Steven Goderis

Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition

High concentrations of iridium have been reported in terrestrial sediments dated at 12.9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the Rancholabrean fauna, changes in the Paleoindian cultures, and the onset of the Younger Dryas cooling [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104:16016–16021]. Here, we report platinum group element (PGE: Os, Ir, Ru, Rh, Pt, Pd), gold (Au) concentrations, and 187Os/188Os ratios in time-equivalent terrestrial, lacustrine, and marine sections to seek robust evidence of an extraterrestrial contribution. First, our results do not reproduce the previously reported elevated Ir concentrations. Second, 187Os/188Os isotopic ratios in the sediment layers investigated are similar to average crustal values, indicating the absence of a significant meteoritic Os contribution to these sediments. Third, no PGE anomalies distinct from crustal signatures are present in the marine record in either the Gulf of California (DSDP 480, Guaymas Basin) or the Cariaco Basin (ODP 1002C). Our data show no evidence of an extraterrestrial (ET)-PGE enrichment anomaly in any of the investigated depositional settings investigated across North America and in one section in Belgium. The lack of a clear ET-PGE signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the Bølling–Allerød/Younger Dryas transition.

Development of an isolation procedure and MC-ICP-MS measurement protocol for the study of stable isotope ratio variations of nickel

Variations in the isotopic composition of Ni resulting from natural mass-dependent processes in terrestrial or extraterrestrial conditions, inhomogeneous distribution of nucleosynthetic components and/or ingrowth from radioactive parent nuclides, help us to further understand the early formation history of Solar System materials and the nature of the processes these materials subsequently experienced. In studies of Ni isotope systematics, mass-dependent variations in the isotopic composition of Ni are often bypassed because of the challenges associated with the sample preparation. At the level of natural variation studied, Ni isotope ratio measurements are extremely sensitive to spectral interference, artificial on-column isotope fractionation and possibly even to the mass bias correction model applied. To adequately address these complications, an isolation procedure and measurement protocol relying on multi-collector ICP-mass spectrometry (MC-ICP-MS) have been designed and validated in this work. The overall reproducibility obtained based on repeated measurement of a Sigma-Aldrich high-purity Ni standard is 0.036‰, 0.049‰, 0.078‰ and 0.53‰ for δ60/58Ni, δ61/58Ni, δ62/58Ni and δ64/58Ni, respectively (n = 14; 2 SD). Nickel isotope ratio variations have been studied in a set of iron meteorites and geological reference materials, and the results obtained, except for those suffering from an elevated 64Zn background, show good agreement with the available literature data. By using the flexible generalized power law with a variable discrimination exponent and the three-isotope method, the processes underlying natural mass fractionation of Ni for terrestrial reference materials were found to have a mixed equilibrium/kinetic nature. Mass-dependent Ni fractionation was observed between sample fractions of the Canyon Diablo iron meteorite, and the extracted fractionation factor β corresponds to isotope partitioning following the power law.

Evaluation of pneumatic nebulization and ns-laser ablation ICP-MS for bulk elemental analysis and 2-dimensional element mapping of iron meteorites

The capabilities and limitations of nanosecond laser ablation ICP-mass spectrometry for bulk and spatially resolved (elemental mapping) analysis of iron meteorites were assessed. The quantitative data obtained were compared to those obtained i. via multi-element solution ICP-MS (after digestion) relying on external calibration and, ii. high-accuracy determination of selected platinum group elements using solution ICP-MS after target element isolation using anion exchange chromatography and deploying isotope dilution. Results generated by the different methods described show good agreement. Significant matrix effects were observed to affect the results of the ns-LA-ICP-MS analysis of iron meteorites, making quantification via a matrix-matched standard a prerequisite. Careless use of intensity distribution maps without proper correction for laser-solid coupling efficiency can lead to incorrect interpretation of the element maps. ns-LA-ICP-MS was shown to be suitable for fast and quasi-nondestructive analysis of iron meteorites, not only homogeneous ataxites and hexahedrites, but also more heterogeneous ones, when considering ablated areas of a sufficient size. In the context of elemental mapping using LA-ICP-MS, Pearsons product–moment correlation analysis was demonstrated to be a powerful tool that can provide valuable information on the fractionation of the elements in the parent bodies of meteorites.

Reproducibility of laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) measurements in mussel shells and comparison with micro-drill sampling and solution ICP–MS

