Integrated benthic foraminiferal and calcareous nannofossil records from IODP site U1506: the biogenic bloom

This Master thesis is a biostratigraphic, biochronologic and paleoenvironmental study of a Miocene-Pliocene section retrieved at Site U1506, drilled during IODP Exp. 371 (Tasman Sea, southwest Pacific). The studied succession spans from 233.50 to 81.75 m CSF-A (Core depth below Sea Floor-A). During the latest Miocene-early Pliocene, increases in the mass accumulation rate (MAR) of biogenic components, major changes in planktic and benthic fauna assemblages, and marked decreases in sedimentary redox conditions have been documented both in the Indian and Pacific Ocean beneath upwelling zones. These conditions are referred to as the “biogenic bloom”. Previous works have suggested that this paleoceanographic event is characterized by a prominent increase in primary productivity between ca. 9.0 and 3.5 Ma. A first result of this thesis is the biostratigraphic classification of the study succession based on calcareous nannofossil biostratigraphy using appearance and disappearance biohorizons of a number of taxa proposed in standard (Martini, 1971; Okada and Bukry, 1980) and alternative biozonations (Backman et al., 2012). According to the biostratigraphic zonation available, the study section extends from Zone NN10 to Zone NN13 (Martini, 1971), from Subzone CN8b to Subzone CN10c (Okada and Bukry, 1980) and from Zone CNM15 to Zone CNPL2 (Backman et al., 2012). From the chronostratigraphic point of view, the analysed section thus spans from the Tortonian (late Miocene) to the Zanclean (early Pliocene). Based on calcareous nannofossil biochronology an integrated age model has been constructed which allows to estimate the time interval spanned from the Base of the section (8.45 Ma) to the Top of the section (4.53 Ma) as well as the sedimentation rates along the section. Moreover, the chronological framework available for Site U1506 also permits to evaluate the precise timing and duration of the biogenic bloom that is from 7.28 to 6.50 Ma (0.78 Myr). This datum consistently shortens the previous estimates for the duration of the event (5.5 Myr). Benthic foraminifera have been used to reconstruct the paleoenvironmental conditions at the sea floor in terms of nutrient availability, trophic conditions and oxygen concentrations along the study section, based on the different paleoecological affinity of the taxa present in the assemblages. The succession has been divided into three intervals based on changes observed in the foraminiferal assemblages and detrended correspondence analysis (DCA). Taxa typical of high nutrient influx and low oxygen conditions, indicative of high productivity environmental conditions, were observed in Interval 1 and are interpreted to document the biogenic bloom between 192.91 and 168.00 m CSF-A. Instead, Interval 2 and 3 are suggestive of more oligotrophic environmental conditions likely characterized by a higher oxygen availability. The next step in order to better understand this event will be to integrate paleontological and geochemical (δ13C) data in order to correlate paleontological changes with modifications in the carbon cycle.