Air‐water gas‐exchange is studied in direct numerical simulations (DNS) of free‐surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface‐shear‐stress and ...the gas‐transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number
Ri=Bν/u*4 where
B,
ν, and
u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise
0<Ri<∞ ranging from convection‐dominated to shear‐dominated cases. The results are used to: (i) evaluate parameterizations of the air‐water gas‐exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, and (iii) find an expression for the gas‐transfer velocity for flows driven by both convection and shear. The evaluated gas transfer‐velocity parametrizations are based on either the rate of turbulent kinetic energy dissipation, the surface flow‐divergence, the surface heat‐flux, or the wind‐speed. The parametrizations based on dissipation or divergence show an unfavorable
Ri dependence for flows with combined forcing whereas the parametrization based on heat‐flux only shows a limited
Ri dependence. The two parametrizations based on wind speed give reasonable estimates for the transfer‐velocity, depending however on the surface heat‐flux. The transition from convection‐ to shear‐dominated gas‐transfer‐velocity is shown to be at
Ri≈0.004. Furthermore, the gas‐transfer is shown to be well represented by two different approaches: (i) additive forcing expressed as
kg,sum=AShearu*Ri/Ric+11/4Sc−n where
Ric=AShear/ABuoy4, and (ii) either buoyancy or shear dominated expressed as,
kg=ABuoyBν1/4Sc−n, Ri>Ric or
kg=Ashearu*Sc−n, Ri<Ric. Here
ABuoy=0.4 and
AShear=0.1 are constants, and
n is an exponent that depends on the water surface‐characteristics.
Key Points:
Air‐sea gas exchange parameterizations are evaluated
Air‐sea gas transfer for flows driven by natural convection and wind
Transition by gas transfer dominated of natural convection versus wind
A national energy strategy in Mexico to 2028, derived from the recent energy reform, projects a considerable shift in the national power generation mix leading to changes on its environmental ...profile. Therefore, this paper aims to compare the potential environmental impacts of the electricity generation under previous, current and projected energy mixes in Mexico at a national and regional scale considering the Yucatan state as the regional case study. These were estimated with a cradle-to-grave Life Cycle Assessment (LCA) following the CML–IA 2000 V2.05 methodology and including the baseline environmental impact categories. Results indicate that the environmental impacts per generated kWh of electricity have decreased in Mexico from 2007 to 2013 and would keep decreasing at a faster rate until 2028 according to current projections. In Yucatan, the environmental impacts increased from 2007 to 2013 due to an increased diesel consumption, but would decrease by 2028 reaching lower levels than in 2007. From 2007 to 2013 almost all the environmental impacts in Yucatan were lower than the country average due to a higher contribution of natural gas in the power generation mix, while in 2028 they would be similar. Overall, there is a significant difference between the national and regional estimations of the potential environmental impacts due to differences in their power generation mix. Furthermore, the potential introduction of shifts derived from the energy reform in Mexico will have a strong positive influence on the environmental impacts from the electricity generation.
•The potential environmental effects of the Mexican energy reform were evaluated.•The environmental impacts of power generation in Mexico and Yucatan were assessed.•A reduction in the environmental impacts of power generation is expected for 2028.•In 2013 all the environmental impacts but GWP were lower for Yucatan than for Mexico.•In 2028 the environmental impacts of Yucatan and Mexico are expected to be similar.
Variations of the mass-independent abundance of
26Mg (
δ
26Mg*) and stable Mg (
δ
25Mg) isotope composition of chondrites are important because they constrain the homogeneity of
26Al and Mg isotopes ...in the proto-planetary disc and the validity of the short-lived
26Al-to-
26Mg chronometer applied to meteorites. We present high-precision Mg isotope data and Al/Mg ratios of chondrites representing nearly all major chondrite classes, including a step-leaching experiment on the CM2 chondrite Murchison.
δ
26Mg* variations in leachates of Murchison representing acid soluble material are ≤
30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in
δ
26Mg* (−0.002 to +
0.118‰). Very small variations in
δ
26Mg* anomalies in bulk chondrites (−0.006 to +
0.019‰) correlate with increasing
27Al/
24Mg ratios and
δ
50Ti, reflecting the variable presence of calcium–aluminium-rich inclusions (CAIs) in some types of carbonaceous chondrites. Similarly, release of radiogenic
26Mg produced by
26Al decay from CAI material in the step-leaching of Murchison best explains the high
δ
26Mg* observed in the last, aggressive, leaching steps of this experiment. Overall, the observed variations in
δ
26Mg* are small and potential differences beyond that which result from the presence of CAI-like material cannot be detected within the analytical uncertainties of this study (±
0.004‰). The results do not allow radical heterogeneity of
26Al (≥±30%) or measurable Mg nucleosynthetic heterogeneity (≥±0.005‰) to have existed on a planetesimal scale in the proto-planetary disc. Combined with published
δ
26Mg* data for CAIs, the bulk chondrite data yield a precise initial (
26Al/
27Al)
0
=
(5.21
±
0.06)
×
10
−
5 and
δ
26Mg*
=
−0.0340
±
0.0016‰ for the Solar System. However, it is not possible with the currently available data to determine with certainty whether CAIs and the material from which planetesimals accreted including chondrite parent bodies had precisely the same initial levels of
26Al, although planetesimals and planets appear to have accreted from material with a mean initial (
26Al/
27Al)
0 in the range of 2.1 to 6.7
×
10
−
5
. The average stable Mg isotope composition of all analysed chondrites, with the exception of a chondrule from the CBa chondrite Gujba (
δ
25Mg
DSM
-
3
=
−0.032
±
0.035‰), is
δ
25Mg
DSM
-
3
=
−0.152
±
0.079‰ (2 sd) and is indistinguishable from that of the Earth's mantle.
