Paleomagnetic secular variation for a 21,000-year sediment sequence from Cascade Lake, north-central Brooks Range, Arctic Alaska

Two > 5-m-long sediment cores from Cascade Lake (68.38° N, 154.60° W), Arctic Alaska, were analyzed to quantify their paleomagnetic properties over the past 21,000 years. Alternating-field demagnetization of the natural remanent magnetization, anhysteretic remanent magnetization, isothermal remanent magnetization, and hysteresis experiments reveal a strong, well-defined characteristic remanent magnetization carried by a low coercivity magnetic component that increases up core. Maximum angular deviation values average < 2°, and average inclination values are within 4° of the geocentric axial dipole prediction. Radiometric ages based on 210Pb and 14C were used to correlate the major inclination features of the resulting paleomagnetic secular variation (PSV) record with those of other regional PSV records, including two geomagnetic field models and the longer series from Burial Lake, located 200 km to the west. Following around 6 ka (cal BP), the ages of PSV fluctuations in Cascade Lake begin to diverge from those of the regional records, reaching a maximum offset of about 2000 years at around 4 ka. Several correlated cryptotephra ages from this section (reported in a companion paper by Davies et al., this volume) support the regional PSV-based chronology and indicate that some of the 14C ages at Cascade Lake are variably too old.

A 62-ka geomagnetic palaeointensity record from the Taymyr Peninsula, Russian Arctic

This work represents the first palaeomagnetic study carried out on the sedimentary record of lake Levinson-Lessing, which is the deepest lake in northern Central Siberia. Palaeomagnetic analyses were carried out on 730 discrete samples from the upper 38 m of the 46 m-long core Co1401, which was recovered from the central part of the lake. Alternating field demagnetisation experiments were carried out to obtain the characteristic remanent magnetisation. The relative palaeointensity is determined using the magnetic susceptibility, the anhysteretic remanent magnetization and the isothermal remanent magnetization for normalization of the partial natural remanent magnetization. The chronology of Co1401 derives from accelerated mass spectrometer radiocarbon ages, optically stimulated luminescence dating, and correlation of the relative palaeointensity of 642 discrete samples with the GLOPIS-75 reference curve. This study focuses on the part >10 ka but although includes preliminary results for the upper part of the core. The record includes the geomagnetic excursions Laschamps and Mono Lake, and resolves sufficient geomagnetic features to establish a chronology that continuously covers ~62 ka. The results reveal continuous sedimentation and high sedimentation rate between 45 and 95 cm ka−1. High lock-in depths are suggested from the low variability of the magnetic record compared to data sets of reference records with lower sedimentation rate. Although the horizontal component of the characteristic remanent magnetization can only be used with caution because Co1401 was cored without core segment overlap, the magnetic record of Co1401 is the only high-resolution record of relative palaeointensity and palaeosecular variations from the Arctic tangent cylinder going back to ~62 ka.

Magnetic Mineralogy of Speleothems From Tropical-Subtropical Sites of South America

Fe-bearing minerals are a tiny fraction of the composition of speleothems. They have their origin in the karst system or are transported from the drainage basin into the cave. Recent studies on the magnetism of speleothems focused on the variations of their magnetic mineralogy in specific time intervals and are usually limited to a single sample. In this study, we describe a database of environmental magnetism parameters built from 22 stalagmites from different caves located in Brazil (South America) at different latitudes, comprising different climates and biomes. The magnetic signal observed in these stalagmites is dominated by low-coercivity minerals (∼20 mT) whose magnetic properties resemble those of the magnetite formed in pedogenic environments. Also, a comparison with few samples from soils and the carbonate from cave’s walls shows a good agreement of the magnetic properties of speleothems with those of soil samples, reinforcing previous suggestions that in (sub-)tropical regimes, the dominant magnetic phase in speleothems is associated with the soil above the cave. Spearman’s rank correlation points to a positive strong correlation between magnetic concentration parameters (mass-normalized magnetic susceptibility, natural remanent magnetization, anhysteretic remanent magnetization, and isothermal remanent magnetization). This implies that ultrafine ferrimagnetic minerals are the dominant phase in these (sub-)tropical karst systems, which extend across a diverse range of biomes. Although the samples are concentrated in the savannah biome (Cerrado) (∼70%), comparison with other biomes shows a higher concentration of magnetic minerals in speleothem underlying savannahs and lower concentration in those underlying moist broadleaf forests (Atlantic and Amazon biome) and dry forests (Caatinga). Thus, rainfall, biome, and epikarst dynamics play an important role in the concentration of magnetic minerals in speleothems in (sub-)tropical sites and indicate they can be an important target for paleoenvironmental research in cave systems.

