Geochemical Perspectives Letters a journal of the European Association of Geochemistry
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  • Short articles (3000 words, all inclusive)
  • Non-profit community journal
  • Highest-quality research in geochemical sciences

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 Top 10 most viewed articles (cumulative count of HTML views) for the last 60 days.

Microplastics contaminate the deepest part of the world’s ocean

Abstract:
Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem.

X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai

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Geochem. Persp. Let. (2018) 9, 1–5 | doi: 10.7185/geochemlet.1829 | Published 27 November 2018

Article views: 744

Sea level rise produces abundant organobromines in salt-affected coastal wetlands

Abstract:
Global sea level rise exposes terrestrially derived natural organic matter to elevated salinities, which may alter the complex biogeochemical cycling of halogens in coastal wetland sediments. Here we show that sea level rise increases the natural production of organobromines in submerged soils and wetland sediments. We compared the concentrations and speciation of sedimentary chlorine and bromine along a salinity gradient in low-lying coastal forested wetlands in Winyah Bay (South Carolina, United States). Sorption differences between chloride and bromide were not observed, but up to 80 % of total retained bromine is organically bound, with the highest fraction of organically bound bromine found in formerly freshwater wetlands inundated by seawater. Wet/dry cycling of soils and the abundance of aromatic-rich natural organic matter in these salt-affected dieback forested wetlands promote bromination of organic matter, as demonstrated by laboratory simulations. Bromination of soil organic matter caused by continued sea level rise thus may be a major source of organobromines in coastal environments and possibly volatile halomethanes.

C. Joe-Wong, D.R. Schlesinger, A.T. Chow, S.C.B. Myneni

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Geochem. Persp. Let. (2019) 10, 31–35 | doi: 10.7185/geochemlet.1911 | Published 23 April 2019

Article views: 328

Calcium stable isotopes place Devonian conodonts as first level consumers

Abstract:
Conodont animals are an extinct group of jawless vertebrates whose hard parts, also known as conodont elements, represent the earliest evidence of a mineralised skeleton in the vertebrate lineage. Conodont elements are interpreted as parts of a feeding apparatus, which together with the presence of eyes and microwear patterns, support the controversial hypothesis that conodont animals were macrophagous predators and/or scavengers. Here, we explore the trophic position of five conodont genera (Palmatolepis, Polygnathus, Ancyrodella, Ancyrognathus and Icriodus) from five contemporary Late Devonian sites distributed worldwide (France, Morocco, Vietnam and Australia) by means of calcium (Ca) stable isotope compositions. The seawater Ca isotope composition was calibrated using contemporary Late Devonian brachiopod isotopic values. By comparison with extant marine trophic chain composed of cartilaginous fish, conodont Ca isotope compositions are indicative of a zooplanktivore - primary piscivore niche, with no genus-specific trophic pattern. The question of active predation or scavenging cannot be resolved definitively but our results strongly suggest that Late Devonian conodonts were first level consumers.

V. Balter, J.E. Martin, T. Tacail, G. Suan, S. Renaud, C. Girard

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Geochem. Persp. Let. (2019) 10, 36–39 | doi: 10.7185/geochemlet.1912 | Published 26 April 2019

Article views: 321

Equilibrium olivine-melt Mg isotopic fractionation explains high δ26Mg values in arc lavas

Abstract:
We determined equilibrium Mg isotope fractionation between olivine and melt (Δ26/24MgOl/melt) in five, naturally quenched, olivine-glass pairs that were selected to show clear textural and chemical evidence of equilibration. We employed a high-precision, critical mixture double-spiking approach to obtain a weighted mean of Δ26/24MgOl/melt = −0.071 ± 0.010 ‰, for values corrected to a common olivine-glass temperature of 1438 K. As function of temperature, the fractionation can be expressed as Δ26/24MgOl/melt = (−1.46 ± 0.26) × 105/T2. The samples analysed have variable H2O content from 0.1 to ∼1.2 wt. %, yet no discernible difference in Δ26/24MgOl/melt was evident. We have used this Δ26/24MgOl/melt to revisit the puzzling issue of elevated Mg isotope ratios in arc lavas. In new Mg isotope data on sample suites from the Lesser Antilles and Mariana arcs, we show that primitive samples have MORB-like Mg isotope ratios while the evolved samples tend to have isotopically heavier compositions. The magnitude of this variability is well explained by olivine fractionation during magmatic differentiation as calculated with our new equilibrium Δ26/24MgOl/melt.

