Geochemical Perspectives Letters is an internationally peer-reviewed journal of the European Association of Geochemistry,
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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: 25,737

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: 23,520

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: 18,689

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: 18,435

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: 16,167

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: 15,095

A cometary origin for martian atmospheric methane

Abstract:
Methane has been reported repeatedly in the martian atmosphere but its origin remains an obstinate mystery. Possible sources include aqueous alteration of igneous rocks, release from ancient deposits of methane/water ice clathrates, infall from exogenous sources such as background interplanetary dust, or biological activity. All of these sources are problematic, however. We hypothesise that delivery of cometary material includes meteor outbursts, commonly known as “meteor showers”, may explain martian methane plumes. Correlations exist between the appearance of methane and near-approaches between Mars and cometary orbits. Additional correlations are seen between these interactions and the appearance of high-altitude dust clouds on Mars, showing that large amounts of material may be deposited on Mars during these encounters. Methane is released by UV breakdown of delivered cometary material. This hypothesis is testable in future Mars/cometary encounters. A cometary origin for methane would reveal formation of methane through processes that are separate from any geological or biological processes on Mars.

M. Fries, A. Christou, D. Archer, P. Conrad, W. Cooke, J. Eigenbrode, I.L. ten Kate, M. Matney, P. Niles, M. Sykes, A. Steele, A. Treiman

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Geochem. Persp. Let. (2016) 2, 10 – 23 | doi: 10.7185/geochemlet.1602 | Published 2 December 2015

Article views: 14,085

Carbon isotope discrimination in C3 land plants is independent of natural variations in pCO2

Abstract:
The δ13C of terrestrial C3 plant tissues and soil organic matter is important for understanding the carbon cycle, inferring past climatic and ecological conditions, and predicting responses of vegetation to future climate change. Plant δ13C depends on the δ13C of atmospheric CO2 and mean annual precipitation (MAP), but an unresolved decades-long debate centres on whether terrestrial C3 plant δ13C responds to pCO2. In this study, the pCO2-dependence of C3 land plant δ13C was tested using isotopic records from low- and high-pCO2 times spanning historical through Eocene data. Historical data do not resolve a clear pCO2-effect (-1.2 ± 1.0 to 0.6 ± 1.0 ‰/100 ppmv). Organic carbon records across the Pleistocene-Holocene transition are too affected by changes in MAP, carbon sources, and potential differential degradation to quantify pCO2-effects directly, but limits of ≤1.0 ‰/100 ppmv or ~0 ‰/100 ppmv are permissible. Fossil collagen and tooth enamel data constrain pCO2-effects most tightly to -0.03 ± 0.13 and -0.03 ± 0.24 ‰/100 ppmv between 200 and 700 ppmv. Combining all constraints yields a preferred value of 0.0 ± 0.3 ‰/100 ppmv (2 s.e.). Recent models of pCO2-dependence imply unrealistic MAP for Cenozoic records.

M.J. Kohn

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Geochem. Persp. Let. (2016) 2, 35 – 43 | doi: 10.7185/geochemlet.1604 | Published 8 January 2016

Article views: 13,923

Tracing Earth’s O2 evolution using Zn/Fe ratios in marine carbonates

Abstract:
Through Earth history, atmospheric oxygen has increased from initial values near zero to its present day level of about 21 % by volume; concomitantly, changes in ocean redox conditions have fundamentally altered global biogeochemical cycles. While there is a reasonable understanding of where oxygen history begins and ends, the quantitative timetable of oxygenation that links the endpoints has proven contentious. Equilibrium between marine surface environments and the overlying atmosphere suggests that carbonate-based redox proxies could refine palaeoredox records in time and space. Here we explore the use of Zn/Fe ratios to infer the evolution of atmospheric O2 through time, based on marine carbonate rocks that are well characterised in terms of depositional age, environmental setting, and diagenetic history. While Fe and Zn in the shallow ocean are mainly sourced from hydrothermal inputs, their redox sensitivities differ significantly, so that geological intervals with higher O2 would be characterised by stepped increases in Zn/Fe as preserved in shallow marine carbonates. Therefore, Zn/Fe analyses of ancient carbonates allow us to constrain past atmospheric pO2 levels, providing a secular record of atmospheric O2 over the past 3.5 billion years. In particular, we corroborate an earlier proposal that for much of the Proterozoic Eon, O2 levels were as low as 0.1-1 % of present atmospheric level. We conclude that Zn/Fe in shallow marine carbonate rocks has potential to provide a quantitative tracer for the long-term redox evolution of the oceans and the rise of atmospheric O2.

X.-M. Liu, L.C. Kah, A.H. Knoll, H. Cui, A.J. Kaufman, A. Shahar, R.M. Hazen

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Geochem. Persp. Let. (2016) 2, 24 – 34 | doi: 10.7185/geochemlet.1603 | Published 2 December 2015

Article views: 13,632

Mononuclear U(IV) complexes and ningyoite as major uranium species in lake sediments

Abstract:
Natural attenuation of uranium in subsurface environments is generally assigned to immobilisation processes due to microbial reduction of U(VI). Recent laboratory studies have established that the end products of such a process include both low solubility biogenic uraninite and more labile non-crystalline U(IV) species. Indeed, biogenic uraninite formation may be inhibited in the presence of organic or inorganic phosphoryl ligands, leading to the formation of non-crystalline U(IV)-phosphate complexes or nanoscale U(IV)-phosphate solids. Such species have been observed in shallow contaminated alluvial aquifers and can thus be suspected to form in other important environments, among which lacustrine sediments have a global environmental significance since they may represent major uranium accumulation reservoirs in riverine watersheds. Here, on the basis of microscopic, spectroscopic and chemical extraction analyses, we report the occurrence of mononuclear U(IV)-phosphate/silicate complexes, accompanied by nano-crystalline ningyoite-like U(IV)-phosphate minerals, as major scavengers for uranium in lacustrine sediments downstream from a former uranium mine in France. This observation reveals that uranium trapping mechanisms during early diagenesis of lacustrine sediments can virtually exclude uraninite formation, which has important implications for better modelling uranium cycling in natural and contaminated freshwaters. Moreover, our results raise issues concerning the long term fate of mononuclear U(IV) complexes and U(IV) phosphate nano-minerals, especially with respect to re-oxidation events.

G. Morin, A. Mangeret, G. Othmane1, L. Stetten, M. Seder-Colomina, J. Brest, G. Ona-Nguema, S. Bassot, C. Courbet, J. Guillevic, A. Thouvenot, O. Mathon, O. Proux, J.R. Bargar

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Geochem. Persp. Let. (2016) 2, 95 – 105 | doi: 10.7185/geochemlet.1610 | Published 16 February 2016

Article views: 13,397