Factors susceptible to alter the original 13Corg signal in Early Palaeozoic marine sediments

PARIS, FLORENTIN; VIDET, BLAISE; GHIENNE, JEAN F.; TANG, PENG; DE LA PUENTE, G. SUSANA

Lille, Francia

Congreso; Palaeozoic Climates International Congress. Closing Meeting of the International Geoscience Programme (IGCP) n° 503 ‘Ordovician Palaeogeography and Palaeoclimate’; 2008

International Geoscience Programme n° 503 Ordovician Palaeogeography and Palaeoclimate

Oxygen and carbon isotopes signals are now commonly used to document global climatic events in Early Palaeozoic sequences. When the positive excursion of the d18O is generally regarded as a proxy for ice cap development, the actual meaning of d13C shifting is more controversial. Many factors may influence the carbon isotopic composition of the oceanic reservoir (e.g. high organic productivity with a huge burial of organic matter, acceleration of the weathering and erosion processes, mass release of carbon dioxide through volcanic activity or other catastrophic processes). All these factors are of global impact. Regional or even very local factors, however, are susceptible to strongly modify the original carbon isotopic values of marine sediments. Thus, when interpreting d13C excursions, one must keep these restrictions in mind. It is expected that technical bias are well controlled enough and thus do not intervene significantly on the obtained values. Other parameters may have much more impact, especially the biological and the mineralogical content of the sediment when bulk samples are used. Land derived plant remains (e.g. spores, tracheids, cuticles) modify the isotopic composition of the bulk organic content of marine sediments with a positive shift of the recorded d13C value of up to 5 per mil depending on the abundance of the terrestrial plant input within the Silurian and Devonian organic residue. The occurrence of large amount of detrital graphite in the processed sample is another factor that also deeply modify the original d13C signal of marine deposits. Chemical etching of the organic matter may have some limited effects too. To prevent such alteration of the original marine d13C signal, we preclude the use of bulk rock sample for the investigations, and we recommend analysing carefully sorted organic fraction, ideally on a single marine organic microfossil group. Chitinozoans constitute an excellent carbon source for such analyses as they are usually abundant, continuously distributed all along the marine sequences, and large enough for a fairly easy sorting. In order to illustrate our statements, the d13C values of more than one hundred samples from various northern Gondwana localities, and ranging from the Early Ordovician to the Late Devonian, have been measured using the chitinozoans as the carbon source.