Assessing sulfate and carbon controls on net methylmercury production in peatlands: An in situ mesocosm approach

The transformation of atmospherically deposited inorganic Hg to the toxic, organic form methylmercury (MeHg) is of serious ecological concern because MeHg accumulates in aquatic biota, including fish. Research has shown that the Hg methylation reaction is dependent on the availability of SO₄ (as an electron acceptor) because SO₄-reducing bacteria (SRB) mediate the biotic methylation of Hg. Much less research has investigated the possible organic C limitations to Hg methylation (i.e. from the perspective of the electron donor). Although peatlands are long-term stores of organic C, the C derived from peatland vegetation is of questionable microbial lability. This research investigated how both SO₄ and organic C control net MeHg production using a controlled factorial addition design in 44 in situ peatland mesocosms. Two levels of SO₄ addition and energetic-equivalent additions (i.e. same number of electrons) of a number of organic C sources were used including glucose, acetate, lactate, coniferous litter leachate, and deciduous litter leachate. This study supports previous research demonstrating the stimulation of MeHg production from SO₄ input alone (∼200 pg/L/day). None of the additions of organic C alone resulted in significant MeHg production. The combined addition of SO₄ and some organic C sources resulted in considerably more MeHg production (∼500 pg/L/day) than did the addition of SO₄ alone, demonstrating that the highest levels of MeHg production can be expected only where fluxes of both SO₄ and organic C are delivered concurrently. When compared to a number of pore water samples taken from two nearby peatlands, MeHg concentrations resulting from the combined addition of SO₄ and organic C in this study were similar to MeHg “hot spots” found near the upland–peatland interface. The formation of MeHg “hot spots” at the upland–peatland interface may be dependent on concurrent inputs of SO₄ and organic C in runoff from the adjacent upland hillslopes.     

Non-adiabatic interactions in excitedC 2 H molecules and their relationship toC 2 formation in comets

A unified picture of the photodissociation of theC ₂ H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC ₂ are formed during the photodissociation of theC ₂ H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC ₂ radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC ₂ fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC ₂ H radical. The photochemically active states are the 2²₊, 2²II, 3²II, and 3²₊, with photodissociation rate constants of 1.0×10^–6, 4.0×10^–6, 0.7×10^–6, and 1.3×10^–6s^–1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×10⁵ s.     

Cape cormorants decrease,move east and adapt foraging strategies following eastward displacement of their main prey

Numbers of Cape cormorants Phalacrocorax capensis breeding in South Africa decreased by nearly 50% from approximately 107 000 pairs in 1977–1981 to 57 000 pairs in 2010–2014. Although four colonies had >10 000 pairs in 1977–1981, there was just one such colony in 2010–2014. Almost all the decrease occurred after the early 1990s off north-west South Africa, between the Orange River estuary and Dassen Island. South of this, the number breeding in the two periods was stable, with some colonies being formed or growing rapidly in the 2000s. The proportion of South Africa’s Cape cormorants that bred south of Dassen Island increased from 35% in 1977–1981 to 66% in 2010–2014, with the opposite situation observed in the north-west. This matched a shift to the south and east in the distributions of two of the Cape cormorant’s main prey species, anchovy Engraulis encrasicolus and sardine Sardinops sagax. In 2014, an apparent scarcity of prey in the north-west resulted in Cape cormorants attempting to take bait from hooks of fishing lines over an extended period, a behaviour not previously recorded. The number of Cape cormorants breeding in the south may be constrained by the absence of large islands between Dyer Island in the west and Algoa Bay in the east. If so, it may be possible to bolster the southern population through the provision of appropriate breeding habitat, such as platforms, or restricting human disturbance at suitable mainland cliff breeding sites.     

Tectonic-hydrothermal pegmatites

The proposed model for the genesis of complex zoned pegmatites depends on two basis conditions: (1) an alkali chloride intergranular fluid phase (of either magmatic or metamorphic origin) is in equilibrium with the solid phases in the host rock, and (2) deformation produces low pressure zones in the host rock. The phase relations of quartz-mica-feldspar in alkali chloride solutions predict that feldspar will form from quartz, muscovite and dissolved alkalies in low pressure zones in quartz-muscovite schist. The reaction is fed by diffusion of hydrothermal alkali chloride solutions into the low pressure zone. Low pH fluids produced during the reaction are out of equilibrium when lithostatic pressure is re-established so that muscovite forms as a late replacement.A critical lowering of pressure is necessary to force the reaction. If the rate of dilation is relatively slow, only quartz will migrate into the low pressure zone, and the critical low pressure will not be attained. A relatively higher rate of dilation will cause the reaction to proceed. This control can explain the quartz core and the occurrence of quartz veins that grade laterally into pegmatite cores. By this model, the quartz core is the first part of the pegmatite to crystallize.The rare elements concentrated in pegmatites are considered to come from four possible sources: (1) hydrothermally introduced solutions, (2) impurity elements adsorbed to mineral grains in the country rock, (3) elements released by recrystallization of the country rock, and (4) elements released by reactions.Other features of pegmatites are shown to be compatible with the proposed model.     

