Vadim A. Kravchinsky

A rock-magnetic record from Lake Baikal, Siberia: Evidence for Late Quaternary climate change

Rock-magnetic measurements of sediment cores from the Academician Ridge region of Lake Baikal, Siberia show variations related to Late Quaternary climate change. Based upon the well-dated last glacial-interglacial transition, variations in magnetic concentration and mineralogy are related to glacial-interglacial cycles using a conceptual model. Interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals. Glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals. The variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods. We also infer an increased contribution of eolian sediment during the colder, windier, and more arid glacial conditions when extensive loess deposits were formed throughout Europe and Asia. Eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals. Variations in magnetic concentration and mineralogy of Lake Baikal sediment correlate to the SPECMAP marine oxygen-isotope record. The high degree of correlation between Baikal magnetic concentration/mineralogy and the SPECMAP oxygen-isotope record indicates that Lake Baikal sediment preserves a history of climate change in central Asia for the last 250 ka. This correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method. Future work must include providing better age control and additional climate proxy data, thereby strengthening the correlation of continental and marine climate records.

An 84-kyr paleomagnetic record from the sediments of Lake Baikal, Siberia

We have conducted a paleomagnetic study of sediment cores obtained from the Selenga prodelta region of Lake Baikal, Russia. This record, which spans approximately the last 84 kyr, contributes to a better understanding of the nature of geomagnetic field behavior in Siberia and is a useful correlation and dating tool. We demonstrate that the Lake Baikal sediments are recording variations in the geomagnetic field. The directional record displays secular variation behavior with a geomagnetic excursion at 20 ka and additional excursions appearing as large-amplitude secular variation at 41, 61, and 67 ka. Smoothing of the geomagnetic excursion behavior occurs in Lake Baikal sediments owing to the intermediate sedimentation rate (13 cm kyr−1). The Lake Baikal relative paleointensity record correlates to absolute paleointensity data for the last 10 kyr and to relative paleointensity records from the Mediterranean Sea and Indian Ocean for the last 84 kyr. This correlation suggests a strong global (i.e., dipole) component to these records and further supports the reliability of sediments as recorders of relative geomagnetic paleointensity. We show that a relative geomagnetic intensity stratigraphy has a potential resolution of 7 kyr by correlating continental and marine records. The geomagnetic intensity stratigraphy helps constrain the age of the difficult to date Lake Baikal sediments.

A PALAEOMAGNETIC STUDY FROM THE MONGOL-OKHOTSK REGION : ROTATED EARLY CRETACEOUS VOLCANICS AND REMAGNETIZED MESOZOIC SEDIMENTS

We collected 47 sites of Upper Triassic to Middle Jurassic siltstones and sandstones and two sites of Lower Cretaceous andesites in a large basin located south of the Mongol‐Okhotsk suture. The suture separates the Siberian craton to the north from the Mongolian and Chinese blocks to the south. Laboratory treatment and analyses identify the same post-folding direction in all rocks. The mean direction (with a corresponding palaeopole at 76.8oN, 152.2oE, A95D 4:2o/ of the overprint component .ND 49 sites) is significantly different at the 95% confidence level from the expected time-averaged Brunhes or present-day field directions. The Lower Cretaceous andesites possess a stable remanent direction at high temperatures that is distinct at 95% confidence limits from the overprint direction. The corresponding pole (58.3oN, 51.0oE, dp=dm D 3.8o=4.6o), based on only fourteen samples, is significantly rotated 78:4o 5:3o counterclockwise and insignificantly displaced 2:7o 3:4o north with respect to the Early Cretaceous reference pole for Siberia. We argue that the rotation is likely tectonic in nature and not due to a chance reading of the palaeosecular variation of the Earth’s magnetic field. Both the palaeomagnetic data and the folding patterns we observed in the field suggest that deformation associated with the suture continued after the Early Cretaceous and involved sinistral shear.

Palaeomagnetic study of Vendian and Early Cambrian rocks of South Siberia and Central Mongolia: was the Siberian platform assembled at this time?

