Browsing by Author "Wurster, CM"
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- ItemThe efficiency of charcoal decontamination for radiocarbon dating by three pre-treatments – ABOX, ABA and hypy(Elsevier, 2014-08-01) Bird, MI; Levchenko, VA; Ascough, PL; Meredith, W; Wurster, CM; Williams, AG; Tilston, EL; Snape, CE; Apperley, DCHere we report results of a direct inter-comparison of the ability of three radiocarbon pre-treatment techniques to remove radiocarbon contamination introduced by exposing known-radiocarbon-free charcoal made at three different temperatures (300, 400 and 500 °C) to environmental contamination on a rainforest floor for one to three years. The initial charcoal had no measureable radiocarbon, but 14C activity increased after environmental exposure to ∼1 pMC after one year (apparent age of ∼40,000 yrs BP) and ∼5 pMC after three years (apparent age of ∼25,000 years). For the 400 and 500 °C samples, all techniques were able to reduce contamination by >90%. Acid–base oxidation (ABOX) provided the most reliable decontamination, reducing the radiocarbon activity of the one year samples to background, and reducing the radiocarbon activity of the three year samples to 0.04 ± 0.02 pMC or less (apparent age >56,900 yrs BP). The performance of ABOX was superior to that of both acid–base–acid (ABA) and hydrogen pyrolysis (hypy) treatments, with ABA performing better than hypy in most cases. No technique was able to fully remove decontamination from the 300 °C charcoal (although ABOX again removed the most contamination), likely due to the incompletely pyrolized nature of the charcoal which is dominated by aromatic clusters of small ring size. All the techniques rely on removing contaminant carbon faster than indigenous carbon and this condition is not met in the case of charcoal produced at temperatures below ∼350 °C. While all pre-treatments are useful for routine sample processing, the results suggest that ABOX is the only technique that can provide reliable decontamination of charcoal of an age close to the dating limit of the radiocarbon dating technique.© 2014, Elsevier B.V.
- ItemHolocene climate–fire–vegetation feedbacks in tropical savannas: insights from the Marura sinkhole, East Arnhem Land, northern Australia(Wiley, 2022-11) Rowe, C; Rehn, E; Brand, M; Hutley, LB; Comley, R; Levchenko, VA; Zwart, C; Wurster, CM; Bird, MIAims Informed management of savanna systems depends on understanding determinates of composition, structure and function, particularly in relation to woody‐plant components. This understanding needs to be regionally based, both past and present. In this study, Holocene plant patterns are explored at a site within the eucalypt savannas of northern Australia. Australian savannas are the least developed globally and uniquely placed to track ecological change. Location Northern Territory, Australia. Methods Palynological analyses were undertaken on a 5‐m sediment core, spanning the last 10,700 calendar years. Pollen was categorised to capture vegetation type, classified further according to plant function and/or environmental response. Detrended Correspondence Analysis was used to quantify ecological dissimilarities through time. Results At the Pleistocene transition, grasses were abundant then declined and remained low relative to increased woody cover from the mid‐late Holocene. Savanna composition gradually transitioned from Corymbia to Eucalyptus dominance until significantly disturbed by a phase of repeated, extreme climate events. Highest non‐savanna variability in terrestrial and wetland plant types formed mixed vegetation communities through the mid‐Holocene. Conclusions Savannas are not homogeneous but the product of plant changes in multiple dimensions. In the Northern Territory, dynamic though restricted non‐eucalypt shifts are embedded within larger, slower eucalypt change processes. Primary climate–vegetation relationships determine the long‐term fire regime. The role of large but infrequent disturbance events in maintaining savanna diversity are significant, in degrees of impact on tree–grass turnover, its form and the extent of vegetation recovery. People's landscape interactions were found to be interwoven within this feedback hierarchy. © 1999-2024 John Wiley & Sons, Inc or related companies. All rights reserved.
