Browsing by Author "Marshall, J"

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    A Holocene subtropical hydroclimate reconstruction from Karboora (Blue Lake), Minjerribah, Queensland
    (Australasian Quaternary Association, 2022-12-06) Maxson, C; Tibby, J; Tyler, JJ; Marshall, J; McGregor, G; Schulz, C; Jacobsen, GE; Klaebe, RM
    Holocene palaeoclimatology provides insights into the climate system, with particular relevance to the next century. This is especially true in sub-tropical Australia due to the under representation of the region in Holocene climate studies. Karboora (Blue Lake), on Minjerribah (North Stradbroke Island), Queensland, Australia is a groundwater window lake of major ecological and cultural significance. The lake’s strong connection with the regional aquifer underpins lake level stability, rendering Blue Lake sediments an ideal tracer of subtle changes in climate. Here, we report a new 5,500-year oxygen isotope record from biogenic (diatom) silica (δ18OBSi) that records variations in rainfall resulting from changes in Pacific atmospheric circulation. These interpretations are supported by modern monitoring over a 20 month period, including the δ18O of lake water, rain water, plant cellulose, and biogenic silica. We link rain δ18O to changes in El Niño Southern Oscillation (ENSO) phases, with phases showing distinct isotopic change that may be linked to tropical or extratropical sources of rainfall. On these grounds, we infer a dominance of extratropical rainfall from 5.5 kyr BP to 3.5 kyr BP, a period of transition from 3.5 kyr BP to 2.5 kyr BP dominated by neither tropical or extratropical rainfall, then a shift to tropically sourced rain from 2.5 kyr BP to the present. The early record (5.5 kyr BP – 3.5 kyr BP) most likely reflects a suppression of summer rainfall caused by a weakened ENSO. This is most likely linked to higher northern hemisphere insolation causing a northward shift in the intertropical convergence zone and westerly wind belt which in turn affected synoptic systems in the Coral and Tasman Seas. The increasing variability in the late record (3.5 kyr BP to present) most likely represents an increase in summer rainfall driven by the intensification of ENSO in the late Holocene.
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    The measurement of swirling flow in HIFAR fuel elements and the effects of fitting flowsplitters
    (Australian Atomic Energy Commission, 1975-01) Marshall, J
    Measurements have been made of swirl in the coolant flow at the inlet of HIFAR fuel elements before and after flowsplitters were fitted in the inlet nozzles. There was considerable swirl before installation of the flowsplitters, and the amplitudes and directions of the swirl formed a symmetrical pattern over the core. The swirl activity distribution appears to have a definite relationship to positions in which some fuel element damage had been noticed. After flowsplitters were fitted, the swirl velocities were found to have been reduced by a factor of about twenty.
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    Measurements of coarse control arm drop characteristics in the materials testing reactor HIFAR
    (Australian Atomic Energy Commission, 1983-02) Marshall, J; Blevins, RJ
    Measurements were made of the angular position/time characteristics of a coarse control arm in the AAEC's materials testing reactor HIFAR following a trip. The method of measurement is described and the results are presented. It is clear that all of the measured transients may be reasonably fitted by a single differential equation.
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    Reading the tea-tree leaves: Melaleuca quinquenervia leaves as a palaeoclimate proxy
    (International Union for Quaternary Research (INQUA), 2019-07-30) Geer, J; McInerney, F; Tibby, J; Hua, Q; Schulz, C; Barr, C; Marshall, J; McGregor, G
    The analysis of organic material preserved in sedimentary records is a useful tool in reconstructing past climatic conditions. It has been suggested that the carbon isotope discrimination (Δ) calculated from the bulk leaf δ13C of the modern Melaleuca quinquenervia tree responds to local precipitation in south-east Queensland, Australia [1]. The preservation of these leaves in lake sediments on Minjerribah (North Stradbroke Island) dating to the mid-Holocene presents the opportunity to produce species-specific stable isotope-based records of precipitation. Here, we test the potential for M. quinquenervia to be used as a palaeoclimate proxy by examining the preservation of the bulk leaf δ13C over time and the relationship of Δ values to historical records of precipitation. Due to the varying rates of degradation of the different chemical constituents of plant matter, it is possible δ13C ratios to be altered by early diagenetic processes before, or during, the incorporation of leaves into the sediment. Therefore, modern studies are needed to establish what factors influence the discrimination derived precipitation record. Focusing on the M. quinquenervia growing at Swallow Lagoon on Minjerribah, we studied the changes to the bulk leaf δ13C ratios of exposed leaves over an eighteen-month field study. We then applied our findings to the measured δ13C ratios of bulk leaf material retrieved from a core taken from the lagoon. The Δ values calculated based on these measurements were then compared to instrumental rainfall data from the past century to test the established relationship with modern precipitation through time. By bisecting each leaf used in this study, we were able to compare the experimentally degraded leaves directly to their corresponding control halves. We observed that decay causes an approximate decrease of 1 ‰ in δ13C, as the leaves become more 13C depleted relative to the control leaf halves that were dried immediately. Quantifying this offset enables adjustment of values to be comparable to the calibration equation established using natural fall leaves from modern M. quinquenervia. Comparing the adjusted Δ values for lake core leaves from the last century to corresponding rainfall data the relationship to local precipitation seems to be preserved. Understanding exactly how early diagenesis changes the stable isotope composition of M. quinquenervia leaf material over time allows us to adjust for the offset between modern and sub-fossil bulk leaf δ13C and advances the potential to use this species as a reliable climate proxy in the future. © The authors.