Browsing by Author "Webb, GE"

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    Large variations in the Holocene marine radiocarbon reservoir effect reflect ocean circulation and climatic changes
    (Elsevier, 2015-04-21) Hua, Q; Webb, GE; Zhao, JX; Nothdurft, LD; Lybolt, M; Price, GJ; Opdyke, BN
    Accurate radiocarbon dating of marine samples requires knowledge of the marine radiocarbon reservoir effect. This effect for a particular site/region is generally assumed constant through time when calibrating marine 14C ages. However, recent studies have shown large temporal variations of several hundred to a couple of thousand years in this effect for a number of regions during the late Quaternary and Holocene. Here we report marine radiocarbon reservoir correction Δ(R) for Heron Reef and Moreton Bay in southwestern (SW) Pacific for the last 8 ka derived from 14C analysis of 230Th-dated corals. Most of our ΔR for the last ∼5.4 ka agree well with their modern value, but large ΔR variability of ∼410 yr (from trough to peak) with possible decadal/centennial fluctuations is evident for the period ∼5.4–8 ka. The latter time interval also has significant variations with similar features in previously published Δ values for other sites in the Pacific, including southern Peru–northern Chile in southeastern (SE) Pacific, the South China Sea, Vanuatu and Papua New Guinea, with the largest magnitude of ∼920 yr from SE Pacific. The mechanisms for these large ΔR variations across the Pacific during the mid-Holocene are complex processes involving (1) changes in the quantity and 14C content of upwelled waters in tropical east Pacific (TEP) (frequency and intensity of ocean upwelling in the TEP, and contribution of Subantarctic Mode Water to the upwelled waters, which is influenced by the intensity and position of southern westerly winds), and (2) variations in ocean circulation associated with climate change (La Niña/El Niño conditions, intensity of easterly trade winds, positions of the Intertropical Convergence Zone and the South Pacific Convergence Zone), which control the spreading of the older upwelled surface waters in the TEP to the western sites. Our results imply the need for employing temporal changes in ΔR values, instead of constant (modern) values, for age calibration of Holocene marine samples not only for the SW Pacific sites but also for other tropical and subtropical sites in the Pacific. Crown Copyright ©2015
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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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    Marine radiocarbon reservoir effect along the north-eastern coast of Australia during the Holocene
    (Accelerator Mass Spectrometry, 2013-10-15) Hua, Q; Webb, GE; Zhao, JX; Nothdurft, LD; Price, GJ
    Radiocarbon dating of surface ocean samples involves estimates of marine radiocarbon reservoir effect (e.g., marine reservoir age (R) and correction (ΔR)). These values for a given location are generally assumed to be constant with time when calibrating marine 14C ages. However, recent studies have reported large variability in the marine radiocarbon reservoir effect of several hundred to a couple of thousand years for various regions in the Pacific, Atlantic and Mediterranean during the Late-glacial and Holocene (Siani et al., 2001; Bondevik et al., 2006; Burr et al., 2009; Hua et al., 2009; Yu et al., 2010; Ortlieb et al., 2011; Sarnthein et al., 2011). These variations result from changes in ocean circulation and the carbon cycle associated with climate change. In this paper we present an investigation of possible variability in the marine radiocarbon reservoir effect along the north-eastern coast of Australia in South-Western (SW) Pacific during the last 8000 years. This study aims to get a better understanding of ocean circulation changes associated with climate change for the study area during the Holocene and to improve radiocarbon dating of marine samples.
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    Variations in marine reservoir corrections for the Great Barrier Reef during the last 7000 years
    (12th International Conference on Accelerator Mass Spectrometry (AMS-12), 2011-03-21) Hua, Q; Webb, GE; Zhao, JX; Nothdurft, LD; Price, GJ
    For calibration of marine 14C ages, marine reservoir correction value (ΔR) of a given location is generally assumed to be constant with time. However, variations in the marine reservoir effect of several hundred to a couple of thousand years have been observed for various regions in the Pacific, Atlantic and Mediterranean during the Late-glacial and Holocene. These variations result from changes in ocean circulation and the carbon cycle associated with climate change. Here we present a new data set of ΔR for the Great Barrier Reef (GBR) in south-western Pacific Ocean during the last 7000 years for the investigation of possible variability in the marine reservoir effect of this area and for improving radiocarbon dating of marine samples. Coral samples used in this study were from shallow cores and storm derived blocks collected on Heron Reef in southern GBR at 23º26'S, 151º55'E. Fifteen unaltered coral samples (based on SEM screening, δ234U and initial 230Th/232Th ratios) were analysed for 230Th/U and 14C. 230Th dates were determined by TIMS Useries in the Radiogenic Isotope Laboratory at the University of Queensland with a precision better than 0.5% (2σ). Aliquots of these dated corals were then taken for radiocarbon analysis. The samples were cut by diamond saw, cleaned in deionised water, oven dried, and homogenised in a mortar and pestle prior to 14C analysis using the STAR AMS facility at ANSTO. A typical precision for 14C analysis is <0.4% (1σ). The results of this investigation will be compared with those of previous studies for the Pacific, and variations in ΔR for the GBR for the mid- to late Holocene and their possible mechanisms will be then discussed. This work was supported by AINSE grant 09/056. Copyright (c) 2011 AMS12.