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.:
- 12C (6 protons, 6 neutrons) vs. 13C (6 protons, 7 neutrons)
Isotope effects: Differences in chemical or physical properties which arise from variations in the atomic mass of an element.
- E.G.: Evaporation and condensation of water concentrate 16O and 18O respectively.
Fractionation: Differential incorporation of isotopes into a molecule as a result of an isotopic effect.
Equilibrium isotope effects: Effect of different atomic masses on bond strength.
When isotopes substitute for one another, the nuclear charge and electron distributions remain the same, but vibrational energies change. Heavier isotopes produce lower vibrational energies. Therefore, heavy isotopes tend to accumulate in the strongest bonds.
Kinetic isotope effects: Differences in reaction rates of isotopic molecules. Normal KIE: The species containing the lighter isotope reacts more quickly. This effect provides information about mechanistic details of reaction pathways (E.G.: Photosynthesis)
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:
- nX refers to the isotope system of interest (eg. 13C or 18O)
- R refers to the ratio of isotopes in that system (eg. 13C/12C or 18O/16O)
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:
- Carbon: PDB (reference material calibrated to a Cretaceous aged belemnite limestone from the Pee Dee formation of S. Carolina)
- Oxygen: PDB, SMOW (standard mean ocean water)
- Sulfur: CDT (Canyon Diablo troilite)
- Nitrogen; air
- Samples with a negative δ value are considered to be depleted in the heavy isotope (i.e. they have more of the light isotope than the standard)
- Samples with a positive δ value are considered to be enriched in the heavy isotope (i.e. they have more of the heavy isotope than the standard)
Stable isotope geochemistry is primarily concerned with variations in isotopic ratios of H, Li, C, N, O, Si, Sr, and S. Their common characteristics:
- Low atomic mass
- Large relative mass difference between rare and abundant isotopes
- Form chemical bonds with high degree of covalent character
- Abundance of rare isotope is sufficiently high to assure precise measurement of isotope ratios by mass spectrometry
- Heavy isotope is usually concentrated in the solid phase in which it is more tightly bound
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
- 16O = 99.76%
- 18O = 0.21%
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
- 12C = 98.89%
- 13C = 1.11%
In practice, fractionation of carbon isotopes is driven by kinetic fractionation of photosynthesis which:
- tends preferentially to take in 12C and therefore yields a depleted signature. Because of this, organic carbon originating in the biosphere tends to be depleted in 13C.
- The burial of biosphere-derived organic matter sequesters 12C in the sediments and rocks, therefore leaving the remaining pool of available C enriched 13C.
- less by temperature and ice volume
- more by processes and environments that control the burial of organic matter.
The isotope record reflects the depletion of organic carbon in 13C, and its movement between two reservoirs:
- Sedimentary organic carbon
- Carbonate rock
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:
- C3: Used by "normal" plants.
- C4: Alternative pathway that requires less water. Used by many arid adapted plants and evolutionary descendants of arid plants. Grasses, especially use the C4 pathway.
- A general negative trend that may reflect the evolution and proliferation of planktonic foraminiferans, who move biologic carbon into the carbonate reservoir
- A negative excursion beginning in the Miocene, that may reflect the rise of C4 plants.
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:
- The low ratio of Jurassic and Cretaceous reflects accelerated sea-floor spreading.
- The high ratios of the Cambrian and Neogene reflect the erosion caused by major orogenies. (Late Neoproterozoic Pan-African orogeny and Neogene Himalayan orogeny.)
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.
- Sulfur (right)