Median Destructive Field Research Articles

Late-Holocene paleomagnetic secular variation records from Lake Turkana, East Equatorial Africa

We have carried out a paleomagnetic study of three piston cores collected from Lake Turkana. The goal is to recover a Holocene paleomagnetic secular variation (PSV) record for this lake and to correlate it with other Holocene PSV records from the East Africa Rift Valley (EARV). All three cores were sampled with u-channels and magnetic measurements of magnetic susceptibility, the natural remanence (NRM), and two artificial remanences, anhysteretic remanence (ARM) and saturation isothermal remanence (SIRM), were made on them. The remanences were routinely step-wise demagnetized and measured at 10 mT steps up to 60 mT. The NRMS had a simple pattern of demagnetization with a characteristic direction (ChRM) removed between 10 and 60 mT. ARM and SIRM demagnetization indicated that the magnetic grains were relatively soft with median destructive fields (MDF) less than 30 mT. We interpret the magnetic grains to be multi-domain (silt-sized) magnetite/titanomagnetite. The resulting magnetic records of all three cores could be correlated. A chronology for these cores was determined from four radiocarbon dates on core 4P. We also estimated the sediment ages by correlating the PSV to two other well-dated PSV records from the same region, Lakes Malawi and Victoria. PSV age estimation indicates that the radiocarbon dates are about 500 year too old; a correction for that offset causes three of the four radiocarbon ages to become consistent with the PSV age estimates. The final composite Lake Turkana PSV record for the last ~4000 years is the highest resolution directional record of PSV (~200 cm/ky) ever to be recovered from Africa.

Rock magnetism of ejected basaltic boulders from Lonar crater, India: Implications for the existence of a short‐lived impact‐generated weak magnetic field

AbstractThe continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angular basaltic boulders of size ≤5 m. These boulders experienced varying level of shock between 2–30 GPa due to impact, as indicated by the extreme fracturing of these basaltic boulders, fragmentation of plagioclase and titanomagnetite constituents of these ejected boulders, and the presence of maskelynite in them. We measure some rock magnetic properties, e.g., NRM/χ (natural remanent magnetization [NRM]/bulk magnetic susceptibility [χ]), REM (=NRM/saturation isothermal remanent magnetization [SIRM] ratio expressed in %), and anisotropy of magnetic susceptibility (AMS) on 53 subsamples from 18 oriented drill cores of the shocked ejected basaltic boulders from the eastern half of ejecta deposit in the present study. The measured data are similar in many respects to our previous observations on Lonar crater rim shocked basalts (Arif et al. 2012b). For example, a small population of the ejected basaltic boulder samples show very high NRM/χ (between 378 and 989 Am−1; n = 7) and REM (between 1.5 and 7%; n = 4) and the AMS axes of these ejected basaltic boulders show triaxial distributions in stereographic projections. Moreover, some of the ejected basaltic boulders show higher values of squareness ratio (Mrs/Ms) and median destructive field (MDF) suggesting permanent changes in the intrinsic magnetic properties due to impact shock pressure. In stereographic plot, the high coercivity and high temperature (HC_HT) magnetization component of these ejected basaltic boulders are distributed in discrete clusters on the periphery of a small enveloping circle whose center (D = 108.0°, I = +69.2°) lies close to the HC_HT cluster of the crater rim shocked basalts. The center of this enveloping circle and the average HC_HT component of Lonar crater rim shocked basalts have the same statistical orientation, although the former has steeper dip. This distribution suggests the possibility that the ejected basaltic boulders, which were deposited during the modification stage of Lonar crater evolution, were magnetized in an impact‐induced magnetic field that was rapidly decaying just after the impact. Our present study suggests that the ejected basaltic boulders and Lonar crater rim shocked basalts experienced high shock pressure (≥2 GPa) magnetization during impact.

