Biostratigrafia e significato paleoecologico dei nannofossili calcarei alla transizione Oligocene-Miocene al Site IODP U1507 (Mare di Tasmania) = Calcareous nannofossil biostratigraphy and paleoecological significance at Oligocene-Miocene transition at Site IODP U1507 (Tasman Sea)

This Master thesis is a biostratigraphic and biochronologic study of an Oligocene-Miocene section retrieved at Site IODP U1507 (Tasman Sea) during the IODP Expedition 371. The interval studied is comprised from 480 to 413.2 mbsf (meter below sea floor). The biostratigraphic analysis carried on the calcareous nannofossil assemblages allows to date the study sedimentary sequence by means biostratigraphic events (e.g., appearance, disappearance, …). To this end, the standard zonations (Martini, 1971; Okada and Bukry, 1980) have been applied. In addition, more recent biostratigraphic schemes (Agnini et al., 2014; Backman et al., 2012) were used to improve the biostratigraphic resolution of onboard data. According to the standard zonations, the study section extends from Zone N25 to Zone NN3 (Martini, 1971), and from Subzone CP19a to Zone CN2 (Okada and Bukry, 1980). If more recent schemes are adopted the section comprises an interval between Zone CNO5 (Agnini et al., 2014) to Zone CNM5 (Backman et al., 2012). From the chronostratigraphic point of view the analyzed section spans from the Chattian (late Oligocene) to the Burdigalian (early Miocene). The development of an integrated age model allows to estimate the interval spanned from the Base of the section (24.62 Ma) to the Top of the section (17.86 Ma), which means ca.7 Myr. In addition, the age model was used to provide new indipendent biochronological data of several biohorizons. A comparison between our results and data already available from literature have highlighted a substantial concistency. However, calibrations available for the Top of S. ciperoensis show a remarkable difference (ca. 0.4 Ma). The Oligo-Miocene boundary (23.03 Ma) is formally defined in the Global Stratotype Section and Point (GSSP) at the Lemme-Carosio section (N Italy). Unfortunately, when comparing calcareous nannofossil biostratigraphic data available from this section and those obtained at Site IODP U1507, there are some inconsistencies. In particular, the Top of S. dephix is observed in different positions in the two successions, if tied to magnetostratitigraphy, and this has obviously implications also for chronostratigraphy. A further comparison with other successions has permitted to point out that data from Site IODP U1507 are consistent with results from the equatorial (Ceara Rise), northern (J-Anomaly Ridge) and southern (Agulhas Ridge) Atlantic Ocean (Pälike et al., 2006; Norris et al., 2014).