Global Geochemical Cycles Research Articles

Pollution of snow cover in the impact zone of enterprises in Norilsk Industrial Area

The content of pollutants in snow samples and their composition have been determined. The quantitative indicators of the content of nickel, copper, and other heavy metals and sulfur are used to assess the nature of spatial pollution of snow cover in the impact zone of enterprises in the Norilsk Industrial Area (NIA). A map of distribution of heavy metals and sulfur in the region is compiled. It is established that the largest part of pollutants entering the atmosphere in the form of dust was deposited on the underlying surface near sources of industrial emissions (up to 7 km from them). The pollutants entering the atmosphere in the form aerosols and gases are transported over considerable distances. A greater concentration of water-soluble forms of sulfur when compared with the background are marked at a distance of more than 400 km. Calculations show that only 2% of all sulfur emissions are deposited within a radius of 250 km and the remaining portion of sulfur is incorporated into the global geochemical cycles.

Sequencing at sea: challenges and experiences in Ion Torrent PGM sequencing during the 2013 Southern Line Islands Research Expedition.

Genomics and metagenomics have revolutionized our understanding of marine microbial ecology and the importance of microbes in global geochemical cycles. However, the process of DNA sequencing has always been an abstract extension of the research expedition, completed once the samples were returned to the laboratory. During the 2013 Southern Line Islands Research Expedition, we started the first effort to bring next generation sequencing to some of the most remote locations on our planet. We successfully sequenced twenty six marine microbial genomes, and two marine microbial metagenomes using the Ion Torrent PGM platform on the Merchant Yacht Hanse Explorer. Onboard sequence assembly, annotation, and analysis enabled us to investigate the role of the microbes in the coral reef ecology of these islands and atolls. This analysis identified phosphonate as an important phosphorous source for microbes growing in the Line Islands and reinforced the importance of L-serine in marine microbial ecosystems. Sequencing in the field allowed us to propose hypotheses and conduct experiments and further sampling based on the sequences generated. By eliminating the delay between sampling and sequencing, we enhanced the productivity of the research expedition. By overcoming the hurdles associated with sequencing on a boat in the middle of the Pacific Ocean we proved the flexibility of the sequencing, annotation, and analysis pipelines.

Quantifying the impact of riverine particulate dissolution in seawater on ocean chemistry

The quantification of the sources and sinks of elements to the oceans forms the basis of our understanding of global geochemical cycles and the chemical evolution of the Earth's surface. There is, however, a large imbalance in the current best estimates of the global fluxes to the oceans for many elements. In the case of strontium (Sr), balancing the input from rivers would require a much greater mantle-derived component than is possible from hydrothermal water flux estimates at mid-ocean ridges. Current estimates of riverine fluxes are based entirely on measurements of dissolved metal concentrations, and neglect the impact of riverine particulate dissolution in seawater. Here we present 87Sr/86Sr isotope data from an Icelandic estuary, which demonstrate rapid Sr release from the riverine particulates. We calculate that this Sr release is 1.1–7.5 times greater than the corresponding dissolved riverine flux. If such behaviour is typical of volcanic particulates worldwide, this release could account for 6–45% of the perceived marine Sr budget imbalance, with continued element release over longer timescales further reducing the deficit. Similar release from particulate material will greatly affect the marine budgets of many other elements, changing our understanding of coastal productivity, and anthropogenic effects such as soil erosion and the damming of rivers.

Geothermobarometric history of subduction recorded by quartz inclusions in garnet

Burial histories of subduction zone rocks are often difficult to accurately constrain, owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages. Knowledge of the depth‐histories of subduction is, however, required for our understanding of global geochemical cycles, subduction‐related seismicity, and the evolution of destructive tectonic boundaries. The high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals. Quartz inclusions in garnet from Sifnos, Greece, preserve such a record of the pressure of garnet growth, allowing detailed reconstruction of the metamorphic evolution of these rocks. Pressure‐dependent Raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth. All measured inclusions suggest that garnet growth occurred between 19 and 20.5 kbars, with little evidence for significant pressure variation during the garnet growth interval, which is interpreted to record ∼100°C of heating. Coupled with thermometry and geochronology, these results show that early, cold burial was followed by a phase of rapid heating, which immediately preceded exhumation. Garnet growth occurred primarily during this heating phase.

