| 1. |
- Fahnestock, M. F., et al.
(författare)
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Mercury reallocation in thawing subarctic peatlands
- 2019
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Ingår i: Geochemical perspectives letters. - : European Association of Geochemistry. - 2410-339X .- 2410-3403. ; 11, s. 33-38
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Tidskriftsartikel (refereegranskat)abstract
- Warming Arctic temperatures have led to permafrost thaw that threatens to release previously sequestered mercury (Hg) back into the environment. Mobilisation of Hg in permafrost waters is of concern, as Hg methylation produced under water-saturated conditions results in the neurotoxin, methyl Hg (MeHg). Thawing permafrost may enhance Hg export, but the magnitude and mechanisms of this mobilisation within Arctic ecosystems remain poorly understood. Such uncertainty limits prognostic modelling of Hg mobilisation and impedes a comprehensive assessment of its threat to Arctic ecosystems and peoples. Here, we address this knowledge gap through an assessment of Hg dynamics across a well-studied permafrost thaw sequence at the peak of the growing season in biologically active peat overlying permafrost, quantifying total gaseous mercury (TGM) fluxes, total mercury (Hg-Tot) in the active layer peat, porewater MeHg concentrations, and identifying microbes with the potential to methylate Hg. During the initial thaw, TGM is liberated, likely by photoreduction from permafrost where it was previously stored for decades to centuries. As thawing proceeds, TGM is largely driven by hydrologic changes as evidenced by Hg accumulation in water-logged, organic-rich peat sediments in fen sites. MeHg in porewaters increase across the thaw gradient, a pattern coincident with increases in the relative abundance of microbes possibly containing genes allowing for methylation of ionic Hg. Findings suggest that under changing climate, frozen, well-drained habitats will thaw and collapse into saturated landscapes, increasing the production of MeHg and providing a significant source of the toxic, bioaccumulative contaminant.
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| 2. |
- Gislason, S.R., et al.
(författare)
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Environmental pressure from the 2014–15 eruption of Bárðarbunga volcano, Iceland
- 2015
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Ingår i: Geochemical Perspectives Letters. - : European Association of Geochemistry. - 2410-3403 .- 2410-339X. ; 1:2015, s. 84 - 93
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Tidskriftsartikel (refereegranskat)abstract
- The effusive six months long 2014-2015 Bárðarbunga eruption (31 August-27 February) was the largest in Iceland for more than 200 years, producing 1.6 ± 0.3 km3 of lava. The total SO2 emission was 11 ± 5 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO2 exceeded the 350 μg m−3 hourly average health limit over much of Iceland for days to weeks. Anomalously high SO2 concentrations were also measured at several locations in Europe in September. The lowest pH of fresh snowmelt at the eruption site was 3.3, and 3.2 in precipitation 105 km away from the source. Elevated dissolved H2SO4, HCl, HF, and metal concentrations were measured in snow and precipitation. Environmental pressures from the eruption and impacts on populated areas were reduced by its remoteness, timing, and the weather. The anticipated primary environmental pressure is on the surfacewaters, soils, and vegetation of Iceland.
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| 3. |
- Guilbaud, R., et al.
(författare)
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Oxygen minimum zones in the early Cambrian ocean
- 2018
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Ingår i: Geochemical Perspectives Letters. - : European Association of Geochemistry. - 2410-339X .- 2410-3403. ; 6, s. 33-38
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Tidskriftsartikel (refereegranskat)abstract
- The relationship between the evolution of early animal communities and oceanic oxygen levels remains unclear. In particular, uncertainty persists in reconstructions of redox conditions during the pivotal early Cambrian (541-510 million years ago, Ma), where conflicting datasets from deeper marine settings suggest either ocean anoxia or fully oxygenated conditions. By coupling geochemical palaeo-redox proxies with a record of organic-walled fossils from exceptionally well-defined successions of the early Cambrian Baltic Basin, we provide evidence for the early establishment of modern-type oxygen minimum zones (OMZs). Both inner- and outer-shelf environments were pervasively oxygenated, whereas mid-depth settings were characterised by spatially oscillating anoxia. As such, conflicting redox signatures recovered from individual sites most likely derive from sampling bias, whereby anoxic conditions represent mid-shelf environments with higher productivity. This picture of a spatially restricted anoxic wedge contrasts with prevailing models of globally stratified oceans, offering a more nuanced and realistic account of the Proterozoic-Phanerozoic ocean transition.
