The initial responses of hot liquid water released under low atmospheric pressures: Experimental insights

Experiments have been performed to simulate the shallow ascent and surface release of water and brines under low atmospheric pressure. Atmospheric pressure was treated as an independent variable and water temperature and vapor pressure were examined as a function of total pressure variation down to low pressures. The physical and thermal responses of water to reducing pressure were monitored with pressure transducers, temperature sensors and visible imaging. Data were obtained for pure water and for solutions with dissolved NaCl or CO₂. The experiments showed the pressure conditions under which the water remained liquid, underwent a rapid phase change to the gas state by boiling, and then solidified because of removal of latent heat. Liquid water is removed from phase equilibrium by decompression. Solid, liquid and gaseous water are present simultaneously, and not at the 611 Pa triple point, because dynamic interactions between the phases maintain unstable temperature gradients. After phase changes stop, the system reverts to equilibrium with its surroundings. Surface and shallow subsurface pressure conditions were simulated for Mars and the icy satellites of the outer Solar System. Freezing by evaporation in the absence of wind on Mars is shown to be unlikely for pure water at pressures greater than c. 670 Pa, and for saline solutions at pressures greater than c. 610 Pa. The physical nature of ice that forms depends on the salt content. Ice formed from saline water at pressures less than c. 610 Pa could be similar to terrestrial sea ice. Ice formed from pure water at pressures less than c. 100 Pa develops a low thermal conductivity and a ‘honeycomb’ structure created by sublimation. This ice could have a density as low as c. 450 kg m⁻³ and a thermal conductivity as low as 1.6 W m⁻¹ K⁻¹, and is highly reflective, more akin to snow than the clear ice from which it grew. The physical properties of ice formed from either pure or saline water at low pressures will act to reduce the surface temperature, and hence rate of sublimation, thereby prolonging the lifespan of any liquid water beneath.     

A nutrient loading budget for Biscayne Bay, Florida

The water quality in Biscayne Bay has been significantly affected by past and continuing coastal and watershed development. The nutrient concentrations in the Bay have been dramatically changed by the conversion of natural creeks and sheet flow freshwater inputs to rapid and episodic canal inputs from the large and rapidly expanding Miami metropolitan area. This study is an evaluation of nutrient loadings to Biscayne Bay for 1994-2002 from canal, atmospheric, and groundwater sources. Dissolved inorganic nitrogen (DIN, as nitrate, nitrite, and ammonium) and total phosphorus (TP) loadings by the canals were influenced by their geographic locations relative to discharge amount, watershed land use, stormwater runoff, and proximity to landfills. Annual budgets showed that canals contributed the bulk of N loading to the bay as 1687.2 metric ton N yr(-1) (88% total load). Direct atmospheric DIN load for Biscayne Bay was only 231.7 ton N yr(-1), based on surface area. Of the canal DIN load, nitrate+nitrite (NO(x)(-)) loading (1294.5 ton N yr(-1)) made up a much greater proportion than that of ammonium (NH(4)(+), 392.6 ton N yr(-1)). In the urbanized north and central Bay, canal DIN load was evenly split between NO(x)(-) and NH(4)(+). However, in the south, 95% of the DIN load was in the form of NO(x)(-), reflecting the more agricultural land use. Contrary to N, canals contributed the only 66% of P load to the bay (27.5 ton P yr(-1)). Atmospheric TP load was 14 ton Pyr(-1). In the north, canal P load dominated the budget while in the south, atmospheric load was almost double canal load. Groundwater inputs, estimated only for the south Bay, represented an important source of N and P in this zone. Groundwater input of N (141 ton N yr(-1)) was about equal to atmospheric load, while P load (5.9 ton P yr(-1)) was about equal to canal load.     

Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)

Geochemical studies of long-lived volcanic complexes are crucial for the understanding of the nature and composition of the subduction component of arc magmatism. The Pichincha Volcanic Complex (Northern Andean Volcanic Zone) consists of: (1) an old, highly eroded edifice, the Rucu Pichincha, whose lavas are mostly andesites, erupted from 1,100 to 150 ka; and (2) a younger, essentially dacitic, Guagua Pichincha composite edifice, with three main construction phases (Basal Guagua Pichincha, Toaza, and Cristal) which developed over the last 60 ka. This structural evolution was accompanied by a progressive increase of most incompatible trace element abundances and ratios, as well as by a sharp decrease of fluid-mobile to fluid-immobile element ratios. Geochemical data indicate that fractional crystallization of an amphibole-rich cumulate may account for the evolution from the Guagua Pichincha andesites to dacites. However, in order to explain the transition between the Rucu Pichincha andesites and Guagua Pichincha dacites, the mineralogical and geochemical data indicate the predominance of magma mixing processes between a mafic, trace-element depleted, mantle-derived end-member, and a siliceous, trace-element enriched, adakitic end-member. The systematic variation of trace element abundances and ratios in primitive samples leads us to propose that the Rucu Pichincha magmas came from a hydrous-fluid metasomatized mantle wedge, whereas Guagua Pichincha magmas are related to partial melting of a siliceous-melt metasomatized mantle. This temporal evolution implies a change from dehydration to partial melting of the slab, which may be associated with an increase in the geothermal gradient along the slab due to the presence of the subducted Carnegie Ridge at the subduction system. This work emphasizes the importance of studying arc-magma systems over long periods of time (of at least 1 million of years), in order to evaluate the potential variations of the slab contribution into the mantle source of the arc magmatism.     

Bromine cycle in subduction zones through in situ Br monitoring in diamond anvil cells

The geochemical partitioning of bromine between hydrous haplogranitic melts, initially enriched with respect to Br and aqueous fluids, has been continuously monitored in situ during decompression. Experiments were carried out in diamond anvil cells from 890 °C to room temperature and from 1.7 GPa to room pressure, typically from high P, T conditions corresponding to total miscibility (presence of a supercritical fluid). Br contents were measured in aqueous fluids, hydrous melts and supercritical fluids. Partition coefficients of bromine were characterized at pressure and temperature between fluids, hydrous melts and/or glasses, as appropriate: D^Br_fluid/melt = (Br)_fluid/(Br)_melt, ranges from 2.18 to 9.2 ± 0.5 for conditions within the ranges 0.66-1.7 GPa, 590-890 °C; and D^Br_fluid/glass = (Br)_fluid/(Br)_glass ranges from 60 to 375 at room conditions. The results suggest that because high pressure melts and fluids are capable of accepting high concentrations of bromine, this element may be efficiently removed from the slab to the mantle source of arc magmas. We show that Br may be highly concentrated in subduction zone magmas and strongly enriched in subduction-related volcanic gases, because its mobility is strongly correlated with that of water during magma degassing. Furthermore, our experimental results suggest that a non negligible part of Br present in the subducted slab may remain in the down-going slab, being transported toward the transition zone. This indicates that the Br cycle in subduction zones is in fact divided in two related but independent parts: (1) a shallower one where recycled Br may leave the slab with a water and silica-bearing “fluid” leading to enriched arc magmas that return Br to the atmosphere. (2) A deeper cycle where Br may be recycled back to the mantle maybe to the transition zone, where it may be present in high pressure water-rich metasomatic fluids.     

Meteoric Be in soil profiles - A global meta-analysis

In order to assess current understanding of meteoric ¹⁰Be dynamics and distribution in terrestrial soils, we assembled a database of all published meteoric ¹⁰Be soil depth profiles, including 104 profiles from 27 studies in globally diverse locations, collectively containing 679 individual measurements. This allows for the systematic comparison of meteoric ¹⁰Be concentration to other soil characteristics and the comparison of profile depth distributions between geologic settings. Percent clay, ⁹Be, and dithionite-citrate extracted Al positively correlate to meteoric ¹⁰Be in more than half of the soils where they were measured, but the lack of significant correlation in other soils suggests that no one soil factor controls meteoric ¹⁰Be distribution with depth. Dithionite-citrate extracted Fe and cation exchange capacity are only weakly correlated to meteoric ¹⁰Be. Percent organic carbon and pH are not significantly related to meteoric ¹⁰Be concentration when all data are complied.The compilation shows that meteoric ¹⁰Be concentration is seldom uniform with depth in a soil profile. In young or rapidly eroding soils, maximum meteoric ¹⁰Be concentrations are typically found in the uppermost 20 cm. In older, more slowly eroding soils, the highest meteoric ¹⁰Be concentrations are found at depth, usually between 50 and 200 cm. We find that the highest measured meteoric ¹⁰Be concentration in a soil profile is an important metric, as both the value and the depth of the maximum meteoric ¹⁰Be concentration correlate with the total measured meteoric ¹⁰Be inventory of the soil profile.In order to refine the use of meteoric ¹⁰Be as an estimator of soil erosion rate, we compare near-surface meteoric ¹⁰Be concentrations to total meteoric ¹⁰Be soil inventories. These trends are used to calibrate models of meteoric ¹⁰Be loss by soil erosion. Erosion rates calculated using this method vary based on the assumed depth and timing of erosional events and on the reference data selected.     