The accumulation of trace elements (Mg, Mn, Sr, Ba) in Unio pictorum L. mussel shells from Lake Balaton has been assessed using a Laser Ablation (LA) system coupled to either a quadrupole-based or a sector-field inductively coupled plasma-mass spectrometer (ICP - MS), as well as by a combination of micro-drill sampling and solution ICP-MS. The LA-ICP-MS measurements were carried out in the holes made by the micro-drilling system. The longitudinal concentration profiles obtained with the different methods show similar patterns. However, the absolute concentrations determined at individual spots (holes) can be quite different. Especially Ba shows erratic peaks at a very small spatial scale. A paired, two-sample t-test between LA-ICP-MS longitudinal profiles and between LA-ICP-MS and micro-drill/solution ICP-MS profiles indicates that, in most cases, there is no significant difference between the concentration profiles of Ba, Mg, Mn and Sr. Average shell concentrations of Mg, Mn, Sr and Ba, as obtained by LA-ICP-MS and micro-drill/solution ICP-MS, compare well with bulk shell concentrations as obtained by acid digestion/ICP-MS of larger shell pieces. Next to the four elements mentioned above, also the concentrations of Cd, Co, Cr, Cu, Ni, Pb and Zn could be determined by bulk shell analysis. The element concentrations in 11 shells, all sampled at the same site, show a relative standard deviation (RSD) between 2% (Ni) and 46% (Zn). LA-ICP-MS and micro-drill solution ICP-MS are not sensitive enough for the determination of ultra-trace elements in Lake Balatons mussel shells. We estimated the amount of shell material necessary to determine Ni, Pb, Cr and Cu by micro-drilling ICP-MS (for a concentration that equals 3 times their limit of detection) at, respectively, 0.04, 0.82, 2.7 and 0.4 mg, while the amount sampled by micro-drilling is about 0.06 mg.

Application of laser ablation-ICP-mass spectrometry for 2-dimensional mapping of element distributions in a Late Archean impact spherule layer

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been successfully applied to visualize the 2-dimensional distribution of various major (Mg, Mn, Fe) and trace (Cr, Co, Ni, Ir, Pt) elements in the Late Archaean Paraburdoo impact spherule layer (PB SL). As this spherule layer is highly enriched in meteoritic material, this study particularly focuses on the distribution of siderophile elements. Multi-elemental maps with a lateral resolution of 15 μm, covering surface areas of approximately 1.5 mm × 1.5 mm and multiple impact spherules, were acquired by LA-ICP-MS using two different ablation – standard (circular) and teardrop – cells. It was observed that with the teardrop cell, the elemental maps follow the mineralogy as displayed by the optical microscopy image more closely, with limited memory and washout effects in comparison to the standard cell. In order to acquire representative quantitative data, 100 μm diameter spot LA-ICP-MS analysis in a vertical transect through the SL was performed additionally. Quantification of element concentrations in the PB SL was carried out using external calibration procedures. The validity of this approach was evaluated by comparison of the averages of the determined transect spot concentrations to bulk rock geochemical data and it was found that the mean concentrations fall within the uncertainty of the bulk geochemical analysis, but the results show biases of 5.9% for Pt, 16% for Ir, 11% for Cr and 38% for Ni. In addition, the bulk rock Pt/Ir elemental ratio obtained upon transect LA-ICP-MS analysis was reported and it was observed that the slope of the regression line (1.06) and the mean value (1.23) for all data points measured with LA-ICP-MS equal the bulk rock Pt/Ir value of 1.08–1.43 measured by fire assay/solution ICP-MS for all PB SL outcrops at the Governor site.

First detection of extraterrestrial material in ca. 2.49 Ga impact spherule layer in Kuruman Iron Formation, South Africa

Thin layers rich in formerly molten spherules interpreted as distal ejecta from large impacts by extraterrestrial bodies have been found in 8 stratigraphic units deposited between ca. 2.63 Ga and 2.49 Ga and attributed to a minimum of 4 separate impacts. Here we report geochemical evidence of extraterrestrial material in the only one of these spherule layers where it has not been previously reported, the Kuruman spherule layer (KSL) in the Kuruman Iron Formation, a banded iron formation (BIF) in the Griqualand West Basin (South Africa). We identified the KSL in 3 drill cores separated by as much as ∼350 km and analyzed 2 core samples that have a mean Ir concentration of ∼12.9 ppb and nearly chondritic interelement ratios of platinum group elements Ir, Ru, Pt, and Rh. This suggests that the samples contain ∼1%–3% by mass extraterrestrial material even though the spherules are highly diluted by ambient sediment. Our geochemical data strongly support the correlation of the KSL with the Dales Gorge spherule layer (DGSL) in a penecontemporaneous BIF in the Hamersley Basin (Western Australia). The KSL and DGSL are close matches in terms of major and various trace element contents and the DGSL has a comparable Ir content of ∼11.5 ppb. Therefore it is very likely the KSL and DGSL are distal ejecta from a ca. 2.49 Ga impact by a single extraterrestrial object >10 km across. The lack of any significant changes in the stratigraphic succession in either basin also implies that large impacts alone are not sufficient to cause long-term changes in Earth’s surface environments.

Strontium isotope analysis on cremated human remains from Stonehenge support links with west Wales

Cremated human remains from Stonehenge provide direct evidence on the life of those few select individuals buried at this iconic Neolithic monument. The practice of cremation has, however, precluded the application of strontium isotope analysis of tooth enamel as the standard chemical approach to study their origin. New developments in strontium isotopic analysis of cremated bone reveal that at least 10 of the 25 cremated individuals analysed did not spend their lives on the Wessex chalk on which the monument is found. Combined with the archaeological evidence, we suggest that their most plausible origin lies in west Wales, the source of the bluestones erected in the early stage of the monument’s construction. These results emphasise the importance of inter-regional connections involving the movement of both materials and people in the construction and use of Stonehenge.

Solar System: Lethal billiards

A huge collision in the asteroid belt 160 million years ago sent fragments bagatelling around the inner Solar System. One piece might have caused the mass extinction that wiped out the dinosaurs 65 million years ago.