Behavior of Greigite‐Bearing Marine Sediments During AF and Thermal Demagnetization and Its Significance

AbstractGyro‐remanent magnetization (GRM) is a frequently occurring yet unwanted remanence contamination for certain samples during alternating field (AF) demagnetization of the natural remanent magnetization. The origin and detailed properties of GRM have not yet been fully understood. In this study, systematic rock magnetic analyses were conducted on marine greigite‐bearing samples of Hole U1433A drilled by the IODP Expedition 349 from the South China Sea. Results show that GRM is mostly acquired above ~55 mT AF demagnetization and can be effectively removed by heating to ~400°C during thermal demagnetization but a secondary tail could remain until ~585°C. In addition, no apparent GRM was observed during the AF demagnetization for the 400°C thermally treated samples. These results strongly suggest that GRM is dominantly carried by single domain (SD) greigite but with minor contributions from SD magnetite. Thus, thermal treatment alone or the hybrid demagnetization (i.e., thermal demagnetization at ~400°C first then systematical AF demagnetization) can efficiently avoid the GRM acquisition and be beneficial for relative paleointensity estimation for greigite‐bearing samples. Besides, GRM carried by greigite has a low thermal stability. Our results also show AF demagnetization spectra of anhysteretic remanent magnetization (ARM) could be strongly distorted by GRM effects due to both have a preference of SD particles. Thus, the median destructive field of ARM is improper to be used as a coercivity proxy for greigite‐bearing samples. Instead, the biplot analysis of AF demagnetization of natural remanent magnetization and ARM can be used to evaluate the relative content of greigite.

Archaeomagnetic knowledge of the Neolithic in Bulgaria with emphasis on intensity changes

Abstract Several archaeological baked clay structures from Bulgarian Neolithic sites were archaeomagnetically studied. According to the ancient firing conditions, the collected materials are dwelling remains and ovens with different heating histories. The variability of magnetic properties is greater in the burnt dwelling remains compared to the ovens. The prevailing magnetic minerals are magnetite/titanomagnetite and epsilon iron oxide. Less often, the presence of hematite is suggested. For one of the ovens, a well-pronounced evolution in magnetic properties was observed between its successive levels. The collected materials possess stable magnetic mineralogy and the success rate of archaeointensity determination experiment is 84%. The experiment completely failed only for the structure where epsilon iron oxide is not identified and where the lowest median destructive fields are determined. The obtained archaeomagnetic results are summarized as 10 new reference points. The structures subjected to the same firing event are combined as one feature. Their dating is based on well-established relative Neolithic chronology and stratigraphic constraints. The new data coincide fairly well with the Bulgarian dataset and with the data available for the neighbouring countries, confirming an intensity maximum around 5550 BC and a minimum around 5400 BC. No indications of geomagnetic ‘spikes’ or ‘jerks’ are detected.

AF demagnetization and ARM acquisition at elevated temperatures in natural titanomagnetite bearing rocks

SUMMARY Understanding the temporal changes of the Earth’s magnetic field intensity is one of the main goals of modern palaeomagnetism. For most palaeointensity methods to yield reliable results, the magnetic minerals must obey a set of rules. One of these rules is the additivity of partial thermal (TRM) or anhysteretic remanent magnetizations (ARM). Additivity was previously shown for partial TRM in single-domain particles and more generally for ARMs. Additivity between these two low-field remanences, however, has not been investigated, yet. This paper presents a series of rock magnetic experiments on natural low Ti titanomagnetites (Curie temperature between 534 °C and 561 °C) examining the effects of high temperatures on alternating field (AF) demagnetization and acquisition of an ARM. One of our sample sets comes from a borehole drilled through the impact melt sheet of the Manicouagan crater (Canada), the other from the Rocche Rosse lava flow on the island of Lipari (Italy). Hysteresis parameters indicate the magnetic carriers in the pseudo-single-domain range showing no evidence for oxidation. Thermal demagnetization at 300 °C and 500 °C before AF demagnetization shifts the coercivity spectra towards higher fields. AF demagnetization experiments at 500 °C show a significant (by a factor between 1.4 and >7.6) reduction in median destructive field and a shift towards lower coercivities. A linear relationship was found between the peak magnetic field required to demagnetize a fraction of a full TRM of a sample at a specific temperature and the one necessary to demagnetize the same fraction at room temperature after heating to that temperature. The comparison of full ARM and partial TRM at successively higher temperatures with a hybrid hTARM reveals that combined additivity between the two kinds of remanences is fulfilled. These results open the possibility to demagnetize highly coercive minerals, such as hematite and goethite, which is often not achievable at elevated temperatures. Furthermore, the additivity of TRM and ARM remanences may be used to develop novel hybrid TRM/ARM palaeointensity methods for samples, where heating is problematic (e.g. in meteorites).

