About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve ...long‐standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long‐standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories.
Key Points
Standardization and correction procedures for carbonate clumped isotopes are reviewed
We propose a carbonate‐based standardization scheme to improve reproducibility
We demonstrate improved interlaboratory comparability of clumped isotope measurements
Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, interlaboratory ...discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale.
Key Points
The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurements
Measurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishable
We propose new consensus values for a set of seven carbonate reference materials and updated guidelines to report clumped isotope measurements
The chemical and thermal stability of organic nitrogen (N) in pyrogenic organic matter (PyOM) was examined by charring casein at 350
°C and 450
°C. The alteration was compared with that observed for ...char derived from lignin, cellulose, grass and wood. With respect to heating, casein showed a considerably higher stability than cellulose. Comparable proportions of carbon (C) and N were recovered, supporting the idea that black nitrogen (BN) represents an integral part of the char structure. Although some amides were still present, they lost importance with increasing temperature. Charring of grass revealed an enrichment in N compounds because of the low thermal stability of cellulose. The similarity in the resulting nuclear magnetic resonance (NMR) spectra to those of casein char confirmed that BN can play a major role in the chemical composition of plant char.
Subjecting the chars to oxidation with acidic dichromate demonstrated that, in spite of their relatively high resistance to heat, the N-containing compounds of the chars were less recalcitrant than the components of the cellulose char. Thus, in soil, N-rich chars are likely to be underestimated on the basis of this method. On the other hand, for an ancient paddy soil whose N-containing char compounds were calculated to account for ca. 25% of the total organic C in the soil. This clearly underlines the pedogenic stability of BN and confirms that it has the potential to contribute significantly to the refractory soil organic matter pool.
In the last thirty years, portable X-ray fluorescence (pXRF) has grown from prototypes to being a key technique for field geochemical analyses, especially for mining and environmental applications. ...This technology provides real-time or near real-time decision support for operational field decisions (exploration, mining, site remediation or waste management), provides a cost-saving alternative to classical laboratory analysis programs, and deals efficiently with remote or harsh field conditions. The ability to rapidly collect a large number of samples and replicate analyses facilitates acquisition of higher data density compatible with geostatistics.
pXRF can be used profitably for sample screening and selection tasks, dynamic sampling plans based on field observations and measurements, grid mapping of a site and determining relative element abundance. In waste management and remediation, pXRF is used to identify unknown waste composition, and verify waste loads before disposal or treatment.
In all applications, having robust QA/QC plans and systematic laboratory controls on selected samples are essential for ensuring reliable measurements. The best overall results are usually achieved through a clever combination of field (pXRF) and lab data, with pXRF providing the bulk of the data at low cost, on larger data sets and potentially with better reliability than lab-based campaigns based on a limited number of samples.
•pXRF is now a key field technique for mining and environmental applications•pXRF provides near real-time decision support for operational field decisions•A cost-saving and rapid alternative to laboratory analysis, useful for screening•pXRF requires robust QA/QC plans and laboratory controls on selected samples•Data quality can be better than laboratory analyses through denser sampling
Paleoredox conditions are commonly evaluated based on elemental proxies but, despite their frequency of use, most of these proxies have received little comparative evaluation or assessment of their ...range of applicability to paleomarine systems. Here, we evaluate 21 elemental proxies, including six proxies based on the C-S-Fe-P system (TOC, S, TOC/S, DOPT, Fe/Al, Corg/P), nine proxies based on trace-metal enrichment factors (CoEF, CrEF, CuEF, MoEF, NiEF, PbEF, UEF, VEF, ZnEF), and six additional proxies from Jones and Manning (1994) (U/Th, Uauth, V/Cr, Ni/Co, Ni/V, (Cu+Mo)/Zn), in 55 Phanerozoic marine formations. We used principal components analysis (PCA) to determine relationships among these 21 proxies in each formation and then sought to identify patterns across the full database. The first principal component (PC1) accounted for 40.1% of total dataset variance on average, with the highest median loadings on trace-metal enrichment factors (NiEF 0.82, MoEF 0.76, all nine >0.50). The next highest median loadings are on C-S-Fe-P proxies (TOC 0.58, DOPT 0.30, Corg/P 0.28), with bimetal proxies yielding uniformly lower loadings (Ni/Co 0.18, V/Cr 0.13). PCA of the factor loadings for the 55 study formations demonstrated associations among the 21 elemental proxies linked to specific sediment host phases: (1) an organic cluster associated with TOC, Mo, V, and Zn, (2) a uranium cluster associated with all U-based proxies, and (3) a sulfide cluster associated with S and Fe as well as the trace metals Co, Cu, Ni, and Pb (i.e., the major and typical minor constituents of diagenetic pyrite).
The findings of the present study have important ramifications for use of elemental proxies for paleoredox analysis. First, all of the proxies examined here are influenced by environmental redox conditions to some degree, although the degree of redox influence on any given proxy can vary considerably from one formation to the next. Second, sedimentary enrichment of most proxies depends on the presence of specific mineral and organic host phases, and evaluation of elemental redox proxy data requires an understanding of how elements are partitioned among those phases. Third, no single proxy is a universally reliable redox indicator, although some are more consistently useful than others—notably, TOC and trace-metal EFs. Fourth, because of this inherent variability in proxy response, adoption of redox proxy thresholds established in earlier published studies is discouraged. Instead, we recommend that future redox studies establish redox thresholds on a formation-specific basis through internal cross-calibration of multiple redox proxies.
•Evaluation of 21 elemental redox proxies, including C-S-Fe-P, trace metal ratios and EFs.•The most effective redox proxies are trace-metal EFs (Re, Ni, Mo) and TOC.•No proxy is universally reliable; all redox analyses should be on a formation-specific basis.•All elemental proxies are linked to a specific sediment fraction (organic, sulfide, etc.).•Different proxies thus inform about redox influences on specific sediment fractions.
► Charcoal aromaticity increases quickly between 200
°C and 400
°C. ► Aromatic condensation of charcoals increases smoothly between 350
°C and 1000
°C. ► Charcoal produced above 700
°C contain ...PAH-like domains with >19 rings. ► NMR and BPCA analyses of charcoals give compatible results.
Quantifying the role of black carbon (BC) in geochemical processes is difficult due to the heterogeneous character of its chemical structure. Chestnut wood charcoal samples produced at heat treatment temperatures (HTT) from 200–1000
°C were analysed using two different solid state
13C NMR techniques. First, aromaticity was determined as the percentage of total signal present in the aromatic region of
13C direct polarisation (DP) spectra. This was found to increase through the low temperature range of 200–400
°C; at higher temperatures, aromaticity was found to be >90%. Second, aromatic condensation was determined through the measurement of the chemical shift of
13C
benzene sorbed to the charcoals, which is influenced by the presence of “ring currents” in the aromatic domains of the charcoals. This technique was less sensitive to molecular changes through the lower temperature range, but showed there was a smooth increase in the degree of condensation of the aromatic structures with increasing temperature through the temperature range 400–1000
°C.
Ab initio molecular modelling was used to estimate the size of aromatic domains in the charcoals based on the strength of the ring currents detected. These calculations indicated that charcoals produced at temperatures below 500
°C contain aromatic domains no larger than coronene (7 ring). At higher temperatures the size of these domains rapidly increases, with structures larger than a 19 ring symmetrical PAH being predominant in charcoals produced at temperatures above 700
°C. Data from this study were found to be generally consistent with previously published measurements using the benzenepolycarboxylic acid (BPCA) molecular marker method on the same set of samples.
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