Brief Explanation of Different Types of Isotope Analyses

Chlorine              

Chlorine has two stable, naturally occurring isotopes, ³⁵Cl and ³⁷Cl, as well as many radioisotopes. Chlorine has 17 protons and the two stable isotopes, ³⁵Cl and ³⁷Cl, have 18 and 20 neutrons, respectively. ³⁵Cl occurs naturally in 75.76 % of chlorine atoms and ³⁷Cl occurs in 24.24 % of chlorine atoms.

³⁷Cl can be used as a hydrologic tracer due to naturally occurring fractionation of chlorine isotopes in water. The stable isotopes of chlorine can be particularly useful conservative tracers for examining surface and ground water mixing. For water sample analysis, chlorine is converted to methyl chloride which is analyzed via IRMS to determine the ratio of ³⁷Cl/³⁵Cl (δ³⁷Cl). This ratio is compared to the ratio of ³⁷Cl/³⁵Cl in the standard SMOC (Standard Mean Ocean Chloride) using standard delta notation. Another use for stable chlorine isotope analysis is for the compound specific isotope analysis (CSIA) of chlorinated organic solvents, along with ¹³C and ²H.

Bromine

Bromine has many radioisotopes, but only two naturally occurring, stable isotopes, ⁷⁹Br and ⁸¹Br. Bromine has 35 protons while⁷⁹Br and ⁸¹Br have 44 and 46 neutrons, respectively. ⁷⁹Br occurs naturally in 50.69 % of bromine atoms, while ⁸¹Br occurs in 49.31 % of bromine atoms.

Although bromine exists naturally in seawater at a mass ratio of ~300:1 to that of chlorine, it can still be used as a hydrologic tracer due to the natural fractionation of δ⁸¹Br ranging from 0.00 to +1.80 in water, relative to SMOB (standard mean ocean bromide). Bromine isotopes in water and solid samples are converted to methyl bromide which is analyzed via IRMS to determine the ratio of ⁸¹Br/⁷⁹Br (δ⁸¹Br).

Strontium

Strontium has four stable, naturally occurring isotopes: ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr, with relative abundances of 0.56 %, 9.86 %, 7.00 % and 82.5 %, respectively. Strontium has 38 protons while ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr have 46, 48, 49, and 50 neutrons, respectively. Although ⁸⁷Sr occurs naturally in 7.00 % of strontium atoms, it is also produced by radioactive decay from ⁸⁷Rb (37 protons and 50 neutrons).

Due to this property, the ratio of ⁸⁷Sr/⁸⁶Sr is useful for the determination of the age of rocks and minerals. ⁸⁷Sr/⁸⁶Sr can also be used to track water flows due to the fact that any strontium found in a groundwater or seawater sample will have the same isotopic composition as the mineral it was weathered from. As a result, naturally observed ranges in ⁸⁷Sr/⁸⁶Sr values, are caused by either differences in mineral composition along different flow paths, or differences in the amount of strontium weathered from the same mineral in different water samples. ⁸⁷Sr/⁸⁶Sr values are often affected by the mixing of several different mineral sources, making it difficult to distinguish these sources using strontium ratios alone. When combined with other isotope data and water chemistry, however, ⁸⁷Sr/⁸⁶Sr can be a very useful tool for solute source apportionment. Strontium isotopes are analyzed using a very precise mass spectrometer called a TIMS (thermal ionization mass spectrometer).

Tritium

³H, or tritium, is a radioactive isotope of hydrogen. Tritium has one proton and two neutrons, possessing a half life of 12.43 years. Due to tritium’s short half life it is an ideal hydrologic tracer for studying processes such as the mixing and flow of water on time scales less than 100 years. The amount of tritium in water samples is expressed in tritium units (TU) where 1 TU is defined as 1 ³H atom in 10¹⁸ hydrogen atoms.

Concentrations of tritium in the atmosphere increased drastically after the advent of thermonuclear bombs and their subsequent testing in the 1950s and 1960s. Before the advent of thermonuclear weapons, the average amount of tritium in the atmosphere was 2-8 TU, but by 1963 the concentration had increased to almost 6000 TU. The concentration has decreased gradually since then, due to the termination of atmospheric thermonuclear testing. Because of nuclear testing, water with higher tritium content must have been derived after 1953 and can be used as a marker for water recharge.

Tritium amounts are determined using a method called liquid scintillation counting. In this method, samples are dissolved in a solvent which contains a surfactant and other additives, known as scintillators. Radioactive emissions known as beta particles are emitted from the tritium in the sample transferring the energy to a solvent molecule in solution. The energy is eventually transferred to a scintillator molecule which emits a photon (light particle). The wavelength of the emitted light is known and can be measured. By counting the number of photons emitted in a given timeframe, the number of tritium atoms can be calculated using the half life.

Carbon

Carbon has two stable isotopes, ¹²C and ¹³C, with relative abundances of 98.89 % and 1.11 %, respectively. Carbon atoms have 6 protons and ¹²C has 6 neutrons while ¹³C has 7 neutrons. Carbon isotopes can be analyzed for a number of natural compounds, including organics, carbonates, dissolved inorganic carbon (DIC), or dissolved organic carbon (DOC).

Applications of carbon isotopes can include the determination of the photosynthetic pathways that a plant uses. Atmospheric δ¹³C is -7 ‰ relative to the international standard VPDB. When plants fix carbon during photosynthesis ¹³C is depleted relative to atmospheric levels resulting in more negative δ¹³C values. Depending on whether a plant uses a C3, C4 or CAM photosynthetic pathway will determine the range of δ¹³C values it will possess. δ¹³C of carbonates can also be used to determine how rocks and minerals are formed.

