Browsing by Author "Ayliffe, LK"

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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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    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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    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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    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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    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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    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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    An investigation of the climatic influences on 14C activity in a Holocene stalagmite from Flores, Indonesia
    (18th INQUA Congress, 2011-07-21) Griffiths, ML; Drysdale, RN; Hua, Q; Hellstrom, JC; Frisia, S; Gagan, MK; Zhao, JX; Fischer, MJ; Ayliffe, LK
    Over the past decade, a number of speleothem studies have used radiocarbon dating to address a range of palaeoclimate problems. These have included the use of the bomb pulse to anchor chronologies over the last 60 years, the combining of U-series and radiocarbon measurements to improve the radiocarbon calibration curve, and linking atmospheric radiocarbon variations with climate changes. 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 from a previously studied speleothem from Flores, Indonesia, to examine DCF variations and its relationship with above-cave climate over the late-Holocene to modern interval. A strong association between the DCF and other hydrologically controlled proxy data clearly shows that more dead carbon is being delivered to the speleothem during periods of higher cave recharge (i.e. lower ?18O, ?13C and Mg/Ca values) and hence a stronger summer monsoon. One possible explanation is a higher contribution from the bedrock under such conditions. Although one might expect a concurrent increase in stable carbon isotope values as DCF increases (not observed here), it is possible that such an increase in ?13C may be more than offset by the effect of increased recharge on the rate of carbon dioxide degassing. But, a higher proportion of bedrock carbon is not the only possible explanation: when the monsoon is stronger, a greater proportion of less mobile ‘older carbon’ may be leached from the soil thus diluting the ‘younger carbon’ fraction. This would produce an ‘apparent’ increase in DCF. Copyright (c) 2011 INQUA 18
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    Post-glacial coupling of the Australasian monsoon and teleconnections to the North Atlantic: new insights from Indonesian speleothems
    (GNS Science, 2009-05-15) Griffiths, ML; Drysdale, RN; Gagan, MK; Zhao, JK; Ayliffe, LK; Hellstrom, JC; Hantoro, WS; Frisia, S; Feng, YX; Cartwright, I; St Pierre, E; Fisher, M; Suwargadi, B
    The Australasian monsoon system orchestrates rainfall variability and terrestrial productivity in the densely populated region of the tropical Indo-Pacific. A clear understanding of the dominant mechanisms governing its variability has been difficult to resolve, partly because we currently lack high-resolution proxy records of past monsoon behaviour, particularly for the southern tropics. Here we provide a radiometrically dated reconstruction of Australian-Indonesian summer monsoon (AISM) rainfall based on oxygen isotopes and trace element data in stalagmites from southern Indonesia. The multi-proxy records are tied to age-depth models constructed from 62 TIMS and MC-ICP-MS U-series ages, covering the period 0 to 12.6 ka B.P. The record shows that the AISM was anti-phased with the East Asian summer monsoon (EASM) on orbital to millennial-centennial timescales over the past 12.6 ka. At the orbital-scale, local summer insolation was an important driver of opposing changes in AISM and EASM rainfall. However, a slight mismatch between the AISM and insolation from 9 to 11 ka B.P. is concurrent with the sharp rise in eustatic sealevel, which apparently increased the supply of northwesterly summer monsoon moisture to the Indonesian maritime continents. At millennial-centennial timescales, the oxygen isotope and trace element records show that periods of weakened North Atlantic meridional overturning circulation and cooling, including the Younger Dryas cold stage, are in phase with sharp increases in AISM rainfall. The connection between the AISM and a cooler North Atlantic is probably due to enhanced outflow from the Asian winter monsoon and associated southward migration of the intertropical convergence zone. These interhemispheric connections were dominant until ~6.5 ka, when the El Niño-Southern Oscillation became the governing influence on AISM variability.