Abstract A palaeomagnetic study of Vendian and Early Cambrian sediments from the Angara block of the Siberian platform: Shaman (52.08°N, 108.83°E) and Minya (58.0°N, 110.0°E) Formations, and the Tuva-Mongolian block: Tsagan-Olom and Bayan-Gol Formations (46.76°N, 96.37°E) isolated three different components of magnetization through thermal demagnetization. The stable high-temperature characteristic remanence directions show both normal and reverse polarities. The mean palaeopoles computed after these high-temperature components are: 32.0°S/71.1°E (dp/dm=6.9°/13.8°) for the Vendian Shaman Formation (10 sites, 80 samples), 33.7°S/37.2°E (dp/dm=8.6°/14.7°) for the Vendian Minya Formation (12 samples), 22.8°S/28.4°E (dp/dm=10.8°/21.6°) for the Vendian Tsagan-Olom Formation (4 sites, 25 samples) and 21.4°S/167.1°E (dp/dm=9.6°/19.1°) for the Early Cambrian Bayan-Gol Formation (6 sites, 49 samples). From a compilation of Vendian and Early Cambrian palaeopoles from the Anabar, Angara and Aldan blocks of the Siberian platform and Tuva-Mongolia block, we propose a model where these blocks were situated in an equatorial to low south palaeolatitude position, with their present-day southern boundaries facing the north pole. From the analysis of the scatter of these poles, we conclude that the Siberian platform might not have fully amalgamated by this time, and that significant rotations occurred after the Early Cambrian. Our new palaeopoles for the Tuva-Mongolia block, together with previously published ones, show that this block was already adjacent to Siberia by the Vendian and Early Cambrian. We propose that the large counterclockwise rotation of the Tuva-Mongolia block with respect to Angara block could mark the end of the closure of the part of the Palaeo-Asian ocean separating these two blocks, and could account for the occurrence of Vendian-Early Cambrian ophiolites in the region.

New Tertiary paleomagnetic poles from Mongolia and Siberia at 40, 30, 20, and 13 Ma: Clues on the inclination shallowing problem in central Asia

We report results of a paleomagnetic study of 490 cores from 59 sites, corresponding to 52 distinct basaltic flows from Mongolia and Siberia: Khaton Sudal (39.4 Ma, 44.5°N/101.4°E), Taatsyn Gol (1, 31.5 Ma, 45.4°N/101.3°E; 2, 28.0 Ma, 45.5°N/101.1°E), Ust Bokson (19.9 Ma, 52.1°N/100.3°E), and Taatsyn Gol (3, 12.7 Ma, 45.5°N/101.0°E). Stepwise thermal and alternating field demagnetizations isolated a stable high-temperature component (HTC) of magnetization in most specimens, which we interpret as the primary magnetization of these basaltic lava flows. The four corresponding paleopoles appear consistent with coeval paleopoles from other Asian effusive formations. However, except for the 12.7 Ma paleopole, the paleopoles are systematically far-sided from the European apparent polar wander path (APWP) with respect to site locations, corresponding to anomalously shallow inclinations in Tertiary Asian effusive formations. In the hypothesis of a dipolar magnetic field in the Tertiary, this indicates a ∼1000–1500 km position of the Siberia craton and Amuria block farther south than expected at 40 and 30 Ma. Tectonically, this interpretation implies decoupling and relative rotations between the western and eastern parts of Eurasia between the Cretaceous and Present. We show that if Siberia were located more to the south, the ∼15°–20° paleolatitude anomaly generally observed in sedimentary formations from central Asia reduces to a more reasonable average value of ∼7°, which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of Asian blocks under the effect of ongoing penetration of India into Eurasia in the Tertiary.

Magnetic record of Lake Baikal sediments: chronological and paleoclimatic implication for the last 6.7 Myr

Magnetic remanence vectors for 1472 samples taken from a 601 m core through Lake Baikal sediments are reported along with a complete magnetic susceptibility profile obtained from a pass-through system. Matching the stable remanence directions to the standard geomagnetic polarity time scale (GPTS) provides a robust chronology from the present back to V6.7 Ma and yields a remarkably constant sediment accumulation rate of 3.9 cm/kyr. For earlier times ^ represented by depths s 270 m ^ correlation to the GPTS is more problematic. Susceptibility fluctuations reflect climatic changes that can be matched to the marine oxygen isotope pattern for the last 6.7 Myr. Spectral analysis of the resulting susceptibility time series then indicates that, for the most part, the Milankovitch obliquity signal dominates. However, when the temporal evolution of the frequency content is investigated by analyzing sequences of time windows, a complex picture emerges in which eccentricity and precession power appear during some intervals. Furthermore, there is persistent evidence for significant power in a ‘non-Milankovitch’ band between 28 and 35 kyr. A 2003 Elsevier Science B.V. All rights reserved.

