Microfossil evidence for salinity events in the Holocene Coorong Lagoon, South Australia

Abstract

Nearly 6 m of uncompacted muddy sediment was recovered from the floor of the northern Coorong Lagoon in the core Coorong #5. Radiocarbon analyses of molluscan shells indicate that sedimentation at the core site commenced before 6830 ± 90 yr cal BP, and the presence of Pinus pollen confirms a modern age for the uppermost 0.5 m. Microfossils extracted from the core sediment samples, 2 cm slices at 10 cm intervals, included the foraminifera Ammonia sp., Elphidium excavatum and Elphidium gunteri; the ostracods Osticythere baragwanathi and Leptocythere lacustris; and charophyte oogonia. Shell fragments of the estuarine bivalve Spisula (Notospisula) trigonella in the lowermost 0.7 m of the core are evidence that these sediments were subject to some marine influence, but the absence of foraminifera and ostracods from this same interval indicates that at the core site salinity was not sufficient to support populations of these organisms. Thus, prior to 6830 ± 90 yr cal BP the Younghusband Peninsula was in place, in part isolating the northern lagoon from the Southern Ocean. The initial recorded salinity event is signified by abundant Ammonia sp. at a core depth of 5.2 m. The duration of this event was relatively brief; foraminifera were mostly absent in the immediately overlying 2 m, representing ca 700 yr of sedimentation. This observation is attributed to substantial inflow of freshwater from the River Murray. In the upper 3.0 m, Ammonia sp. was present in most core samples indicating that for most of the past 6000 years the Coorong Lagoon has been sufficiently saline to support a continuing population of this species. At a core depth of 1.3 m, the sediment sample yielded >2000 tests of Ammonia sp., and they were accompanied by maximum pre-modern numbers of E. excavatum, O. baragwanathi and oogonia. Taken together, these data signify the maximum pre-modern salinity event recorded in the core sediments, probably correlating in time with regional drought conditions at ca 3500 yr BP. Elphidium gunteri is confined to the modern sediments where it is abundant and accompanied by equally large numbers of Ammonia sp., E. excavatum, O. baragwanathi and L. lacustris. These data collectively indicate water conditions that are significantly changed from those that prevailed in the Coorong Lagoon for most of the Holocene.

近6米厚的泥质沉积物从Coorong泻湖北部的湖底掘进的岩心Coorong5号中获得。软体动物壳的放射性碳分析表明，岩心地点的沉积作用始于6830±90年之前，松树类花粉的存在证实了最上部0.5米的现代年龄。从岩心沉积物样品以每10厘米的间隔取得2厘米厚的切块，从中发现的微体化石包括: 有孔虫Ammonia sp.、Elphidium excavatum 及Elphidium gunteri；介形虫Osticythere baragwanathi和Leptocythere lacustris；还有轮藻雌器。岩心最底部0.7米沉积物中的河口双壳类Spisula （Notospisula）trigonella的碎片证实这些沉积物曾受海洋影响，但同层位中缺乏有孔虫和介形虫，表明该地的盐度不足以支持这些动物居群。因此，在6830±90年之前，该地有Younghusband半岛，将泻湖北部与南洋部分隔开。在岩心5.2米深处，丰富的Ammonia sp.标志着一次盐度事件。该事件持续时间较短；之上的两米沉积物中缺失有孔虫，代表着约700年的沉积作用，其原因是Murray河 淡水的大量涌入。在上部3米沉积物中，Ammonia sp.存在于大多数岩心样品中，这说明过去6000年的大部分时间中，Coorong 泻湖盐度相当高，足以支持这个种的持续生存。在岩心深处1.3米处，样品中发现2000多个Ammonia sp. 标本，同时发现的还有近现代分子E. excavatum、O. baragwanathi以及轮藻雌器。这些资料的综合，可标示记录在岩心沉积物中的最大近现代盐度事件，可能与约3500年之前的区域性干旱条件相对应。Elphidium gunteri局限于现代沉积物中，丰富并伴有大量的Ammonia sp.、E. excavatum、O. baragwanathi和L. lacustris。这些资料的综合可表明Coorong 泻湖的主要水条件在全新世的大部分时间发生了极大的变化。

KEY WORDS:

• Coorong Lagoon
• foraminifera
• Holocene
• oogonia
• ostracods
• South Australia

关键词:

• Coorong 泻湖
• 有孔虫
• 全新世
• 轮藻雌器
• 介形虫
• 南澳大利亚

ACKNOWLEDGEMENTS

The analyses of fossil micro-organisms in core Coorong #5 were data acquired by Chantelle Lower as part of her B.App.Sc. Honours research. The core was one of a suite funded by an Australian Research Council Linkage Grant to Peter Gell, David McKirdy and John Tibby, in which the industry partners were the Department of Water, Land and Biodiversity Conservation and the Department of Environment and Heritage, South Australia. Funding for the four radiocarbon analyses of mollusc shells was provided by the University of South Australia. The manuscript was reviewed by Alberto Albani and Peter Gell.