Geochemical recording mechanisms related to kinetic processes during travertine precipitation

Léonora Fleurent – 2015

The response of continental hydrosystems to recent climate fluctuations can be reconstructed via the continuous measurement of groundwater discharge and climatic chronicles. In the case of no available data, recent environmental reconstructions can be reached through various recorders of environmental conditions such as travertine. However, if the relationship between geochemical records in travertine and environmental parameters seems to be accepted, the details of processes including kinetic and degassing, and their respective weights in paleoenvironmental reconstructions are not clearly established. To better constrain the process, a meticulous recognition of precipitation mechanisms and the geochemical signature of registration was performed on travertine –recent and fossil- precipitating from acarbo-gaseous spring in the Massif Central (France). It work couple innovative laboratory experiments and field work seriated.

Rate of degassing of CO₂ in rich spring is considered to influence the rate of calcite precipitation and the stable isotope δ¹⁸O and δ¹³C composition. The isotopic equilibrium is rarely maintained during the deposition of travertine and the rate of CO₂ degassing may be the main controlling factor of the disequilibrium. Because of the lack of knowledge, the fractionation processes (kinetic or equilibrium) occurring between the rich- CO₂ water, gas and travertine, specific pH and temperature-controlled laboratory tests were conducted in two stages: (1) characterization of the only degassing mechanisms, and (2) combining degassing experiments and precipitation to better understand the kinetic processes during precipitation of travertine. In parallel, a specific field work was led by laying Plexiglas substrate along different flow line and has allowed us to know the resolution of travertine precipitation and obtain a better estimation of fractionation factors.

These experimentations confirm that during a degassing leading to travertine precipitation, the speciation is an important parameter to take into account, in fact, a linear relationship between εDIC- CO₂ (g) is observe. We also prove that, for a high rate of degassing, the isotopic equilibrium is not reached because the reaction involving the light isotope is greater than that involving the heavy isotope. Thus, there is different rate of reaction between the species of dissolved inorganic carbon; the reactions than occur in the water are quicker than the reaction between water and gas, which is diffused controlled. These conclusions were applicable to travertine that precipitate on Ours spring (French Massif Central).

Field results help us to demonstrate that changes in precipitation conditions are mainly due to changes in hydrodynamic conditions and secondarily to temperature changes, which could be modeled using the equation defined by Plummer in 1968 and showed the variability of precipitation conditions in situ. In the case of laboratory experiments with precipitation compared with the field data, the εDIC-calcite fractionation factor will depend on the concentrations of each species of CID but also carbon partition between gas phase and solid phase. For oxygen 18, from 2°C to 10°C, the results achieved on current travertine have allowed us to use the right Coplen recently defined (2007) to link the temperature reliably at precipitation time and fractionation factor. However, for higher temperatures, the fractionation factor is more important than it should be and confirming unrepresentative of Coplen equation for determination of precipitation temperature. Although, for precipitation experiments the trend is opposite to that observed for the Ours spring travertine confirming the influence of the speed of degassing and the saturation index of isotopic fractionation. We also confirmed with the experiments, the impact of the precipitation rate on the isotopic contents recorded in travertine. This work has highlighted the complex kinetics and mechanisms that are not completely constrained only by the use of isotopic tools. Using the partitioning of trace elements is an essential tool to support the study of these mechanisms.