X.-N. Liu, R.C. Hin, C.D. Coath, M. van Soest, E. Melekhova, T. Elliott

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Geochem. Persp. Let. (2022) 22, 42–47 | doi: 10.7185/geochemlet.2226 | Published 19 July 2022

Article views: 311

Carbon isotopic signatures of super-deep diamonds mediated by iron redox chemistry

Abstract:
Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases influences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compositions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.

J. Liu, W. Wang, H. Yang, Z. Wu, M.Y. Hu, J. Zhao, W. Bi, E.E. Alp, N. Dauphas, W. Liang, B. Chen, J.-F. Lin

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Geochem. Persp. Let. (2019) 10, 51–55 | doi: 10.7185/geochemlet.1915 | Published 24 May 2019

Article views: 310

New insights into the plumbing system of Santorini using helium and carbon isotopes

Abstract:
We have analysed He, Ne and CO2 concentrations and isotopic ratios in seawater samples collected using a ROV above hydrothermal fields and in CO2-accumulating pools within Santorini caldera (Greece). Our results provide new insights on the plumbing and magmatic sources of this volcano, which is critical to understand and better predict its volcanic eruptions, and its relationships to nearby volcanic systems. The 3He/4He ratio is as high as 6.54 times the atmospheric ratio, and significantly higher than any other samples from Santorini, including the active eruptive centre of Nea Kameni (~3.6 Ra). The 3He/4He ratio, corrected for atmospheric helium dissolved in seawater is ~7 Ra, a value similar to that measured at the nearby Kolumbo submarine volcano crater. The fluids display typical mantle signatures never recorded before in the active centres of the island of Santorini, in which fluids and gases are contaminated at shallow depths. The helium isotopic signature suggests that the source of volatiles for these two volcanoes is similar to the MORB source instead of being the subcontinental lithospheric mantle as suggested previously.

M. Moreira, J. Escartin, L. Scelin, L. Ruzié-Hamilton, P. Nomikou, C. Mével, M. Andreani

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Geochem. Persp. Let. (2019) 10, 46–50 | doi: 10.7185/geochemlet.1914 | Published 20 May 2019

Article views: 308

Thermodynamic controls on redox-driven kinetic stable isotope fractionation

Abstract:
Stable isotope fractionation arising from redox reactions has the potential to illuminate the oxygenation of Earth’s interior, oceans, and atmosphere. However, reconstruction of past and present redox conditions from stable isotope signatures is complicated by variable fractionations associated with different reduction pathways. Here we demonstrate a linear relationship between redox-driven kinetic fractionation and the standard free energy of reaction for aqueous chromium(VI) reduction by iron(II) species. We also show that the intrinsic kinetic fractionation factor is log-linearly correlated with the rate constant of reaction, which is in turn a function of the free energy of reaction. The linear free energy relationship for kinetic fractionation describes both our experimental results and previous observations of chromium isotope fractionation and allows the magnitude of fractionation to be directly linked to environmental conditions such as pH and oxygen levels. By demonstrating that the magnitude of kinetic fractionation can be thermodynamically controlled, this study systematically explains the large variability in chromium(VI) isotope fractionation and provides a conceptual framework that is likely applicable to other isotope systems.

C. Joe-Wong, K.L. Weaver, S.T. Brown, K. Maher

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Geochem. Persp. Let. (2019) 10, 20–25 | doi: 10.7185/geochemlet.1909 | Published 29 March 2019

Article views: 302

Exchange catalysis during anaerobic methanotrophy revealed by 12CH2D2 and 13CH3D in methane

Abstract:
The anaerobic oxidation of methane (AOM) is a crucial component of the methane cycle, but quantifying its role in situ under dynamic environmental conditions remains challenging. We use sediment samples collected during IODP Expedition 347 to the Baltic Sea to show that relative abundances of 12CH2D2 and 13CH3D in methane remaining after microbial oxidation are in internal, thermodynamic isotopic equilibrium, and we attribute this phenomenon to the reversibility of the initial step of AOM. These data suggest that 12CH2D2 and 13CH3D together can identify the influence of anaerobic methanotrophy in environments where conventional bulk isotope ratios are ambiguous, and these findings may lead to new insights regarding the global significance of enzymatic back reaction in the methane cycle.