A Feasibility Study of Microbialites as Paleomagnetic Recorders

Microbialites–layered, organosedimentary deposits–exist in the geologic record and extend back in deep time, including all estimated times of inner core nucleation. Microbialites may preserve magnetic field variations at high-resolution based on their estimated growth rates. Previous studies have shown that microbialites can have a stable magnetization. However, the timing and origin of microbialite magnetization were not well determined, and no study has attempted to evaluate whether actively growing microbialites record the geomagnetic field. Here, we present centimeter-scale magnetization and magnetic property variations within the structure of modern microbialites from Great Salt Lake (GSL), United States, and Laguna Bacalar, Mexico, Pleistocene microbialites from GSL, and a Cambrian microbialite from Mongolia. All samples record field directions close to the expected value. The dominant magnetic carrier has a coercivity of 35–50 mT and unblocking temperatures are consistent with magnetite. A small proportion of additional high coercivity minerals such as hematite are also present, but do not appear to appreciably contribute to the natural remanent magnetization (NRM). Magnetization is broadly consistent along microbialite layers, and directional variations correlate with the internal slope of the layers. These observations suggest that the documented NRM may be primarily detrital in origin and that the timing of magnetization acquisition can be close to that of sediment deposition.

Rock magnetism as a tool for investigating the use of archaeological artefacts from baked clay

&lt;p&gt;Even though multidisciplinary approaches applied to the investigation of archaeological findings are widely used, the use of rock magnetic properties is still poorly exploited in the determination of the use of ancient artefacts. In this study, we present the results of a combined archaeological, morphological and magnetic analyses applied on the ring-shape clay artefacts found at the archaeological site of Villa del Foro, in Northern Italy. The materials studied are dated between the sixth and the first half of fifth century BC and are found in large quantities in different trenches of the archaeological excavation. To investigate their thermal history and to exploit their possible use as kiln supports, cooking stands, or loom weights, we have investigated their natural remanent magnetization (NRM) and the magnetic mineralogy changes occurred during laboratory heating. Magnetic analysis used for the determination of the firing temperatures show thermal stability up to 500-600 &lt;sup&gt;o &lt;/sup&gt;C, while further laboratory heating at 700 &lt;sup&gt;o &lt;/sup&gt;C introduces magnetic alteration. Thermal demagnetization of the samples generally shows a strong and stable thermal remanent magnetization. In few cases, a clear secondary component is present, suggesting partial re-heating or displacement at temperatures ranging from 200 &lt;sup&gt;o&lt;/sup&gt;C to 450 &amp;#176;C. Such secondary magnetic component can be indicative of a secondary heating or of a displacement of the rings from their initial firing position while still hot. Even though the studied rings belong to casually different morphological typologies, no connection among type and magnetic behavior was observed, suggesting that the ring&amp;#8217;s morphology does not define neither their production conditions nor the final use of the artefacts. The estimated firing temperatures of around 600-700 &lt;sup&gt;o&lt;/sup&gt;C are compatible with the heating of the rings during their manufacture rather than related to cooking activities. In combination with the archaeological evidence and the morphological analysis it is thus suggested that the rings were used as weight looms and baked only during their production procedures. Such a pilot study can be used as reference for the identification of similar objects found in Italy and Europe during the Iron Age and confirms the great potential of rock magnetic analysis in the investigation of the technology and use of ancient baked clays.&lt;/p&gt;

Multi-proxy study of the Cretaceous-Paleogene (K-Pg) boundary in deep-sea turbiditic sediments in the Uzgruň section (Outer Flysch Carpathians, Czech Republic)

