ANSTO :: Browsing by Author "Gagan, MK"

Browsing by Author "Gagan, MK"

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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.
Building a future on knowledge from the past: what paleo-science can reveal about climate change and its potential impacts in Australia
(Commonwealth Scientific and Industrial Research Organisation, 2005-06) Harle, KJ; Etheridge, DM; Whetton, P; Jones, R; Hennessy, K; Goodwin, ID; Brooke, BP; van Ommen, TD; Barbetti, M; Barrows, TT; Chappell, J; De Deckker, P; Fink, D; Gagan, MK; Haberle, SG; Heijnis, H; Henderson-Sellers, A; Hesse, PP; Hope, GS; Kershaw, P; Nicholls, N
In Australia, high quality instrumental climate records only extend back to the late 19th century and therefore only provide us with a brief snapshot of our climate, its mean state and its short-term variability. Palaeo-records extend our knowledge of climate back beyond the instrumental record, providing us with the means of testing and improving our understanding of the nature and impacts of climate change and variability in Australia. There is a vast body of palaeo-records available for the Australian region (including Antarctica), ranging from continuous records of sub-decadal up to millennial scale (such as those derived from tree rings, speleothems, corals, ice cores, and lake and marine sediments) through to discontinuous records representing key periods in time (such as coastal deposits, palaeo-channels, glacial deposits and dunes). These records provide a large array of evidence of past atmospheric, terrestrial and marine environments and their varying interactions through time. There are a number of key ways in which this evidence can, in turn, be used to constrain uncertainties about climate change and its potential impacts in Australia.
Century-to-decade scale modulation of ENSO recorded by postglacial laminated sediments from the Peru continental margin
(International Atomic Energy Agency, 2004-10-25) Skillbeck, G; Gagan, MK; Goodwin, I; Watson, M; Fink, D
Cores collected from three sites on the continental margin of Peru during ODP Leg 201 recovered >5 m of LGM-recent sediment. At Site 1227 Holocene sediments are absent, but a well preserved early last glacial-interglacial transition (LGIT) section spanning ~17,200-15,900 cal yrBP is present. The sediments are predominantly diatomaceous oozes with subtle dark and light laminations which may be annual in origin. The chronology of drill-core at this site is well-constrained by five bulk sediment 14C dates that define a linear sedimentation rate of ~270 cm/ka [1]. In contrast, Holocene sediments are well-represented at Sites 1228 and 1229. Sedimentation rates over this period suggest the Holocene can be subdivided into two regimes. The older period spans the early and middle Holocene (~10,000 yrBP to ~2,800 yrBP) during which time the sedimentation rate was relatively slow at 4-6 cm/ka. However, we cannot exclude the possibility of unconformities in this part of the stratigraphic section, and this rate should therefore be considered a minimum. From ~2,800 yrBP to the present day, the chronology at both sites is well defined by multiple 14C ages that allow us to confidently define linear sedimentation rates of 70-100 cm/ka [1]. At both sites, the late Holocene appears to be stratigraphically complete. In order to investigate an El Nino origin for the laminae on this part of the Peru shelf, we have undertaken two independent lines of study. First, high-resolution (0.1 mm per pixel) scanned colour images were analysed for all of the cores. For the early LGIT and the late Holocene, the chronological model indicates that sub-annual layers can be resolved, where present. Accordingly, we have used the red colour intensity band from the scanned images to carry out time series analysis of ENSO-band (2-8 year) variability. Analysis of Hole 1228B shows two cyclicity peaks in the ENSO band over the past 10 ka. One of these, at a peak period of 5.3 yr, dominates over the last 3 ka, with the overall Holocene pattern very similar to that shown by Rodbell et al. [2], Moy et al. [3], and Riedinger et al. [4]. In contrast, spectral analysis of ENSO band data from Hole 1227B, shows a strong decadal variance peaking at 16.5 and 22.5 yr and only weak variance in the ENSO band (4.7 yr). In addition, there is a well-developed oscillation in the variance of these data with cyclicity of about 110 yr which we believe is similar to that proposed by [5]. We have sought to test that the laminations do indeed contain