Analytical methods

for stable isotope content analyses, water, gas and carbonate samples were con-verted into co₂ by direct acidification, and the ¹³c contents of carbon species were measured by mass spectrometry (sIra) at the Ides laboratory (university of paris sud, france). the ¹³C content is reported using conventional δ notation, as a devia-tion from the V-pdB (Vienna-Belemnitela from the pee dee formadevia-tion, north caro-lina, usa).

concerning ¹⁴c measurements, all the dating was performed on in situ depos-ited left remains. The specific treatment for organic samples was applied following the standard alkali-acid-alkali protocol to avoid any contamination either by humic acids or by atmospheric co₂. the co₂ gas produced after burning was reduced on iron powder (Ides laboratory) and graphite sources were measured with the accel-erator mass spectrometry (arteMIs) of the uMs lMc14 in gif-sur-yvette (Insu-cnrs france). the ¹⁴c activities are expressed as percentages of modern carbon (pMC) normalized to a δ¹³c = –25‰.

4. results

the stable isotopic composition of the spring water is –9.6‰ and –62.8‰ vs V-sMoW for ¹⁸o and ²H respectively.

Stable isotope contents of finely laminated carbonates comprise between 5.8‰

and 7.6‰ for ¹⁸o, and from –7.9‰ to –6.7‰ for ¹³c, with mean values of +6.6‰ and –7.2‰ respectively. these results display a ¹³c enrichment (up to +8‰) with respect to the isotopic composition of the spring tdIc (total dissolved Inorganic carbon).

this enrichment may be due to either degassing of the dissolved co₂ from the run-ning spring water, the temperature during the deposition and/or to precipitation frac-tionation during the solid carbonates deposits [1].

BarBecot et al.

FIG. 2. Evolution of the stable isotope contents of carbonated deposits from the Bear Spring with depth. Plain circles represent sampled levels for ²³⁸U/²³⁴U/²²⁶Ra analyses (ongoing).

The grey line indicates the variability of the data after a two-step running mean calculation.

fIg. 3. Evolution of the ¹⁴C activity in atmospheric CO₂ (normalized at 25‰) since 1950.

90 100 110 120 130 140 150 160 170 180 190

1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006 2010

Time (yr) Normalized atmospheric CO2 (pMC)

IAEA-CN-186/121

preliminary results indicate that the organic residues come from c3-plants, with a mean δ¹³C of –26‰ vs PDB [3]. A first ¹⁴c activity of 118 pMc performed on in situ leaves at 65 cm depth indicate that the time period covered by our core is of a few decades (figure 3).

5. dIscussIon and conclusIon

for carbo gaseous waters, the quantity of co₂ dissolved at great depth controls the carbonate system during water recovery to the surface through the calcareous de-posits of the limagne plain. It evolves according to the equilibria of the calco car-bonic system. the increase of ca²⁺ during the rising up of the water is accompanied by a progressive saturation of the solution with respect to calcium carbonate.

The solid carbonates precipitates along the surface flowpath originate from water with a nearly constant ¹⁸o isotopic composition, highlighting the inertia of the system that gives rise to the springs. In such a geochemically homogenized water system, the stable isotope contents of the carbonates issued from precipitation reflect the mean isotopic composition of water, with respect to the thermodependant frac-tionation factors. the deviation between isotopic compositions of water and carbon-ates, taking into account the water-calcite fractionation and its related variability, can give information on the temperature during carbonate precipitation.

Preliminary calculations indicate a temperature significantly lower than the mean annual temperature of this area for carbonate precipitation, which is an aberration for the time lag covered. other processes have then to be considered to explain this anomaly such as (1) a biofilm intimately mixed with carbonates, (2) ki-netic fractionation, (3) the evolution of the isotope signature of the water between the emergence and the zone of precipitating carbonates, due to either evaporation, impact of hydroxide precipitation preceding carbonate precipitation, or the variability of the aquifer reservoir, and/or (4) the recrystallisation or mixing of different carbon-ated phases.

Whatever the processes involved, a significant evolution in the environmental signal is highlighted on δ¹³c signatures. this is clearly related with the geochemical process of rayleigh distillation during water degassing and carbonate precipitation [4–6]. We may infer two superimposed fluctuations: the first one, cyclic, is probably annual to biannual, while the second one, linear, implies geochemical processes in relation with a long term evolution of the system.

as a conclusion, the chronology needs to be more deeply investigated through the comparison of ¹⁴c activity measurements and of ²¹⁰pb/²²⁶ra dating [7–8], in order to discriminate short hydrological events/peaks. Moreover, associated stable isotope contents of these finely laminated carbonated samples will be compared with histori-cal records such as temperature and precipitation, in order to identify the key param-eters of the geochemical differentiation footprinted in the solid carbonates.

BarBecot et al.

REFERENCEs

[1] BelKessa, r., données Hydrogéologiques acquises en limagne, rapport 78 sgn 087 Mce (1978) 29.

[2] Bureau de recHercHes géologIQues et MInIères, dIVIsIon na-tIonale des eauX MInérales et tHerMales, les eaux minérales et le gaz carbonique, note technique dneMt n°10 (1997) 23.

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[6] scott, K.M., lu, X., caVanaugH, c.M., lIu, J.s., optimal methods for esti-mating kinetic isotope effects from different forms of the rayleigh distillation equa-tion, geochim. cosmochim. acta 68 3 (2004) 433– 442.

[7] condoMInes, M., rIHs, s., first ²²⁶ra–²¹⁰pb dating of a young speleothem, earth planet. sci. lett. 250 (2006) 4–10.

[8] condoMInes, M., BrouZes, c., rHIs, s., radium and its daughters in hy-drothermal carbonates from auvergne (french Massif central): origin and dating applications, cras serie II-a (1999), Vol. 328–1, 23–28.

mATRIx PORE WATER IN LOW PERmEABLE