Global Geochemical Cycles Research Articles

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.

Barium isotope fractionation in the global barium cycle: First evidence from barium minerals and precipitation experiments

In this study, we present first results from an ongoing investigation on the stable barium (Ba) isotope fractionation in the natural barium cycle. Stable Ba isotope signatures of international IAEA reference materials (synthetic barium sulfate, IAEA-SO-5, -6, and barium carbonate, IAEA-CO-9), natural Ba minerals and experimental Ba precipitates have been analyzed as a first approach to evaluate potential discriminating processes in the global geochemical barium cycle. Measurements were carried out on a multi-collector ICP-MS applying a 130Ba/ 135Ba double spike. Data are expressed as per mil deviation from a laboratory Ba nitrate standard solution in the δ 137/134Ba notation (external 2σ stdev < 0.1‰). Whereas the various synthetic solid standards and a synthetic barium chloride show very similar isotope results close to the nitrate standard solution, the terrestrial barium gangue minerals (four barites, one norsethite [BaMg(CO 3) 2]) were close to, or depleted in the heavy isotope (δ 137/134Ba values down to −0.2‰), compared to the standard solution. A natural barite from China derived from a mineral collection gave an isotope value of −0.4‰. High δ 34S and δ 18O ratios in this sample point to a formation under influence of microbial sulfate reduction, probably in a marine environment of sedimentary exhalative or diagenetic origin. A value of δ 137/134Ba = − 0.5‰ was found in a diagenetic barite sample from ODP Leg 207. The observed natural discriminations are clearly larger than the analytical uncertainty of the stable isotope measurements, indicating significant isotope discrimination in the natural barium cycle. Precipitation experiments from aqueous barium chloride solutions at temperatures of ~ 21 °C and 80 °C indicate that the light Ba isotope is enriched in pure barium carbonate and barium sulfate compared to the aqueous solution. A maximum isotope fractionation of − 0.3‰ is observed for both barium carbonate and sulfate, that – in the case of BaCO 3 – seems to be influenced by precipitation rate and/or the aqueous speciation, but less by temperature.

Complete genome sequence and proteomic analysis of a thermophilic bacteriophage BV1

Viruses of thermophiles are of great interest due to their roles in gene transfer, global geochemical cycle and evolution of life on earth. However, the thermophilic bacteriophages have not been studied extensively. In this investigation, a typical bacteriophage BV1 was obtained from a thermophilic bacterium Geobacillus sp. 6k512, which was isolated from an inshore hot spring in Xiamen of China. The BV1 contained a double-stranded linear DNA of 35 055 bp, which encodes 54 open reading frames (ORFs). Interestingly, eight of the 54 BV1 ORFs shared sequence similarities to genes from human disease-relevant bacteria. Seven proteins of the purified BV1 virions were identified by proteomic analysis. Determination of BV1 functional genomics would facilitate the better understanding of the mechanism for virus-thermophile interaction.

DETECTING PLANETARY GEOCHEMICAL CYCLES ON EXOPLANETS: ATMOSPHERIC SIGNATURES AND THE CASE OF SO2

We study the spectrum of a planetary atmosphere to derive detectable features in low resolution of different global geochemical cycles on exoplanets - using the sulphur cycle as our example. We derive low resolution detectable features for first generation space- and ground- based telescopes as a first step in comparative planetology. We assume that the surfaces and atmospheres of terrestrial exoplanets (Earth-like and super-Earths) will most often be dominated by a specific geochemical cycle. Here we concentrate on the sulphur cycle driven by outgassing of SO2 and H2S followed by their transformation to other sulphur-bearing species which is clearly distinguishable from the carbon cycle which is driven by outgassing of CO2. Due to increased volcanism, the sulphur cycle is potentially the dominant global geochemical cycle on dry super-Earths with active tectonics. We calculate planetary emission, reflection and transmission spectrum from 0.4 to 40 micrometer with high and low resolution to assess detectable features using current and Archean Earth models with varying SO2 and H2S concentrations to explore reducing and oxidizing habitable environments on rocky planets. We find specific spectral signatures that are observable with low resolution in a planetary atmosphere with high SO2 and H2S concentration. Therefore first generation space and ground based telescopes can test our understanding of geochemical cycles on rocky planets and potentially distinguish planetary environments dominated by the carbon and sulphur cycle.

