Investigation of the stable isotope fractionation in speleothems with laboratory experiments

Wiedner, Elke Anne; Scholz, Denis; Mangini, Augusto; Polag, Daniela; Mühlinghaus, Christian; Segl, Monika

doi:10.1016/j.quaint.2007.03.017

Cited by 70 publications

(44 citation statements)

References 20 publications

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“…3). Using a nucleation seed in the stalagmite growth experiment that consists of a mixture of calcium carbonate polymorphs is favourable to using no seed, simple glass frosting (Mickler et al, 2006) or artificial substrates such as glass fiber (Wiedner et al, 2008) because seeded growth is more representative of natural growth conditions; more specifically because the presence of seed material closely matched to the precipitating material reduces the energy barrier required for solid precipitation to occur (Steefel and Van Cappellen, 1990;Stumm and Morgan, 1996;Lin and Singer, 2005). The seed calcium carbonate was doped with Eu to allow subsequent correction for Ca from the seed when measuring d 44/42 Ca, with Eu/Ca of typically 1.4 Â 10 À6 (atom ratio).…”

Section: Experimental Methodsmentioning

confidence: 99%

Large fractionation of calcium isotopes during cave-analogue calcium carbonate growth

Reynard

Day

Henderson

2011

Geochimica et Cosmochimica Acta

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We have measured d 44/42 Ca of laboratory-precipitated calcite grown in an experimental setup that closely replicates stalagmite formation. Calcium solutions were dripped onto two different substrates in tightly-controlled conditions and calcite precipitated due to rapid CO 2 degassing. With seeded glass slides as the substrate, we observe a Ca isotope ratio in the calcite which is $0.5& per amu lower than that in the growth solution. This fractionation is generally almost twice that observed in previously published calcite growth experiments and indicates a large kinetic effect on Ca isotopes in the stalagmite growth environment. The precipitate forming near the spot where the drip lands shows slightly greater solution-to-precipitate fractionation than calcite further from the drip reflecting a decrease in this kinetic fractionation as precipitation continues. We interpret these results in the context of the model of Fantle and DePaolo (2007) which involves surface entrapment of light Ca isotopes to decrease calcite d 44/42 Ca, and depletion of Ca from the solution in the direct vicinity of the growing calcite to increase calcite d 44/42 Ca. In the stalagmite setting, the second of these effects is minimized so that calcite Ca isotope ratios are unusually light. This interpretation suggests that stalagmite Ca isotope ratios should decrease with the saturation state of the drip water (i.e. with the growth rate of calcite). Ca isotopes might therefore allow reconstruction of surface entrapment of trace metals and isotopes more generally and might, for instance, allow an assessment of the appropriate relationship between oxygen isotope fractionation and temperature for periods of past growth in stalagmites.

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Section: Experimental Methodsmentioning

confidence: 99%

Large fractionation of calcium isotopes during cave-analogue calcium carbonate growth

Reynard

Day

Henderson

2011

Geochimica et Cosmochimica Acta

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“…Calcite growth experiments in a controlled laboratory environment, on the other hand, enable environmental factors to be varied and assessed independently. There have been only three such calcite growth studies in cave-analogue growth conditions (Fantidis and Ehhalt, 1970;Huang and Fairchild, 2001;Wiedner et al, 2008), all of which differ from the natural cave setting in the way in which carbonate is grown. In particular, supersaturation and carbonate growth was induced by combining CaCl 2 with NaHCO 3 .…”

Section: Introductionmentioning

confidence: 99%

Oxygen isotopes in calcite grown under cave-analogue conditions

Day

Henderson

2011

Geochimica et Cosmochimica Acta

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Speleothem oxygen isotopes and growth rates are valuable proxies for reconstructing climate history. There is debate, however, about the conditions that allow speleothems to grow in oxygen isotope equilibrium, and about the correct equilibrium fractionation factors. We report results from a series of carbonate growth experiments in karst-analogue conditions in the laboratory. The setup closely mimics natural processes (e.g. precipitation driven by CO 2 -degassing, low ionic strength solution, thin solution film) but with a tight control on growth conditions (temperature, pCO 2 , drip rate, calcite saturation index and the composition of the initial solution). Calcite is dissolved in water in a 20,000 ppmV pCO 2 environment. This solution is dripped onto glass plates (coated with seed-carbonate) in a lower pCO 2 environment (<2500 ppmV), where degassing leads to calcite growth. Experiments were performed at 7, 15, 25 and 35°C. At each temperature, calcite was grown at three drip rates (2, 6 and 10 drips per minute) on separate plates. The mass of calcite grown in these experiments varies with temperature (T in K) and drip rate (d_r in drips min À1 ) according to the relationship daily growth mass = 1.254 + 1.478 * 10 À9 * e 0.0679T + (e 0.00248T À 2) * (À0.779d_r 2 + 10.05d_r + 11.69). This relationship indicates a substantial increase of growth mass with temperature, a smaller influence of drip rate on growth mass at low temperature and a non-linear relationship between drip rate and growth mass at higher temperatures. Low temperature, fast dripping conditions are found to be the most favourable for reducing effects associated with evaporation and rapid depletion of the dissolved inorganic carbon reservoir (rapid DIC-depletion). The impact of evaporation can be large so caves with high relative humidity are also preferable for palaeoclimate reconstruction. Even allowing for the maximum offsets that may have been induced by evaporation and rapid DIC-depletion, d 18 O measured in some of our experiments remain higher than those predicted by Kim and O'Neil (1997). Our new results are well explained by equilibrium at a significantly higher a calcite-water , with a kinetic-isotope effect that favours 16 O incorporation as growth rate increases. This scenario agrees with recent studies by Coplen (2007) and Dietzel et al. (2009). Overall, our results suggest that three separate processes cause d 18 O to deviate from true isotope equilibrium in the cave environment. Two of these drive d 18 O to higher values (evaporation and rapid DIC-depletion) while one drives d 18 O to lower values (preferential incorporation of 16 O in the solid carbonate at faster growth rates). While evaporation and DIC-depletion can be avoided in some settings, the third may be inescapable in the cave environment and means that any temperature to d 18 O relationship is an approximation. The controlled conditions of the present experiments also display limitations in the use of the Hendy test to identifying equilibrium growth.

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“…In recent years, several studies investigated stable isotope fractionation in speleothems using both in-situ cave (Mickler et al, 2004(Mickler et al, , 2006 and laboratory experiments (Wiedner et al, 2008). In addition, two quantitative models were developed describing the evolution of speleothem d 13 C in dependence on several parameters such as temperature, drip rate, supersaturation with respect to calcite and mixing of the impinging drop and the solution layer on top of the stalagmite (Mü hlinghaus et al, 2007;Romanov et al, 2008a).…”

Section: Introductionmentioning

confidence: 99%