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| 4. |
- Vermeesch, P., et al.
(författare)
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A genetic classification of the tholeiitic and calc-alkaline magma series
- 2021
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Ingår i: Geochemical Perspectives Letters. - : European Association of Geochemistry. - 2410-339X .- 2410-3403. ; 19, s. 1-6
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Tidskriftsartikel (refereegranskat)abstract
- The concept of the ‘magma series’ and the distinction between alkaline, calc-alkaline and tholeiitic trends has been a cornerstone in igneous petrology since the early 20th century, and encodes fundamental information about the redox state of divergent and convergent plate tectonic settings. We show that the ‘Bowen and Fenner trends’ that characterise the calc-alkaline and tholeiitic types of magmatic environments can be approximated by a simple log ratio model based on three coupled exponential decay functions, for A = Na2O + K2O, F = FeOT and M = MgO, respectively. We use this simple natural law to define a ‘Bowen-Fenner Index’ to quantify the degree to which an igneous rock belongs to either magma series. Applying our model to a data compilation of igneous rocks from Iceland and the Cascade Mountains effectively separates these into tholeiitic and calc-alkaline trends. However the simple model fails to capture the distinct dog-leg that characterises the tholeiitic log ratio evolution, which can be attributed to the switch from ferrous to ferric iron-bearing minerals. Parameterising this switch in a two stage magma evolution model results in a more accurate fit to the Icelandic data. The same two stage model can also be fitted in A–T–M space, where ‘T’ stands for TiO2. This produces a new way to identify calc-alkaline and tholeiitic rocks that does not require the conversion of FeO and Fe2O3 to FeOT. Our results demonstrate that log ratio analysis provides a natural way to parameterise physical processes that give rise to these magma series.
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| 5. |
- Vuillemin, A., et al.
(författare)
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Authigenic minerals reflect microbial control on pore waters in a ferruginous analogue
- 2023
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Ingår i: Geochemical Perspectives Letters. - : European Association of Geochemistry. - 2410-339X .- 2410-3403. ; 28, s. 20-26
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Tidskriftsartikel (refereegranskat)abstract
- Ferruginous conditions prevailed in the oceans through much of Earth's history. However, minerals recording these conditions remain difficult to interpret in terms of biogeochemical processes prior to lithification. In Lake Towuti, Indonesia, ferruginous sediments are deposited under anoxic sulfate-poor conditions similar to the Proterozoic oceans, allowing the study of mineralogical (trans)formations during microbial diagenesis.Comprehensive pore water geochemistry, high resolution geochemical core profiles, and electron microscopy of authigenic minerals revealed in situ formation of magnetite, millerite, and abundant siderite and vivianite along a 100 m long sequence. Framboidal magnetites represent primary pelagic precipitates, whereas millerite, a sulfide mineral often overlooked under sulfate-poor conditions, shows acicular aggregates entangled with siderite and vivianite resulting from saturated pore waters and continuous growth during burial. These phases act as biosignatures of microbial iron and sulfate reduction, fermentation and methanogenesis, processes clearly traceable in pore water profiles.Variability in metal and organic substrates attests to environment driven processes, differentially sustaining microbial processes along the stratigraphy. Geochemical profiles resulting from microbial activity over 200 kyr after deposition provide constraints on the depth and age of mineral formation within ferruginous records.
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| 6. |
- Smit, Mattijs, et al.
(författare)
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Formation of Archean continental crust constrained by boron isotopes
- 2019
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Ingår i: Geochemical Perspective Letters. - : European Association of Geochemistry. - 2410-3403. ; , s. 23-26
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Tidskriftsartikel (refereegranskat)abstract
- The continental crust grew and matured compositionally during the Palaeo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11B/10B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicatedifferentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The B- and 11B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga.
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