ALBERT

Description: In most chemical reactions, stable isotopes are fractionated in a mass-dependent manner, yielding correlated isotope ratios in elements with three or more stable isotopes. The proportionality between isotope ratios is set by the triple isotope fractionation exponent θ that can be determined precisely for, e.g., sulfur and oxygen by IRMS, but not for metal(loid) elements due to the lower precision of MC-ICP-MS analysis and smaller isotopic variations. Here, using Mg as a test case, we compute a complete metrologically robust uncertainty budget for apparent θ values and, with reference to this, present a new measurement approach that reduces uncertainty on θ values by 30%. This approach, namely, direct educt-product bracketing (sample–sample bracketing), allows apparent θ values of metal(loid) isotopes to be determined precisely enough to distinguish slopes in three-isotope space. For the example of Mg, we assess appropriate quality control standards for interference-to-signal ratios and report apparent θ values of carbonate–seawater pairs. We determined apparent θ values for marine biogenic carbonates, where the foraminifera Globorotalia menardii yields 0.514 ± 0.005 (2 SD), the coral Porites, 0.515 ± 0.006 (2 SD), and two specimens of the giant clam Tridacna gigas, 0.508 ± 0.007 (2 SD) and 0.509 ± 0.006 (2 SD), documenting differences in the uptake pathway of Mg among marine calcifiers. The capability to measure apparent θ values more precisely adds a new dimension to metal(loid) δ values, with the potential to allow us to resolve different modes of fractionation in industrial and natural processes.

Description: The ratio of 18O to 16O in cherts and other chemical sediments has increased by about 15‰ over geological time, but the cause of this increase is debated. Here, we provide a 1D sediment-column model designed to investigate the role of diagenesis, and specifically the heat flow through marine sediments, in setting the chert oxygen isotope ratios. The model simulates the transformation of amorphous silica (opal-A) to crystalline quartz via an intermediate phase by using a silicon mass balance that is driven by the kinetics and thermodynamics of silica phase dissolution and (re)precipitation. The model demonstrates that heat flow through marine sediments influences the rate, and therefore depths, temperatures, and oxygen isotope compositions, at which cherts form. The implication is that because global heat flow from the solid Earth has decreased through geological time, heat flow is an important contributing factor to the long-term trend in chert oxygen isotope composition. The model is provided as a set of Matlab scripts (".m" files) and assorted input datasets provided as standard plain text files. The model is described in full in the manuscript "Chert oxygen isotope ratios are driven by Earth's thermal evolution" by Michael Tatzel, Patrick J. Frings, Marcus Oelze, Daniel Herwartz, Nils K. Lünsdorf, and Michael Wiedenbeck, and in the online Supporting Information associated with the manuscript. Once downloaded and unzipped, the files should be added to the local Matlab search path. The parameters of interest can be changed in the first few lines of 'chertKineticModel.m'. No other files need to be opened or modified. These files have been tested in Matlab R2020a running on Mac OS X 12.2.1 and in Matlab R2022b on Mac OS X 12.6.1.

Description: The continuous improvement of analytical procedures using multi‐collector technologies in ICP‐mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERM‐AE143, ERM‐AE144, ERM‐AE145, IRMM‐009 and NIST SRM 980 were cross‐calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ25/24Mg values and less than 0.037‰ for the δ26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM‐AE143 anchors all magnesium δ‐scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM‐AE144 and ERM‐AE145, which are product and educt of a sublimation–condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θapp, the fractionation relationship ln α(25/24Mg)/ln α(26/24Mg).

Description: The 18O/16O ratio of cherts (δ18Ochert) increases nearly monotonically by ~15‰ from the Archean to present. Two end-member explanations have emerged: cooling seawater temperature (TSW) and increasing seawater δ18O (δ18Osw). Yet despite decades of work, there is no consensus, leading some to view the δ18Ochert record as pervasively altered. Here, we demonstrate that cherts are a robust archive of diagenetic temperatures, despite metamorphism and exposure to meteoric fluids, and show that the timing and temperature of quartz precipitation and thus δ18Ochert are determined by the kinetics of silica diagenesis. A diagenetic model shows that δ18Ochert is influenced by heat flow through the sediment column. Heat flow has decreased over time as planetary heat is dissipated, and reasonable Archean-modern heat flow changes account for ~5‰ of the increase in δ18Ochert, obviating the need for extreme TSW or δ18Osw reconstructions. The seawater oxygen isotope budget is also influenced by solid Earth cooling, with a recent reconstruction placing Archean δ18OSW 5 to 10‰ lower than today. Together, this provides an internally consistent view of the δ18Ochert record as driven by solid Earth cooling over billion-year timescales that is compatible with Precambrian glaciations and biological constraints and satisfyingly accounts for the monotonic nature of the δ18Ochert trend.