Search

Terrestrial data spanning the Last Glacial Maximum (LGM) and deglaciation from the southern Australian region are sparse and limited to discontinuous sedimentological and geomorphological records with relatively large chronological uncertainties. This dearth of records has hindered a critical assessment of the role of the Southern Hemisphere mid-latitude westerly winds on the region's climate during this time period. In this study, two precisely dated speleothem records for Mairs Cave, Flinders Ranges, are presented, providing for the first time a detailed terrestrial hydroclimatic record for the southern Australian drylands during 23-15 ka. Recharge to Mairs Cave is interpreted from the speleothem record by the activation of growth, physical flood layering, and δ18O and δ13C minima. Periods of lowered recharge are indicated by 18O and 13C enrichment, primarily affecting δ18O, argued to be driven by evaporation of shallow soil/epikarst water in this water-limited environment. A hydrological driver is supported by calcite fabric changes. These include the presence of laminae, visible organic colloids, and occasional dissolution features, related to recharge, as well as the presence of sediment bands representing cave floor flooding. A shift to slower-growing, more compact calcite and an absence of lamination is interpreted to represent reduced recharge. The Mairs Cave record indicates that the Flinders Ranges were relatively wet during the LGM and early deglaciation, particularly over the interval 18.9-15.8 ka. This wetter phase ended abruptly with a shift to drier conditions at 15.8 ka. These findings are in agreement with the geomorphic archives for this region, as well as the timing of events in records from the broader Australasian region. The recharge phases identified in the Mairs Cave record are correlated with, but antiphase to, the position of the westerly winds interpreted from marine core MD03-2611, located 550 km south of Mairs Cave in the Murray Canyons region. The implication is that the mid-latitude westerlies are located further south during the period of enhanced recharge in the Mairs Cave record (18.9-16 ka) and conversely are located further north when greater aridity is interpreted in the speleothem record. A further comparison with speleothem records from the northern Australasian region reveals that the availability of tropical moisture is the most likely explanation driving enhanced recharge, with further amplification of recharge occurring during the early half of Heinrich Stadial 1 (HS1), possibly influenced by a more southerly displaced Intertropical Convergence Zone (ITCZ). A rapid transition to aridity at 15.8 ka is consistent with a retraction of this tropical moisture source. [ABSTRACT FROM AUTHOR]

Terrestrial data spanning the Last Glacial Maximum (LGM) and deglaciation from the southern Australian region are sparse, and limited to discontinuous sedimentological and geomorphological records with relatively large chronological uncertainties. This dearth of records has prevented a critical assessment of the role of the Southern Hemisphere mid-latitude westerly winds on the region's climate during this time period. In this study, two precisely-dated speleothem records for Mairs Cave, Flinders Ranges, are presented, providing a detailed terrestrial hydroclimatic record for the southern Australian drylands during 23-15 ka for the first time. Enhanced recharge to Mairs Cave is interpreted from the speleothem record by the activation of growth, physical flood layering and δ18O and δ13C minima. Periods of lowered recharge are indicated by isotopic enrichment, primarily affecting δ18O, argued to be driven by evaporation of shallow soil/epikarst water in this water-limited environment. A hydrological driver is supported by calcite fabric changes. The Mairs Cave record indicates that the Flinders Ranges were relatively wet during the LGM and early deglaciation, particularly over the interval 18.9-16 ka. This wetter phase ended abruptly with a shift to drier conditions at 15.8 ka. These findings are in agreement with the geomorphic archives for this region, as well as the timing of events in records from the broader Australasian region. The recharge phases identified in the Mairs Cave record are correlated with, but antiphase to, the position of the westerly winds interpreted from a marine core in the Great Australian Bight. The implication is that the mid-latitude westerlies are located further south during the period of enhanced recharge in the Mairs Cave record (18.9-16 ka), and conversely are located further north when greater aridity is interpreted in the speleothem record. A comparison with speleothem records from the northern Australasian region reveals that the availability of sub-tropical/tropical moisture is the most likely explanation driving enhanced recharge, with further amplification of recharge occurring during the early half of Heinrich Stadial 1, possibly influenced by a more southerly-displaced Intertropical Convergence Zone (ITCZ). A rapid transition to aridity at 15.8 ka is consistent with a retraction of this tropical moisture source. [ABSTRACT FROM AUTHOR]

Annual lamination in trace elements content allows to improve speleothem chronology as well as to extract paleoclimate information about fluctuations of the seasonal signal through time. Given the relatively slow growth and textural heterogeneity intrinsic in most speleothems, only high-resolution mapping techniques provide a viable approach to resolve trace elements variability at annual to sub-annual scale. Synchrotron radiation X-ray fluorescence microscopy (SR-μXRF) is, to date, the ideal technique as it provides quantitative, non-destructive fast scanning of large samples at the necessary high spatial resolution (0.5 – 5 μm).Here we present SR-μXRF investigation of U/Th dated stalagmites from caves in different geographic and climate setting including semi-arid (Flinders Ranges, South Australia), temperate (Central Italy) and tropical humid (Cook Islands, Northern Laos) climates.SR-μXRF analyses were coupled with high-resolution petrographic observation and LA-ICP-MS elemental analyses in order to verify the XRF elemental quantification and to test the relationship between fabric and trace element incorporation.All the analysed stalagmites revealed faint to sharp annual lamination in Sr, and, occasionally, in other elements such as Br, Cu and Zn. In climate setting characterised by strong seasonal contrast stalagmites often exhibit fabric changes at annual to centennial scale that are reflected in the internal porosity as well as in the incorporation of trace elements. Fabric control and spatial heterogeneity is also evident in the intensity of the annual cycles and, in some cases, the amplitude of Sr cycles vary considerably from one line scan to the adjacent ones. This poses the question of the significance and reproducibility of trace element analyses in speleothems characterised by fabric heterogeneity. [ABSTRACT FROM AUTHOR]

