Browsing by Author "Leondard, ND"

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    Life and death of Holocene reefs of Moreton Bay, Queensland, Australia
    (Australian Geosciences Council, 2012-08-05) Nothdurft, LD; Major, J; Leondard, ND; Zhao, JX; Price, GJ; Welsh, KJ; Webb, GE; Hua, Q
    Dead fringing coral reefs of Moreton Bay are the southernmost fringing reefs in eastern Australia and are significant examples of marginal environments for subtropical reef growth. Two sites were investigated in the western part of the bay at Wellington Point and Cleveland Point. Seven auger cores and 5 surface transects were made across the dead reef flats to record spatial data on coral distribution and elevation and to provide samples for dating to test the timing and potential causes of reef termination. A total of 78 coral samples were dated using 230Th/U and 14C techniques. 230Th dates were determined by TIMS U-series, and MC-ICP-MS and 14C dates by AMS. Dates of framework corals indicate that reef growth initiated at different elevations by ∼7000 ybp and was predominantly vertical with little or no lateral progradation. Reef growth was continuous until termination at ∼5800 ybp. Micro-atolls (6523, 6680 ybp) suggest a sea level of ∼+1.3m above current lowest astronomical tides and elevated sea level persisted through that interval. Small incipient coral colonies on the dead reef surface range in ages between ∼4850 and 4700 ybp suggesting conditions were again suitable at that time for a recolonisation of the reef flat, but reef growth was not reinitiated. No younger ages were recorded. Temporal changes in Moreton Bay conditions have resulted in periodic phase shifts between favourable and unfavourable conditions for reef growth, coincident with a small sea level fall ∼5800 ybp and including potentially cooler temperatures and decreased water quality within the bay.
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    Variability in the Holocene marine radiocarbon reservoir effect for the Tropical West Pacific
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) Hua, Q; Ulm, S; Yu, K; Clark, T; Nothdurft, LD; Leondard, ND; Pandolfi, JM; Jacobsen, GE; Zhao, JX
    Reliable determination of the marine radiocarbon reservoir effect (e.g., marine radiocarbon reservoir correction (ΔR)) is important for the construction of robust radiocarbon chronologies for marine archives for various research areas including archaeology, palaeoecology, paleoceanography, Quaternary research and climate change studies. In this study, we examined temporal ΔR variability for the South China Sea (SCS) and the Great Barrier Reef (GBR) during the past ~8.1 ka based on ¹⁴ C analysis of ²³⁰ Th-dated corals. Coral samples, including Acropora sp., Cyphastrea sp., Favia sp. and Porites sp., were collected from Xisha (or Paracel) Islands and Yongshu Reef, Nansha (or Pratly) Islands in the SCS; Clerke Island, Gore Island, Haggerstone Island, Alexandra Reef and High Island in the northern GBR; and Great Keppel Island in the southern GBR. A total of 44 unaltered coral samples (based on SEM screening, δ²³⁴ U and initial ²³⁰ Th/²³²Th ratios) were analysed for ²³⁰ Th and ¹⁴ C. ²³⁰ Th dates were determined using a VG Sector-54 thermal ionization mass spectrometer (TIMS) or a Nu Plasma MC-ICP-MS in the Radiogenic Isotope Facility, The University of Queensland, with a precision better than 0.5% (2σ). Subsamples of these dated corals were then taken for radiocarbon analysis using the STAR AMS facility at ANSTO with a typical precision of ~0.3-0.4% (1σ). Results show large ΔR variations of ~410 yr and ~490 yr for the SCS and the northern GBR at ~5.5-8.1 ka and ~5.5-7 ka, respectively, and a smaller ΔR variability of ~200 yr for the SCS at ~2-3.5 ka. Changes in the sources (or ¹⁴ C level) of upwelled waters in the Tropical East Pacific, and variations in Pacific-wide and regional/local ocean circulation associated with climate change might be responsible for these observed ΔR variations [1]. The results of our study also indicate the need for regional marine radiocarbon calibration curves for improved radiocarbon dating of marine samples as the observed Holocene ΔR values for the Tropical Pacific are not fully reproduced by recent modelling work using a 3D ocean model [2], which takes into account climate change effects. Ocean circulation changes were included in the model for the period 11.5-50 ka but possibly not considered or not well represented for the Holocene, which might explain the differences between the observed and modelled ΔR values. © The Authors