Composition law of oblique anhysteretic remanent magnetization and its relation to the magnetostatic interaction

AbstractThe basic properties of oblique anhysteretic remanent magnetization (OARM) acquired in a weak and steady magnetic field with an arbitrary angle to the alternating field direction were studied. OARM and rock‐magnetic experiments were conducted on samples of basalt, granite, and sediment containing noninteracting single domain (SD), interacting SD, pseudo‐single‐domain, and multidomain low‐Ti titanomagnetites. The intensity of OARM (MOARM) systematically increased or decreased with increasing angle between alternating and steady field directions (θSF), while the angle between alternating field and OARM directions (θOARM) increased with increasing θSF for all samples. During stepwise alternating field demagnetization, the OARM vector shows a single component parallel to the steady field direction for θSF = 0° (ARMǁ) and 90° (ARM⊥). The median destructive field of ARM⊥ is larger than that of ARMǁ. For intermediate angles (θSF = 30°, 45°, and 60°), the OARM vector did not parallel the applied steady field but gradually increased θOARM with higher coercivity. These experiments indicate that the OARM vector is approximately given by the sum of two orthogonal magnetizations coinciding with ARMǁ and ARM⊥, respectively. Thus, the OARM vector can be determined by acquisition efficiencies of ARMǁ and ARM⊥ in an individual sample. Based on these experiments and associated rock‐magnetic measurements, noninteracting SD samples show lower ARM⊥/ARMǁ ratios, compared to other samples. This result suggests that OARM can be used as a useful tool to detect noninteracting SD particles in the paleomagnetic samples.

Paleointensity during periods of rapid reversal: A case study from the Middle Jurassic Shamrock batholith, western Nevada

Paleointensity is one of the least deter­ mined parameters in geomagnetism, yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the Earth’s core and lower mantle. We test two important hy­ potheses that affect our understanding of how paleointensity has changed in Earth’s past: (1) the Mesozoic Dipole Low (MDL) hypoth­ esis and (2) the inverse relationship between geomagnetic polarity reversal rate and paleo­ intensity. We report paleointensity results determined using both the Coe modified Thellier method and the IZZI protocol on the medium­grained hornblende­biotite granite of the Middle Jurassic Shamrock batholith in the southern Singatse Range, western Nevada (USA). Previous zircon U­Pb dating gives an age of batholith emplacement at ca. 165.8 ± 0.4 Ma. This age estimate is coincident with a period of unusually high reversal frequency (~10 reversals/Ma) in the Middle Jurassic. Remanence in the Shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase. The labora tory un­ blocking temperature range for the well­de­ fined thermal remanent magnetization is nar ­ row, between ~540 and 575 °C, and median destructive fields are between 40 and 50 mT, thus we infer that the age of rema nence ac­ quisition is approximated by the U­Pb age estimate for pluton emplacement. Two sets of quality criteria were used to assess the paleo­ intensity results. The first set yielded an 87% success rate while the second set using more stringent criteria resulted in only a 20% suc­ cess rate. Mean paleointensity values for the Shamrock batholith using the “loose” and “strict” quality criteria are 34.0 ± 6.4 µT (N = 13) and 33.6 ± 9.6 µT (N = 3) respectively, with errors reported at one standard deviation. After correction for both remanence anisot­ ropy and cooling rate, the mean values that pass “loose” and “strict” criteria are respec­ tively 17.9 ± 2.7 µT and 17.5 ± 2.6 µT. The uncorrected and corrected paleointensity values for the “strict” estimates yield respec­ tive mean virtual dipole moments (VDMs) of 62.7 ± 18 ZAm 2 (Z = 10 21 ) and 32.7 ± 4.9 ZAm 2 . The uncorrected value is within error of the VDMs reported for plagioclase grains isolated from ca. 160 Ma basalts from Ocean Drilling Program Site 801, however these values have an absolute difference ranging from ~6 to 20 ZAm 2 , while the corrected VDM value is more consistent with the esti­ mated average Jurassic field strength of ~29 ZAm 2 . Using a recent estimate for the long­ term stable field strength of ~42 ZAm 2 , our corrected values add further credence to both the MDL and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses.

