Synergetic influence of water masses and Kangaroo Island barrier on foraminiferal distribution, Lincoln and Lacepede shelves, South Australia: a synthesis

Abstract

The mid-latitude, cool-water, high-energy Lincoln and Lacepede Shelves along the southern Australian margin are covered with mixed Holocene and Pleistocene sediments. Foraminiferal specimens of Recent and Pleistocene age from dredge samples are strongly mixed. Eight assemblages in two major groups A and B were recognised from the inner shelf to the upper slope, and they appear to be depth-related. However, some are typical of localised environments such as an upwelling zone (assemblage A2) or areas strongly affected by a high accumulation of relict specimens indicative of previous lagoonal deposition (assemblages A3 and A4). Sediment starvation due to strong wave abrasion is largely responsible for the preservation of this mixed biofacies. The interaction between shelf waters (the Great Australian Bight Current), waters from the Spencer and St. Vincent Gulfs, oceanic waters and waters from the River Murray, is suggested as the main factor controlling the overall distribution pattern of foraminifera. A warmer, nearly oligotrophic condition on the Lincoln Shelf, signalling a warmer regime, is indicated by a higher abundance of warmwater planktonic species and by a higher faunal diversity. On the Lacepede Shelf, the diversity is lower and warm-water forms are less common, implying a cooler or mesotrophic condition. These differences may be due to the position of Kangaroo Island which blocks the passage of the shelf water flowing to the southeast, thereby creating two different water regimes. This barrier would have been more significant during the late Pleistocene, when sealevel was low, the shelves were narrower and a lagoonal environment was present as indicated by different relict faunas on these two shelves. © 1998 Association of Australasian Palaeontologists.