Magnetostratigraphy of ODP Leg 207 sediments

Ocean Drilling Program (ODP) Sites 1257-1261 recovered thick sections of Upper Cretaceous-Eocene oceanic sediments on Demerara Rise off the east coast of Surinam and French Guiana, South America. Paleomagnetic and rock magnetic measurements of ~800 minicores established a high-resolution composite magnetostratigraphy spanning most of the Maastrichtian-Eocene. Magnetic behavior during demagnetization varied among lithologies, but thermal demagnetization steps >200°C were generally successful in removing present-day normal polarity overprints and a downward overprint induced during the ODP coring process. Characteristic remanent magnetizations and associated polarity interpretations were generally assigned to directions observed at 200°-400°C, and the associated polarity interpretations were partially based on whether the characteristic direction was aligned or apparently opposite to the low-temperature "north-directed" overprint. Biostratigraphy and polarity patterns constrained assignment of polarity chrons. The composite sections have a complete polarity record of Chrons C18n (middle Eocene)-C34n (Late Cretaceous).

(Table 1) Vesicularity of basalts recovered at DSDP Holes 49-407, 49-408, and 49-409

Average vesicularity of basalt drilled at three sites on the west flank of the Reykjanes Ridge increases with decreasing age. This change apparently records concomitant decrease in water depth at the ridge crest where the basalt was erupted and suggests substantial upward growth of the crest during the past 35 Myr.

Manganese nodules recovered by R/V Vityaz expedition

Uranium, radium, thorium and ionium were determined directly on seven concretions from three stations in the Indian Ocean, and on two concretions and a manganese-rich crust from two stations in the Pacific Ocean. The uranium content averages 3 to 5 gamma/g and the thorium content varies only slightly, but the Th/U ratio in the concretions is typically 2 to 5 in the Indian Ocean and 5 to 15.5 in the Pacific. The ionium content ranges from 1.0 x 10-9 to 3.6 10**-9 g/g in concretions from both oceans. Radium is more abundant in specimens from the Pacific Ocean (Ra = 3 - 12.7 x 10**-11 g/g) than from the Indian Ocean (1.5 - 5.2 x 10**-11 g/g). Analyses for Ca, Mn, Fe, Si, Ni, P, and ignition loss are also given. Radioactive equilibria between uranium, ionium, and radium are strongly disturbed throughout the concretions, and the RA/U and lo/U ratios generally exceed equilibrium ratios. Migration of radium from interior layers was established, so that neither determination of the ages of the concretions nor of their rates of growth can be considered reliable. The age of the concretions cannot exceed 800,000 years, and all grew within relatively short periods of time; there may have been "dormant" periods during growth. Estimates of growth rates are calculated from the radium and ionium contents; they show marked discordance.

(Figure 2) Range and distribution chart of dinoflagellate cysts recovered from Campanian to Maestrichtian sediments at DSDP Hole 95-612

Fifty-nine samples from the basal 110 m of DSDP Hole 612 (United States Atlantic Margin) were analyzed for palynomorph content. In total, 84 species and subspecies of dinoflagellate cysts were recorded which, on comparison with published data and shipboard analyses, indicate a Campanian to Maestrichtian age for this part of the succession. The Campanian/Maestrichtian contact is taken to occur in the upper part of Core 612-69.

Table I: Schedule of the drilling operation and lengths of the recovered core material for the boreholes on Vernagtferner in 1979

In 1979 a core drilling project was carried out on Vernagtferner in the Oetztal Alps (Austria). This report describes the field work of the drilling project, the recovered core material and the occurrence of water in the boreholes and compiles the succeding investigation program.

Mass-accumulation rates for the non-authigenic, inorganic, crystalline, eolian component of sediments recovered at DSDP Sites 62-464 to 62-466

The mass-accumulation rate (MAR) of the non-authigenic, inorganic, crystalline component of deep-sea sediments from the Pacific aseismic rises apparently reflects influx of eolian sediment. The eolian sediment usually is dominated by volcanic material, except during glacial times. Sediments from Hess Rise provide a discontinuous record of eolian MARs. During Albian to Cenomanian time, the influx of volcanic material was fairly high (0.35-0.6 g/cm**2/10**3 yr), recording the latest stages of the Albian volcanism that formed Hess Rise. From the Campanian through the Paleocene, influx of eolian sediment was low, averaging 0.03 g/cm**2/10**3 yr. None of the four Hess Rise drill sites show evidence of the Late Cretaceous volcanic episode recorded at many sites now in the equatorial to subtropical Pacific. Pliocene to Pleistocene samples record a peak in volcanic influx about 4 to 5 m.y. ago, which has been well documented elsewhere. The several-fold increase in eolian accumulation rates elsewhere which are correlated with the onset of severe northernhemisphere glaciation 2.5 m.y. ago is not obvious in the Hess Rise data.

Characteristic magnetization of upper Cretaceous and lower Tertiary sediments

Lower Campanian to middle Eocene chalks and oozes were recovered at Sites 761 and 762 of Ocean Drilling Program Leg 122 on the Exmouth Plateau, northwest Australia. Paleomagnetic analyses were made on 125 samples from Hole 761B and 367 samples from Hole 762C. Thermal cleaning, alternating field demagnetization, or mixed treatment reveals a stable remanent component of normal or reversed polarity. Correlation of the magnetic polarity sequences established for these holes with the standard magnetic polarity time scale was aided by nannofossil zonation. At Hole 761B, the sequence extends from Subchron C32-N (upper Campanian) through Subchron C17-R (middle Eocene), but given the low sedimentation rate, not all the subchrons of the standard magnetic polarity sequence were recognized. The sequence at Hole 762C extends from Subchron C13-R (middle Eocene) to the boundary between Chrons C33 and C34 (lower Campanian). The sedimentation rate is higher at Hole 762C, and all the magnetic polarity subchrons of the Campanian and Maestrichtian stages were identified. Thus, this hole could be a reference section to refine the Upper Cretaceous time scale.

Major and trace element content of the 17 lithostratigraphic units recovered during DSDP Leg 49 (Table 1)

IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions. It is apparent that few of the recovered basalts have the geochemical characteristics of typical "depleted" midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge. Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb, 87Sr/86Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in "MORB". The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an "enriched" and a "depleted" source, but rather a range of sources heterogeneous on different scales for different elements - to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.