Rock-magnetic proxies of wind intensity and dust since 51,200 cal BP from lacustrine sediments of Laguna Potrok Aike, southeastern Patagonia

The sedimentary archive from Laguna Potrok Aike is the only continuous record reaching back to the last Glacial period in continental southeastern Patagonia. Located in the path of the Southern Hemisphere westerly winds and in the source region of dust deposited in Antarctica during Glacial periods, southern Patagonia is a vantage point to reconstruct past changes in aeolian activity. Here we use high-resolution rock-magnetic and physical grain size data from site 2 of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO) in order to develop magnetic proxies of dust and wind intensity at 52°S since 51,200 cal BP. Rock-magnetic analysis indicates the magnetic mineral assemblage is dominated by detrital magnetite. Based on the estimated flux of magnetite to the lake and comparison with distal dust records from the Southern Ocean and Antarctica, kLF is interpreted as a dust indicator in the dust source of southern Patagonia at the millennial time scale, when ferrimagnetic grain size and coercivity influence are minimal. Comparison to physical grain-size data indicates that the median destructive field of isothermal remanent magnetization (MDFIRM) mostly reflects medium to coarse magnetite bearing silts typically transported by winds for short-term suspension. Comparison with wind-intensity proxies from the Southern Hemisphere during the last Glacial period and with regional records from Patagonia since the last deglaciation including marine, lacustrine and peat bog sediments as well as speleothems reveals similar variability with MDFIRM up to the centennial time scale. MDFIRM is interpreted as a wind-intensity proxy independent of moisture changes for southeastern Patagonia, with stronger winds capable of transporting coarser magnetite bearing silts to the lake.

Tephrostratigraphy of the Potrok Aike Maar Sediment Sequence

The sedimentary archive from Laguna Potrok Aike is the only continuous record reaching back to the last Glacial period in continental southeastern Patagonia. Located in the path of the Southern Hemisphere westerly winds and in the source region of dust deposited in Antarctica during Glacial periods, southern Patagonia is a vantage point to reconstruct past changes in aeolian activity. Here we use high-resolution rock-magnetic and physical grain size data from site 2 of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO) in order to develop magnetic proxies of dust and wind intensity at 52°S since 51,200 cal BP. Rock-magnetic analysis indicates the magnetic mineral assemblage is dominated by detrital magnetite. Based on the estimated flux of magnetite to the lake and comparison with distal dust records from the Southern Ocean and Antarctica, kLF is interpreted as a dust indicator in the dust source of southern Patagonia at the millennial time scale, when ferrimagnetic grain size and coercivity influence are minimal. Comparison to physical grain-size data indicates that the median destructive field of isothermal remanent magnetization (MDFIRM) mostly reflects medium to coarse magnetite bearing silts typically transported by winds for short-term suspension. Comparison with wind-intensity proxies from the Southern Hemisphere during the last Glacial period and with regional records from Patagonia since the last deglaciation including marine, lacustrine and peat bog sediments as well as speleothems reveals similar variability with MDFIRM up to the centennial time scale. MDFIRM is interpreted as a wind-intensity proxy independent of moisture changes for southeastern Patagonia, with stronger winds capable of transporting coarser magnetite bearing silts to the lake.