J.L. Ash, M. Egger, T. Treude, I. Kohl, B. Cragg, R.J. Parkes, C.P. Slomp, B. Sherwood Lollar, E.D. Young

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Geochem. Persp. Let. (2019) 10, 26–30 | doi: 10.7185/geochemlet.1910 | Published 15 April 2019

Article views: 301

Ultrahigh pressure structural changes in a 60 mol. % Al2O3-40 mol. % SiO2 glass

Abstract:
Structure of an Al-containing silicate glass (60 mol. % Al2O3–40 mol. % SiO2, A40S) is investigated up to 131 GPa, a pressure close to that of the Earth’s core-mantle boundary, by using our recently developed double stage large volume cell. The first peak (r1) of the pair distribution function, which corresponds to T–O distance (T = Al, Si), rapidly increases below 16 GPa, indicating an increase of average coordination number (CN) of T–O from ~4 to 6. The r1 linearly decreases in the pressure range of 25–110 GPa, but it displays a slope change and becomes nearly constant above 110 GPa. The slope change may imply a structural change in the A40S glass around 110 GPa, and may be explained by the change in Al–O distance associated with the Al–O CN increase from 6 to >6 as predicted by molecular dynamics simulations (Ghosh and Karki, 2018). Our observations suggest an important role for aluminum in densification of aluminosilicate at the deep lower mantle, which might imply a dense aluminosilicate magma with negative buoyancy.

I. Ohira, Y. Kono, Y. Shibazaki, C. Kenney-Benson, A. Masuno, G. Shen

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Geochem. Persp. Let. (2019) 10, 41–45 | doi: 10.7185/geochemlet.1913 | Published 3 May 2019

Article views: 300

Widespread and intense wildfires at the Paleocene-Eocene boundary

Abstract:
Discovery of impact spherules associated with the onset of the Carbon Isotope Excursion (CIE) that marks the Paleocene-Eocene (P-E) boundary (~56 Ma) indicates that the P-E transition was coincident with an extraterrestrial impact. Charcoal abundances increase >20 times background immediately above the P-E spherule layer at two Atlantic Coastal Plain palaeo-continental shelf localities located >200 km apart. Individual charcoal shards (~100 μm long; 58-83 wt. % carbon) show charred plant features. The carbon isotope ratio of charcoal (δ13Ccharcoal) through the peak shows that it originated from pre-impact vegetation that burned. We consider two scenarios to explain this widespread, synchronous increase in charcoal at the P-E boundary: 1) warming-induced, continental-scale drying; and 2) impact-induced wildfires. Differentiating between these two hypotheses depends critically on the observed sequence of events, which on the western North Atlantic margin is: the impact spherule horizon, followed by the peak in charcoal (derived from vegetation that grew before the CIE and impact), and finally the nadir of the CIE. Importantly, the pre-excursion δ13Ccharcoal remains constant through the CIE onset, requiring a dramatic increase in sedimentation. This work clarifies our understanding of the timing and sequence of events following an extraterrestrial impact at the P-E boundary.

M.K. Fung, M.F. Schaller, C.M. Hoff, M.E. Katz, J.D. Wright

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Geochem. Persp. Let. (2019) 10, 1–6 | doi: 10.7185/geochemlet.1906 | Published 1 March 2019

Article views: 293

 Top 10 most viewed articles (cumulative count of HTML views) for the last 12 months.

Microplastics contaminate the deepest part of the world’s ocean

Abstract:
Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem.

X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai

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Geochem. Persp. Let. (2018) 9, 1–5 | doi: 10.7185/geochemlet.1829 | Published 27 November 2018

Article views: 9402

Carbon isotopic signatures of super-deep diamonds mediated by iron redox chemistry

Abstract:
Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases influences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compositions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.

