Burial Of Sediments Research Articles

Disturbance mechanisms of lacustrine organic carbon burial: Case study of Cuopu Lake, Southwest China

Lakes are important organic carbon (OC) traps in the global carbon cycle. Recent studies have shown that the rate of OC burial in lacustrine sediments is influenced by factors such as climate change, land-use change, and eutrophication. In this study, we use multiproxy methods to reveal the mechanisms of lacustrine sediment OC burial in an alpine lake (Cuopu Lake), in southwest China. Combined with the dating from 210Pbex and n-alkanes distribution analysis using the Positive Matrix Factorization model, the sedimentary history was divided into five stages: religious activity (the 1840s–1880s), earthquake (the 1880s–1910s), garrison (the 1910s–1960s), transition (the 1960s–1990s), and ecotourism (the 1990s–2010s). During the earthquake stage, OC burial was dominated by terrestrial solids (>40%) and co-precipitated algae (>30%), with a rapid deposition rate (>4 mm a−1) and low OC concentration (<4 mg g−1). During the other stages, when the level of disturbance was relatively low, a change in nutrient conditions either promoted or inhibited plant growth, which influenced the type of buried OC. The contribution of OC derived from combustion sources varied from stage to stage. Severe anthropogenic disturbances have led to a significant increase in nutritional levels in the lake water, leading to an increase in the OC burial rate. Climate change, which leads to changes in temperature and rainfall, did not significantly influence OC burial, whereas nitrogen deposition (and associated ecological changes) was a significant determinant. When the general mechanism is dominant, the total nitrogen to inorganic phosphorus ratio is an effective indicator of OC burial due to its selective promotion of different plant types. In conclusion, our results suggest that lacustrine sediment OC burial is closely linked to physical and anthropogenic factors in Cuopu Lake, as well as similar montane lakes.

Sediment distribution and organic carbon burial in a subtropical hydroelectric reservoir

Reservoir sediment is a major player in the terrestrial carbon (C) budget, and the quantification of C burial in sediments is important in assessing the reservoir’s role in the global C budget. However, assessments of C burial in sediments of sub-tropical hydroelectric reservoirs and their potential as C sinks are thus far limited due to a lack of whole reservoir assessments of C burial. In this study, a combined seismic survey with sediment core analysis was conducted in the Xin’anjiang Reservoir, the first large hydroelectric reservoir in China. The total organic carbon (TOC), total nitrogen, isotopic C composition (δ13C), and sediment grain size were analyzed. 210Pb and 137Cs were used to obtain the sediment chronologies. The data suggest that bottom bathymetry of the pre-existing reservoir is an important factor on sediment accumulated within the reservoir. The sediment OC content and trophic state became controlling factors and accelerated the OC burial in the sediment. Based on the interpolated sediment thickness data and total reservoir area, the average TOC burial rate was estimated at 34.0 g C m−2 year−1 (ranging 0–676.7 g C m−2 year−1). This rate is within that estimated by sediment coring (mean 40.4 g C m−2 year−1, ranging 15.8–110.1 g C m−2 year−1) as a result of high heterogeneity of sediment distribution and irregular sedimentation. However, the result was close to the estimates of previous studies, demonstrating that the combined method here can also be an effective way to conduct future estimation of C burial in freshwater reservoirs.

Zinc- and cadmium-isotope evidence for redox-driven perturbations to global micronutrient cycles during Oceanic Anoxic Event 2 (Late Cretaceous)

