Glendonites as Paleoclimatic and Paleoceanographic Indicators: A Case Study from the Glacially Influenced Permian System of Eastern Australia*

Glendonites, pseudomorphs after ikaite (CaCO3. 6H2O), feature prominently in Permian glaciomarine strata of eastern Australia. Because ikaite formation requires near-freezing temperatures, high alkalinity, and elevated orthophosphate, the presence of glendonites implies a particular array of paleoenvironmental conditions. The utility of glendonites as paleoenvironmental proxies was assessed via petrographic and isotopic study. The glendonites possess a granular internal fabric consisting of concentrically zoned calcite grains that “float ” in a matrix of enclosing calcite cements. The zoned calcite grains represent the ikaite replacement phase, based on the ubiquitous presence of such grains in modern and ancient glendonites from many localities and their petrographic relationships with enclosing phases. The ikaite replacement phase possesses the highest δ18O and lowest δ13C values, which lie at one end of a trend toward progressively lower δ18O and higher δ13C values of enclosing phases. The low δ13C values implicate organic matter, and possibly methane, as the main carbon source for ikaite precipitation. The results are consistent with observations from the modern, which suggest that ikaite is an early diagenetic phase that forms in the zone of suboxic diagenesis in organic matter (OM) rich sediments. Given ikaite’s proclivity for forming in OM-rich sediments, glendonites may be considered useful indicators of paleoproductivity, paleo-upwelling, and potential hydrocarbon source rocks. In addition, the δ13C values from the ikaite replacement phase may also be useful indicators of the presence or absence of paleomethane seeps. Interpretations of oxygen isotope data from the ikaite replacement phase reflect precipitation from cold waters, but mor