J. Liu, W. Wang, H. Yang, Z. Wu, M.Y. Hu, J. Zhao, W. Bi, E.E. Alp, N. Dauphas, W. Liang, B. Chen, J.-F. Lin

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Geochem. Persp. Let. (2019) 10, 51–55 | doi: 10.7185/geochemlet.1915 | Published 24 May 2019

Article views: 3277

Calcium stable isotopes place Devonian conodonts as first level consumers

Abstract:
Conodont animals are an extinct group of jawless vertebrates whose hard parts, also known as conodont elements, represent the earliest evidence of a mineralised skeleton in the vertebrate lineage. Conodont elements are interpreted as parts of a feeding apparatus, which together with the presence of eyes and microwear patterns, support the controversial hypothesis that conodont animals were macrophagous predators and/or scavengers. Here, we explore the trophic position of five conodont genera (Palmatolepis, Polygnathus, Ancyrodella, Ancyrognathus and Icriodus) from five contemporary Late Devonian sites distributed worldwide (France, Morocco, Vietnam and Australia) by means of calcium (Ca) stable isotope compositions. The seawater Ca isotope composition was calibrated using contemporary Late Devonian brachiopod isotopic values. By comparison with extant marine trophic chain composed of cartilaginous fish, conodont Ca isotope compositions are indicative of a zooplanktivore - primary piscivore niche, with no genus-specific trophic pattern. The question of active predation or scavenging cannot be resolved definitively but our results strongly suggest that Late Devonian conodonts were first level consumers.

V. Balter, J.E. Martin, T. Tacail, G. Suan, S. Renaud, C. Girard

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Geochem. Persp. Let. (2019) 10, 36–39 | doi: 10.7185/geochemlet.1912 | Published 26 April 2019

Article views: 2984

Emergence of peraluminous crustal magmas and implications for the early Earth

Abstract:
Detrital zircons from the Jack Hills (JH) metasedimentary belt of Western Australia are a record of the first ∼1.5 billion years of Earth history and can be used to help reconstruct the conditions of crust formation and secular changes therein. Beginning as early as ca. 4.3 Ga, but becoming more pronounced in the mid-Archean, a peraluminous signature begins to emerge from the JH zircon record. Combined with trace elements (P, REEs) and Ti-in-zircon thermometry, this increase in peraluminosity is likely the result of deep (>7 kbar) partial melting of hydrous mafic protoliths or partial melting of metasedimentary source material. In a geodynamic context, these results may suggest a gradual shift from a vertical tectonic regime toward a horizontal tectonic regime with potential subduction-like or collisional processes creating the necessary conditions for peraluminous melt generation beginning locally at least by ∼3.6 billion years ago (Ga).

M.R. Ackerson, D. Trail, J. Buettner

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Geochem. Persp. Let. (2021) 17, 50–54 | doi: 10.7185/geochemlet.2114 | Published 14 May 2021

Article views: 2803

Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era

Abstract:
Eukaryotes reached ecological importance in the late Neoproterozoic Era, some one billion years after their emergence. Their slow rise to prominence has been ascribed to prolonged environmental stagnation, but testing this idea requires an appraisal of the evolution of atmospheric and ocean chemistry. Establishing a nuanced geochemical history is, however, challenging due to the paucity of well preserved sedimentary rocks. Here, we present new Mo isotope ratios from black shale units spanning ∼1560 to ∼1170 Ma. These results, combined with literature data, reveal potential episodic expansions of oxygenated and/or mildly reducing conditions during the Mesoproterozoic Era, suggesting fluctuating oxygen availability that could have exerted a crucial control on the evolution of eukaryotes.

Y. Ye, S. Zhang, H. Wang, X. Wang, C. Tan, M. Li, C. Wu, D.E. Canfield

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Geochem. Persp. Let. (2021) 18, 16–21 | doi: 10.7185/geochemlet.2118 | Published 15 June 2021

Article views: 2750

182W evidence for core-mantle interaction in the source of mantle plumes

Abstract:
Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182Hf decayed entirely to 182W in the mantle after metal-silicate segregation. Therefore, the 182W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.

