Influence of diagenesis on the electrical resistivity and the formation factor of deep-sea carbonate sediments

Laboratory measurements of electrical resistivity on two DSDP pelagic carbonate sequences permitted the study of the effect of diagenesis on the electrical and other physical properties such as velocity and porosity. Electrical resistivity and formation factor increase with sediment depth. Changes in porosity with progressive diagenesis (that is, ooze-chalk-limestone) are observed, and changes in pore geometry are inferred. These changes are interpreted in terms of systematic variations in electrical and physical properties. Influence of silica on carbonate diagenesis may result in significant change in pore geometry. This effect inhibits electrical conduction leading to complicated but generally high resistivity values for siliceous and cherty limestones. Compressional velocity correlates well with resistivity for both sequences.     

The influence of the partial pressure of carbon dioxide on the total carbonate of seawater

The dependence of the total carbonate concentration of ocean water on temperature and atmospheric partial pressure of carbon dioxide is calculated. The results show that the increase in total carbonate caused by the increase of carbon dioxide in the atmosphere is ca. 25–50 times larger than the precision in the experimental determination of C_t.     

Effects of organic pollutants on methanogenesis,sulfate reduction and carbon dioxide evolution in salt marsh sediments

Anaerobic salt marsh sediments were amended with a variety of organic pollutants and the effects on methanogenesis, sulfate reduction and carbon dioxide evolution were examined. Addition of 1000 μg g^?1 (dry weight sediment) Arochlor 1221, lindane, endrin, benzene and phenanthrene resulted in no significant effects on the activities studied. Methanogenesis was inhibited by 1000 μg g^?1toxaphene, PCP, chlordane, naphthalene, DDT, Kepone and heptachlor and by 100 μg g^?1 PCP and toxaphene. At 1000 μg g^?1 naphthalene and toxaphene and 100 μg g^?1 PCP, a period of initial inhibition of methanogenesis was followed by stimulation relative to controls. Arochlor 1254 (1000 μg g^?1) and Temik (500 and 10 μg g^?1) stimulated methanogenesis from the outset. Temik at 500 μg g^?1 gave the greatest stimulation of methanogenesis (900% of controls) of any of the compounds studied. Sulfate reduction was inhibited by 1000 μg g^?1 PCP, toxaphene, naphthalene and chlordane and by 500 μg g^?1 atrazine and 100 μg g^?1 heptachlor. Sustained inhibition of sulfate reduction by naphthalene, toxaphene and PCP may have contributed to the stimulation of methanogenesis. Carbon dioxide evolution was not significantly affected by most of the compounds studied except for 100 μg g^?1 PCP and 1000 μg g^?1 aphthalene, each of which gave significant inhibition in only one of three experiments.Concentrations of individual organic pollutants required to cause observable effects were high. It is concluded that, except for highly polluted sediments, methanogenesis, sulfate reduction and CO₂ evolution would not be affected by the compounds studied here at concentrations typically found in the environment.     

Direct determination of organic carbon in modern reef sediments and calcareous organisms after dissolution of carbonate

An improved method for the direct determination of organic carbon in calcareous marine sediments, organisms and particulate matter is described. Samples are dissolved in phosphoric acid to remove carbonates, purged with oxygen to remove CO₂, and the resulting acidic solution is analysed for total (dissolved and particulate) organic carbon. The method is rapid, involves minimal sample manipulation, and is both accurate (better than ± 2%) and precise (better than ± 0.2 mg organic carbon/g sediment). The method is especially suitable for modern carbonate sediments which have low levels of predominantly acid-soluble organic carbon.     

Recent changes of biogenic carbonate deposition in anoxic sediments of the Black Sea: sedimentary record and climatic implication

Recent changes of carbonate deposition were traced in a Black Sea sediment core taken in the western abyssal basin. The sediments were dated from a vertical profile of excess 210Pb. The 210Pb geochronology corresponded well to the 137Cs fallout record. A 20-year cyclic variability of carbon deposition has been traced in the dated sediments and has been related inversely to the long-term changes in temperature of air over the basin, forcing the convection in the upper water column, which may bear influence upon the coccolithophorid blooms by bringing nutrients from deeper water to the surface.     