δ¹³C of dissolved inorganic carbon (DIC) can also be a useful tool to determine information on the weathering of carbonate or silicate materials by acid rain or other processes. This information can be used to understand the reactions affecting alkalinity in water. Using the δ¹³C values in natural systems for assessing the proportion of DIC derived from these processes can be complicated by the various carbon exchange and cycling processes that will induce fractionation and alter these values. δ¹³C values for DIC should be correlated with other tracers, such as, ⁸⁷Sr/⁸⁶Sr and δ¹⁴C to obtain a more detailed understanding of these natural processes. δ¹³C of dissolved organic carbon (DOC) can also be used to gain information on how organic carbon moves during plant and sediment cycles. This information can be used to help determine plant productivity in the past and present. δ¹³C analysis is also a potentially impactful tool in CSIA, for examining: hydrocarbons, chlorinated solvents, and various other organic contaminants.

Sulfur

Sulfur has four stable isotopes: ³²S, ³³S, ³⁴S and ³⁶S with relative abundances of 95.02 %, 0.75 %, 4.21 % and 0.02 %, respectively. Sulfur atoms have 16 protons and ³²S, ³³S, ³⁴S and ³⁶S have 16, 17, 18 and 20 neutrons, respectively. δ³⁴S is reported in ‰ relative to the international standard VCDT and is the ratio of ³⁴S/³²S.

δ³⁴S values can be affected by the reduction of sulfate to sulfide by anaerobic bacteria or by a number of exchange reactions where ³⁴S is concentrated in the compound in the highest oxidation state. δ³⁴S values are often used to understand how sulfide ore deposits are formed, as ore formed by sedimentary processes will have more negative δ³⁴S values, while ore produced by igneous processes will have a δ³⁴S close to 0.0 ‰. Combining the analysis of δ³⁴S-SO₄ with δ¹⁸O-SO₄ increases the sensitivity for sulfate source separation.

Nitrogen

Nitrogen has two stable isotopes: ¹⁴N and ¹⁵N with relative abundances of 99.634 % and 0.366 %, respectively. Nitrogen atoms have seven protons and ¹⁴N and ¹⁵N have 7 and 8 neutrons, respectively. δ¹⁵N values are reported in ‰ relative to the nitrogen composition of the air which stays constant.

δ¹⁵N values can be used for a variety of purposes, including understanding biological reactions such as nitrification and assimilation which results in enrichment of the substrate and depletion of the product material. The isotopic values of δ¹⁵N (along with δ¹⁸O) of nitrates can also be used to look at: the extent of nitrate contamination and cycling in surface and ground waters, assess the physical and biological processes controlling these concentrations, and apportion any unique sources contributing to nitrate loading.

Furthermore, δ¹⁵N analysis can be used on a variety of organic sample types (i.e. plants, soils, sediments, phytoplankton, etc.). These values are used, often with simultaneous δ¹³C determination, to evaluate food web structures, differentiate marine and terrestrial organic matter sources, assess N cycling between biological compartments, and for paleo-environmental reconstructions.

Carbon-14

Carbon-14 is a radioactive isotope of carbon with a half life of 5730 years. It has 6 protons and 8 neutrons and is produced from the reaction of cosmic rays with ¹⁴N in the atmosphere. ¹⁴C values are usually reported as a percentage of modern carbon-14.  ¹⁴C data, along with δ¹³C values, can help to identify and confirm how carbon is transported in streams or groundwater systems. ¹⁴C is also useful in dating groundwater; due to its half life of 5730 years, it is most useful at dating water between 1 000 and 40 000 years.

Oxygen

Oxygen has three stable isotopes: ¹⁶O, ¹⁷O and ¹⁸O with relative abundances of 99.63 %, 0.0375 % and 0.1995 %, respectively. Oxygen has 8 protons while ¹⁶O, ¹⁷O, and ¹⁸O has 8, 9, and 10 neutrons, respectively. δ¹⁸O values are reported relative to VSMOW (Vienna Standard Mean Ocean Water). Oxygen isotopes can be used to better separate samples containing oxygen bonded to another element, including, nitrate, sulfate, carbonates, phosphate, and silicates. The use of two isotopes results in better isotopic separation of different samples, allowing for a greater understanding of nutrient cycling, and water transport processes.

The isotopes of water (δ²H/δD and δ¹⁸O) can be used to track precipitation.  Precipitation data from around the world has been used to produce the Global Meteoric Water Line (GMWL) which shows the relationship between the isotopes of water: δD = 8*δ¹⁸O + 10. The isotopic composition of precipitation is determined by the temperature at which the precipitation condenses, and the amount of water vapour already condensed in the air relative to the initial amount of water vapour in the air. As a precipitation source moves across a landmass it will release the heavier isotopes first, resulting in successively lighter rain events. Since precipitation is the main source of groundwater recharge, hydrogen and oxygen tracers of water are a good way of tracking groundwater movement and recharge potential.

Sources

USGS — Isotope Tracers — Resources. (2003, August). Retrieved from http://wwwrcamnl.wr.usgs.gov/isoig/period/index.html

Sharp, Z. (2007). Principles of stable isotope geochemistry. Upper Saddle River, NJ: Pearson/Prentice Hall.

Eggenkamp, H. (2014). Geochemistry of Stable Chlorine and Bromine Isotopes. Heidelberg: Springer.