Contrasting microfossil preservation and lake chemistries within the 1200–1000 Ma Torridonian Supergroup of NW Scotland

We acknowledge the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. DW acknowledges funding from the European Commission and the Australian Research Council. This is publication number 838 from the Australian Research Council Centre of Excellence for Core to Crust Fluid Systems.Publisher PD

Timing and mechanism for intratest Mg/Ca variability in a living planktic foraminifer

Geochemical observations indicate that planktic foraminifer test Mg/Ca is heterogeneous in many species, thereby challenging its use as a paleotemperature proxy for paleoceanographic reconstructions. We present Mg/Ca and Ba/Ca data collected by laser ablation ICP-MS from the shells of Orbulina universa cultured in controlled laboratory experiments. Test calcite was labeled with Ba-spiked seawater for 12 h day or night calcification periods to quantify the timing of intratest Mg-banding across multiple diurnal cycles. Results demonstrate that high Mg bands are precipitated during the night whereas low Mg bands are precipitated during the day. Data obtained from specimens growing at 20 °C and 25 °C show that Mg/Ca ratios in both high and low Mg bands increase with temperature, and average test Mg/Ca ratios are in excellent agreement with previously published empirical calibrations based on bulk solution ICP-MS analyses. In general, Mg band concentrations decrease with increasing pH and/or [CO2−3] but this effect decreases as experimental temperatures increase from 20 °C to 25 °C. We suggest that mitochondrial uptake of Mg2+ from the thin calcifying fluid beneath streaming rhizopodial filaments may provide the primary locus for Mg2+ removal during test calcification, and that diurnal variations in either mitochondrial density or activity produce Mg banding. These results demonstrate that Mg banding is an inherent component of test biomineralization in O. universa and show that the Mg/Ca paleothermometer remains a fundamental tool for reconstructing past ocean temperatures from fossil foraminifers

Advances in the Analysis of Biogeochemical Interfaces: NanoSIMS to Investigate Soil Microenvironments

Since a NanoSIMS high-resolution secondary ion mass spectrometry instrument was first used for cosmochemistry investigations over a decade ago, both interest in NanoSIMS and the number of instruments available have significantly increased. However, SIMS comes with a set of challenges that are of both technical and conceptual nature, particularly for complex samples such as soils. Here, we synthesize existing research and provide conceptual and technical guidance to those who wish to investigate soil processes at the sub-micron scale using secondary ion mass spectrometry, specifically with NanoSIMS. Our review offers advice resulting from our own operational experience but also intends to promote synergistic research on yet unresolved methodological issues. We identify and describe the basic setup of a NanoSIMS instrument and important issues that may arise as a soil sample specimen is prepared for NanoSIMS analysis. This is complemented by discussions of experimental design, data analysis and data representation. Next to experimental design, sample preparation is the most crucial prerequisite for successful NanoSIMS analyses. We discuss the requirements and limitations for sample preparation over the size range from individual soil particles to intact soil structures such as macroaggregates or intact soil cores. For robust interpretation of data obtained by NanoSIMS, parallel spatial, textural (scanning electron microscopy, atomic force microscopy) or compositional analyses (scanning transmission X-ray microscopy) are often necessary to provide necessary context. We suggest that NanoSIMS analysis is most valuable when applied in concert with other analytical procedures and can provide powerful inference about small scale processes that can be traced via isotopic labeling or elemental mapping.Keywords: Scanning transmission X-ray microscopy, Fluorescent in situ hybridization, Secondary ion mass spectrometry, Isotopic enrichment, Rhizosphere, Transmission electron microscopy, Scanning electron microscopy, Near edge X-ray absorption fine structure spectrometry, Organo-mineral associations, Microaggregate

Subcellular tracking reveals the location of dimethylsulfoniopropionate in microalgae and visualises its uptake by marine bacteria

Phytoplankton-bacteria interactions drive the surface ocean sulfur cycle and local climatic processes through the production and exchange of a key compound: dimethylsulfoniopropionate (DMSP). Despite their large-scale implications, these interactions remain unquantified at the cellular-scale. Here we use secondary-ion mass spectrometry to provide the first visualization of DMSP at sub-cellular levels, tracking the fate of a stable sulfur isotope (34S) from its incorporation by microalgae as inorganic sulfate to its biosynthesis and exudation as DMSP, and finally its uptake and degradation by bacteria. Our results identify for the first time the storage locations of DMSP in microalgae, with high enrichments present in vacuoles, cytoplasm and chloroplasts. In addition, we quantify DMSP incorporation at the single-cell level, with DMSP-degrading bacteria containing seven times more 34S than the control strain. This study provides an unprecedented methodology to label, retain, and image small diffusible molecules, which can be transposable to other symbiotic systems.This work was supported by ANNiMS (Australian Government, Department of Education, Employment and Workplace Relations), the AMMRF Centre for Microscopy, Characterisation and Analysis (UWA) and by Australian Research Council Grant DE160100636

The effect of silica activity on the diffusion of Ni and Co in olivine

The diffusion of Ni and Co was measured at atmospheric pressure in synthetic monocrystalline forsterite (Mg2SiO4) from 1,200 to 1,500 °C at the oxygen fugacity of air, along [100], with the activities of SiO2 and MgO defined by either forsterite + peric

Reassessing the first appearance of eukaryotes and cyanobacteria

The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago and the onset of extreme ice ages. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from ∼2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10-20‰) of 13C relative to bulk sedimentary organic matter. Here we present micrometre-scale, in situ 13C/12C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in 13C, indicating that indigenous petroleum is 10-20‰ lighter than the extracted hydrocarbons. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism ∼2.2 Gyr ago and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78-1.68 Gyr ago and ∼2.15 Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis ∼2.7 Gyr ago and exclude previous biomarker evidence for a long delay (∼300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45-2.32 Gyr ago