This study uses organic-rich sediments from the Tarfaya Basin, Morocco, to assess the Cd- and Zn-isotope response to dramatic global palaeoenvironmental change during the Cenomanian–Turonian interval (Late Cretaceous). These organic-rich continental-margin deposits include an expression of Oceanic Anoxic Event 2 (OAE 2, ∼94 Ma), an interval associated with the spread of low-oxygen marine environments and widespread burial of organic-rich sediments. Due to placement of the Tarfaya Basin in a region of upwelling and relatively constant local environmental conditions, the stratigraphic variations in δ114Cd and δ66Zn values largely reflect changes in the seawater isotopic composition of the sub-surface proto-North Atlantic Ocean. Positive shifts of ∼0.2–0.3‰ away from background values in δ114Cd and δ66Zn are observed during the main phase of the positive carbon-isotope excursion associated with OAE 2. These isotopic shifts are coeval with decreases in Cd/TOC and Zn/TOC ratios and thus imply that drawdown of isotopically light Cd and Zn from seawater inventories was a result of extensive burial of these metals in organic-rich marine sediments globally. Low δ66Zn values during the Plenus Cold Event, a cooler episode during OAE 2, are similar in timing and magnitude to variations found in the English Chalk (Eastbourne, UK) and support the inference of a global control on these isotopic excursions. The δ66Zn values during the Plenus Cold Event are taken to record global oxygenation, possibly including the remobilization of isotopically light Zn from continental-margin sediments. A considerably smaller change in δ114Cd values for this interval implies that the Cd- and Zn-isotope systems can provide information about slightly different environmental processes, with global seawater composition with respect to Zn also being influenced by the magnitude of oxic removal sinks and isotopically light Zn input fluxes from sediments and hydrothermal fluids.

Experimental evidence for lava-like mud flows under Martian surface conditions

Large outflow channels on ancient terrains of Mars have been interpreted as the products of catastrophic flood events. The rapid burial of water-rich sediments after such flooding could have led to sedimentary volcanism, in which mixtures of sediment and water (mud) erupt to the surface. Tens of thousands of volcano-like landforms populate the northern lowlands and other local sedimentary depocentres on Mars. However, it is difficult to determine whether the edifices are related to igneous or mud extrusions, partly because the behaviour of extruded mud under Martian surface conditions is poorly constrained. Here we investigate the mechanisms of mud propagation on Mars using experiments performed inside a low-pressure chamber at cold temperatures. We found that low viscosity mud under Martian conditions propagates differently from that on Earth, because of a rapid freezing and the formation of an icy crust. Instead, the experimental mud flows propagate like terrestrial pahoehoe lava flows, with liquid mud spilling from ruptures in the frozen crust, and then refreezing to form a new flow lobe. We suggest that mud volcanism can explain the formation of some lava-like flow morphologies on Mars, and that similar processes may apply to cryovolcanic extrusions on icy bodies in the Solar System.

Infaunal response during the end-Permian mass extinction

Abstract The end-Permian mass extinction (EPME) profoundly shaped shallow marine ecosystems. Although much has been learned about this event based on the body-fossil record, the global infaunal response to the EPME, as represented by ichnofossils, is much less understood. Here we analyze secular changes in ichnodiversity and ichnodisparity from the late Permian to the Middle Triassic based on a global trace-fossil data set. Results show that, in contrast to the body-fossil record, late Permian global ichnodiversity and ichnodisparity maintained their level until the Griesbachian, followed by a sharp loss in the Dienerian. Notably, the Griesbachian shows an unusual dominance of shallower tiers. The discrepancy between the body- and trace-fossil record is interpreted to be the result of the resurgence of widespread microbial matgrounds in the Griesbachian that aided the preservation of surface, semi-infaunal, and shallow-tier ichnofossils. Our study shows that the EPME strongly affected the sediment mixed layer, allowing the preservation of shallower tier trace fossils. The disappearance of the mixed layer in the earliest Triassic may have enhanced pyrite burial in sediments and inhibited its further re-oxidation, therefore impacting sea water sulfate concentrations.

Mineralogical transformations in polymetallic nodules and the change of Ni, Cu and Co crystal-chemistry upon burial in sediments

