Late Quaternary lake filling and drying in northern Australia's largest freshwater lake

ABSTRACT

Lake Woods, Jurrkulu, is northern Australia's largest freshwater lake filled today in strong Indo–Australian summer monsoons (IASM) to maximum depths of ~5 m. On the western margins of the lake, two large arcuate ridges occur 20 to 40 m above the lake floor and inset within these are other mid‐elevation lake deposits, such as depositional ridges, which suggest past filling events much higher than the modern maximum. We have undertaken the analysis of 20 samples for single‐grain optically stimulated luminescence (OSL) from shoreline and lake‐floor deposits and supplemented this chronology with an analysis of the basin hypsometry, basin‐wide erosion rates and rare earth elements to ascertain the dominant source of the large source‐bordering ridges. The outer and inner arcuate ridges represent both aeolian accumulation and inferred lake‐level highstands due to the presence of fine gravel facies and gastropods contained within them. We document two lake highstands at 195 ± 16 ka and at 98 ± 7 ka (20 and 15 m above the lake floor, respectively; the lake floor is 197 m Australian Height Datum, AHD). We also present a third but undated filling event 18.5 m above the lake floor (215.5 m AHD) but that is bracketed by two OSL ages of 91 ± 5 and 52 ± 3 ka, with a Bayesian modelled boundary age for lake filling of 95.9 to 53.8 ka (95% credible interval; C.I.). We find no evidence of the previously reported late Marine Isotope Stage [MIS] 3 highstand deposits and the prevalence of gully‐filled structureless sediments, dated to the mid to late Holocene, many of which contain archaeological artefacts that do not allow for the quantification of any accurate Holocene lake‐filling episodes. The lake floor is characterised by deeply cracking silts and clays, with single‐grain OSL results supporting significant post‐depositional mixing (maximum ages are <143 ka). Denudation rate data from detrital stream sediment samples indicate some of the lowest rates of denudation in northern Australia, of between 2.05 ± 0.51 and 0.83 ± 0.23 m/Myr, with the latter rate stemming from the dominant catchment area in the Barkly Tablelands. Sediment provenance data indicate that the slowly eroding Barkly Tablelands is the dominant source for the clays and silts in the lake basin. Despite being the largest lake in northern Australia, the slow rates of catchment‐wide erosion coupled with the intermittent filling have only produced a discontinuous record of the IASM, but one that shows exceptional episodic filling in MIS 5 to 4.