Geochemical Perspectives Letters is an internationally peer-reviewed journal of the European Association of Geochemistry, produced by and for the geochemical community:
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Climate driven carbon and microbial signatures through the last ice age

Abstract:
Ice cores preserve diverse materials as millennial-scale proxies for Earth’s history. While major ions and elemental analyses are commonly investigated in palaeoclimate reconstructions, the integration of biological measurements is rapidly developing. Although the limited number of data herein impose constraints on broader generalisations, we show that microbial assemblages and organic matter (OM) composition from Byrd Station and West Antarctic Ice Sheet Divide ice cores may serve as palaeoecological markers from the Last Glacial Maximum (LGM; section ~20.5 ka BP) and last deglaciation periods (LD; section ~14.5 ka BP), reflecting environmental changes. Fluorescent analyses determined OM from both cores to have similar amino acid-like signatures; however, more comprehensive molecular characterisation showed only 12 % overlap in molecular formulae, with Byrd OM being more chemically labile. Microbial diversity in both cores was low, and together with predicted metabolic capabilities, differed significantly between communities. Variation in OM composition and microbial diversity reflects changes in environmental sources and deposition patterns onto the Antarctic Ice Sheet during distinct climate periods, with OM composition potentially shaping microbial communities post-deposition. Combining detailed microbial and OM composition analyses created a unique window into the past, providing a way to characterise carbon composition and potential metabolic processes as a function of environmental change.


J. D’Andrilli, H.J. Smith, M. Dieser, C.M. Foreman
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Geochem. Persp. Let. (2017) 4, 29-34 | doi: 10.7185/geochemlet.1732 | Published 21 September 2017

Running out of gas: Zircon 18O-Hf-U/Pb evidence for Snowball Earth preconditioned by low degassing

Abstract:
The general long-term stability of Earth’s climate over geologic time was punctuated by dramatic excursions. Between ca. 2.5 and 0.5 billion years ago (Ga), these events included the globally extensive glaciations known as Snowball Earths, when ice extended to tropical latitudes. Such anomalous periods of time provide unique opportunities for understanding the mechanisms regulating planetary climate and habitability. However, the causes of these events remain enigmatic, in part because there is little information about fluxes in the global carbon cycle in deep time. We propose that the oxygen stable isotope composition in zircons (δ18Ozircon) contains information about past weathering conditions on the continents, imparted during the time between separation of parent material from the mantle (reflected in the Hf model age) and zircon crystallisation (the U/Pb age). A new compilation of coupled 18O-Hf-U/Pb isotopic data shows that the mean δ18Ozircon value varied particularly between 2.5 Ga and 0.5 Ga. The maximum in the δ18Ozircon record, which we interpret as a time of intense weathering, is associated with the Lomagundi Event (~2.22–2.07 Ga), a dramatic carbon isotope excursion thought to reflect enhanced organic carbon burial facilitated by the release of phosphorous during rock weathering. The onset of the Neoproterozoic Snowball Earth events coincides with the minimum in δ18Ozircon, suggesting low silicate weathering rates at the time. This evidence suggests that long-term decreases in the rate of CO2 release to the atmosphere from solid Earth degassing may have preconditioned the global climate system for intense glaciations.


J. Hartmann, G. Li, A.J. West
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Geochem. Persp. Let. (2017) 4, 41-46 | doi: 10.7185/geochemlet.1734 | Published 15 September 2017

Stable strontium isotopic heterogeneity in the solar system from double-spike data

Abstract:
Strontium isotopic anomalies in meteorites are important in assessing nucleosynthetic sources to, and measuring the timing of, early solar system processes. However, conventional use of a constant 88Sr/86Sr value in correcting for instrumental mass fractionation during analysis renders measurements ambiguous and removes information on mass-dependent fractionation variations. From double-spike techniques we obtain data for the four stable strontium isotopes free of this ambiguity, and report measurements from a range of meteoritic, lunar and terrestrial materials. The Earth, Moon, basaltic eucrites and feldspars from angrites (differentiated samples) follow a single mass-dependent fractionation line and have a common nucleosynthetic origin in terms of their strontium isotopes. In contrast, bulk rock CI, CV3, CM and CO chondrite samples serve to define another mass-dependent fractionation line, displaced by 94 ± 28 ppm to heavier 84Sr/86Sr and/or 88Sr/86Sr ratios than that for the differentiated samples. Our Sr-isotopic data are consistent with a primary contrast in early solar system composition between an outer zone of primitive, mostly undifferentiated, materials and an inner zone of (almost entirely) differentiated materials that accumulated to form the terrestrial planets.