Polymetallic nodules from the Clarion and Clipperton Zone of the equatorial Pacific Ocean were studied using X-ray diffraction, X-ray absorption, Fourier-transformed infrared spectroscopy and transmission electron microscopy. This study includes nodules found at the sediment surface as well as subsurface (14–16 cm sediment depth) and deeply buried (530–985 cm sediment depth) nodules. The surface and subsurface nodules are currently under oxic conditions whereas the deeply buried nodules are under suboxic conditions. Surface nodules consist mainly of turbostratic phyllomanganates (7 Å and 10 Å vernadite and Fe-vernadite); todorokite is a minor phase, if present at all. In contrast, subsurface and especially deeply buried nodules predominantly consist of todorokite, which increases in abundance with depth in the sediment. Thus, upon burial of nodules within the shallow sediment, phyllomanganates transform to todorokite, probably through the combined action of time and change in the ambient chemical conditions. Nodules from deeper sediment depth (>500 cm) consist primarily of todorokite and additionally show signs of dissolution.The transformation of phyllomanganates to todorokite and their further dissolution upon nodule burial under suboxic conditions induces modifications in the crystal-chemistry of Ni, Co, and Cu. In surface nodules, Ni and Co are incorporated in the octahedral sheets of phyllomanganates, whereas Cu mainly is located at the edges of those phyllomanganate sheets. In buried nodules Cu and to a lesser extent Ni are incorporated in todorokite by forming outer-sphere complexes within the tunnels. However, Ni is predominantly incorporated within the octahedra of the newly formed todorokite structure. Co is also enriched in the octahedra of todorokite as a result of dissolution of hydrogenetic vernadite and re-incorporation in the more stable Mn-phase formed during the diagenetic transformation. Co enrichment under suboxic conditions after burial within the sediments is noteworthy since Co in surface nodules is characteristic for oxic conditions.

Calcium isotopes in deep time: Potential and limitations

Calcium is an essential element in the biogeochemical cycles that regulate the long-term climate state of Earth. The removal of CO2 from the ocean-atmosphere system is controlled by the burial of carbonate sediments (CaCO3), ultimately linking the global calcium and carbon cycles. This fundamental link has driven the development of the stable calcium isotope proxy with application to both ancient skeletal and non-skeletal bulk carbonate sediments. Calcium isotope ratios (δ44/40Ca) have been used to track long-term changes in seawater chemistry (e.g., aragonite vs. calcite seas) and to elucidate short-term climatic perturbations associated with mass extinction events. However, developments in the calcium isotope proxy have shown that δ44/40Ca values in carbonate minerals also are sensitive to changes in precipitation rates, mineralogy and diagenesis, thereby complicating the application of the proxy to the reconstruction of global cycles. First, inorganic carbonate precipitation experiments have demonstrated that carbonate δ44/40Ca values are sensitive to precipitation rates with higher rates generally leading to larger fractionation. Second, δ44/40Ca values are sensitive to carbonate mineralogy with inorganic aragonite and calcite being on average ~ 1.5‰ and ~ 0.9‰ depleted relative to contemporaneous seawater, respectively. The effects of both changes in carbonate mineralogy and precipitation rates affect primary and secondary minerals, but are particularly pronounced during carbonate diagenesis where relatively slow rates of recrystallization and neomorphism can lead to significant changes in bulk sediment δ44/40Ca values. Third, changes in faunal composition expressed in skeletal fossil archives can lead to large changes in carbonate δ44/40Ca values that are decoupled from changes in global cycles. Nevertheless, when these factors are appropriately considered the application of calcium isotopes in ancient carbonate sediments becomes a powerful tool for understanding biogeochemical processes that operate over many scales; from diagenetic changes within the sediment pore-space, to regional changes across ancient carbonate platforms, and to global changes in seawater chemistry through time. Importantly, the processes that contribute to variability in carbonate δ44/40Ca values are likely to impact other carbonate-bound proxies, highlighting the potential for calcium isotopes as a tool to better understand the variability of other isotope systems.

The fate of organic carbon burial in the river-dominated East China Sea: Evidence from sediment geochemical records of the last 70 years