Partial self-reversal of TRM in baked soils and ceramics from Ecuador

Partial self-reversed thermoremanent magnetizations (SRTRMs) were observed in samples of baked soils, hearths and ceramics from the Rumipamba archeological site near Quito (Ecuador) and ceramics from sites near the town of Esmeraldas (Ecuador). The SRTRMs were recognized at room temperature on few samples but cooling the samples in liquid nitrogen enhanced the intensity of the SRTRM and measurement at 77°K enables its rapid detection in many samples from these sites. Alternating field demagnetization of the SRTRM indicate median destructive field of the order of 50mT and thermal demagnetization give unblocking temperatures in the temperature range 280–380°C.The magnetic carriers of the SRTRM are stable to heating in air or in vacuum up to 600°C suggesting that titanomaghemite should not be the magnetic carrier of the SRTRM.The studied baked clays and ceramics contain detrital material of mainly volcanic origin. Ti-poor titanomagnetite is the main magnetic carrier identified by strong field data or susceptibility measurements versus temperature. Ilmeno-hematite grains were recognized with microscope observations under reflected light. Scanning electron microscope observations with energy dispersive X-ray spectrometry indicate a Ti/Fe ratio corresponding to an ilmenite content of ∼0.55.We also compared the magnetic properties of the partially self-reversed baked clays with those of the self-reversed Pinatubo pumices. The SRTRMs were measured upon cooling from room temperature to 20°K with the MPMS. Upon cooling to 20°K the SRTRM show a nearly tenfold increase in intensity with respect to the room temperature measurement. The baked clay and ceramics from Ecuador carrying the SRTRM share similar magnetic properties with the Pinatubo pumices (unblocking temperatures, low temperature behavior) supporting the interpretation that detrital hemoilmenite originating from the Holocene activity of the numerous Ecuadorian volcanoes is the main carrier of the SRTRM in the archeological samples.In some samples, anisotropy tensors of SRTRM show large anisotropy degree (>2). The high anisotropy observed when the total magnetization is the sum of a normal thermoremanent magnetization (TRM) and a SRTRM is likely amplified because the anisotropy carried by the SRTRM is larger than the one carried by the normal component.A component of SRTRM may be hidden in archeological materials containing detrital hemoilmenites related to dacitic/andesitic volcanism and this may affect the determination of paleointensity of the earth magnetic field especially when non thermal method are used.

High-resolution paleomagnetic secular variations and relative paleointensity since the Late Pleistocene in southern South America

Paleomagnetic inclination, declination and relative paleointensity were reconstructed from the sediments of Laguna Potrok Aike in the framework of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO). Here we present the u-channel-based full vector paleomagnetic field reconstruction since 51.2 ka cal BP. The relative paleointensity proxy (RPI) was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the ChRM interval (NRM/ARM10–40 mT). A grain size influence on the RPI was removed using a correction based on the linear relationship between the RPI and the median destructive field of the natural remanent magnetisation (MDFNRM). The new record is compared with other lacustrine and marine records and stacks from the mid- to high-latitudes of the Southern Hemisphere, revealing consistent millennial-scale variability, the identification of the Laschamp and possibly the Mono Lake geomagnetic excursions, and a direction swing possibly associated to the Hilina Pali excursion at 20 ka cal BP. Nonetheless, a global-scale comparison with other high-resolution records located on the opposite side of the Earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern South America at 46 ka cal BP.

Palaeoclimatic changes in the NW Iberian ­Peninsula during the last 3000 years inferred from ­diagenetic ­proxies in the Ría de Muros sedimentary record

The Ria de Muros is the northernmost of the 4 Rias Baixas and, like the others, is subject to seasonal upwelling. The high organic carbon content of its sediments (average 2.4%) is due to the consequent high primary production together with significant continental input from river discharge and runoff. This high organic carbon content not only favours early diagenesis, but does so to an extent that depends on the relative proportions of labile marine organic carbon and refractory terres- trial organic carbon. Thus, changes in early diagenesis reflected in the mineralogy and/or composi- tion of the Ria's sediments can throw light on the degree of oceanic influence in the Ria, and hence on changes in the circulation and ventilation of its water masses and/or the climate on shore. In this study of sediment cores taken in the outer Ria de Muros, the combined use of textural analysis, mag- netic properties (magnetic susceptibility (κ), ratio of anhysteretic to isothermal remanent magnetisa- tion (ARM:IRM), and median destructive field (MDF)) and geochemical parameters (total concentra- tions of diagenetically stable and mobile elements in sediment and pore water) allowed the identification of a current redox front and 2 palaeosedimentary redox fronts in the sediment record. These fronts originated during periods of high marine/terrestrial organic matter ratio (as inferred from the ratio of total organic carbon to total nitrogen (TOC:TN) and δ 13 C). Sedimentation rates calcu- lated from 14 C dating results identify these periods as known periods of increased upwelling and reduced continental input due to colder, drier climate in the NW Iberian Peninsula, namely the Little Ice Age, the Dark Ages, and first cold period of the Upper Holocene.

