Delta 66Zn ratios and age of carbonates from ODP Site 138-849 (Table 1)

The carbonate fraction of sediment core ODP 849, leg 138, located in the eastern equatorial Pacific, mostly consisting of coccoliths, was separated and analyzed for its Zn isotopic composition. The overall variation in Zn isotopic composition, as determined by multiple-collector, magnetic-sector, inductively coupled plasma mass spectrometry, was found to be on the order of 1? (expressed in delta66Zn, where deltaxZn=[(xZn/64Zn)sample/(xZn/64Zn)standard -1]*10**3 and x=66, 67 or 68) over the last 175 ka. The analytical precision was 0.04 per mil and the overall reproducibility was usually better than 0.07 per mil. The Zn isotopic composition signal exhibits several marked peaks and a high-frequency variability. A periodogram of the delta66Zn signal showed two periodicities of 35.2 and 21.2 ka. We suggest that the latter is caused by the precession of the Earth's axis of rotation. The periodogram exhibits a minimum at 41.1 ka, thus showing that the Zn isotopic composition is independent of the obliquity in the eastern equatorial Pacific. The range of delta66Zn values observed for the carbonate fraction of ODP 849 overlaps with the range observed for Fe-Mn nodules in the world's oceans, which suggests that seawater/carbonate Zn isotope fractionation is weak. We therefore assume that most of the Zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean. The ODP 849 delta66Zn record seems to follow the changes in the insolation cycles. Changes in the late summer/fall equatorial insolation modulate the intensity of the equatorial upwelling, hence the mixing between deep and surface waters. We propose that during decreased summer/fall equatorial insolation, when a steep thermocline can develop (El Niño-like conditions), the surface waters cannot be replenished by deep waters and become depleted in the lighter Zn isotopes by biological activity, thus resulting in the progressive increase of the delta66Zn values of the carbonate shells presumably in equilibrium with surface seawater.

Oxygen and hydrogen isotopes measured in the Northeast Atlantic

Only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in NE Atlantic water masses, and data are especially sparse for intermediate and deep-water masses. The current study greatly expands this record with 527 d18O values from 47 stations located throughout the mid- to low-latitude NE Atlantic. In addition, dD was analyzed in the 192 samples collected along the GEOTRACES North Atlantic Transect GA03 (GA03_e=KN199-4) and the 115 Iberia-Forams cruise samples from the western and southern Iberian margin. An intercomparison study between the two stable isotope measurement techniques (cavity ring-down laser spectroscopy and magnetic-sector isotope ratio mass spectrometry) used to analyze GA03_e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios. The surface (0-100 m) and central (100-500 m) water isotope data show the typical, evaporation related trend of increasing values equatorward with the exception for the zonal transect off Cape Blanc, NW Africa. Off Cape Blanc, surface water isotope signatures are modified by the upwelling of fresher Antarctic Intermediate Water (AAIW) that generally has isotopic values of 0.0 to 0.5 per mil for d18O and 0 to 2 per mil for dD. Along the Iberian margin the Mediterranean Outflow Water (MOW) is clearly distinguished by its high d18O (0.5-1.1 per mil) and dD (3-6 per mil) values that can be traced into the open Atlantic. Isotopic values in the NE Atlantic Deep Water (NEADW) are relatively low (d18O: -0.1 to 0.5 per mil; dD: -1 to 4 per mil) and show a broader range than observed previously in the northern and southern convection areas. The NEADW is best observed at GA03_e Stations 5 and 7 in the central NE Atlantic basin. Antarctic Bottom Water isotope values are relatively high indicating modification of the original Antarctic source water along the flow path. The reconstructed d18O-salinity relationship for the complete data set has a slope of 0.51, i.e., slightly steeper than the 0.46 described previously by Pierre et al. (1994, J. Mar. Syst. 5 (2), 159-170.) for the tropical to subtropical Northeast Atlantic. This slope decreases to 0.46 for the subtropical North Atlantic Central Water (NACW) and the MOW and to 0.32 for the surface waters of the upper 50 m. The dD-salinity mixing lines have estimated slopes of 3.01 for the complete data, 1.26 for the MOW, 3.47 for the NACW, and 2.63 for the surface waters. The slopes of the d18O-dD relationship are significantly lower than the one for the Global Meteoric Water Line with 5.6 for the complete data set, 2.30 for the MOW, 4.79 for the NACW, and 3.99 for the surface waters. The lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics.

Biostratigraphic datums and magnetic properties of ODP Leg 178 sites

During Ocean Drilling Program (ODP) Leg 178, eight holes were drilled at three sites (1095, 1096, and 1101) on the continental rise along the western Antarctic Peninsula. The rise sediments proved to be good paleomagnetic recorders and provided continuous magnetostratigraphic records at all three sites. Biosiliceous microfossils, particularly diatoms and radiolarians, were present in the upper Miocene through lower Pliocene sections. In the upper Pliocene to Pleistocene sections, biosiliceous microfossils were rare but calcareous nannofossils and foraminifers were present. This paper summarizes the biostratigraphy and magnetostratigraphy of Leg 178 continental rise sites and is the first attempt at direct calibration of Antarctic biostratigraphic events to the geomagnetic polarity timescale in the Pacific sector of the Southern Ocean.

Seafloor magnetic anomalies and ages observed from shipboard and airborne measurements from New Zealand to the Amundsen Sea (south of 64°S)

Geophysical data acquired using R/V Polarstern constrain the structure and age of the rifted oceanic margin of West Antarctica. West of the Antipodes Fracture Zone, the 145 km wide continent-ocean transition zone (COTZ) of the Marie Byrd Land sector resembles a typical magma-poor margin. New gravity and seismic reflection data indicates initial continental crust of thickness 24 km, that was stretched 90 km. Farther east, the Bellingshausen sector is broad and complex with abundant evidence for volcanism, the COTZ is ~670 km wide, and the nature of crust within the COTZ is uncertain. Margin extension is estimated to be 106-304 km in this sector. Seafloor magnetic anomalies adjacent to Marie Byrd Land near the Pahemo Fracture Zone indicate full-spreading rate during c33-c31 (80-68 Myr) of 60 mm/yr, increasing to 74 mm/yr at c27 (62 Myr), and then dropping to 22 mm/yr by c22 (50 Myr). Spreading rates were lower to the west. Extrapolation towards the continental margin indicates initial oceanic crust formation at around c34y (84 Myr). Subsequent motion of the Bellingshausen plate relative to Antarctica (84-62 Myr) took place east of the Antipodes Fracture Zone at rates <40 mm/yr, typically 5-20 mm/yr. The high extension rate of 30-60 mm/yr during initial margin formation is consistent with steep and symmetrical margin morphology, but subsequent motion of the Bellingshausen plate was slow and complex, and modified rift morphology through migrating deformation and volcanic centers to create a broad and complex COTZ.