Uncertainties in determining microbial biomass C using the chloroform fumigation–extraction method

We tested the accuracy of the chloroform fumigation–extraction method, which is commonly used to determine soil biomass C concentrations. Accurate and precise determination of total microbial biomass is important in order to characterize soil properties and to develop predictive metal transport models for soils. Two natural soils, and individual soil components, including silica sand, montmorillonite, kaolinite, a humic acid, and Bacillus subtilis bacterial cells, were fumigated for 24 h. Following the fumigation, C from fumigated and unfumigated samples was extracted using a 0.5 M K₂SO₄ solution. The difference between the C content in the fumigated and unfumigated samples ideally represents C due to biomass because the fumigation procedure should lyse cells and release biomass C. We observed increased C release upon fumigation for bacteria-only samples, confirming the ability of fumigation to lyse cells. There was no difference in extracted C concentration between fumigated and unfumigated samples of silica sand and of humic acid, confirming that the fumigation process does not introduce additional organic C to samples of these soil components. However, the fumigated clay samples both showed increased C release relative to the unfumigated controls, indicating that significant concentrations of the fumigant, chloroform, adsorbed onto the clay minerals studied here. Additionally, we found significant chloroform remaining in the extracts from two fumigated natural soils. Attempts to remove the chloroform from the soils or soil components prior to extraction by increasing the evacuation time, or to remove chloroform in the extracts by sparging them vigorously with nitrogen gas, both failed. This research reveals that chloroform gas may adsorb significantly to clays and the clay fraction of natural soils. Thus, the fumigation–extraction method must be corrected to account for the added chloroform C and accurately assess the concentration of biomass C in soils that contain significant concentrations of clays.     

Formation of syngenetic and early diagenetic iron minerals in the late Archean Mt. McRae Shale, Hamersley Basin, Australia: New insights on the patterns, controls and paleoenvironmental implications of authigenic mineral formation

A section through the late Archean Mt. McRae Shale comprising, in ascending order, a lower shale interval (LSI), a banded iron formation (BIF), an upper shale (USI) and a carbonate (C1) has been analyzed for total Fe and Al contents and authigenic Fe present as carbonate, oxide, sulfide and silicate phases. The authigenic mineralogy is controlled by the episodic addition of Fe from hydrothermal activity and removal of Fe by sulfide, relative to rates of clastic sedimentation. The LSI and BIF have mean Fe_T/Al values of 2 and 5, respectively, that record iron enrichment from hydrothermal sources. Iron was precipitated primarily as siderite accompanied by Fe-rich chlorite from anoxic bottom waters rich in dissolved Fe. Pyrite formation was probably limited by the availability of sulfate, which was present at low concentrations and became rapidly depleted. The USI has generally lower Fe_T/Al values (0.6-1.3), similar to those found in Paleozoic shales, with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle. This interval contains authigenic Fe predominantly as pyrite, where high values for DOP (>0.8) indicate the existence of a water column that became rich in dissolved sulfide (euxinic) when sulfate concentrations increased due to a transient or secular increase in ocean/atmosphere oxygenation. High concentrations of dissolved sulfide maintained low concentrations of dissolved Fe, which allowed only minor amounts of Fe to be precipitated as carbonates and silicates. The USI also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin. The carbonate C1 represents a basinal chemistry where sulfide has been removed and Fe_T/Al values are ∼1 indicating that hydrothermal activity again produced dissolved Fe-rich bottom waters. Detailed iron speciation of the Mt. McRae Shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late Archean Hamersley Basin, just prior to the Paleoproterozoic rise in atmospheric oxygenation, and our refined methods have relevance to all Fe-rich deposits.     

A depositional history of particulate organic carbon in a floodplain lake from the lower Ob’ River, Siberia

Northern, high latitude soils have stored vast amounts of organic carbon (OC) in permafrost and peats for many millennia, however, climate change may mobilize and release this particulate OC (POC) to arctic rivers. Deltaic and floodplain lakes that receive fluvial sediments, primarily during the spring freshet, may provide records of such changes in riverine POC. Here, we examine properties of OC in a sediment core from a lake in the lower floodplain of the Ob’ River, west Siberia, to determine how the properties of OC deposited in this lake varied over many decades and to evaluate use of this sedimentary OC as a recorder of riverine POC load and properties. The core predates the most recent, dramatic changes in arctic climate and hence may serve as a benchmark against which to contrast future variations in fluvial POC discharge. Elemental, stable carbon and radiocarbon isotopic analyses, along with nuclear magnetic resonance (NMR) spectroscopy and molecular-level information (lignin phenol composition), indicate two major sources of OC to most of the sediments in this lake: plant-derived OC and algal-derived OC. However, a mixing model indicates that the nature and ¹⁴C content of these two sources change with depth in the sediment, resulting in three distinct layers: surface horizons, a “high-OC” layer and “mixing” horizons found above and below the high-OC layer. The plant-derived component is significantly aged throughout the core (¹⁴C ages of 1300-3900 years) and appears to derive from primarily local, tundra sources, whereas the algal component is modern. Our analysis suggests that the usual mode of OC deposition, as exemplified by the “mixing” and surface horizons, involved mixing of varying amounts of new algal production (35-65%) with aged permafrost- or peat-derived OC. This deposition was interrupted by an event, such as the collapse of a riverbank, which laid down the compositionally distinct “high-OC” layer in which plant-derived OC mixes with aged mineral-soil-derived OC without clear input from algae. The relative amounts of the plant and algal components in the lake sediments appear to be controlled primarily by local hydrological conditions rather than by river-wide processes, suggesting that comparison of sediment records from multiple lakes within a floodplain will be important to assess changes in POC export by arctic rivers. However, the flux and nature of the higher plant-derived OC may carry important information on the sources and dynamics of OC stored within the drainage basin.     

