Category:Carbon Isotope stratigraphy in the Chalk Group
Carbon isotope stratigraphy in the Chalk Group is concerned with examination of the ratio of the two commonly occurring isotopes of carbon, namely Carbon-12 (12C) and Carbon-13 (13C). Phytoplankton photosynthesis strongly discriminates against 13C, preferentially incorporating 12C into organic matter (Mitchell et al., 1996). Increases in 13C therefore reflect removal of 12C from the carbon cycle, which in turn reflects changes in organic productivity or the oxidation of organic matter (Mitchell et al., 1996). Thus, a peak in 13C might be produced by high organic productivity or by inhibiting the recycling of organic material (e.g. by locking it up in black shale deposits), or both.
Recently, there has been much research on the causes behind changes in the 13C signature. Mitchell et al. (1996) showed that major positive anomalies of 13C in the Cenomanian correlated with sequence boundaries or transgressive surfaces (see Sequence Stratigraphy), and proposed a link between transgressive events, black shale formation and positive excursions of the 13C signature.
Irrespective of the exact causes of variation in the 13C signature, there is widespread agreement that carbon isotope stratigraphy is a powerful correlation tool. The recorded variations of 13C appear to reflect original palaeoenvironmental variation, and have not been strongly affected by diagenetic changes (Jenkyns et al., 1994). Furthermore, the 13C signatures from coeval successions that are widely separated show remarkable similarity. Gale et al (1993) demonstrated synchroneity of changes in the 13C signature with respect to bio-markers at the Cenomanian-Turonian boundary in sections across the UK, and also with the coeval succession at Pueblo, western USA. Jenkyns et al. (1994) pointed out the remarkable similarity of 13C curves for the UK Chalk Group and the coeval Scaglia facies near Gubbio, Italy, but noted that the correlation of the two profiles showed that there were major discrepencies in stage boundary definitions between the sections. Subsequently, Voigt & Hilbrecht (1997) showed that some of these problems were overcome by revisions to the criteria used to recognise stage boundaries in the Italian succession. These authors also demonstrated that the 13C signature could be used to accurately correlate different Cenomanian-Coniacian successions across Europe, and that for the correlation of Boreal and Tethyan provinces, it 'is probably more accurate and reliable than any other stratigraphic technique'.
GALE, A S, JENKYNS, H C, KENNEDY, W J & CORFIELD, R M. 1993. Chemostratigraphy versus biostratigraphy: data from around the Cenomanian-Turonian boundary. Journal of the Geological Society, London, 150, 29-32.
JENKYNS, H C, GALE, A S & CORFIELD, R M. 1994. Carbon- and oxygen-isotope stratigraphy of the English Chalk and Italian Scaglia and its palaeoclimatic significance. Geological Magazine, 131, 1-34.
MITCHELL, S F, PAUL, C R C & GALE, A S. 1996. Carbon isotopes and sequence stratigraphy. In HOWELL, J A & AITKEN, J K (eds), High Resolution Sequence Stratigraphy: Innovations and Applications, Geological Society Special Publication, 104, 11-24.
VOIGT, S & HILBRECHT, H. 1997. Late Cretaceous carbon isotope stratigraphy in Europe: Correlation and relations with sea level and sediment stability. Palaeogeography, Palaeoclimatology, Palaeoecology, 134, 39-59.
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