Isothermal Remanent Magnetization

Sand dunes are unstable ecosystems and are organized at the
juncture between sea and land. They are formed from sand, which is eroded
and ground rock, derived from terrestrial and oceanic sources. This system is
influenced by several factors, such as ocean swell, coastline, shape of beach,
prevailing wind, frequency of storm events, and particle size of the sands
forming the dune. Though not directly controlled by climate, the landward
movement of coastal dunes is facilitated by arid and semi-arid conditions.
Red sand dunes (locally known as ‘teri sand’) occur in the coastal
plains of South India between Muttom on the south west coast (~16km south
west of Nagercoil) and Rameswaram in Tamil Nadu (8°00’ to 9°30’ N;
77°18’ to 79° 00’ E). The teri sands exhibit shades of red with yellow and
brown, colour varying from 2.5YR 3/4 to 10R 3/6 in Munsell colour chart and
in thickness from 1m to greater than 10m.
From the spatial distribution of teri sands, the dunes are identified
as inland teri deposits (ITD) and coastal teri deposits (CTD). The ITD occur
as sand sheets of medium to coarse-grained sand with thin gravel layers at
variable depth, located near to the foothills of Western Ghats and are limited
in number. Whereas the CTD occur as vast discontinuous accumulations of
medium to fine grained sand in the form of dunes and sheets, roughly parallel
to coast and are ~6-8 km away from modern shoreline
The Teri sands from the southeast coast of Tamil Nadu were
examined by magnetic methods, including magnetic susceptibility
measurements, acquisition of isothermal remanent magnetization,
geochemical, X-ray diffraction and optically stimulated luminescence (OSL)
dating. Five sections were excavated up to 5-7m with three inland deposits
iv
(KCV, SKM, TPV, THOP and MUT) and one near coastal (THOP). Magnetic
parameters show the important contribution of hematite structures to the
studied sands, namely in SKM and KCV samples.
In TPV, MUT and THOP samples, magnetic parameters indicate
the presence of multidomain magnetite or mixed mineral contents of
magnetite and antiferromagnetic minerals. The SKM samples present the
highest quantity of hematite and smallest quantity of magnetite, as is shown
by the highest values of HIRM% and smallest of S-300 and SIRM. The positive
correlation between χ with clay in TPV, THOP and KCV sections indicates
that the clay matrix carries magnetic susceptibility.
In SKM, magnetic susceptibility is due to the silt fraction. The
presence of magnetite in THOP and TPV sections is probably of oceanic
origin carried by strong offshore winds. In SKM the wet environments due to
cultivation (associated with a arid climate) may contribute to the prevalence
of the necessary conditions for the dominant presence of hematite.
Marine sediments are extremely sensitive indicators of
chronological variations in concentration and grain size of deposited
terrigenous/lithogenous material. In correspondence with geochemistry data,
the variation of χ depends upon the deposition because they would have been
deposited in both dry and wet periods, and it also depends upon the mode of
haulage.
These outcomes corresponded well with the previous work on Teri
sand available on the southeast coast of India by several authors and this
effort can be used for future comparative studies on Teri deposits. In XRD
data, correlation analysis indicated TPV and MUT sections have a similar
type of deposition and THOP did not show any positive correlation with TPV
and MUT and even with its own deposition. In a comparsion with
v
geochemistry data, χ variation and OSL dates, it was shown that the sample
[MUT 21 (200cm) with an OSL age of 14±2 ka indicated deposition during
humid interval and at [∼17±2 ka to ∼19±2 ka MUT 61 (600cm) depicts the
dry period of deposition.

We report Mössbauer spectra, magnetic hysteresis, acquisition and demagnetization of isothermal remanent magnetization (IRM), and low-temperature IRM warming curves of biotite crystals, as well as a summary of domain observations on magnetic extracts. The biotites are from 1.0 to 1.2 Ga gneisses and 1.85 Ga plutons of the Grenville, Churchill and Bear Provinces (Canada). Most have paramagnetic susceptibilities in the

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.

