Anhysteric Remanent Magnetization Research Articles

Mapping magnetism: Geophysical modelling of stratigraphic features by using in situ magnetic susceptibility measurements at Pinnacle Point 5‐6 North, South Africa

AbstractThis study utilizes geostatistical modelling of magnetic susceptibility (MS) for geophysical prospection of archaeological stratigraphy at the Middle Stone Age rock shelter site of Pinnacle Point 5‐6 North. These models are overlaid onto high‐resolution photography of the stratigraphic sequence to study the lateral and vertical changes within the magnetic signature of the archaeological sequence and correlate these changes to micromorphological interpretations previously made at the site. In situ analysis is reinforced by laboratory magnetic mineralogical analysis utilizing MS; frequency‐dependent susceptibility (χFD); isothermal remanent magnetization; and anhysteric remanent magnetization to understand the composition of the magnetic minerals creating the in situ signature. This study shows that there is consistent variation in the magnetic signatures of the sequence that can be mapped with in situ MS; there is a correlation with laboratory analysis of magnetic mineralogy, which provides insight into changes in human behaviour; and our models correlate well with micromorphological interpretations of the site.

A test of the pseudo-Thellier technique for determining relative palaeointensity in the Tirna Basin, Osmanabad, Maharashtra, India

Abstract A relative palaeointensity determination was obtained using the pseudo-Thellier technique on sediment from Ther, Tirna Basin, Latu-Osmanabad District, Maharashtra, India. The stability of the natural remanent magnetization was investigated by alternating field (AF) demagnetization. Rock magnetic studies suggest that the main carriers of magnetization are ferrimagnetic minerals, predominantly pseudo-single-domain magnetite. To varying degrees, the smoothed palaeoinclination and palaeodeclination patterns of the Tirna Basin are similar to other Asian palaeosecular variation records CALS3k.4, CALS10k.1 and SED3k.1, with an age offset. Measurements of intensity of the natural remanent magnetization left after AF demagnetization v. intensity of anhysteric remanent magnetization gained at the same peak were carried out on a set of samples. A jackknife re-sampling scheme provides error estimates for the palaeointensity. A good agreement pattern can be observed between the Tirna Basin relative palaeointensity proxy and other global curves with an age shift. Although some temporal offsets of palaeointensity features between different records have been recognized, their similar shape suggests that the palaeointensity can give a globally coherent signal and may be used as a relative dating technique. For the first time, relative palaeointensity data for the past 2 kyr from India is presented here, which complement the existing archaeological records but with an additional input.

Investigating the potential of using environmental magnetism techniques as pollution proxy in urban road deposited sediment

The use of mineral magnetic techniques as pollution proxy for road deposited sediment was explored using various statistical approaches. Standard techniques were adopted for measurement of mineral magnetic and geochemical parameters. The analyses of magnetic parameters revealed that the samples were dominated by ferrimagnetic minerals and multidomain grains. This implied that the magnetic fractions in the samples might be of anthropogenic origin. Results also indicate that the samples were dominated by low coercive, magnetically soft minerals. Thermomagnetic curves confirmed magnetite as the remanence bearing magnetic mineral having a Curie point temperature of ~580 °C. The strong association observed between magnetic susceptibility, susceptibility of anhysteric remanent magnetization and saturation isothermal remanent magnetization and aluminum, titanium, manganese, iron, chromium and lead demonstrated that these metals occurred as ferrimagnetic particles of technogenic origin resulting from vehicular sources. Assessment of pollution status of the road deposited sediment identified silicon and lead as the priority pollutants of concern. Generally, pollution load index was <1 (mean, 0.66 ± 0.14), indicating that the samples were not polluted in the overall, but the metals were in the buildup stage requiring constant monitoring. The sources of pollutants from principal component and cluster analyses identified the sources of pollution to be mainly from vehicular emissions such as brake linings, exhaust materials, tire wear, corroded metal parts, abrasion of lubricating oil and road construction materials. This study found that mineral magnetic techniques offer great potential as pollution proxy for soil pollution studies.

