The high-temperature heat capacity of natural calcite (CaCO3)

The heat capacity (C _P) of a natural sample of calcite (CaCO₃) has been measured from 350 to 775 K by differential scanning calorimetry (DSC). Heat capacities determined for a powdered sample and a single-crystal disc are in close agreement and have a total uncertainty of ±1 percent. The following equation for the heat capacity of calcite from 298 to 775 K was fit by least squares to the experimental data and constrained to join smoothly with the low-temperature heat capacity data of Staveley and Linford (1969) (C _P in J mol^?1 K^?1, T in K): $$\begin{gathered} C_p = - 184.79 + 0.32322T - 3,688,200T^{ - 2} \hfill \\ {\text{ }} - (1.2974{\text{ }} \times {\text{ 10}}^{ - {\text{4}}} )T^2 + 3,883.5T^{ - 1/2} \hfill \\ \end{gathered} $$ Combining this equation with the S ₂₉₈ ⁰ value from Staveley and Linford (1969), entropies for calcite are calculated and presented to 775 K. A simple method of extrapolating the heat capacity function of calcite above 775 K is presented. This method provides accurate entropies of calcite for high-temperature thermodynamic calculations, as evidenced by calculation of the equilibrium: CaCO_{3 (s)}=CaO_(s)+CO_{2 (s)}.     

Fluxes of uranium and thorium series isotopes in the Santa Barbara Basin

Samples from the MANOP Santa Barbara Basin sediment trap intercomparison were analyzed for the isotopes of uranium, thorium, radium, lead, and polonium. All of the traps showed approximately the same compositions and isotopic ratios, indicating that they trapped similar materials. The²³⁴Th flux via falling particles was very close to the flux predicted from the production and scavenging rates of²³⁴Th from the water column. The²¹⁰Pb content of the trapped particles and the surface sediments were the same, however, the measured flux of²¹⁰Pb was seven times greater than the predicted flux. Predicted and measured fluxes of²²⁸Th and²¹⁰Po were similarly out of balance. To explain this apparent inconsistency, we suggest (as others have done) that the Santa Barbara Basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale. These two effects increase the apparent area from which the basin derives the longer-lived isotopes but does not increase significantly the supply of the short-lived²³⁴Th.     

Growth of the ice phase in strong cumulonimbus updrafts

A numerical model of ice phase growth in an ascending parcel is used to delineate seeding requirements under the competing embryo and glaciation hypotheses. The strong updraft core is found to remain virtually all liquid until homogeneous freezing occurs, AgI or dry ice seeding having negligible effects with achievable seeding rates. This suggests that the glaciation hypothesis is untenable. Natural hail embryo formation is noted to be limited to updrafts less than 3 to 4 m sec^?1 at cloud base. AgI seeding of such updraft regions at rates currently used is found to produce concentrations of hail embryos sufficient to enhance competition in multi-cell hailstorms, although super-cell storms may require significantly greater seeding rates.     

Lead-210 and pollen geochronologies on Lake Superior sediments

Rates of sedimentation in two Lake Superior deposits were determined by both ragweed pollen and ²¹⁰Pb geochronologies. The former yields an average rate over the time since the first appearance of enhanced concentrations of the pollen as a consequence of human settlement. Sedimentation rates derived on these two bases can be brought into accord if the first appearance of ragweed pollen in the sediments was around 1830 and if the sedimentation rates have been uniform over the past century.     

Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental,Chihuahua, Mexico

The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains 1% K₂O at 60% SiO₂. Trace element abundances of a typical ignimbrite with 73% SiO₂ are Sr 225 ppm, Rb 130 ppm, Y 32 ppm, Th 12 ppm, Zr 200 ppm, and Nb 15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.     

Transposed Climates for Study of Water Supply Variability on the Laurentian Great Lakes

Hydrological models of the Great Lakes basin were used to study the sensitivity of Great Lakes water supplies to climate warming by driving them with meteorological data from four U.S. climate zones that were transposed to the basin. Widely different existing climates were selected for transposition in order to identify thresholds of change where major impacts on water supplies begin to occur and whether there are non-linear responses in the system. The climate zones each consist of 43 years of daily temperature and precipitation data for 1,000 or more stations and daily evaporation-related variables (temperature, wind speed, humidity, cloud cover) for approximately 20–35 stations. A key characteristic of these selected climates was much larger variability in inter-annual precipitation than currently experienced over the Great Lakes. Climate data were adjusted to simulate lake effects; however, a comparison of hydrologic results with and without lake effects showed that there was only minor effects on water supplies.     

