Isotope stratigraphy is a method of determining relative ages of sediments based on measurement of isotopic ratios of a particular element. It works on the principle that the proportions of some isotopes incorporated in biogenic minerals (calcite, aragonite, phosphate) change through time in response to fluctuating palaeoenvironmental and geological conditions. However, this primary signal is often masked by diagenetic alteration of sediments which have secondarily altered the isotopic ratios. Disentangling primary and secondary components of measured isotopic ratios is a difficult and frequently controversial subject. Although isotopes of many elements have been studied oxygen and carbon strontium, are of particularly wide application.
Oxygen isotopes
The ratios in which the two stable isotopes of oxygen (16O and 18O) are precipitated in carbonates and phosphates depends upon the oxygen isotopic composition of the fluid from which the mineral precipitated and also on the temperature at which this took place. However, some organisms incorporate oxygen isotopes that are out of equilibrium with temperature and seawater composition. In addition, primary isotopic values may commonly be altered by diagenetic recrystallisation of carbonate sediments.
Oxygen isotopes can record detailed changes in ocean temperature and ice volume. The most extensive use of oxygen isotopes has been in deep-sea cores of Cenozoic, especially Quaternary sediments, where data from calcitic microfossils, notably foraminifera, record fluctuating temperatures and the growth and decay of ice-sheets, allowing the recognition of oxygen isotope stages. The separate effects of temperature and ice volume are distinguished by comparing isotope ratios in coeval planktonic and benthonic microfossils, mainly foraminifera. Because both parameters were driven by Milankovitch climatic cycles, it has been possible to identify and correlate oxygen isotope stages in detail across the globe, and 18O curves provide a very refined (20 ka resolution) time-scale for Quaternary to Neogene time. In pre-Cenozoic sediments the use of oxygen isotopes in both stratigraphy and palaeoenvironmental studies has been much more limited because much of the carbonate is recrystallised, and only rarely reflects secular changes in oxygen isotope ratios.
For discussion of the Marine Isotope Stage (abbreviated as MIS) time-scale, derived using oxygen isotope measurements, see ‘climatostratigraphy‘.
Carbon isotopes
The two stable isotopes of carbon, 12C and 13C, vary in relative abundance through time in both carbonate minerals and organic matter. The fluctuations in 13C are brought about by changes in the balance of fluxes of the carbon cycle, including inputs of terrestrial carbon and oxidation of marine organic matter, and outputs by production and burial of marine carbonate and organic matter. Because the residence time in the carbon cycle is brief (10 ka), changes in flux are recorded accurately and globally in the sedimentary record. Furthermore, carbon isotopes are relatively robust and resistant to diagenesis.
Strontium isotopes
Strontium isotope stratigraphy relies on measurement of 87Sr/86Sr in marine biogenic carbonate or phosphate. Precipitation of these minerals involves incorporation of strontium from seawater, which will have an 87Sr/86Sr identical to that of oceanic values, which is of the same value globally at any point in time. The 87Sr/86Sr ratio changed systematically through time and it is therefore possible to date samples by placing them on a standard curve. The method works best for periods of time over which there was a long-term unidirectional shift in ratios, as during the Tertiary. Strontium isotope stratigraphy gives a maximum time-resolution of about 1 ma.
Strontium in seawater is derived from three sources: fluvial input of material weathered from continental crust; hydrothermal leaching of oceanic basalts at mid-ocean ridges; and recrystallisation of carbonate minerals. Changing strontium values reflect global changes in these geological processes.