Upper Jurassic benthic foraminiferal assemblages of the Charentes-Maritimes region (Atlantic Coast, France) and their answer to climate changes

Abstract

This study is based on the Upper Jurassic successions of the La Rochelle Platform (western France). There, sedimentary sequences are characterised by periodic changes in depositional environments, inferred from the alternation of calcareous marlstones and limestones and the occurrence of tempestites. The structure and dynamic changes of the benthic foraminiferal assemblages were studied and compared to sedimentological and geochemical proxies in order to detect periodic changes in the ocean/climate system and their effects on microbenthos assemblages. This study was combined with an analysis of the foraminiferal preservation. In the upper Kimmeridgian Aulacostephanus mutabilis and Aulacostephanus eudoxus ammonite zones, high abrasion and dissolution of the tests indicate that benthic foraminifers may have been reworked by bottom currents or by wave energy, a result consistent with the local occurrence of tempestites. More generally, the low diversity of the studied benthic foraminiferal assemblages and the occurrence of highly dominant agglutinated taxa suggest that they were originating from a shallow-water environment. In particular, two blooms of species of the genus Haplophragmoides, identified in the Kimmeridgian Epaspidoceras rupellensis and A. eudoxus ammonite zones, are interpreted to result from the development of highly vegetated biomes close to the coast. These intervals are successfully correlated to geochemical indicators that infer more humid climatic conditions. The stratigraphic distribution of selected benthic foraminiferal species shows significant similarities with the Subboreal foraminiferal biozonation as it has been proposed for the English Channel and adjacent areas. The upper Kimmeridgian foraminiferal assemblages are characterised by an increased proportion of typical species for the Subboreal and Boreal realms. This result is in congruency with geochemical and mineralogical data from the Kimmeridgian North Aquitaine Platform, which indicate a change in oceanic circulation from northward warm to southward cold streams during the lower Kimmeridgian E. rupellensis ammonite Zone. This result further supports an existing climate model simulation in which the observed divergence between (1) pollen assemblages, reflecting periodic warm and arid conditions, and (2) seawater temperature estimates derived from marine organisms indicating cold conditions, is most likely related to extensional tectonic activity and to the late Kimmeridgian Proto-North Atlantic seaway opening.