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Volume 10

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

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

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

Thermodynamic controls on redox-driven kinetic stable isotope fractionation

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

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

A lunar hygrometer based on plagioclase-melt partitioning of water

The Moon was initially covered by a magma ocean. Hydrogen detected in plagioclase of ferroan anorthosites, the only available samples directly crystallised from the lunar magma ocean (LMO), can be used to quantify LMO hydrogen content. We performed experiments to determine plagioclase-melt partition coefficients of water under LMO conditions with water contents of co-existing plagioclase and melt quantified using Fourier-Transform Infrared Spectroscopy. Results indicate lunar plagioclase can incorporate approximately one order of magnitude more water than previously assumed. Using measured water contents of lunar plagioclase, this suggests that ~100 μg/g H2O equivalent was present in the residual magma when 95 % of the initial LMO had crystallised. Our results constrain initial LMO water contents to ~ 5 μg/g H2O equivalent if water was conserved throughout LMO evolution. If on the other hand the initial LMO contained >1000 μg/g water as suggested by experiments on LMO crystallisation, >99 % hydrogen degassing occurred during the evolution of the LMO.

Y.H. Lin, H. Hui, Y. Li, Y. Xu, W. van Westrenen

Geochem. Persp. Let. (2019) 10, 14–19 | doi: 10.7185/geochemlet.1908 | Published 26 March 2019

Onset of new, progressive crustal growth in the central Slave craton at 3.55 Ga

Ancient rock samples are limited, hindering the investigation of the processes operative on the Earth early in its history.  Here we present a detailed study of well-exposed crustal remnants in the central Slave craton that formed over a 1 billion year magmatic history. The tonalitic-granodioritic gneisses analysed here are broadly comparable to common suites of rocks found in Archean cratons globally. Zircon Hf isotope data allow us to identify a major change in the way continental crust was formed in this area, with a shift to distinctly positive εHf starting at ~3.55 Ga. The crust production processes and spatial distribution of isotopic compositions imply variable interaction with older crust, similar to the relationships seen in modern tectonic settings; specifically, long-lived plate margins. A majority of the Slave craton might have been formed by a similar mechanism.

J.R. Reimink, D.G. Pearson, S.B. Shirey, R.W. Carlson, J.W.F. Ketchum

Geochem. Persp. Let. (2019) 10, 8–13 | doi: 10.7185/geochemlet.1907 | Published 26 March 2019

Corrigendum to “Plume-lithosphere interaction, and the formation of fibrous diamonds” by Broadley et al., 2018

M.W. Broadley, H. Kagi, R. Burgess, D. Zedgenisov, S. Mikhail, M. Almayrac, A. Ragozin, A. Pomazansky, H. Sumino

Geochem. Persp. Let. (2019) 10, 7 | doi: 10.7185/geochemlet.1825cor | Published 13 March 2019

Widespread and intense wildfires at the Paleocene-Eocene boundary

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

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