Isotope Geochemistry

Stable isotope fractionation. Many powerful tools in biogeochemistry are based on stable isotopes. Isotopes are atoms of the same element but of different mass, for example ¹⁸O and ¹⁶O. The mass difference leads to small differences in the properties of isotopic molecules, which, for instance, causes small enrichments or depletions of the light or heavy isotope in different chemical compounds. This is called "isotope fractionation". The differences are usually of the order of per mil (‰; Greek symbol 'delta').

For example, there is a little bit more ¹⁸O than ¹⁶O in CaCO₃ compared to the water from which the CaCO₃ formed. The ratio is a function of temperature, which provides a means of taking Earth's temperature in the past (based on fossil CaCO₃). In addition to temperature, also the growth and decay of the continental ice sheets during glacials/interglacials affect the oxygen isotope ratio preserved in carbonates (via the seawater ratio). ¹⁶O is preferentially deposited on the continents and removed from the ocean when sea level drops during glacials.

The oxygen isotope ratio of carbonates allows reconstruction of temperature and continental ice volume changes, which have varied periodically over the past 2 million years (data is available from Lisiecki and Raymo). The pacemaker behind the major ice age terminations is still somewhat of a mystery; see Schulz and Zeebe (2006) under Publications.

Sediment Geochemistry. Sediments play an important role in the global biogeochemical cycling of the elements. For instance, deep-sea carbonate sediments are an essential component of the global carbon cycle. Projections of future fossil fuel neutralization and thus atmospheric CO₂ levels depend on the sea-floor CaCO₃ that is available to dissolution. Other examples include methane hydrate reservoirs in marine sediments, which have recently received strong attention because of their relevance to the carbon cycle and climate.

The term 'methane hydrate' describes a mixture of ice and gas in which methane molecules are trapped within the crystal structure of ice. They can form in deep-sea sediments under certain temperature and pressure conditions and are often found at continental margins. Methane hydrates have been referred to as 'burning ice' (more).

In certain areas of the ocean, methane is released into the porewaters of deep-sea sediments and the water column, where it can be utilized by microbes and converted into CO₂. For example, in areas such as the Hydrate Ridge (Cascadia Margin, off Oregon), the porewater chemistry is dominated by anaerobic oxidation of methane by microbial consortia. In addition to its effect on the chemistry, methane oxidation also dominates the stable carbon isotope composition of dissolved inorganic carbon in porewater (delta-C-13).

Biogenic methane is isotopically 'light', that is, strongly depleted in C-13. When the methane is oxidized to CO₂, also the dissolved carbonate species become isotopically 'light'. The figure shows model results (lines) for the carbon isotope composition of porewater carbonate species and data (d¹³C of TCO₂, squares) at Hydrate Ridge. Due to the effect of methane oxidation, porewater carbonate species can be depleted by more than 50‰ relative to the bottom water; see Zeebe (GCA, 2007) under Publications.

Sediment-modeling studies complement our endeavors to decipher isotope phenomena in biogenic CaCO₃ (see Active Research Grants, National Science Foundation: OCE05-25647).

Molecular Theory. The term isotope, which in Greek means equal places, reflects the fact that different atoms can occupy the same place in the Periodic Table. For example, hydrogen (¹H) and deuterium (D = ²H) have nearly equal chemical properties, but D has twice the mass of H. For many other isotope systems, the mass difference is much smaller (e.g. only 13% for ¹⁸O and ¹⁶O).

Equilibrium isotope fractionation between chemical compounds is caused by differences in the frequencies of the fundamental motions of isotopic molecules - just as the oscillation frequency of two objects connected by a spring depends on the masses of the objects. The animation shows the "out-of-plane" mode of boric acid (calculations: GAMESS visualization: Molekel). From the molecular frequencies, isotope fractionation factors can be calculated that are used in geochemistry; see Zeebe (2005) under Publications for boron fractionation factors.

Current projects focus on understanding and predicting isotope fractionation including elements such as H, B, C, O, P, S, and Ca.

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