&lt;p&gt;The composite profile, with 4 studied sections, is located near the Uzgru&amp;#328; village (Czech Republic) next to a small stream. The profile is composed of Late Maastrichtian to Palaeocene flysch sediments and the K-Pg boundary is set in claystones within this turbiditic setting. Ongoing research of local paleoenvironment and stratigraphy is based on paleo- and rock-magnetic methods, micropaleontology and geochemistry to obtain more detailed view of the local situation during the K-Pg extinction event. Based on biostratigraphy, two dinocyst zones (Bub&amp;#237;k et al., 2002): Palynodium grallator and Carpatella cornuta (first occurrence in the Danian), two calcareous nannofossil zones in the Upper Maastrichtian, and the agglutinated foraminifer zone Rzehakina fissistomata in the Paleogene were distinguished. Biostratigraphic data support the K-Pg boundary interval. The uppermost Maastrichtian is indicated by nannofossil species Micula prinsii, UC26d&lt;sup&gt;TP&lt;/sup&gt; zone. Basal Paleogene non-calcareous strata contain dinocyst Carpatella cornuta and agglutinated foraminifers of Rzehakina fissistomata zone. The presence of low-latitude nannofossil taxa M. prinsii and Ceratolithoides kamptneri show input of warm waters during the uppermost Maastrichtian. Several rock-magnetic methods, such as acquisition of Isothermal remanent magnetization (IRM), acquisition of Anhysteretic remanent magnetization (ARM), Anisotropy of magnetic susceptibility (AMS), Field dependence of magnetic susceptibility (HD) and Frequency dependence of magnetic susceptibility (FD), were applied to estimate behaviour and origin of magnetic particles. Natural remanent magnetization (NRM) values of samples range from 0.09 to 2.48 mA/m. Volume normalized magnetic susceptibility (MS) show values from 130 up to 1197 SI*10-6. There is no increase observed in MS across stratigraphic boundary due to turbiditic evolution of sediment. Due to character of sediments, we applied alternating field (AF) demagnetization and used principal component analysis (PCA; Kirschvink, 1980) for estimation of characteristic remanent component. Most of the K/Pg sections worldwide have well documented Iridium anomaly. In Uzgru&amp;#328;, the preliminary results show that although the values are not as pronounced, the Ir at K-Pg boundary is still higher than in surrounding sediments. For tracing of Deccan traps effect we plan to apply mercury (Hg)/total organic carbon (TOC) stratigraphy. TOC content of 20 pilot samples is low, but not under detection limit of the instrumentation (mean value 0.92 wt%). One sample reached value 4.41 wt% of TOC. Sulphur contents are reaching 1 wt%, but several samples were under detection limit of the instrumentation. Sulphur concentrations suggest more reduction conditions of burial.&lt;/p&gt;&lt;p&gt;Current research is supported by Czech Science Foundation project no. 19-07516S and is in accordance with research plan no. RVO67985831.&lt;/p&gt;&lt;p&gt;Bub&amp;#237;k, M., Adamov&amp;#225;, M., B&amp;#261;k, M., Franc&amp;#367;, J., Gedl, P., Mikul&amp;#225;&amp;#353;, R., &amp;#352;v&amp;#225;benick&amp;#225;, L., &amp; Uchman, A. (2002). V&amp;#253;sledky v&amp;#253;zkumu hranice k&amp;#345;&amp;#237;da/terci&amp;#233;r v magursk&amp;#233;m fly&amp;#353;i u Uzgrun&amp;#283;. Geologick&amp;#233; v&amp;#253;zkumy na Morav&amp;#283; a ve Slezsku, 9, 18&amp;#8211;22&lt;/p&gt;&lt;p&gt;L. Kirschvink (1980), The least-squares line and plane and the analysis of palaeomagnetic data,&amp;#160;Geophysical Journal International, 62(3), 699&amp;#8211;718,&amp;#160;https://doi.org/10.1111/j.1365-246X.1980.tb02601.x&lt;/p&gt;

Multiple pulses in lacustrine turbidites reveal earthquake doublets

&lt;p&gt;Earthquake doublets form a particular challenge for seismic hazard assessment and can provide insights into potentially characteristic fault behaviour. However, knowledge on this type of earthquake sequences is limited to information provided by historical archives as their identification in paleoseismic records is ambiguous. The continuous sedimentation records provided by lacustrine settings might be able to resolve closely-timed earthquakes, but confident identification of earthquake doublets has, up to now, not been made. To reveal the potential of these high-resolution records, we perform a detailed analysis of a multi-pulsed turbidite that has been identified in the sedimentary infill of Lake Singkarak and that was generated by the March 2007 West Sumatra earthquake doublet (i.e. two M&lt;sub&gt;w&lt;/sub&gt;&gt;6 shocks on adjacent fault segments at 2 hours apart). In order to distinguish non-synchronously generated pulses in this turbidite (different earthquake, same turbidite source area) from those that are potentially synchronously-generated (same earthquake, different turbidite source areas), we develop a new methodology that allows analysing paleoflow directions by using grain-size analysis, natural remanent magnetization measurements and high-resolution X-ray computed tomography. Combining these techniques allows us to reveal the absolute geographical orientation of elongated grains, which are considered to be deposited aligned to the dominant paleoflow direction. Application to the 2007 turbidite in Lake Singkarak allows identifying the presence of non-synchronously generated pulses, thus confirming that each earthquake in the 2007 West Sumatra doublet triggered separate turbidity currents in the lake. Our study thus underscores the invaluable sensitivity of lacustrine paleoseismic records and outlines a promising methodology to analyse previously-described multi-pulsed lacustrine turbidites to reveal the occurrence of, up to now, unknown earthquake doublets.&lt;/p&gt;

