Rates of silicate dissolution in deep-sea sediment: In situ measurement using U/U of pore fluids

Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the ²³⁴U/²³⁸U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).The pore water ²³⁴U/²³⁸U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment ²³⁴U/²³⁸U activity ratios are close to 1.0. The ²³⁴U/²³⁸U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess ²³⁴U to the pore fluid by α-recoil injection of ²³⁴Th. The fraction of ²³⁸U decays that result in α-recoil injection of ²³⁴U to pore fluid is estimated to be 0.10 to 0.20 based on the ²³⁴U/²³⁸U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10⁻⁷ to 2 × 10⁻⁶ yr⁻¹. There is significant down-hole variability in pore water ²³⁴U/²³⁸U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 10⁴ times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials.The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (R_d) of soils and deep-sea sediments can be approximately described by the expression R_d ≈ 0.1 Age⁻¹ for ages spanning 1000 to 5 × 10⁸ yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.     

Substitution of Natural Gas for Coal: Climatic Effects of Utility Sector Emissions

Substitution of natural gas for coal is one means of reducing carbon dioxide (CO₂) emissions. However, natural gas and coal use also results in emissions of other radiatively active substances including methane (CH₄), sulfur dioxide (SO₂), a sulfate aerosolprecursor, and black carbon (BC) particles. Will switching from coal to gas reduce the net impact of fossil fuel use on global climate? Using the electric utility sector as an example, changes in emissions of CO₂, CH₄,SO₂ and BC resulting from the replacement of coal by natural gas are evaluated, and their modeled net effect on global mean-annual temperature calculated. Coal-to-gas substitution initially produces higher temperatures relative to continued coal use. This warming is due to reduced SO₂ emissionsand possible increases in CH₄ emissions, and can last from 1 to 30years, depending on the sulfur controls assumed. This is followed by a net decrease in temperature relative to continued coal use, resulting from lower emissions of CO₂ and BC. The length of this period and the extent of the warming or cooling expected from coal-to-gas substitution is found to depend on key uncertainties and characteristics of the substitutions, especially those related to: (1) SO₂ emissions and consequentsulphate aerosol forcing; and (2) the relative efficiencies of the power plantsinvolved in the switch.     

Petrophysical characterization and natural fracturing in an olivine-dolerite aquifer

As part of a study investigating the naturally-occurring fractures in mafic rocks, two holes were drilled 450 m apart through the Palisades dolerite sill in New York. Well-2 is 229 m deep and Well-3 was drilled to 305 m, both penetrating through the sill and into the underlying Triassic sediments of the Newark Basin. Both holes were logged with downhole geophysical tools, including the BHTV, which acoustically images fractures intersecting the well. Understanding the fracture pattern, density, and porosity in the sill is essential for identifying possible zones of active fluid flow and high permeability. Using the BHTV logs, 96 and 203 fractures were digitally mapped within the sill in Well-2 and Well-3, respectively. Most fractures appear to dip steeply (76-78°). There is a shift in fracture orientation, however, and these fractures may or may not be continuous over the short lateral distance between Well-2 and Well-3. The lithology of the sill as identified by drill chips is nevertheless continuous between the holes. Both intersect a 7 m thick olivine-rich layer about 15 m above the bottom of the sill. Several fractures identified in Well-2 have large apparent aperture (>6cm) which correspond to high porosity zones (6-14%) observed in the logs. Resistivity logs were used to compute porosity using Archie's law and match well with the neutron porosity log in Well-2. We use the relationship between porosity and fracture aperture within the sill at Well-2 to infer the porosity in Well-3. High-porosity, large-aperture zones, including the target olivine layer, are identified in both holes. Changes in the temperature gradient log indicate active fluid flow in the sill, although flow appears to be most active in the sediments. Direct field measurements of bulk permeability, hydrologic modeling of fluid flow and calibration of fracture and log porosity will be undertaken in the future.     

Managing cropland and rangeland for climate mitigation: an expert elicitation on soil carbon in California

Understanding the magnitude of and uncertainty around soil carbon flux (SCF) is important in light of California’s efforts to increase SCF (from the atmosphere to soils) for climate change mitigation. SCF depends, to a great extent, on how soils are managed. Here, we summarize the results of an elicitation of soil science and carbon cycle experts aiming to characterize understanding of current SCF in California’s cropland and rangeland, and how it may respond to alternative management practices over time. We considered four cropland management practices—biochar, compost, cover crops, and no-till—and two rangeland management practices, compost and high-impact grazing. Results across all management practices reveal underlying uncertainties as well as very modest opportunities for soil carbon management to contribute meaningfully to California’s climate mitigation. Under median scenarios, experts expect all the surveyed management practices to reverse SCF from negative to positive, with direct carbon additions via biochar and compost offering the best potential for boosting the soil carbon pool.     

Groundmass crystallization of Mount St. Helens dacite, 1980–1986: a tool for interpreting shallow magmatic processes

The 1980–1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe–Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10^-9 cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10^-11 cm/s; plagioclase growth rates then decreased to <1x10^-11 cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981–1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe–Ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930°C and at log f_O2 of -10.8; by 1985–1986 oxide temperatures decreased to 870°C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (<4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980—this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwkck et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5–10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.     

The IPCC AR5 guidance note on consistent treatment of uncertainties: a common approach across the working groups

Evaluation and communication of the relative degree of certainty in assessment findings are key cross-cutting issues for the three Working Groups of the Intergovernmental Panel on Climate Change. A goal for the Fifth Assessment Report, which is currently under development, is the application of a common framework with associated calibrated uncertainty language that can be used to characterize findings of the assessment process. A guidance note for authors of the Fifth Assessment Report has been developed that describes this common approach and language, building upon the guidance employed in past Assessment Reports. Here, we introduce the main features of this guidance note, with a focus on how it has been designed for use by author teams. We also provide perspectives on considerations and challenges relevant to the application of this guidance in the contribution of each Working Group to the Fifth Assessment Report. Despite the wide spectrum of disciplines encompassed by the three Working Groups, we expect that the framework of the new uncertainties guidance will enable consistent communication of the degree of certainty in their policy-relevant assessment findings.