Rock Magnetic Investigations Research Articles

Lahar events in the last 2000 years from Vesuvius eruptions – Part 1: Distribution and impact on densely inhabited territory estimated from field data analysis

Abstract. Lahars represent some of the most dangerous phenomena in volcanic areas for their destructive power, causing dramatic changes in the landscape with no premonitory signs and impacting the population and infrastructure. In this regard, the Campanian Plain turns out to be very prone to the development of these phenomena, since the slopes of the Somma–Vesuvius and Campi Flegrei volcanoes, along with the Apennine reliefs, are mantled by pyroclastic deposits that can be easily remobilized, especially after intense and/or prolonged rainfall. This study focuses on the analysis of pyroclastic fall and flow deposits and of the syn- and post-eruptive lahar deposits related to two sub-Plinian eruptions of Vesuvius in 472 CE (Pollena) and 1631. To begin with, historical and field data from the existing literature and from hundreds of outcrops were collected and organized into a database, which was integrated with several new pieces of data. In particular, stratigraphic, sedimentological (facies analysis and laboratory), and archeological analyses were carried out, in addition to rock magnetic investigations and impact parameter calculations. The new data are also referenced to the finding of ash beds in more distal areas, which were included in new isopach maps for the two sub-Plinian eruptions. The results show that for both eruptions the distribution of the primary deposits is wider than previously known. A consequence of these results is that a wider areal impact should be expected in terms of civil protection, as the sub-Plinian scenario is the reference one for a future large eruption of Vesuvius. Such a distribution of the pyroclastic deposits directly affects the one of the lahar deposits, also because a significant remobilization took place during and after the studied eruptions, which involved distal phreatomagmatic ash. From these integrated analyses, it was possible to constrain the timing of the deposition and the kind of deposits remobilized (pyroclastic fall vs. flow), and it was possible to calculate the velocities and dynamic pressures of the lahars and ultimately infer the lahar transport and emplacement mechanisms. The multidisciplinary approach adopted in this work shows how it is crucial to assess the impact of lahars in densely populated areas even at distances of several to tens of kilometers from active volcanoes. This especially applies to large parts of the densely populated areas around Somma–Vesuvius up to the nearby Apennine valleys.

A first archeomagnetic record from the South Atlantic: Investigation of a lime kiln on St. Helena

The geomagnetic field is anomalously weak in the region of the South Atlantic causing a reduced shielding effect against energetic cosmic particles and malfunctions of satellite communication. The evolution of this so-called South Atlantic Anomaly can be reconstructed for past times by means of archeomagnetic investigations conducted on suitable recorders of the ancient geomagnetic field. Here we present a first archeomagnetic study from the South Atlantic region on a lime kiln operated until the middle of the 19th century on the island of St. Helena. The laboratory experiments comprised basic rock magnetic investigations as well as demagnetization and archeointensity experiments using the MT4 protocol along with cooling rate and anisotropy corrections. Reconstructed field directions show a rather large scatter mainly due to the limited number of samples. While the effects of anisotropy are found to be negligible, cooling rate effects account for ∼11% on average yielding a corrected intensity slightly lower than expected from instrumental measurements performed at St. Helena. Nevertheless, the comparison with historical records and field models yields a good agreement with respect to the obtained archeomagnetic field components and associated uncertainties. Furthermore, the new full-vector record represents an important complement for archeomagnetic databases and, therefore, will constrain archeomagnetic field modelling approaches in the South Atlantic more accurately.

Paleomagnetic constraints on neotectonic deformation within the Southern Tian Shan piedmont and implications for the latest Miocene enhanced aridification in the Tarim Basin

The India-Eurasia collision reactivated the Tian Shan belt of Central Asia and led to crustal shortening which has been accommodated by fold-and-thrust belts on both flanks of the orogen. Southward propagation into the Tarim Basin has been accommodated by the Kuqa fold-and-thrust belts which record a range of well-preserved syntectonic continental sequences. Using existing magnetostratigraphic age constraints, we have conducted an in-depth paleomagnetic and rock magnetic investigation of this fold-and-thrust belt with the aim of linking the neotectonic deformation to the uplift of the Tian Shan orogen to the north. Anisotropy of magnetic susceptibility (AMS) shows “pencil structure” fabrics between levels dated 31.0 and 5.5 Ma. These are succeeded by incipient deformation fabrics at ~5.3 Ma, which dates an abrupt decrease in shortening by synsedimentary strain. This change coincides with the timing of initial syntectonic growth strata in the foreland basin of the Southern Tian Shan and contemporaneous vertical-axis rotations in the Yaha section. Collectively this evidence implies that the Southern Tian Shan piedmont underwent transpressional deformation with rotation accompanying strain partitioning during a regional sinistral shear. The extreme aridification which has occurred in the Tarim Basin since the latest Miocene is interpreted in terms of the deflection of the ambient westerly airstream away from this region due to the growth of the Tian Shan and its final collision with the Pamir.

