Magnetic properties of light and dark sediment layers from the Japan Sea

Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K, Kfd, ARM, SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers, testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations, with glacial stages characterized by a lower magnetic mineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35-0.40 Ma ago.

(Table 1) Anisotropy of magnetic susceptibility parameters of ODP Hole 134-833B

Ocean Drilling Program (ODP) Sites 832 and 833 were drilled in the intra-arc North Aoba Basin of the New Hebrides Island Arc (Vanuatu). High volcanic influxes in the intra-arc basin sediment resulting from erosion of volcanic rocks from nearby islands and from volcanic activity are associated with characteristic magnetic signals. The high magnetic susceptibility in the sediment (varying on average from 0.005 to more than 0.03 SI) is one of the most characteristic physical properties of this sedimentary depositional environment because of the high concentration of magnetites in redeposited ash flows and in coarse-grained turbidites. Susceptibility data correlate well with the high resolution electrical resistivity logs recorded by the formation microscanner (FMS) tool. Unlike the standard geophysical logs, which have low vertical resolution and therefore smooth the record of the sedimentary process, the FMS and whole-core susceptibility data provide a clearer picture of turbiditic sediment deposition. Measurements of Curie temperatures and low-temperature susceptibility behavior indicate that the principal magnetic minerals in ash beds, silt, and volcanic sandstone are Ti-poor titanomagnetite, whereas Ti-rich titanomagnetites are found in the intrusive sills at the bottom of Site 833. Apart from an increase in the concentration of magnetite in the sandstone layer, acquisition of isothermal and anhysteretic remanences does not show significant differences between sandstone and clayey silts. The determination of the anisotropy of magnetic susceptibility (AMS) in more than 400 samples show that clayey siltstone have a magnetic anisotropy up to 15%, whereas the AMS is much reduced in sandstone layers. The magnetic susceptibility fabric is dominated by the foliation plane, which is coplanar to the bedding plane. Reorientations of the samples using characteristic remanent magnetizations indicate that the bedding planes dip about 10° toward the east, in agreement with results from FMS images. Basaltic sills drilled at Site 833 have high magnetic susceptibilities (0.05 to 0.1 SI) and strong remanent magnetizations. Magnetic field anomalies up to 50 µT were measured in the sills by the general purpose inclinometer tool (GPIT). The direction of the in-situ magnetic anomaly vectors, calculated from the GPIT, is oriented toward the southeast with shallow inclinations which suggests that the sill intruded during a reversed polarity period.

Magnetic properties of ODP Hole 116-717C

Dark gray and black mud turbidites cored on ODP Leg 116 commonly yielded large magnetic susceptibility peaks. What is more, these peaks displayed different shapes suggesting variations in sedimentological processes. Consequently, a detailed study of the magnetic properties of two of these turbidites was undertaken to better understand the source of their unusual magnetism. Physical properties were measured as was the demagnetization behavior of sample natural remanent magnetizations (NRMs). Subsequently, an anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) were imparted to the samples, demagnetized, and various grain size tests based on the behavior of these remanences were applied. Finally, magnetic concentrates from two samples were examined with a scanning electron microscope with the capability to do energy dispersive X-ray (EDX) analysis. The turbidites stand out from surrounding layers because of their high susceptibilities, NRMs, ARMs, SIRMs, and ratios of ARM and SIRM to susceptibility. Their alternating field and thermal demagnetization properties and IRM acquisition curves are consistent with titanomagnetite grains as the primary magnetic mineral with some amount of hematite present. These properties are very similar to those published for samples from the Deccan flood basalts and suggest this formation as a possible source of the magnetic grains. Magnetic granulometry tests implied that the magnetic particles behave dominantly as single-domain and pseudo-single-domain grains. Moreover, they also implied that the large variation in susceptibility observed in the black mud turbidites results from a tenfold increase in the concentration of titanomagnetite grains. Electron microscope, EDX, and SIRM analyses revealed detrital titanomagnetites with typical sizes around 8-10 µm, but as large as 20-25 µm. These are probably the dominant magnetic grains in the black mud turbidites; however, ARM and susceptibility frequency-dependence suggested that there may also be a submicrometer fraction present. Most of the observed titanomagnetite grains are tabular and some display exsolution lamellae, accounting for the pseudo-single-domain behavior despite their moderate sizes. We hypothesize that the magnetic mineral concentration variations are brought about by sedimentological factors. The heavier magnetic minerals may tend to sink to the bottom of a turbidite; however, sometimes turbidite turbulence may act to keep these tabular, medium-size grains in suspension longer than some other larger or more equidimensional grains. Consequently, the susceptibility peak shape may reflect the turbidite current velocities as well as other sedimentological factors.

