Browsing by Author "Woodhead, JD"
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- ItemCemented in time: formation of the 20 000 year old Willandra fossil trackway(Geological Society of Australia, 2014-07-07) Graham, IT; Whiteside, E; Ward, C; Cendón, DI; Westaway, M; Cupper, ML; Woodhead, JDThe Willandra Lakes system of southwestern NSW is situated within the Murray Basin, and consists of 19 interconnected relict lake basins. In general, the lake sediments within the Willandra Lakes system consist of wellsorted quartz sands (with typical lacustrine shelly fauna), associated with deeper water clays and sandy clays. In 2003, the region was found to host the largest known in-situ tract of Pleistocene human footprints in the world, with the site located on the shoreline of a small, relict lake basin between Lakes Garnpung and Leaghur. Optically Stimulated Luminescence dating revealed that the sediments were deposited between 19 000 and 20 000 years ago. The area is of great cultural value to the local Aboriginal communities (the traditional tribal groups Paakantyi, Mutthi Mutthi and Ngiyampaa), as well as to both the national and international scientific communities. Detailed mineralogical (quantitative XRD), geochemical (XRF, ICP-MS, stable isotopes) and textural (petrography, SEM) analyses of the sediments were undertaken to help determine their origin and provide a basis for their future conservation. The footprints themselves are impressed into a hardpan unit, surrounded by low sand dunes. Approximately 820 m2 of the hardpan has been excavated and explored. The footprint-bearing sediments are composed of a series of thin laminae totalling 150 mm thick, accumulated over repeated cycles of wetting and drying. These sediments are largely composed of pelloids and intraclasts of authigenic clay-sized particles (<2–8 μm) of ferroan magnesite (or hydromagnesite/palygorskite), eolian-derived fractured quartz grains and minor (<10 wt%) kaolinite/illite. There is a large lateral and vertical variation in the modal mineralogy; the NE corner contains 90.5 wt% ferroan magnesite (and minor smithsonite) while the SW and W parts contain 49 wt% ferroan magnesite. The other sediments are largely composed of eolian quartz (up to 85 wt%), kaolinite, illite, rutile, albite, microcline, hematite, goethite and rare dolomite. In terms of stable isotopes, the ferroan magnesite carbonate has a δ13C of –2.5‰ while the hydromagnesite has δ13C of 0.4‰. The magnesite, hydromagnesite and palygorskite appear to have been derived through precipitation within the lake. Although direct precipitation of these phases is rare under surficial conditions, it can occur if there is a high Mg/Ca, only possible if extensive early precipitation of calcite cements occurred before the waters entered into the lake. As the lake dried-out, there was an increase in salinity resulting in a decrease in the activity of water in solution, thereby increasing the hydrated Mg2+ leading to magnesite precipitation.
- ItemMagnesium in subaqueous speleothems as a potential palaeotemperature proxy(Springer Nature, 2020-10-06) Drysdale, RN; Couchoud, I; Zanchetta, G; Isola, I; Regattieri, E; Hellstrom, JC; Govin, A; Tzedakis, PC; Ireland, T; Corrick, E; Grieg, A; Wong, HKY; Piccini, L; Holden, PJ; Woodhead, JDFew palaeoclimate archives beyond the polar regions preserve continuous and datable palaeotemperature proxy time series over multiple glacial-interglacial cycles. This hampers efforts to develop a more coherent picture of global patterns of past temperatures. Here we show that Mg concentrations in a subaqueous speleothem from an Italian cave track regional sea-surface temperatures over the last 350,000 years. The Mg shows higher values during warm climate intervals and converse patterns during cold climate stages. In contrast to previous studies, this implicates temperature, not rainfall, as the principal driver of Mg variability. The depositional setting of the speleothem gives rise to Mg partition coefficients that are more temperature dependent than other calcites, enabling the effect of temperature change on Mg partitioning to greatly exceed the effects of changes in source-water Mg/Ca. Subaqueous speleothems from similar deep-cave environments should be capable of providing palaeotemperature information over multiple glacial-interglacial cycles. © 2020 The Author(s)
- ItemPartitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem(Elsevier, 2019-11-01) Drysdale, RN; Zanchetta, G; Baneschi, I; Guidi, M; Isola, I; Couchoud, I; Piccini, L; Greig, A; Wong, HKY; Woodhead, JD; Regattieri, E; Corrick, E; Paul, B; Spötl, C; Denson, E; Gordon, J; Jaillet, S; Dux, F; Hellstrom, JCThe trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation. This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients (where the partitioning (D) of element X (DX) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However, dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’ subaqueous calcite from a drill core taken through CD3 to derive DMg, DSr, DBa and DU then compare these with published cave, cave-analogue and seawater-analogue studies. The DMg for CD3 is anomalously high (0.042 ± 0.002) compared to previous estimates at similar temperatures (∼8 °C). The DSr (0.100 ± 0.007) is similar to previously reported values, but data from this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [Na/Sr] might play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa in CD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates (<1 μm a−1), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis of the upper 7 μm of CD3, regarded as ‘modern’ for the purposes of this study, reveals considerable heterogeneity, particularly for Sr, Ba and U, which is potentially indicative of compositional zoning. This reinforces the need to conduct 2D mapping and/or multiple laser passes to capture the range of time-equivalent elemental variations prior to palaeoclimate interpretation. © 2019 Elsevier Ltd
- ItemRadiometrically dated speleothem records of Terminations IV and V and linkages to the North Atlantic(International Union for Quaternary Research (INQUA), 2019-07-28) Pollard, T; Drysdale, RN; Woodhead, JD; Edwards, RL; Hellstrom, JC; Cheng, H; Dux, F; Daëron, M; Li, XL; Wong, HKY; Couchoud, I; Regattieri, E; Zanchetta, G; Isola, IPaleoclimate archives tied to accurate and precise chronologies are crucial for developing a dynamical understanding of the causes and nature of Quaternary glacial terminations [1]. While numerous well-dated archives provide good chronological control through Terminations I and II, there is presently a lack of well-dated records spanning earlier terminations. A notable exception to this is a suite of remarkably well-dated Chinese speleothems that have been used to date the commencement of each termination over the past 640 kyr [2,3]. This has been achieved by correlating weak Asian Monsoon anomalies as captured by speleothem oxygen isotope signals with terminal Heinrich events in the North Atlantic. While this approach is well-suited to testing orbital hypothesis by comparing the timing of termination commencement with insolation metrics, it only provides precise age control at the beginning of each termination. This precludes assessment of the progression of climatic changes over the full course of the termination, and the timing at which full interglacial conditions are reached. Here we present a composite speleothem record spanning glacial terminations IV and V from the Antro del Corchia cave system located on the western coast of central Italy. This record is anchored to a uranium-thorium based chronology and contains numerous proxies representing both local and regional climate, including Δ47-based temperature data derived from a pool carbonate formed under very slow degassing conditions. By taking advantage of established links between speleothem proxies from this cave site and marine proxies from the North Atlantic, we fix the marine sediment data to a radiometric age scale. This allows us to constrain the timing of ocean circulation and SST changes occurring in the North Atlantic throughout the duration of Terminations IV and V, and compare these with terrestrial temperatures in central Italy. © The Authors.
- 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.
- ItemUnderstanding Riversleigh’s geology: what we know in 2006, and where to next(University of New South Wales, 2006) Graham, IT; Price, E; Cendón, DI; Woodhead, JDNot available