The accumulation of organic carbon (OC) in the sediments of river-dominated continental margins plays a critical role in the global carbon budget and is subject to human modification. However, the relationships between human activities in drainage basins and carbon burial in modern sediments are still poorly understood. Here, a well preserved sediment core from a mud zone in the East China Sea (ECS) was analyzed for isotopes (δ13Corg and δ15N), radioisotopes (210Pb and 137Cs), bulk organic carbon and nitrogen, sediment grain size, major elements, trace metals and chemical properties (pH and Eh), in order to reveal the fate of the OC buried in the studied shelf over the last 70 years. The results showed that the input of terrestrial organic matter (TOM) from the Yangtze River into the ECS declined by 15% between 1980 and 2005, in concomitance with decreasing terrestrial-derived Ca and Zr. These changes were mainly due to alterations in the sediment load and cultivated land, induced by river damming and soil conservation practices. The amount of marine organic matter (MOM) was strongly and positively correlated with chlorophyll a concentrations and the loads of dissolved inorganic nitrogen and phosphate. The discharge of riverine nutrients from artificial fertilizers seems to be one of the key factors influencing marine productivity. OC burial fluxes varied between 33.8 and 54.7 g C m−2 yr−1 and were primarily related to changes in the mass accumulation rates (MARs) and OC%. Notably, due to the construction of the Three Gorges Dam (TGD) in 2003, the input of eroded material from the downstream and subaqueous delta of the Yangtze River increased, profoundly altering the local hydrodynamics and sedimentary redox levels. These changes may, in turn, accelerate the degradation of sedimentary OC, preventing carbon deposition and preservation.

Recent magmatism drives hydrocarbon generation in north-east Java, Indonesia

Conventional studies of petroleum basins associate oil generation with the gradual burial of organic-rich sediments. These classical models rely on the interplay between pressure, temperature, and the time required for organic matter transformation to oil and gas. These processes usually occur over geological timescales, but may be accelerated by rapid reactions when carbon-rich sediments are exposed to migrating magmatic fluids. The spectacular Lusi eruption (north-east Java, Indonesia) is the surface expression of the present-day deep interaction between volcanic and sedimentary domains. Here we report the ongoing generation of large amounts of hydrocarbons induced by a recent magmatic intrusion from the neighbouring Arjuno-Welirang volcanic complex. We have investigated a unique suite of oil and clast samples, and developed a detailed conceptual model for the complex hydrocarbon migration history in this part of the basin by integrating multidisciplinary techniques. Our results show that palynology, organic petrology, and chlorite microthermometry are the most sensitive geothermometers for basins affected by recent magmatic activity. These findings further our understanding of the driving mechanisms fueling the world’s largest active mud eruption and provide a unique dataset to investigate modern hydrocarbon generation processes.

Numerical modeling of methane hydrate accumulation with mixed sources in marine sediments: Case study of Shenhu Area, South China Sea

The natural gas hydrates detected in Shenhu Area, South China Sea are considered to have great potential for exploitation and to become a research hotspot. However, details of the dynamic accumulation and evolution processes of gas hydrate over a long time in this area remain unclear. In this study, we developed a numerical model to predict the accumulation of gas hydrates in the marine sediments by introducing the biogenic methanogenesis module to the framework of the existing simulator TOUGH+HYDRATE. A one-dimensional dynamic model was applied to reproduce the formation of gas hydrate at site SH2 in Shenhu Area. The burial of sediments and associated phenomena (e.g., evolution of temperature, sediment compaction and consequent reduction in sediment porosity and permeability, fluid expulsion) are taken into account. Modeling results indicate that with the conventional conversion coefficient (CC) of organic carbon to methane of 0.15, the amount of in-situ biogenic methane is not sufficient to form the highly saturated hydrate detected at this location and the remaining methane (91.4%) is supplied by the upward flow of fluids. The tectonic movement that took place about 1.5 Ma created pathways for the fluids to migrate vertically. The pore water and methane gas fluxes in the upward fluids were determined as 2 × 10−10 kg/s·m2 and 1 × 10−11 kg/s·m2, respectively, using the proved hydrate saturation profile and measured pore water chlorinity from recovered core samples. The distribution of hydrate in the sediments is significantly influenced by pore water salinity. The inhibitory effect of salt on hydrate formation, which shifts the pressure of three-phase (i.e., solid hydrate, methane gas and pore water) equilibrium to actual pore water pressure, could be the major reason for the appearance of a three-phase co-existence zone in the hydrate concentrated zone. Additionally, the water flux and diffusivity of salt have a profound effect on the formation of gas hydrate by affecting the salinity of pore water in the gas hydrate concentrated zone. The results of this study are of great significance for guiding marine gas hydrate exploration and resource evaluation.

