Sedimentation rates in the Makarov Basin, central Arctic Ocean: A paleomagnetic and rock magnetic approach

Three long sediment cores from the Makarov Basin have been subjected to detailed paleomagnetic and rock magnetic analyses. Investigated sediments are dominated by normal polarity including short reversal excursions, indicating that most of the sediments are of Brunhes age. In general, the recovered sediments show only low to moderate variability in concentration and grain size of the remanence-carrying minerals. Estimations of relative paleointensity variations yielded a well-documented succession of pronounced lows and highs that could be correlated to published reference curves. However, together with five accelerator mass spectrometry C-14 ages and an incomplete Be-10 record, still two different interpretations of the paleomagnetic data are possible, with long-term sedimentation rates of either 1.3 or 4 cm kyr(-1) However, both models implicate highly variable sedimentation rates of up to 10 cm kyr(-1), and abrupt changes in rock magnetic parameters might even indicate several hiatuses

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before  ∼  320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments. While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land and intra-lake processes

Evidence for geomagnetic excursions recorded in Brunhes and Matuyama Chron lavas from the trans‐Mexican volcanic belt

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99072/1/arar_methodology.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99072/2/jgrb50214.pd

Archaeomagnetic results on three Early Iron Age salt-kilns from Moyenvic (France)

International audienceVariations of the Earthfs magnetic field during the first millennium BC in western Europe remain poorly constrained, especially archaeointensity changes. Three salt-kilns (MOA, MOB and MOC) sampled in Moyenvic (Lorraine, eastern France) have been studied to provide new reference data. Each kiln has been dated by radiocarbon to originate from the Early Iron Age or Hallstatt period (between VIII and Vth Century BC). Rock magnetic experiments and hysteresis results suggest the predominance of pseudo-single domain (PSD) Ti-poor magnetite. Archaeomagnetic directions obtained by thermal and alternating field demagnetizations have high mean inclination (close to 70.) and declination (between 19 and 31.). A first set of classical Thellier.Thellier experiments was conducted on 46 samples with a laboratory field almost parallel to the direction of the characteristic remanent magnetization (ChRM). Only 24 of these specimens present a linear NRM.TRM plot. For other specimens, NRM.TRM plots are concave-up with positive pTRM checks. The very large dispersion observed between the determined palaeointensity values suggests some artefacts have not been fully recognized. A second set of Thellier experiments was conducted on 34 sister specimens with the laboratory field applied quasi-perpendicular to the ChRM. In these cases,mineralogical evolutions during heating and chemical remanent magnetization acquisitions have been clearly recognized, despite positive pTRM checks. The concave-up shapes of NRM.TRM plots appear mainly due to mineralogical alteration rather than to the presence of PSD.MD grains. For the entire set of samples the success rate of the palaeointensity determinations is very low with 80 per cent of the samples rejected. Nevertheless, reliable mean archaeointensities have been obtained for two of the three kilns (MOA, 80.1 } 14.5 ƒÊT and MOB, 86.6 } 6.9 ƒÊT at the latitude of Paris). The high field strength and the archaeomagnetic directions determined, combined with previous published data, provide further evidence for important changes of the Earth magnetic field in Europe during the first half of the first millennium BC. These large variations of the geomagnetic field during the Iron Ages indicate that archaeomagnetism is highly suitable for dating of structures from this period

Cooling rates of pyroclastic deposits inferred from mineral magnetic investigations: a case study from the Pleistocene Mýtina Maar (Czech Republic)

