Rock magnetic properties of ODP Leg 134 sites

During Leg 134, the influence of ridge collision and subduction on the structural evolution of island arcs was investigated by drilling at a series of sites in the collision zone between the d'Entrecasteaux Zone (DEZ) and the central New Hebrides Island Arc. The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain that extends from the northern New Caledonia Ridge to the New Hebrides Trench. High magnetic susceptibilities and intensities of magnetic remanence were measured in volcanic silts, sands, siltstones, and sandstones from collision zone sites. This chapter presents the preliminary results of studies of magnetic mineralogy, magnetic properties, and magnetic fabric of sediments and rocks from Sites 827 through 830 in the collision zone. The dominant carrier of remanence in the highly magnetic sediments and sedimentary rocks in the DEZ is low-titanium titanomagnetite of variable particle size. Changes in rock magnetic properties reflect variations in the abundance and size of titanomagnetite particles, which result from differences in volcanogenic contribution and the presence or absence of graded beds. Although the anisotropy of magnetic susceptibility results are difficult to interpret in terms of regional stresses because the cores were azimuthally unoriented, the shapes of the susceptibility ellipsoids provide information about deformation style. The magnetic fabric of most samples is oblate, dominated by foliation, as is the structural fabric. The variability of degree of anisotropy (P) and a factor that measures the shape of the ellipsoid (q) reflect the patchy nature of deformation, at a micrometer scale, that is elucidated by scanning electron microscope analysis. The nature of this patchiness implies that deformation in the shear zones is accomplished primarily by motion along bedding planes, whereas the material within the beds themselves remains relatively undeformed.

Granulometry, biogenic silica content, magnetic susceptibility and bulk densities of ODP Site 188-1165

The Prydz Bay area is a key region for studying and understanding the history of the eastern Antarctic Continental Ice Sheet (O'Brien, Cooper, Richter, et al., 2001, doi:10.2973/odp.proc.ir.188.2001). Ocean Drilling Program (ODP) Site 1165 is situated in a water depth of 3357 m on the continental rise offshore from Prydz Bay and lies in front of the outlet for the Lambert Glacier-Amery Ice Shelf system that today drains 22% of East Antarctica. The site was drilled into mixed pelagic and hemipelagic sediments from the southwestern side of the Wild Drift. The drift is an elongate sediment body formed by the interaction of sediment supplied from continental shelf and slope with westward-flowing bottom currents. The sedimentary sequence is characterized by alternations between a generally gray to dark gray facies and a green to greenish gray facies. The greenish facies are structureless diatom-bearing clays with common bioturbation and larger amounts (>15%-20%) of biogenic silica, dispersed clasts, and lonestones than the dark gray facies, which are mostly less bioturbated clay with some silt laminations (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.103.2001). High-quality advanced piston corer and extended core barrel cores containing nearly complete sections of middle Miocene to early Pliocene age allow a detailed characterization of sedimentary cycles and can provide indications for ice advances of the Lambert Glacier system into Prydz Bay, for the extent of sea ice, and for changes in oceanic circulation. The purpose of this work is to provide a data set of coarse-fraction mass percentage (>63, >125, and >250 µm) and biogenic silica content measured on sediments of late Miocene to early Pliocene age drilled at Site 1165. Additionally, high-resolution records of magnetic susceptibility (MS) and gamma ray attenuation (GRA) bulk density are presented. These shipboard data sets were edited postcruise. Furthermore, I provide a high-resolution dry bulk density record that is derived from GRA bulk density and can be used for the calculation of mass accumulation rates. These sedimentological and physical parameters will be used in future work to understand the depositional pattern of alternating biogenic and terrigenous sediments that was observed at Site 1165 (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.103.2001).

Magnetic susceptibility of deep-sea cores from the Scotia Sea

We studied two deep-sea cores from the Scotia Sea to reconstruct past atmospheric circulation in the southern hemisphere and to resolve a long-standing debate on the interpretation of magnetic susceptibility (MS) records in Southern Ocean (SO) sediment. High-sedimentation sites MD07-3134 (0.2 - 1.2 m/kyr) and MD07-3133 (0.3 - 2 m/kyr) cover the last 92.5 kyr and 36 kyr, respectively. Both exhibit a one-to-one coupling of the MS and Ca2+ signal to the non-sea salt (nss) Ca2+ signal of the EDML ice core, clearly identifying atmospheric circulation as means of distribution. Comparison of additional proxies also excludes major influence by volcanic sources, sea-ice, icebergs, or oceanic current transport. The close resemblance of the dust proxies over the last glacial cycle, in turn, allows for the establishment of an age model of unprecedented resolution and precision for SO deep-sea sediment because atmospheric transport involves no major leads or lags. This is of particular importance because MS is routinely measured on deep-sea cores in the SO but the sediments usually lack biogenic carbonate and therefore had only limited stratigraphic control so far. Southern South America (SSA) is the likely source of eolian material because Site MD07-3133, located closer to the continent, has slightly higher MS values than Site MD07-3134, and also the MS record of Patagonian Site SALSA shows comparable variability. Patagonia was the dust source for both the Scotia Sea and East Antarctica. Dust fluxes were several times higher during glacial times, when atmospheric circulation was either stronger or shifted in latitude, sea level was lowered, shelf surfaces were exposed, and environmental conditions in SSA were dominated by glaciers and extended outwash plains. Hence, MS records of SO deep-sea sediment are reliable tracers of atmospheric circulation, allowing for chronologically-constrained reconstructions of the circum Antarctic paleoclimate history.

