Fig 1 - uploaded by Gina Frost
Content may be subject to copyright.
Source publication
A rock magnetic study was performed on sediment cores from four sites in the South Atlantic off the western coast of Africa, which were taken during the Ocean Drilling Program Leg 175 (Sites 1078, 1082, 1084, and 1085). The sites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentratio...
Contexts in source publication
Context 1
... ODP Leg 175, sediment cores were taken at thir- teen sites off the western coast of Africa between 5 • and 32 • S ( Wefer et al., 1998). The main purpose of the leg was to investigate the paleoceanography of the Angola-Benguela currents. Sediments from four sites out of thirteen, Sites 1078, 1082, 1084, and 1085 ( Fig. 1), were used in this study. This area contains one of the major upwelling regions in the world, and is characterized by organic-rich greenish sedi- ments with intense diagenetic activity. Site 1078 is in the Mid-Angola Basin off Lobito, and its water depth is 427 m. The site is between the high- productivity regions to both the north and ...
Context 2
... 8(a)). This supports the inference that the low-field magnetic susceptibility of these sites is dominated by paramagnetic minerals. After subtracting the high-field slope (paramagnetic behavior), we could determine the hys- teresis parameters of the ferrimagnetic fraction of the sedi- ments (Fig. 8(b)). Magnetic domain state was estimated from Fig. 10(a)). Results of sample 1082A-6H3-135 are displayed. Irreversibility of cooling and warming curves above about 100 K is indicative of magnetite. The difference in the two curves before sediments were thawed (thin black curves) is larger than that a few months after thawing (thicker gray ...
Context 3
... origin contribute more to the sediments from the Congo Basin and the Angola Basin than to the sediments off the African coast at and to the south of the Walvis Ridge, where biogenic magnetites of single domain size dominate. Remanent magnetization in a temperature range between 6 and 250 K or 300 K was measured on frozen and unfrozen samples (Fig. 10). Changes of remanent magnetization were obscured by a strong magnetic moment at low temperature from the dominant paramagnetic component, which was in- duced by a small residual field in the sample space of the low-T susceptometer (∼0.05 mT, varies with temperature and varies each time after generating a strong field) and by the ...
Context 4
... and varies each time after generating a strong field) and by the remanent magnetization of blocked paramagnetic materials at low temperature. None the less, the presence of magnetites in both frozen and unfrozen samples can be concluded from the irreversibility of magnetization curves during zero-field low-temperature cycling of 250 K IRM (Fig. 10(b)). Magne- tization during cooling down was slightly larger than during warming up above about 100 K. The loss of remanent mag- netization during the low-temperature cycle is most likely caused by passing through the magnetic isotropic point of magnetite (T I ) and the Verwey transition. The temperature of T I is at about 130 K for pure ...
Context 5
... loss of remanent mag- netization during the low-temperature cycle is most likely caused by passing through the magnetic isotropic point of magnetite (T I ) and the Verwey transition. The temperature of T I is at about 130 K for pure magnetite, and lowered by substitution of Ti 4+ (Dunlop and¨Ozdemirand¨ and¨Ozdemir, 1997). The Ver-wey transition is a crystallographic phase transition known to occur at 110-120 K for pure magnetite (Verwey, 1939). ...
Context 6
... 1939). The magnetization loss of frozen samples during the low- temperature cycle was significantly larger than that of the thawed samples. Another sign of the presence of magnetite is a slight increase of the slope of the thermal demagnetization curves around 100 K, which was detected as a small increase in the derivative (−M/T ) (an arrow in Fig. 10(a)). This could be a manifestation of the Verwey transition. Pyrrhotite cannot be expected here because magnetic transition at 30 to 34 K, indicative of pyrrhotite ( Rochette et al., 1990), was not ...
Similar publications
A rock magnetic cyclo-stratigraphy was measured for the upper Barremian-lower Aptian Cupido Formation of northeast Mexico. Anhysteretic remanent magnetization (ARM; 97 muT DC field, 100 mT peak AC field) was used to measure magnetic mineral concentration variations for the uppermost 150 m of the Cupido Fm (the `Cupidito'). The Cupido Fm is characte...
Very fine samples from the mineralized zones of the Jacupiranga complex at the Cajati mine were selected for crystallographic identification of Ti-magnesioferrite (TMf) nanostructures embedded in titanomagnetite (TM) using high-resolution transmission electron microscopy (TEM). A magnetic concentrate obtained of pyroxenite samples (sites 4 to 7) wa...