Magnetic properties of LiZnCu ferrite synthesized by the microwave sintering method

Abstract Lithium ferrites have attracted considerable attention because they have been used as replacements for garnets due to their low cost. A series of polycrystalline ferrite samples were prepared with the composition of Li X Zn (0.6−2 X ) Cu 0.4 Fe 2 O 4 ( X =0.05, 0.1, 0.15, 0.2, 0.25, 0.3) at chemical reaction temperature 150 °C by sintering with microwave assisted combustion method. The characterization shows the formation of single phase cubic structure when carried out by using the X-rays technique and I – R technique. Magnetization parameters such as saturation magnetization, coercivity, magnetic moment were calculated by using the Hysteresis graph. The Curie temperature obtained using the susceptibility data are found to be in the range 350–700 °C. Anhysteric remanent magnetization is used for estimating the grain size and domain structure of the composition. An attempt has been made to synthesis the nano-particles at lower reaction temperature by using non-conventional microwave sintering method. The advantage of this method is its lower sintering temperature and time compared to the conventional ceramic technique and direct formation of nano-ferrites without ball-milling.

Enhanced methane flux event and sediment dispersal pattern in the Krishna–Godavari offshore basin: Evidences from rock magnetic techniques

Detailed rock magnetic measurements such as susceptibility (χ), susceptibility of anhysteric remanent magnetization (ΧARM), saturation isothermal remanent magnetization (SIRM), soft-isothermal remanent magnetization (soft-IRM) and hard-IRM (HIRM) were carried out on the topmost sediment samples of 73 gravity cores collected in the Krishna–Godavari (KG) offshore basin to understand the distribution of magnetic parameters and the associated diagenetic signatures related to upward methane flux. The variation in magnetic parameters in the KG offshore basin fluctuates around the average value (Χ = 105 × 10−8 m3 kg−1, ΧARM = 1.22 × 10−5 m3 kg−1, SIRM = 2040 × 10−5 Am2 kg−1, soft-IRM = 1807 and HIRM = 82) except at the location GC-07 (15.767°N, 81.814°E) where the magnetic parameters Χ, ΧARM and SIRM drop to ∼1/6th, ∼1/14th and ∼1/8th of the average near surface value indicating substantial loss in the concentration of primary magnetic iron oxide minerals. Anomalous low value of ΧARM/Χ and ΧARM/SIRM at GC-07 suggests that the magnetic remanence property is governed by coarser pseudo-single domain (PSD) or multi-domain (MD) grains. To understand the nature of the anomalous magnetic parameters at GC-07, complete downcore variation of magnetic parameters was also studied. The magnetic parameters were found to be even more depleted at all depths with susceptibility dropping (at 5.2 mbsf) to ∼1/200th of its average near surface value. Interestingly, this core shows a rapid increase in the concentration of dissolved methane from ∼0 nM at surface to ∼153 nM at around 5.2 mbsf where the sulfate concentration is also reduced to zero. The anaerobic oxidation of methane (AOM) and the sulfate methane transition zone (SMTZ) occur between ∼4.2 and 5.2 mbsf based on the present day sulfate concentration profile. The close association between the dissolution of magnetic iron oxide minerals and AOM at GC-07 suggests that the reduced magnetization is primarily due to the release of HS− by AOM. The magnetic iron oxide minerals are dissolved not only at SMTZ but also above the current depth of the SMTZ. It is likely that the methane flux was higher in the past leading to upward shift of the SMTZ, and the HS− released by AOM facilitated the dissolution of magnetic iron oxide minerals at shallower depths. Alternatively, the HS− diffusing upward from the current depth of SMTZ might have lead to dissolution at shallower depth. Therefore, additional geological/geochemical investigations are warranted at GC-07 to establish the dissolution mechanism of magnetic iron oxide minerals.The variability in different magnetic parameters within the slope basin and channels shows systematic decrease in the concentration of magnetic minerals and increase in the abundance of stable single domain (SSD) magnetic grains with water depth. The observed strong correlation between the silt fraction and the magnetic susceptibility for the slope basin and channels suggests that the variation in magnetic parameters is predominantly controlled by the sediment sorting/transport process as finer particles are deposited in deeper water.

