School of Geography, Earth and Atmospheric Sciences - Research Publications
Permanent URI for this collection
Search Results
1 - 9 of 9
-
ItemNo Preview AvailableElevated Southern Hemisphere moisture availability during glacial periods(NATURE PORTFOLIO, 2024-02-08)Late Pleistocene ice-age climates are routinely characterized as having imposed moisture stress on low- to mid-latitude ecosystems1-5. This idea is largely based on fossil pollen evidence for widespread, low-biomass glacial vegetation, interpreted as indicating climatic dryness6. However, woody plant growth is inhibited under low atmospheric CO2 (refs. 7,8), so understanding glacial environments requires the development of new palaeoclimate indicators that are independent of vegetation9. Here we show that, contrary to expectations, during the past 350 kyr, peaks in southern Australian climatic moisture availability were largely confined to glacial periods, including the Last Glacial Maximum, whereas warm interglacials were relatively dry. By measuring the timing of speleothem growth in the Southern Hemisphere subtropics, which today has a predominantly negative annual moisture balance, we developed a record of climatic moisture availability that is independent of vegetation and extends through multiple glacial-interglacial cycles. Our results demonstrate that a cool-moist response is consistent across the austral subtropics and, in part, may result from reduced evaporation under cool glacial temperatures. Insofar as cold glacial environments in the Southern Hemisphere subtropics have been portrayed as uniformly arid3,10,11, our findings suggest that their characterization as evolutionary or physiological obstacles to movement and expansion of animal, plant and, potentially, human populations10 should be reconsidered.
-
ItemNo Preview AvailableDQPB : software for calculating disequilibrium U-Pb ages(COPERNICUS GESELLSCHAFT MBH, 2023-04-20)Abstract. Initial radioactive disequilibrium amongst intermediate nuclides of the U decay chains can have a significant impact on the accuracy of U–Pb ages, especially in young samples. For samples that can reasonably be assumed to have attained radioactive equilibrium at the time of analysis, a relatively straightforward correction may be applied. However, in younger materials where this assumption is unreasonable, it is necessary to replace the familiar U–Pb age equations with more complete expressions that account for growth and decay of intermediate nuclides through time. DQPB is software for calculating U–Pb ages while accounting for the effects of radioactive disequilibrium among intermediate nuclides of the U decay chains. The software is written in Python and distributed as both a pure Python package and a stand-alone graphical user interface (GUI) application that integrates with standard Microsoft Excel spreadsheets. The software implements disequilibrium U–Pb equations to compute ages using various approaches, including concordia intercept ages on a Tera–Wasserburg diagram, U–Pb isochron ages, Pb*/U ages based on single aliquots, and 207Pb-corrected ages. While these age-calculation approaches are tailored toward young samples that cannot reasonably be assumed to have attained radioactive equilibrium at the time of analysis, they may also be applied to older materials where disequilibrium is no longer analytically resolvable. The software allows users to implement a variety of regression algorithms based on both classical and robust statistical approaches, compute weighted average ages and construct customisable, publication-ready plots of U–Pb age data. The regression and weighted average algorithms implemented in DQPB may also be applicable to other (i.e. non-U–Pb) geochronological datasets.
-
ItemNo Preview Available
-
ItemCave opening and fossil accumulation in Naracoorte, Australia, through charcoal and pollen in dated speleothems(SPRINGERNATURE, 2022-09-26)Abstract Caves are important fossil repositories which provide records extending back over million-year timescales. While the physical processes of cave formation are well understood, the timing of initial cave development and opening—a more important parameter to studies of palaeontology, palaeoanthropology and archaeology—has proved more difficult to constrain. Here we investigate speleothems from the Naracoorte Cave Complex in southern Australia, with a rich record of Pleistocene vertebrate fossils (including extinct megafauna) and partly World Heritage-listed, using U-Th-Pb dating and analyses of their charcoal and pollen content. We find that, although speleothem formation began at least 1.34 million years ago, pollen and charcoal only began to be trapped within growing speleothems from 600,000 years ago. We interpret these two ages to represent the timing of initial cave development and the subsequent opening of the caves to the atmosphere respectively. These findings demonstrate the potential of U-Th-Pb dating combined with charcoal and pollen as proxies to assess the potential upper age limit of vertebrate fossil records found within caves.
-
ItemPartitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem(PERGAMON-ELSEVIER SCIENCE LTD, 2019-11-01)The 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.