Stratigraphy, structure and host-rock chemistry are dominant controls on the location of Au in Archaean greenstone-hosted Au deposits, but the stratigraphy in such deposits is seldom obvious due to ...the monotonous nature of the host rocks or pervasive alteration associated with Au mineralisation. Portable, hand-held, X-ray fluorescence (pXRF) spectrometry provides a method to rapidly collect large amounts of whole-rock geochemical data that can yield new insights into both stratigraphy and Au localisation. Here we present results of pXRF analyses of samples from a representative section through Au-mineralised amphibolite-facies metabasaltic rocks at Plutonic Gold Mine, Western Australia. These data illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence, confirming previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrating for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au commonly occurs along basalt flow boundaries. The elemental concentration data clearly demonstrates for the first time the stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. The methods described in this paper are readily applied, and have the potential to enhance the understanding of otherwise unclear stratigraphy and its control on mineralisation in many different types of deposits worldwide.
► Portable, hand-held, X-ray fluorescence (pXRF) spectrometry provides a method to rapidly collect large amounts of whole-rock geochemical data. ► Element concentration data obtained by pXRF reveals, for the first time, the stratigraphy of basalt flows in an amphibolite-facies metamorphosed greenstone belt sequence at Plutonic Gold Mine, Western Australia. ► Gold is shown to be localised along basalt flow boundaries. ► The pXRF data support previous interpretations that the sequence in the study area is overturned, and show that the basalts become increasingly evolved down-section, without significant structural repetition. ► The methods described here are readily applied, and have the potential to enhance the understanding of otherwise unclear stratigraphy and its control on mineralisation in many different deposits types.
We report new compositional and isotopic data for submarine lavas erupted on the Rumble V Ridge cross chain behind the Kermadec Arc at ∼36°S and for locally subducting sediment. In order to constrain ...cross‐arc changes in the melt source, Havre Trough ambient mantle wedge isotope and trace element characteristics are interpreted from regional back‐arc basalts that are relatively free of slab‐derived components. They have MORB‐like trace element concentrations and are isotopically “Pacific” but define greater heterogeneity in 206Pb/204Pb and 176Hf/177Hf than previously known within the Havre Trough. In contrast with the ambient mantle, all Rumble V Ridge lavas have trace element and isotopic characteristics consistent with subduction zone contributions despite their rear‐arc setting but are less fluid‐enriched than at the Kermadec volcanic front. A broad trend in Nd‐Hf isotopic space and the correlation between 176Hf/177Hf and Hf concentration anomaly for Rumble V Ridge lavas is explained by across‐arc changes in (1) the mantle wedge component, (2) the nature of the subduction component, and (3) the mass fraction of subduction components added. Samples from the distal cross chain tend to have lower 176Hf/177Hf at similar 143Nd/144Nd compared with samples closer to the arc, suggesting that a low‐176Hf/177Hf component is preferentially removed from the mantle wedge during trenchward advection. Isotope trends suggest that locally subducting sediment is the primary slab component for Rumble V Ridge magmas, but bulk mixing of ambient mantle with sediment or slab‐derived fluids cannot account for cross‐arc trace element ratios. Instead, cross‐chain isotope and trace element characteristics are explained by the addition of 0.05%–2.0% sediment melts where trace zircon, monazite, and rutile are residual. The cross chain tracks an east to west increase in the mass fraction of a common subduction component. A particularly enriched subset of eastern basalts is interpreted as being derived from the addition of an even higher mass fraction of a different subduction component with greater apparent stability of refractory trace phases during sediment melting. An implication of this study is that both Nd and Hf can be mobile in sediment‐rich subduction zones, but the relative mobility depends on the sediment composition and depth of melting, and the absolute mobility is small.
Bodies of water provide essentials for both human society as well as natural ecosystems. To expand the services this water provides, hybrid food-energy-water systems can be designed. This paper ...reviews the fields of floatovoltaic (FV) technology (water deployed solar photovoltaic systems) and aquaculture (farming of aquatic organisms) to investigate the potential of hybrid floatovoltaic-aquaculture synergistic applications for improving food-energy-water nexus sustainability. The primary motivation for combining electrical energy generation with aquaculture is to promote the dual use of water, which has historically high unused potential. Recent advances in FV technology using both pontoon and thin film structures provides significant flexibility in deployment in a range of water systems. Solar generated electricity provides off-grid aquaculture potential. In addition, several other symbiotic relationships are considered including an increase in power conversion efficiency due to the cooling and cleaning of module surfaces, a reduction in water surface evaporation rates, ecosystem redevelopment, and improved fish growth rates through integrated designs using FV-powered pumps to control oxygenation levels as well as LED lighting. The potential for a solar photovoltaic-aquaculture or aquavoltaic ecology was found to be promising. If a U.S. national average value of solar flux is used then current aquaculture surface areas in use, if incorporated with appropriate solar technology could account for 10.3% of total U.S. energy consumption as of 2016.