Carbon Deposition and Burial in Estuarine Sediments of the Contiguous United States

AbstractEstuaries represent the primary linkage between the terrestrial and marine carbon cycles, and estuarine processing of riverine and coastal carbon plays a disproportionately large role in the global carbon cycle relative to the small areal extent of the estuarine environments. However, knowledge of the rate of organic carbon deposition and burial in estuarine sediments is lacking at regional scales. Data on surficial total organic carbon, linear sedimentation, and bulk density of estuarine sediments were compiled and categorized via a cluster analysis in order to estimate carbon deposition within the contiguous United States (CONUS). The cluster analysis broadly grouped estuaries by geography, but exceptions to geographic clustering highlighted differences within regions. A transfer function from deposition to burial based on linear sedimentation rate was used to estimate burial efficiency, and thus the rate of carbon burial within each cluster. We estimate organic carbon deposition rates within CONUS estuarine sediments to be 161 [121–217, 95% confidence] g C/m2/yr with a burial efficiency estimated at 38 [34–42, 95% confidence] %, which yields a long‐term burial rate of 64 [44–97, 95% confidence] g C/m2/yr. Spatially integrated organic deposition and burial rates are 11.3 [8.5–15.2, 95% confidence] and 4.5 [3.1–6.8, 95% confidence] Tg C/yr, respectively. Our findings allow a more thorough understanding of coastal carbon cycling, which is critical for both management purposes as well as for the assessment of the role of estuaries in past and future climate change.

Enhanced Organic Carbon Burial in Sediments of Oxygen Minimum Zones Upon Ocean Deoxygenation

Oxygen minimum zones (OMZs) in the ocean are expanding. This expansion is attributed to global warming and may continue over the next 10 to 100 kyrs due to multiple climate CO$_\text{2}$-driven factors. The expansion of oxygen-deficient waters has the potential to enhance organic carbon burial in marine sediments, thereby providing a negative feedback on global warming. Here, we study the response of dissolved oxygen in the ocean to increased phosphorus and iron inputs due to CO$_\text{2}$-driven enhanced weathering and increased dust emissions, respectively. We use an ocean biogeochemical model coupled to a general ocean circulation model (the Hamburg Oceanic Carbon Cycle model, HAMOCC 2.0) to assess the impact of such regional deoxygenation on organic carbon burial in the modern ocean on time scales of up to 200~kyrs. We find that an increase in input of phosphorus and iron leads to an expansion of the area of the OMZ impinging on continental margin sediments and a significant decline in bottom water oxygen in the open ocean relative to pre-industrial conditions. The associated increase in organic carbon burial could contribute to the drawdown of $\sim$1600~Gt of carbon, which is equivalent to the total amount of CO$_\text{2}$ in the atmosphere predicted for the year 2100 in a business as usual scenario, on time scales of up to 50 kyrs. The corresponding areal extent of sediments overlain by bottom waters with little or no oxygen as estimated by the model is not very different from the minimum area estimated for two major oceanic anoxic events in Earth's past. Such events were associated with major perturbations of the oceanic carbon cycle, including high rates of organic carbon burial. We conclude that organic carbon burial in low oxygen areas in the ocean could contribute to removal of anthropogenic CO$_\text{2}$ from the atmosphere on long time scales.