H. Rizo, D. Andrault, N.R. Bennett, M. Humayun, A. Brandon, I. Vlastelic, B. Moine, A. Poirier, M.A. Bouhifd, D.T. Murphy

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Geochem. Persp. Let. (2019) 11, 6–11 | doi: 10.7185/geochemlet.1917 | Published 20 June 2019

Article views: 2719

Widespread and intense wildfires at the Paleocene-Eocene boundary

Abstract:
Discovery of impact spherules associated with the onset of the Carbon Isotope Excursion (CIE) that marks the Paleocene-Eocene (P-E) boundary (~56 Ma) indicates that the P-E transition was coincident with an extraterrestrial impact. Charcoal abundances increase >20 times background immediately above the P-E spherule layer at two Atlantic Coastal Plain palaeo-continental shelf localities located >200 km apart. Individual charcoal shards (~100 μm long; 58-83 wt. % carbon) show charred plant features. The carbon isotope ratio of charcoal (δ13Ccharcoal) through the peak shows that it originated from pre-impact vegetation that burned. We consider two scenarios to explain this widespread, synchronous increase in charcoal at the P-E boundary: 1) warming-induced, continental-scale drying; and 2) impact-induced wildfires. Differentiating between these two hypotheses depends critically on the observed sequence of events, which on the western North Atlantic margin is: the impact spherule horizon, followed by the peak in charcoal (derived from vegetation that grew before the CIE and impact), and finally the nadir of the CIE. Importantly, the pre-excursion δ13Ccharcoal remains constant through the CIE onset, requiring a dramatic increase in sedimentation. This work clarifies our understanding of the timing and sequence of events following an extraterrestrial impact at the P-E boundary.

M.K. Fung, M.F. Schaller, C.M. Hoff, M.E. Katz, J.D. Wright

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Geochem. Persp. Let. (2019) 10, 1–6 | doi: 10.7185/geochemlet.1906 | Published 1 March 2019

Article views: 2677

Ultrahigh pressure structural changes in a 60 mol. % Al2O3-40 mol. % SiO2 glass

Abstract:
Structure of an Al-containing silicate glass (60 mol. % Al2O3–40 mol. % SiO2, A40S) is investigated up to 131 GPa, a pressure close to that of the Earth’s core-mantle boundary, by using our recently developed double stage large volume cell. The first peak (r1) of the pair distribution function, which corresponds to T–O distance (T = Al, Si), rapidly increases below 16 GPa, indicating an increase of average coordination number (CN) of T–O from ~4 to 6. The r1 linearly decreases in the pressure range of 25–110 GPa, but it displays a slope change and becomes nearly constant above 110 GPa. The slope change may imply a structural change in the A40S glass around 110 GPa, and may be explained by the change in Al–O distance associated with the Al–O CN increase from 6 to >6 as predicted by molecular dynamics simulations (Ghosh and Karki, 2018). Our observations suggest an important role for aluminum in densification of aluminosilicate at the deep lower mantle, which might imply a dense aluminosilicate magma with negative buoyancy.

I. Ohira, Y. Kono, Y. Shibazaki, C. Kenney-Benson, A. Masuno, G. Shen

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Geochem. Persp. Let. (2019) 10, 41–45 | doi: 10.7185/geochemlet.1913 | Published 3 May 2019

Article views: 2646

New insights into the plumbing system of Santorini using helium and carbon isotopes

Abstract:
We have analysed He, Ne and CO2 concentrations and isotopic ratios in seawater samples collected using a ROV above hydrothermal fields and in CO2-accumulating pools within Santorini caldera (Greece). Our results provide new insights on the plumbing and magmatic sources of this volcano, which is critical to understand and better predict its volcanic eruptions, and its relationships to nearby volcanic systems. The 3He/4He ratio is as high as 6.54 times the atmospheric ratio, and significantly higher than any other samples from Santorini, including the active eruptive centre of Nea Kameni (~3.6 Ra). The 3He/4He ratio, corrected for atmospheric helium dissolved in seawater is ~7 Ra, a value similar to that measured at the nearby Kolumbo submarine volcano crater. The fluids display typical mantle signatures never recorded before in the active centres of the island of Santorini, in which fluids and gases are contaminated at shallow depths. The helium isotopic signature suggests that the source of volatiles for these two volcanoes is similar to the MORB source instead of being the subcontinental lithospheric mantle as suggested previously.

