An unrecognised Holocene palaeo-lake at the terminus of the Murray-Darling Basin: a palaeo-discharge record and implications for current climate reconstructions

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.