Comparison and calibration of nonheating paleointensity methods: A case study using dusty olivine

We present a comparative study of nonheating paleointensity methods, with the aim of determining the optimum method for obtaining paleointensities from “dusty olivine” in chondritic meteorites. The REM method, whereby thermoremanent magnetization (TRM) is normalized by saturation isothermal remanent magnetization (SIRM), is shown to “over normalize” TRM in dusty olivine due to the transformation of stable single‐vortex (SV) states to metastable single‐domain (SD) states in a saturating field. The problem of over normalization is reduced in the REMc and REM' methods, which more effectively isolate the high‐coercivity stable SD component of remanence. A calibration factor of f = 1600 (1000 &lt; f &lt; 2900) is derived for the REM' method. Anhysteric remanent magnetization (ARM) is shown to be a near perfect analogue of TRM in the stable SD component of dusty olivine. ARM normalization of the high‐coercivity (100–150 mT) remanence with a calibration factor fARM = 0.91 (0.7 &lt; fARM &lt; 1.2) yields paleofield estimates within ± 30% of the actual field values for SD dominated samples. A Preisach method for simulating TRM acquisition using information extracted from first‐order reversal curve (FORC) diagrams is shown to work well for SD dominated samples, but fails when there is a large proportion of SV remanence carriers. The failure occurs because (1) SV states are not properly incorporated into the Preisach distribution of remanence carriers, and (2) the acquisition of TRM by SV states is not properly modeled by the underlying SD thermal relaxation theory.

Pre- and post-monsoon variations in the magnetic susceptibilities of soils of Mumbai metropolitan region: implications to surface redistribution of urban soils loaded with anthropogenic particulates

Mumbai metropolitan region (MMR) in India represents one of the most industrialized and thickly populated areas of the monsoon dominated Asian region. We present here pre- and post-monsoon magnetic susceptibility variations in the top-soils representing sampling domains of industrial, heavy traffic and forested areas within MMR. The rock magnetic studies (including isothermal and anhysteric remanent magnetization and hysteresis loop analysis) infer predominant pseudo single domain to multi domain grains in an overall ferrimagnetic dominant mineralogy of the soils. The susceptibility-temperature variations (from −190 to 700°C) infer maghemite (γ-Fe2O3) as the chief mineral component of pedogenic origin, and the pure magnetite (Fe3O4) is of anthropogenic nature. Spatial distribution of ferrimagnetic concentration is in agreement with polluting sources. The post-monsoon redistribution pattern is greatly controlled by the surface runoff and topographic conditions. The study demonstrates that in a ferrimagnetically reach substrate like MMR, the spatial distribution patterns derived from routine concentration- and grain-size-dependent rock magnetic parameters integrated with topographic and seasonal attributes yield significant information on the style and surface re-distribution of anthropogenically loaded soils and sediments to identify its seasonal dumping. Alternatively, knowing the source of signal, the magnetic susceptibility can be further used as a robust parameter to produce detailed maps to monitor the pollutions in urban areas.

Application of the pseudo-Thellier technique to a paleomagnetic record from lake El Trébol (Patagonia, Argentina)

&#x0D; &#x0D; &#x0D; The pseudo-Thellier technique was applied to obtain relative paleointensity determinations using a sediment core from Lake El Tre?bol (Patagonia, Argentina).&#x0D; &#x0D; &#x0D; &#x0D; &#x0D; Measurements of intensity of natural remanent magnetization left (NRMleft) after AF demagnetization versus intensity of anhysteric remanent magnetization gained (ARMgained) at the same peak were carried out on a set of samples. Two versions of a jackknife resampling scheme were used to get error estimates on the paleointensity.&#x0D; &#x0D; &#x0D; &#x0D; &#x0D; The pseudo-Thellier paleointensity records were compared with the authors previous results where the remanent magne- tization at 20mT (NRM20mT) has been normalized using the anhysteric remanent magnetization at 20mT (ARM20mT), the satu- ration of the isothermal remanent magnetization at 20mT (SIRM20mT) and the low field magnetic susceptibility (k) (Gogorza et al., 2006). The pseudo-Thellier record shows a reasonable agreement with the standard method of normalization (NRM20mT/ ARM20mT).&#x0D; &#x0D; &#x0D;

Magnetic measurements and pollutants of sediments from Cauvery and Palaru River, India