New model of Si balance in the Late Cretaceous epicontinental European Basin

The Si, which is an important element within the Earth's crust, mostly occurs as a monomeric silicic acid (DSi) in seawater and is utilised by marine organisms to build their skeletons made of opaline silica (BSi). It is also an essential component for many minerals formed in marine environments. The general model of the Si balance, mostly derived from studies of the current oceans, assumes Si input from the weathering of the Earth's crust, its flux in the water column under biological control, and the output via burial in marine sediments. In the Campanian-Maastrichtian (Late Cretaceous), the extremely high sea levels caused the reduction of the continents area and triggered the development of widely spread epicontinental seas. This, combined with extensive volcanic activity, spreading of the ocean floor, and active ocean ridges, elevated the overall Si concentration in the seawater, affected the proportion of Si input from terrestrial weathering and deep-sea fluxes, and influenced the development of the siliceous organisms. The model presented in this study reconstructs the Si balance in the Campanian-Maastrichtian European Basin based on geological records, characterizes the main sources of the DSi input, biological flux, and output via diagenesis. The data presented indicate the significant role of siliceous sponges in the biologically controlled part of the cycle and the DSi-rich inflow from the oceans, which were the principal sources of the (net) inputs of DSi. This data has linked the Si cycle to the bottom zone of the basin, which contrasts with the recent model in which the cycle is concentrated on the subsurface zone of the ocean. The data derived from petrographic studies of chalk and opoka lithofacies broadly show how the Si cycle had affected rock formation and highlighted the role of the mineralogical composition of the seabed mud in the output of DSi from the cycle. The sediment pathway of the DSi burial in the presented Cretaceous model was initiated by the early transformations of biogenic opal-A (BSi) into a more stable form of opal-CT a few centimetres below the seafloor during the early stages of diagenesis, impacting Si recycling. The new model of the Late Cretaceous Si cycle better recognises the role of siliceous sponges and diagenesis in the Si balance in modern environments and provides a paleoecological interpretation of the formation of chalk, opoka, and siliceous nodules during the Late Cretaceous in the European Basin. • In Late Cretaceous the main DSi input was by upwelling currents from deep oceans. • The siliceous sponges play a main role in DSi uptake from water column and its sink. • The Si recycling was limited by its fast burial and diagenetic transformations. • The Si cycle in Late Cretaceous epicontinental seas differs from the modern model.

Organic Carbon Origin, Benthic Faunal Consumption, and Burial in Sediments of Northern Atlantic and Arctic Fjords (60–81°N)

AbstractFjords have been recently recognized as hot spots of organic carbon (Corg) sequestration in marine sediments. This study aims to identify regional and local drivers of variability of Corg burial in north Atlantic and Arctic fjords. We provide a comparative quantification of Corg, δ13C, photosynthetic pigments content, benthic biomass, consumption, Corg accumulation, and burial rates in sediments in six fjords (60–81°N). Higher sediment Corg content in southern Norway reflected longer phytoplankton growth season and higher productivity. Higher contributions of terrestrial Corg were noted in temperate/southern Norway (dense land vegetation and high precipitation) and Arctic/Svalbard (glacial erosion) than in subarctic/northern Norway locations. Benthic biomass and carbon consumption were best correlated to δ13C and photosynthetic pigments content indicating control by quality rather than quantity of available food. Benthic faunal consumption did not seem to affect the variability in Corg burial. Regional environmental factors (water temperature and latitude) combined with local factors (Corg, grain size, and pigment concentration) explained 94% of Corg burial variability. Based on the present study and literature data on Corg content, origin, and burial rates, the fjords were classified into four categories: temperate, subarctic, Arctic with glaciers, and Arctic without glaciers. The variability in marine productivity, terrestrial inflows, and carbon sequestration in fjords must be considered for global estimates of their role in blue carbon storage and for building scenarios of future changes in the course of climate warming.

Confusing a Pollen Grain with a Parasite Egg: an Appraisal of "Paleoparasitological Evidence of Pinworm (Enterobius Vermicularis) Infection in a Female Adolescent Residing in Ancient Tehran".

There is often the risk of confusing pollen grains with helminth eggs from archaeological sites. Thousands to millions of pollen grains can be recovered from archaeological burial sediments that represent past ritual, medication and environment. Some pollen grain types can be similar to parasite eggs. Such a confusion is represented by the diagnosis of enterobiasis in ancient Iran. The authors of this study confused a joint-pine (Ephedra spp.) pollen grain with a pinworm egg. This paper describes the specific Ephedra pollen morphology that can be confused with pinworm eggs.

