Browsing by Author "Hellstrom, JC"

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    A 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, JC
    Cave 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.
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    A 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, JC
    Cave 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.
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    Assessment of climatic influences on 14C activity in a Holocene stalagmite from Flores, Indonesia
    (University of Newcastle, 2010-06-30) Griffiths, ML; Drysdale, RN; Hua, Q; Hellstrom, JC; Frisia, S; Gagan, MK; Zhao, JX; Ayliffe, LK
    In the last decade, a number of speleothem studies have used radiocarbon dating to address a range of paleoclimate problems. These have included the use of the bomb pulse to anchor chronologies over the last 60 years (Mattey et al 2008), the combining of U-series and radiocarbon measurements to improve the radiocarbon calibration curve (Beck et al. 2001), and linking atmospheric radiocarbon variations with climate changes (McDermott et al. 2008). Central to a number of these studies is how to constrain, or interpret variations in, the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that contributes to a speleothem radiocarbon measurement. In this study, we use radiocarbon measurements, stable isotope and trace element geochemistry, and U-series ages to examine DCH variations between 2.4 and 2.8 ka in a previously studied (Griffiths et al. 2009; 2010) speleothem from Liang Luar, Flores, Indonesia.
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    Characterising the expression of sub-millennial scale climate events in western Europe during the early last glacial period using multi-proxy speleothem records
    (Australasian Quaternary Association Inc., 2022-12-06) Corrick, E; Drysdale, RN; Hellstrom, JC; Couchoud, I; Wong, HKY; Didier, C; Hai, C; Jaillet, S; Tocino, S
    Past abrupt climate changes act as critical analogues for understanding how the climate system may respond to future abrupt changes. One of the best examples of naturally occurring abrupt climate change is the series of millennial-scale Dansgaard-Oeschger (D-O) events that took place during the last glacial period (115,000 – 11,500 years ago). D-O events are clearly recorded in ice-cores from Greenland, with coincident climate changes detected in marine and terrestrial records spanning a range of climate zones. Greenland ice cores also record shorter-lived ‘sub-millennial’ scale events that occur within the main D-O event sequence, particularly during the early last glacial period. To what extent these sub-millennial events were expressed outside of Greenland is currently poorly understood. Here we characterise the response to sub-millennial scale climate changes in western Europe using five multi-proxy (δ18O, δ13C, Mg and Sr) speleothem records from Saint-Marcel and Orgnac Caves, France, that collectively span 127 – 87 kyr BP. The replicated speleothem records clearly preserve both millennial D-O events and sub-millennial events, demonstrating the strong coupling between the climate of south-east France and the North Atlantic across both millennial and sub-millennial timescales. Interestingly, the multiproxy record reveals a decoupling between broad temperature (indicated by δ13C) and precipitation changes (indicated by δ18O) during some of these sub-millennial scale events. This suggests that climate teleconnections operating during sub-millennial events were in some ways different to those during the stronger millennial-scale D-O events.
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    Chronology, stratigraphy and palaeoenvironmental interpretation of a Late Pleistocene to mid-Holocene cave accumulation on Kangaroo Island, South Australia
    (Wiley-Blackwell, 2013-10-01) McDowell, MC; Bestland, EA; Bertuch, F; Ayliffe, LK; Hellstrom, JC; Jacobsen, GE; Prideaux, GJ
    Chronological, sedimentological and geochemical analyses of a clastic infill from Kelly Hill Cave (5K1), Kangaroo Island, document a palaeoenvironmental record that spans from the Late Pleistocene to the middle Holocene. We AMS radiocarbon-dated bone collagen and U-Th-dated speleothem to determine that fossiliferous sediments were deposited between >20ka and 7ka ago. Most of the 15 sedimentary layers are dominated by sand- and silt-sized quartz that is physically and geochemically comparable with surface soils in the Kelly Hill area. Late Pleistocene and Last Glacial Maximum strata are represented primarily by homogeneous, poorly sorted quartz-rich sediments that contain little organic matter, but include a thin layer composed largely of silt-sized clay pellets that resemble sediments deflated from playa lakes. Microstructures observed in petrographic slides indicate that, with the exception of one layer, all sediments experienced little reworking once deposited in the cave. Some layers display pedogenic microstructures such as redeposited clays and opaline silica infilling that indicate postdepositional modification; that is, cave-floor soil development. Overlying Holocene-aged sediments also consist mainly of quartz but have much greater organic matter content. Some of these sediments have been strongly influenced by re-precipitated organic matter that appears to have been transported into the cave via vadose drip water. The presence of dissolved organic matter in soil/vadose waters suggests a high vegetation density and acidic soils, which are congruent with the more equitable climatic conditions characteristic of the Holocene. The sediments described here provide a valuable palaeoenvironmental record that will facilitate future interpretation of associated vertebrate fossils. © 2013, Wiley-Blackwell.
