Isotope Ratio Mass Spectrometry (IRMS) Case Studies
Anatoly Chernyshev, Khaled Melghit, Z. Naturforsch. C, May 2015
Dyerophytum indicum Kuntze (Arabic: ‘mellah’ — salty), belonging to the Plumbaginaceae family, is an erect shrub up to 2 m tall, distributed in the Arabian Peninsula and Western India. In Oman, it is a common plant of wadis (desert and mountain erosion valleys of intermittent streams) at altitudes of up to 1800 m.
Leaves of D. indicum are covered with friable white mineral coating (see above) described in the literature as a mere salt, even usable for cooking. However, based on our own experience in the field, the coating is tasteless, insoluble in water, and contains carbonates. The apparent discrepancy of the mineral composition with that reported in the literature led us to perform an initial chemical characterization of this coating.
The samples of mineral coating from leaves of D. indicum were collected from two sites in the Sultanate of Oman: the water-bearing floor of Wadi Muaiden (N23º 0.0'; E57º 39.5'), and from a wadi near Muscat (N23º 27.7'; E58º 20.5'), in December 2010. Average daily temperatures at the time of collection were +15ºC to +27ºC. The coating was separated from the leaf surface with a soft brush, and did not undergo any further treatment to avoid solid phase reactions. The analyses were performed within a week from the collection of samples.
We performed IRMS analysis for 13C/12C and 18O/16O isotope ratios in the mineral phase collected from two plant specimens and compared the results with those obtained for wadi water evaporite, surrounding mineral dust and calcareous rocks, as well as with literature data on atmospheric CO2 (see table below). The concentration of either 13C or 18O in the coating is higher than in any possible environmental carbonate source, which indicates involvement of the plant in the formation of the mineral.
Based on the critical analysis of XRD and EDS data (see the original article), the composition of D. indicum mineral coating can be best described as having three major mineral phases: monohydrocalcite (~13 wt.%), nesquehonite (~38 wt.%), and calcite (~45 wt.%); a minor hydromagnesite phase, and traces of silica and sylvite. The presence of lansfordite, aragonite and dolomite, though possible, requires additional experimental evidence.
Based on our and published data, we suggest the following scheme for D. indicum biomineralization:
First, the plant produces an exudate with elevated concentrations of calcium or magnesium, and hydrocarbonate. If the major cation is Ca2+, monohydrocalcite precipitates first, according to the mechanism suggested by Fischbeck and Müller in 1971, leading to an increased Mg2+/Ca2+ ratio. This ratio facilitates the precipitation of nesquehonite (or lansfordite at low temperatures) and small amounts of aragonite. Contact with air is reported to be essential for monohydrocalcite formation. When brine contained high concentrations of Mg2+, the major precipitate was nesquehonite, in the same proportion as in the D. indicum mineral coating, i.e. 38%. Monohydrocalcite then gradually dehydrates either to calcite, or to aragonite. Aragonite forms in wet atmosphere, at temperatures from –2 to +50ºC. In dry atmosphere, or in wet atmosphere containing ammonia, the major product of dehydration is calcite. Further maturation of the mineral mixture might involve such reactions as transformation of nesquehonite to hydromagnesite, dissolution of nesquehonite in the exudate and subsequent crystallization of monohydrocalcite and dolomite, and lastly, repetitive recrystallization of the participating mineral phases leading to heavy isotope enrichment.
Overall, the discovered case of plant biomineralization is highly unusual, both with respect to the mineral phase and isotopic compositions. The mineral mixture resembles mostly the product expected from a physical crystallization of a mixed cation carbonate solution. However, its isotopic content strictly calls for the participation of the host plant in the process.