Comprehensive magnetic analysis of the tephras in Middle-Late Pleistocene loess records of Serbia, and implications for tephra identification, correlation and loess chronology

Tephras are highly valuable for achieving precise chronologies and secure correlations for sedimentary sequences. Rock magnetic investigations provide a simple, rapid tool for tephra identification especially for weathered tephra, cryptotephra (hidden tephra), thin and dispersed distal tephra. But so far, the most diagnostic magnetic properties for tephra identification and for differentiation of specific tephras are still uncertain. Three laterally continuous and representative tephras/cryptotephras embedded in Serbian loess deposits provide a rare opportunity to explore application of magnetic approaches in tephrostratigraphy. Systematic rock magnetic and petrographic comparison of tephra and its host loess reveal that the characteristic magnetic mineral of the studied tephras is moderate-Ti titanomagnetite of magmatic origin, with a dominant magnetic grain size of non-interacting single domain (SD) and pseudo-single domain (PSD) state. We identify the most diagnostic magnetic properties for tephra identification in loess and many other sediments. The Curie temperature of titanomagnetite is the most diagnostic property of tephras. A low-temperature alternating current (AC) magnetic susceptibility peak at ∼20–100 K is a sensitive indicator of coarse-grained titanomagnetite. SIRM-χfd and χARM-χfd plots can easily differentiate tephras from loess-paleosol sequences and possibly other pedogenesis-influenced sediments. Furthermore, our results suggest Curie temperatures can be a promising tool of regional tephra correlation. Our magnetic evidence and mineral composition analysis support the same origin of Tephra-L5 as Tephra-L4 from Alban Hills Volcanic District, possibly from the ∼457 ka eruption, which would promote a robust chronostratigraphic model of Danube loess.

Global warming drove the Mid-Miocene climate humidification in the northern Tibetan Plateau

The Miocene is a key transitional period for the Asian monsoon evolution and establishment of modern-like geomorphic landscapes in the northern Tibetan Plateau. Elucidating the Miocene paleoenvironmental changes and their relationships with climate and tectonics are significant in understanding future hydrological changes in the northern Tibet under a warming climate. In this study, detailed lithological and rock magnetic investigations were conducted on a Miocene fluvial-lacustrine sediment sequence from the Linxia Basin in Northeast Tibet, to understand the paleoenvironmental and palaeohydrological response to the tectonics and global climatic change, especially to the effects of the Middle Miocene Climatic optimum (MMCO). The results demonstrated that strong waterlogged reducing environments during MMCO precluded the occurrences of pedogenesis in the Linxia Basin owing to highly humid climates. As the precipitation decreased significantly before and after MMCO, pedogenesis occurred increasingly owing to temperate and well-drained conditions. A synthesis of the lithological and magnetic variations in Miocene fluvial-lacustrine sediments further indicated that climate humidification prevailed in most areas of the northern Tibetan Plateau during MMCO. These observations demonstrate that global climate change, rather than the tectonic uplift, significantly influenced the Mid-Miocene paleoenvironmental evolution in northern Tibet.

Bending of the Western Mongolian Blocks Initiated the Late Triassic Closure of the Mongol‐Okhotsk Ocean and Formation of the Tuva‐Mongol Orocline

AbstractBending of ribbon continents or arcs is an important tectonic process for reconstructing central Asia. Formation of the striking Tuva‐Mongol Orocline by closure of the Mongol‐Okhotsk Ocean (MOO) has long been proposed, but coupling of the two geological events has not been well illustrated, hindering our understanding of tectonic evolution of central Asia. In order to constrain age of initial closure of the MOO in its western segment and formation of the Tuva‐Mongol Orocline, we performed paleomagnetic studies on the Late Triassic clastic rocks in the Amuria Block (AMB; 17 sites) and the Tarvagatay Block (TVB; 10 sites) of the western Mongolian blocks (WMB) on both sides of the Mongol‐Okhotsk Suture. Rock magnetic investigations show hematite and magnetite as main magnetic carriers of the characteristic remanent magnetization (ChRM). The ChRM directions from both regions pass fold and/or reversal tests and can be considered as primary remanent magnetization. Combining the overlapped new paleomagnetic poles (AMB: λ/φ = 70.4°N/233.8°E, A95 = 4.6°; TVB: λ/φ = 70.5°N/248.2°E, A95 = 9.0°) with geological evidence, we propose that the bending of the WMB following its collision with the Siberia Craton resulted in the Late Triassic closure of the MOO in its western segment, and initiated the formation of the Tuva‐Mongol Orocline.