A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 2. Experimental testing

[1] This paper reports an empirical test of a new nonheating Preisach-based protocol for determining the absolute ancient magnetic field intensity (paleointensity) using a selection of synthetic samples and a large suite of modern lavas and pyroclastic lithic samples. Generally, the Preisach paleointensity estimates compare favorably with the expected field intensities: (1) for the synthetic samples displaying single-domain-like behavior, the method returned the correct result for the known field, while multidomain samples yielded an underestimate, and (2) averaging 168 post-1850 A.D. lavas yielded a value <6% within the expected field value. These Preisach paleointensity estimates are also compared with paleointensity determinations made on the same suite of samples using standard Thellier-type heating protocols and the nonheating remanence (REM) method. The Preisach paleointensity estimates compare favorably for samples that yielded correct Thellier-type determinations, including the synthetic single-domain-like samples. In addition, the Preisach method produced estimates for cases where the Thellier-type estimates failed. A possible selection criterion was identified (median destructive field), which was found to improve the paleointensity estimates in some sample suites. For the investigated sample set the Preisach method was found to be much more accurate than the REM method. The importance of cooling rate on the Preisach paleointensity estimate is also examined.

Geomagnetic paleointensity between 1300 and 1750 A.D. derived from a bread oven floor sequence in Lübeck, Germany

Geomagnetic paleointensities have been determined from a single archaeological site in Lübeck, Germany, where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today. Age dating confines the time interval from about 1300 A.D. to about 1750 A.D. Paleomagnetic directions have been published from each oven floor and are updated here. The specimens have very stable directions and no or only weak secondary components. The oven floor material was characterized rock magnetically using Thellier viscosity indices, median destructive field values, Curie point determinations, and hysteresis measurements. Magnetic carriers are mixtures of SD, PSD, and minor MD magnetite and/or maghemite together with small amounts of hematite. Paleointensity was measured from selected specimens with the double‐heating Thellier method including pTRM checks and determination of TRM anisotropy tensors. Corrections for anisotropy as well as for cooling rate turned out to be unnecessary. Ninety‐two percent of the Thellier experiments passed the assigned acceptance criteria and provided four to six reliable paleointensity estimates per oven floor. Mean paleointensity values derived from 22 oven floors show maxima in the 15th and early 17th centuries A.D., followed by a decrease of paleointensity of about 20% until 1750 A.D. Together with the directions the record represents about 450 years of full vector secular variation. The results compare well with historical models of the Earth's magnetic field as well as with a selected high‐quality paleointensity data set for western and central Europe.

Impact of diagenesis on the environmental magnetic record from a Holocene sedimentary sequence from the Chukchi–Alaskan margin, Arctic Ocean

We present a high-resolution Holocene sedimentary record of environmental variability from the eastern Chukchi Sea. An ice-rafted debris bearing silty-clay marks the deglacial to post-glacial Holocene transition at this site and is dated at 9.7 ka. An interval of oscillating magnetic parameters from 9.5 to 8.7 ka coincides with the Holocene Thermal Maximum in the western Arctic, and is manifested at the study area as pulses of fine-grained magnetite input every 180–230 years, possibly from increased river discharge or stronger currents flowing over the core site. The magnetic mineral assemblage is very uniform over the last 8.2 ka and consists of a mixture of magnetite, titanomagnetite, and a magnetic phase that we tentatively identify as the magnetic iron sulfide greigite. The amount of magnetic iron sulfides increases up through the Holocene, a trend that is controlled by the amount of marine organic matter available to fuel bacterial sulfate reduction. The median destructive field of the Natural Remanent Magnetization (MDF NRM) displays centennial to millennial scale cycles with significant variance at periods of 900–1300 and 1700–2700 years, with intervals of high MDF NRM values coinciding with indicators of greater sea ice cover [McKay, J., de Vernal, A., Hillaire-Marcel, C., Not, C., Polyak, L., Darby, D., 2008. Holocene fluctuations in Arctic sea-ice cover: Dinocyst-based reconstructions for the eastern Chukchi Sea. Can. J. Earth Sci. 45, 1399–1415]. The MDF NRM is controlled by the variable abundance of iron sulfides formed during early diagenesis. We interpret intervals of high MDF NRM values as times of stronger water column stratification, during which the pyritization process was interrupted by the lack of marine organic matter and lack of reactive iron. Intervals of low MDF NRM values, which coincide with indicators of reduced sea ice cover, are interpreted as times of stronger vertical mixing of the water column, which allows fresh marine organic matter and reactive iron to reach the seafloor, driving the pyritization process to completion.

