Magnetic properties of ODP Hole 109-670A

Magnetic fabrics of serpentinized peridotites are related to anisomorphic magnetite formed during serpentinization. In the less serpentinized facies they are, however, mainly mimetic of the high temperature deformation prior to serpentinization. In more serpentinized peridotites, the magnetic fabrics, related to magnetite veins which are more developed in this case, are superimposed on mimetic fabrics. Remanent properties, hysteresis loop parameters, and Curie temperatures were measured. Natural remanent magnetizations (NRM) have crystallization remanent magnetic (CRM) origin. Measured magnetic parameters suggest that pseudo-single domain (PSD) grains of magnetite are present in samples with low degree of serpentinization. The samples with high degree of serpentinization contain mainly multi-domain (MD) magnetite grains.

Mineralogy and diagenesis: Their effect on acoustic and electrical properties of pelagic clays at DSDP Leg 86 Holes

Analysis of pelagic clay samples from Sites 576, 578, and 581 shows that physical, acoustic, and electrical trends with increasing burial depth are related to mineralogical and diagenetic changes. The properties of interest are bulk density (roo), porosity (phi), compressional-wave velocity (Vp) and velocity anisotropy (Ap), and electrical resistivity (Ro) and resistivity anisotropy (Ar). In general, as demonstrated in particular for the brown pelagic clay, the increase in roo, Vp, Ro, and to a lesser extent Ap and Ar with increasing depth is primarily caused by decreasing phi (and water content) as a result of compaction. The mineralogy and chemistry of the pelagic clays vary as a function of burial depth at all three sites. These variations are interpreted to reflect changes in the relative importance of detrital and diagenetic components. Mineralogical and chemical variations, however, play minor roles in determining variations in acoustic and electrical properties of the clays with increasing burial depth.

Diagenetic development of deep-sea carbonate sediments from DSDP Holes 30-228, 30-289 and 33-316

Laboratory measurements of ultrasonic velocity (VP, VS) and attenuation (QP**-1, QS**-1) in deep-sea carbonate sequences at DSDP Sites 288, 289 and 316 in the equatorial Pacific were made in conjunction with studies of sediment density, porosity and pore geometry in order to investigate the role of diagenesis in the development of physical properties. Bulk porosity decrease appears to be related more significantly to depth of burial than to age of strata. Both depth of burial and age, however, are important factors controlling the modal pore diameter. In deep-burial diagenesis the modification of pore geometry is influenced by the presence of silica during diagenesis. In carbonate sequences at the three DSDP sites studied, shear wave attenuation anisotropy (QSHH**-1/QSHV**-1) correlates with the shear wave velocity anisotropy. Pore orientation, resulting from overburden pressure and other deep-burial diagenetic processes, is an important factor controlling the increase of VP anisotropy with age and depth of burial. On the basis of observed minor changes in anisotropy values with increasing pressure for some samples, other contributions to VP anisotropy such as grain orientation and bedding lamination cannot be ruled out.

Magnetic properties and paleomagnetism of basalts from ODP Leg 107 basalts

The basalts recovered at Holes 651A and 655B appear to carry a single component remanent magnetization, which is generally of reversed polarity. These magnetizations are consistent with eruption during the Matuyama (651A) and Gilbert (655B) polarity epochs. The blocking temperature spectra and the Js/T curves indicate that titanomaghemite is the principal remanence carrier. The lower mean destructive field (MDF) and higher susceptibility at 651A probably indicates a lower mean oxidation state at this hole relative to 655B, which may simply reflect the age difference between the two basalt sequences. At both holes, a decreasing downcore trend both in natural remanent magnetization (NRM) and susceptibility probably indicates that maghemitization (from primary titanomagnetite) increases downcore. An interval of high coercivity at hole 655B (119.80-151.45 mbsf) appears to define a magnetically distinct unit within the basalt sequence.

Physical properties nd chemistry of igneous rocks from the Pigafetta and East Mariana Basins