Process‐based modeling of silicate mineral weathering responses to increasing atmospheric CO2 and climate change

A mathematical model describes silicate mineral weathering processes in modern soils located in the boreal coniferous region of northern Europe. The process model results demonstrate a stabilizing biological feedback mechanism between atmospheric CO2 levels and silicate weathering rates as is generally postulated for atmospheric evolution. The process model feedback response agrees within a factor of 2 of that calculated by a weathering feedback function of the type generally employed in global geochemical carbon cycle models of the Earth's Phanerozoic CO2 history. Sensitivity analysis of parameter values in the process model provides insight into the key mechanisms that influence the strength of the biological feedback to weathering. First, the process model accounts for the alkalinity released by weathering, whereby its acceleration stabilizes pH at values that are higher than expected. Although the process model yields faster weathering with increasing temperature, because of activation energy effects on mineral dissolution kinetics at warmer temperature, the mineral dissolution rate laws utilized in the process model also result in lower dissolution rates at higher pH values. Hence, as dissolution rates increase under warmer conditions, more alkalinity is released by the weathering reaction, helping maintain higher pH values thus stabilizing the weathering rate. Second, the process model yields a relatively low sensitivity of soil pH to increasing plant productivity. This is due to more rapid decomposition of dissolved organic carbon (DOC) under warmer conditions. Because DOC fluxes strongly influence the soil water proton balance and pH, this increased decomposition rate dampens the feedback between productivity and weathering. The process model is most sensitive to parameters reflecting soil structure; depth, porosity, and water content. This suggests that the role of biota to influence these characteristics of the weathering profile is as important, if not more important, than the role of biota to influence mineral dissolution rates through changes in soil water chemistry. This process‐modeling approach to quantify the biological weathering feedback to atmospheric CO2 demonstrates the potential for a far more mechanistic description of weathering feedback in simulations of the global geochemical carbon cycle.

Anthropogenic osmium in rain and snow reveals global-scale atmospheric contamination

Osmium is one of the rarer elements in seawater, with typical concentration of approximately 10 x 10(-15) g g(-1) (5.3 x 10(-14) mol kg(-1)). The osmium isotope composition ((187)Os/(188)Os ratio) of deep oceans is 1.05, reflecting a balance between inputs from continental crust (approximately 1.3) and mantle/cosmic dust (approximately 0.13). Here, we show that the (187)Os/(188)Os ratios measured in rain and snow collected around the world range from 0.16 to 0.48, much lower than expected (>1), but similar to the isotope composition of ores (approximately 0.2) that are processed to extract platinum and other metals to be used primarily in automobile catalytic converters. Present-day surface seawater has a lower (187)Os/(188)Os ratio (approximately 0.95) than deep waters, suggesting that human activities have altered the isotope composition of the world's oceans and impacted the global geochemical cycle of osmium. The contamination of the surface ocean is particularly remarkable given that osmium has few industrial uses. The pollution may increase with growing demand for platinum-based catalysts.

Magmatic filtering of mantle compositions at mid-ocean-ridge volcanoes

The Earth's mantle constitutes over 80% of the planet's volume, and is therefore a key reservoir in global geochemical cycling. The magnitudes and length scales of heterogeneities in the composition of the mantle are an important aspect of this reservoir, but are inaccessible to direct sampling. Mid-ocean-ridge basalt (MORB), the dominant eruptive product of ridges, provides a geographically widespread first-order snapshot of the mantle in terms of the distribution of major and trace elements and its isotopic composition. However, a range of processes occur between melt generation at depth and eruption on the sea floor that modulate the chemical signals of mantle heterogeneity in MORB, making it an imperfect sample. Detailed observations over the past few years have revealed that regional mantle heterogeneity is generally preserved in MORB most accurately where the melt supply is low, and that timescales between melt generation and MORB eruption are relatively short. Nevertheless, because of the variety of volcanic and magmatic processes that act to preserve or destroy signatures of mantle heterogeneity in MORB, a much broader base of observations from different locations will be required to faithfully reconstruct upper mantle heterogeneity.