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    Comparison of oxygen isotope records from radiocarbon dated groundwater and U-Th dated flowstone
    (20th International Mass Spectroscopy Conference, 2014-08-24) Adler, L; Treble, PC; Meredith, KT; Baker, AA; Hellstrom, JC
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    Comparison of δ18O in groundwater and a cave flowstone: improving the interpretation of the speleothem δ18O paleoclimate proxy
    (International Union for Quaternary Research (INQUA), 2019-07-30) Adler, L; Priestley, SC; Treble, PC; Baker, AA; Hellstrom, JC; Griffiths, AD; Meredith, KT
    Speleothems are high-resolution records that can be used for terrestrial paleoclimate reconstruction from their oxygen and carbon isotopes (δ18O and δ13C), and whose deposition is directly related to the groundwater recharge process. Groundwater δ18O records have the potential to provide an important long-term record of past climate, but they are low-resolution records as the isotope signal can be altered during flow within the aquifer. In this study we compare measured δ18O values from both a groundwater record and speleothem record from a flowstone over the past 12,000 years for the first time from south west Western Australia. Flowstones normally form from cave streams or fast dripping seepages and their oxygen isotope composition is sensitive to the extent of kinetic fractionation, determined by water flow rate, as well as, streams or seepages water δ18O composition. In this study comparison of δ18O values from a groundwater record and flowstone enables the source water oxygen isotopic composition to be constrained in order for a more complete interpretation of the higher resolution speleothem record, including the site specific kinetic processes and climatic changes. The flowstone δ18O values appear consistent with millennial variability in recharge δ18O predicted using the groundwater values, although the flowstone oxygen isotopes are generally enriched in 18O compared to the regional groundwater record before ~7.4ka. This offset between the records indicates that flowstone calcite deposition was not in isotopic equilibrium with its source water likely due to low flow regimes and extensive degassing of CO2. According to a model of isotopic disequilibrium that is driven by water supply, the Holocene portion of the flowstone record contains periods of relatively lower isotopic disequilibrium indicating possible higher water supply, and periods of relatively higher isotopic disequilibrium indicating possible lower water supply consistent with local and regional archives.
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    Continental aridification and the vanishing of Australia's megalakes
    (Geological Society of America, 2011-02) Cohen, TJ; Nanson, GC; Jansen, JD; Jones, BG; Jacobs, Z; Treble, PC; Price, DM; May, JH; Smith, AM; Ayliffe, LK; Hellstrom, JC
    The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The final disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridification that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water filling these lakes at different times during the last full glacial cycle. © 2011, Geological Society of America
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    Enigmatic human remains from the Pleistocene-Holocene transition of southwest China and the complex evolutionary history of east Asians
    (Australian Geosciences Council, 2012-08-05) Curnoe, D; Ji, X; Herries, AIR; Bai, K; Taçon, PSC; Bao, Z; Fink, D; Zhu, Y; Hellstrom, JC; Luo, Y; Cassis, G; Su, B; Wroe, S; Hong, S; Parr, WCH; Huang, S; Rogers, N
    The Upper Pleistocene hominin fossil record of East Asia is poorly known due to a scarcity of well-described, reliably classified and accurately dated fossils. In 2008, we started a joint project involving six Chinese and five Australian institutions to examine the later Pleistocene human record from southwest China. The region has been identified from genetic research as a hotspot of human diversity, and has yielded a number of human remains thought to derive from Pleistocene deposits. We prepared, reconstructed, described and dated a new partial skeleton from a consolidated sediment block collected in 1979 from the site of Longlin Cave (Guangxi Province). We also undertook new excavations at Maludong (Yunnan Province) to clarify the stratigraphy and dating of a large sample of mostly undescribed human remains from the site. Both samples probably derive from the same population, exhibiting an unusual mixture of modern human traits, characters probably plesiomorphic for later Homo, and some unusual features. We dated charcoal with AMS radiocarbon dating and speleothem with the Uranium-series technique and the results show both samples to be from the Pleistocene-Holocene transition: ∼14.3–11.5 ka. At nearby sites, some modern humans had already begun the economic transition to agriculture by this time. We suggest the Longlin-Maludong humans represent either a late-surviving archaic population or a previously unknown modern human group who colonised East Asia probably without leaving descendants. The archaeological record of Maludong indicates these humans were engaging in complex behaviours, including ochring bones and shells and manufacturing containers from human skulls.