Magnetic susceptibility stratigraphy for Pleistocene and Pliocene sediments of ODP Leg 110 samples

Whole-core magnetic susceptibility measurements define a detailed stratigraphy that enables correlation between the various Pleistocene, Pliocene, and upper Miocene sections cored on ODP Leg 110, near the Tiburon Rise. The magnetic susceptibility in these sections is primarily related to the content of volcanic ash, rich in titanomagnetite, and also inversely related to calcium carbonate content. The high resolution of the susceptibility record enables correlations with a resolution of about 0.3 m of sediment thickness, and the identification of minor faults not definable by biostratigraphic means. Reverse and normal faults identified in Hole 672A are probably a result of normal oceanic sediment dewatering and compaction processes. This work indicates some of the problems of using visible ash layers as time-stratigraphic markers.

Geochemical data and magnetic measurements of IODP Expedition 308, Hole U1319A from the Gulf of Mexico

A 160 m mostly turbiditic late Pleistocene sediment sequence (IODP Expedition 308, Hole U1319A) from the Brazos-Trinity intraslope basin system off Texas was investigated with paleo- and rock magnetic methods. Numerous layers depleted in iron oxides and enriched by the ferrimagnetic iron-sulfide mineral greigite (Fe3S4) were detected by diagnostic magnetic properties. From the distribution of these layers, their stratigraphic context and the present geochemical zonation, we develop two conceptual reaction models of greigite formation in non-steady depositional environments. The "sulfidization model" predicts single or twin greigite layers by incomplete transformation of iron monosulfides with polysulfides around the sulfate methane transition (SMT). The "oxidation model" explains greigite formation by partial oxidation of iron monosulfides near the iron redox boundary during periods of downward shifting oxidation fronts. The stratigraphic record provides evidence that both these greigite formation processes act here at typical depths of about 12-14 mbsf and 3-4 mbsf. Numerous "fossil" greigite layers most likely preserved by rapid upward shifts of the redox zonation denote past SMT and sea floor positions characterized by stagnant hemipelagic sedimentation conditions. Six diagenetic stages from a pristine magnetite-dominated to a fully greigite-dominated magnetic mineralogy were differentiated by combination of various hysteresis and remanence parameters.

Magnetic measurements of ODP Hole 178-1095B on the Pacific continental rise of the West Antarctic Peninsula

In this study we present a late Miocene - early Pliocene record of sixty-four zones with prominent losses in the magnetic susceptibility signal, taken on a sediment drift (ODP Site 1095) on the Pacific continental rise of the West Antarctic Peninsula. The zones are comparable in shape and magnitude and occur commonly at glacial-to-interglacial transitions. High resolution records of organic matter, magnetic susceptibility and clay mineral composition from early Pliocene intervals demonstrate that neither dilution effects nor provenance changes of the sediments have caused the magnetic susceptibility losses. Instead, reductive dissolution of magnetite under suboxic conditions seems to be the most likely explanation. We propose that during the deglaciation exceptionally high organic fluxes in combination with weak bottom water currents and prominent sediment draping diatom ooze layers produced temporary suboxic conditions in the uppermost sediments. It is remarkable that synsedimentary suboxic conditions can be observed in one of the best ventilated open ocean regions of the World.

Magnetic properties of two cores from the Iberian Margin

Magnetic properties of late Quaternary sediments on the SW Iberian Margin are dominated by bacterial magnetite, observed by transmission electron microscopy (TEM), with contributions from detrital titanomagnetite and hematite. Reactive hematite from eolian dust, together with low organic matter concentrations and the lack of sulfate reduction, lead to dissimilatory iron reduction and availability of Fe(II) for abundant magnetotactic bacteria. Magnetite grain-size proxies (kARM/k and ARM/IRM) and S-ratios (sensitive to hematite) vary on stadial/interstadial timescales, contain orbital power, and mimic planktic d18O. The detrital/biogenic magnetite ratio and hematite concentration are greater during stadials and glacial isotopic stages, reflecting increased detrital (magnetite) input during times of lowered sea level, coinciding with atmospheric conditions favoring hematitic dust supply. Magnetic susceptibility, on the other hand, has a very different response being sensitive to coarse detrital multidomain (MD) magnetite associated with ice-rafted debris (IRD). High susceptibility and/or magnetic grain size coarsening, mark Heinrich stadials (HS), particularly HS2, HS3, HS4, HS5, HS6 and HS7, as well as older Heinrich-like detrital layers, indicating the sensitivity of this region to fluctuations in the position of the polar front. Relative paleointensity (RPI) records have well-constrained age models based on planktic d18O correlation to ice-core chronologies, however, they differ from reference records (e.g. PISO) particularly in the vicinity of glacial maxima, mainly due to inefficient normalization of RPI records in intervals of enhanced detrital/eolian hematite input.

