Combined use of conventional and clumped carbonate stable isotopes to identify hydrothermal isotopic alteration in cave walls

Abstract

Alteration of conventional carbonate stable isotopes (δ18O, δ13C) in cave walls has been shown to be a useful tool to identify cave formation driven by deep-seated processes, i.e., hypogene karstification. If combined with a prior information on the paleowater stable isotope composition, further insights can be obtained on the temperature and the source of the paleowater. Clumped isotope composition (Δ47) of carbonates is an independent measurement of temperature, and if combined with the conventional stable isotopes, can provide information on the paleowater stable isotope composition. On the example of Provalata Cave (N. Macedonia), we apply for the first time, both conventional and clumped stable isotope analysis, and identify two different isotope alteration trends, reflecting two distinct hydrothermal events: an older, hotter one, where isotope alteration was likely related to isotope diffusion, lowering the δ18O values of the carbonate; and a younger one, related to the cave formation by low-temperature CO2-rich thermal waters, with dissolution-reprecipitation as the alteration mechanism, causing decrease in δ18O values, and unexpected increase in δ13C values. The findings are further corroborated by additional insight from optical petrography and cathodoluminescence microscopy, as well as fluid inclusion analysis of secondary calcite crystals related to the cave forming phase.

Hydrothermal silicification and hypogene dissolution of an exhumed Neoproterozoic carbonate sequence in Brazil: Insights from fluid inclusion microthermometry and silicon‐oxygen isotopes

“…Fluid–rock interaction resulting in modifications of the pristine isotope ratios of cave walls is a common phenomenon in hypogene speleogenesis (Plan et al, 2012; Spötl et al, 2021; Temovski et al, 2022). The most commonly observed pattern of isotopic alteration in carbonate rocks is the lowering of the bedrock δ 18 O values near the cave wall because of the interaction with low‐δ 18 O high‐temperature meteoric waters, as well as the lowering of bedrock δ 13 C values due to interaction with dissolved inorganic carbon with lower δ 13 C values (Dublyansky, 1995; Temovski et al, 2022). Interaction of low‐δ 30 Si and high‐temperature thermal solutions with silica material would result in changes in the Si isotopic ratio, usually towards lower δ 30 Si values (Chakrabarti et al, 2012).…”