Climate-controlled organic matter accumulation as recorded in the Upper Jurassic Argiles de Chatillon Formation, a shallow-marine counterpart of the Kimmeridge Clay Formation

Mudstones from the Argiles de Chatillon Formation exposed in the Boulonnais region of Northern France represent a proximal lateral equivalent of the organic-rich Kimmeridge Clay Formation. The Argiles de Chatillon Formation is composed of two subunits that straddle the Kimmeridgian-Tithonian boundary. Each subunit contains an organic-rich interval. The two conspicuous organic-rich intervals have been linked to either periods of high sea level or greenhouse warming. Here, we use palynology to further understand climate and environmental mechanisms that drove organic matter enrichment. We use bulk organic carbon isotope records (delta C-13(org)) to correlate the Boulonnais sections with those of the Kimmeridge Clay Formation. The palynological results suggest that the stratigraphically lower organic-rich interval (Kimmeridgian) was deposited under suboxic to anoxic stratified conditions. A large-scale climate shift from cooler/humid to warmer/arid conditions marked the Kimmeridgian-Tithonian boundary, influencing organic matter enrichment in the stratigraphically higher organic-rich interval (Tithonian). In contrast with the lower organic-rich interval, there are no indications of stratified conditions for the higher organic-rich interval. Within this thicker organic-rich interval, cyclic variations in amorphous organic matter distribution, total organic carbon and delta C-13(org) trends on a 2 m scale are observed. They co-occur with fluctuations of the palynological assemblages, indicative of more humid versus arid climate conditions, likely alternating on a similar to 100 kyr eccentricity timescale. Our results show that under the most humid phases of these overall arid climate conditions, sulfurization of carbohydrates was the dominant control on organic matter preservation. This climate-controlled process that drives organic matter enrichment in the Tithonian can be recognized on a basin-wide scale.