First record of early Aptian Oceanic Anoxic Event 1a from the Paris Basin (France) - climate signals on a terrigenous shelf

Abstract In 2013, Andra (French National Radioactive Waste Management Agency) drilled boreholes to the south-east of the Paris Basin, to characterise Aptian and Albian clayey formations, including the ‘Argiles a Plicatules’ Formation dated as early Aptian. One of these boreholes intersected this formation with an excellent recovery allowing detailed biostratigraphy (ammonites), sedimentology, clay mineralogy, isotope geochemistry (δ13Corg) and Rock-Eval analyses to be performed. The base of the formation corresponds to transgressive dark-grey silty clays with iron oolites and plant debris indicating a coastal environment evolving up-section to upper offshore environments. Higher in the succession, clays with less than 4% CaCO3 and less than 0.8% of organic matter were deposited in lower offshore environments. The occurrence of ammonites can be used to draw a biostratigraphic scheme, and in particular, to recognise the deshayesi and furcata ammonite Zones. The clay mineral assemblages are composed of illite and kaolinite associated with minor amounts of chlorite, vermiculite and smectite. The δ13Corg values range between -24 and -25‰ except in a particular interval characterised by a prominent negative excursion of about -3‰, that is recognised worldwide and precedes Oceanic Anoxic Event 1a (OAE1a). This interval is also characterised by an abundance of kaolinite and the absence of smectite suggesting an acceleration of the hydrological cycle and enhanced runoff and hydrolysing conditions over the emerged landmasses just before and during the onset of OAE1a. This climate change is consistent with isotope and palynological data indicating warm and humid climate conditions before and during the onset of OAE1a. Surprisingly, the interval corresponding to OAE1a is not significantly enriched in organic matter and its lithology is not different from the rest of the core suggesting that Tethyan anoxic water masses did not reach the relatively shallow epicontinental environments of the Paris Basin.