Browsing by Author "Blaxell, M"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Beryllium isotope signatures of ice shelves and sub-ice shelf circulation
    (Elsevier, 2019-01-01) White, DA; Fink, D; Post, AL; Simon, KJ; Galton-Fenzi, BK; Foster, S; Fujioka, T; Jeromson, MR; Blaxell, M; Yokoyama, Y
    Be isotopes are a useful tracer of sediment source and transport pathways but have not been widely tested in glacio-marine environments. We measured Be isotopes in a range of depositional environments from open marine, sub-ice shelf and subglacial settings throughout Prydz Bay, one of Antarctica's largest ice drainage systems. We find that strong sub-ice shelf and bottom current circulations can advect 10Be-rich open marine sediments into an ice shelf cavity, and 10Be-poor terrestrial sediments onto the continental shelf at the ice shelf outflow, meaning that 10Be concentrations reflect sub-ice shelf circulation patterns rather than depositional environment. However, HCl-extractable 10Be/9Be ratios can provide a more robust discrimination of sediment deposited in open marine and sub-ice shelf settings. Thus, Be isotopes are a useful tracer of both environmental setting and sub-ice shelf circulation strength in both modern and paleo-ice sheet margins. Crown Copyright © 2018 Published by Elsevier B.V.
  • Thumbnail Image
    Item
    The distribution and fractionation of beryllium isotopes in various reactive phases of Antarctic marine sediments
    (Australian Nuclear Science and Technology Organisation, 2021-11) Jeromson, MR; Fujioka, T; Fink, D; Post, A; Simon, KJ; Sánchez-Palacios, JT; Blaxell, M; Enge, TG; Wilcken, KM; White, DA
    Beryllium isotopes, ¹⁰ Be and ⁹ Be, in Antarctic marine sediments are increasingly being applied as paleoenvironmental proxies and indicators of past ice shelf extent. The evidence base for interpreting meteoric-¹⁰ Be concentrations and ¹⁰ Be/⁹ Be 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-¹⁰ Be is geochemically adsorbed onto sediment grains in the reactive phase during transport from the atmosphere to deposition on the seafloor. Unlike meteoric-¹⁰ Be, ⁹ Be 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 ¹⁰ Be and ⁹ Be 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 NH₂ OH-HCl in 1M HCl, iv) organic leached by 0.01M HNO₃ and H₂ O₂ , 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 ¹⁰ Be, about 90% of total ¹⁰ Be, with the remaining 10% being in the mineral/residual phase. For ⁹ Be, the oxide phases contained only 10-30%, the majority of ⁹ Be 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 ⁹ Be was close to equally split across the two oxide phases, 80% of reactive ¹⁰ Be 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. ⁹ Be 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 ¹⁰ Be 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-¹⁰ Be, and for understanding the processes that drive the source and distribution of different isotopes around ice shelf systems. © The Authors
  • Thumbnail Image
    Item
    Extracting 10Be and 9Be from Antarctic marine sediments – a comparison of different extraction techniques
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) Jeromson, MR; Fujioka, T; Fink, D; Post, AL; Simon, KJ; Sánchez-Palacios, JT; Blaxell, M; Enge, TG; Wilcken, KM; White, DA
    Application of meteoric-¹⁰ Be (M¹⁰ Be) in sediments and soils from diverse geomorphic settings has been active for many decades. In some cases, M¹⁰ Be is normalized by the reactive ⁹ Be from the same sediment sample. Given the complexities in geochemical pathways that M¹⁰ Be is incorporated in the reactive mineral phase of such sediments, very different Be isotope chemistry extraction techniques have been developed. Measurement of M¹⁰ Be and the reactive phase of ⁹ Be in coastal Antarctic marine sediments has increasingly become promising as a paleo-proxy for the presence (or absence) of past ice shelves, and/or subglacial meltwater discharge from grounded outlet glaciers draining the ice sheet. However, published works select different methods to chemically leach Be isotopes from the reactive phase of Antarctic marine sediment and few studies have quantitively compared the efficacy of different leaching recipes. This is problematic because comparisons of ¹⁰ Be/⁹ Be ratios across different Antarctic sites assumes the same chemical fractionation of Be isotopes regardless of the leaching method. We examined three large-volume sediment grabs from near the Amery Ice Shelf front in East Antarctica