Single-cell genome and metatranscriptome sequencing reveal metabolic interactions of an alkane-degrading methanogenic community

Microbial interactions have a key role in global geochemical cycles. Although we possess significant knowledge about the general biochemical processes occurring in microbial communities, we are often unable to decipher key functions of individual microorganisms within the environment in part owing to the inability to cultivate or study them in isolation. Here, we circumvent this shortcoming through the use of single-cell genome sequencing and a novel low-input metatranscriptomics protocol to reveal the intricate metabolic capabilities and microbial interactions of an alkane-degrading methanogenic community. This methanogenic consortium oxidizes saturated hydrocarbons under anoxic conditions through a thus-far-uncharacterized biochemical process. The genome sequence of a dominant bacterial member of this community, belonging to the genus Smithella, was sequenced and served as the basis for subsequent analysis through metabolic reconstruction. Metatranscriptomic data generated from less than 500 pg of mRNA highlighted metabolically active genes during anaerobic alkane oxidation in comparison with growth on fatty acids. These data sets suggest that Smithella is not activating hexadecane by fumarate addition. Differential expression assisted in the identification of hypothetical proteins with no known homology that may be involved in hexadecane activation. Additionally, the combination of 16S rDNA sequence and metatranscriptomic data enabled the study of other prevalent organisms within the consortium and their interactions with Smithella, thus yielding a comprehensive characterization of individual constituents at the genome scale during methanogenic alkane oxidation.

Turekian reflections

Karl Karekin Turekian (1927–2013) was a man of remarkable scientific breadth, with innumerable important contributions to marine geochemistry, atmospheric chemistry, cosmochemistry, and global geochemical cycles. He was mentor to a long list of students, postdoctorate scholars, and faculty (at Yale and elsewhere), a leader in geochemistry, a prolific author and editor, and had a profound influence in shaping his department at Yale University. Karl was a true scholar and interested in everything, scientific and nonscientific. As his Yale colleague George Veronis reflected, “Karl plunged into any topic, very often far removed from his own specialty, and had heated discussions about all kinds of issues. His discussions were heated because that’s how he got himself worked up to carry on the way he did. Very often he would say something like, ‘I don’t know anything about so and so,’ when a topic was raised, and quite often he didn’t. He wanted to learn, and his own ignorance would not deter him from carrying on.” Karl’s autobiography (1) and a recent interview (2) tell his story far better than we can and make clear his vast contributions to science, especially his role in the development and advancement of the field of geochemistry. What is especially notable about Karl is his impact on people, the unanimity of the love for Karl by all who knew him well, and his unique character. We’d like to recount a little of what it was like to be on the receiving end of his warmth and wisdom.

Archaea in Biogeochemical Cycles

Archaea constitute a considerable fraction of the microbial biomass on Earth. Like Bacteria they have evolved a variety of energy metabolisms using organic and/or inorganic electron donors and acceptors, and many of them are able to fix carbon from inorganic sources. Archaea thus play crucial roles in the Earth's global geochemical cycles and influence greenhouse gas emissions. Methanogenesis and anaerobic methane oxidation are important steps in the carbon cycle; both are performed exclusively by anaerobic archaea. Oxidation of ammonia to nitrite is performed by Thaumarchaeota. They represent the only archaeal group that resides in large numbers in the global aerobic terrestrial and marine environments on Earth. Sulfur-dependent archaea are confined mostly to hot environments, but metal leaching by acidophiles and reduction of sulfate by anaerobic, nonthermophilic methane oxidizers have a potential impact on the environment. The metabolisms of a large number of archaea, in particular those dominating the subsurface, remain to be explored.