M. Moreira, J. Escartin, L. Scelin, L. Ruzié-Hamilton, P. Nomikou, C. Mével, M. Andreani

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Geochem. Persp. Let. (2019) 10, 46–50 | doi: 10.7185/geochemlet.1914 | Published 20 May 2019

Article views: 2586

Exchange catalysis during anaerobic methanotrophy revealed by 12CH2D2 and 13CH3D in methane

Abstract:
The anaerobic oxidation of methane (AOM) is a crucial component of the methane cycle, but quantifying its role in situ under dynamic environmental conditions remains challenging. We use sediment samples collected during IODP Expedition 347 to the Baltic Sea to show that relative abundances of 12CH2D2 and 13CH3D in methane remaining after microbial oxidation are in internal, thermodynamic isotopic equilibrium, and we attribute this phenomenon to the reversibility of the initial step of AOM. These data suggest that 12CH2D2 and 13CH3D together can identify the influence of anaerobic methanotrophy in environments where conventional bulk isotope ratios are ambiguous, and these findings may lead to new insights regarding the global significance of enzymatic back reaction in the methane cycle.

J.L. Ash, M. Egger, T. Treude, I. Kohl, B. Cragg, R.J. Parkes, C.P. Slomp, B. Sherwood Lollar, E.D. Young

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Geochem. Persp. Let. (2019) 10, 26–30 | doi: 10.7185/geochemlet.1910 | Published 15 April 2019

Article views: 2576

 Top 10 most viewed articles (cumulative count of HTML views) for all time.

Microplastics contaminate the deepest part of the world’s ocean

Abstract:
Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem.

X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai

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Geochem. Persp. Let. (2018) 9, 1–5 | doi: 10.7185/geochemlet.1829 | Published 27 November 2018

Article views: 45410

Global climate stabilisation by chemical weathering during the Hirnantian glaciation

Abstract:
Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled “weathering thermostat” has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (δ7Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive δ7Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO2 degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO2 drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat.

P.A.E. Pogge von Strandmann, A. Desrochers, M.J. Murphy, A.J. Finlay, D. Selby, T.M. Lenton

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Geochem. Persp. Let. (2017) 3, 230–237 | doi: 10.7185/geochemlet.1726 | Published 15 June 2017

Article views: 33632

Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation

Abstract:
The differentiation of Earth into a metallic core and silicate mantle left its signature on the chemical and isotopic composition of the bulk silicate Earth (BSE). This is seen in the depletion of siderophile (metal-loving) relative to lithophile (rock-loving) elements in Earth’s mantle as well as the silicon isotope offset between primitive meteorites (i.e. bulk Earth) and BSE, which is generally interpreted as a proof that Si is present in Earth’s core. Another putative light element in Earth’s core is sulphur; however, estimates of core S abundance vary significantly and, due to its volatile nature, no unequivocal S isotopic signature for core fractionation has thus far been detected. Here we present new high precision isotopic data for Cu, a chalcophile (sulphur-loving) element, which shows that Earth’s mantle is isotopically fractionated relative to bulk Earth. Results from high pressure equilibration experiments suggest that the sense of Cu isotopic fractionation between BSE and bulk Earth requires that a sulphide-rich liquid segregated from Earth’s mantle during differentiation, which likely entered the core. Such an early-stage removal of a sulphide-rich phase from the mantle presents a possible solution to the long-standing 1st terrestrial lead paradox.

P.S. Savage, F. Moynier, H. Chen, J. Siebert, J. Badro, I.S. Puchtel, G. Shofner

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Geochem. Persp. Let. (2015) 1, 53–64 | doi: 10.7185/geochemlet.1506 | Published 4 June 2015

Article views: 26175

182W evidence for core-mantle interaction in the source of mantle plumes

Abstract:
Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182Hf decayed entirely to 182W in the mantle after metal-silicate segregation. Therefore, the 182W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.