Physical properties of basalts from Ocean Drilling Program Sites 800 and 801 in the Pigafetta Basin and Site 802 in the East Mariana Basin, including porosity, wet-bulk density, grain density, compressional wave velocity, and thermal conductivity, were measured aboard JOIDES Resolution during Leg 129. The ranges for the properties are large, as typified by the velocity, which varies from 3.46 to 6.59 km/s. Extensively altered basalts immediately above and below a silicified hydrothermal deposit (60-69 m sub-basement depth) at Site 801 display the highest porosity, and lowest bulk density, velocity, and thermal conductivity, whereas the slightly altered rocks from Site 802 and the lowermost part of Site 801 represent the other extreme in physical properties variations. In order to better establish the relationship between physical properties and alteration of the rocks, the compressional wave velocities were compared with results from major and trace elemental analyses and petrographic examination of select samples. For the Leg 129 basalts, velocity displays a generally consistent decrease with increasing K2O, H2O+, loss on ignition, and Rb contents and the value of Fe3+/FeT and decreasing concentrations of SiO2, FeOT, CaO, MgO, and MnO. These trends are consistent with trends documented for the progressive alteration of oceanic crust and indicate that on a laboratory sample scale, basalt alteration is largely responsible for the variation of the physical properties of basalts sampled at Sites 800, 801, and 802.

Magnetic measurements on ODP holes 178-1095A, 178-1095B and 178-1101A

We have determined the azimuth of bottom-current flow in drift deposit sediments recovered at ODP Sites 1095 and 1101, Antarctic Peninsula, using paleomagnetic reorientation of anisotropy of magnetic susceptibility (AMS) ellipsoids. A total of 38 cores from the two ODP sites have been measured, providing spatial and directional information on the physical record of the ACC (Antarctic Circumpolar Current) in the Plio-Pleistocene. Declination and inclination of the paleomagnetic vector of each core segment were used to reorient the AMS principal axes to the geographic coordinates. The cores were reoriented using the measured direction of the characteristic remanent magnetization (ChRM) with respect to a common reference line for the core, from which we are able to determine the orientation of the paleocurrent flow for Sites 1095 (Drift 7) and 1101 (Drift 4) relative to the geographic coordinates. Both sites have paleocurrent directions trending ~NW-SE, which in the former locality are parallel to a sediment wave field. Our study shows that a combination of magnetic fabric analysis and paleomagnetism allows deep-sea sedimentary fabric to be used as a long-term proxy of bottom-current flow history.

Paleomagnetic and rock magnetic properties of IODP Expedition 318 cores

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well-behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.

Grain size and magnetic fabric analyses

The combined use of grain size and magnetic fabric analyses provides the ability to discriminate among depositional environments in deep-sea terrigenous sediments. We analyzed samples from three different depositional settings: turbidites, pelagic or hemipelagic interlayers, and sediment drifts. Results indicate that sediment samples from these different environments can be distinguished from each other on the basis of their median grain size, sorting, as well as the intensity and shape of magnetic fabric as determined from an examination of the anisotropy of magnetic susceptibility. We use these discriminators to interpret downcore samples from the Bermuda Rise sediment drift. We find that the finer grains of the Bermuda Rise (relative to the Blake Outer Ridge) do not result from lower depositional energy (current speed) and so may reflect a difference in the nature of sediment being delivered to the site (i.e., distance from source) between the two locations.

Seismic velocities and elastic properties of gabbroic rocks from ODP Hole 118-735B

The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 735B, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, Vp and Vs were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. Measured velocities reflect the mineralogical makeup and microstructures acquired during the tectonic history of Hole 735B. Most of the undeformed and unaltered gabbros are approximately 65:35 plagioclase/clinopyroxene rocks plus olivine or oxide minerals, and the observed densities and velocities are fully consistent with the Voigt-Reuss-Hill (VRH) averages of the component minerals and their proportions. Depending on their olivine content, the predominant olivine gabbros at 200 MPa have average Vp = 7.1 ± 0.2 km/s, Vs = 3.9 ± 0.1 km/s, and grain densities of 2.95 ± 0.5 g/cm3. The less abundant iron-titanium (Fe-Ti) oxide gabbros average Vp = 6.75 ± 0.15 km/s, Vs = 3.70 ± 0.1 km/s, and grain densities of 3.22 ± 0.05 g/cm3, reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (1) directional variations in Vp, both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among Vp values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of Vs of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.

Late Miocene to Pleistocene paleoceanographic records from the Feni and Gardar Drifts: Pliocene reduction in abyssal flow

Magnetic fabric analyses from two North Atlantic drift deposits provide proxies for determining relative variations in the strength of abyssal flow over the last 10 my. The data show a cessation of current-controlled sedimentation at the shallower Feni Drift (2417 m) at the time of onset of Northern Hemisphere glaciation (2.6 Ma). Drift formation ended nearly 2 my earlier (4.2 Ma) at the deeper Gardar Drift (3220 m), implying stepwise reduction in deep-water flow. Relatively light delta18O values at the deeper Gardar Drift indicate a warmer, thus also more salty, water mass site prior to 6 Ma. We interpret this as representing Mediterranean Sea water, which flowed north at depths greater than that of the Feni Drift Site. The supply of Mediterranean Water to the North Atlantic was shut off as the Gibraltar Straits closed, causing the Messinian salinity crisis, and never returned to that position in the water column after the Mediterranean opened again.