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    Environmental change in the Adriatic region over the last 365 kyr from episodic deposition of Modrič Cave (Croatia) speleothems
    (International Union for Quaternary Research (INQUA), 2019-07-25) Surić, M; Lončarić, R; Columbu, A; Bajo, P; Lončar, N; Drysdale, RN; Hellstrom, JC
    The Adriatic Sea, as the northernmost and practically landlocked part of the Mediterranean Sea, is influenced by both Atlantic and Mediterranean air masses, so the isotopic records from coastal cave speleothems are expected to reveal local and regional responses to global climate changes on that bordering region. In this study we present a 365-kyr long composite isotopic record of three speleothems from Modrič Cave located 120 m from the shoreline on the central part of the eastern Adriatic coast. Results of a 4-year monitoring program of the surface and cave microclimate, rain and drip water stable isotope composition and rain and drip intensities, show cave air temperature variations within 1 °C, a relative humidity of 100%, dripwater O and H stable isotopic composition (range 0.4‰) very well buffered against the seasonal rainfall range (6.8‰). Such conditions enable calcite deposition without strong kinetic isotopic disequilibrium effects, as validated by Hendy tests, and give us confidence that the isotopic signal is faithfully recording climate changes. All three speleothems (MOD-31, MOD-32, MOD-33) were active, collected from their growth position, so Holocene signals can be traced in all of them. Within their older parts, U-Th dating reveals the following growth periods: MOD-31 from MIS 10 to MIS 7, MOD-32 from MIS 4 to MIS 2, and MOD-33 from MIS 6 to MIS 5. Relatively large δ18O and δ13C ranges (-8.16‰ – -2.97‰ and -13.11‰ – -1.00‰, respectively) indicate significant environmental changes whose comparison with stable isotope time series of adjacent speleothem records and other regional proxies provides the longest eastern Adriatic speleothem-based record. © 2019 The Authors.
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    Evidence for Holocene changes in Australian-Indonesian monsoon rainfall from stalagmite trace element and stable isotope ratios
    (Elsevier, 2010-03-15) Griffiths, ML; Drysdale, RN; Gagan, MK; Frisia, S; Zhao, JX; Ayliffe, LK; Hantoro, WS; Hellstrom, JC; Fischer, MJ; Feng, YX; Suwargadi, BW
    Trace element and stable isotope ratios from an active stalagmite (LR06-B1) recovered from Liang Luar Cave on the island of Flores (eastern Indonesia) are used to reconstruct the position of the austral summer inter-tropical convergence zone and Australian-Indonesian summer monsoon variability during the Holocene. Uranium-series dating of the stalagmite shows that it commenced growth 12,640 years ago , with hiatuses spanning 8,560 to 6,420 and 3,670 to 2,780 years ago. Stalagmite Mg/Ca and Sr/Ca ratios correlate significantly with one another, and with δ18O and δ13C, throughout the record. This suggests that the Mg/Ca and Sr/Ca ratios are dominated by prior calcite precipitation, a process whereby degassing in the vadose zone during periods of low recharge causes deposition of calcite and disproportionate loss of Ca2+ ions (relative to Mg2+ and Sr2+) ‘upstream’ of the stalagmite. The degree of initial 234U/238U disequilibrium also appears to have been controlled by recharge to the overlying aquifer. Together with the Mg/Ca, Sr/Ca, and δ18O values, the initial uranium isotope activity ratios ([234U/238U]I) imply a generally drier early Holocene, coincident with a lower sea level and lower Southern Hemisphere summer insolation. Comparison of speleothem δ18O time-series from Flores and Borneo shows that they vary in unison for much of the Holocene. However, there is a significant decrease in the Borneo δ18O record ~6,000 to 4,000 years ago that does not occur in the Flores record. This anomaly may be related to a change in the Australian-Indonesian summer monsoon circulation in response to a protracted positive phase of the Indian Ocean Dipole. Under this scenario, stronger upwelling off of western Indonesia would, based on present-day effects, result in reduced summer convective activity over Flores and a subsequent northward shift of the intertropical convergence zone. © 2010, Elsevier Ltd.