The geochemistry of primary and weathered oil shale and coquina across the Julia Creek vanadium deposit (Queensland, Australia)

A significant resource of vanadium and molybdenum exists near Julia Creek, Australia, where the middle Cretaceous organic-rich Toolebuc Formation lies between 0 and 25 m of the surface. We present and discuss a comprehensive geochemical study of the Toolebuc Formation and its enclosing stratigraphy near Julia Creek to understand this ore deposit. V and Mo contents in fresh facies are strongly associated with total organic carbon (TOC) contents, but not with Al or CaCO₃; this suggests that V and Mo were originally concentrated in the organic fraction. However, chemical extractions using H₂O₂ indicate that Mo was originally concentrated in pyrite. The data also suggest that V was mobilised from organic matter during early diagenesis and became associated with clays as little V was extracted by H₂O₂ in the fresh samples. TOC contents in the Toolebuc Formation were removed during weathering, residually enriching trace metals including V and Mo, and as a result, the TOC relationship with V and Mo disintegrates. With weathering, both V and Mo predominantly became associated with iron oxide/hydroxide phases (and possibly other unidentified phases) as these elements in the weathered facies were highly soluble in the sodium citrate–sodium dithionite digestion. Large shale-hosted V and Mo deposits such as Julia Creek offer a potentially viable alternative to the currently mined magnetite-hosted deposits. A thorough understanding of the formation and host mineral phases for V and Mo of these shale deposits, however, is critical to ensure that these valuable metals can be feasibly extracted.     

Holocene paleoenvironmental changes inferred from diatom assemblages in sediments of Kusawa Lake, Yukon Territory, Canada

The southwest Yukon Territory, Canada, is an important region for recovering sensitive records of Holocene paleoclimatic change. More information is needed, however, to constrain the timing of the major Holocene climatic transitions, and to understand associated impacts on different ecosystems. For example, paleolimnological studies have focused on small lakes and ponds, but the history of large lakes has received little study. We analyzed diatom assemblages, species richness, valve concentrations, and biogenic silica, in the sediments of Kusawa Lake (60°16.5'N; 136°10.9'W; 671 m a.s.l.) to reconstruct the responses of this large (surface area = 142 km²), deep (Z_max = 135 m) freshwater ecosystem to Holocene climatic transitions. Diatoms colonized the lake soon after ice retreat, around 11,000 cal yr BP; assemblages throughout the record were dominated by planktonic types. Diatom concentrations and biogenic silica were high during the Holocene Thermal Maximum between 10,700 and 7300 cal yr BP, then began to decrease in response to cooling associated with orbitally driven reductions in insolation. Diatom assemblages shifted towards taxa with lower surface water temperature optima after 8300 cal yr BP, perhaps in response to abrupt and progressive cooling. Our study confirms that diatom assemblages in large lakes are sensitive to regional-scale paleoclimatic changes.     

Chemostratigraphy of the Tamengo Formation (Corumbá Group,Brazil): A contribution to the calibration of the Ediacaran carbon-isotope curve

The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most complete Ediacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution in southwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids (Vendotaenia, Eoholynia), soft-bodied metazoans (Corumbella) and skeletal fossils (Cloudina, Titanotheca). The Tamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostratigraphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the Tamengo Formation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopic analyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive δ¹³C excursion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to −3.5‰ associated with a marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateau characterized by δ¹³C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upper part of the δ¹³C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age ( Babinski et al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excursion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), the Dengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negative excursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly, although δ¹³C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur just beneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the origin of these breccias is a glacioeustatic sea-level fall, but a tectonic interpretation cannot be completely ruled out.     

