An exceptionally weak Devonian geomagnetic field recorded by the Viluy Traps, Siberia

The detection of anomalous time averaged geomagnetic behaviour is crucial for understanding past magnetospheric shielding and inferring deep Earth evolution. Links have been suggested between geomagnetic field variation over timescales of tens to hundreds of millions of years and processes near the core–mantle boundary (CMB); however, this becomes difficult to establish prior to the Permo-Carboniferous Reversed Superchron (PCRS; 267–319 Ma) due to a lack of reliable data. To improve the record prior to the PCRS, we present multi-method produced paleointensity results from nines dykes and lava flows from the Viluy Traps, Siberia, emplaced during the Upper Devonian between 376.7 ± 1.7 Ma and 364.4 ± 1.7 Ma. These sites have previously been published as part of two paleodirectional studies, one of which produced the accepted 360 Ma pole for Siberia (Q factor 6). All of the sites produced very weak field values ranging from 4.3–14.9 Z A m2, in close agreement with other recent results from Mid-Lower Devonian Siberian samples. QPI criteria have been used to illustrate the reliability of these new, low paleointensities, confirming the period of weak field suggested by other recent Siberian work, and the period of implied increased incidence of solar wind radiation, extended into the Upper Devonian. Along with evidence for moderate-high reversal frequencies and a potentially significant multipolar component during the Devonian, these weak field values also suggest a significantly different pattern of heat flow across the CMB relative to more recent times

Paleomagnetism of the Precambrian Eastern Sayan rocks: Implications for the Ediacaran-Early Cambrian paleogeography of the Tuva-Mongolian composite terrane

The Tuva-Mongolian Precambrian composite terrane is located within the complex Central Asian fold belt, which separates the Siberian portion of the Eurasian continent from other incorporated continental blocks. This terrane is one of the key elements that must be accurately modeled to reconstruct the Neoproterozoic-Paleozoic tectonic history of Eurasia. The reconstruction of the terrane's paleoposition after breaking up of supercontinent Rodinia relatively to other continental blocks has fundamental importance, as it is one of the missing blocks in most present day reconstructions.We present a paleomagnetic study of rocks from several formations of the Eastern Sayan region south of Siberian platform (representative location Λ=52.0°N, Φ=100.5°E), within the Tuva-Mongolian composite terrane. Sections of siltstone, fine grained sandstone and associated sills were sampled at several localities in this complex region. These units have been dated as Precambrian to Early Cambrian in age. The resulting collection was taken from three formations, representing a total of 33 sites, collected from this previously unsampled and remote region. Generally 6 to 11 samples per site were collected. Stepwise thermal demagnetization was completed using between 10 and 18 heating steps to up to temperatures of 680°C. Principal component analysis of the stepwise thermal demagnetization data was successful in isolating two characteristic remanent magnetizations. The lower unblocking temperature component, component A, fails the fold test, is always of downward directed magnetic inclination, and may correspond to the present day Earth's magnetic field. The higher unblocking temperature magnetic component (B), was observed in the Dunzhugur Formation (BDF, N=8 sites) and the Bokson Formation (BBF, N=11 sites from 5 localities). The B component differs significantly from component A, and is recorded by sites of downward and upward directed magnetic inclinations in the Bokson Formation. Component BDF is most likely a result of the regional remagnetization of the sediments and sills during multiple tectonic events in the area. Component BBF yields a positive fold test and one site has reversed polarity direction. A virtual geomagnetic pole calculated from component BBF, after rotating along a small circle, is coincident with other Ediacaran to Early Cambrian aged poles reported from nearby Mongolia and Siberia. This observation supports the earlier interpretation of Kravchinsky et al. (2001) postulating an adjacent position of the Tuva-Mongolian composite terrane and the Siberian continent in Ediacaran-Early Cambrian times. © 2010 Elsevier B.V

Evolution of the Mongol-Okhotsk ocean as constrained by new palaeomagnetic data from the Mongol-Okhotsk suture zone, Siberia