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    Exploring the dating of “dirty” speleothems and cave sinters using radiocarbon dating of preserved organic matter
    (Elsevier, 2017-04) Blyth, AJ; Hua, Q; Smith, AM; Frisia, S; Borsato, A; Hellstrom, JC
    Speleothems and other carbonate deposits such as tufa containing high proportions of detrital material can be difficult to chemically date due to detrital thorium levels causing a high level of error in conventional U-Th disequilibrium dating. Here we investigate the use of an alternative technique centring on radiocarbon dating of organic matter preserved within the detrital fraction. Non-acid soluble humic, particulate and detritally absorbed organic matter was recovered from eight samples from a flowstone sinter formed within a roman aqueduct at Trento in Italy with a maximum age of 100 CE (1850 cal yr BP), and two repeat samples from a dripstone formed within the 20th Century on a wire fence at Lilly-Pilly Cave, Buchan Caves Reserve in Victoria, Australia. In the aqueduct samples the median calibrated 14C ages ranged from 2232 to 2889 cal yr BP, with 95.4% probability age range in the youngest and oldest samples of 2153–2337 and 2342–3449 cal yr BP respectively. The median age of the more modern dripstone was 336 cal yr BP, with a 95.4% probability age range of 148–486 cal yr BP. These results provide very approximate ball-park estimates of the age of the sample, but are consistently too old when compared to the known maximum ages of formation. It is hypothesised that this offset is due to a combination of the nature of the organic carbon transported from the source organic matter pools, and reworking of both modern and old organic carbon by in situ microbial communities. ©2017 Elsevier B.V
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    Fast high-resolution synchrotron micro-XRF mapping of annually laminated stalagmites
    (Copernicus GmbH, 2019-04-11) Borsato, A; Frisia, S; Hellstrom, JC; Treble, PC; Johnson, K; Howard, DL; Greig, A
    Annual lamination in trace elements content allows to improve speleothem chronology as well as to extract paleoclimate information about fluctuations of the seasonal signal through time. Given the relatively slow growth and textural heterogeneity intrinsic in most speleothems, only high-resolution mapping techniques provide a viable approach to resolve trace elements variability at annual to sub-annual scale. Synchrotron radiation X-ray fluorescence microscopy (SR-μXRF) is, to date, the ideal technique as it provides quantitative, non-destructive fast scanning of large samples at the necessary high spatial resolution (0.5 – 5 μm). Here we present SR-μXRF investigation of U/Th dated stalagmites from caves in different geographic and climate setting including semi-arid (Flinders Ranges, South Australia), temperate (Central Italy) and tropical humid (Cook Islands, Northern Laos) climates. SR-μXRF analyses were coupled with high-resolution petrographic observation and LA-ICP-MS elemental analyses in order to verify the XRF elemental quantification and to test the relationship between fabric and trace element incorporation. All the analysed stalagmites revealed faint to sharp annual lamination in Sr, and, occasionally, in other elements such as Br, Cu and Zn. In climate setting characterised by strong seasonal contrast stalagmites often exhibit fabric changes at annual to centennial scale that are reflected in the internal porosity as well as in the incorporation of trace elements. Fabric control and spatial heterogeneity is also evident in the intensity of the annual cycles and, in some cases, the amplitude of Sr cycles vary considerably from one line scan to the adjacent ones. This poses the question of the significance and reproducibility of trace element analyses in speleothems characterised by fabric heterogeneity. © Author(s) 2019. CC Attribution 4.0 license.
