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.