- ItemHolocene savanna hydroclimate record from Kinrara Lake, north-east Queensland, Australia(Elsevier, 2024-03) James, J; Comley, R; Wurster, CM; Levchenko, VA; Gadd, PS; Bird, MIWe present a record of hydroclimatic change over the Holocene from Kinrara Lake, in a seasonally dry savanna location in north-eastern Australia. The record is derived from the oxygen (δ18O) and carbon stable isotope (δ13C) composition of endogenic and biogenic (gastropod) carbonate. The stable isotope proxy records are complemented by elemental geochemical (Itrax) and sedimentological proxy data providing a record of hydrologic and climate change, spanning 10.5 ka to the present day. Two main forms of endogenic carbonate occur in the lake sediments; (i) carbonate associated with biofilms during the early-Holocene, under drier-than-modern conditions, (ii) photosynthetic and evaporatively-enriched precipitates in the late-Holocene, associated with enhanced climate variability inducing drought periods. Strong relationships between negative δ18O values and increased Ti, Rb, Fe/Mn, inc/coh, are linked with strengthened monsoon conditions, while enhanced periods of dryness are inferred from more positive δ18O values, increased Ca/∑Fe, Ti, Al, and subsequent intensifications in lake productivity (higher Si/Ti, S/Ti, Mn/Ti). Three distinct phases can be identified in palaeohydrological history of the lake; (i) a relatively stable drier-than-modern phase during the early-Holocene (10.5 to 8.2 ka), (2) a significantly wetter-than-modern, monsoon-dominated phase through the mid-Holocene into the late-Holocene (8.2–2.8 cal yr BP), and (3) after 2.8 ka, increased intensity of ENSO-related rainfall variability during the late-Holocene, continuing into the present. © 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
- ItemImproved pretreatment method for the isolation and decontamination of pyrogenic carbon for radiocarbon dating using hydrogen pyrolysis(Elsevier, 2021-02-01) Orr, TJ; Wurster, CM; Levchenko, VA; Ascough, PL; Bird, MIPyrogenic carbon (charcoal, black carbon, elemental carbon) is one of the most common materials used for radiocarbon dating of terrestrial samples. However, exogenous carbon contamination can compromise the accuracy of radiocarbon ages. This study presents the results of two chemical pretreatments prior to hydrogen pyrolysis (hypy) as improved protocols for the isolation and decontamination of pyrogenic carbon, i) a simple acid-oxidation step (A-Ox/hypy) and ii) acid-base-acid (ABA/hypy). The A-Ox/hypy protocol uses HNO3 and H2O2, while ABA/hypy uses HCl and NaOH. Both pretreatments remove labile and inorganic carbon before hypy, decreasing the potential for in situ production of pyrogenic carbon during the hypy reaction. The effectiveness of each protocol was directly measured on charcoal artificially produced at 350 °C, 450 °C and 550 °C from radiocarbon-free wood, and exposed to environmental contamination for 1–3 yrs. The results show a >94% reduction in carbon contamination for the 450 °C and 550 °C charcoal samples occurred using A-Ox/hypy, but this treatment was less effective for the 350 °C charcoal. A >99% reduction in carbon contamination in all charcoal samples examined occurred using ABA/hypy. The A-Ox/hypy protocol was further tested on cave guano sediments, which had previously reported erroneous dates following simple organic solvent extraction followed by ABA pretreatment. Effective decontamination was achieved using A-Ox/hypy on the guano, which corrected a radiocarbon age reversal. Overall, ABA/hypy effectively decontaminated the charcoals and was a more efficient pretreatment for charcoal than A-Ox/hypy, however resulting in larger sample mass loss. Therefore, ABA/hypy is the recommended protocol for older (>30,000 14C yr BP) charcoal or sediment samples, or where date accuracy is imperative, while A-Ox/hypy represents an improved protocol for the quick and cost-effective measurement of younger samples (<30,000 14C yr BP) when sample size is of concern. © 2021 Elsevier B.V.
- ItemIndigenous impacts on north Australian savanna fire regimes over the Holocene(Springer Nature, 2021-12-01) Wurster, CM; Rowe, C; Zwart, C; Sachse, D; Levchenko, VA; Bird, MIFire is an essential component of tropical savannas, driving key ecological feedbacks and functions. Indigenous manipulation of fire has been practiced for tens of millennia in Australian savannas, and there is a renewed interest in understanding the effects of anthropogenic burning on savanna systems. However, separating the impacts of natural and human fire regimes on millennial timescales remains difficult. Here we show using palynological and isotope geochemical proxy records from a rare permanent water body in Northern Australia that vegetation, climate, and fire dynamics were intimately linked over the early to mid-Holocene. As the El Niño/Southern Oscillation (ENSO) intensified during the late Holocene, a decoupling occurred between fire intensity and frequency, landscape vegetation, and the source of vegetation burnt. We infer from this decoupling, that indigenous fire management began or intensified at around 3 cal kyr BP, possibly as a response to ENSO related climate variability. Indigenous fire management reduced fire intensity and targeted understory tropical grasses, enabling woody thickening to continue in a drying climate. © 2024 Springer Nature Limited. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
