Z. V. Sharonova

Extraterrestrial iron in the Cretaceous-Danian sediments

The composition and distribution of particles of native iron in eight sections of the Cretaceous-Danian sediments in the Caucasus, Crimea and Kopet Dagh were studied using thermomagnetic analysis up to 800°C. Iron particles are found in 330 of 571 tested samples, their percentage varies from 10−5 to 0.05%, and their distribution is bimodal. It was established that the Santonian sediments of the Caucasus and Kara-Kala are enriched with the iron particles; the upper boundary of these sediments is marked by a sharp drop in the iron content at approximately 84 Ma, which coincides with the upper boundary of the Dzhalal hyperchron. The variations in the Curie point of iron from 680°C up to 780°C reflect the fluctuations of the nickel admixture. A peak of the elevated iron content with nearly constant nickel of 5% was found in all studied sections, i.e., this is a global effect. The global pattern of the distribution and composition of the iron particles clearly indicates that their origin is associated with cosmic dust. At the same time, the particles of Ni-Fe alloy and pure nickel are very rare, and their concentration does not correlate with the content of iron particles. Apparently, there are very few Ni-Fe and pure nickel particles in cosmic dust, and, most likely, the particles of Ni-Fe alloy are mainly due to impact events.

Extraterrestrial magnetic minerals

Thermomagnetic and microprobe analyses are carried out and a set of magnetic characteristics are measured for 25 meteorites and 3 tektites from the collections of the Vernadsky Geological Museum of the Russian Academy of Sciences and Museum of Natural History of the North-East Interdisciplinary Science Research Institute, Far Eastern Branch of the Russian Academy of Sciences. It is found that, notwithstanding their type, all the meteorites contain the same magnetic minerals and only differ by concentrations of these minerals. Kamacite with less than 10% nickel is the main magnetic mineral in the studied samples. Pure iron, taenite, and schreibersite are less frequent; nickel, various iron spinels, Fe-Al alloys, etc., are very rare. These minerals are normally absent in the crusts of the Earth and other planets. The studied meteorites are more likely parts of the cores and lower mantles of the meteoritic parent bodies (the planets). Uniformity in the magnetic properties of the meteorites and the types of their thermomagnetic (MT) curves is violated by secondary alterations of the meteorites in the terrestrial environment. The sediments demonstrate the same monotony as the meteorites: kamacite is likely the only extraterrestrial magnetic mineral, which is abundant in sediments and associated with cosmic dust. The compositional similarity of kamacite in iron meteorites and in cosmic dust is due to their common source; the degree of fragmentation of the material of the parent body is the only difference.

Native iron and other magnetic minerals in the sediments of the northwestern Atlantic: Thermomagnetic and microprobe evidence

The thermomagnetic and microprobe analyses of sedimentary samples from DSDP 386, 387, 391A, and 391C boreholes in the northwestern Atlantic reveal the ubiquitous occurrence of particles of native iron. The concentrations of native iron are bimodal everywhere with the zero mode necessarily present. The nickel admixture in native iron forms two groups, one represented by pure iron and the comprising native iron with 5–6% Ni. The redeposition of iron particles manifests itself in the correlation between the concentrations of these particles and terrestrial minerals (magnetite), as well as in the equalization and reduction of the concentration of the iron particles. Pyrite and pyrrhotite are widespread in the studied sediments, and the distribution of native iron does not depend on the presence of pyrite (i.e., on redox conditions) in them. At the same time, the distributions of pyrite and particles of magnetite + titanomagnetite are inversely correlated, which can probably be accounted for by the partial dissolution of magnetite and titanomagnetite in the reducing conditions. The increased concentration of particles of volcanogenic homogeneous titanomagnetite is revealed in the volcanoclastic turbidites of the Oligocene and early and middle Miocene age at the base of the Bermuda Rise (borehole 386). The titanomagnetite composition is characteristic of the basalts of plume magmatism; it corresponds to the depth of the magmatic source in the interval of 50–25 km.

Thermomagnetic Evidence of Native Iron in Sediments

The paper summarizes the results of thermomagnetic analysis concerning the distribution of metallic iron in the sediments ranging in age from Miocene to Early Cretaceous sampled from the following sections: Gams (Austria); Verkhorech’e and Sel’bukhra (the Crimea); Kvirinaki and Tetritskaro (Georgia); Aimaki, Dzhengutai, Madzhalis, and Gergebil (Ciscaucasia, Russia); Klyuchi and Teplovka (Volga region, Russia); Koshak (Kazakhstan); and Khalats and Kara-Kala (Turkmenia). Small amounts of native iron (from 10−5% to 0.05%) are identified in 521 samples of 921 studied; i.e., iron particles are almost pervasive. This fact traces the origin of these particles to cosmic dust. Some established features point to the heterogeneous character of the cosmic dust: (a) the samples clearly fall into two groups. One group comprises the rocks that contain iron particles; the rocks of the other group are iron-free. In the first group, four intervals are distinguished where the sediments are globally enriched with iron with constant nickel content (5–6%); (b) in terms of composition, the iron particles are divided into three groups. The first group contains pure iron; the particles pertaining to the second group contain iron with a minor amount of nickel typical for kamacite; and the third group comprises the particles of Fe-Ni alloy with more than 20% nickel. The first and the second groups are ubiquitous; the particles of the third group are spread locally. They bear no relation to cosmic dust and are probably associated with the meteoritic impacts.