B.L.A. Charlier, I.J. Parkinson, K.W. Burton, M.M. Grady, C.J.N. Wilson, E.G.C. Smith
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Geochem. Persp. Let. (2017) 4, 35-40 | doi: 10.7185/geochemlet.1733 | Published 15 September 2017

A secular increase in continental crust nitrogen during the Precambrian

Abstract:
Recent work indicates the presence of substantial geologic nitrogen reservoirs in the mantle and continental crust. Importantly, this geologic nitrogen has exchanged between the atmosphere and the solid Earth over time. Changes in atmospheric nitrogen (i.e. atmospheric mass) have direct effects on climate and biological productivity. It is difficult to constrain, however, the evolution of the major nitrogen reservoirs through time. Here we show a secular increase in continental crust nitrogen through Earth history recorded in glacial tills (2.9 Ga to modern), which act as a proxy for average upper continental crust composition. Archean and earliest Palaeoproterozoic tills contain 66 ± 100 ppm nitrogen, whereas Neoproterozoic and Phanerozoic tills contain 290 ± 165 ppm nitrogen, whilst the isotopic composition has remained constant at ∼4 ‰. Nitrogen has accumulated in the continental crust through time, likely sequestered from the atmosphere via biological fixation. Our findings support dynamic, non-steady state behaviour of nitrogen through time, and are consistent with net transfer of atmospheric N to geologic reservoirs over time.


B.W. Johnson, C. Goldblatt
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Geochem. Persp. Let. (2017) 4, 24-28 | doi: 10.7185/geochemlet.1731 | Published 1 September 2017

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

Abstract:
Multiphase inclusions represent microenvironments where the interaction between fluid and host mineral is preserved during the rock geological path. Under its peculiar chemical-physical constraints, the entrapped solute-rich fluid might follow a crystallisation mechanism which is not predictable through simple equilibrium arguments. In this letter, by the modelling of solid-solution equilibrium and the application of principles of mass conservation, we demonstrate that cavities in mantle garnet filled with slab-derived fluids can re-equilibrate to a pyrope + spinel + chlorite assemblage at the same high P-T of their formation. The basis of this occurrence is a dissolution-reprecipitation mechanism, triggered by a dilute, non-equilibrated slab fluid.


M. Campione, S. Tumiati, N. Malaspina
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Geochem. Persp. Let. (2017) 4, 19-23 | doi: 10.7185/geochemlet.1730 | Published 23 August 2017

Microbial nitrogen cycle enhanced by continental input recorded in the Gunflint Formation

Abstract:
Nitrogen isotope compositions (δ15N values) of kerogen in the sedimentary rocks from the 1878 Ma Gunflint Formation were analysed to understand the relationships among microbial activities, ocean chemistry, and tectonic evolution in the Animikie Basin. In the present study, the stepwise combustion analysis, performed on 13 kerogen samples, indicates that the δ15N values of kerogen can be sub-divided into two fractions with discrete values depending on the combustion temperatures: a lower-temperature fraction (from 500 to 575 ˚C), and a higher-temperature fraction (higher than 575 ˚C). A positive correlation was observed between the δ15N values of the lower-temperature fractions and Pr/Sm ratios, which represent contributions from the continental input. In contrast, no correlation was observed between the δ15N values of the higher-temperature fractions and the Pr/Sm ratios. This relationship between the δ15N values and the continental inputs is explained by the isotopic fractionation effects of the biological nitrogen cycle associated with the enhanced microbial activity, triggered by the active tectonic settings in the Animikie basin.


A. Ishida, K. Hashizume, T. Kakegawa
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Geochem. Persp. Let. (2017) 4, 13-18 | doi: 10.7185/geochemlet.1729 | Published 5 August 2017

No direct contribution of recycled crust in Icelandic basalts

Abstract:
Using Melt-PX to model the decompression melting of a heterogeneous mantle, I investigated the role of major-element composition of the lithologies present in the source on magmatic productivity, and trace element and isotopic melt compositions, independently of the bulk mantle composition. My calculations demonstrate that the volume of magma produced is not significantly affected by the nature of the lithological heterogeneity, but depends on the bulk mantle composition. However, an isochemical bulk mantle can produce contrasting trace element and isotopic melt compositions depending on the major-element compositions of the lithologies present in the source. Results show that the observed crust thickness of the Icelandic rift zones is consistent with about 10 % of recycled crust in the source, but also demonstrate there is no need to involve the contribution of melts derived from a recycled basalt component to explain the compositional variability of the Icelandic basalts in rift zones, and rather advance the contribution of olivine-bearing hybrid lithologies formed by solid-state reactions between the recycled crust and the peridotite.


