Matthias Sinnesael

Trace element analyses of carbonates using portable and micro-X-ray fluorescence: performance and optimization of measurement parameters and strategies

Variations in elemental abundances in carbonate archives offer a wealth of information that can be used as a proxy for the palaeoenvironment and diagenetic history. The state-of-the-art portable handheld X-ray Fluorescence (pXRF) and laboratory micro X-ray Fluorescence (μXRF) instruments provide a relatively inexpensive, fast and non-destructive way of acquiring these trace element composition data. However, there are well-known issues and limitations regarding the method of spectrum acquisition and the conversion of XRF spectra into quantitative elemental mass fractions. This study offers a guideline for the appropriate use of these XRF techniques for the study of carbonates. Using certified calcium carbonate and dolomite standards, accuracy and reproducibility of a pXRF (Bruker AXS Tracer IV) and a μXRF (Bruker M4 Tornado) device are tested under various measurement conditions. The experimental set-up allowed for the variation of several parameters, including the measurement area, integration time, quantification method and measurement strategy. The effects on the accuracy and reproducibility of the quantified elemental abundance results are examined to assess the optimal performance conditions for both devices for the determination of trace element abundances in natural carbonates. The limits of detection and quantification are evaluated for both instruments for a range of trace elements commonly used as palaeoenvironmental proxies (e.g. Sr, Mn and Fe). The quality of the XRF spectra is evaluated using spectral processing software. As a result, two new methods for the determination of optimized parameter combinations are proposed for a range of commonly used elements. The Time of Stable Reproducibility (TSR) is based on optimizing the measurement reproducibility by examining the change of the relative standard deviation per time unit and proposing an integration time threshold for reproducible measurements. The Time of Stable Accuracy (TSA) is based on optimizing the measurement accuracy by studying changes in accuracy as a function of increasing integration time and defining an integration time threshold for accurate measurements. An overview table including minimum integration times by which a reliable measurement is achieved is provided for all analyzed elements and experimental set-ups for this study. However, the methodological approach that is developed here is applicable to other (carbonate) materials as well. A comparison between the two X-ray fluorescence instruments allows the evaluation of their respective advantages and disadvantages. Finally, we recommend optimal measurement strategies and techniques for specific research questions.

Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity and obliquity

The Late Devonian envelops one of Earth’s big five mass extinction events at the Frasnian–Famennian boundary (374 Ma). Environmental change across the extinction severely affected Devonian reef-builders, besides many other forms of marine life. Yet, cause-and-effect chains leading to the extinction remain poorly constrained as Late Devonian stratigraphy is poorly resolved, compared to younger cataclysmic intervals. In this study we present a global orbitally calibrated chronology across this momentous interval, applying cyclostratigraphic techniques. Our timescale stipulates that 600 kyr separate the lower and upper Kellwasser positive δ13C excursions. The latter excursion is paced by obliquity and is therein similar to Mesozoic intervals of environmental upheaval, like the Cretaceous Ocean-Anoxic-Event-2 (OAE-2). This obliquity signature implies coincidence with a minimum of the 2.4 Myr eccentricity cycle, during which obliquity prevails over precession, and highlights the decisive role of astronomically forced “Milankovitch” climate change in timing and pacing the Late Devonian mass extinction.Understanding of Late Devonian mass extinction mechanisms is poor due to imprecise stratigraphies. Here, using cyclostratigraphic techniques, the authors present a global orbitally-calibrated chronology and reveal the key role of astronomically-forced Milankovitch climate change.

Stratigraphic record of the asteroidal Veritas breakup in the Tortonian Monte dei Corvi section (Ancona, Italy)

The discovery of elevated concentrations of the cosmogenic radionuclide ^3He in deep-sea sediments from Ocean Drilling Program (ODP) Site 926 (Atlantic Ocean) and ODP Site 757 (Indian Ocean) points toward accretion of extraterrestrial matter, probably as a result of the catastrophic disruption of a large asteroid that produced the Veritas family of asteroids at ca. 8.3 ± 0.5 Ma, and which may have had important effects on the global climatic and ecologic systems. Here, we investigated the signatures possibly related to the Veritas event by performing a high-resolution multiproxy stratigraphic analysis through the late Tortonian−early Messinian Monte dei Corvi section near Ancona, Italy. Closely spaced bulk-rock samples through a 36-m-thick section, approximately spanning from ca. 9.9 Ma to ca. 6.4 Ma, show an ∼5-fold ^3He anomaly starting at ca. 8.5 Ma and returning to background values at ca. 6.9 Ma, confirming the global nature of the event. We then analyzed, at 5 cm intervals, bulk-rock samples for sedimentary and environmental proxies such as magnetic susceptibility, calcium carbonate content, total organic carbon, and bulk carbonate δ^(18)O and δ^(13)C, through a 21-m-thick section encompassing the ^3He anomaly. Available high-resolution sea-surface temperature data (via alkenone analyses) for this site show a temperature decrease starting exactly at the inception of the ^3He anomaly. Cyclostratigraphic fast-Fourier-transform spectral analyses of the proxies indicate an age of 8.47 ± 0.05 Ma for the inception of the ^3He anomaly. A search for impact ejecta (analogous to what is present in the late Eocene, where both a ^3He anomaly and large-scale impact events are recorded) was not successful. Detailed cyclostratigraphic analyses of our data suggest that the changes in the stable isotope series and environmental proxy series through this late Tortonian time interval had a common forcing agent, and that perturbations of orbitally forced climate cycles are present exactly through the interval with the enhanced influx of extraterrestrial ^3He. Thus, the chemostratigraphic evidence for a collisional event that created the Veritas family of asteroids, coinciding with climate perturbations on Earth, suggests yet another form of interaction between Earth and the solar system.