Magnetolithologic and magnetomineralogical characteristics of sediments at the Mesozoic/Cenozoic boundary: The Koshak section (Mangyshlak peninsula)

Results of a detailed petromagnetic study of sediments of the Koshak section, including the Mesozoic/Cenozoic (K/T) boundary, are presented. The rocks are shown to have a very low magnetization. A relatively high magnetization is characteristic of two thin clayey beds, one at the K/T boundary and the other 0.6 m above it: x up to 2.5 × 10−9 m3/kg, Ms up to 0.6 × 10−3 A m2/kg, and Mrs up to 0.4 × 10−3 A m2/kg. This is related to relatively high concentrations of hemoilmenite (up to 0.2%), magnetite (up to 0.01%), and goethite (up to 0.24%) in these beds. It is evident that the distribution of these magnetic minerals is lithologically controlled (the predominant occurrence in clayey beds), which is expressed, in particular, in the relation between the paramagnetic (clayey) and diamagnetic (carbonate) contributions to the magnetization of the sediments. Thus, clayey interbeds are sharply distinguished by the value of the paramagnetic magnetization (Mp = (83–86) × 10−5 A m2/kg) as compared with purely diamagnetic chalk (Md = −(26–35) × 10−5 A m2/kg). Minor concentrations of metallic iron (up to ∼0.002%) discovered in the sediments have a lithologically uncontrolled distribution (metallic iron is more often observed near the K/T boundary rather than in clayey beds). Most probably, magnetite, hemoilmenite, and goethite were accumulated mostly with clay and other terrigenous material, while fine particles of iron are likely to have been dispersed by air. The whole set of the data of this work suggests that the K/T boundary is not distinguished by characteristic magnetomineralogical and magnetolithologic features.

A relationship between the concentration of native iron particles in sediments and the rate of their accumulation: A synthesis of thermomagnetic data

The data on the distribution of native iron particles in sediments yielded by thermomagnetic analysis are explored. It is shown that the accumulation of iron particles in sediments is statistically inversely linked with the rate of sedimentation and the frequency of geomagnetic reversals and statistically directly linked to the intensity of the geomagnetic field. These dependences highlight the predominance of cosmic iron particles in the studied sediments.

Direction and relative paleointensity of the geomagnetic field in the middle and later pleistocene from paleomagnetic data of the Roxolany section (Ukraine)

The lower part of the Roxolany section (Ukraine) is studied, and paleomagnetic and petromagnetic characteristics of rock samples are obtained for the time interval 300-75 ka. Detailed curves of variations in angular parameters of the geomagnetic field are constructed in the entire time interval, and curves of variations in the relative paleointensity are obtained for the interval 300-180 ka. Using the values of the angular parameters and the deviations of the virtual geomagnetic pole from the position of the stationary field, anomalous directions compatible with the Jamaica excursion are identified in the intervals 250-249 and 221-220 ka. The geomagnetic field evolution is studied by methods of wavelet analysis, and the field generation process is shown to vary in the interval 300-180 ka.

Paleomagnetic studies of Paleolithic site deposits in the Matzuka Cave (Northern Caucasus)

As a result of detailed paleomagnetic and magnetic studies of Paleolithic site deposits in the Matuzka Cave, a record of the Matuzka geomagnetic excursion in lithologic layer 7 has been discovered and studied. Such characteristic features as the geomagnetic field direction, position of the virtual geomagnetic pole, geomagnetic field intensity (roughly estimated) after and during the excursion, and climatic conditions coeval with its existence make the Matuzka excursion similar to the ∼130-ka Blake excursion. This dates at ∼130 ka the formation of layer 7 with ancient archaeological findings.

On the synchronism in the events within the core and on the surface of the earth: the changes in the organic world and in the polarity of the geomagnetic field in the phanerozoic

The data on geomagnetic reversals are compared with the changes in the organic world and with the lower-mantle plumes. The times of the formation of plumes and the times of their appearance on the Earth’s surface relate to the intervals characterized by the different frequencies of geomagnetic reversals, i.e., there is no interrelation between the formation of plumes and the frequency of the changes in the geomagnetic field polarity. At the same time, a certain synchronism is observed between the frequency of the geomagnetic reversals and the boundaries of the biostratigraphic ages, i.e., the changes in the organic world in the long-period range. A hypothesis is proposed, which explains the change in the sign of the geomagnetic field by the combined effect of the irregular rotation of the internal core relative to the mantle and the changes in the slope angle of the axis of the Earth’s rotation, which, in turn, results in synchronous events on the Earth’s surface: the rates of changes in the organic world.

Study of Changes in the Rock Formation Medium of the Karadzha Section (Azerbaijan) from the Results of Petromagnetic Investigations

The petromagnetic characteristics of marine and subaqueous deposits on the upper marine terrace of the Karadzha section (Azerbaijan) are investigated. These deposits correspond to the great Khvalynsk transgression of the Paleocaspian. Thirteen meters of the section encompassing an age interval of ∼45–20 ka are sampled continuously. Regular along-section changes are revealed, and their relation to environmental changes is established. Magnetite, maghemite, and hematite that were formed in the process of alteration of the maternal rocks in the Paleocaspian wash-down basin are the main ore minerals of the studied rocks. The ratios of these minerals in the deposits depend on the degree of alteration of the maternal rocks. Both the total amount of ore mineral (K and SIRM parameters) and the rigidity parameter Bcr regularly increase in the basal part of the section and in the sediments reflecting finer variations in the basin level. An increased magnetic rigidity and the sensitivity of petromagnetic parameters to weak variations in the sea level are characteristic features of sediments in this part of the Paleocaspian compared with normal marine sediments.