- ItemThe influence of C3 and C4 vegetation on soil organic matter dynamics in contrasting semi-natural tropical ecosystems(European Geosciences Union, 2015-08-26) Saiz, G; Bird, MI; Wurster, CM; Quesada, CA; Ascough, PL; Domingues, T; Schrodt, F; Schwarz, M; Feldpausch, TR; Veenendaal, E; Djabeletey, G; Jacobsen, GE; Hien, F; Compaore, H; Diallo, A; Lloyd, JVariations in the carbon isotopic composition of soil organic matter (SOM) in bulk and fractionated samples were used to assess the influence of C3 and C4 vegetation on SOM dynamics in semi-natural tropical ecosystems sampled along a precipitation gradient in West Africa. Differential patterns in SOM dynamics in C3/C4 mixed ecosystems occurred at various spatial scales. Relative changes in C=N ratios between two contrasting SOM fractions were used to evaluate potential site-scale differences in SOM dynamics between C3- and C4-dominated locations. These differences were strongly controlled by soil texture across the precipitation gradient, with a function driven by bulk 13C and sand content explaining 0.63 of the observed variability. The variation of 13C with soil depth indicated a greater accumulation of C3-derived carbon with increasing precipitation, with this trend also being strongly dependant on soil characteristics. The influence of vegetation thickening on SOM dynamics was also assessed in two adjacent, but structurally contrasting, transitional ecosystems occurring on comparable soils to minimise the confounding effects posed by climatic and edaphic factors. Radiocarbon analyses of sand-size aggregates yielded relatively short mean residence times ( ) even in deep soil layers, while the most stable SOM fraction associated with silt and clay exhibited shorter in the savanna woodland than in the neighbouring forest stand. These results, together with the vertical variation observed in 13C values, strongly suggest that both ecosystems are undergoing a rapid transition towards denser closed canopy formations.However, vegetation thickening varied in intensity at each site and exerted contrasting effects on SOM dynamics. Thisstudy shows that the interdependence between biotic and abiotic factors ultimately determine whether SOM dynamics of C3- and C4-derived vegetation are at variance in ecosystems where both vegetation types coexist. The results highlight the far-reaching implications that vegetation thickening may have for the stability of deep SOM. © 2015, Copernicus Publications.
- ItemLoss and gain of carbon during char degradation(Elsevier, 2017-03-01) Bird, MI; McBeath, AV; Ascough, PL; Levchenko, VA; Wurster, CM; Munksgaard, NC; Smernik, RJ; Williams, AAWe report results of a study examining controls on the degradation of chars produced at 300, 400 and 500 °C from radiocarbon-free wood, deployed for three years in a humid tropical rainforest soil in north Queensland, Australia. The chars were subjected to four treatments (i) no litter (ii) covered by leaf litter, (iii) covered by limestone chips to alter local pH, and (iv) covered by limestone chips mixed with leaf litter. Radiocarbon, stable isotope and proximate analyses indicate significant ingress of exogenous (environmental) carbon and mineral material, strongly correlated with loss of indigenous (char) carbon from the samples. While indigenous carbon losses over three years were generally <8% for the char produced at 500 °C char under any treatment, chars formed at lower temperatures lost 5–22% of indigenous carbon accompanied by ingress of up to 7.5% modern exogenous carbon. The data provide clear evidence of a direct link between the ingress of exogenous carbon, likely at least partly due to microbial colonization, and the extent of char decomposition. Failure to account for the ingress of exogenous carbon will lead to a significant under-estimate of the rate of char degradation. © 2016, Elsevier Ltd.
- ItemStable isotope composition of cave guano from eastern Borneo reveals tropical environments over the past 15,000calyrBP(Elsevier, 2017-03-24) Wurster, CM; Rifai, H; Haig, J; Titin, J; Jacobsen, GE; Bird, MIInsular southeast Asia is a key driver for global atmospheric and oceanic circulation, is a hotspot for biodiversity and conservation, and is likely to have played a unique and important role in early human dispersals. Despite this, partially due to its vast size and remote tropical location, very few continuous palaeoenvironmental records exist, especially in eastern Borneo. Therefore, we investigated δ13C and δ15N values, and geochemistry of two cave guano deposits to reconstruct palaeoenvironments in eastern Borneo. Firstly, a profile was recovered from Gomantong caves, Sabah, reflecting a continuous deposit over ~ 15 cal kyr BP. Secondly, a profile was recovered from Bau Bau cave, East Kalimantan, that ranged from ~ 15–5 cal kyr BP. The geochemical signature of each deposit confirmed the material to be ancient guano. δ13C values revealed that a continuous dense rainforest persisted over at least the last 15 cal kyr BP around the Gomantong site that was relatively insensitive to regional climate change. By contrast, δ13C values at Bau Bau indicate that, although rainforest remained dominant in the record, a significant drying occurred between 7.7 and 6.3 cal kyr BP, with up to 25% grasses present. Although most regional models suggest that sea-level rise and increased Holocene insolation led to an increase in monsoonal moisture, we find some evidence of more regional variability, and that a reduction in monsoonal precipitation could have occurred. However, we cannot discount the implementation of an anthropogenic fire regime that opened the canopy allowing more grasses to occur. © 2017 Elsevier B.V. All rights reserved.