Institutskolloquium

Title: Quantification of ^239,24xPu in environmental samples – method development, applications and perspectives

Summary: The measurement of ^239,240Pu in environmental samples can play a key role in investigating Earth (sub-)surface processes. Earth’s outermost skin has been enriched in such anthropogenic fallout radionuclides (FRNs) as a consequence of atmospheric nuclear weapon tests conducted in the 1950s and 1960s, providing distinct geochronological markers. The application of FRNs is well established, with ¹³⁷Cs (and the naturally occurring ²¹⁰Pb) being most commonly measured. However, ^239,240Pu activities are more decay-insensitive (t_1/2 ²³⁹Pu: ~24.1 ka; ²⁴⁰Pu: ~6.6 ka), and there is less soil inventory contamination arising from nuclear power plant accidents compared to ¹³⁷Cs. Additionally, only a few grams of sample material are required for a measurement.

Several years of research at the University of Cologne (UoC) have resulted in the development of tailored sample processing protocols that enable the routine extraction of ^239,24xPu from environmental samples. The set of isotopes in focus is completed by ²⁴⁴Pu (t_1/2 = 81.1 Myrs), whose natural abundances are of primordial origin with minimal interstellar influx (<1000 atoms cm^-2 Myr^-1). Any measurement of ^239,24xPu can be calibrated against the Cologne Pu-multiisotope Standard (ColPuS). Furthermore, a significant increase in measurement precision of isotopic ^239,24xPu concentrations has been achieved through Accelerator Mass Spectrometry (AMS), as compared to earlier methods such as conventional mass spectrometry or decay counting techniques. The development of such measurement capabilities at the UoC Centre for Accelerator Mass Spectrometry (CologneAMS) has enabled the exploration of the numerous potential applications of ^239,240Pu in deciphering modern Earth (sub-)surface processes in different settings. The sample processing workflow, combined with the precision of AMS measurements, allows for resolving specific ²³⁹Pu activities below ~5 mBq kg^-1 (~10^-15og/g). Consequently, spatial focus is placed on study sites where ultra-high precision measurements are required, such as in drylands in the southern hemisphere. A recent study traces dust influx into CaSO₄ crust in the Atacama Desert in northern Chile, while another application focuses on agroecosystems located in the Free State Province of South Africa. Here, plutonium concentrations indicate that SOM loss in arable land can primarily be caused by wind erosion dominating over other factors at play, such as mineralisation processes.

With the foundation for successful measurements of ^239,24xPu being laid, future work aims to further increase the measurement precision of ²⁴⁰Pu at CologneAMS. Additionally, an alternative measurement method employing a next generation MS/MS MC-ICP-MS (Thermo Fisher Neoma) is being considered to improve measurement sensitivities as compared to older systems. Together with the prospective implementation of further advances made in chemical processing, the overall sample throughput for samples containing ultra-low concentrations of ^239,24xPu may be increased.

Dr. Borhan Bagherpour

Privat

Title: Permian-Triassic Mass Extinction: Insights From Paleoenvironmental Studies and Stable Isotopes In China and Iran.

Summary: The largest mass extinction of the Phanerozoic, which occurred at the Permian-Triassic Boundary (PTB), was linked to significant paleoenvironmental shifts, including perturbation in the global carbon cycle, global warming, and changes in sedimentary regimes. In this talk, I present a multi-disciplinary approach to studying these paleoenvironmental changes during the Late Permian and earliest Triassic. This research focuses on the exceptional sedimentary records from South China and Central Iran, which offer a unique opportunity to examine the effects of these changes across a variety of marine depositional environments.

The occurrence of microbial limestones on platforms and the coeval development of oxygen-depleted black shales in deeper marine settings provide insight into the prevailing environmental conditions at different depths, as well as the importance of microbial communities in the recovery. Additionally, I discuss the paleoenvironmental impacts of the Emeishan large igneous province and the factors contributing to an earlier extinction event around the end-Guadalupian (mid-Permian), which may have also contributed to the severity of the PTB extinction. Our high-resolution data suggest that volcanic activity and carbon cycle disruptions during the Late Permian had more localized effects, in contrast to the widespread environmental changes observed at the PTB in the Early Triassic.

Ms. Elena Rogmann MSc.

Privat

Title: To garnet and beyond – The fate of aluminium in the Earth’s mantle

Summary: Subduction zones transport a variety of elements, such as aluminium and alkali earth metals, into the deep Earth. These elements, particularly aluminium, are more concentrated in subducting sediment and oceanic crust (MORB) compared to the surrounding mantle, which leads to distinct mineralogical differences. In the upper mantle and mantle transition zone, aluminium is primarily stored in garnet. However, as slabs descend through to the lower mantle, garnet becomes unstable, and aluminium is hosted by other minerals.

Two key lower mantle phases, the New Aluminous Phase (NAL) and Calcium-ferrite Type Phase (CF), are the hosts of aluminium in the lower mantle. While their stabilities in simple chemical systems (NaAlSiO₄–MgAl₂O₄) are well understood (Imada et al., 2011), more realistic chemical systems have only recently been explored. This presentation will focus on how potassium—a significant component of subducting sediments—affects the stability of NAL and CF (Rogmann et al., 2024).

