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The Devonian to Carboniferous (DC) transition coincided with a green-to-ice house climatic shift, anoxia, disappearance of lower latitude carbonate banks, and turnover from warm-to-cool water carbonate factories. In western Laurentia, the switch to carbonate factories dominated by cool-water biota was contemporaneous with a tectonically driven palaeogeographic change. To investigate this depositional shift and infer the relative impact of climate vs tectonics, a continental-scale sedimentological and geochemical study was conducted on twelve stratigraphic sections of DC strata from western Canada to southern Nevada (USA). The spatial–temporal distribution of microfacies records the turnover from [i] a Famennian lime mud-rich, shallow warm-water carbonate ramp with low sedimentation rates, mesotrophic conditions and tabular geometry to [ii] Tournaisian to Viséan lime mud-depleted and grainstone dominated cool-water carbonate ramp with anomalous high sedimentation rates, oligotrophic conditions and a pronounced slope. Positive excursions of δ18Ocarb (+ 2‰ V-PDB) and δ13Ccarb (+ 4‰ V-PDB) of Lower Mississippian carbonates likely correspond to the first cooling peak of the Carboniferous-Permian icehouse climate, following carbon withdrawal during black shale deposition during the late Famennian and early Tournaisian. However, late Tournaisian return of photozoan elements and their persistence throughout the Viséan suggests that warmer surface water existed, revealing a decoupling of the lower latitude ocean and the atmosphere. Shoaling of the thermocline was likely a result of cold-water upwelling along an open coast, as the Antler orogen no longer provided an oceanic obstruction to the west. This study shows that carbonate platforms are more susceptible to regional changes than global shifts. [ABSTRACT FROM AUTHOR]

• Non-zero I/(Ca + Mg) values were detected in Archean and Paleoproterozoic shallow marine carbonates. • The samples are from several locations and range in age from 2.93 to 2.31 Ga. • These non-zero values indicate the original localized presence of dissolved marine oxygen. Marine oxygenation during the Archean and Paleoproterozoic is poorly constrained. Iodine in carbonate is a possible oxygen indicator. We obtained molar ratios of iodine to calcium-magnesium [I/(Ca + Mg)] for 2.93–2.31 Ga from shallow marine Ca-carbonate (limestone, dolostone) platform sediments at several locations in the southern Superior Province in Canada and the USA. Maximum I/(Ca + Mg) ratios obtained are 0.13, 0.26, 0.05 and 0.22 μmol/mol, for 2.93, 2.86, 2.80 and 2.31 Ga samples, respectively. These non-zero carbonate I/(Ca + Mg) ratios indicate localized oxygen in Archean and Paleoproterozoic shallow marine environments. These data support an early origin for biological oxygen-production and, specifically, the idea that mid-late Archean oxygenation was sufficient to facilitate Ca-carbonate precipitation in shallow marine 'oxygen oases' adjacent to anoxic iron-rich seas [ABSTRACT FROM AUTHOR]