that represent a range of grainsize and environmental conditions. For Be extraction, homogenised materials from each of the three samples were treated with four different leaching procedures, 1–3 targeting the reactive phase: 1) 6M HCl; 2) 0.5M HCl followed by 1M hydroxylamine hydrochloride in 1M HCl; 3) 0.04M hydroxylamine hydrochloride in 25% acetic acid solution 4) a total extraction dissolving in HF, HNO₃ , and HClO₄ . We also selected one grab to assess the effect of grainsize within the following fractions: <38 um, 38–63 um, 63–90 um, 90–125 um, and >125 um. Each fraction was leached with 6M HCl for 24 hours at room temperature. We found that both the 6M HCl and the 1M hydroxylamine procedures leached the same amount of ¹⁰ Be as the total extraction, while the 0.04M hydroxylamine treatment leached only two thirds. Interestingly, the 6M HCl and the 0.04M hydroxylamine procedures leached the same relative proportion of ⁹ Be to ¹⁰ Be, and thus gave the same ¹⁰ Be/⁹ Be ratio, while the 1M hydroxylamine procedure leached relatively more ⁹ Be in relation to ¹⁰ Be, resulting in a lower ¹⁰ Be/⁹ Be than the other two methods. As shown in previous studies, our results indicate that Be-isotope concentrations varied inversely with grainsize, in our case increasing 4- fold from coarsest to finest fractions, critically showing that the ¹⁰ Be/⁹ Be ratio remained constant across all grainsizes. Hence, grainsize can be normalised by applying the reactive ¹⁰ Be/⁹ Be ratio. We conclude that differences in leaching procedures, can lead to significant variations in efficiencies in extracting Be isotopes from the reactive phase of sediment, whereas the ¹⁰ Be/⁹ Be ratio appears to remain the same. This study highlights the importance of careful method selection and its consistent application to allow for comparison between studies and more robust interpretation.
  • Thumbnail Image
    Item
    Short duration glacial advances: implications for cosmogenic dating chronologies
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) White, DA; Fülöp, RH; Fink, D; Fujioka, T; Blaxell, M; Jeromson, MR; Codilean, AT
    Cosmogenic nuclides are now the mainstay technique for constraining past ice advances in terrestrial environments. However, limited erosion of bedrock and recycling of older glacial sediments complicates the interpretation of dated records. Where younger deposits or striated bedrock surfaces contain nuclides inherited from previous periods of exposure, cosmogenic dates overestimate deglaciation by tens to hundreds of thousands of years. This is especially problematic in Antarctica, where low erosion rates and cold-based glaciation produce widespread inheritance in erratics or bedrock surfaces. Studies using long-lived nuclides such as Be-10 have identified sites where <5% of erratics are inheritance-free, making it logistically difficult to measure ice sheet retreat. To address this issue, the short-lived nuclide in-situ C -14 is being applied in Antarctic deglaciation studies. The relatively recent deglaciation across most parts of Antarctica mean inheritance in erratics can be detected by discordant Be-10 and C-14 ages. Bedrock may also be dateable using C-14 if the inherited nuclides decay below measurement uncertainties while ice cover is present. However, both these approaches, particularly direct bedrock dating, make assumptions of the duration of the glacial advance. To test these approaches we measured C-14 in bedrock surfaces from sites across East Antarctica between Enderby Land (48oE) and Vincennies Bay (107oE). At each site, the true deglaciation age was well established via Be-10 ages on erratics, and in some cases, coincident with the onset of biogenic sedimentation in lake and marine basins. We find that the assumption of a long-duration Last Glacial Maximum (LGM) is not widely met. The two inland sites appear to have been buried long enough to re-set the C-14 signal. However, at least four sites along the East Antarctic coastline displayed bedrock surfaces that were saturated with C-14, despite clearly having been covered by ice during the early Holocene. These results mean that in-situ C-14 ages must be carefully interpreted, especially when used on bedrock. Bedrock surfaces that provide discrete ages can be used to indicate that deglaciation has occurred since the global Last Glacial Maximum. However, those with saturated surfaces provide equivocal results unless supporting evidence can support the expected duration of the LGM. © The Authors