Rock-magnetic techniques have become a useful tool in environmental issues; in particular, magnetic studies constitute an alternative way to study pollution in different media. The present contribution focuses on magnetic parameters as pollution indicators, especially from their relationship with contents of heavy metals. The work was carried out in two Indian rivers located in Tamil Nadu, southern India. Several sediment samples were collected and studied in the laboratory using magnetic techniques, magnetic susceptibility, anhysteric remanent magnetization, isothermal remanent magnetization, and chemical techniques to determine contents of heavy metals. Magnetic mineralogy indicates the predominance of ferrimagnetic minerals; although magnetite-like minerals are the main magnetic carriers, antiferromagnetic minerals can be present as subordinate carriers. Concentration-dependent magnetic parameters revealed noticeable differences between both rivers, e.g. magnetic susceptibility is four times higher in Cauvery than in Palaru River. Moreover, such increase can be interpreted as “magnetic enhancement” and therefore related to the pollution status. This magnetic enhancement indicated a different pollutant contribution in both rivers, and also, a different spatial distribution along these rivers, where critical (or more polluted) sites were identified. On the other hand, univariate and multivariate statistical analyses—e.g. PCoordA, Multifactorial Analysis of distance, PCA and RDA—were examined, revealing a link between magnetic and chemical variables. Among magnetic parameters, the concentration-dependent magnetic parameters (e.g. magnetic susceptibility) seem to be the most relevant for this study.

Magnetic studies of natural goethite samples from Tharsis, Huelva, Spain

Las propiedades magnéticas de la goethita no han sido ampliamente estudiadas. Las muestras naturales frecuentemente contienen mezclas complejas de minerales. En el presente trabajo se estudió material de Tharsis (Huelva, España), con el propósito de corroborar la presencia de goethita, a partir de mediciones de magnetismo de rocas. A tal fin se realización mediciones de susceptibilidad magnética, magnetización remanente isotérmica (MIR), anhistérica (MRA), y se efectuó la desmagnetización por campos alternos y por temperatura. Difracción de rayos X y microscopía de barrido electrónico también fueron efectuados. Los resultados obtenidos son comparados y discutidos con relación a estudios previos de goethitas naturales y sintéticas. A partir de los estudios térmicos de MRI y no magnéticos, se observó la presencia de hematita magnéticamente más dura.

Relative paleointensity of the geomagnetic field over the last 21,000 years BP from sediment cores, Lake El Trébol (Patagonia, Argentina)

Abstract Four cores from Lake El Trébol (Patagonia, Argentina) have been used to estimate regional geomagnetic paleointensity. The rock magnetic studies indicate that the magnetic mineralogy of the clay-rich sediments is dominated by pseudo-single domain magnetite in a range of grain sizes and concentration which are suitable for paleointensity studies. The remanent magnetisation at 20 mT (NRM20 mT) has been normalised using the an-hysteric remanent magnetisation at 20 mT (ARM20 mT), the saturation of the isothermal remanent magnetisation at 20 mT (SIRM20 mT) and the low field magnetic susceptibility (k). Coherence function analysis indicates that the normalised records are free of environmental influences. Our paleointensity (NRM20 mT/ARM20 mT) versus age curve shows good agreement with published record from another lake in Argentina and with records from other parts of the world, suggesting that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar signal.

Paleosecular variations recorded by Holocene-Pleistocene sediments from Lake El Trébol (Patagonia, Argentina)

Paleomagnetic studies carried out on six cores from the bottom sediments from lake El Trébol, South Argentina are described. Measurements of directions declination and inclination ( D and I) and intensity of natural remanent magnetisation (NRM), magnetic susceptibility at low and high frequency (specific, X and volumetric, k), isothermal remanent magnetisation (IRM), saturation isothermal remanent magnetisation (SIRM), back field and anhysteric remanent magnetisation (ARM) were carried out. Stability of the NRM was investigated by alternating-field demagnetisation. Rock magnetic studies suggest that the main carriers of magnetisation are ferrimagnetic minerals, predominantly pseudo single domain magnetite. Records of NRM and k show a good correlation with the sedimentological description, and also with those of shorter cores previously studied. The tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs. Due to their different chronological meaning and their rather bad behaviour as magnetic recorder, these layers were removed from the sequence and the gaps that were produced along the profiles by the removal were closed, obtaining a shortened depth. Radiocarbon age estimates from these cores and from earlier studies allow us to construct a paleosecular variation (PSV) record for the past 24,000 years. These results show good correlation with the previous one. The spectrum obtained from spectral analysis shows major peaks at periodicities of about 1150, 1500, 2490, 3630, 5240 and 11,810 years. Both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of the Bauer plots, with a preponderance of clockwise movement.

