Browsing by Author "Constantine, M"
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- ItemComparing 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, H; 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, CAThe 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.
- ItemDifferentiating between the d13C signature from environmental conditions and SOM cycling in eastern Australian peat sediments(Australasian Quaternary Association (AQUA), 2021-07-08) Forbes, MS; Cohen, TJ; Marx, SK; Sherborne-Higgins, B; Cadd, H; Francke, A; Cendón, DI; Peterson, MA; Mooney, SD; Constantine, M; Boesl, F; Kobayashi, Y; Mazumder, DThe analysis of stable carbon isotopes is commonly used in Quaternary science to reconstruct the environmental conditions and vegetation contributions to sedimentary sequences. However, the measured d13C signature of the total organic matter (OM) pool can also reflect other complexities within depositional environments. The peats of the Thirlmere Lakes system in the southern section of the Blue Mountains World Heritage Area provides an excellent opportunity to closely scrutinise such d13C dynamics. These deposits are rich in TOC (20-40%) meaning analytical techniques such as 13C-NMR, used to characterise the OM pool, can be applied effectively. Furthermore, the identification of several peat units deposited over the last ~130 ka allows for temporal comparisons. d13C values determined for a 7 m sediment sequence from Lake Couridjah representing both the MIS 1 and MIS 5e interglacial periods vary by up to 4 to 6‰. These trends were subsequently identified in two other sediment sequences (Lake Baraba and Lake Werri Berri) proximal to Lake Couridjah. Initially we interpreted our results as reflecting a C3 dominated vegetation environment with MIS 1 wetter than MIS 5e, following the established relationship between water stress and d13C enrichment. However, spectral analysis of the OM pool indicates that d13C is driven by changing OM dynamics rather than large changes in environmental conditions. In these environments, the greater presence of carbohydrates (i.e. cellulose) in MIS 1 result in more depleted d13C values. In contrast, the MIS 5e peat is dominated by relative inert OM C fractions including charcoal and lipids (such as leaf waxes), which influences environmental proxies such as C/N. Thus, it is likely that the older MIS 5e peat is a more decomposed version of the active MIS 1 peat, and thus differentiating environmental conditions between the two using d13C alone is not particularly illuminating. To overcome this, we describe the d13C values for a coarse charcoal and high temperature hydrogen pyrolysis fractions, modern vegetation, catchment POC and DOC, and n-alkanes composition and generate catchment carbon models for both MIS 1 and MIS5e. Finally comparing the size of the OM pools of both interglacial deposits can provide useful information in estimating the carbon storage capacity of peat deposits in eastern Australia over these time scales. © The Authors.
- ItemDifferentiating between the d13C signature from environmental conditions and SOM cycling in eastern Australian peat sediments(Australasian Environmental Isotope Conference, 2022-11-14) Forbes, MS; Cohen, TJ; Marx, SK; Sherborne-Higgins, B; Cadd, H; Francke, A; Cendón, DI; Peterson, MA; Mooney, SD; Constantine, M; Boesl, F; Kobayashi, Y; Mazumder, DThe analysis of stable carbon isotopes is commonly used in Quaternary science to reconstruct the environmental conditions and vegetation contributions to sedimentary sequences. However, the measured d13C signature of the total organic matter (OM) pool can also reflect other complexities within depositional environments. The peats of the Thirlmere Lakes system in the southern section of the Blue Mountains World Heritage Area provides an excellent opportunity to closely scrutinise such d13C dynamics. These deposits are rich in TOC (20-40%) meaning analytical techniques such as 13C-NMR, used to characterise the OM pool, can be applied effectively. Furthermore, the identification of several peat units deposited over the last ~130 ka allows for temporal comparisons. d13C values determined for a 7 m sediment sequence from Lake Couridjah representing both the MIS 1 and MIS 5e interglacial periods vary by up to 4 to 6‰. These trends were subsequently identified in two other sediment sequences (Lake Baraba and Lake Werri Berri) proximal to Lake Couridjah. Initially we interpreted our results as reflecting a C3 dominated vegetation environment with MIS 1 wetter than MIS 5e, following the established relationship between water stress and d13C enrichment. However, spectral analysis of the OM pool indicates that d13C is driven by changing OM dynamics rather than large changes in environmental conditions. In these environments, the greater presence of carbohydrates (i.e. cellulose) in MIS 1 result in more depleted d13C values. In contrast, the MIS 5e peat is dominated by relative inert OM C fractions including charcoal and lipids (such as leaf waxes), which influences environmental proxies such as C/N. Thus, it is likely that the older MIS 5e peat is a more decomposed version of the active MIS 1 peat, and thus differentiating environmental conditions between the two using d13C alone is not particularly illuminating. To overcome this, we describe the d13C values for a coarse charcoal and high temperature hydrogen pyrolysis fractions, modern vegetation, catchment POC and DOC, and n-alkanes composition and generate catchment carbon models for both MIS 1 and MIS5e. Finally comparing the size of the OM pools of both interglacial deposits can provide useful information in estimating the carbon storage capacity of peat deposits in eastern Australia over these time scales.
- ItemThe evolution of Thirlmere lakes: a long-term sedimentary record of climate and fire dynamics in the Sydney Basin(Australasian Quaternary Association Inc., 2018-12-10) Cohen, TJ; Marx, SK; Barber, E; Forbes, MS; Gadd, PS; Tyler, JJ; Haines, HA; Woodward, C; Zamora, A; Mooney, SD; Constantine, MThe Thirlmere lakes are located 40 km from the coast and are at ~300 m elevation and fall within the Greater Blue Mountains World Heritage area. The series of five lakes sit within a narrow and sinuous former river valley within the Hawkesbury sandstone with surrounding dry sclerophyll forest. Recent declines in water levels have prompted the NSW Office of Environment and Heritage to fund research about the history of Thirlmere lakes, the sub-surface characteristics and the potential frequency of past drying. This research builds on some existing work and has highlighted the extraordinary potential for the region for a long-term archive for palaeoenvironmental research.To date we have taken multiple vibracores across three lakes to depths of 7 m and we have supplemented this with some preliminary deep drilling to depths of 14 m. Our initial chronology is based on radiocarbon and OSL and we have employed a raft of geochemical and palaeoecological techniques to investigate changes through time. The lakes contain excellent organic preservation with deposition of the ‘modern’ peat environments commencing ~11 ka across two of the lakes investigated. This phase is represented by the upper 2 -3 m of organic rich peat (50% TOC). The underlying sediments are a mix of weakly bedded organic clays and oxidised clay facies that represent lake-wide drying intervals, a sequence that is repeated down profile. All five lakes are separated by alluvial sills that are comprised of medium to well-sorted sands, interbedded with organic ‘marker’ horizons that indicate these separate lakes were once joined, prior to the Last Glacial Maximum. The sandy sills that separate the lakes are derived from tributary alluvial fans accumulating progressively over the Holocene and effectively blocking and separating the lakes into their current configuration. This paper provides a preliminary overview of the chrono-stratigraphic history of Thirlmere lakes. © The Authors