Tephra layers of the Mýtina Maar, Czech Republic, contain ferrimagnetic Mg–Al-rich titanomagnetite, which is suggested to originate from a fractionated alkaline CO$_{2}$-rich lithospheric mantle melt. We investigated the magnetic mineralogy and Curie temperature (T$_{C}$) from tephra deposits of two drill cores (< 9 m depth). T$_{C}$ calculated (208 ± 14 °C) from chemical composition (Fe$^{2+}$$_{0.8}$Mg$_{0.5}$Fe$^{3+}$$_{1.1}$Al$_{0.3}$Ti$_{0.3}$O$_{4}$) is in accordance with TC retrieved from cooling curves of temperature-dependent magnetic susceptibility measurements (195–232 °C). However, thermomagnetic curves are irreversible either with lower (type I) or higher (type II) TC in the heating curve. All curves show transition temperatures above ca. 390 °C, indicating maghemitization. We interpret the irreversibility of TC (ΔT$_{C}$) in terms of different degrees of cation ordering, overprinted or masked by different degrees of maghemitization, which is a low-temperature phenomenon. Negative ΔT$_{C}$ indicates that original deposited titanomagnetite has cooled faster and, therefore, has stored a lower degree of cation ordering compared to heating/cooling rate of 11 °C/min in the Kappabridge. Type II with positive ΔT$_{C}$ indicates higher degree of cation ordering, and, therefore, slower cooling rate. The central part of this deposit shows most severe maghemitization, indicating rather wet emplacement. We, therefore, suggest different eruption styles for deposition of type I pyroclastics with more phreatomagmatic and type II pyroclastics with more phreato-Strombolian eruption styles. Our study is a new approach to discriminate different cooling histories in maar deposits using the Curie temperature of titanomagnetite. We suggest that this method has the potential to discriminate different emplacement modes resulting from different eruption styles

El uso de parámetros magnéticos en estudios paleolimnológicos en Antártida

En esta contribuci&oacute;n se describen las distintas t&eacute;cnicas y mediciones magn&eacute;ticas utilizadas en Magnetismo Ambiental y Paleomagnetismo. Tales mediciones ofrecen &uacute;tiles indicadores para realizar estudios relacionados con cambios clim&aacute;ticos y ambientales, as&iacute; como herramientas de dataci&oacute;n. Si bien es ampliamente conocida la utilidad de la susceptibilidad magn&eacute;tica, en primer lugar se discute el potencial y necesidad del uso de par&aacute;metros adicionales obtenidos a partir de mediciones de magnetizaciones remanentes (natural, anhist&eacute;rica e isot&eacute;rmica), hist&eacute;resis magn&eacute;tica y estudios termomagn&eacute;ticos. A continuaci&oacute;n se presentan resultados magn&eacute;ticos obtenidos en sedimentos lacustres del Archipi&eacute;lago James Ross (NE de la Pen&iacute;nsula Ant&aacute;rtica) como un caso de estudio. Se complementa con estudios sedimentol&oacute;gicos, hidroqu&iacute;micos, geoqu&iacute;micos y de estad&iacute;stica multivariada, pero se pone &eacute;nfasis en los par&aacute;metros magn&eacute;ticos y su relaci&oacute;n con los distintos procesos que ocurren en los sistemas lacustres ant&aacute;rticos. Se analiza adem&aacute;s el uso de las paleointensidades relativas como herramienta de dataci&oacute;n en lagunas ant&aacute;rticas

ASSESSING LOCK-IN DEPTH AND ESTABLISHING A LATE HOLOCENE PALEOMAGNETIC SECULAR VARIATION RECORD FROM THE MONGOLIAN ALTAI

Although paleomagnetic secular variations (PSV) often corroborate radiocarbon (14C)-based lacustrine sediment chronologies, this is not the case at the high-altitude site Khar Nuur in the Mongolian Altai Mountains. Our results show that the inclination pattern resembles those from a regional reference record from Shireet Naiman Nuur and global geomagnetic field models very well, but with a constant offset of 730 ± 90 yr. Possible reservoir effects from terrestrial pre-aging and hardwater effects can be excluded as the cause of the ∼730-yr offset because the different dated compounds correspond very well to each other, and modern reservoir effects are negligible. Instead, the constant ∼730-yr offset in the PSV pattern is likely the result of a constant lock-in depth of 26 ± 2 cm below the sediment-water interface at Khar Nuur. This assumption is supported by comparison of paleoclimatological proxies from Shireet Naiman Nuur, where similarities are obvious for the 14C-based chronology of Khar Nuur without a ∼730-yr adjustment. Therefore, the previously published 14C-based chronology of Khar Nuur provides a reliable age control. Accepting the lock-in depth of 26 ± 2 cm, the good consistency in inclination between Khar Nuur and global geomagnetic field models highlights the reliability of the latter even in a paleomagnetically understudied area

Evaluation of the multispecimen parallel differential pTRM method: a test on historical lavas from Iceland and Mexico

Accepted versio