Relevant Magnetic Parameters and Heavy Metals from Relatively Polluted Stream Sediments - Vertical and Longitudinal Distribution Along a Cross-City Stream in Buenos Aires Province, Argentina

Previous magnetic and geochemical studies of stream sediments in the Northeast of Buenos Aires Province, in the Greater La Plata area, have revealed a high level of trace elements, especially lead and zinc. Continuing these preliminary works, new sampling sites were considered in order to improve and increase the magnetic mapping of this area. This part of the study focuses especially on the magnetic properties of sediments from a cross-city stream. Such stream receives a variable pollutant load from urban, industrial and diffuse sources. Concentration-dependent magnetic parameters, magnetic carriers (para, anti and ferrimagnetic) and their features (e.g. softer or harder magnetic carrier, grain size distribution, etc.) have been calculated using magnetic measurements. According to magnetic parameters and heavy metal loads two main distinctive groups of sediment-cores are distinguished. The vertical distribution of sediments reveals a recent anthropogenic influence, possibly, belonging to the last 20-40 years. Four out of all the magnetic parameters (magnetic susceptibility, anhysteric remanent magnetisation, S-ratio and κ ARM/κ) were chosen in order to investigate their relationship with contents of heavy metals. In all the cases, good positive correlations were obtained using linear regression. However, more significant correlation factors were achieved for grain size and magnetic feature-dependent parameters (κ ARM/κ and S-ratio) than for magnetic concentration-dependent parameters. Therefore, the κ ARM/κ and the S-ratio seem to be the most relevant magnetic parameters to describe the vertical and longitudinal distribution of heavy metals present in these stream sediments. The non-significant relationship between heavy metals and concentration-dependent parameters could be explained taking into account the discrimination of the two distinctive groups. The group belonging to the Coastal Plain shows a clear linear trend between heavy metals and magnetic susceptibility and anhysteric remanent magnetisation.

Paleointensity studies on Holocene–Pleistocene sediments from lake Escondido, Argentina

Relative changes in geomagnetic field intensity over the last 16,000 years BP were recovered from the study of four cores obtained from Lake Escondido (South Argentina). Rock magnetic analysis shows that the main magnetic mineral is magnetite, the concentrations being mainly between 0.01 and 0.1%, and the grain size between 1 and 8 μm. In order to fulfil the criteria for assessing the reliability of paleointensity data derived from sediments [Rev. Geophys. 31 (1993) 319], the samples whose concentrations and size are beyond this range were rejected. The remanent magnetisation at 20 mT (NRM 20 mT ) was normalised using the anhysteric remanent magnetisation (ARM 100 mT ), the saturation of the isothermal remanent magnetisation (SIRM) and the low field magnetic susceptibility ( k). Coherence function analysis indicates that the records are not significantly affected by local environmental conditions. This suggests that the variations in normalised remanence are mostly likely due to geomagnetic paleointensity fluctuations. ARM 100 mT is the best normaliser for the Lake Escondido sediments. The record of relative paleointensity (NRM 20 mT /ARM 100 mT ) shows peaks and troughs whose amplitudes are similar to those in the St. Lawrence Estuary, Lake Baikal, Lake Pepin and Larsen-A Ice shelf data set and a stack of archeomagnetic data given as virtual axial dipole moment (VADM).