Paleomagnetic and rock-magnetic studies on lake baikal sediments -BDP96 borehole at academician ridge-

Publisher Summary This chapter presents two paleomagnetic and rock–magnetic studies conducted on two sedimentary cores, BDP96-1 and BDP96-2, drilled at the Academician Ridge of Lake Baikal. The comparison of the paleomagnetic inclination records with the geomagnetic polarity time scale showed that the sedimentary sequence covers the age of the past 5 million years. The study is conducted on discrete samples and on quarter-core samples. Path-through measurement of the quarter core samples revealed detailed geomagnetic variation, such as the double polarity transitions around the B/M boundary. The average sedimentation rate from the depth-age relation is estimated to be 3.8 cm/kyr, with a correlation coefficient of 0.997–0.999. This high correlation suggests that the sedimentation at Academician Ridge during the past 5 million years is continuous in a quiet environment. Magnetic susceptibility is closely related to changes in the content of biogenic silica and shows a clear correlation with glacial–interglacial change. Susceptibility measurement is relatively quick and nondestructive, making it a valuable means of paleoclimatic study of Lake Baikal sediment, however, further study of the mechanism of the susceptibility changes in Lake Baikal associated with paleoclimate are needed.

Paleomagnetism and Paleoenvironmental Magnetism Studied on BDP-98 Sedimentary Cores from Lake Baikal

Paleomagnetic and rock-magnetic studies were conducted on two sedimentary cores, BDP-98-1 (200 m in length) and BDP-98-2 (ca. 410 m in length from 191 m to 600 m deep), drilled at the Academician Ridge of Lake Baikal. The magnetic inclination records assigned to the geomagnetic polarity time scale (GPTS) indicate that BDP-98-1 attained an age of 5 Ma. For BDP-98-2, owing to the long reversed interval between 290 m and 350 m, it became difficult to find a correlation to GPTS. A correlation tested in this study suggests that the basal age is over 11 Ma at 600 m deep. The average sedimentation rate for BDP-98-1 and the upper 50 m of BDP-98-2 is estimated as 4.1cm/kyr. The sedimentation over the long reversed interval is as high as 29.3cm/kyr, and below this depth, to 600 m, the rate is about 4.5cm/kyr.

Derivation of paleolongitude from the geometric parametrization of apparent polar wander path: Implication for absolute plate motion reconstruction

Obtaining ancient longitude position of continents in the past has always been a challenge for plate tectonic reconstructions. Paleomagnetism has been commonly used to reconstruct paleolatitudes and relative rotations but not paleolongitudes. In this work, we present a synthesized method to derive paleolongitude by geometrically parametrizing apparent polar wander path (APWP). Great and small circle modeling are implemented concurrently to the identified APWP tracks to calculate the paleomagnetic Euler parameters (stage rotation pole and rotation angle). From the Euler parameters of the optimal fitting option, the absolute motion history can be restored for the reference geometries. Using our method as well as the results from relative plate motion studies, we reevaluate the dispersion history of East Gondwana since 140 Ma. To further test the validity of our method, we compare the predictions from four other absolute motion models mainly in paleolatitude movement, longitudinal variation, and great circle distance, which suggest the most similarity with the global hybrid reference frame.

PMTec: A new MATLAB toolbox for absolute plate motion reconstructions from paleomagnetism

Abstract Established on Euler rotations (rotation poles and angles), quantitative representation of plate motion history has been one of the focus fields in geoscience since the beginning of the age of plate tectonics. Here we present a new MATLAB based toolbox PMTec primarily developed for (1) construction of apparent polar wander paths (APWP, in the form of running means and spherical splines) from paleomagnetic data and (2) absolute plate motion calculations through APWP geometric parameterizations. We choose to build the graphical users interface of PMTec using MATLAB considering its powerful mapping toolbox for geospatial data visualization and its rising popularity in the geoscience community. Theoretical background, functioning modules, and data and file management in PMTec are formulated in this paper. The computational and graphical capabilities of PMTec are demonstrated using published data to provide an overview about its operation procedure and potential applications. The PMTec package and associated tutorials are available for download from the website: http://www.ualberta.ca/~vadim/software.htm . PMTec is a freeware for plate tectonic research and education purposes and allowed to be redistributed among users.