H. Rizo, D. Andrault, N.R. Bennett, M. Humayun, A. Brandon, I. Vlastelic, B. Moine, A. Poirier, M.A. Bouhifd, D.T. Murphy

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Geochem. Persp. Let. (2019) 11, 6–11 | doi: 10.7185/geochemlet.1917 | Published 20 June 2019

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Release of subducted sedimentary nitrogen throughout Earth’s mantle

Abstract:
The dynamic process of subduction represents the principal means to introduce chemical heterogeneities into Earth's interior. In the case of nitrogen (N) - atmosphere's most abundant gas - biological-activity converts N2 into ammonium ions (NH4+), which are chemically-bound within seafloor sediments and altered oceanic crust that comprise the subducting slab. Although some subducted N re-emerges via arc-related volcanism (Sano et al., 1998), the majority likely bypasses sub-arc depths (150-200 km) and supplies the deeper mantle (Li et al., 2007; Mitchell et al., 2010; Johnson and Goldblatt, 2015; Bebout et al., 2016). However, the fate of subducted N remains enigmatic: is it incorporated by the shallow convecting mantle - the source of ridge volcanism, or is the deeper mantle - nominally associated with mantle plumes - its ultimate repository? Here, we present N-He-Ne-Ar isotope data for oceanic basalts from the Central Indian Ridge (CIR)-Réunion plume region to address this issue. All on-axis samples with depleted MORB mantle (DMM) affinities (3He/4He = 8 ± 1 RA; Graham, 2002) have low N-isotopes (mean δ15N = -2.1 ‰), whereas those with plume-like 3He/4He display higher values (mean δ15N = 1.3 ‰). We explain these data within the framework of a new mantle reference model to predict a time-integrated net N regassing flux to the mantle of ~3.4 × 1010 mol/yr, with the plume-source mantle representing the preferential destination by a factor of 2-3. The model has implications for the present-day imbalance between N subducted at trenches and N emitted via arc-related volcanism, the N-content of Earth's early atmosphere, as well as relationships between N2 and the noble gases in mantle reservoirs, including 3He/4He-δ15N relationships in plume-derived lavas.

P.H. Barry, D.R. Hilton

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Geochem. Persp. Let. (2016) 2, 148–159 | doi: 10.7185/geochemlet.1615 | Published 3 May 2016

Article views: 23208

Oxygenation of the mid-Proterozoic atmosphere: clues from chromium isotopes in carbonates

Abstract:
Chromium (Cr) isotopes in marine sedimentary rocks can be used as a sensitive proxy for ancient atmospheric oxygen because Cr-isotope fractionation during terrestrial weathering only occurs when pO2 exceeds a threshold value. This is a useful system when applied to rocks of mid-Proterozoic age, where fundamental questions persist about atmospheric pO2 and its relationship to biological innovation. Whereas previous studies have focused on temporally limited iron-rich sedimentary rocks, we present new Cr-isotope data from a suite of mid-Proterozoic marine carbonate rocks. Application of the Cr-isotope proxy to carbonate rocks has the potential to greatly enhance the temporal resolution of Proterozoic palaeo-redox data. Here we report positive δ53Cr values in four carbonate successions, extending the mid-Proterozoic record of Cr-isotope fractionation – and thus pO2 above threshold values – back to ~1.1 Ga. These data suggest that pO2 sufficient for the origin of animals was transiently in place well before their Neoproterozoic appearance, although uncertainty in the pO2 threshold required for Cr-isotope fractionation precludes definitive biological interpretation. This study provides a proof of concept that the Cr-isotopic composition of carbonate rocks can provide important new constraints on the oxygen content of the ancient atmosphere.

G.J. Gilleaudeau, R. Frei, A.J. Kaufman, L.C. Kah, K. Azmy, J.K. Bartley, P. Chernyavskiy, A.H. Knoll

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Geochem. Persp. Let. (2016) 2, 178–187 | doi: 10.7185/geochemlet.1618 | Published 24 May 2016

Article views: 22939

Environmental pressure from the 2014–15 eruption of Bárðarbunga volcano, Iceland

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 11.8 ± 5 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO2 exceeded the 350 µg m3 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 surface waters, soils, and vegetation of Iceland.