Upper and lower Jaramillo polarity transitions recorded in IODP Expedition 303 North Atlantic sediments: Implications for transitional field geometry

Sediments collected during the Integrated Ocean Drilling Program (IODP) Expedition 303 in the North Atlantic provide records of polarity transitions and geomagnetic excursions at a high resolution. Here we investigate polarity transitions at the upper and lower boundaries of the Jaramillo subchronozone at Sites U1305, U1304, and U1306. The sediments carry strong natural remanent magnetizations (NRM) with median destructive fields consistent with magnetite as the dominant magnetic carrier. Both polarity transitions are characterized by low values of relative paleointensity. The U1305 record of the lower Jaramillo reversal exhibits a marked cluster of virtual geomagnetic poles (VGPs) over southern South America, and a secondary accumulation in the region of NE Asia/North Pacific. The main South American VGP cluster is also visible at Site U1304, which documents a less complex pattern, possibly because of a higher degree of smoothing. Records of the upper Jaramillo polarity transition document a VGP loop over the Americas, followed by north to south motion including a secondary VGP accumulation near India. The similarity between several records of the upper Jaramillo transition obtained at various sites in the North Atlantic is a strong indication that geomagnetic field changes have been faithfully captured. Results suggest that a transverse, possibly dipolar, field component has fluctuated during these polarity reversals, and that these fluctuations combined with a reduced axial dipole component yielded the observed field at the Earth’s surface during the polarity transitions. The lower Jaramillo transition also exhibits VGP clusters in the vicinity of South America and eastern Asia but these clusters are shifted relative to the upper Jaramillo VGP clusters, implying a memory exerted by the upper mantle on polarity transition geometry.

Holocene Sediment Magnetic Properties along a Transect From Ísafjardardjúp to Djúpáll, Northwest Iceland

ABSTRACTHolocene changes in terrestrial provenance and processes of sediment transport and deposition are tracked along a fjord-to-shelf transect adjacent to Vestfirdir, Iceland, using the magnetic properties of marine sediments. Magnetic susceptibility (MS) profiles of 10 cores (gravity and piston) were obtained onboard using a Bartington MS loop. Remanent magnetizations were measured at 1-cm intervals from u-channel samples taken from six cores on a cryogenic magnetometer. Between six and nine alternating field demagnetization steps were used to isolate the characteristic magnetization directions. The chronologies of the cores used in this study were determined from AMS 14C dates on mollusks and foraminifera and constrained by the regional occurrence of the 10,200 ± 60 cal yr. BP Saksunarvatn tephra. Correlative fluctuations in magnetic concentration are noted between the fjord and shelf sites, though these fluctuations are partially masked by regional variations in carbonate content. The onset of Neoglaciation is interpreted by changes in magnetic properties including an increase in mass magnetic susceptibility that began approximately 3000 cal yr. BP. The maximum angular deviation and the median destructive field (generally <20 mT) suggest that the natural remanent magnetization is carried by a coarse ferrimagnetite mineralogy, likely magnetite or titano-magnetite. Reproducible paleomagnetic inclination values are observed in several records, including a nearly vertical inclination around 8000 cal yr. BP, suggesting that the magnetic pole may have been proximal to Iceland, followed by an interval of much shallower inclination (6000–7000 cal yr. BP).

LTD‐Thellier paleointensity of 1.2 Ga Nova Floresta mafic rocks (Amazon craton)

Absolute paleointensity at 1.2 Ga was obtained on basalts and gabbros from Nova Floresta Formation (Amazon craton, Brazil). Magnetic carriers in these rocks are single‐domain to multi‐domain pure (to nearly pure) magnetite. They present a strong decrease in remanence after low‐temperature demagnetization (LTD) accompanied by an increase in median destructive fields. Incorporation of LTD to Thellier double‐heating measurements improved paleointensity estimates and enabled paleofield recovery for otherwise sterile rock samples, giving twenty seven successful determinations for six sites (45% success rate). Site‐mean paleofield values are within 4.90 μT and 15.55 μT (mean at 8.37 ± 4.07 μT). These results correspond to a mean VDM of 2.08 ± 0.94 × 1022 Am2, thus increasing the number of low‐field determinations that dominate the (still scarce) Precambrian database. Our study exemplifies the need for alternative approaches in the study of ancient rocks.

