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Browsing by Author "Griffiths, ML"

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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.
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.
Human impact on the environment in Western Flores, Indonesia during the late Holocene: identifying agricultural transitions
(18th INQUA Congress, 2011-07-21) St Pierre, E; Zhao, JX; Aplin, K; Drysdale, RN; Golding, SD; Griffiths, ML; Hua, Q
Limestone caves can act as excellent repositories of palaeoenvironmental information and past human activities. This paper presents a multi-proxy record of late Holocene palaeoclimate and palaeoenvironmental change derived from environmental archives in Liang Luar (Steam Cave), western Flores, Indonesia. Liang Luar, located ~1 km from Liang Bua (the discovery site of the hominid species Homo floresiensis), is a ~1.6km long passage with several large chambers and numerous speleothems (stalagmites and stalactites). A palaeoclimate record was compiled using stable carbon and oxygen isotope ratios from a stalagmite precisely dated to 0-1800 yr BP using U/Th dating. The stalagmite isotope record preserves an episode of rapid vegetation change c. 800 yr BP marked by a large shift in carbon and decoupling of the carbon from the oxygen isotopes, and thus thought to be unrelated to climate. Excavated owl pellet deposits in the entrance of Liang Luar dated by 14C AMS on charcoal, reveal continuous sediment deposition from at least 2400 yr BP to the present. The 14C chronology demonstrates a sudden increase in depositional rates at the cave entrance as well as an increase in the abundance of charcoal fragments, at the time of the vegetation change inferred from the stalagmite record. Faunal remains indicate the commensal species Rattus exulans, arrives early in the sequence, while Rattus rattus appears much later. A surprisingly late appearance of two rodents associated with irrigated rice fields, together with a surge in frog remains, indicates wet rice farming was only recently introduced to the area. These paleoenvironmental records act as a basis from which to understand the timing and intensity of human impacts on late Holocene environments in western Flores, and the relationship of this incursion to changing land use patterns. Copyright (c) 2011 INQUA 18
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.
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
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.
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
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
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.
Rainfall variability and temporal changes in the dead carbon fraction in an Indonesian speleothem
(Australasian Quaternary Association Inc., 2016-01-01) Hua, Q; Griffiths, ML; Drysdale, RN; Bajo, P; Jenkins, D; Hellstrom, JC; Johnson, KR; Gagan, MK; Zhao, JX
The number of speleothem-based paleoclimate records has increased significantly in recent years. To assess the potential hydrological control on speleothem radiocarbon variability, we constructed a high-resolution dead carbon fraction (DCF) record from a speleothem from Flores, Indonesia for two different periods, the Younger Dryas (YD) chronozone and the Last Millennium. A total of thirty-four 14C analyses were conducted on calcite extracted from U-Th dated stalagmite LR06-B1. To better characterise the paleoclimate and environmental changes, highresolution stable-isotope (δ18O, δ13C) and trace-element (Mg/Ca, Sr/Ca) measurements were also conducted along the same stalagmite sections. Broad comparison of the DCF record with the hydrologically-controlled proxy data suggests that rainfall increases were matched by DCF increases. In line with a previous interpretation of DCF variability for the same specimen, but during the interval 2.4-2.8 ka 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. It then follows that more abundant monsoon rainfall in Flores resulted in the soil-karst system being 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. Our results indicate that DCF in tropical speleothems can be used as a proxy of past rainfall and consequently monsoon variability.

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