Rapid growth of an Archean continent by arc magmatism

The voluminous Meso- to Neoarchean rocks exposed in the Beartooth Mountains of the northern Wyoming Province of western North America comprise the Long Lake Magmatic Complex (LLMC), a variably metamorphosed and deformed association of igneous and meta-igneous plutonic rocks with SiO₂ ranging from at least 52 to 78 wt%. Within this compositional range, rock types include lineated amphibolites to hornblende-bearing gneisses of intermediate composition and multiple generations of foliated to unfoliated granitoids. Emplacement ages range between approximately 2.79 and 2.83 Ga, based on U–Pb zircon geochronology (SHRIMP). Field relations, elemental compositions, and geochronology indicate that these rocks do not represent a single fractional crystallization sequence, but rather, the LLMC was constructed by injection of numerous, discrete magmas as sill-like bodies over an ∼40 Ma period. Although there is a continuum of compositions in the LLMC, trace element abundances can be used to distinguish distinct sources and petrogenetic processes that can be broadly extrapolated to at least 3 compositional groupings: (1) trondhjemitic to granitic intrusive rocks with SiO₂ >70 wt%, (2) variably metamorphosed granodioritic orthogneisses with SiO₂ between 63 and 70 wt%, and (3) amphibole-bearing mafic to intermediate gneisses with SiO₂ between 52 and 63 wt%. Despite the range of SiO₂ contents, maximum LREE abundances are similar across the compositional range and, consequently, exhibit a wide range of (La/Yb)_n ratios (∼20–130). All LLMC rocks share a relative depletion in HFSE abundances similar to modern convergent margin magmas. Initial Sr and Nd isotopic compositions across the compositional range are consistently offset from typical bulk silicate earth (BSE) values and preclude unaltered derivation from primitive or depleted mantle. Common Pb isotopic data define a single array that lies above model crustal growth curves and, along with the Nd and Sr data, suggest relatively uniform interaction with, or derivation from, older lithosphere. The combined isotopic and elemental data suggest the LLMC resulted from simultaneous, rapid, and voluminous production of diverse magmas that represent melting of isotopically similar, but compositionally distinct, crustal and mantle sources. Dynamically, Meso- to Neo-archean crustal growth in the northern Wyoming Province appears to require an environment similar to a modern ocean–continent convergent margin with a comparable rate of crustal production and diversity of magma series. The resultant crust and associated mantle lithosphere (keel) appear to have suffered little-to-no modification prior to Laramide (Cretaceous) uplift and exposure.     

Aquifer Storage Recovery (ASR) of chlorinated municipal drinking water in a confined aquifer

About 1.02 × 10⁶ m³ of chlorinated municipal drinking water was injected into a confined aquifer, 94–137 m below Roseville, California, between December 2005 and April 2006. The water was stored in the aquifer for 438 days, and 2.64 × 10⁶ m³ of water were extracted between July 2007 and February 2008. On the basis of Cl⁻ data, 35% of the injected water was recovered and 65% of the injected water and associated disinfection by-products (DBPs) remained in the aquifer at the end of extraction. About 46.3 kg of total trihalomethanes (TTHM) entered the aquifer with the injected water and 37.6 kg of TTHM were extracted. As much as 44 kg of TTHMs remained in the aquifer at the end of extraction because of incomplete recovery of injected water and formation of THMs within the aquifer by reactions with free-chlorine in the injected water. Well-bore velocity log data collected from the Aquifer Storage Recovery (ASR) well show as much as 60% of the injected water entered the aquifer through a 9 m thick, high-permeability layer within the confined aquifer near the top of the screened interval. Model simulations of ground-water flow near the ASR well indicate that (1) aquifer heterogeneity allowed injected water to move rapidly through the aquifer to nearby monitoring wells, (2) aquifer heterogeneity caused injected water to move further than expected assuming uniform aquifer properties, and (3) physical clogging of high-permeability layers is the probable cause for the observed change in the distribution of borehole flow. Aquifer heterogeneity also enhanced mixing of native anoxic ground water with oxic injected water, promoting removal of THMs primarily through sorption. A 3 to 4-fold reduction in TTHM concentrations was observed in the furthest monitoring well 427 m downgradient from the ASR well, and similar magnitude reductions were observed in depth-dependent water samples collected from the upper part of the screened interval in the ASR well near the end of the extraction phase. Haloacetic acids (HAAs) were completely sorbed or degraded within 10 months of injection.