El Niño Southern Oscillation (ENSO) is an internal forcing of the climate system. This event has an actual frequency of 2 to 8 years. Evidence from a paleoclimate proxy database of gray scale (GS), in samples from Pallcacocha lake in Ecuador, indicates that the ENSO had a frequency of 35 to 75 years during the Late Pleistocene. In this work we expl...
[1] Studies of the petrology, mineral chemistry and rock-magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal rema...
The Ocean Drilling Program Leg 158 drill holes from the Trans‐Atlantic Geotraverse hydrothermal field are investigated to understand the rock magnetic signatures of hydrothermal mineralization. A composite columnar section has been constructed through hole correlation to understand the stratigraphic variation of magnetomineralogy within the stockwo...
Citations
... Rapid loss of magnetization during storage has been reported for organic-rich lake sediment cores (Oldfield et al. 1992) and for anoxic marine sediments Roberts et al. 1999;Yamazaki et al. 2000). Such modifications of magnetic properties are known as "storage diagenesis" (Oldfield et al. 1992). ...
To examine limnological conditions in Lake Ogawara on the Pacific coast of northwestern Japan, we investigated the magnetic properties of dredged bottom sediment originally collected from the lake in the summer of 2011. We used non-destructive methods to measure the low-field magnetic susceptibility shortly after sampling, and anhysteretic remanent magnetization (ARM) was assessed in 2012 and 2015. The ARM acquisition and demagnetization curves from littoral sites showed several patterns that reflect the provenance of the sediments. At water depths below 10 m, the magnetic susceptibility and ARM of greenish black mud with high organic content decreased considerably with the increase in water depth, but ARM increased slightly at water depths greater than 16 m. We also found that the magnetic concentrations of mud samples were reduced markedly during a period of storage for about 3 years. We attributed these reductions to diagenetic loss of magnetic minerals, which had been enhanced at deeper sites. It is possible that the ARM carriers in deeper areas were derived from authigenic formation of iron sulfide or from deposition of suspended matter in the hypolimnion water. We propose that the magnetic properties of surficial sediments are controlled by limnological stratification of the brackish lake water, thus possibly providing an analog for down-core variations of magnetic parameters associated with the modification of magnetic minerals during reductive diagenesis.
... Second, many of the intervals initially measured on the ship are subsequently sampled and consumed in a variety of other types of studies and thus are not available for further paleomagnetic investigations. Third, the shipboard measurements may be the only record of the paleomagnetic signal in some intervals because alteration of magnetic minerals can be rapid in some sediments when they are exposed to oxygen after being buried in reducing conditions [e.g., Richter et al., 1999; Yamazaki et al., 2000]. Thus obtaining reliable remanence measurements rapidly and continuously for every core maximizes the paleomagnetic information that can be gleaned from the relative rare core collections and helps direct more detailed shore-based analyses to intervals of greatest interest for paleomagnetic investigations. ...
We use paleomagnetic data from cores collected at Ocean Drilling Program Site 1062 on the Bahama Outer Ridge, as well as observations from prior cruises, to quantify the nature and to assess the causes of drilling overprints. The largest overprint arises from an isothermal remanent magnetization that is imparted to the core during coring or core retrieval. A second type of overprint is apparent only in soft sediments collected with a piston corer and has coercivities and unblocking temperatures that completely overlap those of the primary remanent magnetization, which means it cannot be removed by standard demagnetization methods. This demagnetization-resistant overprint can produce significant biases in paleomagnetic observations, particularly those from split core sections. We hypothesize that shearing, which occurs as a result of friction on the inside wall of the piston corer as it rips through sediments and cuts a core, deflects the ancient magnetization. We develop and test a model that predicts how shear deformation can rotate remanence-carrying grains and deflect the paleomagnetic remanence of a split core section. Using the shear model predictions and directions measured on split core sections and U channel samples from the undeformed core center, we estimate the average deformation sustained and then correct the split core data for biases caused by shear deformation. The reduction in the deviations between corrected split core data and U channel data is statistically significant, indicating that the shear model is capable of accounting for a significant portion of the systematic biases observed in paleomagnetic observations obtained from piston cores.