Rock Magnetism and Magnetic Fabric Study of the Icelandite and Rhyodacite Long Volcanic Sequence at Mauna Kuwale, Wai’anae Volcano, Oahu, Hawaii, USA

In order to understand further the emplacement (i.e., volcanic growth) of 22 Icelandite and 3 Rhyodacite cooling units in one of the long volcanic sequences known as Mauna Kuwale of the Wai’anae volcano (ca. 3.3 Ma), Oahu Hawaii we have conducted appropriate rock magnetic experiments described below as well as anisotropy of magnetic susceptibility (AMS) studies of such 25 units. We have undertaken rock magnetic investigations such as continuous and partial thermo-magnetic cycles of low field magnetic susceptibility versus temperature dependence, (k-T) curves experiments. We classified the k-T heating-cooling dependence of susceptibility in three groups A, B and C. Type A: yielded two components of titano-magnetite with a predominat Ti rich phase and occasionally a relevant magnetite component phase. Type B: samples are characterized by Ti poor magnetites. Magnetite dominates as the main magnetic carrier. Type C: k-T curves show one single phase of titanomagnetite, and Ti poor magnetite. The coercivity or remanence, determined by back field magnetization is always <60 mT, which suggest the predominance of magnetic components of low coercivity, like magnetite. Usually, two coercivity components are identified in the specimens. In addition we also conducted magnetic granulometry analyses on 27 specimens to determine the domain state of the flows. The ratio of hysteresis parameters (Mr/Mrs versus Hcr/Hc) show that overall samples fall in the Pseudo-Single Domain (PSD) region with high values of Mr/Mrs and very low values of Hcr/Hc. Only two samples from cooling units 17 and specially 22 show a Single Domain (SD) magnetic behavior and a sample from one unit approaches the SD-MD mixture region. We measured the magnetic susceptibility of all cooling units and we found out that in all analyzed units the magnetic susceptibility is low 13.7 ± 8.8 (10−3 SI). Magnetic anisotropy/magnetic fabric is used as a tool in rock fabric analyses to investigate the preferred orientation of magnetic minerals in rocks. Magnetic anisotropy is low on all (measured) flows from the Icelandite cooling units from 1 to 17 (mean P’ = 1.010), but becomes noticeably distinct and high in rhyodacite cooling units 23, 24 and 25 (mean P’ = 1.074). Four units show a magnetic fabric with k3 axes vertical to sub-vertical which may be denoted as normal for the horizontal to sub horizontal units. Two Icelandite cooling units display oblate shapes and two other cooling units triaxial shapes. K1 axes are horizontal but point in different directions, i.e., NE and NW. Remaining cooling units show different magnetic fabric. Units 17, 23, 24 and 25, despite important variations in anisotropy (low for units 25 and high for units 23 and 24) and shape of ellipsoid (oblate in cooling unit 23, prolate in 24 and triaxial in 25) the k3 axes show the same orientation, SW to SSW dipping around 45° and a very steady magnetic lineation azimuth NW nearly horizontal to sub horizontal. The magnetic mineralogy and magnetic fabric indicate that both the Icelandite and Rhyodacite cooling units the magmatic evolution during the shield stage of the entire Wai’anae volcano and that such growth was not affected by tectonic deformation.