Magnetic susceptibility of bottom sediments from the Middle and South Caspian Sea

Dependence of magnetic susceptibility of bottom sediments from the Caspian Sea on composition of magnetoactive minerals contained in the heavy subfraction of fine-grained sand (0.125-0.100 mm grain size fraction) was established. Changes in the curve shape and magnetic susceptibility values reflect a pulsating pattern of input of different (in magnetic properties) magmatic and metamorphic clastic minerals into sediments, as well as different intensities of formation of authigenic magnetoactive iron sulfides under conditions of multiple alternation of transgressive and regressive phases in marine basins. Values of magnetic susceptibility and shapes of magnetic susceptibility curves for studied sedimentary sequences show that sediments in the South and Middle Caspian Basins are characterized by different specific features.

Magnetic properties of igneous rocks recovered at ODP Leg 127 sites

Measurements of natural remanent magnetization (NRM), initial susceptibility (K), anisotropy of magnetic susceptibility, frequency dependent susceptibility (Xfd), and viscous remanent magnetization (VRM) are reported from volcanic rocks recovered during ODP Leg 127 in the Japan Sea. The results indicate a significant difference between the basalts drilled in the Yamato Basin (Site 794 and 797) and in the Japan Basin (Site 795). The Koenigsberger ratios (Q) show very low values in the Yamato Basin attesting that the remanence is not dominant over the induced magnetization. This evidence could explain why no magnetic anomaly pattern has been recognized in this basin. Experiments of VRM acquisition and decay show that both the processes are multistage with the acquisition process proceeding more rapidly and deviates more from a log (t) law than the corresponding decay. The sediments interlayered with the basalts in the acoustic basement of the Yamato Basin show processes of remagnetization related to the emplacement of the dikes. Temperatures of heating between 200° and 250°C were estimated from the different unblocking temperatures of the two components of magnetization.

Magnetic susceptibility and ice-rafted debris in surface sediments of the Nordic Sea

Magnetic susceptibility and ice-rafted debris of surface sediments in the Nordic Seas were investigated to reconstruct source areas and recent transport pathways of magnetic minerals. From the distribution of magnetic susceptibility and ice-rafted debris and published data on petrographic tracers for iceberg drift, we reconstructed a counter-clockwise iceberg drift pattern during cooler phases in the Holocene, which is similar to conceptual and numerical models for Weichselian iceberg drift. The release of basaltic debris at Scoresby Sund played a significant role for the magnetic signature of stadial/interstadial events during isotope stage 3 recorded in sediment cores of the Nordic Seas.

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.

(Table 1) Magnetic susceptibility, lithologic composition and stable isotope record for ODP Hole 132-810C

Site 810 was drilled atop Shatsky Rise during Ocean Drilling Program (ODP) Leg 132. The principal objective at Site 810 was to drill interbedded cherts and chalks of Mesozoic age using the diamond coring system (DCS). The objective was not achieved because of difficulties in setting up the reentry cone on the seafloor; however, a shortened section of Cretaceous-Cenozoic nannofossil ooze was recovered with the advanced piston corer (APC). Although the section is interrupted by hiatuses, the upper 50 m carry detailed information relating to biogenic productivity, water chemistry, and eolian input during the Pliocene and Pleistocene. Four holes were drilled at Site 810. Hole 810A consists of a single mud-line core for an ongoing ODP geriatric study. The second hole (Hole 810B) was washed to 60 mbsf (without core recovery) to provide information required for setting the 16-in. casing attached to the reentry cone. Hole 810C penetrated 136.1 mbsf, mostly with the APC, with a total recovery of 143.81 m of nannofossil ooze. A reentry cone was placed over Hole 810D but no casing was successfully suspended in the hole and no sediment was cored. This data report presents the results of shore-based high-resolution analyses of carbonate and oxygen isotopic variations in the upper 50 m of the section at Site 810 and compares these variations with the shipboard determinations of magnetic susceptibility and GRAPE bulk density from the multisensor track.