This paper presents new data from palaeomagnetic investigations on the Upper Palaeozoic and Mesozoic geological units from the Siberian platform and the Mongol-Okhotsk suture zone. Within the southern portion of the Mongol-Okhotsk suture we collected palaeomagnetic samples from the Late Permian Belektuy formation (Borzya region; 50.7°N, 116.9°E) and the Middle-Late Jurassic Shadaron formation (Unda-Daya; 51.5°N, 117.5°E). We sampled the Late Permian Alentuy formation (Khilok region; 50.8°N, 107.2°E), the Early to Middle Jurassic Irkutsk sedimentary basin (ISB; 52.0°N, 104.0°E), the Late Jurassic Badin formation (Mogzon region; 51.8°N, 112.0°E), and the Early Cretaceous Gusinoozesk formation (Gusinoc Lake region; 51.2°N, 106.5°E) additionally in the northern region of the Mongol-Okhotsk suture. Apart from the results of the ISB and Gusinoozersk formations, which show very large ellipses of confidence and might be the present-day geomagnetic field overprint, our results allow us to constrain the evolution of the Mongol-Okhotsk Ocean palaeomagnetically from the Late Permian to the Middle-Late Jurassic. They confirm that this large Permian ocean closed during the Jurassic, ending up in the late Jurassic or the beginning of the Cretaceous in the eastern end of the suture zone, as suspected on geological grounds. However, although geological data suggest a Middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data show evidence of a still large palaeolatitude difference between the Amuria and Siberia blocks. This is interpreted as a result of the quite fast closure of the ocean after the Middle Jurassic. Finally, our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes, which we interpret as probably arising both from collision processes and from a left-lateral shear movement along the suture zone, due to the eastward extrusion of Mongolia under the effect of the collision of India into Asia

Magneto- and cyclostratigraphy in the red clay sequence:New age model and paleoclimatic implicationfor the eastern Chinese Loess Plateau

The Chinese Loess Plateau red clay sequences display a continuous alternation of sedimentary cycles that represent recurrent climatic fluctuations from 2.58 Ma to the Miocene. Deciphering such a record can provide us with vital information on global and Asian climatic variations. Lack of fossils and failure of absolute dating methods made magnetostratigraphy a leading method to build age models for the red clay sequences. Here we test the magnetostratigraphic age model against cyclostratigraphy. For this purpose we investigate the climate cyclicity recorded in magnetic susceptibility and sedimentary grain size in a red clay section previously dated 11 Myr old with magnetostratigraphy alone. Magnetostratigraphy dating based on only visual correlation could potentially lead to erroneous age model. In this study the correlation is executed through the iteration procedure until it is supported by cyclostratigraphy; i.e., Milankovitch cycles are resolved in the best possible manner. Our new age model provides an age of 5.2 Ma for the Shilou profile. Based on the new age model, wavelet analysis reveals the well-preserved 400 kyr and possible 100 kyr eccentricity cycles on the eastern Chinese Loess Plateau. Further, paleomonsoon evolution during 2.58–5.2 Ma is reconstructed and divided into three intervals (2.58–3.6 Ma, 3.6–4.5 Ma, and 4.5–5.2 Ma). The upper part, the youngest stage, is characterized by a relatively intensified summer monsoon, the middle stage reflects an intensification of the winter monsoon and aridification in Asia, and the earliest stage indicates that summer and winter monsoon cycles may have rapidly altered. The use of cyclostratigraphy along with magnetostratigraphy gives us an effective method of dating red clay sequences, and our results imply that many presently published age models for the red clay deposits should be perhaps re-evaluated

Link between global cooling and mammalian transformation across the Eocene-Oligocene boundary in the continental interior of Asia

Evidence in the world&#39;s ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene-Oligocene boundary at 33.9 Ma. The remarkable cooling period in the ocean, called the Eocene-Oligocene transition (EOT), is correlated with pronounced mammalian faunal replacement as shown in terrestrial fossil records. For the first time within Asia, a section is magnetostratigraphically dated that also produces mammalian fossils that span the Late Eocene-Early Oligocene transition. Three fossil assemblages revealed through the EOT (34.8, 33.7, and 30.4 Ma) demonstrate that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and that changes in mammalian communities were accelerated by aridification in central Asia. Three fossil assemblages (34.8, 33.7, and 30.4 Ma) within the north Junggar Basin (Burqin section) tied to this magnetostratigraphically dated section, reveal that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and that changes in mammalian communities were accelerated by aridification in central Asia. The biotic reorganization events described in the Burqin section are comparable to the Grande Coupure in Europe and the Mongolian Remodeling of mammalian communities. That is, the faunal transition was nearly simultaneous all over the world and mirrored global climatic changes with regional factors playing only a secondary role.</p