Climate change during the Triassic and Jurassic

The transition from the Triassic to Jurassic is associated with dramatic changes in Earth's climate. Pangaea was breaking up as North America rifted away from Africa, the Central Atlantic Magmatic Province erupted, and the concentration of atmospheric carbon dioxide increased dramatically. This article summarises the changes in Earth's climate associated with this transition, including a discussion of the various impacts of the increased carbon dioxide on the Earth system, the question of whether the wet episode in the Carnian was a global or regional event, the formation of bauxite deposits, and how dinosaur distributions changed over time. Palaeoclimate model simulations reveal the spatial changes in climate between the Triassic and Jurassic, illustrating the subtropics becoming slightly cooler and wetter despite the warming trend for the Earth's average temperature.     

Scale effects of hydrostratigraphy and recharge zonation on base flow

Uncertainty regarding spatial variations of model parameters often results in the simplifying assumption that parameters are spatially uniform. However, spatial variability may be important in resource assessment and model calibration. In this paper, a methodology is presented for estimating a critical basin size, above which base flows appear to be relatively less sensitive to the spatial distribution of recharge and hydraulic conductivity, and below which base flows are relatively more sensitive to this spatial variability. Application of the method is illustrated for a watershed that exhibits distinct infiltration patterns and hydrostratigraphic layering. A ground water flow model (MODFLOW) and a parameter estimation code (UCODE) were used to evaluate the influence of recharge zonation and hydrostratigraphic layering on base flow distribution. Optimization after removing spatial recharge variability from the calibrated model altered base flow simulations up to 53% in watersheds smaller than 40 km(2). Merging six hydrostratigraphic units into one unit with average properties increased base flow residuals up to 83% in basins smaller than 50 km(2). Base flow residuals changed <5% in watersheds larger than 40 and 50 km(2) when recharge and hydrostratigraphy were simplified, respectively; thus, the critical basin size for the example area is approximately 40 to 50 km(2). Once identified for an area, a critical basin size could be used to guide the scale of future investigations. By ensuring that parameter discretization needed to capture base flow distribution is commensurate with the scope of the investigation, uncertainty caused by overextending uniform parameterization or by estimating extra parameter values is reduced.     

Application of Regional Phase Amplitude Tomography to Seismic Verification

v--vWe have applied tomographic techniques to amplitude data to quantify regional phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to apply path corrections into regions devoid of calibration events, as well as raising levels of confidence in the corrections because of their more physical basis. Our tomography technique solves for resolvable combinations of attenuation, source-generation, site and spreading terms. First difference regularization is used to remove singularities and reduce noise effects.¶In initial tests the technique was applied to a data set of 1488, 1.0 Hz, P_g/L_g amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power-law distance correction traditionally applied to amplitude ratios. Relative P_g/L_g attenuation varied with geologic region, with low values in Tibet, intermediate values in basins and high values for platforms and older crust. Spatial patterns were consistent with previous path effect studies in Asia, especially local earthquake coda-Q. Relative spreading was consistent with expected values for P_g and L_g. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident.¶Tomography residuals followed systematic trends with distance, which may result from the evolution from direct to coda phases, focusing, model tradeoff or data windowing effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects. The residual patterns often follow geological boundaries, which could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These results will guide efforts to reparameterize tomography models to more effectively represent regional wave attenuation and blockage. The interpolated residuals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.     

A model for phlogopite development in low magnesium rocks

Phlogopites containing anomalously high values of Zn, Mn and Li have been identified in low-magnesium feldspathic schists and quartzites from the Precambrian of northern New Mexico. The phlogopite is volumetrically minor (<2%) and coexists with muscovite. The presence of hematite and piemontite in the phlogopite-bearing samples indicates high oxidizing conditions, but their presence in phlogopite-free mica schists nearby demonstrates that high fo₂ is insufficient to account for the phlogopite occurrences. A model for phlogopite development is presented which is based upon (1) high fo₂ conditions, (2) limited H⁺ during metamorphism, and (3) early mobilization of Mg (and Zn, Mn, Li) during incipient metamorphism. The absence of phlogopite in associated rocks which refect high fo₂ conditions is due to the larger amount of H⁺ available during metamorphism, resulting in the development of abundant muscovite rather than rare phlogopite.