-
ItemNo Preview AvailableLow impact sampling of speleothems - reconciling scientific study with cave conservation(SOCIETA SPELEOLOGICA ITALIANA, 2022-01)Speleothems are increasingly valued as important paleoclimate archives and yet the removal of samples from caves can come at a cost to natural heritage, impacting delicate environments with limited mechanisms for repair. Conservation of cave environments is a key responsibility for scientists and, with this in mind, we are working to develop and implement techniques that allow us to extract valuable scientific data, with minimal impact. In this study, we demonstrate the utility of low-impact reconnaissance dating surveys on caves in southern Tasmania and southwest Western Australia as a precursor to the removal of stalagmites for paleoclimate reconstruction. Small flakes of calcite were discretely extracted from the base and tip of fallen stalagmites and dated using U-Th techniques. We specifically targeted stalagmites that have naturally fallen or been previously broken by human interference, to further reduce our impact on the caves. This approach provides maximum and minimum age constraints for each stalagmite and valuable information of growth frequencies without the need to remove whole samples from the cave. Selecting the most appropriate samples to analyze based on reconnaissance ages greatly reduces the quantity of speleothem material to be removed from a cave to locate a desired interval of past time, mitigating the impacts of the research. Moreover, the reconnaissance age data enable us to build an archive of speleothem ages from the cave for future scientific research and to provide information on the age and nature of cave development, useful for cave management purposes and other studies. To assess the accuracy of this method we compared the reconnaissance age with the results of a detailed age evaluation on a small number of stalagmites removed from the caves. We have found this method to be effective and has allowed us to successfully identify several stalagmites suitable for our scientific objectives.
-
Item
-
ItemNo Preview AvailableSouthern Hemisphere subtropical drying as a transient response to warming(NATURE PUBLISHING GROUP, 2019-03)Climate projections1–3 and observations over recent decades4,5 indicate that precipitation in subtropical latitudes declines in response to anthropogenic warming, with significant implications for food production and population sustainability. However, this conclusion is derived from emissions scenarios with rapidly increasing radiative forcing to the year 21001,2, which may represent very different conditions from both past and future ‘equilibrium’ warmer climates. Here, we examine multi-century future climate simulations and show that in the Southern Hemisphere subtropical drying ceases soon after global temperature stabilizes. Our results suggest that twenty-first century Southern Hemisphere subtropical drying is not a feature of warm climates per se, but is primarily a response to rapidly rising forcing and global temperatures, as tropical sea-surface temperatures rise more than southern subtropical sea-surface temperatures under transient warming. Subtropical drying may therefore be a temporary response to rapid warming: as greenhouse gas concentrations and global temperatures stabilize, Southern Hemisphere subtropical regions may experience positive precipitation trends.
-
ItemSpeleothem Records of the Hydroclimate Variability throughout the Last Glacial Cycle from Manita pec Cave (Velebit Mountain, Croatia)(MDPI, 2021-08)We present stable carbon (δ13C) and oxygen (δ18O) isotope records from two partially coeval speleothems from Manita peć Cave, Croatia. The cave is located close to the Adriatic coast (3.7 km) at an elevation of 570 m a.s.l. The site experienced competing Mediterranean and continental climate influences throughout the last glacial cycle and was situated close to the ice limit during the glacial phases. U-Th dating constrains the growth history from Marine Isotope Stage (MIS) 5 to MIS 3 and the transition from MIS 2 to MIS 1. 14C dating was used to estimate the age of the youngest part of one stalagmite found to be rich in detrital thorium and thus undatable by U-Th. On a millennial scale, δ18O variations partly mimic the Dansgaard–Oeschger interstadials recorded in Greenland ice cores (Greenland Interstadials, GI) from GI 22 to GI 13. We interpret our δ18O record as a proxy for variations in precipitation amount and/or moisture sources, and the δ13C record is interpreted as a proxy for changes in soil bioproductivity. The latter indicates a generally reduced vegetation cover towards MIS 3–MIS 4, with shifts of ~8‰ and approaching values close to those of the host rock. However, even during the coldest phases, when a periglacial setting and enhanced aridity sustained long-residence-time groundwater, carbonic-acid dissolution remains the driving force of the karstification processes. Speleothem morphology follows changes in environmental conditions and complements regional results of submerged speleothems findings. Specifically, narrow sections of light porous spelaean calcite precipitated during the glacial/stadial sea-level lowstands, while the warmer and wetter conditions were marked with compact calcite and hiatuses in submerged speleothems due to sea-level highstands. Presumably, the transformation of this littoral site to a continental one with somewhat higher amounts of orographic precipitation was a site-specific effect that masked regional environmental changes.