El Teniente porphyry copper deposit, the world’s greatest intrusion-related Cu–Mo ore body, is hosted within basaltic–andesitic volcanic and gabbroic rocks (mafic complex). This ore body is strongly affected by multiple events of alteration/mineralization with pervasive potassic and chloritic alteration and coetaneous with associated copper mineralization. We present paleomagnetic results obtained from oriented samples at four locations within the mine and from two drill cores, 200 and 400 m long, respectively. Samples are representative of all the main hydrothermally altered rock units, with emphasis on the mafic host rock and dacitic (Teniente dacite porphyry) and dioritic porphyry intrusions. Magnetic experiments [hysteresis loop, isothermal remanent magnetization (IRM), k–T curves, thermal, and alternating field demagnetization] show the presence of prevailing magnetite. Microscope and SEM observations show two families of magnetite, (a) large multidomain magnetite grains, associated with biotite and chlorite of various different hydrothermal alteration events, and (b) abundant small to medium grain-size magnetite (−4–10−3 Am−1), the mineralized mafic host rocks have usually high susceptibility (>0.01 and up to 0.2 SI) with NRM in the range 0.1–2 Am−1. Most mafic complex rock samples have univectorial magnetizations during alternating field or thermal demagnetization. Within the mine, the magnetic polarity is spatially distributed. In the northern part of the deposit, the Teniente dacite porphyry, the associated hydrothermal breccias, and the hosting mafic complex record a reverse polarity magnetization, also observed in the El Teniente sub-6 mine sector immediately to the east and southeast. In the eastern part of the deposit, a normal polarity is observed for samples of the mafic complex from the two long drill cores. There is no evidence for superimposed magnetizations of opposite polarities in samples of the mafic complex. Anhysteretic remanent magnetization (ARM) in a DC field of 40 μT and NRM have similar magnitude and comparable behavior upon alternating field demagnetization. The well-defined strong remanent magnetizations associated with high unblocking temperatures (>500°C) indicate an acquisition of remanent magnetization during mineralization by circulating high temperature fluids related with ore deposition. Paleomagnetic results and the recorded polarity zonation suggest multiple mineralization events occurred at El Teniente, each one with its own evolution stages, superimposed within the district. These results indicate that a simplified broad four-stage model for El Teniente, as presented and overly employed by many authors, divided in (1) late magmatic, (2) main hydrothermal, (3) late hydrothermal, and (4) posthumous stage, does not recognize various short-lived single mineralization events, some superimposed and some distinctly separated in time and space. There is no paleomagnetic evidence for post-mineralization deformation

Different grain-size fractions of 12 lithologically defined and 2 non-defined Weichselian glacial stage diamicton units from Nordaustlandet, Svalbard were studied for their mineral magnetic properties. The applied measurements included magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), 'backfield' isothermal remanent magnetization (IRM) and several of their parametric ratios. The results indicated a poor correlation between the magnetic fingerprint of dry-sieved and wet-sieved diamicton samples. The dry-sieved samples had higher χ, ARM and SIRM values, suggesting a higher concentration of ferrimagnetic grains. This was probably because washing of the samples resulted in the loss of the most fine-grained ferrimagnetic material. Both sieving methods seem to be capable of discriminating different diamictons, but they cannot be used contemporaneously. The results only partially support previous findings of the chronostratigraphic correlation of diamictons. They indicate that in suitable conditions and within a limited area it is possible to discriminate (and group) glacigenic sequences based on the magnetic signature of their fine-grained fractions in the sample. For example and based on the mineral magnetic fingerprint, a diamicton unit that was described from the marine core (MUR 09/sor1/125-150) obtained from Isvika region most probably represents diamicton material that were derived from provenance areas related to the Middle Weichselian glacial stage. It was also noted that certain mineral magnetic parameters, such as SIRM/χ and SIRM/ARM ratio, had greater potential in discriminating diamicton units than others.

A rock magnetic cyclo-stratigraphy was measured for the upper Barremian-lower Aptian Cupido Formation of northeast Mexico. Anhysteretic remanent magnetization (ARM; 97 μT 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 characterized by meter-scale, peritidal and subtidal carbonate cycles representative of inner and middle platform settings. Isothermal remanent magnetization (IRM) acquisition experiments and SEM examination suggest that micron-sized detrital magnetite derived from eolian dust carries the ARM signal. At the sampled sections (Potrero Garcia and Potrero Chico, 30 km apart), ARM records a synchronous 30-35 m oscillation with maxima coinciding with 4th-order sequence boundaries; this oscillation is superimposed by prominent higher-frequency variability. Tuning the 30-35 m oscillation to a 405-kyr periodicity (long eccentricity) focuses the higher frequencies into the short eccentricity, obliquity, and precession bands; the precession-band signal modulates with a pronounced eccentricity signature. The ARM signal is tightly correlated between the two sections, but is decoupled from the 5th-order meter-scale cycles. ARM amplitudes diminish up-section as the facies record progressive deepening. We interpret that this deepening was associated with regional and/or global increases in temperature and humidity that would result in a decrease in the influx of eolian-derived magnetic grains to the study area. This Mexican ARM rock magnetic cyclo-stratigraphy can be correlated with Milankovitch cycles recorded in coeval Tethyan pelagic deposits at Gorgo a Cerbara and Cismon, Italy suggesting it is global in origin.