Magnetic properties of tidal flat sediments of the Yangtze Estuary and its relationship with particle size

This paper considers the magnetic properties of tidal flat sediments of the Yangtze Estuary and its relationship with particle size. The results indicate that magnetite of multi-domain (MD)/pseudo-single domain (PSD) in size dominates the magnetic properties of the samples. In addition to detrital magnetite, spherical magnetic particles are found in the magnetically strongest sediments near Shidongkou sewage outlet, which are ascribed to the product of industrial pollution. Particle size plays a strong role on the magnetic properties of the sediments. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) are positively correlated with the proportion of the fraction of 8-16 μm. However, magnetic parameters indicating fine-grained ferrimagnetic minerals, e.g. susceptibility of anhysteric remanent magnetization (X ARM), are highly correlated with the fine fraction of <4 μm and even the fraction of <32 μm. The results indicate that, whenX is used as a proxy for pollutant content, it is necessary to consider the possible influence of particle size on it. Due to the significant relationship betweenX ARM and the fine component proportion,X ARM can be applied to the normalization of pollutant content for particle size effect.

Preliminary Results of Magnetic Measurements on Stream-Sediments from Buenos Aires Province, Argentina

A wide urban area in the Northeast of Buenos Aires Province (Argentina) is exposed to anthropogenic influence, mainly due to industrial and urban activity. In this area two streams were chosen, Del Gato and El Pescado streams. They have similar characteristics, although the first one runs through the built-up area and the other one through a rural area; consequently, Del Gato stream has a higher input of pollutants (fluvial effluents, fly ashes, solid wastes, etc.) than El Pescado stream. Magnetic measurements and geochemical studies were carried out on five sediment cores taken out from both streams with the aim of establishing a link between concentration of ferrimagnetic materials and contents of heavy metals. In order to investigate concentration of magnetic carriers and its intrinsic features (present magnetic minerals and grain size distribution) several magnetic parameters were measured and relations between them were analyzed. The magnetic characteristics for both sites indicate the presence of magnetically soft minerals in both streams, predominantly in Del Gato stream and with the additional presence of relatively hard minerals in El Pescado stream. Besides, a higher concentration of secondary ferrimagnetic minerals was found in Del Gato stream than in El Pescado stream. Chemical studies show (in some cases) a high concentration of some heavy metals (lead, copper, zinc, nickel and iron) on the upper 22 cm. Magnetic parameters, especially anhysteric remanent magnetization and contents of some heavy metals, showed a very good correlation.

Late Holocene Changes in Precipitation and Hydrography Recorded in Marine Sediments from the Northeastern Caribbean Sea

We present a record of climate variability spanning the last 2000 years obtained from sediment cores retrieved south and west of Puerto Rico in the northeastern Caribbean Sea. The records include lithological and mineral magnetic parameters as well as planktonic foraminifer data. For chronostratigraphic control, AMS 14C and 210Pb/137Cs measurements were made. Harmonic analysis of the values of the mineral magnetic parameters “saturation isothermal remanent magnetization” (SIRM), “anhysteric remanent magnetization divided by magnetic susceptibility” (ARM/χ), and “saturation isothermal remanent magnetization divided by magnetic susceptibility” (SIRM/χ) indicate the existence of a ∼200-year-long climate cycle in the northeastern Caribbean during the last 2000 years. The detected cycle may reflect changes in precipitation patterns over the low-latitude North Atlantic Ocean and surrounding continental areas. Higher organic carbon contents appear in the sediments both off southern and western Puerto Rico before and at the onset of the Little Ice Age around A.D. 1300 to 1500. This is indicative of increased run off and/or enhanced surface productivity possibly associated with more intense wind-induced upwelling. Major changes in the geochemical and mineral magnetic records around A.D. 850–1000 concur with changes in other records from the Caribbean and North African regions indicating a shift toward a more humid climate over the low-latitude North Atlantic.

Insights into Permian Pedogenesis and Climate: Magnetic Stratigraphy of Four Stacked Paleosols, Lower Permian (Wolfcampian) Roca Shale, Central Kansas: ABSTRACT