Biogenic silica: a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon

AbstractOn geological time‐scales (≥106 years), the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric CO2 through chemical weathering of Ca‐ and Mg‐silicate minerals in continental rocks. Rivers transport the soluble products of weathering (cations, alkalinity and silicic acid) to the oceans, where they are utilized by marine ecosystems. On decadal to glacial–interglacial time‐scales, however, large biotic fluxes and storages of Si within terrestrial and freshwater ecosystems need to be taken into account. Recent studies have emphasized the importance of Si‐accumulating plants, which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths. These include grasses, sedges, palms, some temperate deciduous trees and conifers, and many tropical hardwoods. Landscapes dominated by accumulator plants, such as tropical rainforests, grasslands, herbaceous wetlands and bamboo forests, act as ‘silica factories’. Important ‘silica bioengineers’ in freshwater ecosystems comprise diatoms, sponges and chrysophytes. This paper reviews the biological role of Si in higher plants, the impact of vegetation on rates of chemical weathering, the fluxes of Si through catchment ecosystems, lakes and rivers, and the potential contribution of new geochemical and isotopic tracers to Si biogeochemistry. Multiproxy investigations of lake sediments will provide novel insights into past variations in Si biocycling from terrestrial to aquatic realms on 10–106 year time‐scales. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Multi-spectral imaging of rhizobox systems: New perspectives for the observation and discrimination of rhizosphere components

In this issue of Plant and Soil Nakaji et al. (Plant Soil, this volume, 2008) report a novel approach for automatically identifying roots and other rhizosphere components in rhizosphere images acquired using a multi-spectral (visible—VIS- and near-infrared—NIR-) imaging system. The images are acquired through a root-window observation device and the study highlights the perspectives offered by this imaging system. An outstanding outcome of this research is that the new approach can be applied to effectively separate soil litter from the purely mineral phase and distinguish root tissues that differ in physiological status, i.e. live (different age classes), senescent and dead. If achievable routinely, such a detailed classification of rhizosphere components could greatly improve our appraisal of root turnover and associated organic matter input to the soil, information of paramount importance for an improved understanding of many essential processes such as global geochemical cycles. Minirhizotrons (MR) systems have been increasingly used in global change studies because they are a convenient way to frequently and nondestructively quantify root length production and mortality (Norby and Jackson, New Phytol, 147:3–12, 2000; Hendrick and Pregitzer, Ecology, 73:1094–1104, 1992). However, the MR technique still has many limitations, including the lack of a standard, accurate and rapid procedure to extract and classify rhizosphere components from the MR images obtained. The recent work by Nakaji et al. (Plant Soil, this volume, 2008) provides convincing evidence that the inclusion of a VIS-NIR multi-spectral capability into conventional MR systems could substantially improve this method, and extend its adoption by the wider plant scientist community as a standard research tool.

Earth's copper resources estimated from tectonic diffusion of porphyry copper deposits

Improved estimates of global mineral endowments are relevant to issues ranging from strategic planning to global geochemical cycling. We have used a time-space model for the tectonic migration of porphyry copper deposits vertically through the crust to calculate Earth's endowment of copper in mineral deposits. The model relies only on knowledge of numbers and ages of porphyry copper deposits, Earth's most widespread and important source of copper, in order to estimate numbers of eroded and preserved deposits in the crust. Model results indicate that ∼125,895 porphyry copper deposits were formed during Phanerozoic time, that only ∼47,789 of these remain at various crustal depths, and that these contain ∼1.7 × 1011 tonnes (t) of copper. Assuming that other types of copper deposits behave similarly in the crust and have abundances proportional to their current global production yields an estimate of 3 × 1011 t for total global copper resources at all levels in Earth's crust. Thus, ∼0.25% of the copper in the crust has been concentrated into deposits through Phanerozoic time, and about two-thirds of this has been recycled by uplift and erosion. The amount of copper in deposits above 3.3 km, a likely limit of future mining, could supply current world mine production for 5500 yr, thus quantifying the highly unusual and nonrenewable nature of mineral deposits.