a climate signal by undertaking several geochemical and mineralogical studies. The sediments are dominantly diatomaceous oozes, but contain other minor components, including terrigenous sediment and organic matter. We analysed d13C of bulk organic matter, d18O of planktonic foraminifer tests, organic carbon content, carbonate content, and terrigenous sediment (quartz/feldspar) percentages of representative dark and light laminae in upper part of Hole 1227B. Our hypothesis is that El Nino events will produce warmer and wetter conditions (lower foraminiferal d18O) and increased terrestrial sediment input (increased quartz and feldspar and lower d13C of organic matter). At the same time, the bulk organic carbon and carbonate data will reflect a decline in water-column productivity driven by the suppression of upwelled nutrients during El Niño events. Taken together, our preliminary analyses indicate that the dark laminae represent periods of warmer sea surface temperatures and increased terrestrial runoff associated with El Nino events. Independent time series and geochemical analysis of LGM-Holocene sediments from the Peru continental margin indicate the presence of an El Nino/La Nina climate signal preserved in alternating dark and light laminae. Time-series analysis of long (>1000 yr), stratigraphically continuous sequences from the late Holocene and the early LGIT reveal decadal (~15-20 yr) and century (~110 yr) scale modulation of ENSO climate variability. We would like to thank Dennis Mather and the staff at The Australian Institute of Nuclear Science and Engineering (AINSE) for grants supporting radiocarbon dating in this project (Nos 02/169 & 04/139), and the Australian ODP Office for their assistance in providing funds for collection and analysis of material.
Century-to-decade scale modulation of ENSO recorded by postglacial laminated sediments from the Peru continental margin
(International Atomic Energy Agency, 2006) Skillbeck, G; Gagan, MK; Goodwin, I; Watson, M; Fink, D
Cores collected from three sites on the continental margin of Peru during ODP Leg 201 recovered >5 m of LGM-recent sediment. At Site 1227 Holocene sediments are absent, but a well preserved early last glacial-interglacial transition (LGIT) section spanning ~17,200- 15,900 cal yrBP is present. The sediments are predominantly diatomaceous oozes with subtle dark and light laminations which may be annual in origin. The chronology of drill-core at this site is well-constrained by five bulk sediment 14C dates that define a linear sedimentation rate of ~270 cm/ka [1]. In contrast, Holocene sediments are well-represented at Sites 1228 and 1229. Sedimentation rates over this period suggest the Holocene can be subdivided into two regimes. The older period spans the early and middle Holocene (~10,000 yrBP to ~2,800 yrBP) during which time the sedimentation rate was relatively slow at 4-6 cm/ka. However, we cannot exclude the possibility of unconformities in this part of the stratigraphic section, and this rate should therefore be considered a minimum. From ~2,800 yrBP to the present day, the chronology at both sites is well defined by multiple 14C ages that allow us to confidently define linear sedimentation rates of 70-100 cm/ka [1]. At both sites, the late Holocene appears to be stratigraphically complete. In order to investigate an El Nino origin for the laminae on this part of the Peru shelf, we have undertaken two independent lines of study. First, high-resolution (0.1 mm per pixel) scanned colour images were analysed for all of the cores. For the early LGIT and the late Holocene, the chronological model indicates that sub-annual layers can be resolved, where present. Accordingly, we have used the red colour intensity band from the scanned images to carry out time series analysis of ENSO-band (2-8 year) variability. Analysis of Hole 1228B shows two cyclicity peaks in the ENSO band over the past 10 ka. One of these, at a peak period of 5.3 yr, dominates over the last 3 ka, with the overall Holocene pattern very similar to that shown by Rodbell et al. [2], Moy et al. [3], and Riedinger et al. [4]. In contrast, spectral analysis of ENSO band data from Hole 1227B, shows a strong decadal variance peaking at 16.5 and 22.5 yr and only weak variance in the ENSO band (4.7 yr). In addition, there is a well-developed oscillation in the variance of these data with cyclicity of about 110 yr which we believe is similar to that proposed by [5]. We have sought to test that the laminations do indeed contain a climate signal by undertaking