Past changes to isotopic and solute balances in a continental playa: cluesfrom stable isotopes of lacustrine carbonates

In many salt lakes around the world, the relative abundance of preserved au thigenic minerals is different from that predicted from solute mass balance calculations. Conventional mass balance models assume that chloride behave s conservatively over long periods of time and fail to take into account th e role of diffusion, deflation and fractional crystallisation/dissolution o f salts. An alternative approach is to use oxygen isotopes as these reflect directly water molecule rather than solute concentrations and have the add ed advantage of providing a palaeohydrological record in lacustrine carbona tes. We present a steady-state, stable-isotope model, in conjunction with s table isotopic measurements of sub-surface brines, regional groundwaters an d carbonate deposits from the Lake Malata-Lake Greenly plays complex in Sou th Australia, to estimate the apparent leakage and palaeoleakage from these superficially closed playa lakes. The steady-state model calculations, usi ng the present delta(18)O and delta(2)H compositions of the lake brines and inflowing groundwater, suggest that the apparent present-day leakage for t he complex is 75 to 90% of inflow (similar to 35 times that calculated from Cl- and Br-). Under such conditions, only low magnesian calcite precipitat es and the lake water experiences reduced effects of evaporation, gas and v apour exchange and, consequently, reduced isotopic and chemical enrichment. Further, our model shows that calcite becomes increasingly Mg-rich until l eakage is reduced to similar to 55 to 70% of inflow - a condition favourabl e for dolomitisation. delta(18)O and delta(13)C of the lacustrine carbonate s show excursions on the order of 5 parts per thousand over the length of a 2.3 m core, indicating thar the lake complex has varied from being through flow dominated (presence of low Mg-calcite relatively depleted in delta(18) O and delta(13)C) to evaporation dominated thigh Mg-calcite/dolomite relati vely enriched in delta(18)O and delta(13)C) throughout late Quaternary. Our estimates of leakage fractions are consistent with the observed mineralogi cal suite, but there remains a discrepancy between apparent closure indicat ed by the presence of the highly saline brine reservoir (< 1% leakage) and high rates of throughflow inferred from stable isotope data. We propose tha t the brine was formed by winter time re-solution of a seasonal halite crus t which forms during summer dominated evaporative discharge. Recirculation of the secondary brine, and mixing with regional groundwater may decouple t he solute cycle from the water cycle in many playa lakes. The end result is a partially mixed brine characterised by nearly conserved solutes but with isotopic signatures indicative of brine-rainfall-groundwater interactions. (C) 2000 Elsevier Science B.V. All rights reserved.