Stable isotopes of carbon and sulphur as indicators of environmental change: past and present

Stable isotopes of carbon ( 12 C and 13 C) and sulphur ( 32 S and 34 S) are fractionated during some of the transformations between reservoirs in the global carbon and sulphur cycles. The main processes controlling the isotopic composition of Earth surface reservoirs are, for carbon, photosynthesis, respiration and equilibration between oceanic dissolved inorganic carbon and atmospheric CO 2 , and, for sulphur, bacterial sulphate reduction and sulphide reoxidation. Thus the different reservoirs of carbon and sulphur in the global cycles attain characteristic isotopic compositions that can be used as a ‘fingerprint’ for different carbon and sulphur sources and to track changes in the balance of carbon and sulphur stored in different reservoirs. These isotopic signals can be used both to examine contemporary anthropogenic impacts on the cycling of carbon and sulphur and, where isotopic compositions are faithfully preserved in ancient sediments, to examine historical changes in global geochemical cycles through geological time. A number of case studies are discussed to illustrate how isotopic techniques represent one of the most powerful and important tools available in the study of both the causes and affects of ancient and modern global environmental change.

Low- and high-temperature weathering budgets in Kamchatka peninsula

In the general attempt to link chemical weathering, global geochemical cycles and climate, the study of riverine erosion in the active volcanic arc of Kamchatka provides important informations. As it was shown by several authors (Louvat and Allegre, 1997 and Dessert, 2003), chemical weathering of silicate rocks, and in particular basalts, is acting as a major contribution to the global fluxes derived from continental weathering and then to the global carbon cycle. In such a case, the atmospheric CO2 consumption flux derived from basalts represents 30% of the global flux from all silicates and acts as an important regulation system of the Earth's environment (Dessert, 2003). The global budget investigated is based on CO2 consumption taking place during low-temperature chemical weathering of basaltic rocks occurring in soils and river systems. But a substantial amount of alkalinity of river waters in volcanic context is derived from higher temperature water–rock interactions occurring during hydrothermal circulation. However, in all the previous river studies, the river chemistry was systematically corrected from the high-temperature component to evaluate the CO2 consumption rate, although in certain rivers the alkalinity derived from high-temperature water–rock interaction was not negligible. Here, we adopt another approach and, by focusing on a volcanic province particularly impacted by hydrothermalism, we chose not to subtract the hydrothermal component to the weathering budget and to consider both fluxes of “low-temperature” and “high-temperature” chemical interactions. As an example, the high-temperature weathering explains almost 30% of the global cationic rate in Kamchatka. Depending on the origin of CO2 (atmospheric or volcanic) and the abundance of sulfuric acid as a proton donor (oxidation of sulfide minerals and degassing), the impact of the two chemical weathering processes have distinct consequences in the long-term carbon cycle perspective.