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    Further geological and palaeoanthropological investigations at the Maludong hominin site, Yunnan Province, Southwest China
    (Springer Nature, 2013-08-28) Ji, X; Curnoe, D; Bao, Z; Herries, AIR; Fink, D; Zhu, Y; Hellstrom, JC; Luo, Y; Tacon, PSC
    Three-dimensional mapping and section work undertaken by us in 2008 have identified 11 stratigraphic units at Maludong site. AMS radiocarbon dating of charcoal established an accurate and internally consistent age profile for the sequence of 17.8±0.2 ka to 13.2±0.1 ka. Archaeomagnetic analysis showed changes in externally derived pedogenically enhanced material consistent with a warming in climate between the cold period of Henrich Event 1 and the Bølling-Allerød interstadial. Human remains recovered during the 1989 excavation were derived from a deposit dating to this interstadial, or between 14.3±0.3 ka and 13.5±0.1 ka. Anthropogenic features, including burnt rocks, baked sediment and thick charcoal and ash layers, were identified and examined through archaeomagnetic analysis. Two monkey fossils are described here, one of them being reassigned from Macaca robustus to M. aff. M. assamensis. They confirm the young age of the site and also show signs of anthropogenic alteration in the form of burning. Additional human cranial remains are reported for the first time and new data are provided for some specimens described previously. A range of new features is identified that strengthen the affinities of the Maludong remains to archaic humans. The presence of this globally unique mosaic of archaic and modern features raises important questions about human evolutionary history in East Asia during the Late Upper Pleistocene. © The Authors - Open Access
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    High-resolution stalagmite reconstructions for Australian-Indonesian monsson rainfall variability during Heinrich stadial 3 and Greenland interstadial 4
    (Elsevier, 2011-02-15) Lewis, S; Gagan, MK; Ayliffe, LK; Zhao, JX; Hantoro, WS; Treble, PC; Hellstrom, JC; LeGrande, AN; Kelley, M; Schmidt, GA; Suwargadi, BW
    Little is known about the possible teleconnections between abrupt climatic changes originating in the North Atlantic and precipitation dynamics in the Australian-Indonesian summer monsoon (AISM) domain. We examine the climatic impacts of Heinrich stadial 3 (HS3) and Greenland interstadials 4 and 3 (GIS4/3) on AISM-associated precipitation through a high-resolution analysis of stable isotope (delta(18)O, delta(13)C) and trace element (Mg/Ca, P/Ca) ratios in a stalagmite from Liang Luar cave, Flores, Indonesia. Sixteen high precision (230)Th dates indicate that stalagmite LR07-E1 grew rapidly (similar to 0.3-1.0 mm/yr) in two phases between similar to 31.5-30.1 ka and similar to 27.8-25.6 ka, separated by a similar to 2.3 kyr unconformity. Temporally consistent abrupt responses occur in the Flores record during HS3 and GIS4, which are coherent with changes in stalagmite delta(18)O records from China and Brazil. The response of low-latitude precipitation to HS3 cooling and GIS4 warming, as demonstrated by the widely separated sites, comprises three distinct simplified phases: (1) a strong southward migration of the ITCZ during HS3 is associated with a decrease in rainfall at Liang Luar cave and in China, while wetter conditions are reconstructed from Brazil, (2) represents the peak of HS3 impacts and an extended hiatus begins in the Flores record and (3) where suggested dry conditions at Liang Luar throughout GIS4 form part of a coherent north-south anti-phasing in precipitation changes. The reconstructed changes are also broadly consistent with NASA GISS ModelE-R simulations of a Heinrich-like freshwater perturbation in the North Atlantic basin, which produces a southward shift in the ITCZ. The relationship between the palaeoclimate records indicates that atmospheric teleconnections rapidly propagate and synchronise climate change across the hemispheres during periods of abrupt climate change. Our findings augment recent proposals that large-scale atmospheric re-organisations during stadials and interstadials play a key role in driving changes in atmospheric CO(2) concentration, air temperature and global climate change. (C) 2011 Elsevier B.V.