Stable isotope ratio measurements of Cu and Zn in mineral dust (bulk and size fractions) from the Taklimakan Desert and the Sahel and in aerosols from the eastern tropical North Atlantic Ocean

Accurate characterization of the stable isotope composition of Cu and Zn in major global mineral dust sources and in aerosols is central to the application of these isotope systems to the studies of global geochemical processes and cycles. We test here for the first time Cu and Zn isotope ratios within a well-defined source-receptor setting on the continent–ocean interface and determine the isotope composition of (i) bulk surface soil dust samples from the Sahel region, (ii) individual size fractions of surface dust samples from the Taklimakan Desert, and (iii) aerosols collected in the equatorial eastern North Atlantic region. This is achieved reducing the blank contribution during the ion exchange step using small resin and acid volumes and using a second ion exchange passage to purify the Cu fraction. We find no significant correlation between size fractions and isotope ratios in the two samples analyzed from the Taklimakan Desert. Mass balance calculations suggest that isotope ratios of bulk samples are within the analytical precision of the <4μm size fraction and can be used to characterize atmospheric long range transport of Cu and Zn in mineral dust from the Taklimakan Desert. The <1µm size fractions of two aerosol samples collected over the equatorial eastern North Atlantic region have Cu and Zn isotope ratios that are different to Sahel surface soil dust suggesting important non-crustal sources, in line with calculated enrichment factors, and possibly of anthropogenic origin. Using previously reported δ66Zn values for anthropogenic emission from Europe, preliminary calculations suggest that up to 55% of Zn arriving at the sampling points in the equatorial eastern North Atlantic region could be of anthropogenic origin.

Conceptual Advances in Fossil Plant Biology: Introduction and Dedication

Conceptual Advances in Fossil Plant Biology: Introduction and Dedication

Mass-balance modeling of mineral weathering rates and CO2consumption in the forested, metabasaltic Hauver Branch watershed, Catoctin Mountain, Maryland, USA

Mineral weathering rates and a forest macronutrient uptake stoichiometry were determined for the forested, metabasaltic Hauver Branch watershed in north-central Maryland, USA. Previous studies of Hauver Branch have had an insufficient number of analytes to permit determination of rates of all the minerals involved in chemical weathering, including biomass. More equations in the mass-balance matrix were added using existing mineralogic information. The stoichiometry of a deciduous biomass term was determined using multi-year weekly to biweekly stream-water chemistry for a nearby watershed, which drains relatively unreactive quartzite bedrock. At Hauver Branch, calcite hosts ~38 mol% of the calcium ion (Ca2+) contained in weathering minerals, but its weathering provides ~90% of the stream water Ca2+. This occurs in a landscape with a regolith residence time of more than several Ka (kiloannum). Previous studies indicate that such old regolith does not typically contain dissolving calcite that affects stream Ca2+/Na+ ratios. The relatively high calcite dissolution rate likely reflects dissolution of calcite in fractures of the deep critical zone. Of the carbon dioxide (CO2) consumed by mineral weathering, calcite is responsible for approximately 27%, with the silicate weathering consumption rate far exceeding that of the global average. The chemical weathering of mafic terrains in decaying orogens thus may be capable of influencing global geochemical cycles, and therefore, climate, on geological timescales. Based on carbon-balance calculations, atmospheric-derived sulfuric acid is responsible for approximately 22% of the mineral weathering occurring in the watershed. Our results suggest that rising air temperatures, driven by global warming and resulting in higher precipitation, will cause the rate of chemical weathering in the Hauver Branch watershed to increase until a threshold temperature is reached. Beyond the threshold temperature, increased recharge would produce a shallower groundwater table and reduced chemical weathering rates. Copyright © 2013 John Wiley & Sons, Ltd.

