We determined the solubility of ikaite from 0° to 25°C to model its saturation state in natural waters and test the hypothesis that it is the precursor of the calcite pseudomorphs in thinolite tufa of Quaternary Lake Lahontan. Reversible solubility at buffered $P_{CO_{2}}$ yields the following expression for the dissolution constant of ikaite: $log K_{ikaite} = 0.15981 - 2011.1/T$. Where $T = ^{\\circ}K$, and $0^{\\circ}C \\leq t \\leq 25^{\\circ}C$. Derived standard state properties are $\\Delta G_{ikaite}^{\\circ} = -2541.9 kJmol^{-1} \\pm 0.66; \\Delta H_{ikaite}^{\\circ} = -2973.4 kJmol^{-1} \\pm 1.02; S_{ikaite}^{\\circ} = 306.6 JT^{-1}mol^{-1} \\pm 1.2$. Modeling shows that ikaite is undersaturated at all temperatures in seawater and in alkaline lakes, but that it rapidly approaches saturation near 0°C. Its precipitation in near-freezing marine sediments requires large additions of $HCO_{3}$ to pore fluids from the diagenetic decomposition of organic matter. Its crystallization in tufas of alkaline lakes, however, requires only small additions of Ca from springs. Simple kinetic experiments show that ikaite is stabilized in natural environments by orthophosphate, which prevents the crystallization of the more stable anhydrous forms of $CaCO_{3}$ but does not interact with the ikaite. Therefore, the presence of ikaite or its pseudomorphs is an indicator of near-freezing conditions in environments with high concentrations of orthophosphate. If ikaite is the precursor of thinolite tufa, then the thinolite likely grew below the sediment-water interface at the site of sublacustrine springs during prolonged cold periods.