several geochemical and mineralogical studies. The sediments are dominantly diatomaceous oozes, but contain other minor components, including terrigenous sediment and organic matter. We analysed d13C of bulk organic matter, d18O of planktonic foraminifer tests, organic carbon content, carbonate content, and terrigenous sediment (quartz/feldspar) percentages of representative dark and light laminae in upper part of Hole 1227B. Our hypothesis is that El Nino events will produce warmer and wetter conditions (lower foraminiferal d18O) and increased terrestrial sediment input (increased quartz and feldspar and lower d13C of organic matter). At the same time, the bulk organic carbon and carbonate data will reflect a decline in water-column productivity driven by the suppression of upwelled nutrients during El Niño events. Taken together, our preliminary analyses indicate that the dark laminae represent periods of warmer sea surface temperatures and increased terrestrial runoff associated with El Nino events. Independent time series and geochemical analysis of LGM-Holocene sediments from the Peru continental margin indicate the presence of an El Nino/La Nina climate signal preserved in alternating dark and light laminae. Time-series analysis of long (>1000 yr), stratigraphically continuous sequences from the late Holocene and the early LGIT reveal decadal (~15-20 yr) and century (~110 yr) scale modulation of ENSO climate variability. We would like to thank Dennis Mather and the staff at The Australian Institute of Nuclear Science and Engineering (AINSE) for grants supporting radiocarbon dating in this project (Nos 02/169 & 04/139), and the Australian ODP Office for their assistance in providing funds for collection and analysis of material.
Coral microatoll reconstructions of El Niño-southern oscillation: new windows on seasonal and interannual processes
(PAGES Past Global Changes, 2013-08-01) McGregor, HV; Woodroffe, CD; Fischer, MJ; Gagan, MK; Fink, D
Porites corals are the most commonly used genus for reconstructing El Niño- Southern Oscillation (ENSO). This hermatypic coral is found in all tropical reef environments(Veron 2000) with a variety of growth forms. Climate reconstructions of a century or more have been obtained from the most common, dome-shaped Porites growth form, whereby the colonies, beginning from the substrate, grow outward and upward towards the ocean surface(Knutson et al. 1972). Domed structures, however, are not the only Porites growth form. © 2013, Authors.
El Niño-Southern Oscillation (ENSO) from 0-2 ka
(University of Melbourne, 2010-05-31) McGregor, HV; Phipps, SJ; Woodroffe, CD; Gagan, MK; Fink, D
Environmental control of the oxygen isotope composition of Porities coral microatolls
(Elsevier, 2011-07-15) McGregor, HV; Fischer, MJ; Gagan, MK; Fink, D; Woodroffe, CD
Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the delta(18)O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for delta(18)O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean delta(18)O values of transects that represent the same time periods is 0.03 parts per thousand, and is within measurement error for a single analysis (0.04 parts per thousand); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07 parts per thousand and; (3) comparison of the average delta(18)O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04 parts per thousand and 0.11 parts per thousand for different time periods. These differences in mean delta(18)O are within the range for intra-and inter-colony differences in seasonal and interannual delta(18)O reported for dome-shaped Porites. Based on these results, a stacked microatoll delta(18)O record was constructed for the period 1978-2007 for comparison with published coral delta(18)O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in delta(18)O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of delta(18)O, and there is a strong correlation (r = -0.71) between microatoll delta(18)O and local sea surface temperature (SST). The delta(18)O-SST slope relationship for microatolls is -0.15 parts per thousand/degrees C, very similar to that reported for fast-growing domed corals (-0.18 parts per thousand to -0.22 parts per thousand/degrees C). Statistical analysis of the stacked microatoll delta(18)O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal delta(18)O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain delta(18)O signals that can significantly extend instrumental records of tropical climate variability. Crown copyright (C) 2011 Published by Elsevier Ltd.
