Category:Oxygen Isotope stratigraphy in the Chalk Group

Oxygen has two commonly occurring stable isotopes, ¹⁶O and ¹⁸O. Calcium carbonate, of which chalk is overwhelmingly composed, naturally concentrates ¹⁸O relative to the water it it is precipitated from (Leeder, 1982). Since the latter process is inversely proportional to temperature, the relative amount of ¹⁸O in chalk is a potentially valuable guide to palaeotemperature (Leeder, 1982).

Palaeotemperature studies in Mesozoic strata using oxygen isotopes are usually considered problematic because of the distorting effects of diagenetic alteration on the relative proportions of ¹⁶O and ¹⁸O. However, Jenkyns et al. (1994) produced O¹⁸ signatures for chalk successions in the UK that suggested minimal diagenetic influence. These authors also showed that there was striking similarity between the O¹⁸ signatures for UK Chalk Group successions and that obtained from coeval strata at Gubbio in northern Italy, despite the obviously contrasting diagenetic histories of the two areas. Consequently, the ¹⁸O signature obtained from the Chalk Group is inferred to reflect original palaeoclimatic variation.

Jenkyns et al. (1994) showed that Late Cretaceous sea-surface temperatures peaked in the Upper Cenomanian, and that from the Early Turonian to the end of the Cretaceous there was progressive global cooling. The onset of this cooling is approximately coincident with a major positive excursion of the ¹³C signature (see: Carbon Isotope Stratigraphy) at the Cenomanian / Turonian boundary linked to the global deposition of black shales. This apparent sedimentological evidence for a widespread poorly oxygenated marine environment has been described as an Oceanic Anoxic Event. Jenkyns et al. (1994) suggested that the global deposition of black shales at this time and presumed consequent decline in atmospheric carbon dioxide (since less carbon was available for oxidative recycling to the atmosphere), might have been an important factor in initiating this cooling phase ('inverse greenhouse effect').

Both Jenkyns et al. (1994) and Jeans et al. (1991) recorded O¹⁸ evidence for a cooling event within the Plenus Marls. This corresponds with macrofossil changes previously linked to the influence of cooler water temperatures (Jefferies, 1963). However, Jeans et al. (1991) suggest that this evidence negates the case for the conventional model of the Cenomanian Oceanic Anoxic Event, in which transgression and expansion of shelfal regions is linked with high organic productivity, oxygen depleation (exacerbated by high sea-surface temperatures) and black shale formation (Gale, 2000). Instead, they proposed that the cooling episode was evidence of significant glaciation, which caused lowered sea levels, regression, and enhanced detrital input from terrestrial sources (thereby forming the Plenus Marls). In this scenario, high organic matter input coupled with poor water circulation resulting from regression is envisaged as causing oxygen depleation and the formation of black shales (Jeans et al., 1991). Jenkyns et al. (1994) preferred to explain this O¹⁸ data in terms of cold water upwelling or the influence of a colder water mass.

References

GALE, A. S. 2000. Late Cretaceous to Early Tertiary pelagic deposits: deposition on greenhouse Earth. In WOODCOCK, N &

JEANS, C V, LONG, D, HALL, M A, BLAND, D J & CORNFORD, C. 1991. The geochemistry of the Plenus Marls at Dover, England: evidence of fluctuating oceanographic conditions and of glacial control during the development of the Cenomanian-Turonian δ¹³C anomaly. Geological Magazine, 128, 603-632.

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.

LEEDER, M R. 1982. Sedimentology - Process and Product. (George Allen & Unwin: London.).