Environmental Reconstruction from the Identification of Magnetic Minerals in the Upper Sedimentary Infill of the Gran Dolina Cave (Burgos, Spain)

The cave system in the Sierra de Atapuerca holds one of the most important archaeological sites for the understanding of early human occupation in Europe. Among the different cavities and galleries, the Gran Dolina cave yielded a new hominin species coined as Homo antecessor of an Early Pleistocene age. Encouraged by our previous results in Gran Dolina, we carried out a study to extend and deepen our rockmagnetic investigation of the paleoenvironmental reconstruction of the upper Gran Dolina cave based on experiments that include composition, relative concentration, and grain size of the magnetic iron oxides present in the sediments. Based on the rockmagnetic experiments, we identified magnetite, hematite, goethite, and possibly maghemite in changeable amounts along the profile, which allows us to complement the existing shortage in the literature on the palaeoenvironmental reconstruction of the site. We tentatively interpret the rockmagnetic changes recorded in the cave sediments in terms of glacial/interglacial conditions, furnishing the base for a better understanding for the formation conditions of this unprecedented archaeological site.

Magnetostratigraphy of palaeolake sequence from Kumaun Lesser Himalaya, India: Implications on young geomagnetic excursions

Geomagnetic excursions are short‐lived episodes when Earth's magnetic field deviates into an intermediate polarity state. Understanding the origin, frequency, amplitude, duration, and field behaviour associated with excursions is a forefront research area within solid Earth geophysics. Excursion events typically last a few thousand to tens of thousands of years; unlike full reversals, excursions are generally not recorded around the globe. We present here a new palaeomagnetic record (Geomagnetic excursions) from a lacustrine sediment sequence (at times, punctuated by undersized horizons of the gravels) with extraordinarily stable signals. Palaeomagnetic and rock magnetic investigations were performed on a 17‐m‐thick profile of unconsolidated sediments from Bagwalipokar palaeolake (Lat. 29°43′18″; Long. 79°28′42″; altitude 4167 ft), Binta Basin in the Kumaun Lesser Himalaya, situated in the zone of the active North Almora Thrust (NAT). A total of 540 oriented palaeomagnetic samples from 108 horizons (five samples per horizon) were collected from the section. The extrapolation of the optically stimulated luminescence (OSL) date indicates that the whole section was deposited between 45 and 2.5 ka. Magnetic remanence is predominantly carried by magnetite alone. Virtual Geomagnetic Pole (VGP) latitudes were determined using mean declinations and inclinations. Two geomagnetic excursions were observed at younger levels [11.6–10.5 m (15.5–14.7 ka) and 15–11.7 m (8.0–2.85 ka)]. These geomagnetic excursions correlate with the recently reported younger Hawaiian lava flow excursions. The youngest excursion from 8 ka may be attributed to the climatic event beginning at 8.2 ka cooling event of an oxygen isotope record of biogenic carbonate from palaeolake Riwasa in north‐western India that provides a history of the Indian Summer Monsoon (ISM) from ∼11 to 6 ka BP and also the North Atlantic glacial outburst flood and the slowdown of Atlantic meridional overturning circulation. However, further work is needed to verify this interpretation.

Sedimentary Rock Magnetic Response to Holocene Environmental Instability in the Pearl River Delta

Located on the northern coast of the South China Sea, the densely populated Pearl River Delta has experienced the combined effects of sea-level change, monsoon-driven discharge, and especially human activity, since the late Holocene. However, how these factors have regulated the regional environmental and sedimentary evolution remains unclear. To better understand these processes, we conducted a high-resolution rock magnetic investigation of the Holocene sediments of core DS01, drilled in the vicinity of the West River channel in the head area of the Pearl River deltaic plain. The magnetic grain-size proxy of the ARM/κlf ratio (the ratio of anhysteretic remanent magnetization to low-field magnetic susceptibility) indicates a long-term fining trend of the magnetite grain size, which may be a response to an increase in the weathering intensity in the Asian monsoon region during the Holocene. An interval with an enhanced concentration of magnetic minerals (mainly magnetite and hematite) occurred during 7.7–4.8 kyr BP (calendar years before 1950), coinciding with a period of delta progradation. During the marine transgression in the early Holocene, two similar intervals of magnetic enrichment may reflect regional hydrodynamic shifts associated with cooling events at ∼9.5–9.3 kyr BP and 8.2 kyr BP. The subsequent 4.2 kyr BP cooling event possibly induced a cold and dry environment in the sediment source area. From ∼800 yr BP onward, there was a major increase in the sedimentary magnetic mineral content, likely in response to intensified agricultural and industrial activities.