Quantitative description of the geomagnetic field during the Matuyama-Brunhes polarity transition

A model of the reversing geodynamo based on the assumptions (1) that reversals start in a localized region of the core and (2) that upon its onset this reversed region extends, or “floods”, both north-south and east-west until the entire core is affected, has recently been shown to provide a generally successful simulation of existing paleomagnetic records of the Matuyama-Brunhes transition (Hoffman, 1979). In this paper the modelled solution is analyzed so as to reveal the behavior of the dominant Gauss coefficients during the transition. At the time of total axial dipole decay the controlling components are found to be a zonal octupole (g₃⁰) and a non-axisymmetric quadrupole (g₂¹, h₂¹). Given the distribution of sites corresponding to the available records of the Matuyama-Brunhes, the existence of a significant zonal quadrupole field component cannot be ruled out; however, the role of any equatorial dipole component can be neglected.Due to the presence of a significant low-order non-axisymmetric term in the analyzed transition field, the predicted minimum intensity experienced during the Matuyama-Brunhes is found to be dependent on both site latitude and longitude. In particular, over a mid-northern circle of latitude, the predicted minimum intensity is found to vary by more than a factor of three, averaging about 10% of the full polarity field strength.Although not a unique solution, the applicability of the findings from this analysis is not tied to the phenomenological model from which they were derived. More specifically, whether the above two-component non-dipole transitional field arises from assumed configurational changes of the reversing geodynamo (as is the case for the flooding model) or, alternatively, is considered to be a stationary (non-reversing) portion of the field during axial dipole decay and regeneration, has little effect on either the calculated path locality of the virtual geomagnetic pole or the minimum intensity experienced at a given site. These two possible situations, in principle, should be distinguishable given the future attainment of detailed paleomagnetic data corresponding to back-to-back (R → N and N → R) polarity transitions.     

Hanford tank residual waste – Contaminant source terms and release models

Residual waste is expected to be left in 177 underground storage tanks after closure at the US Department of Energy’s Hanford Site in Washington State, USA. In the long term, the residual wastes may represent a potential source of contamination to the subsurface environment. Residual materials that cannot be completely removed during the tank closure process are being studied to identify and characterize the solid phases and estimate the release of contaminants from these solids to water that might enter the closed tanks in the future. As of the end of 2009, residual waste from five tanks has been evaluated. Residual wastes from adjacent tanks C-202 and C-203 have high U concentrations of 24 and 59 wt.%, respectively, while residual wastes from nearby tanks C-103 and C-106 have low U concentrations of 0.4 and 0.03 wt.%, respectively. Aluminum concentrations are high (8.2–29.1 wt.%) in some tanks (C-103, C-106, and S-112) and relatively low (<1.5 wt.%) in other tanks (C-202 and C-203). Gibbsite is a common mineral in tanks with high Al concentrations, while non-crystalline U–Na–C–O–P ± H phases are common in the U-rich residual wastes from tanks C-202 and C-203. Iron oxides/hydroxides have been identified in all residual waste samples studied to date. Contaminant release from the residual wastes was studied by conducting batch leach tests using distilled deionized water, a Ca(OH)₂-saturated solution, or a CaCO₃-saturated water. Uranium release concentrations are highly dependent on waste and leachant compositions with dissolved U concentrations one or two orders of magnitude higher in the tests with high U residual wastes, and also higher when leached with the CaCO₃-saturated solution than with the Ca(OH)₂-saturated solution. Technetium leachability is not as strongly dependent on the concentration of Tc in the waste, and it appears to be slightly more leachable by the Ca(OH)₂-saturated solution than by the CaCO₃-saturated solution. In general, Tc is much less leachable (<10 wt.% of the available mass in the waste) than previously predicted. This may be due to the coprecipitation of trace concentrations of Tc in relatively insoluble phases such as Fe oxide/hydroxide solids.