Browsing by Author "Bajo, P"
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- ItemEnvironmental change in the Adriatic region over the last 365 kyr from episodic deposition of Modrič Cave (Croatia) speleothems(International Union for Quaternary Research (INQUA), 2019-07-25) Surić, M; Lončarić, R; Columbu, A; Bajo, P; Lončar, N; Drysdale, RN; Hellstrom, JCThe Adriatic Sea, as the northernmost and practically landlocked part of the Mediterranean Sea, is influenced by both Atlantic and Mediterranean air masses, so the isotopic records from coastal cave speleothems are expected to reveal local and regional responses to global climate changes on that bordering region. In this study we present a 365-kyr long composite isotopic record of three speleothems from Modrič Cave located 120 m from the shoreline on the central part of the eastern Adriatic coast. Results of a 4-year monitoring program of the surface and cave microclimate, rain and drip water stable isotope composition and rain and drip intensities, show cave air temperature variations within 1 °C, a relative humidity of 100%, dripwater O and H stable isotopic composition (range 0.4‰) very well buffered against the seasonal rainfall range (6.8‰). Such conditions enable calcite deposition without strong kinetic isotopic disequilibrium effects, as validated by Hendy tests, and give us confidence that the isotopic signal is faithfully recording climate changes. All three speleothems (MOD-31, MOD-32, MOD-33) were active, collected from their growth position, so Holocene signals can be traced in all of them. Within their older parts, U-Th dating reveals the following growth periods: MOD-31 from MIS 10 to MIS 7, MOD-32 from MIS 4 to MIS 2, and MOD-33 from MIS 6 to MIS 5. Relatively large δ18O and δ13C ranges (-8.16‰ – -2.97‰ and -13.11‰ – -1.00‰, respectively) indicate significant environmental changes whose comparison with stable isotope time series of adjacent speleothem records and other regional proxies provides the longest eastern Adriatic speleothem-based record. © 2019 The Authors.
- ItemHydrological 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, JXThe 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
- ItemPast hydroclimatic variability from southwest Australian speleothems during the last millennium(Australiasian Quaternary Association Inc., 2018-12-10) Treble, PC; Baker, AA; Griffiths, AD; Hellstrom, JC; Bajo, P; Abram, NJ; Fairchild, IJ; Borsato, A; Markowska, M; Gagan, MKSpeleothems from Golgotha Cave in SW Western Australia have been investigated to extend our knowledge of past climate variability for this region during the last millennium. O isotopic datasets, the primary paleoclimate proxy used for speleothems, were constructed for four stalagmites. A challenge in their interpretation has been the disagreement between these records, despite representing coeval growth from within the same cave. Resolving this conundrum has necessitated the characterisation of the hydrology, hydrochemistry, rainfall isotopes7 and development of proxy system forward models1,8 for Golgotha Cave. The findings of these studies will be summarised as a conceptual model in order to present the main karst hydrological features that give rise to each stalagmite’s isotopic response to hydroclimatic forcing. The paleoclimate interpretation will focus on the two continuous stalagmite records that were fed predominately by diffuse flow. This will be supported by evidence from the two stalagmites predominantly fed by fracture flow, which has resulted in a non-linear response to hydroclimatic forcing.
- ItemRainfall 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, JXThe 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.
- ItemStalagmite carbon isotopes and dead carbon proportion (DCP) in a near-closed-system situation: An interplay between sulphuric and carbonic acid dissolution(Elsevier, 2017-08-01) Bajo, P; Borsato, A; Drysdale, RN; Hua, Q; Frisia, S; Zanchetta, G; Hellstrom, JC; Woodhead, JDIn this study, the ‘dead carbon proportion’ (DCP) calculated from combined U-Th and radiocarbon analyses was used to explore the carbon isotope systematics in Corchia Cave (Italy) speleothems, using the example of stalagmite CC26 which grew during the last ∼12 ka. The DCP values in CC26 are among the highest ever recorded in a stalagmite, spanning the range 44.8–68.8%. A combination of almost closed-system conditions and sulphuric acid dissolution (SAD) are proposed as major drivers in producing such a high DCP with minor contribution from old organic matter from the deep vadose zone. The long-term decrease in both DCP and δ13C most likely reflects post-glacial soil recovery above the cave, with a progressive increase of soil CO2 contribution to the total dissolved inorganic carbon (DIC). Pronounced millennial-scale shifts in DCP and relatively small coeval but antipathetic changes in δ13C are modulated by the effects of hydrological variability on open and closed-system dissolution, SAD and prior calcite precipitation. Hence, the DCP in Corchia Cave speleothems represents an additional proxy for rainfall amount. © 2017 Elsevier Ltd.
- ItemUbiquitous karst hydrological control on speleothem oxygen isotope variability in a global study(Springer Nature, 2022-02-15) Treble, PC; Baker, AA; Abram, NJ; Hellstrom, JC; Crawford, J; Gagan, MK; Borsato, A; Griffiths, AD; Bajo, P; Markowska, M; Priestley, SC; Hankin, SI; Paterson, DJSpeleothem oxygen isotopic (δ18O) records are used to reconstruct past hydroclimate yet records from the same cave do not always replicate. We use a global database of speleothem δ18O to quantify the replicability of records to show that disagreement is common worldwide, occurs across timescales and is unrelated to climate, depth or lithology. Our global analysis demonstrates that within-cave differences in mean speleothem δ18O values are consistent with those of dripwater, supporting a ubiquitous influence of flowpaths. We present a case study of four new stalagmite records from Golgotha Cave, southwest Australia, where the isotopic differences between them are informed by cave monitoring. It is demonstrated that karst hydrology is a major driver of within-cave speleothem and dripwater δ18O variability, primarily due to the influence of fractures on flowpaths. Applying our understanding of water movement through fractures assists in quantitative reconstruction of past climate variability from speleothem δ18O records. © The Authors - Open Access under a Creative Commons Attribution 4.0 International License.
- ItemUbiquitous karst hydrological control on speleothem oxygen isotope variability in a global study(Springer Nature, 2022-02-15) Treble, PC; Baker, AA; Abram, NJ; Hellstrom, JC; Crawford, J; Gagan, MK; Borsato, A; Griffiths, AD; Bajo, P; Markowska, M; Priestley, SC; Hankin, SI; Paterson, DJSpeleothem oxygen isotopic (δ18O) records are used to reconstruct past hydroclimate yet records from the same cave do not always replicate. We use a global database of speleothem δ18O to quantify the replicability of records to show that disagreement is common worldwide, occurs across timescales and is unrelated to climate, depth or lithology. Our global analysis demonstrates that within-cave differences in mean speleothem δ18O values are consistent with those of dripwater, supporting a ubiquitous influence of flowpaths. We present a case study of four new stalagmite records from Golgotha Cave, southwest Australia, where the isotopic differences between them are informed by cave monitoring. It is demonstrated that karst hydrology is a major driver of within-cave speleothem and dripwater δ18O variability, primarily due to the influence of fractures on flowpaths. Applying our understanding of water movement through fractures assists in quantitative reconstruction of past climate variability from speleothem δ18O records. © Crown 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.