(Table 1) Viscosity coefficients at various temperatures of samples from ODP Hole 176-735B

Ocean Drilling Program (ODP) Hole 735B was drilled to a depth of 1.5 km in a tectonic window of gabbroic lower oceanic crust created at the Southwest Indian Ridge. The gabbros have a very stable natural remanent magnetization (NRM) of reversed polarity with most unblocking temperatures slightly below the Curie temperature of magnetite. The NRM includes a drilling-induced overprint but its intensity decays strongly towards the interior of the drill core. The demagnetization data yield no or only a very small secondary magnetization component acquired during the present Brunhes chron or an earlier normal chron, suggesting cooling through most of the blocking temperature range during chron C5r and a strong resistance against the acquisition of thermoviscous magnetization. A novel furnace has been designed to measure magnetizations and their time dependences at high temperatures (up to 580 deg C) inside a commercial SQUID magnetometer. Magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550 deg C to determine the time and temperature stability of remanent magnetization. Viscosities are generally small and increase little with temperature below the main blocking temperature, where the increase becomes almost an order of magnitude. Extrapolations to geological times infer viscous acquisitions that would be 5-25% of a thermoremanence in 100 kyr and at temperatures of 200-500 deg C. At ocean bottom temperature the predicted magnetization of one sample acquired in the present Brunhes chron should be 10% of the NRM. However, this is not recognized during NRM demagnetization and partial thermoremanent magnetization (pTRM) acquisitions at 250 deg C are also much smaller than predicted. It thus appears that the NRMs are generally magnetically harder than magnetizations acquired after heating to 570 deg C in the laboratory. Susceptibility changes during heating are small (<5%) indicating a seemingly stable magneto-mineralogy, but conspicuous minima occur after heating to 520 deg C. Also, quasi paleointensity experiments reveal characteristic patterns in the NRM/pTRM ratios and also large increases in pTRM capacity after heating to 570 deg C. Moreover, anhysteretic remanent magnetization acquisition in the low field range (<=10 mT) is strongly enhanced after heating by factors up to three. The alteration of the magneto-mineralogy is interpreted to result from the annealing of defects in magnetite that originate from tectonically induced strain. The oceanic gabbros of Hole 735B are thus ideal source layer material for marine magnetic anomalies, and secondary thermoviscous acquisition, as a possible cause for anomalous skewness, is essentially absent.

(Table 1) Magnetic properties of specimens from DSDP Hole 68-503A

Measurements were made of the magnetic properties of 13 sediment samples from cores spanning the entire depth of Hole 503A. The principal aim was to make a preliminary assessment of the magnetic fabric of material obtained from hydraulic piston coring (HPC) which, though considerably bioturbated, might retain substantial traces of any depositional alignment of magnetic grains. Earlier measurements on Deep Sea Drilling Project cores (Rees, 1971; Rees and Frederick, 1974; Hailwood and Sayre, 1979) suggested that the improved HPC sampling technique should, other things being equal, provide good magnetic fabric information. The Hole 503A sediments were known from shipboard measurements to possess comparatively strong stable remanence and therefore seemed likely subjects for this assessment.

Stable isotopes, core logging data, relative paleointensity, dry bulk density, biogenic opal, CN-data (TC, TOC, CaCO3, TN), element concentrations, and coarse fraction of three sediment cores from the western Bering Sea

We used piston cores recovered in the western Bering Sea to reconstruct millennial-scale changes in marine productivity and terrigenous matter supply over the past ~180 kyr. Based on a geochemical multi-proxy approach, our results indicate closely interacting processes controlling marine productivity and terrigenous matter supply comparable to the situation in the Okhotsk Sea. Overall, terrigenous inputs were high, whereas export production was low. Minor increases in marine productivity occurred during intervals of Marine Isotope Stage 5 and interstadials, but pronounced maxima were recorded during interglacials and Termination I. The terrigenous material is suggested to be derived from continental sources on the eastern Bering Sea shelf and to be subsequently transported via sea ice, which is likely to drive changes in surface productivity, terrigenous inputs, and upper-ocean stratification. From our results we propose glacial, deglacial, and interglacial scenarios for environmental change in the Bering Sea. These changes seem to be primarily controlled by insolation and sea-level forcing which affect the strength of atmospheric pressure systems and sea-ice growth. The opening history of the Bering Strait is considered to have had an additional impact. High-resolution core logging data (color b*, XRF scans) strongly correspond to the Dansgaard-Oeschger climate variability registered in the NGRIP ice core and support an atmospheric coupling mechanism of Northern Hemisphere climates.