Abstract Diffuse pollution is of concern given the impacts on aquatic ecosystems and the introduction of legislation such as the European Water Framework Directive. In order to reduce or halt such pollution it is essential to elucidate its provenance. Furthermore, there ...

The hypothesis of a climate variability within the last interglacial (Eemian), suggested by oxygen isotope data of a Greenland ice core (GRIP) was later supported by magnetic susceptibility, organic matter and pollen signatures in the Eemian lake sediments of the Lac du Bouchet (France). In the present paper, we analyse in detail the rock-magnetic properties of these sediments in order to determine the origin of the magnetic susceptibility variations. Furthermore, in order to verify the empirical relation between magnetic susceptibility and climate, we studied two Eemian sequences from neighbouring lakes (Lac St. Front and Ribains maar lake). Rock-magnetic investigations mainly consisted of analyses of anhysteretic and isothermal remanent magnetizations along with hysteresis measurements carried out on pilot samples. Lac du Bouchet susceptibility variations are at least partly affected by postdepositional magnetite dissolution. Lac St. Front sediments show a complex behaviour resulting in partly contradicting interpretations of rock-magnetic parameters. Possible explanations may be the presence of a very fine-grained magnetite or the presence of maghemite. Ribains maar lake sediments show a homogeneous magnetic mineralogy without major variations in most of the rock-magnetic parameters. Susceptibility records of the three lakes were compared under the simplified assumption that magnetic susceptibility mainly reflects variations of the detrital (titano-)magnetite input. Only a poor correlation between the three Eemian susceptibility sequences is observed, suggesting that local effects (including dissolution and dilution effects) may have altered the original detrital (climatic) signal. The purely detrital origin of the Lac du Bouchet Eemian susceptibility variations must thus be questioned but it nevertheless appears to be the only lake whose susceptibility variations can tentatively be correlated with the GRIP Eemian oxygen isotope record.

During the Miocene-Pliocene, the Carpathian region represented the westernmost part of the so-called Eastern Paratethys, a palaeobioprovince that covered central and eastern Europe as well as parts of southwest Asia. Previous palaeomagnetic investigations provide a high-resolution magnetochronology for the sedimentary sequences of the Carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides, in Chron C3r. Here, we demonstrate using detailed rock magnetic investigations and scanning electron microscope (SEM) analyses that the major magnetic iron sulphide mineral is greigite (Fe3S4). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 °C and SEM observations indicate octahedral grain morphologies and Fe:S ratios that are indicative of greigite. Hysteresis loops have `rectangular' shapes, which are typical of single domain behaviour, with coercivity and coercivity of remanence at values of Bc = 35-45 mT and Bcr = 52-67 mT, respectively. First-order reversal curve diagrams have contours that close around single domain peaks with Bc values of 45-90 mT. Isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component (dispersion parameter, DP ~ 0.15 log units) indicating a narrow grain size distribution. A positive fold test, inclination shallowing and two positive reversal tests are additional arguments for the syn-depositional formation of this greigite. We thus argue that (most of) the greigite was formed under early diagenetic conditions, that is, within 1000 yr of deposition of the sediment in this setting, and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation. The appearance of greigite during Chron C3r (between 6.0 and 5.5 Ma) in the Carpathian foredeep is most likely related to regional tectonic and/or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions.

... Several textural and geochemical properties were measured (grain size, total organic carbon [TOC] content, and CaCO 3 content, all using standard procedures (Guitián and Carballás, 1976). Elemental analysis was performed by X-ray fluorescence (XRF). ...

Our rock magnetic analysis of core Ph05 from the West Philippine Sea demonstrates that the core preserves a strong, stable remanent magnetization and meets the magnetic mineral criteria for relative paleointensity (RPI) analyses. The magnetic minerals in the sequence are dominated by pseudosingle-domain magnetite, and the concentration of magnetic minerals is at the same scale. Both the conventional normalizing method and the pseudo-Thellier method were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization. Susceptibility (χ), anhysteretic remnant magnetization (ARM) and saturation isothermal remnant magnetization (SIRM) were used as the natural remanent magnetization normalizer. However, coherence analysis indicated that only ARM is more suitable for paleointensity reconstruction. The age model of core is established based on oxygen isotope data and AMS14C data, which is consistent with the age model estimated from RPI records. The relative paleointensity data provide a continuous record of the intensity variation during the last 200 ka, which correlates well with the global references RPI stacks. Several prominent low paleointensity values are identified and are correlated to the main RPI minima in the SINT-200 record, suggesting that the sediments have recorded the real changes of geomagnetic field.