Single Atom Counting of Stellar and r-Process Nuclei in Time-Resolved Deep-Sea Archives

Stars are the major element factories in the universe. In 1999, live supernova Fe-60 (t1/2 =2.6 Myr)
was detected in a deep-sea ferromanganese crust (1 ) indicating the accumulation of supernova
dust on Earth about 2 million years ago. This was followed by several projects reinforcing the
initial evidence for a global influx of supernova Fe-60. Recently, a much younger continuous
influx was found in Antarctic snow and in deep-sea sediments (2 –4 ) and an older peak around
6 - 8 Myr in deep-sea crusts (5 , 6 ).
In contrast to the well-known production mechanism and synthesis site of Fe-60, the long-lived
plutonium isotope Pu-244 (t1/2 =80 Myr) is a pure r-process nucleus. The nucleosynthesis site for
the astrophysical r-process is still debated in the astrophysics community. Potential candidates
involve rare supernovae and neutron star mergers. To date no evidence was presented that would
point to an exclusive r-process site and combinations of different sites are considered.
Experimentally, we can search for Pu-244 signatures in samples with known Fe-60 signatures to
test for either common influx patterns or independent Pu-244 influxes disentangled from stellar
Fe-60. Accordingly, this information provides a unique and direct experimental approach for
identifying the production site of the heavy elements.
Based on the recent publication of the first detection of interstellar Pu-244 in a ferromanganese
crust with a time resolution of 4.5 Myr (integrating over much shorter Fe-60 influxes) (6 ), we are
now working on a highly time-resolved profile of Fe-60 and Pu-244 in the large ferromanganese
crust VA13/237KD. This direct experimental input will further constrain models for r-process
nucleosynthesis in the galaxy. The recently determined profile of Fe-60 clearly shows two influxes,
one at 2 Myr, the other at 7 Myr, confirming and refining previous results. Preliminary data on
Pu-244 and an outlook for future measurement campaigns will be given.

References
1. K. Knie et al., Phys. Rev. Lett. 83, 18–21 (1999).
2. D. Koll et al., Phys. Rev. Lett. 123, 072701 (2019).
3. A. Wallner et al., Proceedings of the National Academy of Sciences 117, 21873–21879 (2020).
4. D. Koll et al., EPJ Web Conf. 232, 02001 (2020).
5. A. Wallner et al., Nature 532, 69–72 (2016).
6. A. Wallner et al., Science 372, 742–745 (2021).