Abstract
A chemostratigraphic study (δ13C and δ18O) of the Late Cenomanian and Turonian chalk succession from the “Craie 701” Poigny borehole (near Provins in the Paris Basin, France) provides new high-resolution stable carbon and oxygen isotope data. Correlation of the bentonite horizons and the isotopic trends from Poigny with its English Chalk equivalent allows the development of a precise stratigraphic framework. δ13C and δ18O variations are synchronous and widespread throughout the European epicontinental seas and Tethyan Ocean. In the Poigny borehole, the Oceanic Anoxic Event 2 (OAE 2) is marked by a large and brief positive excursion of carbon isotopes (a carbon isotope excursions: CIE of 3‰ amplitude) without any apparent anoxia in the Late Cenomanian Chalk. Comparisons between different key sections on a North-South transect from the Anglo-Paris Basin to the Umbria-Marche Basin (Gubbio Section, Italy) and the Vocontian Basin (South-East France), suggests that the OAE 2 is linked to an increase in marine organic matter production, modulated by a regional effect on the organic carbon burial rate. Thus, the large positive carbon isotope increase spanning the Middle Cenomanian through to the Middle Turonian, including the salient CIE associated with the OAE 2, reflects a global scale increase in marine productivity that would be concomitant with a major long-term sea level rise. The stratigraphic position of the Turonian-Coniacian boundary can also be better defined by this isotopic study. A comparison of δ18O data between the Anglo-Paris Basin and Tethyan Basin shows high-amplitude, long-term synchronous variations reflecting primary paleo-environmental changes which are independent of local facies, sediment thickness and diagenesis. In particular, a negative shift (−1‰ of amplitude) reflects a warmer climate regime, marking the onset of OAE 2. Two colder phases (+1‰ amplitude each) occurred in the Early Turonian and the beginning of the Late Turonian.
Highlights
Large paleo-climatic and paleo-oceanographic disturbances occurred during the LateCretaceous
The aim of this work is to highlight these paleo-oceanographic events in an epicontinental sea domain, in the central part of the Paris basin, which is in an intermediate position between the English chalk domain and the Tethyan realm (SE of France and central Italy)
If we examine the major positive excursion around the Cenomanian-Turonian boundary at each location, we observe that its thickness is 8 m in England, 20 m in the southeast Paris Basin, 40 m in the Vocontian Basin and 3 m in Umbria-Marche
Summary
Most of these perturbations, such as oceanic anoxic events (OAEs; Schlanger and Jenkyns, 1976; Jenkyns, 2010), are recorded by the pronounced carbon isotope excursions (CIE). The largest of these perturbations is the OAE. 2, which took place at the Cenomanian-Turonian transition (e.g., Tsikos et al, 2004; Grosheny et al, 2006; Erba et al, 2013; Aguado et al, 2016) This event is regionally expressed by particular facies that are rich in organic matter and have local names such as the ‘Black Band’ in Yorkshire (England), the ‘Thomel level’ in the Vocontian Basin (France) or the ‘Bonarelli level’. The detail of the 13C profile of OAE 2 shows several distinct maxima (a, b, and c) which seem to be related to anoxic conditions in most oceanic sites (Voigt et al, 2007; Joo et Sageman, 2014; Jarvis et al, 2015)
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