Equatorial Pacific coral geochemical records show recent weakening of the Walker Circulation
(American Geophysical Union, 2014-11-10) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, D
Equatorial Pacific ocean-atmosphere interactions affect climate globally, and a key component of the coupled system is the Walker Circulation, which is driven by sea surface temperature (SST) gradients across the equatorial Pacific. There is conflicting evidence as to whether the SST gradient and Walker Circulation have strengthened or weakened over the late twentieth century. We present new records of SST and sea surface salinity (SSS) spanning 1959–2010 based on paired measurements of Sr/Ca and δ18O in a massive Porites coral from Butaritari atoll in the Gilbert Islands, Republic of Kiribati, in the central western equatorial Pacific. The records show 2–7 year variability correlated with the El Niño–Southern Oscillation (ENSO) and corresponding shifts in the extent of the Indo-Pacific Warm Pool, and decadal-scale signals related to the Pacific Decadal Oscillation and the Pacific Warm Pool Index. In addition, the Butaritari coral records reveal a small but significant increase in SST (0.39°C) from 1959 to 2010 with no accompanying change in SSS, a trend that persists even when ENSO variability is removed. In contrast, larger increases in SST and SSS are evident in coral records from the equatorial Pacific Line Islands, located east of Butaritari. Taken together, the equatorial Pacific coral records suggest an overall reduction in the east-west SST and SSS gradient over the last several decades, and a recent weakening of the Walker Circulation. © 2014, American Geophysical Union. All Rights Reserved.
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.
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.
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
Integration of ice-core, marine and terrestrial records for the Australian Last Glacial Maximum and Termination: a contribution from the OZ INTIMATE group
(Wiley, 2006-10) Turney, CSM; Haberle, SG; Fink, D; Kershaw, AP; Barbetti, M; Barrows, TT; Black, M; Cohen, TJ; Corrège, T; Hesse, PP; Hua, Q; Johnston, R; Morgan, VI; Moss, PT; Nanson, GC; van Ommen, TD; Rule, S; Williams, NJ; Zhao, JX; D'Costa, D; Feng, YX; Gagan, MK; Mooney, SD; Xia, Q
The degree to which Southern Hemisphere climatic changes during the end of the last glacial period and early Holocene (30-8 ka) were influenced or initiated by events occurring in the high latitudes of the Northern Hemisphere is a complex issue. There is conflicting evidence for the degree of hemispheric ‘teleconnection’ and an unresolved debate as to the principle forcing mechanism(s). The available hypotheses are difficult to test robustly, however, because the few detailed palaeoclimatic records in the Southern Hemisphere are widely dispersed and lack duplication. Here we present climatic and environmental reconstructions from across Australia, a key region of the Southern Hemisphere because of the range of environments it covers and the potentially important role regional atmospheric and oceanic controls play in global climate change. We identify a general scheme of events for the end of the last glacial period and early Holocene but a detailed reconstruction proved problematic. Significant progress in climate quantification and geochronological control is now urgently required to robustly investigate change through this period. © 2006 John Wiley & Sons, Ltd.
Interpreting past climate using southwest Australian speleothems
(Australian Meterological & Oceanographic Society, 2012-01-31) Treble, PC; Azcurra, CS; Baker, AA; Bradley, C; Wood, A; Fischer, MJ; Fairchild, IJ; Hellstrom, JC; Gagan, MK
There is an identified need to extend our baseline climate information beyond the relatively short duration of instrumental records in Australia. An improved knowledge of natural rainfall variability would assist in our understanding of climate change. SW Australia (SWWA) is one region that has been identified as having a changing climate since the 1970s. Speleothems (cave stalagmites) are an effective archive of past climate variability and caves from the coastal region of SWWA are being studied for paleoclimate records. The modern speleothem record from this region has been assessed and shown to record the post-1970s rainfall decrease (Treble et al., 2003; 2005; Fischer and Treble, 2008). The extension of the speleothem record is currently underway, however, a long-term cave monitoring program was also deemed necessary to separate the climatic from non-climatic signals i.e. to reduce uncertainty when interpreting these records. This presentation outlines what we have learnt about the possible hydrological modification of the climate signal in speleothems. In particular, we present results from a five-year long monitoring study of rainfall and cave drip water O isotopes (!18O) from Golgotha Cave, SWWA. From this study, we have been able to characterize the probable flow paths feeding stalagmites in our monitored cave. These flow paths range from slow diffuse flow of isotopically-averaged rainfall to preferential routing of high-magnitude, 18O-depleted, events along fast flow routes into the cave. Hence, we offer a possible explanation for why paleoclimate records from coeval speleothems in our cave may differ. Our study suggests that this disagreement may simply be due to different flow paths resulting in a bias towards the preservation of high or low magnitude rainfall events.
