Mineralogy and stable isotope composition of Recent carbonate sediments and waters from five small lakes, south-eastern South Australia

Abstract

The Robe-Woakwine corridor in southeast South Australia, contains a scattering of lakes of varying size. The coastal lakes selected for study are amongst the smallest Recent carbonate sediment (sub-aqueous surface and sub-surface sediment) and water samples were collected monthly over 6 months and analysed for their stable carbon and oxygen isotopic composition and mineralogy.

Lake Woolley is an ephemeral lake. Its water is hyper-saline(TDS {total dissolved solids} 40g/L). The 18ODIC (DIC= dissolved inorganic C) and 13CDIC of the lake water ranges from -4.2%0 to approximately +l .3%0and the 18O and 13C of the carbonate sediment ranges from +2.2%0 to +4.2%0. The mineralogy of the sediment is aragonite and low Mg-calcite (LMC) with high Mg-calcite (HMC).

Fresh Dip's water is the least saline of the lakes studied (TDS 4g/L). The 18ODIC and 13CDIC of the waters have remained relatively constant at approximately +I%0 and -1%0 respectively, as have the 18O and 13C of the carbonate sediment, with values of approximately +2%0 and +4%0 respectively. Mineralogy is predominantly aragonite.

Old Man Lake's water is hyper-saline (TDS 30g/L) with 18ODIC values of +5%0 to -5%0, whereas 13CDIC remains constant at -2%0. 18O and 13C of the sediment range between +3%0 and +4.7%0 and 0.5%0 to +2.3%0 respectively. Mineralogy is intermediate Mg-calcite (IMC), LMC and aragonite.

Lake Amy and Ghost Lake are both hyper saline (TDS ~70 & 25 g/L respectively). Ghost Lakes is an ephemeral lake, being completely dry in April. 18ODIC and 13CDIC values range between -2.2%0 to 0%0 and -4.4%0 to 0%0 respectively. 18O and 13C of the carbonate sediment range from +1.8%0 to +3.5%0 and -5%0 to 5.3%0 respectively. Mineralogy is predominantly HMC with LMC and aragonite. Lake Amy's water's 18ODIC and 13CDIC range between -4.1%0 and +2.3%0 whereas the sediment 18O remains constant at +2%0 and the 13C range between +1.0%0 and +4.1%0. Mineralogy is predominantly HMC with LMC and aragonite. Traces of the unstable carbonate monohydrocalcite (MHC) also occur.

Each lake is influenced by differing combinations of water inflow. These control the salinity, lake level, precipitate mineralogy, biological activity and isotopic signature of the various components.

The DIC isotopic composition of the waters generally represent regional groundwaters which have undergone atmospheric equilibrium fractionation ( CO2(g) - HCO3-). The stable isotopic signature of the carbonate sediments corresponds to values representing inorganic precipitation at an intermediate rate. Calculated sediment equilibrium isotopic values generally compared unfavourably with those observed, reflecting the unique nature of each lake.