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Greig, Alan
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Greig, Alan
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- PublicationOpen AccessEnvironmental variability between the penultimate deglaciation and the mid Eemian: Insights from Tana che Urla (central Italy) speleothem trace element record(2016)
; ; ; ; ; ; ; ; ; ; ;; ;; ; ; ; ; ;A trace element record (Mg, Sr, Ba, Al, Si, P, Y, Zn) covering the ca. 133 ka to ca. 124 ka time interval was acquired from a flowstone core from Tana che Urla Cave (central Italy). It was compared with stable isotope data to investigate the environmental evolution in response to regional and extra-regional climate changes in the period corresponding to the latter part of the Penultimate Deglaciation and the first part of the Last Interglacial (Eemian). Trace element geochemical changes on centennial and millennial time scales responded to changes in hydrological input, variations in the supply and transport of catchment erosion products to the cave, including those linked to intense rainfall events, and to the state of the overlying soil and vegetation. Abrupt increases in precipitation and the progressive development of soil and vegetation occurred at ca. 132 ka, in response to the development of the global deglacial phase. The major changes in trace element composition are coherent with the previous hydrological interpretation of speleothem oxygen stable isotope composition (δ18O) as predominantly a rainfall-amount proxy. However, reduced growth rate persisted until ca. 130 ka, suggesting still depressed temperatures. An abrupt event of climatic deterioration, with marked decrease in precipitation and soil degradation, is apparent between ca. 131 and 130 ka. Cool-wet conditions between ca. 132 and 131 ka and the subsequent dry period may represent the local hydrological response to an interval of North Atlantic cooling and ice-rafted-debris (IRD) deposition known as Heinrich event 11 (H11). After 129.6 ka there was a rapid recovery according to all of the studied speleothem properties, indicating the onset of full interglacial conditions. A minor amplitude event of reduced precipitation is recorded within the LIG at ca. 127 ka. The record highlights the growing regional evidence for a complex penultimate deglacial climate involving major reorganization of oceanic and atmospheric patterns.105 173 - PublicationRestrictedPrecise microsampling of poorly laminated speleothems for U-series dating(2012)
; ; ; ; ; ; ; ; ; ; ; ; ;Drysdale, R. N.; Department of Resource Management and Geography, University of Melbourne, Parkville 3010, Victoria, Australia ;Bence, T. P.; School of Earth Sciences, University of Melbourne, Parkville 3010, Victoria, Australia ;Hellstrom, J. C.; School of Earth Sciences, University of Melbourne, Parkville 3010, Victoria, Australia ;Couchoud, I.; Department of Resource Management and Geography, University of Melbourne, Parkville 3010, Victoria, Australia ;Greig, A.; School of Earth Sciences, University of Melbourne, Parkville 3010, Victoria, Australia ;Bajo, P.; Department of Resource Management and Geography, University of Melbourne, Parkville 3010, Victoria, Australia ;Zanchetta, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Isola, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Spötl, C.; Institut für Geologie und Paläontologie, Universität Innsbruck, 6020 Innsbruck, Austria ;Baneschi, I.; Istituto di Georisorse e Geoscienza-CNR, Pisa 56100, Italy ;Regattieri, E.; Department of Earth Sciences, University of Pisa, Pisa 56100, Italy ;Woodhead, J. D.; School of Earth Sciences, University of Melbourne, Parkville 3010, Victoria, Australia; ; ; ; ; ; ; ; ; ; ; One of the principal reasons why speleothems are recognised as important palaeoclimate archives is their suitability for accurate and precise uranium-series (U-series) age determination. Sampling speleothem sections for U-series dating is straightforward in most cases because visible growth layers are preserved. However, this is not always the case, and here we describe a sampling strategy whereby growth layers are resolved from trace-element images produced by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We apply this method to a section of an Italian subaqueous speleothem (CD3) that lacks persistent visible growth layering. Trace-element imaging revealed growth layers that are strongly non-planar in their geometry owing to the speleothem's pronounced euhedral crystal terminations. The most prominent trace-element layers were first digitized as x, y vector contours. We then interpolated these in the growth-axis direction to generate a series of contour lines at ∼250-μm increments. The coordinates of these contours were used to guide the sampling via a computerised micromilling lathe. This produced a total of 22 samples for U-series dating by multi-collector ICP-MS. The dating results returned ages in correct stratigraphic order within error. Close inspection of the U-series data and the derived depth–age model suggests that the main source of model-age uncertainty is unrelated to the contour sampling but instead more associated with how closely spaced the model ages are in time, i.e. the model age density. Comparisons between stable oxygen and carbon isotope profiles derived from aliquots of the dating samples and two other stable isotope profiles from CD3 spanning the same time period compare very favourably. Taken together, this suggests that our trace-element contouring method provides a reliable means for extracting samples for dating (and other geochemical analyses), and can be applied to similar speleothems lacking visible growth layering.289 18 - PublicationOpen AccessThe 4.2 ka event in the central Mediterranean: new data from a Corchia speleothem (Apuan Alps, central Italy)(2019)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ; ;We present new data on the 4.2 ka event in the central Mediterranean from Corchia Cave (Tuscany, central Italy) stalagmite CC27. The stalagmite was analyzed for stable isotopes (delta13C and delta18O) and trace elements (Mg, U, P, Y), with all proxies showing a coherent phase of reduced cave recharge between ca. 4.5 and 4.1 ka BP. Based on the current climatological data on cyclogenesis, the reduction in cave recharge is considered to be associated with the weakening of the cyclone center located in the Gulf of Genoa in response to reduced advection of air masses from the Atlantic during winter. These conditions, which closely resemble a positive North Atlantic Oscillation (NAO) type of configuration, are associated with cooler and wetter summers with reduced sea warming, which reduced the western Mediterranean evaporation during autumn–early winter, further reducing precipitation.402 209 - PublicationOpen AccessMagnesium in subaqueous speleothems as a potential palaeotemperature proxy(2020)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;Few 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.199 14 - PublicationOpen AccessPartitioning of Mg, Sr, Ba and U into a subaqueous calcite speleothem(2019)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; 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 lm a 1), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis of the upper 7 lm 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.183 69