Estimation of the uncertainty of future changes in atmospheric carbon dioxide concentration and its radiative forcing

An ensemble experiment with the IAP RAS CM was performed to estimate future changes in the atmospheric concentration of carbon dioxide, its radiative forcing, and characteristics of the climate-carbon cycle feedback. Different ensemble members were obtained by varying the governing parameters of the terrestrial carbon cycle of the model. For 1860–2100, anthropogenic CO₂ emissions due to fossil-fuel burning and land use were prescribed from observational estimates for the 19th and 20th centuries. For the 21st century, emissions were taken from the SRES A2 scenario. The ensemble of numerical experiments was analyzed via Bayesian statistics, which made the uncertainty range of estimates much narrower. To distinguish between realistic and unrealistic ensemble members, the observational characteristics of the carbon cycle for the 20th century were used as a criterion. For the given emission scenario, the carbon dioxide concentration expected by the end of the 21st century falls into the range 818 ± 46 ppm (an average plus or minus standard deviation). The corresponding global instantaneous radiative forcing at the top of the atmosphere (relative to the preindustrial state) lies in the uncertainty range 6.8 ± 0.4 W m^?2. The uncertainty range of the strength of the climate-carbon cycle feedback by the end of the 21st century reaches 59 ± 98 ppm in terms of the atmospheric carbon dioxide concentration and 0.4 ± 0.7 W m^?2 in terms of the radiative forcing.     

The study of mixing rates and sedimentation rates in Meizhou Bay sediments

-The applicability of the doubie-layer model for 210Pb chronology in coastal marine environments was discussed,which is successfully used in the study of mixing rates and sedimentation rates in Meizhou Bay. Differences among sedimentation rates deduced from 210Pb, 210Po, and 137Cs were compared. Mixing rates in the sediment surface layer were determined by means of excess 234Th.     

Methane, carbon monoxide, and carbon dioxide concentrations measured in the atmospheric surface layer over continental Russia in the TROICA experiments

The principal statistical regularities typical of the behaviors of the CH₄, CO, and CO₂ concentrations in the atmospheric surface layer over the continental Russian territory are revealed from the measurements performed in 1997–2004 along the Trans-Siberian Railroad from Moscow to Khabarovsk with a mobile laboratory. The data obtained under the conditions of the atmosphere free of anthropogenic pollutants are analyzed. For near-background conditions, the typical continental methane, carbon monoxide, and carbon dioxide concentrations and characteristic features of their large-scale spatial distributions and daily variations, including those caused by surface inversions, are determined. Variations in the concentrations of these trace gases over industrial regions are analyzed. Our results are compared to the data obtained at background stations of the world network of atmospheric monitoring and to the data of a numerical simulation.     

The influence of seagrass beds on carbonate sediments in the Bahamas

Chemical variables were measured in calcium-carbonate-rich sediments inhabited by the dominant tropical seagrass, Thalassia testudinum, and in adjacent seagrass-free sediments at several locations in the Bahamas Islands. Pore-water alkalinity and p_CO2 were consistently greater, while pH was consistently lower in sediment-pore waters within seagrass beds. The ammonium and molybdate-reactive phosphate concentrations in sediment-pore water were variable for vegetated, compared with unvegetated, sample locations.Thalassia testudinum can generate very large amounts of organic matter within calcium-carbonate-rich sediments. However, little of the organic matter is retained in the sediment and the effect of that organic matter on pore water chemical factors appears to be surprisingly small. These observations are markedly different from those for seagrass beds in high latitude clastic sediments and in Syringodium filiforme seagrass beds near San Salvador Island, where major influences of the seagrass beds on sediment chemistry have been observed. The generally coarser grain size of the carbonate sediments may be a primary factor contributing to these differences.     

Geochemistry and palaeo-oceanography of metalliferous and pelagic sediments from the Late Cretaceous Oman ophiolite

Metalliferous and pelagic sediments are exposed within and above the extrusive successions of the Upper Cretaceous Oman ophiolite which, on the basis of mostly geochemical evidence, is believed to have formed in an incipient marginal basin setting located above a NE-dipping subduction zone. The ophiolitic extrusives document various volcano-tectonic settings which include the axial zones of a spreading ridge, fault-controlled seamounts and off-axis volcanic edifices. Most of the Fe, Mn and trace metal-enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high-temperature sulphide-precipitating vents. The trace element content (e.g. REE and Sr) was largely scavenged from seawater. The sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges, and the ‘basal’ sediments of DSDP wells and of other ophiolite complexes (e.g. Troodos, Cyprus).Distinctive mound structures located low in the lavas are attributed to percolation of sulphide-rich solutions into already deposited metalliferous oxide sediments. The resulting iron-silica rock was probably originally precipitated as ferruginous silicates.Major massive sulphides formed off-axis at the base of intermediate-basic edifices of volcanic arc affinities. Fe, Mn and trace metal enrichment in the sediment cover of a flat-topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away (Lasail). Small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones. The metalliferous sediments, locally precipitated near these vents, are ferromanganiferous, but trace metal-depleted.The metalliferous and pelagic sediment cover of the extrusive successions, generally, documents waning hydrothermal input after volcanism ended in the area.A model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the Arabian continental margin. With progressive subduction this crust approached the margin. Initially, calcareous sediment accumulated above the calcite compensation depth (CCD), but then non-calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the CCD was higher and marginal upwelling possibly enhanced productivity. As the edge of the Arabian continental margin entered the trench, the over-riding ophiolite was regionally uplifted allowing short-lived chalk accumulation above the CCD. This was followed by volcaniclastic deposition related to the tectonic emplacement.     

