Did Port Phillip Bay nearly dry up between ∼2800 and 1000 cal. yr BP? Bay floor channelling evidence, seismic and core dating

Imagery of meandering river-like channel features up to 5 m deep and 100 m wide on the floor of Port Phillip Bay have been revealed by recent multibeam surveys. They are in water depths to −22 m below the present bay water level. This young channelling overlies older infilled Pleistocene–early Holocene channels considered to be cut by the Yarra and Werribee rivers during the last glacial or previous low stand minima >10 000 cal. yr BP and infilled during the Holocene transgression. Vibrocoring data indicate the young channels incise shelly mud. Twelve 14C shell dates in vibrocores from bay floor to 3.5 m are in two units—a lower unit range between 2880 cal. yr BP at 0.7 m to 9491 cal. yr BP at 3.5 m, and an upper unit ∼0.5 to 0.7 m thick <869 cal. yr BP forming the present bay floor. Seismic and vibrocore evidence suggests the post-869 cal. yr BP upper unit also covers the base of the young channels. This evidence implies that Port Phillip dried out to −22 m between ∼2800 and ∼1000 cal. yr BP at a time of stable present day sea-levels. The most likely cause is a sand blockage to the Port Phillip Bay channel entrances, coupled with high evaporation rates. Palynological and foraminiferal evidence suggests variable salinities occurred immediately prior to the sedimentary hiatus, with restoration of marine conditions after. The results have wide implications on Port Phillip's water balance, salinity, pre-European climatic events and Aboriginal occupation.