Browsing by Author "Hodge, E"
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- ItemA 1,000 year rainfall record for SE Australia using speleothem hydrological proxies(Geological Society of Australia, 2010-07-04) McDonald, J; Drysdale, RN; Hodge, E; Hua, Q; Fischer, MJ; Treble, PC; Greig, A; Hellstrom, JCCave drip water studies at Wombeyan Caves (34°19’S, 149°59’E) demonstrated a marked hydrochemical response to wet/dry (McDonald et al., 2004; 2007). Three 20th Century coeval active speleothems (WM4, WM6 and WM20) were geochemically analysed and changes in trace element and stable isotope values were related to the instrumental record. These climate‐geochemical relationships were interpolated to two longrecord speleothems (WM7 and WM9) which grew deeper within the same cave system. Two major difficulties were encountered. Unlike other sub‐annual/annual climate records derived from speleothems using trace elements (Treble et al., 2003; Johnson et al., 2006); stable isotopes (Treble et al., 2005; Johnson et al., 2006) and annual laminae (Borsato et al., 2007) where annual cycles were shown to be consistent, at Wombeyan frequent droughts perturb predicted rainfall patterns and rainfall variability is high. Here, droughts can span several years, or extend over winter, diminishing or obliterating the expected winter recharge signal. Alternatively, wet summers can sustain ‘unseasonal’ high discharge and lessen the expected prior calcite precipitation (PCP) signal. Thus an annual wet/dry geochemical signal is often absent. Secondly, due to the young age of the speleothems and very low uranium concentrations (~10 ppb) the use of U‐series disequilibrium dating was ineffective to produce a robust chronology. Trace element cycle counting only gave the minimum age due to the non‐expression of many cycles. The 14C bomb pulse was successfully defined in two modern stalagmites (WM4 and WM6) and maximum 14‐C activity was around 134 per cent modern carbon (pMC) for both speleothems, indicating rapid transfer of 14‐C from atmosphere, to soil, to drip water during the bomb‐pulse period. A dampened 14C bomb pulse was detected in WM7 (where pMCmax was 112 per cent modern carbon) reflecting the greater degree of mixing within the thicker bedrock. Carbon‐14 AMS analyses were utilised together with trace element cycle counting to obtain preliminary chronological control. Despite these difficulties, palaeohydrological records using multiple proxies: Mg/Ca (aridity), P and Y (wet), δ18O (dominant air mass and rainfall amount signals), and based on 14C ages were constructed for the longerrecord, slower growing stalagmites. Over the last 1,000 years there have been several sustained episodes of wet/arid or highly variable phases. A sustained wet phase occurred ~ 900–1300 AD and followed by ~ 200 years of highly variable wet/dry conditions. From ~ 1500 to 1800 AD a dry phase is indicated. The last 150 years support a drying phase, but the negative IPO (1944–1978 [wettest period in 20thC]) is not indicated by a negative anomaly and further, sustained positive δ18O anomalies in this period suggest that other factors maybe influencing this part of the record (temperature?). The δ18O record indicates changing frequency from ~ centennial to pentadecadal time‐scales in the longer‐time scale oscillations. Within the longer‐time scale oscillations, higher resolution (~ 2–5 years) variability is evident replicating the trend shown by modern annually resolved stalagmites at this site.
- ItemA 1,500 year south Australian rainfall record based on speleothem hydrological proxies(AMOS National Conference, 2013-02-11) McDonald, J; Drysdale, RN; Hua, Q; Hodge, E; Treble, PC; Greig, A; Fallon, SJ; Lee, S; Hellstrom, JCCave drip water studies at Wombeyan Caves (34°19’S, 149°59’E) demonstrated a marked hydrochemical response to wet/dry phases (McDonald Et al. 2004; 2007). Geochemical Variations in three 20th Century coeval active Speleothems were able to be linked to the instrumental record. Subsequently geochemical relationships were investigated in a long record speleothem(WM7) which grew deeper within the same cave system. Obtaining a robust chronology proved to be challenging, due to the young age of the speleothem and very low uranium concentrations (~10 ppb) the use of U‚Aeseries disequilibrium dating was ineffective to produce a robust chronology. Chronology for WM7 was based on a dense sequence of DCF corrected ages using three different age-‐depth models: Clam (Classical method), and Bacon and OxCal (Bayesian statistical approach) (Hua et al. 2012).The new chronology indicated that WM7 began growth around 4400 cal BP(171 mm). However, since sampling from 0-‐50mm was most intensive, the model is based on this part of the stalagmite and indicates that the top 50 mm of WM7 grew during the past 1360 and 1740 years. An aridity index based on Sr,P, Y, La, and Ba shows that over the last 1,500 years several sustained episodes of wet/arid and otherwise variable phases have occurred. Two sustained wet phases ~ 700-‐880 AD and ~ 900-‐ 1250 AD were followed by ~ 400 years of variable wet/dry conditions, although from ~1300 to 1600 AD a drying trend is indicated, but punctuated by several wetter episodes. The last 200 years indicate sustained drying phases. The OE￥13C record is anomalous from ~ 1880 to present and attributed to the stalagmite’s recording of increasing contribution of fossil fuel to CO2 concentrations. Within the longer-‐time scale oscillations, higher resolution (~ 2-‐5 years) variability is evident, replicating the trend shown by modern annually resolved stalagmites at this site.
