Browsing by Author "Hantoro, WS"

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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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    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
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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.
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    Rapid measurement of strontium in speleothems using core-scanning micro x-ray fluorescence
    (Elsevier, 2018-05-25) Scroxton, N; Burns, S; Dawson, P; Rhodes, JM; Brent, K; McGee, D; Heijnis, H; Gadd, PS; Hantoro, WS; Gagan, MK
    Speleothem trace element ratios such as Mg/Ca and Sr/Ca are increasingly used in speleothem paleoclimatology as a supplement to stable oxygen and carbon isotope ratios as proxies for past variability in the hydrologic system. Using multiple proxies together allows for a better understanding of both the local and distal hydrologic changes recorded in speleothem chemistry, and therefore of changes in past rainfall. Despite the potential benefits, trace element analysis of speleothems has yet to become widespread, which is likely due to the significant time and costs required by traditional trace element analytical techniques. In this study, we present an in-depth investigation into a rapid, relatively non-destructive and competitively priced technique for measuring Sr/Ca in speleothems: Core-Scanning micro X-ray Fluorescence (CS-μXRF). We show that CS-μXRF reliably and precisely records Sr concentration in speleothems. Ratioed to near-stoichiometric Ca, the Sr/Ca ratio accounts for variations in beam strength and machine settings, producing a more reliable reported measurement for both intra- and inter-run comparisons. CS-μXRF compares favorably with more conventional trace element procedures such as Quadrupole ICP-MS and ICP-AES, giving confidence in the ability of CS-μXRF to produce paleoclimatologically significant Sr/Ca results. We also identify secondary issues relating to speleothem crystallinity, the dominance of Ca spectral peaks, and comparatively lower energy X-rays that can interfere with precise CS-μXRF analyses. If these can be overcome then CS-μXRF may provide an even more useful method of trace element analysis in speleothem studies. © 2018 Elsevier B.V.
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    What drives vegetation changes in South Sulawesi during the MIS 5e transition?
    (Copernicus Publications, 2024-04-17) Kimbrough, A; Gagan, MK; Dunbar, GB; Treble, PC; Hantoro, WS; Zhao, JX; Edwards, RL; Shen, CC; Suwargadi, B; Wong, HKY; Rifai, H
    Sulawesi speleothem carbon isotopes (δ13C) are found to co-vary with deglacial warming and atmospheric CO2 measured from Antarctic ice cores. This co-variation has thus far been attributed to speleothem δ13C recording changes in vegetation productivity and microbial activity in the soils overlaying caves as vegetation and microbes respond to glacial-interglacial changes in temperature and atmospheric CO2 (Kimbrough et al., 2023; Krause & Kimbrough et al., in press). However, the relationship between speleothem δ13C and regional environmental change is complex and deconvolving the effect of different environmental drivers is difficult. To further investigate the ecosystem response in the Indo-Pacific Warm Pool to substantial warming and CO2 rise during the penultimate deglaciation/marine isotope stage 5e (~127 kyrs ago) we use complimentary geochemical proxies extracted from stalagmite CaCO3. These proxies include phosphorus and sulphur which respond to nutrient uptake by forest biomass above the cave (Treble et al., 2016). The relative abundance of metals such as copper, iron, zinc, and lead are assessed as another means to track biomass/soil regeneration via selective element delivery to the stalagmites by organic colloids flushed from the soil zone (Borsato et al., 2007). These vegetation proxies are compared with the speleothem δ13C and δ18O records and corresponding high-resolution fluorescence mapping of organics via confocal laser scanning (fluorescence) microscopy (Sliwinski & Stoll, 2021). The comparison of transition metals to stable isotopes (δ18O, δ13C) in the Sulawesi speleothem records makes it possible to distinguish between periods in the record where vegetation productivity increased in response to a rise in temperature and CO2 verses periods where changing hydroclimate played a more dominant role. Characterising the appropriate drivers and proxy response is critical to accurately interpret tropical paleoclimate records where interpretations rely on assumptions that rainfall is the primary driver of vegetation change. © Author(s) 2024. This work is distributed under the Creative Commons Attribution 4.0 License.