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    Human remains from the Pleistocene-Holocene transition of Southwest China suggest a complex evolutionary history for East Asians
    (PLOS, 2012-03-14) Curnoe, D; Ji, X; Herries, AIR; Kanning, B; Taçon, PSC; Zhende, B; Fink, D; Yunsheng, Z; Hellstrom, JC; Yun, L; Cassis, G; Bing, S; Wroe, S; Shi, H; Parr, WCH; Shengmin, H; Rogers, N
    Background Later Pleistocene human evolution in East Asia remains poorly understood owing to a scarcity of well described, reliably classified and accurately dated fossils. Southwest China has been identified from genetic research as a hotspot of human diversity, containing ancient mtDNA and Y-DNA lineages, and has yielded a number of human remains thought to derive from Pleistocene deposits. We have prepared, reconstructed, described and dated a new partial skull from a consolidated sediment block collected in 1979 from the site of Longlin Cave (Guangxi Province). We also undertook new excavations at Maludong (Yunnan Province) to clarify the stratigraphy and dating of a large sample of mostly undescribed human remains from the site. Methodology/Principal Findings We undertook a detailed comparison of cranial, including a virtual endocast for the Maludong calotte, mandibular and dental remains from these two localities. Both samples probably derive from the same population, exhibiting an unusual mixture of modern human traits, characters probably plesiomorphic for later Homo, and some unusual features. We dated charcoal with AMS radiocarbon dating and speleothem with the Uranium-series technique and the results show both samples to be from the Pleistocene-Holocene transition: ∼14.3-11.5 ka. Conclusions/Significance Our analysis suggests two plausible explanations for the morphology sampled at Longlin Cave and Maludong. First, it may represent a late-surviving archaic population, perhaps paralleling the situation seen in North Africa as indicated by remains from Dar-es-Soltane and Temara, and maybe also in southern China at Zhirendong. Alternatively, East Asia may have been colonised during multiple waves during the Pleistocene, with the Longlin-Maludong morphology possibly reflecting deep population substructure in Africa prior to modern humans dispersing into Eurasia. © 2012 Curnoe et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License
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    Hydrological control on the dead-carbon content of a tropical Holocene speleothem
    (Elsevier, 2012-12-01) Griffiths, ML; Fohlmeister, J; Drysdale, RN; Hua, Q; Johnson, KR; Hellstrom, JC; Gagan, MK; Zhao, JX
    Over the past decade, a number of speleothem studies have used radiocarbon (14C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse 14C to anchor chronologies over the last 60 years, the combination of U-Th and 14C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric 14C variations with climate change. An issue with a number of these studies is how to constrain, or interpret, variations in the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that reduces radiocarbon concentrations in speleothems. In this study, we use 14C, stable-isotopes, and trace-elements in a U-Th dated speleothem from Flores, Indonesia, to examine DCF variations and their relationship with above-cave climate over the late Holocene and modern era. A strong association between the DCF and hydrologically-controlled proxy data suggests that more dead carbon was being delivered to the speleothem during periods of higher cave recharge (i.e. lower δ18O, δ13C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with 14C measurements through the bomb pulse to disentangle the dominant components governing DCF variability in the speleothem. We find that the DCF is primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than kinetic fractionation and/or variations in the age spectrum of soil organic matter above the cave. Therefore, we infer that periods of higher rainfall resulted in a higher DCF because the system was in a more closed state, which inhibited carbon isotope exchange between the karst water dissolved inorganic carbon and soil-gas CO2, and ultimately led to a greater contribution of dead carbon from the bedrock. © 2020 Elsevier B.V.