- ItemCoupled U-series and radiocarbon dating of a Chinese stalagmite from 15 to 33 ka: testing calibration applicability and dead carbon correction variability(Elsevier, 2006-08) Hodge, E; Zhao, JX; Feng, YX; Wu, J; Fink, D; Hua, QA high purity stalagmite from South China with minimal detrital component (based on 230Th/232Th ratios >4000) and an average growth rate of ∼50 mm/ka has been carefully sampled along its growth axis for both U-series and radiocarbon measurements in a study to investigate its applicability for improving the database of radiocarbon calibration for the pre-dendro period (∼12.4 ka). Our preliminary assessment is based on a set of 15 high-precision AMS 14C-and 6 TIMS U-series samples spaced over the 37 cm length of the stalagmite to confirm an age range of 15 to 33 ka. TIMS U-series dates over this age range can be obtained to 0.5% at 2σ errors and as speleothems are composed of dense crystalline calcite, they are often less vulnerable to post-depositional alteration than corals. However an issue of serious concern in such an analysis is to evaluate whether the variability of the dead carbon fraction (DCF) over this time range reduces the reliability and quality of a speleothem-based calibration of atmospheric radiocarbon. The DCF represents the fraction of carbon derived from host limestone surrounding the cave that contains negligible 14C and therefore offsets the 14C date towards older ages. An assumption of a constant DCF, estimated by others to be ∼16% for speleothems (based on matching to a well-constrained radiocarbon calibration curve from 11 to 15 ka) requires case-by-case verification. Calendar ages for the positions taken for AMS 14C samples were interpolated from adjacent U-series dates on the growth curve. These absolute ages were compared to the measured AMS 14C ages and then overlain on the IntCal04 calibration curve. In broad terms, our preliminary results indicate that the growth rate, although continuous, was not linear over the period from 15 to 33 ka. In order to minimise the difference between our 14C-ages and the IntCal04 curve from 26 ka to 15.6 ka, we required an average DCF of 18%. However, this value causes the younger half (<22 ka) to be ‘too old’ and the upper section (>22 ka) to be too ‘young’ indicating that DCF over the LGM period was probably not constant. To further qualify the status of this stalagmite and decouple growth rate variability from that of the DC, a new set of 30 paired 230Th–AMS 14C-ages are in progress. Copyright © 2006 Elsevier Ltd.
- ItemHolocene 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, MThe 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.
- ItemMid-holocene variability in the marine C-14 reservoir age for northern coastal Papua New Guinea(Elsevier, 2008-08) McGregor, HV; Gagan, MK; McCulloch, MT; Hodge, E; Mortimer, GEChanges in oceanic radiocarbon (C-14) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Nino events. Reservoir ages were calculated using paired U-Th and conventional C-14 dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS C-14 and MC-ICPMS U-Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U-Th and conventional C-14 ages. The combined results show average reservoir ages of 185 +/- 30 C-14 yr (n = 4) for 7220-5850 yr BP compared to 420 C-14 yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air-sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Nino activity around this time would have served to enhance the intrusion of C-14-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Nino-Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region. © 2008, Elsevier Ltd.
- ItemUsing the 14C bomb pulse to date young speleothems(University of Arizona, 2011-04-01) Hodge, E; McDonald, J; Fischer, MJ; Redwood, D; Hua, Q; Levchenko, VA; Drysdale, RN; Waring, CL; Fink, DMany factors may influence the radiocarbon age results of lacustrine sediments, among which the hardwater effect is particularly important. Daihai Lake is a closed lake located in the semi-arid region of Inner Mongolia, China. High concentrations of (HCO3)- and (CO3)2- and high pH values in the lake water imply that there is a hardwater effect when using bulk lacustrine sediment samples for 14C dating. To correct the apparent 14C age, we present a pilot study based on a series of 14C ages of lake surface sediment, lake water, submerged aquatic plant (Myriophyllum), fish bone (Cyprinus carpio), and surface soil samples from and around Daihai Lake. Assuming that the relationship between the 14C/12C ratio of DIC and of atmospheric CO2 was constant (at 0.816), the hardwater effect ages calculated for the past 8000 yr would have varied from 949 to 1788 yr. Together with the reservoir effect and soil organic matter input, the hardwater effect is a major factor causing changes in apparent age when using bulk organic matter for 14C dating. © 2011, University of Arizona.