Preliminary results from a continuous record of atmospheric gaseous mercury at the coastal station Dumont d’Urville in Antarctica

While the tropospheric reactivity of mercury (Hg) in the Arctic is more and more documented only a few attempts were made to study the Hg cycle in the Southern Polar Regions. The role of the Antarctic continent and its influence on the global geochemical cycle of mercury is unclear today, and is certainly under evaluated by current models. Here, we present the first continuous high-time-resolution measurements of atmospheric gaseous elemental mercury (GEM) in East Antarctica from February 2010 to March 2011 at the coastal research station Dumont d’Urville (DDU) (66°40’S, 140°01’E, 43 m asl). We report an annual mean level of 1.062 ± 0.321 ng/m3 with well-marked daily fluctuations from October to January. An intense reactivity originated from the atmospheric boundary layer of the Antarctic plateau under sunlight conditions is observed at DDU. Partly GEM-depleted air masses are exported from the continent and dramatically influence the GEM record at DDU. From November to January, surface waters of the Southern Ocean are an important source of GEM.

Geochemistry of deep sea sediments at cold seep sites in the Nankai Trough: Insights into the effect of anaerobic oxidation of methane

Two types of sediment were recovered from “methane seep” sites in the Nankai Trough: “Cold seep” sediment affected by anaerobic oxidation of methane (AOM), and unaffected “Typical oxic marine sediment,” Concentrations of carbon, sulfur, iron, and trace metals were analyzed in these sediments. The results showed that the sulfur contents of “cold seep” sediments are higher than those of typical oxic marine sediments, although the organic carbon contents of the two types of sediment are identical. The high sulfur contents of seep sediments suggest that AOM was the dominant process at the “cold seep” sites; H2S produced as a result of AOM was fixed as reduced sulfur such as iron sulfide.The highest manganese concentrations (approx. 1000ppm) were found in surface sediments at non-seep sites, whereas a strong enrichment of Mn was not observed in the “cold seep” sediments. Manganese might be released from sediments as soluble Mn2+ ion in the reduced environment produced by methane-bearing seepage. The molybdenum content in “cold seep” sediments (up to 28.3ppm) is about 30 times higher than that in the typical oxic marine sediments. However, a correlation between Mo and organic carbon contents, which is characteristic of anoxic sediment, was not observed in our samples. “Cold seep” sediments are a potential sink in the global geochemical cycle of Mo.

Microbial utilization of abiogenic carbon and hydrogen in a serpentinite-hosted system

Mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system, in which interaction with seawater to produce serpentinite has major consequences for lithospheric cooling, global geochemical cycles, and microbial activity. Serpentinite-hosted hydrothermal activity is exemplified by the Lost City Hydrothermal Field (30°N, Mid-Atlantic Ridge) where fluid–rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen, methane, C2+ alkanes, and formate. Such systems have been proposed as possible analogs to the Early Earth environments that gave rise to the first biochemical pathways. Thus, characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance. Here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important, up to 50% of the microbial biomass is synthesized from mantle carbon. Conversely, mantle carbon contributes only ∼10% of the biomass in areas with minimal sulfate reduction. We attribute this difference to greater incorporation of formate or methane by the dominant microbial species, the Lost City Methanosarcinales, in locations where sulfate reducers are able to facilitate this assimilation. The ability of autotrophic communities at Lost City to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments.

