- Copyright © 2016, SEPM (Society for Sedimentary Geology)
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 CO2(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.