... We thus consider these sediments may preserve paleomagnetic field behavior. Severe and rapid loss of remanent magnetization occurred during storage of the Site 1082 cores, with less than 10% of the initial intensity remaining a few months after core recovery (Yamazaki et al., 2000). The loss of magnetization was found commonly in the sediment cores taken during ODP Leg 175. ...
... The loss of magnetization was found commonly in the sediment cores taken during ODP Leg 175. A rock magnetic study of Yamazaki et al. (2000) indicated the presence of magnetite in the sediments even after the completion of sulfate reduction, and magnetization attributable to magnetite decreased with the loss of magnetization. It is hence considered that the transformation of magnetite into non-magnetic phases after core recovery is responsible for the loss of magnetization, although contribution of iron-sulfides like greigite is not clear. ...
... Remanent magnetization of the Site 1082 sediments, as well as other cores of ODP Leg 175, was very weak, 10 −3 to 10 −5 A/m, which was an order of magnitude larger before the loss of magnetization (Yamazaki et al., 2000). Magnetic susceptibility of the Site 1082 sediments is dominated by paramagnetic materials. ...
A paleomagnetic study was performed on Hole 1082C sediment cores taken during the Ocean Drilling Pro-gram (ODP) Leg 175 in the South Atlantic in order to obtain a high-resolution Brunhes-Matuyama (B/M) polarity transition record. An average sedimentation rate was as high as 10 cm/kyr. The cores consist of strongly anoxic sediments, which is common for the areas of large material supply. Anoxic sediments, which are geochemically quite active, were considered to be unsuitable for studies on detailed behavior of the geomagnetic field such as polarity transitions. For global site distribution, however, it is necessary to make efforts to retrieve paleomagnetic records from such sediments. A continuous record of directional changes around the transition was obtained from U-channel samples after cleaning by stepwise alternating-field (AF) demagnetization. Consistency of the record was checked using discrete samples taken from the other half of the cores. The coring-induced magnetic overprint of radial-inward direction, which has often been reported from ODP piston-cores, was negligibly small in our cores. Relative paleointensity variation was estimated from remanent intensities of the discrete samples normalized by ar-tificial remanences. Our record shows following features of the B/M transition similar to those already reported by previous studies. A zone of large directional fluctuations with low paleointensities occurs just before the main transition (788 to 795 ka based on the oxygen-isotope stratigraphy), which would correspond to the "precursor" of Hartl and Tauxe (1996). The virtual geomagnetic poles (VGPs) at the precursor lie along the so-called preferred longitudinal bands over the north-south Americas and Australia-east Asia. After the main transition from the re-versed to normal polarity, VGPs stayed in the middle-to-high latitudes over the North America with an intermediate paleointensity for about 5 kyrs, and then moved in the vicinity of the North Pole with full recovery of intensity. Such behavior was reported by Oda et al. (2000). These similarities suggest that the anoxic sediments at Site 1082 could record the behavior of the geomagnetic field rather faithfully, although the remanence may be of chemical origin.
A paleomagnetic study has been conducted on three gravity cores obtained from the Ontong-Java Plateau (OJP) in the western equatorial Pacific in order to investigate long-term secular variations during the Brunhes Chron. The cores consist of nanofossil foraminiferal ooze, and covers the last 500,000–800,000 years with average sedimentation rates of 6–9 m/m.y. Downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less, and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters, ratio of anhysteretic remanent magnetization (ARM) to saturation isothermal remanent magnetization (SIRM), and S ratio. These results indicate that the sediments are rock-magnetically homogeneous in general, and are hence suitable for relative paleointensity estimation. I chose SIRM as a normalizer for relative paleointensity. The three cores could be precisely correlated with each other using relative paleointensity variations. Depth-to-age conversion was carried out by tying to the Sint-800 paleointensity stack (Guyodo and Valet, 1999). The ages are consistent with the oxygen-isotope stratigraphy available for one of the three cores. Inclination variations were obtained after alternating-field demagnetization. Large inclination anomaly, about −7° in average, was observed as has been suggested to occur in the western equatorial Pacific by some studies. The inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity. Long-term inclination variations of the order of 10,000–100,000 years were recognized. The inclinations tend to deflect toward negative when relative paleointensity is low. The variations are similar in general to the record from the West Caroline Basin (Yamazaki and Oda, 2002), which supports their model that long-term inclination variation is controlled by relative strength of the geocentric axial dipole (GAD) and persistent non-dipole components.
