sediment mixing. We also found that: (i) higher plant biomarkers were overrepresented in the SOM, and (ii) bulk SOM indicators (e.g., TOC/N, δ13CSOM, δ15NSOM) provided more coherent paleoenvironmental information on the lake studied than hydrocarbon and n-alkanol biomarkers. From stable C isotope composition of SOM it emerged that changes in salinity affected the sources of C used for production of autochthonous SOM. During freshwater phases the production of the SOM was mainly fueled by CO2 delivered from the lake catchment while during saltwater periods marine-derived bicarbonates were primarily used for photosynthesis.

Mapping seafloor habitats in the Bay of Fundy to assess megafaunal assemblages associated with Modiolus modiolus beds

Dense aggregations of horse mussels exist in the Bay of Fundy and are thought to be associated with high biodiversity compared to surrounding habitats. Previous research show correlations between these aggregations and long narrow flow-parallel bedforms. In this study, high resolution seafloor photographs were compared to multibeam echosounder data to investigate benthic community and biodiversity patterns across the Bay of Fundy. Biophysical seafloor types (benthoscapes) were mapped with 71% accuracy using Object Based Image Analysis (OBIA) with fuzzy c-means classification. Community patterns were found to shift gradually across benthoscapes and mega-epifaunal biodiversity was highest in areas of mixed sediments and silty gravel and anemones. Horse mussels occurred mainly within the sand benthoscape where the flow parallel bedforms exist. Community patterns were also assessed at fine scales (5 m) using a by-eye interpretation of the flow parallel bedforms, where gradational shifts across the bedforms and surrounding areas were evident and abundance of horse mussels were found to be significantly greater on the bedforms. These findings will ultimately facilitate decisions around fisheries management, marine spatial planning, and monitoring of the horse mussel habitats and the surrounding benthoscapes within the Bay of Fundy.

Pollen Geochronology from the Atlantic Coast of the United States during the Last 500 Years

Building robust age–depth models to understand climatic and geologic histories from coastal sedimentary archives often requires composite chronologies consisting of multi-proxy age markers. Pollen chronohorizons derived from a known change in vegetation are important for age–depth models, especially those with other sparse or imprecise age markers. However, the accuracy of pollen chronohorizons compared to other age markers and the impact of pollen chronohorizons on the precision of age–depth models, particularly in salt marsh environments, is poorly understood. Here, we combine new and published pollen data from eight coastal wetlands (salt marshes and mangroves) along the Atlantic Coast of the United States (U.S.) from Florida to Connecticut to define the age and uncertainty of 17 pollen chronohorizons. We found that 13 out of 17 pollen chronohorizons were consistent when compared to other age markers (radiocarbon, radionuclide 137Cs and pollution markers). Inconsistencies were likely related to the hyperlocality of pollen chronohorizons, mixing of salt marsh sediment, reworking of pollen from nearby tidal flats, misidentification of pollen signals, and inaccuracies in or misinterpretation of other age markers. Additionally, in a total of 24 models, including one or more pollen chronohorizons, increased precision (up to 41 years) or no change was found in 18 models.

Persistence of Biogeochemical Alterations of Deep‐Sea Sediments by Bottom Trawling

AbstractBottom trawling grounds have been expanding to deeper areas of the oceans since the mid‐XXth century, and mitigating strategies aimed to protect fish stocks, such as temporal trawling closures, have recently been implemented. Here we investigated the biogeochemical properties of sediment from a deep‐sea trawling ground in Palamós Canyon (NW Mediterranean) to assess the effects of a 2‐months trawling closure in the recovery of sedimentary organic matter. In comparison to untrawled areas, the continuous erosion and sediment mixing in trawling grounds led to coarser reworked sediments impoverished in organic carbon (∼30% loss) and promoted the degradation of labile compounds (52–70% loss). These impacts persisted after the temporal trawling closure, highlighting that this management strategy is insufficient to restore the seafloor. Considering the continuous expansion of bottom trawling grounds, this activity could have significant and irreversible biogeochemical impacts on ocean margins at a global scale, hampering their carbon burial capacity.

Depositional characteristics of a lacustrine mixed sediment system: A case study from the eastern Shijiutuo Uplift, offshore Bohai Bay Basin, eastern China

Mixed siliciclastic-carbonate sediments have been widely studied, especially those of marine origins. However, it is rarely investigated in continental settings. Boreholes from the Qinhuangdao-A (QHD-A) block in central Bohai Bay Basin sampled lacustrine mixed sediments during the Eocene period (32–38 Ma), providing an opportunity to facilitate this aspect of research. Here, an integrated study of seismic, core data, and microscopic analysis revealed the formation and evolution of the mixed sediment system. Core observations and microfossil identifications show that mixed sediments characterizing volcaniclastic sand and gravel mixed with bioclastic debris are composed of shallow-depth biota assemblages. The relatively warm climate, low tectonic activity and low siliciclastic influx may result in prevailing occurrences of nearshore biota. Preexisting siliciclastics featuring a well-rounding shape experienced secondary transportation to mix with the nearshore biota, forming the major mixing type (compositional mixing). Seismic data, well-logs, and lithofacies logs divided mixed sediments into three evolutional units (U1-U3). U1 and U2 were dominated by “compositional mixing”. Within the U1, it occurred locally due to high sediment input during the low-level period. Peak took place in the U2 because lake-level rise led to increasing accommodation and decreasing siliciclastic influx. With further deepening of water depth in the U3, shallow-depended biota was not well developed and carbonates increased. Lake-level change results in a distinct depositional sequence of mixed sediments that bioclastic particles mixed with siliciclastics in shallow period and centimeter-scale carbonates interbedded with siliciclastic beds in the highstand. These findings bring new insights into mixed deposits, especially in continental settings.

Bioturbation of Thalassinoides from the Lower Cambrian Zhushadong Formation of Dengfeng area, Henan Province, North China

Bioturbation plays a critical role in sediment mixing and biogeochemical cycling between sediment and seawater. An abundance of bioturbation structures, dominated by Thalassinoides, occurs in carbonate rocks of the Cambrian Series 2 Zhushadong Formation in the Dengfeng area of western Henan Province, North China. Determination of elemental geochemistry can help to establish the influence of burrowing activities on sediment biogeochemical cycling, especially on changes in oxygen concentration and nutrient regeneration. Results show that there is a dramatic difference in the bioturbation intensity between the bioturbated limestone and laminated dolostone of the Zhushadong Formation in terms of productivity proxies (Baex, Cu, Ni, Sr/Ca) and redox proxies (V/Cr, V/Sc, Ni/Co). These changes may be related to the presence of Thalassinoides bioturbators, which alter the particle size and permeability of sediments, while also increase the oxygen concentration and capacity for nutrient regeneration. Comparison with modern studies shows that the sediment mixing and reworking induced by Thalassinoides bioturbators significantly changed the primary physical and chemical characteristics of the Cambrian sediment, triggering the substrate revolution and promoting biogeochemical cycling between sediment and seawater.