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    Hydrological influence on the dead carbon fraction in a tropical speleothem during the Younger Dryas and the Last Millennium
    (American Geophyical Union, 2015-11-16) Griffiths, ML; Hua, Q; Drysdale, RN; Bajo, P; Jenkins, D; Hellstrom, JC; Johnson, KR; Gagan, MK; Zhao, JX
    The number of paleoclimate records derived from speleothems has increased significantly in recent years. In addition, speleothems have been used for calibration of the radiocarbon timescale beyond the range of the tree-ring record. One critical issue for reliable speleothem-based radiocarbon calibration and 14C dating of speleothems is constraining the temporal variations in the radioactively dead carbon (i.e. dead carbon fraction (DCF)) that is incorporated into this archive and to determine the potential mechanisms driving such changes. While some studies have shown insignificant variations in DCF through time and highlighted the potential utility of speleothems to extend/improve the radiocarbon calibration curve, others have reported significant temporal variability in speleothem DCF associated with changes in cave recharge. To further assess the potential hydrological control on speleothem radiocarbon variability, we constructed a new high-resolution DCF record from a speleothem from Flores, Indonesia for two different time periods, the Younger Dryas (YD) chronozone and the Last Millennium. A total of thirty-four 14C analyses (twenty for the YD and fourteen for the Last Millennium) were conducted on pieces of calcite extracted from stalagmite LR06-B1, which was well-dated by ~90 U-Th ages. To better characterize the paleoclimate and environmental changes, high-resolution stable-isotope (δ18O, δ13C) and trace-element (Mg/Ca, Sr/Ca) measurements were also conducted along the same sections of stalagmite. Broad comparison of the DCF record with the hydrologically-controlled proxy data suggests that increases in rainfall were matched by DCF increases. In line with a previous interpretation of DCF variability for the same specimen, but during the time interval 2.4-2.8 cal kyr BP and the post-bomb period, we interpret the DCF during the YD and the Last Millennium to have been primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than other potential factors such as kinetic fractionation and/or variations in the age-spectrum of soil organic matter above the cave. American Geophysical Union, Fall Meeting 2015
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    Increasing Australian-Indonesian monsoon rainfall linked to early Holocene sea-level rise.
    (Nature Publishing Group, 2009-09) Griffiths, ML; Drysdale, RN; Gagan, MK; Zhao, JX; Ayliffe, LK; Hellstrom, JC; Hantoro, WS; Frisia, S; Feng, YX; Cartwright, I; Pierre, ES; Fischer, MJ; Suwargadi, BW
    The Australian-Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo-Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation(1-3), but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak(4). Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise(5-7). We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone(8). However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago. © 2009, Nature Publishing Group.
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    Indo-Pacific hydroclimate over the past millennium and links with global climate variability
    (American Geophysical Union, 2016-01-01) Griffiths, ML; Drysdale, RN; Kimbrough, AK; Hua, Q; Johnson, KR; Gagan, MK; Cole, JE; Cook, BI; Zhao, JX; Hellstrom, JC; Hantoro, WS
    The El Niño-Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation (IPO) are the dominant modes of hydroclimate variability in the tropical Pacific and have far-reaching impacts on Earth’s climate. Experiments combining instrumental records with climate-model simulations have highlighted the dominant role of the Pacific Walker circulation in shaping recent trends in global temperatures (Kosaka and Xie, 2013, 2016). However, the paucity of high-resolution terrestrial paleoclimate records of deep atmospheric convection over the Indo-Pacific Warm Pool (IPWP) precludes a comprehensive assessment as to role of the tropical Pacific in modulating radiative-forced shifts in global temperature on multidecadal to centennial timescales. Here we present a suite of new high-resolution oxygen-isotope records from Indo-Pacific speleothems, which, based on modern rainfall and cave drip-water monitoring studies, along with trace element (Mg/Ca, Sr/Ca) analyses, are interpreted to reflect changes in Australasian monsoon variability during the Common Era (C.E.). Our results reveal a protracted decline in southern Indonesian monsoon rainfall between ~1000-1400 C.E. but stronger between ~1500-1900 C.E. These centennial-scale patterns over southern Indonesia are consistent with other proxy records from the region but anti-phased with records from India and China, supporting the paradigm that Northern Hemisphere cooling increased the interhemispheric thermal gradient, displacing the Australasian ITCZ southward. However, our findings are also compatible with a recent synthesis of paleohydrologic records for the Australasian monsoon region, which, collectively, suggest that rather than moving southward during the LIA, the latitudinal range of monsoon-ITCZ migration probably contracted equatorward (Yan et al., 2015). This proposed LIA ITCZ contraction likely occurred in parallel with a strengthening of the Walker circulation (as indicated through comparison with our hydroclimate records from the central-eastern equatorial Pacific Ocean and western Indian Ocean, and eastern Australia), and thus, the tropical Pacific may have played a critical role in amplifying the radiative-forced global cooling already underway. © 2016. American Geophysical Union