Abstract

Travertine are found in ophiolite massifs in association with bicarbonate-depleted hyperalkaline spring waters (pH up to 11.9), in contrast with most continental carbonates (e.g., travertine, tufa, speleothems) that precipitate from calcium bicarbonate-enriched waters. Here travertines formed from bicarbonate-depleted hyperalkaline spring water were subjected to a multidisciplinary and multi-scale approach to evaluate their potential as proxies of past climatic records and sequestration of atmospheric CO₂.

Two mechanisms of calcium carbonate precipitation were apparent: 1) hydration-hydroxylation reaction due to the mixing of hyperalkaline and surface runoff waters, or 2) dissolution of atmospheric CO_2(g) into hyperalkaline waters.

For two sites, the bulk chemical signature of travertines (Mg, Ca, and Sr wt%) are consistent with “prior calcite precipitation” (PCP) processes and thus likely records the environmental conditions at the time of their formation. However, for the third site, the trace-element concentrations in the various carbonate fabrics indicate some recrystallization. Constant δ¹⁸O values indicate that hydration and hydroxylation reactions completely buffer the oxygen isotope composition of the water (equilibrium state) from which a paleo-temperature can be estimated. In contrast, δ¹³C values reflect potential carbon sources, either from surface runoff waters or atmospheric CO₂.

Within the framework of continental carbonate, calcium carbonate formation in bicarbonate-depleted hyperalkaline environments results in a linear and positive co-variation of δ¹⁸O and δ¹³C values and defines a unique and distinctive stable-isotope field on a δ¹⁸O–δ¹³C plot, in contrast to carbonates formed in more typical bicarbonate-enriched environments. Moreover, the combined variations in δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr between laminae document the changes in the paleo-activity of hyperalkaline spring and surface runoff waters on the time scale of formation. The ⁸⁷Sr/⁸⁶Sr ratio represents a tracer for quantifying surface runoff water contribution. Furthermore, the amount of CO₂ sequestrated in travertine has been estimated following different scenarios of formation. The calculated CO₂ sequestrated for these deposits ranges from 9 kgCO₂ yr^–1 to 522 kgCO₂ yr^–1.