Modeling open system metamorphic decarbonation of subducting slabs

Fluids derived from the devolatilization of subducting slabs play a critical role in the melting of the mantle wedge and global geochemical cycles. However, in spite of evidence for the existence and mobility of an aqueous fluid phase during subduction metamorphism, the effect of this fluid on decarbonation reactions in subducting lithologies remains largely unquantified. In this study we present results from thermodynamic modeling of metamorphic devolatilization of subducted lithologies for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent lithologies. This open system approach in which fluid flow is an intrinsic component of the chemical model offers an alternative to closed system models of subduction zone decarbonation. In general, our models simulating pervasive fluid flow in subducting lithologies predict CO2 fluxes measured from volcanic arcs more closely than models which assume purely channelized flow. Despite the enhanced effect of H2O‐rich fluid infiltration on subduction decarbonation, our results support the hypothesis that CO2 is returned to the deep mantle at convergent margins, particularly in cool and intermediate subduction zones. Our results demonstrate that for most subduction zones, a significant proportion of the CO2 derived from the slab is lost beneath the fore arc, and therefore CO2 flux estimates based on measurements within the volcanic arc alone may significantly underestimate the slab‐derived CO2 flux for individual margins. Nevertheless, our predicted global slab‐derived CO2 flux from convergent margins of 0.35–3.12 × 1012 mols CO2/yr is in good agreement with previous estimates of global arc volcanic flux. Because our predicted global slab‐derived CO2 flux is significantly less than atmospheric CO2 drawdown by chemical weathering, significant CO2 emission from other geologic regimes (e.g., hot spots) would be required to balance the global carbon cycle.

Oscillations in land surface hydrological cycle

Abstract Hydrological cycle is the perpetual movement of water throughout the various component of the global Earth's system. Focusing on the land surface component of this cycle, the determination of the succession of dry and humid periods is of high importance with respect to water resources management but also with respect to global geochemical cycles. This knowledge requires a specified estimation of recent fluctuations of the land surface cycle at continental and global scales. Our approach leans towards a new estimation of freshwater discharge to oceans from 1875 to 1994 as recently proposed by Labat et al. [Labat, D., Godderis, Y., Probst, JL, Guyot, JL, 2004. Evidence for global runoff increase related to climate warming. Advances in Water Resources, 631–642]. Wavelet analyses of the annual freshwater discharge time series reveal an intermittent multiannual variability (4- to 8-y, 14- to 16-y and 20- to 25-y fluctuations) and a persistent multidecadal 30- to 40-y variability. Continent by continent, reasonable relationships between land–water cycle oscillations and climate forcing (such as ENSO, NAO or sea surface temperature) are proposed even though if such relationships or correlations remain very complex. The high intermittency of interannual oscillations and the existence of persistent multidecadal fluctuations make prediction difficult for medium-term variability of droughts and high-flows, but lead to a more optimistic diagnostic for long-term fluctuations prediction.

Kinetics of Sedimentation of a Coagulating Suspension

The main role in the formation of a suspension in an aqueous medium is played by the coagulation of particles with the accompanying sedimentation of growing aggregates. Under natural conditions, it leads to migration of dispersed organic matter from the bulk of the water into bottom sediments [1]. In view of the large volume of the hydrosphere, this contributes significantly to the global geochemical cycles of carbon and accompanying elements. In chemical engineering, one of the widely used processes for producing an amorphous solid phase is precipitation from solutions by reagents. This process is widely applied in water treatment for removing harmful substances by their conversion into insoluble form. The amorphous phase obtained in coagulant hydrolysis is also used for the absorption of contaminants and their removal from the bulk of the water during suspension settling [2]. The processes of mixing, hydrolysis, and coagulation occurring therein were analyzed previously [3].

Viruses in the sea

Viruses exist wherever life is found. They are a major cause of mortality, a driver of global geochemical cycles and a reservoir of the greatest genetic diversity on Earth. In the oceans, viruses probably infect all living things, from bacteria to whales. They affect the form of available nutrients and the termination of algal blooms. Viruses can move between marine and terrestrial reservoirs, raising the spectre of emerging pathogens. Our understanding of the effect of viruses on global systems and processes continues to unfold, overthrowing the idea that viruses and virus-mediated processes are sidebars to global processes.