S. Lambart
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Geochem. Persp. Let. (2017) 4, 7-12 | doi: 10.7185/geochemlet.1728 | Published 12 July 2017

Tracking continental-scale modification of the Earth’s mantle using zircon megacrysts

Abstract:
Metasomatism, the chemical alteration of rocks by a variety of melts and fluids, has formed a key concept in studies of the Earth’s mantle for decades. Metasomatic effects are often inferred to be far-reaching and yet the evidence for their occurrence is usually based upon individual hand specimens or suites of rocks that display considerable heterogeneity. In rare cases, however, we are offered insights into larger-scale chemical modifications that occur in the mantle. Here we utilise the Lu–Hf systematics of zircon megacrysts erupted in kimberlite magmas to discern two temporally and compositionally discrete metasomatic events in the mantle beneath southern Africa, each having an influence extending over an area exceeding one million km2. These data provide unambiguous evidence for metasomatic processes operating at continental scales and seemingly unperturbed by the age and composition of the local lithospheric mantle. The most recent of these events may be associated with the major Jurassic-Karoo magmatism in southern Africa.


J. Woodhead, J. Hergt, A. Giuliani, D. Phillips, R. Maas
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Geochem. Persp. Let. (2017) 4, 1-6 | doi: 10.7185/geochemlet.1727 | Published 10 July 2017

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

Not so non-marine? Revisiting the Stoer Group and the Mesoproterozoic biosphere

Abstract:
The Poll a’Mhuilt Member of the Stoer Group (Torridonian Supergroup) in Scotland has been heralded as a rare window into the ecology of Mesoproterozoic terrestrial environments. Its unusually high molybdenum concentrations and large sulphur isotope fractionations have been used as evidence to suggest that lakes 1.2 billion years ago were better oxygenated and enriched in key nutrients relative to contemporaneous oceans, making them ideal habitats for the evolution of eukaryotes. Here we show with new Sr and Mo isotope data, supported by sedimentological evidence, that the depositional setting of this unit was likely connected to the ocean and that the elevated Mo and S contents can be explained by evapo-concentration of seawater. Thus, it remains unresolved if Mesoproterozoic lakes were important habitats for early eukaryotic life.


E.E. Stüeken, E.J. Bellefroid, A. Prave, D. Asael, N.J. Planavsky, T.W. Lyons
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Geochem. Persp. Let. (2017) 3, 221-229 | doi: 10.7185/geochemlet.1725 | Published 13 June 2017

Reorganisation of Earth's biogeochemical cycles briefly oxygenated the oceans 520 Myr ago

Abstract:
The Phanerozoic radiation of bilaterian animals has been linked to oxygenation of Earth's oceans, due to the oxygen demand of the evolving animal ecosystems. However, how early animals may have regulated Earth’s surface oxygen budget via self-stabilising feedbacks is poorly understood. Here, we report parallel positive uranium, carbon, and sulphur isotope excursions from carbonate successions in Siberia that document a brief global oxygenation episode 521–520 Myr ago, at the onset of diversification of larger arthropods known from the fossil record. Our data and model indicate that an abrupt increase in the sinking rate of marine organic matter expanded the oxygenated zone in the oceans and that reducing conditions returned 1.3 ± 0.8 Myr after the onset of this transient oxygenation episode, necessitating a strong negative feedback to the increasing levels of oxygen. We speculate that larger zooplankton could have sourced both oxygen and food to the seafloor, fueling bioturbation over wider areas and, thereby, stabilising O2-rich habitats in the oceans. Thus, this reorganisation exemplifies how animal ecosystems might have influenced oxygen availability in Earth's surface environment soon after their establishment.


T.W. Dahl, J.N. Connelly, A. Kouchinsky, B.C. Gill, S.F. Månsson, M. Bizzarro
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Geochem. Persp. Let. (2017) 3, 210-220 | doi: 10.7185/geochemlet.1724 | Published 15 June 2017