Temporal variation in sediment C, N, and P stoichiometry in a plateau lake during sediment burial

Variation in C/N/P stoichiometry during sediment burial is related to elemental biogeochemistry cycles from the hydrosphere to the lithosphere. However, C/N/P stoichiometry change in lakes is still unclear although lakes are an important burial site of terrestrial matter. The purpose of this study was to determine the variation in C/N/P stoichiometry and its influencing factors in the burial processes of lake sediments. Two sediment cores (cores 4 and 5) were collected from a plateau lake (Dianchi Lake, China) in July 2014. The CRS 210Pb dating model of was used to estimate the vertical chronological distribution for each layer. Total, organic, and inorganic carbon (TC, TOC, TIC), nitrogen (TN, TON, TIN), and phosphorus (TP, TOP, TIP) were analyzed to determine variation patterns in the C/N (RCN, ROCN, RICN), C/P (RCP, ROCP, RICP), and N/P ratios (RNP, RONP, RINP) in lake sediments. The results showed that TOC, TON, and TIP were the main constituents of TC, TN, and TP in both sediment cores, respectively. After 1960, the burial rates of TOC and TON in core 4 exhibited rapidly increasing trends, coincident with TC and TN. In core 5, the increase occurred after 1980. Significant differences in C/N/P stoichiometry were observed in cores 4 and 5, dominated by allochthonous and autochthonous sources, respectively. RCP and RNP in core 4 were higher than in core 5; the difference in RCN was insignificant. RCN exhibited irregular large fluctuations in the middle layer, with mean values of 20.9 ± 4.0 and 17.3 ± 8.1 for cores 4 and 5, respectively. RCP and RNP declined rapidly with time, indicating that the losses of C and N might be higher than that of P. ROCN, ROCP, and RONP followed the same vertical decreasing pattern as RCN, RCP, and RNP, but RICN and RINP did not. Mineralization could be the primary driver and different sources played a regulatory role in the temporal variation of stoichiometry. Sediment stoichiometry (RCN, RCP, RNP) decreased with time might be mainly due to selective outcome of mineralization and be regulated by different organic matter sources. RCP, ROCP, RNP, and RONP showed significantly decreasing trends with age, whereas RCN and ROCN were relatively consistent. Vertical distribution of inorganic ratios differed from that of organic and total proportions. Further research is needed to determine influencing factors on these ratios and to clarify the role of C/N/P stoichiometry in freshwater ecosystems.

Glacial Runoff Promotes Deep Burial of Sulfur Cycling-Associated Microorganisms in Marine Sediments.

Marine fjords with active glacier outlets are hot spots for organic matter burial in the sediments and subsequent microbial mineralization. Here, we investigated controls on microbial community assembly in sub-arctic glacier-influenced (GI) and non-glacier-influenced (NGI) marine sediments in the Godthåbsfjord region, south-western Greenland. We used a correlative approach integrating 16S rRNA gene and dissimilatory sulfite reductase (dsrB) amplicon sequence data over six meters of depth with biogeochemistry, sulfur-cycling activities, and sediment ages. GI sediments were characterized by comparably high sedimentation rates and had “young” sediment ages of <500 years even at 6 m sediment depth. In contrast, NGI stations reached ages of approximately 10,000 years at these depths. Sediment age-depth relationships, sulfate reduction rates (SRR), and C/N ratios were strongly correlated with differences in microbial community composition between GI and NGI sediments, indicating that age and diagenetic state were key drivers of microbial community assembly in subsurface sediments. Similar bacterial and archaeal communities were present in the surface sediments of all stations, whereas only in GI sediments were many surface taxa also abundant through the whole sediment core. The relative abundance of these taxa, including diverse Desulfobacteraceae members, correlated positively with SRRs, indicating their active contributions to sulfur-cycling processes. In contrast, other surface community members, such as Desulfatiglans, Atribacteria, and Chloroflexi, survived the slow sediment burial at NGI stations and dominated in the deepest sediment layers. These taxa are typical for the energy-limited marine deep biosphere and their relative abundances correlated positively with sediment age. In conclusion, our data suggests that high rates of sediment accumulation caused by glacier runoff and associated changes in biogeochemistry, promote persistence of sulfur-cycling activity and burial of a larger fraction of the surface microbial community into the deep subsurface.