Abstract This study focuses on the rock magnetic character of four stacked paleosols (P1 at base, P4 at top) in the Lower Permian (Wolfcampian, Council Grove Group) Roca Shale exposed near Manhattan, Kansas. Magnetic characterization of Quaternary soils has proven a valuable tool for evaluating climatic conditions and pedogenic processes, yet no systematic study of pre-Quaternary soils has been undertaken. The Roca paleosols are protosols, vertisols, calcic vertisols, and calcisols. Cross-cutting relationships and superposition principles suggest that macro- and micro-climatic conditions evolved as soil formation progressed. In particular: 1) in P2, clasts of the lower (vertic) part are entrained in the upper (calcic, leached) part suggesting a shift from subhumid to semiarid; and 2) in P3 and P4, carbonate nodules have argillic coats that are at times overlain by (hematitic) ferrans, suggesting climate transitions from semiarid to subhumid back to semiarid. Rock magnetic data provide further insight into pedogenesis and climate. A preserved magnetic signature is suggested by: l) distinct magnetic susceptability, natural remanent magnetization (NRM), anhysteric remanent magnetization (ARM) and isothermal remanent magnetization (IRM) in B, Bt, Bk, E, and R horizons; and 2) carbonate nodules and adjacent subangular blocky structure which preserve the same susceptability trends. Overall, susceptability ranges from <2.0 to 11.6 x 10{-8}m{3}/kg, but within P2, P3, and P4, there is a general upwards decrease. PI contains upwards increasing susceptability. These changes correspond to a shift in S-ratios (IRM[0.3T]/SIRM) ratios from ~0.9 to ~0.4, reflecting a possible change from magnetite to hematite upwards through the paleosol profile. NRM values range from almost 1 x 10{-8}`to 5 x 10{-5} Am{2}/kg. NRM/susceptability and SIRM/susceptability ratios (proxies for ferrimagnetic vs. paramagnetic contributions to susceptability) decrease from PI to P4 and the position of the highest ratio within each paleosol profile is lowered from the B (P1) to top Bt (P2) to base Bt (P3, P4) horizons. Rock magnetic data and field observations suggest that weathering intensity increased through time from the development of P1 to P3 (P3 represents the most leached and oxidized paleosol profile), but decreased during development of P4. The oxidizing conditions prevalent in upper soil horizons led to increased lessivage in these intervals and subsequent concentration of clays and iron oxides in lower, less oxidizing horizons. Gleyed intervals are characterized by (SD?) magnetite. The results of this study show that coupled field observation and rock magnetic characteristics of pre-Quaternary paleosols potentially will yield significant climatic and pedogenic information.

Palaeomagnetism of the montagnais diabase dikes of the west central sector of the Labrador trough

Eighty-six specimens were collected from eighteen different sites on the numerous north-striking thin diabase dikes of middle-Proterozoic (Helikian) age in the west central portion of the Labrador trough. These diabase dikes, which are the only intrusive rocks found in the area investigated (longitude 66°00′–67°30′W, latitude 54°00′–55°30′N), consist mainly of plagioclase feld-spars, olivine, pyroxenes and magnetite. Magnetite is the magnetic memory carrier, its Curie point (580°C) was determined with an electromagnetic balance. The volume ferromagnetic susceptibility, the intensity and orientation of natural remanent magnetization (N.R.M.) were measured in the presence of DC magnetic fields of 0.5 to 1 oersted. In order to obtain information relative to the stability of the N.R.M. component, stepwise alternating field (AF) demagnetization was conducted at intensities of 50, 100, 150, 200, 250, 300, 400 and 500 oersted (peak-to-peak) on twenty-one specimens, that is roughly 25% of the total number. A palaeomagnetic stability index test allowing one to choose the AF intensity range for which the rate of change in remanence direction of a diabase specimen is minimized was used. The optimum AF intensity, at which the primary thermoremanent component is most thoroughly isolated after most of the isothermal remanent magnetization (IRM) has been removed and before significant anhysteric remanent magnetization (ARM) is introduced, is located in the 100–200 oersted range. The remaining 75% of the specimens was then demagnetizaed at steps of 100, 150 and 200 oersteds. The decrease in intensity of the N.R.M. component in the course of AF demagnetization is quite rapid between 0.5 and 150 oersteds and relatively insignificant between 150 and 500 oersteds or more. A mean paleomagnetic pole position of 137oW, 10oS (α.95=14o,d p =08o,d m =15o,K=17), with at least one reversal, indicates that the Labrador trough was near the equator (Central North Pacific Ocean) at the time of intrusion. the possible implications of the results of this study are discussed in relation to hypotheses on polar wandering and on continental drift.