Browsing by Author "Clarke, SL"

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    Discovery of a palaeolake at the terminus of the Murray-Darling Basin: a Holocene record of southeastern Australia’s hydroclimate and implications for current palaeoclimate reconstructions
    (International Union For Quaternary Research (INQUA), 2015-07-27) De Carli, E; Hubble, T; Penny, D; Petley, DN; Clarke, SL; Hamilton, RJ; Gadd, PS; Brand, HEA
    The 1.073 million km2 Murray-Darling River Basin (MDB) drains 14% of Australia’s landmass, incorporates Australia’s most economically important agricultural region, and presents one of Australia’s most important and contentious water security challenges. In this study we report the discovery of a hitherto unrecognised terminal palaeolake system ’Lake Mannum’ that developed during the middle to late Holocene, as evidenced by an extensive sequence of laminated muds. The deposit contains grey coloured laminae rich in Fe and smectite, and olive-black coloured laminae K and illite rich, representing high discharge events from both the Darling and Murray River catchments respectively, providing a high-resolution proxy record of MDB hydroclimatic variability during the Holocene. Given the strong influence of major oceanic-atmospheric synoptic circulation features (ENSO, IOS-SAM, IPO, IDO) over the river system, variability in MDB discharge and delivery of suspended sediment flux to the continental shelf have been used as proxy indicators for southeastern Australian palaeoclimate during the Holocene. The existence of a lake system at the terminus of the MDB suggests that discharge of terrigenous sediment to the Southern Ocean was strongly suppressed during this time, meaning that Holocene climate reconstructions for SE Australia which rely on the marine sediment record require re-evaluation.
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    The lower Murray River's mannum muds: a holocene age lacustrine deposit In a bedrock gorge
    (American Geophysical Union (AGU), 2014-12-12) Hubble, T; De Carli, E; Clarke, SL; Penny, D; Hamilton, RJ; Petley, DN; Gadd, PS
    Middle to Late Holocene age, horizontally laminated clays and muds of lacustrine origin predominate the uppermost layers of the valley-fill sequence deposited in the lower Murray River’s bedrock gorge upstream of the set of lakes which separate Australia's largest river system, The Murray-Darling, from its discharge point to the Southern Ocean. The top surface of the Coonambidgal Formation muds is developed at a constant elevation approximately coincident with the Holocene sea-level maximum and the mud deposit thins progressively in thickness upstream from ~30 m to ~10 m over a distance of 150 km due to a gradual, upstream rise in the elevation of the unit's base. Radiocarbon ages for wood and charcoal fragments recovered from two cores indicates the uppermost four to five metres of these muds were deposited after the mid-Holocene sea-level maximum, at below sea-level elevations indicating that the discharge of the Murray-Darling fluvial system was contained and effectively dammed by an obstruction developed downstream of Lake Alexandrina where the present-day river mouth is located. This feature is suspected to be the precursor of the present-day dune and beach-barrier system which occasionally blocks the river mouth and diverts fresh-water flow into the Coorong Wetlands. Muddy sediment from the entire Murray-Darling catchment was effectively trapped in the lower Murray Gorge palaeolake, herein named Lake Mannum, during the mid to late Holocene. High rates of sedimentation (one to two meters per thousand years) produced exquisitely fine-scaled (1 mm to 1 cm) laminations in the upper Coonambidgal Formation. This material has not been disturbed by bioturbation and presents a sediment record with the potential to yield a high-resolution record of the Murray-Darling catchment’s discharge for much of the Holocene. The present-day lower Murray River channel currently presents a meandering but constant planform geometry upstream of Lake Alexandrina that developed as a thalweg incised and entrenched within the Coonambidgal muds, as a somewhat delayed response to the two metre fall in sea-level after the mid-Holocene maximum. The onset of this incision apparently occurred towards the end of the Holocene.
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    An unrecognised holocene palaeo-lake at the terminus of the Murray-Darling Basin: a palaeo-discharge record and implications for current climate reconstructions
    (American Geophysical Union, 2015-12-14) De Carli, E; Hubble, T; Penny, D; Petley, DN; Clarke, SL; Hamilton, RJ; Gadd, PS; Brand, HEA
    The 1.073 million km2 Murray-Darling River Basin (MDB) drains 14% of Australia’s landmass, incorporates Australia’s most economically important agricultural region, and presents one of Australia’s most important and contentious water security challenges. The twin Murray and Darling catchments extend from the sub-tropics to the mid latitudes, with catchment precipitation driven by synoptic-scale oceanic-atmospheric processes that include the Australian Monsoon, SAM, IPO, PDO, IOD and ENSO. In this study we report the discovery of a hitherto unrecognised terminal palaeo-lake system ‘Lake Mannum’ that existed during the middle to late Holocene, as evidenced by an extensive sequence of laminated muds. The deposit contains gray laminae enriched in smectite and Nd/Ti, diagnostic of palaeo-discharges originating from the Darling catchment. These gray laminae are set within olive-black background muds enriched in illite, K and Rb, diagnostic of palaeo-discharges originating from the Murray Catchment. The deposit reflects the hydrological regime of the MDB, representing the first in-situ palaeo-discharge record for the MDB and a proxy record for south-eastern Australia’s precipitation and hydroclimate. Given the strong influence of major oceanic-atmospheric synoptic circulation over the river system, variability in MDB discharge and delivery of suspended sediment flux to the continental shelf have been used as proxy indicators for south-eastern Australia’s palaeo-climate during the Holocene. The existence of palaeo-lake Mannum at the terminus of the MDB suggests that discharge of terrigenous sediment to the Southern Ocean was strongly suppressed during this time, meaning that Holocene climate reconstructions which rely on the marine sediment record require re-evaluation. © 2014 American Geophysical Union.