Debate about sedimentary structures found in intertidal zones of highly evaporative environments such as sabkhas, has commonly been focused on the interpretation of their morphology as purely abiotic or microbial. This is relevant to the study of early signs of Life on Earth, of the resilience of Life in extreme environments and for the search of signs of Life on Mars and exoplanets. Critically, ancient rocks likely underwent multiple diagenetic phases that may have partially overprinted primary features, yet microbially-influenced structures may be stabilized by micrite and micrite itself can be stabilized by organic molecules.The aim of the present study is to investigate the best chemical tracer that highlights primary (or very early diagenetic) depositional features by utilising Synchrotron Radiation based micro-X Ray Fluorsecence (SR-µXRF) mapping. Here we present the first SR-µXRF investigation of living microbial mats collected in the Dohat Faishakh Sabkha (DF) of Qatar compared with a similar fossil counterpart, the loferite facies from the Late Triassic Dolomia Principale (DP) in the Brenta Dolomites (Italy).In the modern DF sample we found that As, Br, Fe and Mn were associated with the dark, microbial laminae (organic part), whilst Sr was associated with the "mineral" part, along with Ca and Rb. This pattern has been already observed by SR-µXRF investigation of continental microbial mats.In the Late Triassic DP laminated facies - commonly interpreted as fossilized cyanobacterial microbial mats that originally thrived in an evaporative environment similar to that of the Qatar sabkha - the laminae consist of dolomite, whose genesis is a matter of debate. However, novel nanostructural investigations of the laminites suggest that some primary structures are preserved and, critically, the SR-µXRF maps revealed that Sr is associated with micrite outlining "microbial laminae".It is well known that Sr is immobilized by calcification and totally related to the mineral parts of the mats. By contrast, other elements, such as Br are in the "organic part", and are preferentially carried away during diagenesis. Sr distribution in the DP is very similar to that in the DF samples, which, coupled with modern growth rates of the mat, hints to the idea that it marks primary carbonate deposition as related to annual cycles in environmental parameters. At the same time, it is interesting to note that all the elements associated with "organics" are not detectable in the Triassic samples.This finding provides a new tool for both assessing existence of a primary mineral component of a microbial mat in fossil samples and for understanding annual-scale processes that drove the formation of "lofer" facies. [ABSTRACT FROM AUTHOR]

Crystallization pathways are at the core of incorporation of chemical species by calcium carbonate minerals. This is particularly important in speleothems where the classical equilibrium precipitation approach and the theoretical partition coefficient are not always applicable. In fact, in several speleothems non classical nucleation plays an active role and an empiric "speleothem enrichment factor" better describes the incorporation of different chemical species. Here we present the first attempt to calibrate chemical parameters on the base of speleothem fabrics for active and Late Holocene stalagmites taken from two caves in Atiu, Cook Islands. Our multi-proxy analysis integrates high-resolution petrography, trace element and stable isotope analysis of stalagmites characterised by growth rates ranging from 20 up to 1000 µm/yr, which have been dated by U-Th series and lamina counting. In-situ calcite farming experiments yielded calcite crystals of the order of ten nanometer from the slow drips, whereas colloidal calcite particulate (2-3 nm crystals) associated to organic and inorganic colloids formed from the fast drips. Fast/variable and slow/constant drip rate end members generate active forming stalagmite fabrics with fast growing opaque open/porous calcite and translucent compact columnar calcite respectively. In the compact fabric soluble species are incorporated within the calcite lattice, whilst OM colloids are expelled or isolated in the crystal defects. This allows using a partition coefficient for soluble elements, but not for elements adsorbed on colloids. In the open/porous fabric calcite crystals seem to grow by attachment of particles and there is a large number of sites where colloids, particulate and even solute elements can be adsorbed and incorporate outside the calcite lattice. This results in an enhanced incorporation of trace elements and thus, the theoretical partition coefficient cannot be utilized to reconstruct the original chemistry of the dripwater.In two Late Holocene stalagmites LA-ICP-MS analyses reveal specific patterns in the elemental distribution. Most of the solute elements (Mg, Sr, Ba, Na) show a positive covariance and can be potentially be utilized as palaeo-hydrological proxy. This is corroborated by the negative correlation with colloidally transported elements such as Y and P, and the general trend of δ13C that is consistent with Mg and Sr. During wet condition (low Mg and Sr), δ13C shifts toward more negative values (down to -11‰). In contrast, dry conditions are characterised by less negative δ13C and higher concentration of Mg and Sr suggesting the influence of kinetic effects and/or evapotranspiration. However, the higher incorporation of solute elements during dry phases is further enhanced in the open/porous fabric and a fabric-specific elemental enrichment factor has to be taken into account. [ABSTRACT FROM AUTHOR]