Rock magnetic properties of ODP Hole 109-670A

The magnetic properties of 11 samples from Site 670 of Leg 109, 3 harzburgites and 8 highly serpentinized peridotites, have been studied. Reflected light microscopy and Curie temperatures confirm that magnetite is the dominant magnetic mineral in all samples. However, both rock types show different magnetic behavior. Susceptibility, saturation magnetization, and NRM are higher for the serpentinites, because of the higher magnetite content. The hysteresis parameters indicate magnetite particles with pseudosingle domain structure for both rock types. For the remarkable anisotropy of the magnetic susceptibility no definite explanation could be found, because of the complex texture of the samples. In both rock types the presence of maghemite, a product of low temperature oxidation of magnetite, has been indicated by reflected light microscopy and by thermomagnetic analysis. As the maghemite converts to hematite at temperatures above 350°C, the temperature during the serpentinization was below this value assuming that the maghemitization took place at the same time.

(Table 1) Summary of horizon, lithology, and paleomagnetic of ODP Hole 126-791B basaltic mousse samples

A paleomagnetic study was made on the highly vesiculated basaltic tuff breccia (the basaltic mousse) drilled by Ocean Drilling Program Leg 126 from the Izu-Bonin backarc, Sumisu Rift, to estimate the mode of its emplacement. Thirty-four 10-cm**3 minicore samples were collected from almost all the horizons of the basaltic mousse. Stepwise thermal and alternating-field demagnetization experiments show that the natural remanent magnetizations of many samples are mainly composed of a single stable component. Although remanence inclinations are not expected to be disturbed by rotary drilling, the measured inclinations of remanence show a random directional distribution as a whole. The remanence inclinations, however, show directional consistency on a smaller scale. High-density sampling and measurements from a limited interval of drilled cores, and the measurement of small disks cut from a single minicore sample show that there is directional consistency over several centimeters. Strong and stable remanent magnetization, the existence of remanence direction consistency, and the fresh lithology of the samples suggest the thermal origin of remanence. Combining the paleomagnetic results with other geological, petrographical, and paleontological characters, the Hole 791B basaltic mousse can be interpreted as a subaqueous explosion breccia produced by deep-sea pyroclastic fountaining.

Rock magnetic, element, and color data of three sediment cores along the Senegalese continental margin

We present a suite of new high-resolution records (0-135 ka) representing pulses of aeolian, fluvial, and biogenic sedimentation along the Senegalese continental margin. A multiproxy approach based on rock magnetic, element, and color data was applied on three cores enclosing the present-day northern limit of the ITCZ. A strong episodic aeolian contribution driven by stronger winds and dry conditions and characterized by high hematite and goethite input was revealed north of 13°N. These millennial-scale dust fluxes are synchronous with North Atlantic Heinrich stadials. Fluvial clay input driven by the West African monsoon predominates at 12°N and varies at Dansgaard-Oeschger time scales while marine productivity is strongly enhanced during the African humid periods and marine isotope stage 5. From latitudinal signal variations, we deduce that the last glacial ITCZ summer position was located between core positions at 12°26' and 13°40'N. Furthermore, this work also shows that submillennial periods of aridity over northwest Africa occurred more frequently and farther south than previously thought.

Rock magnetic of serpentinite seamounts in the Mariana and Izu-Bonin regions

During Leg 125, scientists drilled two serpentinite seamounts: Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. Grain densities of the serpentinized peridotites range from 2.44 to 3.02 g/cm**3. The NRM intensity of the serpentinized peridotites ranges from 0.01 to 0.59 A/m and that of serpentine sediments ranges from 0.01 to 0.43 A/m. Volume susceptibilities of serpentinized peridotites range from 0.05 * 10**-3 SI to 9.78 * 10**-3 SI and from 0.12 * 10**-3 to 4.34 * 10**-3 SI in the sediments. Koenigsberger ratios, a measure of the relative contributions of remanent vs. induced magnetization to the magnetic anomaly, vary from 0.09 to 80.93 in the serpentinites and from 0.06 to 4.74 in the sediments. The AF demagnetization behavior of the serpentinized peridotites shows that a single component of remanence (probably a chemical remanence carried by secondary magnetite) can be isolated in many samples that have a median destructive field less than 9.5 mT. Multiple remanence components are observed in other samples. Serpentine sediments exhibit similar behavior. Comparison of the AF demagnetization of saturation isothermal remanence and NRM suggests that the serpentinized peridotites contain both single-domain and multidomain magnetite particles. The variability of the magnetic properties of serpentinized peridotites reflects the complexity of magnetization acquired during serpentinization. Serpentinized peridotites may contribute to magnetic anomalies in forearc regions.

Paleomagnetism of basalts from ODP Hole 109-648A

Paleomagnetic parameters of 55 basalt samples from Hole 648B on the Mid-Atlantic Ridge were studied. Negative NRM inclinations were found for 10 samples between 10 and 25 m sub-bottom depth. Several hypotheses related to this phenomenon are discussed. NRM intensities, susceptibilities, median destructive fields, and Koenigsberger ratios are slightly different for pillow and massive basalts. One can suggest from measured parameters that magnetic carriers for massive and pillow basalts are PSD titanomagnetite grains more or less close to PSD-MD threshold size with a low degree of alteration.