Depositional History and Palaeoenvironments of the Lake Mulurulu Lunette, Willandra Lakes World Heritage Area, New South Wales

Abstract

The Willandra Lakes form a dry lake system consisting of a number of ancient, formerly perennial, lakes in the western Murray basin. The area has significant scientific value, providing detailed palaeoenvironmental records of arid, ice-age Australia as well as a rich and unique archaeological record. Lake Mungo, resting place of Australia’s oldest dated aboriginal remains, is a terminal lake where studies of the lake system are concentrated. Lake Mulurulu, a flow-through lake in the northern part of the system, is relatively understudied despite abounding potential and a differing hydrological regime. A range of geochronological techniques, combined with stratigraphic and isotope palaeoecological methods, inform the palaeoenvironmental history for the Lake Mulurulu lunette. A comprehensive suite of quartz optically stimulated luminescence (OSL) ages is combined with detailed sedimentological analyses to build a chronostratigraphic framework. Mussel shells and fish otoliths are radiocarbon dated, and wombat teeth are analysed for electron spin resonance (ESR) and uranium series dating. Oxygen isotope analyses on fish otoliths and wombat teeth are used in an attempt to ascertain high-resolution palaeoenvironmental records for the area. The quartz was well suited for OSL dating. Very little evidence for partial bleaching was observed, though sediment mixing proved to be relatively common. A small aliquot multi-grain OSL methodology allowed the identification of sediment mixing and grain transport across unit boundaries. Many samples were found to be very young (< 200 years), and were disproportionately affected by recuperation issues, low precision and high overdispersion. Dating reveals the Mulurulu lunette comprises five major units. Unit A, dating from 60 to over 110 ka, is clay and carbonate rich with a thick palaeosol, and is equivalent to the Golgol Unit at Lake Mungo. Unit B is a clean quartz sand representing an early lake full stage, initiating around 60 ka and ending around 40 ka at the southern end of the lunette and around 32 ka at the northern end of the lunette. Unit C comprises a pelletal clay representing a drying phase, dated to 40-32 ka and 32-28 ka at the southern and northern ends of the lunette, respectively. Unit D is a 28-17 ka quartz sand representing another lake full stage. A thin, previously unrecognised, pelletal clay layer at the southern end of the lunette caps this unit. Unit E comprises laminated quartz sands derived from reworked lunette materials dating to less than 200 years old, indicating a significant amount of recent lunette remobilisation. Unconsolidated mobile sands are present on the crests and leeward side of the dunes. Oxygen isotope analyses of fish otoliths provided evidence of flood events and potentially a lake drying signal that was not expected in this flow-through lake. The oxygen isotope analyses of wombat teeth on the other hand, proved less successful for gaining insight into high-resolution palaeoenvironmental events.

This research provides a new chronology of deposition for the Lake Mulurulu lunette. This forms a similar but distinctive local hydrological history compared to the previous regional understanding, built primarily on the events recorded at Lake Mungo, a terminal lake situated further downstream. Show more