Effects of carbon dioxide on deep-sea harpacticoids revisited

As part of the evaluation of the environmental impact of sequestering carbon dioxide in the deep ocean, we exposed the sediment-dwelling fauna at a station in Monterey Submarine Canyon (36.378°N, 122.676°W, 3262 m) to carbon dioxide-rich seawater and found that most of the harpacticoid copepods were killed. In an expanded, follow-on experiment on the continental rise nearby (36.709°N, 123.523°W, 3607 m), not only did harpacticoids survive exposure to carbon dioxide-rich seawater, but we found no evidence from seven additional metrics that the harpacticoids had been affected. We infer that during the second experiment the harpacticoids were not exposed to a stressful dose. During the second experiment, carbon dioxide-rich seawater appears to have been produced more slowly than in the first, probably because of differences in the near-bottom flow regimes. We conclude that local physical circumstances can substantially influence the results of experiments of this type and will complicate the evaluation of the environmental consequences of deep-ocean carbon dioxide sequestration.     

Dry and wet atmospheric deposition of organic carbon in coastal and water areas of the northeastern part of the Sea of Azov

The paper reports on new data focusing on organic carbon contained in aeolian material and precipitation falling on coastal and water areas of the northeastern part of the Sea of Azov. Atmospheric deposition was sampled in 2006–2014. The particulate organic carbon content varied from 4 to 27% in aeolian dust samples. The concentration of the organic carbon dissolved in rainwater was from 1.6 to 4.3 mg C/L, and from 0.9 to 16.6 mg C/L in snow. The particulate organic carbon content varied from 2 to 43% in snow. Intensity of aeolian dust settling decreased from 178 to 33 mg/m² per day with distance from a source of dust; in contrast, the relative content of organic matter increased. In a spring–summer season the aeolian organic carbon fluxes varied from 12 to 18 mg C/m² per day in Rostov-on-Don, from 28 to 48 mg C/m² per day on the Gulf of Taganrog coast, and from 20 to 80 mg C/m² per day in the water area of the Gulf of Taganrog.     

Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys

 Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s^-1; compressional 1670–1725 m s^-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s^-1; compressional 1520–1570 m s^-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments, carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather than changes due to the effects of biogeochemical processes, is responsible for these differences.     

Sources,deposition rates and decomposition of organic carbon in recent sediment of Sachem Head Harbor,Long Island Sound

The vertical distributions of excess ²¹⁰Pb and fall out ^{239, 240}Pu imply a uniform sedimentation rate of 1·4–1·6 cm year^?1 from 0 to 105–110 cm. This sediment accumulation rate is compatible with sulfate reduction rate data from this location. Below 70 cm only ‘aged’ refractory carbon is present (C_R = 1·8% C) with an age of approximately 2400 years. This phase is present in a number of locations across Long Island Sound. Planktonic carbon (C_P) is present above the 60–67 cm horizon. A value of 1·0 for A_P (¹⁴C activity) at 32–37 cm was taken, A_P = 1·285 was used for contemporary plankton. This was obtained by correcting the measured A_P of a plankton tow sample for admixed refractory carbon. These values were then used to calculate C_R, C_P and C_F (fossil carbon) at 32–37 cm and 6–12 cm. The only values compatible with the known sulfate reduction rate data are C_R equal to pre 60–67 cm levels (1·6–1·8% C), C_F being 0·3% C at both depths, and C_P decreasing with depth from 0·3 to 0·4% C at 6–12 cm to close to zero at 32–37 cm.     

