The Cenomanian/Turonian Boundary Event (CTBE) at Tarfaya, Morocco, northwest Africa: Eccentricity controlled water column stratification as major factor for total organic carbon (TOC) accumulation: Evidence from marine palynology

Abstract

From the exploration core S75 that covers the Cenomanian/Turonian Boundary Event (CTBE) and lower Turonian at Tarfaya, Morocco, northwest Africa, 96 samples from a section 24.5 m thick was analysed palynologically. The palynological data are discussed with various data on isotope- and bio-geochemistry as well as on time series analyses published by Kolonic et al. (2005, Paleoceanography 20, PA 1006) and Kuhnt et al. (2005, Int. J. Earth Sci. 94, 147–159). The dinocyst assemblage is of low to moderate diversity and strongly dominated by peridinioids throughout most of the section. It is suggested, that potentially eccentricity controlled cycles of increased total organic carbon (TOC) accumulation are closely related to increases of the ratio in proportions of peridinioid to gonaulacoid dinocysts (p/g ratio) and proportions of Botryocccus as well as of Bosedinia spp., a peridinioid cyst of probable strongly reduced-salinity water. At higher resolution, however, episodic increases in the p/g ratio are more punctuated in distribution and largely inversely correlated to abundances of Bosedinia spp. It is suggested that warm-humid climatic boundary conditions prevailed in the Tafaya region, episodically superimposed by upwelling events, reflected by increased proportions of Palaeohystrichophora spp. and Subtilisphaera spp. Accordingly, enhanced preservation, supported by strong episodic production pulses, is proposed for the TOC accumulation in the Tarfaya Basin. The p/g ratio peaks decrease both in prominence and frequency when pre-excursion values of δ13C isotopes are re-established after the positive anomaly. This may reflect a moderate shift in control from production to preservation of TOC following the CTBE proper. With respect to δ18O isotopes, episodes of pre-excursion values, possibly reflecting moderate cooling intervals and eustatic sea-level fall, are characterised by the appearance of dinocyst taxa diagnostic of neritic waters. This may reflect the gradual change from pelagic sedimentation characteristic of the plateau- and early recovery-period of δ13C isotopes towards neritic conditions within the late recovery and post-excursion period. Biomarkers suggestive of photic zone oxygen deficiency show only a limited relationship to the distribution of the different algal groups. However, according to the quantitative distribution of prasinophytes, considered to be critically controlled by ammonium availability, denitrification was more prominent during the late Cenomanian interval of the section and decreased significantly with the onset of the Turonian.