Browsing by Author "Yu, K"

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    Holocene marine 14C reservoir age variability: evidence from 230Th-dated corals in the South China Sea
    (American Geophysical Union, 2010-09) Yu, K; Hua, Q; Zhao, JX; Hodge, E; Fink, D; Barbetti, M
    The South China Sea (SCS) is well connected with the western Pacific and influenced by the East Asian monsoon. We have examined temporal variations in radiocarbon marine reservoir ages (R) and regional marine reservoir corrections (ΔR) of the SCS during the Holocene using paired measurements of AMS 14C and TIMS 230Th on 20 pristine corals. The results show large fluctuations in both R and ΔR values over the past 7500 years (yrs) with two distinct plateaus during 7.5–5.6 and 3.5–2.5 thousand calendar years before present (cal ka BP). The respective weighted mean ΔR values of these plateaus are 151 ± 85 and 89 ± 59 yrs, which are significantly higher than its modern value of −23 ± 52 yrs. This suggests that using a constant modern ΔR value to calibrate 14C dates of the SCS marine samples will introduce additional errors to the calibrated ages. Our results provide the first database for the Holocene R and ΔR values of the SCS for improved radiocarbon calibration of marine samples. We interpret the two ΔR plateaus as being related to two intervals with weakened El Niño - Southern Oscillation (ENSO) and intensified East Asian summer monsoon (EASM). This is because the 14C content of the SCS surface water is controlled by both the 14C concentration of the Pacific North Equatorial Current (NEC) which is in turn influenced by ENSO-induced upwelling along the Pacific equator and vertical upwelling within the SCS as a result of moisture transportation to midlatitude region to supply the EASM rainfall. © 2010 American Geophysical Union.
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    Temporal variability in the Holocene marine radiocarbon reservoir effect for the Tropical and South Pacific
    (Elsevier, 2020-12-01) Hua, Q; Ulm, S; Yu, K; Clark, TR; Nothdurft, LD; Leonard, ND; Pandolfi, JM; Jacobsen, GE; Zhao, JX
    Understanding the marine radiocarbon reservoir effect (i.e., marine radiocarbon reservoir age (R) and/or 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 14C analysis of 230Th-dated corals. Results show large ΔR variations of ∼410 yr and ∼490 yr for the SCS and the northern GBR for ∼5.5–8.1 ka and ∼5.5–7 ka, respectively, and a smaller ΔR variability of ∼200 yr for the SCS for ∼2–3.5 ka. Our data, together with those previously published for the Tropical and South Pacific, indicate that variability in ocean upwelling bringing old subsurface waters to the surface and/or changes in the sources (or 14C level) of the upwelled waters are responsible for seeing large ΔR variations in coastal areas along the eastern Pacific and the Tropical East Pacific (TEP) of several hundred to a thousand years mostly during the Early to Middle Holocene. ΔR variations in the central and western Pacific of several and a couple of hundred years during the Middle and Late Holocene, respectively, might be due to variability in Pacific-wide ocean circulation associated with climatic changes controlling the spread of upwelled waters from the TEP to the west. This mechanism together with local/regional effects, such as changes in ocean upwelling in the SCS resulting from East Asian monsoon variability and changes in upwelling and/or horizontal advection in the northern GBR associated with variability in the El Niño Southern Oscillation, might be responsible for the observed ΔR variations in these areas. 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 and South Pacific are not fully reproduced by a recent modelling work using a 3D ocean model, which takes into account climate change effects. Ocean circulation changes were included in the model for the period of 11.5–50 cal kyr BP but possibly not considered or not well represented for the Holocene, which might explain the differences between the observed and modelled ΔR values. Crown Copyright©2020 Published by Elsevier Ltd
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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