The heterogeneous nature of Fe delivery from melting icebergs

Abstract:
The micronutrient iron (Fe) can be transported from marine terminating glaciers to the ocean by icebergs. There are however few observations of iceberg Fe content, and the flux of Fe from icebergs to the offshore surface ocean is poorly constrained. Here we report the dissolved Fe (DFe), total dissolvable Fe (TdFe) and ascorbic acid extractable Fe (FeAsc) sediment content of icebergs from Kongsfjorden, Svalbard. The concentrations of DFe (range 0.63 nM – 536 nM, mean 37 nM, median 6.5 nM) and TdFe (range 46 nM – 57 µM, mean 3.6 µM, median 144 nM) both demonstrated highly heterogeneous distributions and there was no significant correlation between these two fractions. FeAsc (range 0.0042 to 0.12 wt. %) was low compared to both previous measurements in Kongsfjorden and to current estimates of the global mean. FeAsc content per volume ice did however, as expected, show a significant relationship with sediment loading (which ranged from < 0.1 – 234 g L-1 of meltwater). In the Arctic, icebergs lose their sediment load faster than ice volume due to the rapid loss of basal ice after calving. We therefore suggest that the loss of basal ice is a potent mechanism for the reduction of mean TdFe and FeAsc per volume of iceberg. Delivery of TdFe and FeAsc to the ocean is thereby biased towards coastal waters where, in Kongsfjorden, DFe (18 ± 17 nM) and TdFe (mean 8.1 µM, median 3.7 µM) concentrations were already elevated.


M.J. Hopwood, C. Cantoni, J.S. Clarke, S. Cozzi, E.P. Achterberg
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Geochem. Persp. Let. (2017) 3, 200-209 | doi: 10.7185/geochemlet.1723 | Published 7 June 2017

A magma mixing redox trap that moderates mass transfer of sulphur and metals

Abstract:
Mixing and juxtaposition of chemically distinct magmatic systems are key processes for the evolution of Earth’s crust. Yet, the physicochemical nature at mixing interfaces remains poorly described, as crystallisation, melting, heat transfer, and diffusion are interconnected and lead to complex mass transfer processes driving unique patterns of element fractionation. Here, we use diffusion couple experiments between felsic and mafic magmas (melt + crystals ± volatiles) to document the formation of large gradients in oxygen fugacity at the magma-magma mixing interface. Reducing and oxidising boundary layers at the interface develop rapidly and remain in dynamic disequilibrium for days to possibly weeks. We suggest that the observed transient redox gradient is caused by cation transfer across the interface where the required counter flux of electron holes is insufficient to compensate an evolving electron hole gradient. Such boundary layer redox effects may control fractionation of polyvalent and chalcophile elements and moderate, for example, Cu/Au ratios in arc-related porphyry ore deposits.


A. Fiege, P. Ruprecht, A. Simon
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Geochem. Persp. Let. (2017) 3, 190-199 | doi: 10.7185/geochemlet.1722 | Published 15 May 2017

Temporal variation in relative zircon abundance throughout Earth history

Abstract:
Zircon is the preeminent chronometer of deep time on Earth, informing models of crustal growth and providing our only direct window into the Hadean Eon. However, the quantity of zircon crystallised per unit mass of magma is highly variable, complicating interpretation of the terrestrial zircon record. Here we combine zircon saturation simulations with a dataset of ∼52,000 igneous whole rock geochemical analyses to quantify secular variation in relative zircon abundance throughout Earth history. We find dramatically increasing zircon abundance per mass of magma through geologic time, suggesting that the zircon record underestimates past crustal volume even if preservation bias is eliminated. Similarly, zircons were even less likely to crystallise from low-silica magmas in early Earth history than they are today, together suggesting that the observed Hadean zircon record may require a larger volume of generally felsic Hadean crust than previously expected.


C.B. Keller, P. Boehnke, B. Schoene
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Geochem. Persp. Let. (2017) 3, 179-189 | doi: 10.7185/geochemlet.1721 | Published 20 April 2017

The cosmic molybdenum-neodymium isotope correlation and the building material of the Earth

Abstract:
The isotopic similarity of enstatite chondrites and Earth has often been cited as evidence that the Earth is made of enstatite chondrite-like material. Here we show, however, that enstatite chondrites exhibit nucleosynthetic molybdenum (Mo) isotope anomalies and, therefore, cannot represent the sole building blocks of the Earth. Enstatite and ordinary chondrites together with the Earth’s mantle plot on a cosmic Mo-Nd isotope correlation line that reflects varying proportions of s-process matter in these samples. This correlation indicates that the nucleosynthetic makeup of Earth’s building material did not change over time and that Earth, on average, accreted from bodies that originated closer to the Sun and were enriched in s-process matter compared to known chondrites. As such, any contribution of chondrites to Earth’s accreting material must be compensated by the addition of s-process enriched bodies. This material is not present in our meteorite collections, but may have been sampled by Venus or Mercury. The s-process enriched nature of the Earth can fully account for its higher 142Nd compared to chondrites, which therefore does not require an early differentiation of Earth’s mantle.


J. Render, M. Fischer-Gödde, C. Burkhardt, T. Kleine
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Geochem. Persp. Let. (2017) 3, 170-178 | doi: 10.7185/geochemlet.1720 | Published 14 April 2017