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).

The Cambrian explosion, the rapid appearance of most animal phyla in the geological record, occurred concurrently with bottom seawater oxygenation. Whether this oxygenation event was triggered through enhanced nutrient supply and organic carbon burial forced by increased continental weathering, or by species engaging in ecosystem engineering, remains a fundamental yet unresolved question. Here we provide evidence for several simultaneous developments that took place over the Ediacaran–Cambrian transition: expansion of siliceous sponges, decrease of the dissolved organic carbon pool, enhanced organic carbon burial, increased phosphorus removal and seawater oxygenation. This evidence is based on silicon and carbon stable isotopes, Ge/Si ratios, REE-geochemistry and redox-sensitive elements in a chert-shale succession from the Yangtze Platform, China. According to this reconstruction, sponges have initiated seawater oxygenation by redistributing organic carbon oxidation through filtering suspended organic matter from seawater. The resulting increase in dissolved oxygen levels potentially triggered the diversification of eumetazoans.

Various lines of evidence suggest that material isotopically similar to enstatite chondrites may have accreted to the terrestrial planets. However, the enrichment of light Si isotopes in bulk enstatite chondrites is not easy to reconcile with the heavy Si isotopic composition of the Bulk Silicate Earth (BSE). To investigate the origin of the light Si isotopic composition of enstatite chondrites, we have obtained in-situ Si isotope data and simultaneously major-and trace element data in silicate and metal phases of chondrules, a metal-troilite spherule, and matrix from the enstatite chondrites Sahara 97072 (EH3) and Indarch (EH4) using laser ablation split stream-ICP-MS, which combines femto-second LA-MC-ICP-MS and Quadrupole-ICP-MS. Silicates in chondrules show variations in δ 30 Si (‰ variations of 30 Si/ 28 Si relative to NBS-28) ranges from-1.06 ± 0.13‰ (2 S.E.) to-0.38 ± 0.11‰. δ 30 Si in matrix silicates ranges from-0.96 ± 0.18‰ to-0.22 ± 0.12‰. The δ 30 Si-value of silicate phases varies independently of Mg/Si, ruling out simple equilibrium condensation from nebular gas. Some silicates in both enstatite chondrites have δ 30 Si-values like CI chondrites, whereas Si in other silicates is isotopically lighter, suggesting that the precursor materials of EH chondrites were already depleted in heavy Si isotopes. The metal phases in the matrix show average δ 30 Si of-6.0 ± 0.6‰. In spite of different metamorphic grades, the fractionation of Si isotopes between matrix metal and silicates in Sahara 97072 and Indarch shows no systematic differences, and thus no re-equilibration of Si 2 isotopes occurred between silicates and metal at metamorphic temperatures below 900 K. The δ 30 Si-value of metal from a metal-troilite spherule from Sahara 97072 (-8.24 ± 0.12‰) is lower than that of matrix metals. These differences were likely inherited from different formation environments of matrix-and spherule metal. If metal formation occurred under equilibrium conditions, then matrix metal may have formed at higher temperatures than the MTS metal. or at similar temperatures but slightly lower oxygen fugacities, or the MTS metal equilibrated with gas or silicates which were not incorporated into EH chondrites because they were lost from the EH chondrite formation region. Alternatively, the differences in δ 30 Si of different metals could also reflect variable kinetic isotope fractionation during the formation of metal and exsolution of perryite. The considerably lower δ 30 Si-values of bulk EH chondrites compared to CI-and other chondrites partly reflects the presence of Si bearing metal with isotopically light Si and partly silicates with isotopically light Si. The latter indicate loss of a heavy Si-rich silicate component from the EH3 formation region, presumably together with refractory elements. Although the Si isotopic composition of bulk EH chondrites precludes that these represent major building material of the Earth, the combination of complementary heavy Si isotope-and refractory element-enriched reduced materials and ca...

Si. We suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert. We infer that past environmental conditions can be reconstructed with high fidelity from the stable Si isotope composition of chert when initial seawater Si concentrations were high (such as in the Precambrian). Exchange of Si between layers during phase transformation (from opal-A to opal-CT and from opal-CT to quartz) is impeded when variable amounts of detrital minerals are present, because they control rates of silica phase transformation and hence the timing of dissolution-reprecipitation during burial.