Paleomagnetic and rock magnetic evidence for a secondary yet early magnetization in large sandstone pipes and host Late Middle Jurassic (Callovian) Summerville Formation and Bluff Sandstone near Mesita, west central New Mexico

Processes responsible for the acquisition of ancient yet secondary magnetizations are important facets of the geologic history of rocks and, when the age of such magnetizations can be estimated with confidence, provide useful information on the ancient geomagnetic field. In west central New Mexico near Mesita, on the Colorado Plateau, hematitic sandstone and siltstone beds of the Middle Jurassic (Callovian) Summerville Formation and overlying Bluff Sandstone are host to numerous large (up to 100 m2 in map area) pipe‐like sandstone bodies. The pipes are as strongly cemented by hematite (colors range from 10R 6/6 to 10R 3/4) as the host strata; paleomagnetic data from them and their host strata are interpreted to indicate that these rocks have been remagnetized, probably in association with sandstone pipe formation. Reverse polarity magnetizations isolated in both alternating field and thermal demagnetization from pipes are well grouped and are similar to, and not statistically distinct from, those in adjacent host strata. The grand‐mean direction for 16 sites (7 sites in sandstone pipes and 9 in host strata), corrected for slight (5°) west‐northwest tilt of the strata, is D = 163.0°, I = −44.3° (α95 = 2.7°, k = 169). This direction yields a pole position of 72.8°N, 135.7°E (dp = 2.1°, dm = 3.4°). Assuming a modest (i.e., ∼5°) clockwise rotation of the Colorado Plateau, the pole lies at 68.7°N, 143.8°E. Median destructive fields for the remanence in pipes and host strata are typically 40–50 mT; over 90% of the remanence is “unblocked” or removed during changes in the magnetic mineralogy by temperatures of ∼400–450°C. Isothermal remanent magnetization (IRM) acquisition data, and thermal demagnetization of “saturation” IRM, however, demonstrate that the dominant magnetic phase is of high coercivity and relatively high (above 600°C) laboratory unblocking temperatures in both sandstone pipes and host strata, yet it does not appear to contribute significantly to the characteristic remanent magnetization. The similarity in demagnetization properties between pipes and adjacent host strata, the absence of a well‐defined high unblocking temperature remanence that is more typical of hematite‐cemented detrital strata, and the essentially uniform reverse polarity of the remanence are all interpreted to indicate that pipes and host strata contain secondary, yet early acquired magnetizations and that magnetization acquisition continued after pipe injection. We propose that acquisition of the secondary magnetization took place in the presence of alkaline, high pH brines formed by the dissolution of the underlying gypsum‐dominated Lower Jurassic Todilto Formation strata and therefore the remanence is early in age. On the basis of a comparison with Summerville and Morrison (Middle and Late Jurassic) paleomagnetic poles from rocks on the Colorado Plateau, we interpret the secondary remanence in Summerville strata and sandstone pipes near Laguna to be latest Middle to Late Jurassic in age. If realistic, this interpretation further emphasizes the importance of fluid‐rock interaction in the acquisition of secondary magnetizations.

Results of rock magnetic investigations and relative paleointensity determinations on lacustrine sediments from Birkat Ram, Golan Heights (Israel)

Detailed rock magnetic investigations, including the analysis of the laboratory installed magnetizations, the measurement of hysteresis parameters, and the high temperature‐dependency of the saturation magnetization, were carried out on sediments from Birkat Ram, Golan Heights, Israel. The 550 cm long composite profile, composed of three single cores, is supposed to span the Late Holocene, i.e., the last 4400 years, at least. Titanomagnetites in the pseudo single‐domain range were identified as the main magnetic carrier minerals in the homogenous sediments. The variations in concentration and grain size of the magnetic fraction are only partly reflected in the sediment lithology. The relative paleointensity was estimated by normalizing the intensity of the natural remanent magnetization (NRM) after demagnetization at 20 mT (JNRM(20 mT)) by different concentration parameters. The median destructive field of the NRM (MDFNRM) is coherent with the relative paleointensity estimates, indicating that a sedimentary effect has not sufficiently been removed by the normalization process. The morphologies of these estimates, however, correspond to the archeomagnetic records from Bulgaria and Greece. Therefore a second normalization, based on the linear relationship between the paleointensity estimates and the MDFNRM, was applied, resulting in an improvement of the records.