• Ca isotopes do not fractionate during magmatic differentiation of carbonate-rich magmas. • Carbonated silicate melts have δ44/40Ca lower than MORB. • K-richterite and carbonate in mantle sources produce low δ44/40Ca values of magmas. • Ca isotopes are a robust tracer of volatile-induced mantle lithological variation. The volatile components CO 2 and H 2 O induce mantle melting and thus exert major controls on mantle heterogeneity. Primitive intraplate alkaline magmatic rocks are the closest analogues for incipient mantle melts and provide the most direct method to assess such mantle heterogeneity. Given the considerable Ca isotope differences among carbonate, clinopyroxene, garnet, and orthopyroxene in the mantle (up to 1 ‰ for δ44/40Ca), δ44/40Ca of alkaline rocks is a promising tracer of lithological heterogeneity. We present stable Ca isotope data for ca. 1.4 Ga lamproites, 590–555 Ma ultramafic lamprophyres and carbonatites, and 142 Ma nephelinites from Aillik Bay in Labrador, eastern Canada. These primitive alkaline rock suites are the products of three stages of magmatism that accompanied lithospheric thinning and rifting of the North Atlantic craton. The three discrete magmatic events formed by melting of different lithologies in a metasomatized lithospheric mantle column at various depths: (1) MARID-like components (mica-amphibole-rutile-ilmenite-diopside) in the source of the lamproites; (2) phlogopite-carbonate veins were an additional source component for ultramafic lamprophyres during the second event; and (3) wehrlites at shallower depths were an important source component for nephelinites during the final event. The Mesoproterozoic lamproites show lower δ44/40Ca values (0.58 to 0.66 ‰) than MORBs (0.84 ± 0.03 ‰, 2se). This cannot be explained by fractional crystallization or melting of the clinopyroxene-dominated source but can be attributed to a source enriched in the alkali amphibole K-richterite, which has characteristically low δ44/40Ca. The δ44/40Ca values of the ultramafic lamprophyre suite during the second rifting stage are remarkably uniform, with overlapping ranges for primary carbonated silicate melts (aillikite: 0.67 to 0.75 ‰), conjugate carbonatitic liquids (0.71 to 0.82 ‰) and silicate-dominated damtjernite liquid (primary damtjernite: 0.68 to 0.72 ‰). This suggests negligible Ca isotope fractionation during liquid immiscibility of carbonate-bearing magmas. Combined with previously reported δ44/40Ca values for carbonatites and kimberlites, our data suggest that carbonated silicate melts in Earth's mantle have δ44/40Ca compositions resolvably lower than those for MORBs (0.74 ± 0.02 ‰ versus 0.84 ± 0.03 ‰, 2se). The δ44/40Ca values of the Cretaceous nephelinites (0.72 to 0.78 ‰) are homogenous and similar to those of the 590–555 Ma ultramafic lamprophyres, suggesting that the wehrlitic source component for the nephelinites formed by mantle metasomatism during interaction with rising aillikite magmas during the second rifting stage. Our results highlight that both K-richterite and carbonate components in mantle sources can result in the systematically low δ44/40Ca values of alkaline magmas, which may explain previously reported low δ44/40Ca values of alkaline rocks and some carbonatites. Our study indicates that Ca isotopes are a robust tracer of lithological variation caused by volatiles in the Earth's upper mantle. [ABSTRACT FROM AUTHOR]

• Field observations and equilibrium modeling indicate three coeval and confluent water masses associated with mid-Archean carbonate platforms. • Chemocline mixing zones separated offshore anoxic Fe-carbonate precipitation from onshore oxygenated Ca-carbonate precipitation. • Oxygenic Fe2+ removal at chemocline prompted aragonite, followed by calcite, precipitation. The removal of dissolved iron by oxygen was a key control on marine Ca-carbonate precipitation during the Archean, but many details remain poorly resolved. We examined thick carbonate platforms, ∼2.9–2.8 Ga, in the western Superior Province of Canada. Interlayered Fe and Ca-carbonate minerals suggest platform margin precipitation, both as seafloor crusts and in the water column, during mixing of anoxic offshore and oxygenated onshore water masses in dynamic chemoclines. Platform interiors were subject to short-lived influxes of Fe-rich anoxic offshore waters that variously precipitated aragonite, herringbone calcite, Fe-dolomite, and rare siderite. Overall, these observations are consistent with chemical equilibrium analysis assuming three co-existing confluent carbonate water masses: (i) offshore anoxic iron-rich seawater, precipitating Fe-carbonate, (ii) a chemocline mixing zone variously precipitating interlayered aragonite, calcite and Fe oxide, (iii) onshore iron-poor seawater precipitating calcite and aragonite in an 'Oxygen Oasis'. In this system, Fe2+, at ∼ 3 orders of magnitude lower level, supersedes the effect of Mg2+ on Ca-carbonate mineralogy. Only when sufficient Fe2+ was removed by oxygenation did the relative concentrations of Mg2+ and Ca2+ determine CaCO 3 mineralogy. Platform margin chemoclines created by oxygen production likely persisted throughout the Archean. [ABSTRACT FROM AUTHOR]

