Marine diagenesis of ikaite: Implications from the isotopic and geochemical composition of glendonites and host concretions (Palaeogene–Neogene sediments, Sakhalin Island).

Glendonites represent pseudomorphs after calcium carbonate hexahydrate (ikaite) and can be used as indicators of past cold climates, because ikaite only naturally occurs in cold environments (<7°C) in the modern. The results of a multi‐proxy study of 11 glendonite and host concretion samples from Palaeogene (Gennoishi Formation) and Neogene (Bora and Vengeri formations) sediments from Sakhalin Island, Russian Far East are reported here. Petrographic, cathodoluminescence, powder X‐ray diffraction and scanning electron microscope analyses reveal that glendonites are composed of several successive mineralogical phases: low‐magnesium ikaite‐derived calcite, high‐magnesium acicular cement (calcite and dolomite) and blocky calcite or authigenic quartz filling the remaining pore space. Host concretions comprise sandy limestones cemented by high‐magnesium calcite. The δ13C and δ18O values for host concretions and enclosed glendonites are very similar, with δ13C ranging from –20.3 to +1.9‰ Vienna Pee‐Dee Belemnite and δ18O ranging from –3.4 to +3.4‰ Vienna Pee‐Dee Belemnite. Such isotopic values suggest that seawater was the main source of oxygen, while dissolved inorganic carbon and decaying organic matter were the main carbon sources for ikaite growth, concretion and glendonite cementation. The 87Sr/86Sr values within the glendonites and host concretions are significantly lower compared with coeval Palaeogene–Neogene marine carbonates, suggesting an influence of continental runoff on their isotopic composition. Post Archean Australian Shale normalized rare earth element patterns display negative Ce anomalies and positive Eu anomalies, with a depletion in light rare earth elements or bulge in middle rare earth elements. Such spectra of rare earth elements indicate that ikaite growth and replacement occurred very close to the aerobic–anaerobic boundary, with pore waters derived from trapped seawater and/or ikaite dehydration. Since Mg/Ca ratios and alkalinity increase with depth below the sediment–water interface, subsequent mineral phases show enrichment in Mg, while the lack of diagenetic alteration by basinal fluids enabled preservation of the primary isotopic and geochemical characteristics of ikaite within the recrystallized pseudomorph. [ABSTRACT FROM AUTHOR]