Carbon isotope stratigraphy - potential, problems and questions
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Holding Date||21 September 2008|
Chemostratigraphy uses chemical fingerprints stored in sediments and sedimentary rocks for stratigraphic correlation. Stable isotope signatures fixed in sedimentary inorganic and organic matter are among the most powerful methods used in chemostratigraphy. The oxygen isotope record established over the last few million years provides a history of multiple glacial-interglacial cycles showing cyclicities of 20ky and 40ky and, over the last 800 ky of 100 ky.
These cycles record the beat of orbital variations and climate change triggered by changes in insolation through time. While diagenesis increasingly alters the oxygen-isotopic signature of deeply buried marine sediments, C-isotope geochemistry is less influenced by diagenesis in pelagic settings. This explains why lithified pelagic sedimentary successions store very good information for C-isotope stratigraphy. The Early Cretaceous carbon isotope stratigraphy established in pelagic limestone successions form the Southern Alps (N. Italy) is often used as an informal reference curve. It has been calibrated against bio- and magnetostratigraphy. With carbon isotope stratigraphy established in ammonite-dated hemipelagic sediments from the Vocontian Trough (S. France) a tool is available for correlation of ammonite biozones with magnetostratigraphy.
The beginning of the Valanginian carbon isotope excursion starts in campylotoxus ammonite zone Ct4, and the boundary between campylotoxus and verrucosum zones falls into magnetozone CM11. This correlation differs by about one magnetozone form other published correlations. The Barremian-Aptian carbon-isotope stratigraphy from the Vocontian Trough differs in absolute values and in the excursion pattern from the carbon-isotope stratigraphy established at the locality Cismon (S. Alps, N. Italy). Differences can be explained with sedimentary gaps in studied sections and with differences in oceanography and depositional conditions.