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
An isotopic and modelling study of flow paths and storage in quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records
(Elsevier, 2013-03-15) Treble, PC; Bradley, C; Wood, A; Baker, AA; Jex, CN; Fairchild, IJ; Gagan, MK; Cowley, J; Azcurra, CS
We investigated the distinctive shallow sub-surface hydrology of the southwest Western Australia (SWWA) dune calcarenite using observed rainfall and rainfall δ18O; soil moisture, cave drip rate and dripwater δ18O over a six-year period: August 2005–March 2012. A lumped parameter hydrological model is developed to describe water fluxes and drip δ18O. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation δ18O signal and discuss the implications for speleothem paleoclimate records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct δ18O values and recharge responses (‘low’ and ‘high’ flow sites). Dripwaters exhibit δ18O variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored in a secondary reservoir which is associated with high-flow dripwater that is 1‰ lower than our monitored low-flow sites (δ18O). One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). Additionally, our results yield a better understanding of the vadose zone hydrology and dripwater characteristics in Quaternary age dune limestones. We show that flow to our monitored sites is dominated by diffuse flow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock. © 2013, Elsevier Ltd.
Mid-holocene variability in the marine C-14 reservoir age for northern coastal Papua New Guinea
(Elsevier, 2008-08) McGregor, HV; Gagan, MK; McCulloch, MT; Hodge, E; Mortimer, GE
Changes in oceanic radiocarbon (C-14) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Nino events. Reservoir ages were calculated using paired U-Th and conventional C-14 dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS C-14 and MC-ICPMS U-Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U-Th and conventional C-14 ages. The combined results show average reservoir ages of 185 +/- 30 C-14 yr (n = 4) for 7220-5850 yr BP compared to 420 C-14 yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air-sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Nino activity around this time would have served to enhance the intrusion of C-14-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Nino-Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region. © 2008, Elsevier Ltd.
Millennial to seasonal scale views of El Niño-Southern Oscillation from central Pacific corals
(Australasian Quaternary Association, 2022-12-06) McGregor, HV; Wilcox, P; Fischer, MJ; Phipps, SJ; Gagan, MK; Wittenberg, A; Felis, T; Kölling, M; Wong, HKY; Devriendt, L; Woodroffe, CD; Zhao, JX; Fink, D; Gaudry, JJ; Chivas, AR
El Niño-Southern Oscillation (ENSO) is naturally highly variable on interannual to decadal scales making it difficult to detect a possible response to climate forcing. Despite the high variability, several lines of evidence from tropical corals, mollusc, lake sediments, and foraminifera suggest that 5,000-3,000 years ago ENSO variance was on average reduced by 60-80% compared to the present day. We investigate the seasonal-to-centennial variation in ENSO amplitude and tropical climate during this ENSO ‘quiet period’ 5,000-3,000 years ago using a new Sr/Ca SST record from a 175-year-long 4,300-year-old coral, and new d18O and Sr/Ca results from a similar-aged ~180-year-long Porites sp. coral. Both corals were discovered on Kiritimati (Christmas) Island, an optimal ENSO ‘centre of action’ in the central tropical Pacific. Together, these corals confirm a reduction in ENSO amplitude and that ENSO amplitude is modulated on multi-decadal scales. Composites of month-by-month changes in Sr/Ca-SST show an unprecedented view of ENSO and detail which seasonal-scale features of ENSO are an inherent part of the system, and which are subject to change under altered climate states. We also investigate the millennial timescale changes in ENSO variance using combine coral oxygen isotope (18O) data from central Pacific corals and a suite of forced and unforced simulations conducted using the CSIRO Mk3L and GFDL CM2.1 climate system models. On millennial timescales, the coral data reveal a statistically significant increase in ENSO variance over the past 6,000 years. This trend is not reproduced by the unforced model simulations but can be reproduced once orbital forcing is accounted for. Together these views of past ENSO may contribute to advances in understanding the response of ENSO to future changes in climate forcings.

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