Subgrain boundaries and related microstructural features in EDML (Antarctica) deep ice core

Subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization. Statistical data obtained from the EPICA Dronning Maud Land (EDML) deep ice core drilled in East Antarctica cannot detect a depth region of increased subgrain-boundary formation. Grain-boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths. The data do not support the classical view of a change of dominating recrystallization regimes with depth. Three major types of subgrain boundaries, reflecting high mechanical anisotropy, are specified in combination with crystal-orientation analysis.

Physical properties and AMS characteristics of ODP Site 185-1149 sediments

During Ocean Drilling Program Leg 185, we studied progressive changes of microfabrics of unconsolidated pelagic and hemipelagic sediments in Holes 1149A and 1149B in the northwest Pacific at 5818 m water depth. We paid particular attention to the early consolidation and diagenetic processes without tectonic deformation before the Pacific plate subduction at the Izu-Bonin Trench. Shape, size, and arrangement of pores were analyzed by scanning electron microscope (SEM) and were compared to anisotropy of magnetic susceptibility (AMS) data. The microfabric in Unit I is nondirectional fabric and is characterized by large peds of ~10-100 µm diameter, which are made up of clay platelets (mainly illite) and siliceous biogenic fragments. They are ovoid in shape and are mechanically packed by benthic animals. Porosity decreases from 0 to 60 meters below seafloor (mbsf) in Unit I (from 60% to 50%) in association with macropore size decreases. The microfabric of coarser grain particles other than clay in Unit II is characterized by horizontal preferred orientation because of depositional processes in Subunit IIA and burial compaction in Subunit IIB. On the other hand, small peds, which are probably made of fragments of fecal pellets and are composed of smectite and illite (3-30 µm diameter), are characterized by random orientation of clay platelets. The clay platelets in the small peds in Subunit IIA are in low-angle edge-to-face (EF) or face-to-face (FF) contact. These peds are electrostatically connected by long-chained clay platelets, which are interconnected by high-angle EF contact. Breaking of these long chains by overburden pressure diminishes the macropores, and the clay platelets in the peds become FF in contact, resulting in decreases in the volume of the micropores between clay platelets. Thus, porosity in Subunits IIA and IIB decreases remarkably downward. The AMS indicates random fabric and horizontal preferred orientation fabric in Units I and II, respectively. This result corresponds to that of SEM microfabric observations.In Subunit IIB, pressure solutions around radiolarian tests and clinoptilolite veins with normal displacement sense are seen distinctively below ~170 mbsf, probably in correspondence to the transition zone from opal-A to opal-CT.

Seismic data from Colle Gnifetti glacie saddle around borehole KCI, Swiss Alps

Two seismic surveys were carried out on the high-altitude glacier saddle, Colle Gnifetti, Monte Rosa, Italy/Switzerland. Explosive and vibroseismic sources were tested to explore the best way to generate seismic waves to deduce shallow and intermediate properties (<100 m) of firn and ice. The explosive source (SISSY) excites strong surface and diving waves, degrading data quality for processing; no englacial reflections besides the noisy bed reflector are visible. However, the strong diving waves are analyzed to derive the density distribution of the firn pack, yielding results similar to a nearby ice core. The vibrator source (ElViS), used in both P- and SH-wave modes, produces detectable laterally coherent reflections within the firn and ice column. We compare these with ice-core and radar data. The SH-wave data are particularly useful in providing detailed, high-resolution information on firn and ice stratigraphy. Our analyses demonstrate the potential of seismic methods to determine physical properties of firn and ice, particularly density and potentially also crystal-orientation fabric.

Paleomagnetic properties of ODP Hole 178-1101A

Sediments recovered from a drift deposit located on the Pacific side of the Antarctic Peninsula (ODP Leg 178, Site 1101) give a physical record of a bottom current, sourced from the Weddell Sea Deep Water, for the past 3 Ma. Sediment grain size and magnetic fabric analyses indicate a contourite depositional environment and little change in the average intensity of this current. Terrigenous fluxes decreased around the time of the onset of Northern Hemisphere Glaciation, which we interpret as a freezing of the base of the Antarctic Peninsula Ice Cap. Terrigenous fluxes have increased since 1.7 Ma implying a possible return of the Antarctic Peninsula Ice Cap to a more wet-based ice sheet.