Holocene foraminifera as indicators of relative estuarine-lagoonal and oceanic influences in estuarine sediments of the River Murray, South Australia

In southeastern South Australia, the River Murray debouches through a coast al barrier separating euryhaline estuarine-lagoonal waters from the Souther n Ocean. Depending upon the relative freshwater outflow of the river and in gress of the ocean, water salinity varies greatly within the lower estuary. Ammonia beccarii and Elphidium articulatum are euryhaline species of foram inifera that characterize the estuary and back-barrier Coorong Lagoon. The inner-shelf marine environment hosts an assemblage in which Discorbis dimid iatus, E. crispum, E. macelliforme, and various cibicidid species predomina te. In cored sediments recovered from the shallow lower estuary, the relati ve abundance of A. beccarii + E. articulatum was compared with that of D. d imidiatus + E. crispum + E. macelliforme + other species. These data, and A MS radiocarbon ages determined for foraminifera and ostracods, provide evid ence of a change from maximum oceanic influence (5255 +/- 60 yr B.P,) to ma ximum estuarine influence (3605 +/- 70 yr B.P.). Over this same time interv al, sea level fell relatively by about 2 m. However, the event was also con temporaneous with falling water levels in several Victorian lakes, and it i s thus attributed to onset of climatic aridity. Reduced precipitation in th e River Murray catchment and reduced freshwater outflow enhanced developmen t of the flood-tide delta and constriction of the mouth. (C) 2000 Universit y of Washington.