chemostratigraphy

Chemostratigraphy is a branch of stratigraphy that uses chemical and isotopic variations within rocks to interpret their relative ages and to correlate stratigraphic units. It relies on robust geochemical signals preserved in sediments, rocks, or fossil material. Proxies include stable isotope ratios such as carbon (δ13C) and oxygen (δ18O), strontium isotopes (87Sr/86Sr), sulfur isotopes (δ34S), as well as elemental abundances and ratios of trace elements and rare earth elements; organic geochemical markers may also be used.

Data are obtained through sampling and high-precision analytical techniques such as isotope ratio mass spectrometry, multi-collector ICP-MS, ICP-OES, and GC-MS for biomarkers. The resulting geochemical profiles are plotted against stratigraphic depth or age, and correlated between sections by identifying distinctive excursions, chemofacies, or monotonic trends. Chemostratigraphic correlations can be global, reflecting worldwide signals recorded in seawater chemistry, or regional, reflecting local diagenetic or sedimentary processes.

Common applications include refining stratigraphic correlations in sedimentary basins, defining sequence boundaries, and aiding in hydrocarbon exploration. It complements biostratigraphy, magnetostratigraphy, and radiometric dating, and is valuable for marking global events such as perturbations in the carbon cycle or shifts in seawater composition.

Limitations include diagenetic alteration, weathering, sediment mixing, and post-depositional overprinting, which can modify original signals. Proper interpretation requires careful assessment of diagenesis and cross-validation with other dating methods.