The distribution and fractionation of beryllium isotopes in various reactive phases of Antarctic marine sediments
Mr Matthew Jeromson1, Dr. Toshiyuki Fujioka2, Dr. David Fink3, Dr. Alix Post4, Ms. Krista Simon3, Dr. José Tonatiuh Sánchez-Palacios5, Mr. Marcello Blaxell1, Dr. T Gabriel Enge6, Dr. Klaus Wilcken3, Dr. Duanne White1
1University Of Canberra, Canberra, Australia, 2Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain, 3Australian Nuclear Science and Technology Organisation, Sydney, Australia, 4Geoscience Australia, Canberra, Australia, 5Murdoch University, Perth, Australia, 6Research School of Earth Sciences, Australia National University, Canberra, Australia
Beryllium isotopes, ¹⁰Be and ⁹Be, in Antarctic marine sediments are increasingly being applied as paleoenvironmental proxies and indicators of past ice shelf extent. The evidence base for interpreting meteoric-¹⁰Be concentrations and ¹⁰Be/⁹Be ratios has largely been derived from examining their spatial distribution in modern depositional environments, or by correlation with other proxies in paleo-records, such as diatom abundance.
Meteoric-¹⁰Be is geochemically adsorbed onto sediment grains in the reactive phase during transport from the atmosphere to deposition on the seafloor. Unlike meteoric-¹⁰Be, ⁹Be is both available within the reactive phase after crustal weathering and native within mineral lattice in significant quantities. The complexity in fixing and preserving the Be isotopes onto grain surfaces leads to uncertainties in selecting the chemistry methods to consistently extract the reactive phases of ¹⁰Be and ⁹Be in different sediments. This gap in understanding the physical behaviour and geochemical forms of reactive Be in Antarctic sediments limits their utility in reconstruction of paleoenvironmental conditions.
We conducted a sequential leach procedure on three homogenised sediment grab samples from the front of the Amery Ice Shelf that span a range of water masses. Using different chemical reagents, from very weak to very strong, five phases of Be isotope signatures were extracted sequentially, including : i) water soluble, ii) amorphous oxides leached by 0.5M HCl, iii) crystalline oxides leached by 1M NH₂OH-HCl in 1M HCl, iv) organic leached by 0.01M HNO₃ and H₂O₂, and v) mineral/residual phase dissolved by HF– with the water through to organic leach making the reactive phase. We found that the amorphous and crystalline oxide phases contained the largest fraction of ¹⁰Be, about 90% of total ¹⁰Be, with the remaining 10% being in the mineral/residual phase. For ⁹Be, the oxide phases contained only 10-30%, the majority of ⁹Be being in the residual phase. The water-soluble and organic chemical treatments were inefficient in extracting any significant reactive Be. This distribution has been observed in other deep marine and continental riverine sediments. However, the proportional distribution of the two isotopes between the amorphous and crystalline oxides differed for our Antarctic sediments compared to those other studies. While reactive ⁹Be was close to equally split across the two oxide phases, 80% of reactive ¹⁰Be was located within the amorphous phase, with the remainder within the crystalline oxide phase.
The difference in fractionation provides evidence for different sources of each isotope and different processes affecting their deposition. ⁹Be is sourced primarily from the Earth’s crust and is likely segregated into the different fractions during the process of subglacial chemical weathering. Open water ¹⁰Be is processed in the water column, where interaction with biogeochemical processes likely segregates it into the more labile phases. These findings inform decisions regarding the selection of procedures for efficient and reproducible extraction of meteoric-¹⁰Be, and for understanding the processes that drive the source and distribution of different isotopes around ice shelf systems.
Matthew is a final year PhD student at the University of Canberra and is interested in geochemical and cosmogenic proxies applied to Antarctic ice shelf environments, spending time both developing the meteoric-10Be proxy and applying it across multiple Antarctic sectors.
He comes from a sedimentological background, having looked at both clastic and carbonate Pleistocene turbidite sequences off the coast of New Zealand during his Honours and Masters at the University of Auckland.