Chemostratigraphy:

Chemostratigraphy, broadly speaking, is the application of global, regional, and local variations in geochemistry over time to the decipherment of the rock record. This embraces a range of techniques. A well-known example being the identification of the Cretaceous-Paleogene boundary by means of a global iridium spike (right). More commonly, however, chemostratigraphy deals with variation in stable isotope ratios of common elements.

Stable isotopes:

Certain isotopes are stable in nature (i.e. they do not radiometrically decay) and have well defined ratios in oceans and atmosphere. Physical and biological processes can redistribute isotopes, providing clues to ancient environments, climates, and biologic communities and changes in them. Thus, when environmental changes are "secular" (global in scope) they can facilitate detailed correlation of widely separated deposits.

Isotopes: Atoms whose nuclei contain the same # of protons, but different # of neutrons E.G.:

There are ~300 stable isotopes in nature, ~1200 unstable (radiogenic). Only 21 elements are "pure" and have just one stable isotope. All others have ≥2.

Isotope effects: Differences in chemical or physical properties which arise from variations in the atomic mass of an element.

Not directly observable, but can be inferred from its effect on isotopic abundances.

Fractionation: Differential incorporation of isotopes into a molecule as a result of an isotopic effect.

Delta notation:

Isotopic data usually considered in terms of ratios of heavy to light isotope. Most differences between natural samples occur at and beyond the third significant figure of the isotope ratio, therefore a differential notation is used:

δnX = [(Rsample/Rstd)-1] x 103 Where:

E.G.: For carbon: δ13C = [((13C/12sample)/(13C/12Cstd)) -1] x 103

Standards: Delta notation compares the ratios of isotopes in a sample to a standard with a known ratio:

Delta notation is expressed as parts per thousand, or permil units (‰)

E.G.:
  • a limestone with a δ18O value of -2.1‰ has more 16O than PDB
  • a foraminiferan with a δ18O value of +10.5‰ has more 18O than PDB and the limestone.

    Stable isotope geochemistry is primarily concerned with variations in isotopic ratios of H, Li, C, N, O, Si, Sr, and S. Their common characteristics:

    Note: Do not confuse δ notation with Δ ("cap-del") notation. The latter measures the distance of the measured ratio of a sample for two isotopes from a regression-line.

    Oxygen isotope chemostratigraphy

    Relative concentrations:

    In 1947, Urey and Emiliani discovered that O isotopes fractionate depending on temperature. They examined shells of foraminiferans throughout the Pleistocene. The O isotopes in the shells appeared to be responding to temperature changes associated with the ice ages, with oceanic sediments becoming isotopically heavy during glaciations.

    Later it was determined that the relationship was not quite straightforward. Normally, when water evaporates, molecules with 16O more readily enter the vapor phase. Typically this makes no difference as the water soon condenses and is back in the oceans. When it is captured by continental glaciers, however, the oceans become isotopically heavy i.e. enriched in 18O.

    The same principle applies to atmospheric water. Condensation preferentially draws H218O out of the atmosphere. Results in 16O-rich (or 18O-depleted) polar snow.

    During times of high glacial activity, ocean waters are very enriched in 18O, organisms that incorporate oxygen-bearing molecules (such as CaCO3) into shells will also be enriched in 18O. Therefore, a δ18O curve from a stratigraphic column is a direct result of ice volume, but this is a good proxy for temperature changes.

    Of course, δ18O can be obtained directly from cores of continental ice sheets. See data from the Antarctic Vostok ice core.

    Carbon isotope chemostratigraphy

    Relative concentrations:

    In practice, fractionation of carbon isotopes is driven by kinetic fractionation of photosynthesis which:

    Global climatic and oceanographic factors that control burial of organic matter can therefore be deduced from a stratigraphic column of δ13C values.

    Carbon isotopes in carbonates thus reflect depositional and oceanographic processes. These are affected: Therefore carbon isotope distributions are primarily a reflection of oceanographic and climatic changes on a global scale. Strong excursions (positive or negative deflections) are often good isochronous time markers.

    For example:

    The isotope record reflects the depletion of organic carbon in 13C, and its movement between two reservoirs:

    Why would this happen? Under normal circumstances, organic carbon(depleted in 13C) and the products of its decay accumulate in the ocean floor and in deep water. When major reorganizations of ocean circulation cause ocean water layers to overturn, this carbon is returned to the photic zone, where it can be incorporated in carbonate, resulting in a secular negative excursion of δ13C.

    Photosynthetic pathways: Carbon can also be used as a Cenozoic time marker due to the spread of C4 plants. There are two main photosynthetic pathways:

    Each pathway fractionates carbon differently as it incorporates it into sugars. Therefore, C4 plants have very different δ13C values than C3 plants. The Cenozoic δ13C record shows:

    C4 plants became ubiquitous in North America with the spread of great grasslands ~7 ma.

    Strontium isotope chemostratigraphy

    Stable isotopes include: 88Sr, 87Sr, 86Sr, 84Sr. Percentages vary in nature. Sr substitutes readily for Ca in limestones (especially in aragonite), producing a readily available record. Common Sr isotope in oceans is 86Sr, with 87Sr in lesser abundance.

    87Sr/86Sr in modern ocean is 0.7090. 87Sr is produced by the radiometric decay of 87Rb. This ratio (0.7090) fluctuates through time, but shows a steady increase since the Jurassic. What's up?

    Essentially, the 87Sr/86Sr reflects the balance between the leaching of oceanic basalt through hydrothermal activity, resulting in the depletion of 87Sr, and the weathering of continental rocks, causing enrichment in 87Sr. Thus:


    Since the Eocene, the increase in 87Sr/86Sr is almost linear. Thus, a given sample's 87Sr/86Sr ratio can actually be used to date it.

    Many other stable isotope systems, including: Have similar utility. Radioactive isotopes are useful, too, but in a different way. Stay tuned!