ALBERT

Description: An integrated magneto-biostratigraphic framework is presented for Middle Miocene sediments of DSDP Site 372 located in the Western Mediterranean. Detailed biostratigraphic analysis shows a nearly complete sequence of early Middle Miocene calcareous plankton bioevents in the Mediterranean, including the LCO (Last Common Occurrence) of the nannofossil Sphenolithus heteromorphus which has been astronomically dated in the Ras il Pellegrin (RIP) section on Malta Island [Abels, H.A., Hilgen, F.J., Krijgsman, W., Kruk, R.W., Raffi, I., Turco, E., Zachariasse, W.J., 2005. Long-period orbital control on middle Miocene global cooling: integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta, Paleoceanography, 20, PA4012. doi:10.1029/2004PA001129. 11 pp]. Thermal demagnetization of discrete samples revealed a characteristic low-temperature component with dual polarities despite a weak paleomagnetic signal. The resultant magnetostratigraphic record, combined with the calcareous plankton biostratigraphy, is straightforwardly correlated to the geomagnetic polarity time scale (CK95) of Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, J. Geophys. Res., 100, 6093-6095] and the Astronomical Tuned Neogene Time Scale (ATNTS04) of Lourens et al. [Lourens, L.J., Hilgen, F.J., Laskar, J., Shackleton, N.J., Wilson, D., 2004. The Neogene Period. In: Gradstein F.M., Ogg J.G., Smith A.G. (Eds.), A Geologic Time Scale, Cambridge Univ. Press, pp. 409-440]. The subchrons recorded in Site 372 succession range from C5Br up to C5AAr. To confirm the magnetostratigraphic calibration, 40Ar/39Ar dating was performed on feldspar of two volcanic ash layers. The radio-isotopic dating indicates a younger age for these two ash layers compared to the magnetostratigraphic calibrated ages according to the CK95 and ATNTS04 age models. However, if the astronomically calibrated age of 28.21 +/- 0.04 Ma is used for the Fish Canyon standard (FCs), the age for the older ash layer exactly matches its ATNTS04 age. Ages for bioevents were calculated assuming constant sedimentation rates between magnetostratigraphic age-tie points. The S. heteromorphus LCO has an age of 13.54 Ma and 13.63 Ma according to CK95 and ATNTS04, respectively, which is consistent with the astronomical tuned age of 13.65 Ma determined at RIP section [Abels, H.A., Hilgen, F.J., Krijgsman, W., Kruk, R.W., Raffi, I., Turco, E., Zachariasse, W.J., 2005. Long-period orbital control on middle Miocene global cooling: integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta, Paleoceanography, 20, PA4012. doi:10.1029/2004PA001129. 11 pp]. We therefore conclude that the magnetostratigraphic calibration of DSDP Site 372 is correct and that, for the time being, this site can be considered as a reference section for the early Middle Miocene of the Mediterranean region.

Description: A quantitative analysis was carried out of planktonic diatoms (biogenic opal) and calcareous nannofossils (biogenic calcite) in late Quaternary sediments (MIS 1–6) from four cores along a N–S transect east of New Zealand from 39°50'S to 50°04'S across the E–W-trending submarine ridge, the Chatham Rise. This was done to trace movements of oceanic fronts and to improve calcareous nannofossil stratigraphy for the last 130 000 yr in the SW Pacific. Sites ODP 1123 and Q 858 are below present day subtropical surface waters north of Chatham Rise. Site DSDP 594 is below present-day mixed temperate–subantarctic surface water south of the rise, and site ODP 1120 is below subantarctic surface water. The more diverse and opportunistic planktonic diatoms provided marker species for subtropical surface waters (Alveus marina, Fragilariopsis doliolus, Rhizosolenia bergonii and Azpeitia nodulifer) and others for subantarctic surface waters (Nitzschia kerguelensis, Thalassiosira lentiginosa). Application of these tracers permits the following conclusions: (1) subtropical conditions persisted north of Chatham Rise throughout the past 130 000 yr, in spite of the cooling of surface waters during colder periods; (2) during warm times (MIS 5 and MIS 3, and in MIS 1), the sporadic occurrence of subtropical species south of Chatham Rise indicates occasional admixture of subtropical surface waters that far south; (3) subantarctic waters extended to the southern slopes of the Chatham Rise during MIS 5b, late MIS 5a to early MIS 4, during the warmer time intervals in early MIS 3, and during latest MIS 3 to early MIS 2; (4) subantarctic frontal conditions existed over southern Chatham Rise during early MIS 4 and late MIS 3 to early MIS 2; and (5) it is probable that during cooler times, MIS 6, MIS 5b, and in MIS 2, intensified particle transport from the Bounty Trough to the northern flank of Chatham Rise occurred by intensified boundary currents. The larger abundance fluctuations in both microfossil groups at the sites south of Chatham Rise than north of Chatham Rise reflect northward shifts of the Circumpolar Subantarctic Water (CSW) and a contemporaneous disappearance of Australasian Subantarctic Water (ASW), implying an elevated temperature gradient between the surface water masses north and south of the Chatham Rise at the times of such northward shifts of CSW. Calcareous nannofossils are less diverse than diatoms, and are less specialised. Some calcareous nannofossil species show abundance shifts at the same time at different latitudes. Two of these abundance shifts can be used for correlation between subtropical and subantarctic sediments in the SW Pacific: (1) reversal in the relative abundance of Calcidiscus leptoporus and Coccolithus pelagicus associated with the MIS 2/1 boundary; and (2) drop in abundance of Gephyrocapsa muellerae or medium-sized Gephyrocapsa at the MIS 4/3 boundary. An additional abundance shift seems to be restricted to subtropical to mixed temperate–subtropical–subantarctic surface waters: (3) increase in abundance of G. muellerae or medium-sized Gephyrocapsa at the beginning of MIS 2 below the Okareka tephra.