Fluid expulsion from the Dvurechenskii mud volcano (Black Sea)Part I. Fluid sources and relevance to Li, B, Sr, I and dissolved inorganic nitrogen cycles

Significant sediment–ocean chemical fluxes are produced by the expulsion of sedimentary fluids at continental margins. Although such fluxes could play a role in global geochemical cycles, few quantitative estimates of their global, or even regional, significance exist. We carried out a pore water geochemical study of fluids expelled from the Dvurechenskii mud volcano (DMV) in the Black Sea, with the aim of understanding the role played by mud volcanoes in Black Sea geochemical cycles. The DMV is presently expelling highly saline fluids particularly enriched in geochemically important species such as Li + (1.5 mM), B (2.17 mM), Ba 2+ (0.57 mM), Sr 2+ (0.79 mM), I (0.4 mM) and dissolved inorganic nitrogen (DIN) (22 mM). A combination of geochemical indicators shows that this geochemical signature was acquired via organic matter and silicate alteration processes in the subsurface down to 3-km depth and near-surface gas hydrate formation. We used a simple transport model to estimate the benthic fluxes of these solutes at the DMV. Our results show that the DMV is expelling fluids at a rather low seepage rate (8–25 cm year −1 ) resulting in a total water flux of 9.4×10 −5 km 3 year −1 . This gentle regime of fluid expulsion results in Li + , B, Sr 2+ , I and DIN fluxes between 3.8×10 4 and 2.1×10 6 mol year −1 . Surface biogeochemical processes affect the benthic fluxes of Ba 2+ such that the deep Ba 2+ flux is completely consumed through the precipitation of authigenic barite (BaSO 4 ) in surface sediments. The Black Sea I cycle is likely to be affected by mud volcanism, if the 50 known Black Sea mud volcanoes share the rather sluggish activity of the DMV. Mud volcano fluxes of Li, B, Sr and DIN, instead, are too small to affect Black Sea geochemical cycles. On a global scale, mud volcanism could play a role in the marine cycles of Li, B, Sr, I and DIN if current estimates of mud volcano abundance are correct.

Fluid expulsion from the Dvurechenskii mud volcano (Black Sea): Part I. Fluid sources and relevance to Li, B, Sr, I and dissolved inorganic nitrogen cycles

Significant sediment–ocean chemical fluxes are produced by the expulsion of sedimentary fluids at continental margins. Although such fluxes could play a role in global geochemical cycles, few quantitative estimates of their global, or even regional, significance exist. We carried out a pore water geochemical study of fluids expelled from the Dvurechenskii mud volcano (DMV) in the Black Sea, with the aim of understanding the role played by mud volcanoes in Black Sea geochemical cycles. The DMV is presently expelling highly saline fluids particularly enriched in geochemically important species such as Li + (1.5 mM), B (2.17 mM), Ba 2+ (0.57 mM), Sr 2+ (0.79 mM), I (0.4 mM) and dissolved inorganic nitrogen (DIN) (22 mM). A combination of geochemical indicators shows that this geochemical signature was acquired via organic matter and silicate alteration processes in the subsurface down to 3-km depth and near-surface gas hydrate formation. We used a simple transport model to estimate the benthic fluxes of these solutes at the DMV. Our results show that the DMV is expelling fluids at a rather low seepage rate (8–25 cm year −1) resulting in a total water flux of 9.4×10 −5 km 3 year −1. This gentle regime of fluid expulsion results in Li +, B, Sr 2+, I and DIN fluxes between 3.8×10 4 and 2.1×10 6 mol year −1. Surface biogeochemical processes affect the benthic fluxes of Ba 2+ such that the deep Ba 2+ flux is completely consumed through the precipitation of authigenic barite (BaSO 4) in surface sediments. The Black Sea I cycle is likely to be affected by mud volcanism, if the 50 known Black Sea mud volcanoes share the rather sluggish activity of the DMV. Mud volcano fluxes of Li, B, Sr and DIN, instead, are too small to affect Black Sea geochemical cycles. On a global scale, mud volcanism could play a role in the marine cycles of Li, B, Sr, I and DIN if current estimates of mud volcano abundance are correct.