Abstract: The government of British Columbia and Spectra Energy are partnering to explore the potential of CCS in British Columbia to reduce emissions from Spectra Energy’s Fort Nelson gas processing facility. This work is part of a site characterization study, which aims to assess the viability of such a project in the province. Dissolution rates of Keg River carbonates immersed in Keg River brines collected in May 2009 were measured. Experimental conditions simulating injection and formation conditions were CO2 at 50 bar and 105 °C. Results are higher but within an order of magnitude of literature values at similar pressure temperature conditions with values of Log R up to −8.45 mol⋅cm−2⋅s−1. Experiments showed a strong effect of rock and fluid composition on reaction kinetics. The same experimental materials and conditions but using a synthetic 0.1M NaCl brine produced elevated reaction rates, potentially due to a lack of compositional inhibition that are caused by more complex mixtures. A full reactive transport simulation using these results would help constrain the spatial extent dissolution throughout the saline formation being considered for injection. [Copyright &y& Elsevier]

Abstract: The relict Holocene Hot Creek carbonate spring deposit in southeast British Columbia is characterized by excellent preservation of soft-tissue organisms (e.g. cyanobacteria), but poor preservation of organisms with hard-tissue (e.g. wood, diatoms). The deposit is formed mainly of calcified cyanobacteria, with fewer mineralized macrophytes (plants), bryophytes (mosses), wood, and diatoms. Cyanobacteria grew as solitary filaments (Lyngbya) and as radiating hemispherical colonies (Rivularia). Both were preserved by encrustation and encapsulation while alive, and as casts after filament death and decay. Sheath impregnation was rare to absent. Filament encrustation, whereby calcite crystals nucleated on, and grew away from the sheath exterior, produced moulds that replicated external filament morphology, but hastened filament decay. Filament encapsulation, whereby calcite nucleated in the vicinity of, and grew towards the encapsulated filament, promoted sheath preservation even after trichome decay. Subsequent calcite precipitation inside the hollow sheath generated sheath casts. The inability of mineralizing spring water to penetrate durable cell walls meant that bryophytes, macrophytes, and most wood was preserved by encrustation. Some wood resisted complete decay for several thousand years, and its lignified cell walls allowed rare permineralizations. Diatoms were not preserved in the relict deposit because the frustules were dissolved by the basic spring water. Amorphous calcium carbonate produced by photosynthetic CO2 removal may have acted as nucleation sites for physicochemically precipitated calcite. Thus, metabolic activities of floral organisms probably initiated biotic mineralization, but continuous inorganic calcite precipitation on and in flora ensured that soft tissues were preserved. [Copyright &y& Elsevier]

The mineral thaumasite (CaSiO3 · CaCO3 · CaSO4 · 15H2O) has been observed by the authors in a number of concrete samples including (i) an 80 year-old aqueduct in Manitoba, (ii) a 33 year-old pavement in Ontario, (iii) test samples exposed to marine (tidal) conditions, and (iv) laboratory samples exposed to wet–dry cycles in sulphate solution. The source of carbonate differed for these cases being variously derived from de-dolomitization of dolostone aggregate, carbonate ions in the seawater, or atmospheric CO2 through the process of carbonation. In some cases the thaumasite mineral was found in close association with the mineral ettringite (3CaO · Al2O3 · 3CaSO4 · 31H2O).The paper discusses the results of detailed analyses using optical and electron microscopy. Although the mineral thaumasite can be readily identified by these techniques, it is possible that the thaumasite form of sulphate attack is frequently misdiagnosed as conventional sulphate attack due to the similarities in crystal structure and appearance of the minerals thaumasite and ettringite. [Copyright &y& Elsevier]

Among methods for determining the abundance of carbonate minerals insolid samples, dissolution in acid is appealing because it is cheap and can be quick, but it suffers from several sources of uncertainty.The weight loss due to evaporation of water can be corrected for easily by measuring the weight loss from blank vessels as well as samplevessels. However, a further correction, for excess evaporation due to heat liberated by dissolution, is desirable; we show how to calculate it. Certain puzzling results point to a need for further investigation of interfering factors. [ABSTRACT FROM AUTHOR]