The western cordilleras of the Northern Andes (north of 5°S) are constructed from allochthonous terranes floored by oceanic crust. We present
40Ar/
39Ar and fission-track data from the Cordillera ...Occidental and Amotape Complex of Ecuador that probably constrain the time of terrane collision and post-accretionary tectonism in the western Andes. The data record cooling rates of 80–2 °C/my from temperatures of ∼540 °C, during 85 to 60 Ma, in a highly tectonised mélange (Pujilí unit) at the continent–ocean suture and in the northern Amotape Complex. The rates were highest during 85–80 Ma and decelerated towards 60 Ma. Cooling was a consequence of exhumation of the continental margin, which probably occurred in response to the accretion of the presently juxtaposing Pallatanga Terrane. The northern Amotape Complex and the Pujilí unit may have formed part of a single, regional scale, tectonic mélange that started to develop at ~85 Ma, part of which currently comprises the basement of the Interandean Depression. Cooling and rotation in the allochthonous, continental, Amotape Complex and along parts of the continent–ocean suture during 43–29 Ma, record the second accretionary phase, during which the Macuchi Island Arc system collided with the Pallatanga Terrane. Distinct periods of regional scale cooling in the Cordillera Occidental at ∼13 and ∼9 Ma were synchronous with exhumation in the Cordillera Real and were probably driven by the collision of the Carnegie Ridge with the Ecuador Trench. Finally, late Miocene–Pliocene reactivation of the Chimbo–Toachi Shear Zone was coincident with the formation of the oldest basins in the Interandean Depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since ∼6 Ma.
Climate change is altering patterns of precipitation, cryosphere thaw, and land-ocean influxes, affecting understudied Arctic estuarine tidal flats. These transitional zones between terrestrial and ...marine systems are hotspots for biogeochemical cycling, often driven by microbial processes. We investigated surface sediment bacterial community composition and function from May to September along a river-intertidal-subtidal-fjord gradient. We paired metabarcoding of in situ communities with in vitro carbon-source utilization assays. Bacterial communities differed in space and time, alongside varying environmental conditions driven by local seasonal processes and riverine inputs, with salinity emerging as the dominant structuring factor. Terrestrial and riverine taxa were found throughout the system, likely transported with runoff. In vitro assays revealed sediment bacteria utilized a broader range of organic matter substrates when incubated in fresh and brackish water compared to marine water. These results highlight the importance of salinity for ecosystem processes in these dynamic tidal flats, with the highest potential for utilization of terrestrially derived organic matter likely limited to tidal flat areas (and times) where sediments are permeated by freshwater. Our results demonstrate that intertidal flats must be included in future studies on impacts of increased riverine discharge and transport of terrestrial organic matter on coastal carbon cycling in a warming Arctic.
Oblique to strike geological segmentation in the Andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and ...continental plates, as well as the upper-plate expressions of differing along-strike subducted slab age, strength and composition. We present new white mica and biotite 40Ar/39Ar and zircon and apatite fission-track data from several traverses across the Cordillera Real of Ecuador in the northern Andes that reveal distinct along-strike differences in the timing of accelerated crustal cooling during the Cenozoic. The data record elevated cooling rates from temperatures of ∼380°C during ∼65–55 and ∼43–30 Ma from all sampled regions of the Cordillera Real and at ∼15 Ma and since ∼9 Ma in the northern Cordillera Real. Each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust. Elevated cooling rates of up to ∼30–20°C/Myr were initiated during the Palaeocene and Eocene–early Oligocene along the entire contemporaneous margin of Ecuador and were driven by the accretion of the oceanic Pallatanga Terrane and Piñon–Macuchi Block, respectively, onto northwestern South America. Both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the Caribbean Plateau and accreted onto the margin during the approximately northeastward migration of the Plateau into its current position. Within Ecuador the development of higher topography and elevated cooling rates of up to 50°C/Myr at ∼15 Ma and since ∼9 Ma are restricted to the region north of 1°30′S and is situated above the postulated subducted flat-slab section of the aseismic Carnegie Ridge. Plate convergence rate calculations suggest the Carnegie Ridge collided with the Ecuador Trench at ∼15 Ma, which caused the pre-existing coastal provinces to displace to the northeast, subsequently driving extension and marine ingressions in southern Ecuador and compression and uplift in northern Ecuador.
The American Stamp Goldblatt, Laura; Handler, Richard
2023, 2023-02-13
eBook
Examining the canon of nineteenth- and twentieth-century American stamps, Laura Goldblatt and Richard Handler show how postal iconography and material culture offer a window into the contested ...meanings and responsibilities of U.S. citizenship.
PDF