Rock magnetic fingerprint of Mt Etna volcanic ash

SUMMARYA detailed rock magnetic study was conducted on ash samples collected from different products erupted during explosive activity of Mount Etna, Italy, in order to test the use of magnetic properties as discriminating factors among them, and their explosive character in particular. Samples include tephra emplaced during the last 18 ka: the benmoreitic Plinian eruptions of the Pleistocene Ellittico activity from marine core ET97-70 (Ionian Sea) and the basaltic Holocene FG eruption (122 BC), the Strombolian/Phreatomagmatic/sub-Plinian eruptions (namely, the Holocene TV, FS, FL, ETP products and the 1990, 1998 eruptions) collected from the slope of the volcano, and the Recent explosive activity (lava fountains referred to as ‘Ash Rich Jets and Plumes’, or ARJP) that occurred in the 2001–2002 period, related to flank eruptions. Mössbauer spectrometry informs that a single magnetic mineral dominates the three groups, which are characterized by variable magnetic grain sizes and composition. Detailed rock-magnetic investigations, ranging from low temperature to high temperature remanence and susceptibility experiments, indicate that the more explosive products of the Plinian eruptions and ARJP activity tephra, are characterized by oxidized Ti-rich titanomagnetites, with dominant Curie Temperatures between 230 and 330 °C. The FG and ARJP tephra are also characterized by contrasting, yet overall higher, coercivity distributions and higher magnetizations and susceptibilities, including below room temperature. In contrast, most of the Strombolian/sub-Plinian eruptions have a magnetic signature dominated by less coercive magnetite and/or Ti-poor titanomagnetite. Magnetic differences observed between the Late Pleistocene and Holocene FG Plinian eruptions can be attributed to the different composition of the former eruptions, which were fed by more evolved magmas, whereas geochemical variations characterizing the products erupted in the last few decades can be responsible for the differences between the Holocene and recent Strombolian/sub-Plinian products. Importantly, detailed magnetic investigation of sideromelane and tachylite clasts, the two end members of the juvenile fraction extracted from the ash of the most explosive products, determines that the tachylite fraction is responsible for the magnetic signature of the Plinian FG and ARJP tephra, and is attributed to the intense fragmentation that characterizes these activities, likely resulting from undercooling processes. Moreover, the abundant superparamagnetic grains associated with these eruptive styles are believed to represent the nanolite fraction responsible for the increasing viscosity of these magmas, and to be responsible for their explosive character. The distinctive magnetic properties that characterize the tachylite-bearing tephra, representative of the fragmentation process that distinguishes the most explosive activities, provides a useful magnetic tool that can complement traditional volcanological investigations.

Diagenetic dissolution, maghemitization and sulphidization of magnetic minerals in rapidly deposited gas hydrate bearing sediments from the Bay of Bengal

We conducted a comprehensive rock magnetic investigation complemented by a scanning electron microscopy analyses on a 148.8 m sediment core (NGHP-01-20A) from the methane-hydrate bearing site in the Bay of Bengal to systematically evaluate how magnetic minerals selectively respond to a range of diagenetic processes in the sulfidic and methanic sediments. For diagenetic analysis, we defined five distinct sediment magnetic zones (I–V) based on the sediment magnetic signatures. Rock magnetic property is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, silicate hosted magnetic inclusions), altered (titanomaghemite, titanohematite) and fine-grained diagenetic (pyrite) and authigenic (greigite) iron sulfide minerals. Three distinct bands of enhanced magnetic susceptibility in Z-II, Z-III and Z-V mainly comprised of detrital magnetite-rich minerals delivered by the peninsular river systems as a result of higher sedimentation events in the Krishna-Godavari (K-G) basin. Progressive diagenetic dissolution of primary iron oxide and consequent formation of iron sulfide in the uppermost sulfidic zone (Z-I) is clearly mirrored in the magnetic susceptibility (χlf) profile. Presence of moderately altered coarse-grained detrital Fe–Ti particles in the deeper methanic and magnetically enhanced zones (Z-II, Z-III, Z-V) suggest that sediment diagenesis at this site was significantly constrained by the multiple events of rapid sedimentation which might have restricted the complete dissolution of the primary minerals. Decline in magnetic grain size diagnostic proxy (ARM/IRM2.5T) in the uppermost sulfidic zones (Z-I, Z-II) can be attributed to the preferential dissolution of fine-grained magnetite. Further, downcore increases could be linked with the authigenic formation of fine-grained magnetite in methanic zones (Z-III to Z-V). Ferrimagnetic iron sulfide (greigite) indicative (SIRM/χlf) parameter showed sudden rise in Z-II and remain fairly constant throughout, suggesting the presence of authigenic greigite. Scanning electron microscopy in-line with energy dispersive X-ray spectroscopy analyses of magnetic particles from different sediment magnetic zones revealed a range of ferrimagnetic (titanomagnetite, titanomaghemite), antiferromagnetic (titanohematite) iron oxides and iron sulfide (pyrite). Detrital titanomagnetite with angular, irregular, spherical and well-defined octahedral with micron and submicron sized particles mainly derived from Deccan Volcanic Province (DVP) dominates the rock magnetic signal of bulk sediments in the studied core. We noticed several evidence of diagenetic dissolution, sulphidization and maghemitization of magnetic particles in the studied sediment core. Few nanometre-sized detrital magnetic particles occur as inclusions within the silicate minerals and remains protected against the sulfidic dissolution fronts through geologic times. Optical microscopy revealed the dominance of diagenetic pyrite occurring as overgrowth on the ferrimagnetic particles. We demonstrate that the rockmagnetic methods in combination with focussed electron microscope analyses provide valuable insights into the mineralogy, genesis and state of preservation of magnetic particles in sulfidic and methanic environments and thereby enhances the interpretative value of rock magnetic proxies.

