Travertines Associated With Hyperalkaline Springs: Evaluation As A Proxy For Paleoenvironmental Conditions And Sequestration of Atmospheric CO 2

Abstract

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 CO 2 . 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 δ 18 O 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, δ 13 C values reflect potential carbon sources, either from surface runoff waters or atmospheric CO 2 . Within the framework of continental carbonate, calcium carbonate formation in bicarbonate-depleted hyperalkaline environments results in a linear and positive co-variation of δ 18 O and δ 13 C values and defines a unique and distinctive stable-isotope field on a δ 18 O–δ 13 C plot, in contrast to carbonates formed in more typical bicarbonate-enriched environments. Moreover, the combined variations in δ 18 O, δ 13 C, and 87 Sr/ 86 Sr between laminae document the changes in the paleo-activity of hyperalkaline spring and surface runoff waters on the time scale of formation. The 87 Sr/ 86 Sr ratio represents a tracer for quantifying surface runoff water contribution. Furthermore, the amount of CO 2 sequestrated in travertine has been estimated following different scenarios of formation. The calculated CO 2 sequestrated for these deposits ranges from 9 kgCO 2 yr –1 to 522 kgCO 2 yr –1 .