S.R. Gíslason, G. Stefánsdóttir, M.A. Pfeffer, S. Barsotti, Th. Jóhannsson, I. Galeczka, E. Bali, O. Sigmarsson, A. Stefánsson, N.S. Keller, Á. Sigurdsson, B. Bergsson, B. Galle, V.C. Jacobo, S. Arellano, A. Aiuppa, E.B. Jónasdóttir, E.S. Eiríksdóttir, S. Jakobsson, G.H. Guðfinnsson, S.A. Halldórsson, H. Gunnarsson, B. Haddadi, I. Jónsdóttir, Th. Thordarson, M. Riishuus, Th. Högnadóttir, T. Dürig, G.B.M. Pedersen, Á. Höskuldsson, M.T. Gudmundsson

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Geochem. Persp. Let. (2015) 1, 84–93 | doi: 10.7185/geochemlet.1509 | Published 29 June 2015

Article views: 21552

Molecular hydrogen in mantle minerals

Abstract:
Current models assume that hydrogen was delivered to Earth already in oxidised form as water or OH groups in minerals; similarly, it is generally believed that hydrogen is stored in the present mantle mostly as OH. Here we show by experiments at 2-7 GPa and 1100-1300 °C that, under reducing conditions, molecular hydrogen (H2) has an appreciable solubility in various upper mantle minerals. This observation suggests that during the accretion of the Earth, nebular H2 could have been delivered to the growing solid planet by direct dissolution in a magma ocean and subsequent incorporation in silicates. Moreover, the presence of dissolved molecular H2 in the minerals of the lower mantle could explain why magmas sourced in this region are rich in hydrogen, despite the fact that lower mantle minerals contain almost no OH groups.

X. Yang, H. Keppler, Y. Li

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Geochem. Persp. Let. (2016) 2, 160–168 | doi: 10.7185/geochemlet.1616 | Published 18 March 2016

Article views: 21172

Rapid response of silicate weathering rates to climate change in the Himalaya

Abstract:
Chemical weathering of continental rocks plays a central role in regulating the carbon cycle and the Earth's climate (Walker et al., 1981; Berner et al., 1983), accounting for nearly half the consumption of atmospheric carbon dioxide globally (Beaulieu et al., 2012). However, the role of climate variability on chemical weathering is still strongly debated. Here we focus on the Himalayan range and use the lithium isotopic composition of clays in fluvial terraces to show a tight coupling between climate change and chemical weathering over the past 40 ka. Between 25 and 10 ka ago, weathering rates decrease despite temperature increase and monsoon intensification. This suggests that at this timescale, temperature plays a secondary role compared to runoff and physical erosion, which inhibit chemical weathering by accelerating sediment transport and act as fundamental controls in determining the feedback between chemical weathering and atmospheric carbon dioxide.

A. Dosseto, N. Vigier, R. Joannes-Boyau, I. Moffat, T. Singh, P. Srivastava

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Geochem. Persp. Let. (2015) 1, 10–19 | doi: 10.7185/geochemlet.1502 | Published 20 February 2015

Article views: 20976

The effect of warming climate on nutrient and solute export from the Greenland Ice Sheet

Abstract:
Glacial meltwater runoff is likely an important source of limiting nutrients for downstream primary producers. This has particular significance for regions surrounding the Greenland Ice Sheet, which discharges >400 km3 of meltwater annually. The Arctic is warming rapidly but the impact of higher discharge on nutrient export is unknown. We present four years of hydrological and geochemical data from a large Greenland Ice Sheet catchment that includes the two highest melt years on record (2010, 2012). Measurements reveal significant variation in dissolved solute (major ion) and estimated dissolved macronutrient (nitrogen, phosphorus and silica) fluxes, with increases in higher melt years. Labile particulate macronutrients dominate nutrient export, accounting for ~50 % of nitrogen and >80 % of both phosphorus and silica. The response of ice sheet nutrient export to enhanced melting is largely controlled by particle bound nutrients, the future supply of which is uncertain. We propose that the Greenland Ice Sheet provides an underappreciated and annually dynamic source of nutrients for the polar oceans, with changes in meltwater discharge likely to impact marine primary productivity in future decades.

J.R. Hawkings, J.L. Wadham, M. Tranter, E. Lawson, A. Sole, T. Cowton, A.J. Tedstone, I. Bartholomew, P. Nienow, D. Chandler, J. Telling

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Geochem. Persp. Let. (2015) 1, 94–104 | doi: 10.7185/geochemlet.1510 | Published 23 June 2015

Article views: 20020