Atomic scale structure of a plagioclase – magnetite interface

&lt;p&gt;Magnetite (Mt) is the foremost carrier of rock natural remanent magnetization (NRM). Needle- and lath shaped Mt micro-inclusions in plagioclase (Pl) from gabbro often have systematic crystallographic- and shape orientation relationships (CORs, SORs) with the Pl host. The SORs of Mt leads to magnetic anisotropy which may bias the NRM of the Mt-Pl inclusion-host assemblage. Thus, the origin of the CORs and SORs between Mt and Pl is important for paleomagnetic reconstructions. In this context, the atomic structures of Mt-Pl interfaces are of particular interest.&lt;/p&gt;&lt;p&gt;The CORs and SORs between Mt and Pl were reported earlier and the underlying systematics was revealed from correlated optical and scanning electron microscopy (SEM) including electron back scattered diffraction (EBSD) analyses [1] (and references therein). The so-called plane normal type Mt micro-inclusions extend parallel to the Mt&lt;111&gt; direction, which is perpendicular to the densely packed Mt{222} oxygen layers that are parallel to one of seven Pl lattice planes with nearly identical d-spacings, namely Pl(112), Pl(-312), Pl(1-50), Pl(150), Pl(100), Pl(31-2) and Pl(1-12). Direct imaging of Mt-Pl interfaces has rarely been reported due to the beam sensitivity of Pl. Here we present the microscopic structure of a Mt-Pl interface along the inclusion elongation direction using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast STEM (iDPC-STEM) techniques.&lt;/p&gt;&lt;p&gt;The TEM foil was prepared using a focused Ga-ion beam (Ga-FIB) from a lath-shaped Mt micro-inclusion of 23 &amp;#956;m x 17 &amp;#956;m x 0.1 &amp;#956;m extending perpendicular to Mt{111}/Pl(-312). The foil is oriented so that the Mt&lt;111&gt;/Pl(-312)-pole are parallel and Mt{110}/Pl(150) planes are perpendicular to the foil.&lt;/p&gt;&lt;p&gt;The STEM images show that the Mt-Pl interface is perfectly straight and parallel to Mt{110}/Pl(150) and that it is devoid of steps. Electron diffraction patterns confirm that the elongation direction of the micro-inclusions is determined by the good fit of oxygen layers across the Pl-Mt interface. A 2.4% difference in the d-spacings between Pl(-312) and Mt{222} is likely accommodated by every about 42'nd Mt{222} plane forming an edge dislocation at the Mt-Pl interface. In addition, elastic strain is indicated by a deviation of d&lt;sub&gt;111&lt;/sub&gt;/d&lt;sub&gt;110&lt;/sub&gt; of Mt from the strain free reference lattice. Moreover, lattice fringes in iDPC-STEM images reveal coherence between Pl(22-1) and Mt{111} planes without misfit dislocations. This additional coherence may explain the particularly strong alignment of Mt{111} and Pl(-312) reflected by the EBSD data.&lt;/p&gt;&lt;p&gt;In summary, the elongation directions of the Mt inclusions are determined by the alignment of important oxygen layers of both phases across the Mt-Pl interface, which is parallel to oxygen-rich lattice planes in both phases. Misfit dislocations are presumably introduced to compensate the 2.4% lattice misfit along the elongation direction. The well-organized interface structure ensures a low interfacial energy and is a viable explanation for the observed Mt-Pl CORs and SORs. &amp;#160;&lt;/p&gt;&lt;p&gt;Acknowledgement&lt;/p&gt;&lt;p&gt;Funding by FWF project I 3998-N29 and RFBR project 18-55-14003 is acknowledged.&lt;/p&gt;&lt;p&gt;Reference&lt;/p&gt;&lt;p&gt;[1] Ageeva et al (2020) Contrib. Mineral. Petrol. 175(10), 1-16.&lt;/p&gt;

Remagnetization of Permian Emeishan basalts: Constraints on the timing of native copper mineralization in northeast Yunnan Province, China

New paleomagnetic results from the Permian Emeishan basalts in the Zhaotong area, NE Yunnan province, China show four natural remanent magnetization components. Detailed stepwise thermal demagnetization of basaltic samples from 16 flows from the Dadi section, which represent basalt units III and IV, isolated two groups of characteristic remanent magnetizations. Samples in unit IV (five flows) record a southwest declination and a moderate downward inclination that is considered to be a partial remagnetized remanence. The bottom flows from unit III (11 flows) record a normal polarity direction, interpreted as a remagnetization, which yields a tilt-corrected mean direction of Ds/Is = 8.8°/31.6° (N = 9, ks = 39.7, α95 = 8.3°), with a corresponding paleomagnetic pole at 77.1°N, 240.0°E (K = 49.2, A95 = 7.4°). The secondary directions have steeper inclinations than primary ones that have been successfully recovered from other studies in this area of the Emeishan basalts. By comparison with the Phanerozoic paleomagnetic poles of the South China Block, the preferred timing of remagnetization is the Lower-Middle Jurassic. Field relationships suggest that the remagnetization of the Emeishan basalts is coeval with the spatially related, but localized, copper mineralization. Thus the timing of the main copper mineralization hosted in the Emeishan basalts is hypothesized to occur in the Early-Middle Jurassic.