Organic carbon accumulation and sulfate reduction rates in slope and basin sediments of the Ulleung Basin,East/Japan Sea

This study investigated the organic carbon accumulation rates (OCARs) and sulfate reduction rates (SRRs) in slope and basin sediments of the Ulleung Basin, East/Japan Sea. These sediments have high organic contents at depths greater than 2,000 m; this is rare for deep-sea sediments, except for those of the Black Sea and Chilean upwelling regions. The mean organic carbon to total nitrogen molar ratio was estimated to be 6.98 in the Ulleung Basin sediments, indicating that the organic matter is predominantly of marine origin. Strong organic carbon enrichment in the Ulleung Basin appears to result from high export production, and low dilution by inputs of terrestrial materials and calcium carbonate. Apparent sedimentation rates, calculated primarily from excess ²¹⁰Pb distribution below the zone of sediment mixing, varied from 0.033 to 0.116 cm year⁻¹, agreeing well with previous results for the basin. OCARs fluctuated strongly in the range of 2.06–12.5 g C m⁻² year⁻¹, these rates being four times higher at the slope sites than at the basin sites. Within the top 15 cm of the sediment, the integrated SRRs ranged from 0.72 to 1.89 mmol m⁻² day⁻¹, with rates approximately twice as high in the slope areas as in the basin areas. SRR values were consistently higher in areas of high sedimentation and of high organic carbon accumulation, correlating well with apparent sedimentation rates and OCARs. The sulfate reduction rates recorded in the basin and slope sediments of the Ulleung Basin are higher than those reported for other parts of the world, with the exception of the Peruvian and Chilean upwelling regions. This is consistent with the high organic carbon contents of surface sediments of the Ulleung Basin, suggesting enhanced organic matter fluxes.     

Variable deposition rates on the slope and rise off the Mid-Atlantic States

Pronounced variations in slope topography and offshelf spillover have produced a three-fold difference (9 to 25 cm/1,000 yr) in Late Quaternary deposition rates among slope cores southeast of the Mid-Atlantic States. Upper rise cores revealed a high rate but lower core-to-core variability (20 to 44 cm/1,000 yr), largely the result of slope bypassing, i.e., effects of gravity-controlled downslope transport. A transport model suggests temporary slope sediment storage and higher accumulation of mud in more distal rise regions. Eustatic change accounts for the four-fold decrease in slope deposition rate between the Late Pleistocene and Holocene.     

Comparison of pelagic and nepheloid layer marine snow: implications for carbon cycling

Marine snow from upper and mid-water (i.e., pelagic) depths on the California margin is texturally and compositionally different from that traveling in the nepheloid layer. Transmission electron microscopy shows that pelagic marine snow consists primarily of bioclasts (e.g., diatom frustules, foram tests), organic matter, and microbes. These components are entrained as discrete particles or small aggregates μm in diameter) in a loose network of exocellular, muco-polysaccharide material. Clays are infrequent but, when present, are constituents of comparatively compact organic-rich microaggregates. Microbes are abundant and appear to decrease in number with increasing water depth. In contrast, marine snow aggregates collected from just above the sea floor in the nepheloid layer are assemblages of clay particles, clay flocs, and relatively dense clay–organic-rich microaggregates in an exocellular organic matrix. Bioclasts and microorganisms occur only rarely. The prevalence of clay–organic-rich aggregates in the nepheloid layer suggests that, prior to final deposition and burial, marine snow from the pelagic zone is subject to disaggregation and recombination with terrigenous detrital material near or at the sea floor. Results have significant implications for the accumulation and burial rates of organic carbon on continental margins and the aging and bioavailability of sedimentary organic matter. Samples examined were collected offshore of northern and central California.     

Structure of time variations in carbon dioxide in the atmospheric thickness over central Eurasia (Issyk Kul Monitoring Station)

The results are presented of statistical analysis of the data obtained from the 1980–2006 systematic measurements of the volume concentration of carbon dioxide in the atmospheric thickness over central Eurasia. The trends of both monthly and yearly means of CO₂ concentration are determined. During these 26 years, the yearly mean concentration increased by ～42 ppm at a mean rate of (1.56 ± 0.18) ppm per year and reached ～382.7 ppm. General statistical characteristics are found. The distribution function of the monthly mean concentrations of CO₂ is characterized by the presence of a second maximum and a bias of the principal mode toward large values, and the mean (over the measurement time) monthly concentration and the median almost coincide. The distribution function of the yearly mean concentrations of CO₂ is close to a normal distribution, and the mean (over the measurement time) yearly concentration, the median, and the mode also coincide. The trends of short-and long-period variations in the carbon dioxide concentration and their possible relation to a number of geophysical phenomena are revealed. Spectral analysis of the measuring data on CO₂ revealed oscillations with periods of 4, 6, 12, 15, 21, 29, 40, 53, 84, and 183 months. A statistical model with the parameters of these oscillations describes the experimental monthly mean concentrations of carbon dioxide with an rms deviation of 2.3 ppm (±0.6% of the mean over the entire period 361.9 ppm) and the yearly mean concentrations with an rms deviation of 0.9 ppm (～±0.3%).     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.