Magnetic susceptibility and documentation of sediment cores from the Antarctic Peninsula Pacific margin

The effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the Antarctic Peninsula Pacific margin. Nineteen sediment cores were visually described, logged for magnetic susceptibility, and X-radiographed. About 1000 analyses were performed for grain size, clay minerals and biostratigraphy (foraminifera, nannofossils and diatoms). Four sediment types associated with distinct sedimentary processes are recognised based on textural/compositional analysis. (1) Hemipelagic mud forms the bulk of the interglacial sediment, and accumulated from the pelagic settling of bioclasts and ice-rafted/windtransported detritus. (2) Terrigenous mud forms the bulk of the glacial sediment, and accumulated from a combination of sedimentary processes including turbidity currents, turbid plumes, and bottom current reworking of nepheloid layers. (3) Silty deposits occurring as laminated layers and lenses, represent the lateral spillout of lowdensity turbidity currents. (4) Lastly, glacial/interglacial gravelly mud layers derive from settling of ice-rafted detritus. Five depositional settings are interpreted within sediment Drift 7, each characterised by the dominance/interaction of one or several depositional processes. The repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages (glaciation, glacial, deglaciation, and interglacial), each one characterised by a distinctive clay mineral assemblage and bioclastic content. Variations in clay mineral assemblage within interglacial stage 5 (core SED-06) suggest minor colder climatic fluctuations, possibly correlatable with substages 5a to 5e.

Low-temperature magnetic remanence and hysteresis measurements on sediment core GeoB4901-8 from the Niger deep-sea fan

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300-5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe-Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field (BC). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6-13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in BC for the suboxic samples contrasts a 20-25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in BC at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe Kalpha and Ti Kalpha lines of the Fe-Ti oxides may be used to differentiate between members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (~19%), titanomagnetite (~62%), hemoilmenite (~17%) and ~2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (~66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ~5% titanomagnetite was detected. The remaining ~28% comprises Ti-rich particles such as pseudobrookite or rutile.

Paleomagnetic and rock magnetic data from IODP Site U1308

Integrated Ocean Drilling Program (IODP) Site U1308 (central North Atlantic) records paleomagnetic directional and relative paleointensity (RPI) variations for the last 1.5 Myr, in 110 m of the sediment sequence at a mean sedimentation rate of 7.3 cm/kyr. A detailed benthic oxygen isotope record was combined with RPI to produce an integrated, high-resolution magneto-isotopic stratigraphy for Site U1308. Apart from the well-known polarity reversals in this interval, the Punaruu excursion is recorded at 1092 ka and the Cobb Mountain Subchron in the 1182-1208 ka interval. The paleointensity proxies are determined as slopes of NRM versus ARM and NRM versus ARMAQ (ARM acquisition) with linear correlation coefficients to monitor the quality of the linear fit. The RPI record for Site U1308 is compared with the three other paleointensity records (one from the Western Equatorial Pacific and two from the North Atlantic) that cover the same time interval and have accompanying oxygen isotope records. The Match protocol of Lisiecki and Lisiecki (2002) is used to optimize the correlation of paleointensity records. Beginning with the original (published) age models for each record, the Match routine is used to optimize the RPI correlations to Site U1308, with checks to ensure compatibility with oxygen isotope records. Squared wavelet coherence (WTC) indicates significant improvement in RPI (and oxygen isotope) correlations after matching each RPI record to Site U1308, particularly for periods > 10 kyr. The level of coherence for the Atlantic RPI records and the lower resolution Pacific record implies synchronous global variability (at scales > 10 kyr) that can be attributed to the axial dipole geomagnetic field.