Lake Tyrrell is fed by three different water bodies: reflux brines, regional groundwaters, and flow from Tyrrell Creek, which flows only intermittently, entering the lake at its southern tip. Salinity of the surface water typically ranges from 250-330 g/L, while reflux brines contained in the sub-oxic Parilla Sand aquifer beneath the lake floor have a total dissolved solid (TDS) content of ~250 g/L. Both the surface water and the reflux brines are near neutral pH. Regional groundwater, which enters the lake in the spring zone along the northwestern shore, is oxic with a pH <4, and is of approximately seawater salinity (~36 g/L). Reflux brines from Lakes Timboram and Wahpool enter Lake Tyrrell through springs along the eastern shore; these brines are typically sub-oxic to anoxic, of near neutral pH, and are less saline than Tyrrell brines – about 120 g/L TDS. Tyrrell Creek, although it contributes only slightly to the lake’s water budget, is the most acidic of all waters – its pH is usually 2.3-2.75, and it is also highly saline, ~330 g/L.

    A final feature of the Tyrrell waters is the relatively high iron content. Tyrrell creek water contains ~200 mg/L of soluble iron, and the springs on the western shore up to 40 mg/L Tyrrell brines contain 6-7 mg/L Fe, which is 1000 times higher than in seawater It is primarily this feature, along with the acidity of some of the brines, that renders Lake Tyrrell partially analogous to ancient Martian acid-saline systems.

    The ferricretes that form along the southwestern shore are the result of a mixing of waters: low pH, iron-rich, low salinity waters resulting from a mixing of regional groundwaters and Tyrrell Creek water contact the reflux brines of the lake in the subsurface, resulting in a Ghyben-Herzberg lens. The lower salinity water ‘floats’ on top, and emerges along the shore as seeps.

    We sampled the concretions to determine whether their formation was completely abiotic, or was enhanced or mediated by biotic action. This work is ongoing under the purview of Eric Roden at the University of Wisconsin.