Antimony in recent, ombrotrophic peat from Switzerland and Scotland: Comparison with natural background values (5,320 to 8,020 14C yr BP) and implications for the global atmospheric Sb cycle

The lowest concentrations, atmospheric fluxes, and enrichments of Sb in a Swiss bog were found in peat samples dating from 8,020 to 5,320 14C yr BP when Sb inputs were proportional to those of Sc and effectively controlled by deposition of soil dust. For comparison with these ancient samples, modern peat samples from five rural areas of Switzerland and two remote areas of Scotland and Shetland are highly contaminated with Sb, with enrichments of the order of 30 to 80 times. “Lithogenic” Sb concentrations calculated using the Sc concentrations and background Sb/Sc ratio are dwarfed at all sites by “anthropogenic” Sb. The chronology and intensity of the Sb enrichments are in many ways similar to those of Pb which indicates that (1) Sb, like Pb, is well preserved in ombrotrophic peat and (2) the extent of human impacts on the geochemical cycle of Sb is comparable to that of Pb. The similar distribution of Sb and Pb can be explained in terms of their chemical and mineralogical associations, with most lead minerals being rich in Sb. Assuming that the “background” Sb flux (0.35 μg/m2/yr) from the Swiss bog is representative of preanthropogenic deposition rates worldwide, the global flux of natural Sb is estimated at 154 T/a. Using the natural Pb flux published by Patterson and Settle [1987] of 2600 T/a and the “background” Pb/Sb ratio (29) of the preanthropogenic peat samples, the global flux of natural Sb is estimated at 90 T/a. Either way, these values (90 to 154 T/a) are considerably lower than the current estimate of natural Sb to the global atmosphere (2400 T/a) published by Pacyna and Pacyna [2001]. Assuming that the current estimate of anthropogenic Sb to the global atmosphere (1600 T/a) is correct [Pacyna and Pacyna, 2001], the ratio of anthropogenic to natural Sb emissions is on the order of 10 to 18. Taken together, the data from modern and ancient peat samples suggests that the impact of human activities on the global geochemical cycle of Sb may have been underestimated by an order of magnitude. Like Pb, Sb has no known biological function, has a similar toxicity, and is a cumulative poison. The environmental geochemistry of Sb therefore may have a relevance to human and environmental health comparable to that of Pb.

Calcium Isotopic Variations Produced by Biological, Kinetic, Radiogenic and Nucleosynthetic Processes

Study of the isotopic variations of calcium is of interest because Ca is important in geochemical and biochemical processes, and is one of few major cations in rocks and minerals with demonstrated isotopic variability. Calcium is critical to life and a major component of the global geochemical cycles that control climate. Studies to date show that biological processing of calcium produces significant isotopic fractionation (4 to 5‰ variation of the 44Ca/40Ca ratio has been observed). Calcium isotopic fractionation due to inorganic processing at high (e.g., magmatic) temperatures is small. There are few studies of calcium isotope fractionation behavior for low-temperature inorganic processes. The Ca isotopic variations observed in nature, both biological and inorganic, are mostly attributed to kinetic effects, but this inference cannot be confirmed until equilibrium Ca isotope fractionation is more thoroughly investigated. Evaporation of silicate liquids into vacuum at high temperature is expected to produce kinetic Ca isotopic fractionation, as is diffusion of calcium in silicate liquids and aqueous solutions. The evaporation effects have been observed in meteorite samples. Diffusion effects have been observed in the laboratory but not yet in natural samples. The potential value of Ca isotopic studies has barely been tapped. Improvements in measurement precision would increase the attractiveness of Ca isotopes as a geochemical tool, but such improvements have been slow in coming. Calcium is composed mainly of the isotope 40Ca, which is a highly stable, doubly magic nuclide (both the number of neutrons and the number of protons represent closed nuclear shells). There are a total of six stable isotopes covering a mass range from 40 to 48. Ca has been studied for isotopic variations in three ways. The major isotope, 40Ca is the primary radioactive decay product of radioactive 40K. About 89.5% of the 40K …