Intermediate field directions recorded in Pliocene basalts in Styria (Austria): evidence for cryptochron C2r.2r-1

Pliocene volcanic rocks from south-east Austria were paleomagnetically investigated. Samples were taken from 28 sites located on eight different volcanoes. Rock magnetic investigations revealed that magnetic carriers are Ti-rich or Ti-poor titanomagnetites with mainly pseudo-single-domain characteristics. Characteristic remanent magnetization directions were obtained from alternating field as well as from thermal demagnetization. Four localities give reversed directions agreeing with the expected direction from secular variation. Another four localities of the Klöch–Königsberg volcanic complex (3) and the Neuhaus volcano (1) have reversed directions with shallow inclinations and declinations of about 240° while the locality Steinberg yields a positive inclination of about 30° and 200° declination. These aberrant directions cannot be explained by local or regional tectonic movements. All virtual geomagnetic pole positions are located on the southern hemisphere. Four virtual geomagnetic poles lie close to the geographic pole, while all others are concentrated in a narrow longitude sector offshore South America (310°–355°) with low virtual geomagnetic pole latitudes ranging from − 15° to − 70°. The hypothesis that a transitional geomagnetic field configuration was recorded during the short volcanic activity of these five localities is supported by 9 paleointensity results and 39Ar/40Ar dating. Virtual geomagnetic dipole moments range from 1.1 to 2.9·1022 Am2 for sites with low VGP latitudes below about 60° and from 3.0 to 9.3·1022 Am2 for sites with higher virtual geomagnetic pole latitudes. The new 39Ar/40Ar ages of 2.51 ± 0.27 Ma for Klöch and 2.39 ± 0.03 Ma for Steinberg allow for the correlation of the Styrian transitional directions with cryptochron C2r.2r-1 of the geomagnetic polarity time scale.Graphic abstract

A Holocene paleomagnetic record from Küçükçekmece Lagoon, NW Turkey

Lake sediments are very useful for providing high-resolution records of geomagnetic field variations, especially for the Holocene period. We present high-resolution paleomagnetic records from three cores (KCL12P1, P2, and P3) recovered from Küçükçekmece Lagoon, located at the northern shoreline of the Sea of Marmara, Turkey. The cores were subjected to a comprehensive paleomagnetic and rock magnetic investigation using oriented samples. According to the age-depth model, based on radiocarbon dating and X-ray fluorescence-derived Ca/Ti element ratios, tuned to available oxygen isotope records based on an absolute calendar-year time-scale, we obtained a new paleomagnetic record for the last 3800 years. Low paleointensities were found during 1500-2000 BP. Stacked paleomagnetic directions from Küçükçekmece Lagoon were correlated to regional geomagnetic field models. This correlation proved that the paleomagnetic records (paleointensity, inclination) obtained from the Küçükçekmece Lagoon sediments considerably agree with the data from the surrounding region over the past 3800 years.