Experimental study on the existence of nano-scale pores and the evolution of organic matter in organic-rich shale

Abstract To explore the characteristics of nanopores in organic-rich shale, the shale of the Upper Yangtze area under different maturity are studied. Scanning electron microscopy (SEM), low temperature nitrogen experiment (LP-N2-GA) and the Frenkel-Halsey-Hill (FHH) fractal model were used. Results show that the intergranular pores and organic pores are most developed, and the pore size is distributed from a few nanometers to several tens of micrometers. The SEM gray scale images make the pore development characteristics of shale be observed more intuitively. Based on the results of low temperature nitrogen adsorption test and FHH fractal model, the results show the fractal dimension of shale is positively correlated with TOC and quartz, but negatively related to clay minerals and feldspar. There is a good relation between maturity and pore volume.With releasing of pressure, the pore evolution gradually returns to the normal compaction trend, and the pore volume of the shale decrease. Graphical abstract: There are many nano-scale pores in the shale, and the pore characteristics determine the characteristics of shale reservoirs. At the initial stage of sedimentary burial, when Ro is less than 0.7%, the pore volume and specific surface area of the initial shale are large. At the medium maturity stage (0.7%&lt;Ro&lt;1.3%), the pore volume and pores caused by mechanical compaction to some extent. As the degree of thermal evolution deepens to the high maturity stage (1.3% &lt;Ro &lt; 2.6%), the shale is generating large amount of gas. The shale belongs to the low-porosity and low-permeability reservoirs, and the generated oil and gas cannot be effectively transported in a short period of time. The overpressure of the reservoir and the inter-mineral pore characteristics caused by the organic pores and organic acids formed by the hydrocarbon generation are more complicated. When shale is in the stage of over-maturation (Ro&gt;2.6%), as the increase of buried depth, the end of hydrocarbon generation process and long-term pressure release, the under-compacting effect gradually disappear. Porosity evolution gradually returns to normal compaction trend in the shale.

Effects of Damming on River Nitrogen Fluxes: A Global Analysis

AbstractDamming creates biogeochemical hotspots along rivers that modify the riverine flow of nutrients, including nitrogen (N). Here, we quantify the impact of dams on global riverine N fluxes using a reservoir N mass balance model. In‐reservoir processes represented in the model include primary production, mineralization of organic N, denitrification, and sedimentary burial. In addition, we explicitly account for N fixation as a source of N, assuming that the N to phosphorus (P) ratio of the inflow regulates the magnitude of N fixation in reservoirs. The model is scaled up via a Monte Carlo analysis that yields global relationships between N elimination in reservoirs, either by denitrification or burial, and the hydraulic residence time. These relationships are then combined with N loads to the world's dam reservoirs generated by the Global‐NEWS model and the estimated N fixation fluxes. According to the results, in year 2000, worldwide N fixation in reservoirs was on the order of 70 Gmol yr−1, while denitrification and burial in reservoirs eliminated around 270 Gmol yr−1, equal to 7% of N loading to the global river network. The latter is predicted to double to 14% by 2030, mainly as a result of the current boom in dam building. The results further imply that, largely due to N fixation in reservoirs, damming causes a global upward shift in riverine N:P ratios, thus lessening N limitation in receiving water bodies.

A Well‐Posed Alternative to the Hirano Active Layer Model for Rivers With Mixed‐Size Sediment

The active layer model (Hirano, 1971) is frequently used for modeling mixed‐size sediment river morphodynamic processes. It assumes that all the dynamics of the bed surface are captured by a homogeneous top layer that interacts with the flow. Although successful in reproducing a wide range of phenomena, it has two problems: (1) It may become mathematically ill‐posed, which causes the model to lose its predictive capabilities, and (2) it does not capture dispersion of tracer sediment. We extend the active layer model by accounting for conservation of the sediment in transport and obtain a new model that overcomes the two problems. We analytically assess the model properties and discover an instability mechanism associated with the formation of waves under conditions in which the active layer model is ill‐posed. Numerical simulations using the new model show that it is capable of reproducing two laboratory experiments conducted under conditions in which the active layer model is ill‐posed. The new model captures the formation of waves and mixing due to an increase in active layer thickness. Simulations of tracer dispersion show that the model reproduces reasonably well a laboratory experiment under conditions in which the effect of temporary burial of sediment due to bedforms is negligible. Simulations of a field experiment illustrate that the model does not capture the effect of temporary burial of sediment by bedforms.