April 2012: This Month in JoVE

The April issue of the Journal of Visualized Experiments (JoVE) includes an article that follows a lab from the University of Miami of Ohio to Lake Bonney - a permanently ice-covered saline lake in Antarctica. Few organisms can survive the harsh conditions under the ice, where light, warmth, and carbon sources are in very short supply. Single celled microorganisms called protists are the major class of microbes living beneath Lake Bonney, and are capable of bringing carbon, via photosynthesis, into the circle of life. After determining the appropriate sampling site, our authors drill and melt through the ice so that they can collect samples from various depths within the lake’s chemically stratified water column. Once collected, samples are taken to a field laboratory for filtration and stabilization. After processing, microbes present in the water sample are cultured for further experimentation. For example, our authors show that their enrichment cultures retain photosynthetic activity, and future experiments should help better understand protist physiology, as well as how such organisms can affect global geochemical cycling. In Clinical and Translational Medicine, JoVE turns its attention to stereotactic body radiotherapy, or SBRT, for the treatment of gynecological tumors. SBRT is a minimally invasive form of cancer treatment that relies on radiation, delivered by a linear accelerator mounted on a robotic arm, to biologically inactivate tumors. Our authors, from Case Medical Center, explain how gold soft tissue fiducial markers are placed around the cancer target at various depths based on computed tomography imaging. During a treatment planning session, CT and PET images are taken and then co-registered. Guided by these images, both radiation and gynecological oncologists designate the cancer volumes that will be targeted during therapy as well as contour surrounding normal structures that should be left unharmed. At the start of treatment, a target localizing system detects the fiducial markers via cross-plane radiographic imaging and marker location is compared with previously obtained imaging data. During treatment, light emitting diodes track a patient’s movement, in order to correlate their breathing pattern to the location of the fiducial markers, so that radiation can be delivered accurately. Overcoming the complications associated with patient immobilization, our authors show that their procedure for stereotaxic body radiotherapy can precisely deliver radiation to the target volume while limiting dosage to nearby structures. In our Bioengineering section, JoVE presents an article, which involves making biocompatible substrates from silkworm cocoons. First, cocoons are screened for contamination and processed so that silk extract is produced. Extract is placed atop a PDMS mold, which has been designed to create a pattern in the silk film drying on its surface. Once dried, the silk film is subjected to a process referred to as water annealing, which renders the film transparent and perfect for microscopy. Following cell seeding on the films, our authors show that tracking cell migration on the micro-patterned silk substrate is just as easy as on more conventional tissue culture surfaces. The biocompatibility of silk films as well as their potential to be chemically and structurally modified makes them attractive candidate materials for in vivo applications, like medical device design. Surprisingly, primary neuronal culture makes its way into our Immunology and Infection section, because this April, JoVE documents an innovative primary sympathetic neuron culture system used to study herpes simplex virus latency and activation. Peripheral neurons act like reservoirs for the herpes virus that are the source of reactivation events. To establish a peripheral neuron culture, superior cervical ganglia are isolated from embryonic mice and following digestion and dissociation are plated in 96 well plates. At around day 6 in vitro, herpes viruses encoding a viral-GFP fusion protein are added to the cells along with an inhibitor of lytic replication. Upon inhibitor withdrawal, latent virus will reactivate and GFP expression will be observable via fluorescent microscopy. By counting the number of wells exhibiting GFP fluorescence, the ability of small molecules or gene silencing oligonucleotides to either suppress or induce viral reactivation can be tested, making this assay a potential tool to uncover new therapeutics to help manage HSV-1 infection. This brief summary describes only four of the fifty articles JoVE will release in April. Other notable releases include methods for quantifying obesity in rats using x-ray computed tomography, inducing myocardial infarction in the zebrafish heart, and retrograde labeling of the excised spinal cord. Stay tuned.

A road map for the development of community systems (CoSy) biology

Microbial interactions are essential for all global geochemical cycles and have an important role in human health and disease. Although we possess general knowledge about the major processes within a microbial community, we are presently unable to decipher what role individual microorganisms have and how their individual actions influence others in the community. We also have limited knowledge with which to predict the effects of microbial interactions and community composition on the environment and vice versa. In this Opinion article, we describe how community systems (CoSy) biology will enable us to decode these complex relationships and will therefore improve our understanding of individual members of the community and the modes of interactions in which they engage.

An internally consistent, probabilistic, determination of ridge-axis hydrothermal fluxes from basalt-hosted systems

High-temperature hydrothermal systems at mid-ocean ridges play a critical role in global geochemical cycles. Most of the magmatic heat driving hydrothermal circulation along the global mid-ocean ridge system is released into basalt-hosted systems exemplified by intermediate- to fast-spreading ridges. Constraints on the fluid and chemical fluxes associated with these systems come from the compositions of high-temperature hydrothermal vent fluids and the compositions of altered sheeted dikes. Global data compilations of both of these data types, including 20 mineral components, 10 major fluid species, 13 trace elements and 6 isotope ratios, have been jointly fit using a numerical Bayesian inversion procedure. This new approach provides the first internally consistent estimates of fluid and chemical fluxes, with rigorously quantified uncertainties, for a broad suite of elements. The resulting global fluid flux is 8.0±2.1×1012kgyr−1, at a water-dike mass ratio of 0.92, sufficient to cool the sheeted dike complex from magmatic to hydrothermal temperatures and remove ~50% of the latent heat of crystallization of the plutonic complex. For conservative elements, the fluxes represent inputs into the ocean and should allow more accurate models of the controls on modern and paleo-seawater chemistry to be developed.

