Browsing by Author "Swallow, E"

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    Busting the dust: evaluating local vs distal sources in Quaternary sediments at Thirlmere Lakes
    (Elsevier, 2024-10) Forbes, MS; Marx, SK; Cohen, TJ; Sherborne-Higgins, B; Francke, A; Peleckis, G; Jones, BG; Dosseto, A; Cadd, HR; Swallow, E; Raven, M; Cendón, DI; Peterson, MA
    The Quaternary sediments preserved within the Thirlmere Lakes system, Greater Blue Mountains World Heritage Area, Australia, are an important regional environmental record representing at least the last two interglacials. Understanding the source and evolution of these sediments, both temporally and spatially, is an essential component of the site's reconstruction. In this study, we evaluate this question using physical, mineralogical, elemental, and isotopic analytical techniques. Four distinct lake sediment facies all show bi-modal distributions of coarse sand and clay to fine silts, representing various mixtures of catchment Hawkesbury Sandstone and Ashfield Shale. Clays are predominantly kaolinite-dickite, however, the 7 Å dehydrated form of halloysite is prominent in an orange-yellow oxidised lake facies unit. The relative distribution and concentration of rare earth elements, including yttrium (REY), is heterogeneous across all the lake facies, varying between both lakes and with depth. This variability suggests a geochemical signature reflecting a combination of mixed sources and secondary mineral precipitation, driven by catchment geomorphology and specific site conditions. Slightly positive Ce anomalies in the oxidised lake facies, combined with the greater halloysite representation, represents a period of dry conditions and sub-aerial exposure. Evaluation of catchment, regional and continental REY ratios, Eu anomalies and εNd data implies a predominant internal catchment source signature, with any external contributions restricted to the local Bringelly Shale and the immediate south-eastern Australia, including the Murray River Basin. Geochemical and isotopic values for these proposed internal and external sediment sources predicts that an aeolian source from outside the immediate catchment represents a maximum of 30% of the fine-grained sediment fraction. © 2024 Crown Copyright © 2024 Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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    Comparing interglacials in eastern Australia: a multi-proxy investigation of a new sedimentary record
    (Elsevier, 2021-01-01) Forbes, MS; Cohen, TJ; Jacobs, Z; Marx, SK; Barber, E; Dodson, JR; Zamora, A; Cadd, HR; Franke, A; Constantine, M; Mooney, SD; Short, J; Tibby, J; Parker, A; Cendón, DI; Peterson, MA; Tyler, JJ; Swallow, E; Haines, HA; Gadd, PS; Woodward, CA
    The widespread formation of organic rich sediments in south-east Australia during the Holocene (Marine Isotope Stage [MIS] 1) reflects the return of wetter and warmer climates following the Last Glacial Maximum (LGM). Yet, little is known about whether a similar event occurred in the region during the previous interglacial (MIS 5e). A 6.8 m sediment core (#LC2) from the now ephemeral Lake Couridjah, Greater Blue Mountains World Heritage Area, Australia, provides insight into this question. Organic rich sediments associated with both MIS 1 and 5e are identified using 14C and optically stimulated luminescence (OSL) dating techniques. Also apparent are less organic sedimentary units representing MIS 6, 5d and 2 and a large depositional hiatus. Sediment δ13C values (−34 to −26‰) suggests that C3 vegetation dominates the organic matter source through the entire sequence. The pollen record highlights the prevalence of sclerophyll trees and shrubs, with local hydrological changes driving variations in the abundance of aquatic and lake-margin species. The upper Holocene sediment (0–1.7 m) is rich in organic matter, including high concentrations of total organic carbon (TOC; 20–40%), fine charcoal and macrophyte remains. These sediments are also characterised by a large proportion of epiphytic diatoms and a substantial biogenic component (chironomids and midges). These attributes, combined with low δ13C and δ15N values, and C:N ratios of approximately 20, indicate a stable peat system in a swamp like setting, under the modern/Holocene climate. In comparison, the lower organic rich unit (MIS 5e-d) has less TOC (5–10%), is relatively higher in δ13C and δ15N, and is devoid of macrophyte remains and biogenic material. Characterisation of the organic matter pool using 13C-NMR spectroscopy identified a strong decomposition signal in the MIS 5e organic sediments relative to MIS 1. Thus the observed shifts in δ13C, δ15N and C:N data between the two periods reflects changes in the organic matter pool, driven by decompositional processes, rather than environmental conditions. Despite this, high proportions of aquatic pollen taxa and planktonic diatoms in the MIS 5e–d deposits, and their absence in the Holocene indicates that last interglacial Lake Couridjah was deeper and, or, had more permanent water, than the current one. ©2020 Elsevier Ltd.