Recordings of cosmogenic beryllium in marine sediments during the Laschamps geomagnetic dipole low: implications for synchronization of paleo archives and geomagnetic field reconstructions

Cosmogenic beryllium (10Be) is produced in the Earth’s atmosphere during the nuclear cascade initiated by the collision of galactic cosmic rays with the nuclei of atmospheric atoms. The production rate of 10Be is therefore globally influenced by the solar and geomagnetic fields. It exhibits both a latitudinal and an altitudinal gradient, with the highest production rate in the polar stratosphere and the lowest in the equatorial troposphere. 10Be is removed from the atmosphere, primarily through wet deposition, and is globally recorded in various paleo archives, such as ice or sediment cores. Given the correlation between the 10Be production rate and Earth’s magnetic shielding and solar activity, 10Be is a valuable tool for the reconstruction of both in the past. Additionally, the global modulation of its production rate renders 10Be an effective synchronization tool for diverse paleo archives. Yet, additional factors may also affect the recorded production rate signal. Recent atmospheric mixing models suggest a latitude-dependent influence of geomagnetic field variations on 10Be records. Consequently, depending on the selected record, the global 10Be production rate change may be either over- or underestimated. In marine sediments, several other influences must be accounted for, including the residence time of beryllium in the water column, changes in sedimentation rates or scavenging efficiency, benthic fluxes of beryllium, bioturbation, and potential sediment re-deposition. Additionally, especially for synchronization purposes, it is imperative to measure marine sediment cores with the highest possible resolution.

To begin with, a beryllium purification protocol for measuring 10Be with accelerator mass spectrometry (AMS) was developed. This protocol ensures high sample throughput at a low cost while simultaneously yielding samples that perform well in the AMS. This method employs precipitation reactions and utilizes only standard laboratory materials and equipment. A comparative analysis of this method was conducted against an established protocol based on hydroxide precipitations and column chromatography. Although the method resulted in slightly lower sample purity and yield, this did not adversely affect the AMS measurement. In contrast to the established method, the approach is significantly more cost-effective and enables a two- to threefold increase in sample throughput.

To gain a better understanding of the various influences on the recorded 10 Be signal, several marine sediment cores were analyzed. To assess influences on the 10Be recording in marine sediments, three cores were analyzed for their 10Be/9Be ratios during the Laschamps geomagnetic dipole low (∼ 41 ka BP). Although all cores are influenced by the Antarctic Circumpolar Current (ACC) and exhibit high sedimentation rates, differing depositional conditions and influences are present. A rapid and distinct increase in the 10Be/9Be ratios by 50 – 80 percent was observed in all cores during the Laschamps event, which corresponds with changes in the respective paleomagnetic inclinations. However, when compared to 10Be data from ice cores, it was found that all sediment cores were influenced to varying extents by oceanic residence times of 10Be. The short oceanic residence time of beryllium at core PS67/197-1 is consistent with published data for the region, while the longer one of core PS75/054-1 suggests that at least a portion of the scavenged beryllium originates from the pelagic Pacific. In core PS97/085-3, indications of a temporary increase in terrigenous influence were observed. Other sedimentary processes did not significantly affect the 10Be/9Be ratios of the cores. Despite the attenuation effects of residence time and the variable terrigenous influence observed in core PS97/085-3, it was found that the 10Be/9Be ratios are fundamentally suitable for synchronizing different regions and archives.

The latitude-dependent distribution of 10Be deposition was investigated by comparing published 10Be sediment data and ice core records during the Laschamps event with the data in this thesis. Variations in 10Be deposition during the Laschamps event were examined in relation to latitude and compared with modeled 10Be deposition rate changes derived from different geomagnetic field reconstructions (LSMOD.2, GGFSS70, Black Sea, GLOPIS-75), in conjunction with the atmospheric mixing model GEOS-Chem. It was found that the results align well with the atmospheric mixing model, which postulates incomplete mixing of 10 Be in the atmosphere before deposition, revealing a lower amplitude of deposition at higher latitudes compared to lower ones. Furthermore, the global deposition rate change during the Laschamps event was estimated, yielding a value that supports the geomagnetic field model LSMOD.2, while indicating that the production rate change derived from GLOPIS-75 may be overestimated. Overall, the gap between geomagnetic field models and atmospheric transport models of 10Be, as well as data-based reconstructions, was narrowed by this study. Notably, it was demonstrated that the global 10Be production rate increased by 117 – 133 percent during the Laschamps event. Although this figure exceeds previous estimates derived from ice core data, it remains insufficient to fully reconstruct the production rate changes of 14C based on 14C measurements.