A Combined Rock Magnetic and Meteorological Investigation of the Precipitation Boundary Across the Tibetan Plateau

AbstractCharacterizing the spatial distribution of precipitation across the Tibetan Plateau (TP) is important for understanding how the topography and atmospheric circulation influence the climatic patterns of the region. We collected 143 surface soil samples from the central and southeastern TP (SETP) for rock magnetic characterization of the spatial distribution of the pedogenic intensity. Comparison with meteorological data shows that pedogenic intensity decreases significantly along the pathway of moist air across the SETP toward the interior, and that there is a positive correlation between pedogenic magnetite/maghemite content and mean annual precipitation. By contrast, the samples from the central TP show a low degree of pedogenic alteration, consistent with the limited precipitation within this area. The results indicate the possibility of using magnetic properties of surface soils to determine the precipitation distribution in areas lacking meteorological stations. These observations, combined with published soil magnetic results, help to define the climatic boundary in the TP.

A full-vector paleomagnetic secular variation record from 55,000 to 33,000 cal. years BP from Río Valdéz glaciolacustrine outcrop (Tierra Del Fuego, Argentina)

High-resolution paleomagnetic investigations were performed on Río Valdez outcrop at the Fagnano Lake, central Isla Grande de Tierra del Fuego, southernmost South America. Our aims were to develop a full-vector paleomagnetic secular variation (PSV) record and establish paleomagnetic chronostratigraphy for the outcrop based on radiocarbon dating and the correlation of the PSV record with other dated PSV records in the region. We detected two distinct anomalous directional intervals at 35,400 and 33,800 cal. years BP and at 41,500 and 39,000 cal. years BP, which could be associated with Mono Lake and Laschamp excursions. Rock magnetic investigations revealed slight changes in concentration, mineralogy, and magnetic grain size along the sedimentary sequence, but the reconstruction of past direction and relative paleointensity variations in the geomagnetic field remained feasible. The record provides new insights into the behavior of the geomagnetic field at high latitudes in the Southern Hemisphere, about which very little is currently known.

Remagnetization of Jutal dykes in Gilgit area of the Kohistan Island Arc: Perspectives from the India–Asia collision

SUMMARY The Kohistan Island Arc (KIA) occupies the northwestern region of the Himalayan Mountains, sandwiched between Asia and India plates. Its formation, collision with plate boundaries, and evolution has been controversially discussed for a couple of decades. To better understand this, a palaeomagnetic study has been conducted on the Jutal dykes (ca. 75 Ma), intruded in the northeastern part of the KIA. Comprehensive rock magnetic investigations reveal that the magnetic carrier minerals are pyrrhotite and magnetite. An intermediate temperature component (ITC) predominates the natural remanent magnetization and shows good coincidence within-site; it is carried by pyrrhotite and is considered reliable, yielding a mean direction at Dg/Ig = 11.5°/39.9° (kg = 28.4, α95 = 3.5°) before and Ds/Is = 8.6°/12.1° (ks = 5.1, α95 = 9.1°) after tilt correction. A high-temperature component that is carried by magnetite exhibits random distribution within-site. The fold test for the ITC is negative, indicating a post-folding origin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy indicates that the magnetic carrier minerals were influenced by metamorphism or thermochemical fluids. The comparison of mean palaeolatitude (22.6 ± 3.5°N) of the ITC with the collisional settings and thermal history of the study area implies that the remagnetization occurred at ∼50–35 Ma, consistent with the previous reported palaeomagnetic data of the KIA. We propose a tectonic model that shows the evolution of the Jutal dykes, supporting the concept that India collided with the KIA first, followed by a later collision with Asia.

An Inclination Shallowing‐Corrected Early Triassic Paleomagnetic Pole for the North China Craton: Implication for the Mesozoic Geography of Proto‐Asia

AbstractThe Early Mesozoic is a key period for the formation of the Asia continent. In this study, in order to reconstruct the Early Mesozoic paleogeography of Proto‐Asia, we carried out a paleomagnetic study on the Early Triassic strata from the Chengde Basin in the northeastern North China Craton (NCC). Rock magnetic investigations reveal magnetite and hematite as the main magnetic carriers. Characteristic remanent magnetization directions from 21 sites show normal and reversed polarities and site‐mean directions pass fold tests. After inclination shallowing (IS) correction with the elongation/inclination method, a mean direction is calculated at Ds/Is = 322.5°/39.5° (ks = 32.6, α95s = 5.7°) in stratigraphic coordinates. The corresponding Early Triassic paleomagnetic pole (λ/φ = 53.5°N/9.5°E, A95 = 5.3°) overlaps with coeval poles that have also been corrected for IS reported from the western part of the NCC. This result indicates that there is no relative movement between the eastern and western parts of the NCC. With all IS‐corrected paleomagnetic poles, a mean Early Triassic pole is recalculated for the NCC at λ/φ = 59.4°N/13.4°E (A95 = 4.7°). We use this pole to reconstruct the paleogeography of Proto‐Asia and confirm previous studies that did not use IS corrections, in which two “triangle bay‐shaped” oceans, the Paleo‐Tethys Ocean and the Mongol‐Okhotsk Ocean, existed along the southern and northern margins of the unified NCC‐Mongolian Block (MOB). Clockwise rotations of the South China Craton and Siberia Craton relative to the NCC‐MOB during the Early Mesozoic led to the final closure of both oceans.

