Methods and procedures applied


I. Stable isotope ratio measurements, δ 13C, δ 18O, δ 15N, δ 34S, δ D - (‰ )

  1. Water
    • a.) CO2-H2O isotope exchange method
    • b.) Zn reduction method
  2. Carbonate - acidic digestion of calcite, dolomite and aragonite
  3. Organic material
    • a.) conversion in presence of CuO
    • b.) conversion by the elemental analyzer
    • c.) conversion by the calorimeter bomb
  4. Sulphate and sulphide
    • a.) direct, high
    • b.) temperature conversion of sulphate sulphur or sulphide sulphur to SO2
  5. NH4 and NO3
    • a.) distillation followed by NH3 trapping and N2 production

By combination of the most abundant isotopes of oxygen, 16O (99.7 %) and 18O (0.2%), and those of hydrogen, 1H (99.9 %) and deuterium (2H or D, 0.015%), water molecules (1H216O, 1H2H16O, and 1H218O etc.) of differing mass are produced.

These water molecules have differing vapor pressure During a change of phase (evaporation, condensation, sublimation) enrichment of the lighter isotope will occur in the more volatile phase as opposed to the less volatile phase because these water molecules have different vapor pressure. This effect, known as isotope fractionation, is strongly temperature dependent.

The isotopic composition of a water sample is related to an international standard (SMOW - Standard Mean Ocean Water) and it is expressed as a delta value in parts per thousand (permille). For oxygen the relation can be written as following:

where R is the ratio of the atomic abundance of 18O to 16O.

Ocean water has values of δ18O and δ2H around 0 ‰ relative to SMOW standard. In general, fresh water has negative delta values, as during the evaporation of ocean water the lighter molecules preferentially enter atmospheric water vapor. The δ18O values can, however, also become positive.

In continental precipitation, the δ2H and δ18O values show a linear dependency. This linear slope is called Meteoric Water Line (MWL) and for continental rain has a value of 8 on a δ2H/δ18O plot. The 2H excess expressed in the equation by d, is defined as:

D = δ2H - 8·δ18O = +10 ‰

(intercept on the y axis). Near the coast the value of d is smaller than +10 ‰, in Antarctica δ »0 ‰. In areas where the relative humidity immediately is below the present mean value, d is greater than +10‰. Gat (1970) showed that the deuterium excess at present lies at +22‰ (in the Mediterranean). Examining these results we can say that d can be regarded as a palaeoclimatic indicator.

Seasonal and local variability of δ2H and δ18O values provide indications whether groundwater recharge occurs in a defined recharge area, or diffusely over the entire extent of the aquifer. Paleowater can often be distinguished from recently recharged water. The hydrogen or oxygen isotopic composition may also be used to quantitatively estimate the input of river or irrigation water to groundwater.

Recharge or evaporation rates can be determined from isotope depth profiles of soil moisture, which cannot be done, or much less accurately with other methods.

The principle of stable isotope ratio measurements:

Prior to stable isotope ratio measurements different samples has to be brought in gaseous form. A host of preparation methods have been developed and improved to convert different sample compounds to an appropriate gas including H2, CO2, N2, and SO2.

δ18O measurement

The δ18O value of the water sample is derived from that of the CO2, with which there is an equilibrium. Therefore CO2-H2O isotope exchange method is used by equilibrating the water first with CO2 and then analyzing the CO2. The sample should have pH<4,5 to ensure the fast exchange (approx 3 hours) of oxygen between water and carbon dioxide, through the reaction:

H218O+C16O16O Þ H216O+C18O16O

CO2 in equilibrium with water is about 41.2 ‰ enriched in 18O. Analytical precision on δ18O is usually ≠0.2 ‰.

δ D measurement

Deuterium in water is measured by reducing the water to elemental hydrogen, using Zn. 250 mg BDH ANALAR Zn is introduced into a teflon capped Pyrex ampoule and heated, while the system is evacuated. Cooling the Zn at temperature of -196° C, the water is trapped on its surface. Afterthat the ampoule is evacuated and heated for an hour to 460° C. The hydrogen obtained by reduction can be introduced directly into the mass spectrometer. Analytical precision on δD is usually ≠2-3 ‰.

δ 13C measurement

The measurement of isotopes in carbonate minerals is done on CO2 gas that normally is produced by acidification. The sample is loaded into one of the two “fingers” of an ampoule. The other “finger” contains 3 ml 100% phosphoric acid. The ampoule is evacuated, closed and the phosphoric acid poured onto the sample. The temperature of the vessel is kept at 25o C for 24 hours. The products of the reaction are water, which is trapped cryogenically at -60o C, and carbon dioxide, which is trapped at liquid nitrogen temperature:

CaCO3 + 2H3PO4 Þ CO2 + H2O + Ca(PO4)2

At this point, after pressure adjustment carbon dioxide can be analyzed by the mass spectrometer. This conversion is quantitative for C, and thus as long as all of the CO2 is recovered, there can be no fractionation for 13C. Analytical precision on δ13C is usually ≠0.2 ‰.

δ 15N measurement (NO3-, NH4+)

During sample distillation the dissolved nitrate is reduced to NH3 by addition of Dewarda alloy. The dissolved ammonium is liberated with MgO and in both methods the evolved NH3 is trapped in diluted H2SO4. Oxidation of ammonium sulphate is made by sodium hypobromite. The evolved N2 gas is then analyzed directly on the mass spectrometer against AIR. Analytical precision on δ15N is usually ≠ 1 ‰.

δ 34S measurement

Sulphate sample and NaPO3 are added into a fused quartz tube, covered with quartz wool and heated to 610° C. The copper added into the colder part of the tube will reduce the formed SO3 to SO2. The temperature of the furnace is increased to 1100o C. The resulted gases (SO2, O2, H2O) are trapped into liquid nitrogen. The purification of SO2 is achieved by replacing the liquid nitrogen with a gas-trap at -40o C, in which contaminants are trapped and SO2 can be transferred into the measurement ampoule and connected to the mass spectrometer.

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