The effects of uranium speciation on the rate of U(VI) reduction by Shewanella oneidensis MR-1

We measured the kinetics of U(VI) reduction by Shewanella oneidensis MR-1 under anaerobic conditions in the presence of variable concentrations of either EDTA or dissolved Ca. We measured both total dissolved U and U(VI) concentrations in solution as a function of time. In separate experiments, we also measured the extent of U(VI) adsorption onto S. oneidensis in order to quantify the thermodynamic stabilities of the important U(VI)-bacterial surface complexes. In the EDTA experiments, the rate of U(IV) production increased with increasing EDTA concentration. However, the total dissolved U concentrations remained constant and identical to the initial U concentrations during the course of the experiments for all EDTA-bearing systems. Additionally, the U(VI) reduction rate in the EDTA experiments exhibited a strong correlation to the concentration of the aqueous U⁴⁺-EDTA complex. We conclude that the U(VI) reduction rate increases with increasing EDTA concentration, likely due to U⁴⁺-EDTA aqueous complexation which removes U(IV) from the cell surface and prevents UO₂ precipitation.In the Ca experiments, the U(VI) reduction rate decreased as Ca concentration increased. Our thermodynamic modeling results based on the U(VI) adsorption data demonstrate that U(VI) was adsorbed onto the bacterial surface in the form of a Ca-uranyl-carbonate complex in addition to a number of other Ca-free uranyl complexes. The observed U(VI) reduction rates in the presence of Ca exhibit a strong negative correlation to the concentration of the Ca-uranyl-carbonate bacterial surface complex, but a strong positive correlation to the total concentration of all the other Ca-free uranyl surface complexes. Thus, the concentration of these Ca-free uranyl surface complexes appears to control the rate of U(VI) reduction by S. oneidensis in the presence of dissolved Ca. Our results demonstrate that U speciation, both of U(VI) before reduction and of U(IV) after reduction, affects the reduction kinetics, and that thermodynamic modeling of the U speciation may be useful in the prediction of reduction kinetics in realistic geologic settings.     

Multidimensional thermal structure of magnetized neutron star envelopes

Recently launched X-ray telescopes have discovered several candidate isolated neutron stars. The thermal radiation from these objects may potentially constrain our understanding of nuclear physics in a realm inaccessible to terrestrial experiments. To translate the observed fluxes from neutron stars into constraints, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We describe models of the thermal structure of the envelopes of neutron stars with magnetic fields up to 10¹⁴ G. Unlike earlier work, we infer the properties of envelope models in two dimensions and precisely account for the quantization of the electron phase-space. Both dipole and uniformly magnetized envelopes are considered.     

Geomagnetic field analysis—III. Magnetic fields on the core—mantle boundary

Summary. The method of stochastic inversion, previously applied to secular variation data, is applied to main field data. Adaptations to the method are required: non-linear, as well as linear, data are used; allowance is made for crustal components in the observatory data; and the prior information is specified differently. The requirement that the models should satisfy a finite lower bound on the Ohmic heating in the core provides strong prior information and gives finite error estimates at the core—mantle boundary.
The new method is applied to data from the epochs 1969.5 and 1980.0. The resulting field models are very much more complex than other models, such as the IGRF models extrapolated to the core, and show considerable small-scale detail which, on the basis of the error analysis, can be believed.
The flux integral over the northern hemisphere is computed at each epoch; the difference between the two epochs is approximately one standard deviation, suggesting that the question as to whether the decay of the dipole is consistent with the frozen-flux hypothesis has been resolved in favour of the hypothesis.     

Tidal torques and galactic warps

%We study the evolution of galactic disks subject to tidal torques motivated by cosmological N-body simulations using analytic and numerical techniques. We find that self-gravitating disks subject to these torques resemble observed warped galaxies. The warps develop at a local surface density of 70 M _⊙ pc^-2 and move out through the disk at a rate that depends on the surface density of the disk. This revised version was published online in August 2006 with corrections to the Cover Date.     

THE NOMENCLATURE OF POLYMICT BASALTIC ACHONDRITES

The subgroups within the basaltic achondrite suite are defined using the structural criterion of Wahl (1952). The ‘monomict’ meteorites are samples of a single lithology while the polymict meteorites are those containing two or more lithologies. The ‘monomict’ subgroups eucrites, cumulate eucrites and diogenites are subdivided into both brecciated and unbrecciated meteorites. The polymict achondrites sample a petrological-compositional continuum that contains both mafic and ultramafic rock types and may be subdivided into several groups. Two groups of polymict basaltic achondrites, the polymict eucrites and howardites are separated using an arbitrarily defined criterion. The recommended criterion is based on the amount of magnesian ortho-pyroxenite (diogenite) component in the meteorite. Howardites contain more than 10% and polymict eucrites contain less than 10%. The criteria proposed (perhaps with minor variations), appear to reconcile the ambiguities caused by the polymict eucrites. These meteorites, using earlier structural criteria, are howardites, but using mineralogical-chemical criteria are eucrites. As a subgroup of the polymict achondrites, their relationship with the howardites is clear, and the preservation of the term ‘eucrite’ in their name highlights their modal affinity to the monomict eucrites.