Calcium isotope evidence for suppression of carbonate dissolution in carbonate-bearing organic-rich sediments

Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (δ 44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of δ 44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, δ 44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the δ 44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.

Clumped-isotope geochemistry of carbonates: A new tool for the reconstruction of temperature and oxygen isotope composition of seawater

Abstract Clumped-isotope geochemistry deals with the state of ordering of rare isotopes in molecules, in particular with their tendency to form bonds with other rare isotopes rather than with the most abundant ones. Among its possible applications, carbonate clumped-isotope thermometry is the one that has gained most attention because of the wide potential of applications in many disciplines of the earth sciences. In particular, it allows reconstructing the temperature of formation of carbonate minerals without knowledge of the isotopic composition of the water from which they were formed. In addition, the O isotope composition of the waters from which they were formed can be calculated using the δ 18 O of the same carbonate sample. This feature offers new approaches in paleoclimatology for reconstructing past global geochemical cycles. In this contribution two applications of this method are presented. First the potential of a new analytical method of measurement of clumped isotopes on small samples of foraminifera, for high-resolution SST and seawater δ 18 O reconstructions from marine sediments is shown. Furthermore the potential of clumped isotope analysis of belemnites, for reconstructing seawater δ 18 O and temperatures in the Cretaceous is shown.

New host for carbon in the deep Earth

The global geochemical carbon cycle involves exchanges between the Earth's interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO(2). The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing (CO(4))(4-) tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008) Novel high-pressure structures of MgCO(3), CaCO(3) and CO(2) and their role in Earth's lower mantle. Earth Planet Sci Lett 273:38-47]. This high-pressure polymorph of carbonates concentrates a large amount of Fe((III)) as a result of intracrystalline reaction between Fe((II)) and (CO(3))(2-) groups schematically written as 4FeO + CO(2) → 2Fe(2)O(3) + C. This results in an assemblage of the new high-pressure phase, magnetite and nanodiamonds.

Controlling factors of sheet erosion under degraded grasslands in the sloping lands of KwaZulu-Natal, South Africa

The current increase in the global demand for food and fresh water and the associated land use changes or misuses exacerbate water erosion which has become a major threat to the sustainability of the soil and water resources. Soil erosion by rainfall and runoff is a natural and geologic phenomenon, and one of the most important components of the global geochemical cycle. Despite numerous studies on crop lands, there is still a need to quantify soil sheet erosion (an erosion form that uniformly removes fertile upper soil horizons) under grasslands and to assess the factors of the environment that control its spatial variation. For that purpose, fifteen 1 m2 micro-plots installed within a 23 ha catchment under pasture in the sloping lands of KwaZulu-Natal (South Africa) were monitored during the 2007–2008 rainy season to evaluate runoff (R) and sediment losses (SL). Soil losses computed from the 37 rainfall events with soil erosion averaged 6.45 ton ha−1 year−1with values from 3 to 13 ton ha−1 year−1. SL were significantly correlated with the proportion of soil surface coverage by the vegetation (P < 0.01) whereas the slope gradient, and soil characteristics such as bulk density or clay content were not correlated. R and SL increased as the proportion of soil surface coverage decreased and this trend was used to predict the spatial variations of sheet erosion over the 23 ha catchment. Greater sheet erosion occurred at the catchment plateau and at the vicinity of gully head cuts probably in relation to regressive erosion. Mitigating sheet erosion would require an appropriate management of the soil cover through appropriate management of cattle grazing, especially at places where “natural” erosion is likely to occur.