We performed high-pressure experiments using a laser-heated diamond anvil cell (DAC) in the KAlSiO₄–NaAlSiO₄–MgAl₂O₄ system, under pressures corresponding to the uppermost to mid-lower mantle. To constrain the elasticity of the CF phase, we conducted Brillouin scattering experiments on CF single crystals in a DAC at an array of pressures. These measurements allow us to model seismic wave velocities, which can be compared to measured seismic data of the deep Earth and can help constrain mineralogical models of the deep Earth.

We find our measurements of the elastic moduli of CF to deviate significantly from those reported in current mineral physical databases based on theoretical calculations. We also observe the onset of the spin transition in ferric iron, expected for CF between 25 and 35~GPa (Wu et al., 2017). Additionally, we find that the addition of potassium significantly broadens the stability range of NAL by more than 25 GPa. We also observe a significant potassium solubililty in CF at high pressure, which may hold the entire MORB potassium budget.

Imada, Saori, Kei Hirose, and Yasuo Ohishi. "Stabilities of NAL and Ca-ferrite-type phases on the join NaAlSiO 4-MgAl 2 O 4 at high pressure." Physics and Chemistry of Minerals 38 (2011): 557-560.

Rogmann, Elena-Marie, et al. "The effect of potassium on aluminous phase stability in the lower mantle." Contributions to Mineralogy and Petrology 179.5 (2024): 1-18.

Wu, Ye, et al. "Spin transition of ferric iron in the calcium‐ferrite type aluminous phase." Journal of Geophysical Research: Solid Earth 122.8 (2017): 5935-5944.

Title: Dates and rates of Earth surface processes revealed through trapped charge dating

Summary: The interactions between rates of Earth surface processes, climate and tectonics shape landscapes from mountains to oceans. A robust chronology is essential for understanding the timing of environmental change, its main drivers and how quickly landscapes have evolved over the recent past (i.e., the last few million years). Trapped charge dating techniques, such as electron spin resonance (ESR) and luminescence dating, provide valuable insights into Earth surface processes and landscape change over timescales of 10²-10⁶ years, offering critical information on the timing and pace of environmental change for evaluating processes that are important to society. In this talk, I will highlight the potential of trapped charge dating as a geochronological tool to constrain the timing and rates of landscape evolution, illustrated by selected geomorphological studies from around the world.

Title: Quantification of ^239,24xPu in environmental samples – method development, applications and perspectives

Summary: The measurement of ^239,240Pu in environmental samples can play a key role in investigating Earth (sub-)surface processes. Earth’s outermost skin has been enriched in such anthropogenic fallout radionuclides (FRNs) as a consequence of atmospheric nuclear weapon tests conducted in the 1950s and 1960s, providing distinct geochronological markers. The application of FRNs is well established, with ¹³⁷Cs (and the naturally occurring ²¹⁰Pb) being most commonly measured. However, ^239,240Pu activities are more decay-insensitive (t_1/2 ²³⁹Pu: ~24.1 ka; ²⁴⁰Pu: ~6.6 ka), and there is less soil inventory contamination arising from nuclear power plant accidents compared to ¹³⁷Cs. Additionally, only a few grams of sample material are required for a measurement.

Several years of research at the University of Cologne (UoC) have resulted in the development of tailored sample processing protocols that enable the routine extraction of ^239,24xPu from environmental samples. The set of isotopes in focus is completed by ²⁴⁴Pu (t_1/2 = 81.1 Myrs), whose natural abundances are of primordial origin with minimal interstellar influx (<1000 atoms cm^-2 Myr^-1). Any measurement of ^239,24xPu can be calibrated against the Cologne Pu-multiisotope Standard (ColPuS). Furthermore, a significant increase in measurement precision of isotopic ^239,24xPu concentrations has been achieved through Accelerator Mass Spectrometry (AMS), as compared to earlier methods such as conventional mass spectrometry or decay counting techniques. The development of such measurement capabilities at the UoC Centre for Accelerator Mass Spectrometry (CologneAMS) has enabled the exploration of the numerous potential applications of ^239,240Pu in deciphering modern Earth (sub-)surface processes in different settings. The sample processing workflow, combined with the precision of AMS measurements, allows for resolving specific ²³⁹Pu activities below ~5 mBq kg^-1 (~10^-15og/g). Consequently, spatial focus is placed on study sites where ultra-high precision measurements are required, such as in drylands in the southern hemisphere. A recent study traces dust influx into CaSO₄ crust in the Atacama Desert in northern Chile, while another application focuses on agroecosystems located in the Free State Province of South Africa. Here, plutonium concentrations indicate that SOM loss in arable land can primarily be caused by wind erosion dominating over other factors at play, such as mineralisation processes.

With the foundation for successful measurements of ^239,24xPu being laid, future work aims to further increase the measurement precision of ²⁴⁰Pu at CologneAMS. Additionally, an alternative measurement method employing a next generation MS/MS MC-ICP-MS (Thermo Fisher Neoma) is being considered to improve measurement sensitivities as compared to older systems. Together with the prospective implementation of further advances made in chemical processing, the overall sample throughput for samples containing ultra-low concentrations of ^239,24xPu may be increased.