Magnetostratigraphy and Chronology of the Lower Pleistocene Primate Bearing Dafnero Fossil Site, N. Greece

This paper aims to contribute to the stratigraphic and geochronological evaluation of the primate bearing Dafnero fossil site of Northern Greece by means of lithostratigraphic, paleomagnetic and paleontological analyses. The 60 m thick fossiliferous deposits of fluviatile origin are recognized as representing a typical braided-river sequence unconformably overlying molassic sediments. Rock magnetic investigations indicate the presence of both medium and low coercivity minerals. Paleomagnetic sampling of the Dafnero sediments yielded a stable magnetic remanence, and the characteristic remanent magnetization directions pass reversal test with dual polarity. Based on calibration from mammal fossils, the normal polarity magnetozone N1 located in the upper third of the studied section could correlate with chron C2n (the Olduvai subchron), suggesting that the fossil horizon is within C2r with an extrapolated age of 2.4–2.3 Ma and rather closer to the upper age limit. The results allow the re-calibration of several middle Villafranchian assemblages of S. Balkans and the correlation of the corresponding mammal fauna with the environmental shifts of Praetiglian, as it is recorded in climatostratigraphic data from the Black Sea.

Late Holocene Moisture Changes in the Core Area of Arid Central Asia Reflected by Rock Magnetic Records of Glacier Lake Kalakuli Sediments in the Westernmost Tibetan Plateau and their Influences on the Evolution of Ancient Silk Road

AbstractThe evolution of Ancient Silk Road (ASR) was deeply influenced by late Holocene moisture changes in Arid Central Asia (ACA). Nevertheless, controversies in Holocene moisture change pattern of ACA and poorly–constrained age models of related studies have made the discussion about late Holocene moisture changes in ACA and their influences on the evolution of ASR difficult. Recently, a high–resolution age model during the late Holocene was established for Kalakuli Lake, a small glacier lake located in the core area of ACA. A thorough rock magnetic investigation was carried out on Kalakuli Lake sediments based on this age model. The magnetic mineral assemblage of Kalakuli Lake sediments is still dominated by primary magnetite despite minor diagenetic effects. Comparisons of rock magnetic records to parameters previously used as indicator of glacier fluctuations suggest that clastic input to Kalakuli Lake was high (low) and magnetic grain size is relatively larger (smaller), when glaciers on Muztagh Ata advanced (retreated). The ARM/SIRM ratio, a magnetic grain size proxy, is directly related to lake hydrodynamics, which are ultimately controlled by glacier fluctuations on Muztagh Ata as the result of regional moisture changes. Late Holocene moisture changes indicated by the ARM/SIRM ratio are consistent with cool/wet and warm/dry oscillations indicated by the unweighted average of biomarker hydrogen isotopic data of the C26 and C28 n–alkanoic acids in a previous study about Kalakuli Lake, most moisture change records of the core area of ACA and winter insolation of the Northern Hemisphere, but opposite to Asian monsoon evolution. Given Asian monsoon and the westerlies are mutually inhibited, we propose that late Holocene moisture changes in the core area of ACA were controlled by the intensity of Asian monsoon versus the westerlies under the governance of solar insolation. Generally increased moisture since the late Holocene indicated by the ARM/SIRM ratio favored cultural exchange and integration between the western and the eastern Eurasia, which paved the way for the formation of ASR. Coincidence between significant increase in moisture at ~200 BC suggested by the ARM/SIRM ratio and the formation of ASR indicates moisture as an important factor that facilitated the formation of ASR. The onsets of three prosperity periods of ASR in the history generally correspond to periods when moisture was relatively high, nevertheless, stagnant periods of ASR do not coincide with periods when moisture was relatively low in the core area of ACA. Disorganized correlations between stagnant periods of ASR and moisture changes in the core area of ACA suggest that moisture is not the decisive factor influencing the evolution of ASR.