A 10-million year time profile of interstellar influx to Earth mapped through supernova Fe-60 and r-process Pu-244
Creators: name: Koll, Dominik
Publisher: name: The Australian National University
Year: 2023

description: Interstellar radionuclides are contemporary witnesses of nucleosynthesis in the universe and can be used to constrain nucleosynthesis models as well as to investigate interstellar medium dynamics and transport of interstellar dust. Live traces of the supernova-produced radionuclide 60Fe were detected in several geological archives on Earth and on the Moon, demonstrating the influx of interstellar dust from near-Earth supernovae within the last 10 Myr. The recent discovery of the r-process radionuclide 244Pu opened the door to study the time-resolved influx of both radionuclides onto Earth. In this thesis, a large piece of the ferromanganese crust VA13/2-237KD from the Pacific Ocean was characterised, chemically processed and analysed for interstellar 60Fe and 244Pu with superior time resolution and sensitivity compared to previous investigations. The deep-sea ferromanganese crust was optically 3D-scanned with sub-mm resolution, facilitating the generation of a 3D computer model and the construction of a full-scale acrylic 3D model. The growth of the ferromanganese crust was investigated by a 3D micro-CT scan and cosmogenic 10Be dating. Strong evidence for a change in growth in lateral direction as well as with depth was found. Three drill-holes were taken with a depth resolution of 1-2mm for 10Be, 26Al, 53Mn and 60Fe measurements, whereas the remaining crust was sampled in ~1 Myr time-resolved layers for 182Hf, 244Pu and 247Cm. An element separation protocol was developed to obtain highly purified samples for accelerator mass spectrometry (AMS). The high sensitivity AMS measurements for 10Be, 60Fe and 244Pu were carried out at DREAMS, HIAF and VEGA, respectively. The precision 10Be measurements allowed to establish a depth-age relation of the crust and to investigate the growth pattern of the crust over more than the last 15 Myr. An unexpected 10Be anomaly was discovered at (10.5+-1.0) Myr in both drill-holes and hints for a corresponding anomaly were found in the literature after re-evaluation. The measurements of interstellar 60Fe yielded the well-established distinct influx peaking at (2.4+-0.2) Myr, in perfect agreement with the literature. Furthermore, the second, older 60Fe influx was clearly observed in the time interval 6.8-7.9 Myr with superior time resolution compared to previous work. The timing of the maximum influx could be constrained to (7.3+-0.8) Myr when combining the results from this work and the literature data. The measurements in this work represent the highest sensitivity 60Fe measurements worldwide with an unprecedented background ratio of 60Fe /Fe=(0.21+-0.15)x10-16 for processed samples. The interstellar origin of the detected 60Fe could be unequivocally proven by ruling out potential background contributions. A 10-million year time profile of interstellar 60Fe and 244Pu influx with high time resolution, capable of resolving both radionuclide influxes, could be established in this work for the first time. A background level of (90+-60) 244Pu atoms per measured cathode and the so far highest detection efficiency for interstellar 244Pu worldwide with up to 1.1% for processed samples were achieved. This corresponds to a sensitivity of 1 out of 90 244Pu atoms or 36 zeptograms per sample. The detected 244Pu concentrations indicate a continuous influx to Earth over the last 10 Myr. The 244Pu influx, with fluences into the crust of (10.1+-3.1) 244Pu atoms cm-2 Myr-1 and (4.4+-1.6) 244Pu atoms cm-2 Myr-1 for the time intervals 1.19-4.05 Myr and 4.05-9.45 Myr, respectively, and both significant around the 3-sig-level, does not indicate a strong peak-like pattern in contrast to 60Fe. Eventually, the deposition of interstellar radionuclides was discussed in light of a potential inhomogeneous global distribution. It could be shown that the data of this work combined with existing literature data are compatible with an inhomogeneous with latitudinal differences modulated by atmospheric and oceanic processes.