The Cenomanian/Turonian Boundary event (CTBE) at Tarfaya, Morocco: Palaeoecological aspects as reflected by marine palynology

Abstract

Two sections from the Cenomanian–Turonian boundary event (CTBE) at Tarfaya, northwest Africa, were analysed fully quantitatively by marine palynology. The results are compared and integrated with published data from micropalaeontology, total organic carbon content (TOC), isotope geochemistry and, for selected intervals, the distribution of chlorobactanes. A rise in eustatic sea level, composed of several third-order transgression pulses, is reflected by various palynologic proxies. A combination of productivity and preservation is proposed as the main control on TOC accumulation preceding and across the CTBE. Strong episodic upwelling events, reflected by significant increases of the peridinioid/gonyaulacoid (p/g) ratio of dinocysts, are documented at the onset of eustatic transgression and closely preceding the onset as well as within the plateau stage of the positive δ 13C anomaly. These events are considered as initial and sustaining causes respectively for TOC accumulation. The peak abundance of both green algal groups, Botryococcus and prasinophytes, is coincident with pelagic sedimentation and is strongly related to the increase of both oxygen deficiency in the water column and total organic carbon content. An episodic halocline stratification of the water column is suggested, possibly related to both a globally intensified hydrologic cycle and a current induced input of high- to middle-latitude surface waters into the Tarfaya Basin. This is corroborated by the prominent episodic appearance of Bosedinia, a peridinioid dinocyst with probable freshwater/reduced salinity affinity, as well as several euryhaline taxa common within late Mesozoic high to mid-latitude waters. A slight correlation with contents of chlorobactanes is documented at least for the prasinophyte distribution, which may reflect a moderate availability of reduced nitrogen within photic zone waters. However, maxima of chlorobactanes are related to palynologically barren or depleted intervals, possibly reflecting the sulfidic stage of redox conditions within photic zone waters. The almost coeval appearance of the suggested high-latitude taxon ? Ginginodinium sp. close to the onset of the δ 13C anomaly both at Tarfaya and at Wunstorf, northwest Germany, corroborates a connection of surface water circulation between high and low latitude sites during the eustatic sea-level high.