Factors susceptible to alter the original 13Corg signal in Early Palaeozoic marine sediments
Résumé
Oxygen and carbon isotopes signals are now commonly used to document global climatic events in Early Palaeozoic sequences. However, when positive excursions of the 18O are generally regarded as a proxy for depicting a global cooling (i.e., ice cap development), the actual meaning of 13C 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 on continents, 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. When interpreting 13C excursions, these restrictions must be kept 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 analysed 13C value of up to 5 per mil depending on the contribution of the terrestrial plant input to the Silurian or Devonian organic residues. The occurrence of large amount of detrital graphite in the processed samples is another factor that also deeply modifies the original 13C signal of marine deposits. The carbon of these graphite flakes has 13C values significantly less negative than the carbon of the associated marine organic remains. Consequently, the positive shift of the 13C values in bulk rock samples may mostly reflect the dramatic increase of the inherited graphite in the sediment. Chemical etching of the organic matter during the processing has some limited effects. To prevent an alteration of the original marine 13C signal, we preclude the use of bulk rock samples for carbon isotope investigations. As an alternative, we recommend to analyse a carefully sorted organic fraction, ideally from 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 13Corg values of more than one hundred samples from various northern Gondwana localities ranging from the Early Ordovician to the Late Devonian, have been measured using the chitinozoans as the unique carbon source.