The extent of Greater India with precise and accurate chronological control is a key issue that concerns the spatio-temporal pattern and tectonic models of the India-Asia collision. Here we carried out a detailed magnetostratigraphic and paleomagnetic study on the Upper Cretaceous oceanic red beds (CORBs) (Chuangde Formation) exposed in the Tethyan Himalaya terrane. The high temperature (650‒690°C) magnetic components are isolated from two separated sections at Cailangba and display both normal and reverse polarities, which were used to construct magnetic polarity sequences of the sections that can be subsequently correlated to the geomagnetic polarity time scale (GPTS) to better estimate the age of the rocks. With the aid of previously published biostratigraphy by Chen et al. (2011, Sedimentary Geology), the polarity magnetozones of the Cailangba B section are correlated to chron C32r.2r (74.3–74.0 Ma) and the upper part of chron C33n (79.9–74.3 Ma), and the single normal polarity magnetozone of the Cailangba A section is correlated to the upper part of chron C33n (79.9–74.3 Ma). As a result, the CORBs in the Cailangba A and B sections represent the time interval of 76.2–74.0 Ma by magnetobiostratigraphy. Two independent methods of inclination shallowing correction were tested, which all indicate a bias inclination of ~70%. After inclination shallowing correction, the mean inclination increased to ‒35.0°, giving what we propose to be a high-quality Late Cretaceous paleopole of 40.8°N/256.3°E, A₉₅ =1.8°. Our findings indicate that the Indian passive continental margin was situated at a paleolatitude of 19.4° ± 1.8°S at ~75 Ma. These data suggest that Greater India extended about 715 ± 374 km farther north from the present northern margin of India in the Late Cretaceous, implying a latitudinal width of 3641 ± 308 km for the Neo-Tethys Ocean that still separated the Lhasa terrane of southern part of the Asian plate and the Greater India.

How to cite: Yuan, J., Yang, Z., and Deng, C.: Paleomagnetism of the Upper Cretaceous oceanic red beds in southern Tibet, China: Implications for the extent of Greater India at ~75 Ma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3685, https://doi.org/10.5194/egusphere-egu21-3685, 2021.

Deep-sea sediment sometimes lacks biostratigraphic or radiometric age constraints. Chemical stratigraphy and magnetostratigraphy is useful for dating it. Oxic pelagic clay contains Fe-Mn oxyhydroxides that can retain seawater ¹⁸⁷Os/¹⁸⁸Os values, and its age can be estimated by fitting the isotopic ratios to the seawater ¹⁸⁷Os/¹⁸⁸Os curve. On the other hand, the stability of Fe-Mn oxyhydroxides is sensitive to redox change, and it is not clear whether the original ¹⁸⁷Os/¹⁸⁸Os values are always preserved in sediments. However, due to the lack of independent age constraints, the reliability of ¹⁸⁷Os/¹⁸⁸Os ages of pelagic clay have never been tested. Magnetostratigraphy is often unsuccessful for pelagic clay older than a few Ma, which has been attributed to diagenesis. Here we report multiple polarity reversals in ca. 35 Ma pelagic clay around Minamitorishima Island, which is inconsistent with a ¹⁸⁷Os/¹⁸⁸Os age model. In a ~5 m thick interval, previous studies correlated ¹⁸⁷Os/¹⁸⁸Os data to a brief (<1 million years) isotopic excursion in the late Eocene. Paleomagnetic measurements revealed at least 12 polarity zones in the interval, indicating a >2.9 – 6.9 million years duration. Quartz and feldspars content showed that while the paleomagnetic chronology gives reasonable eolian flux estimates, the ¹⁸⁷Os/¹⁸⁸Os chronology leads unrealistically high values. These results suggest that the low ¹⁸⁷Os/¹⁸⁸Os signal has diffused from an original thin layer to the current ~5 m interval, causing an underestimate of the deposition duration. The preservation of the polarity patterns indicates that a mechanical mixing such as bioturbation cannot be the main process for the diffusion, so diagenetic re-distribution of Fe-Mn oxyhydroxides and associated Os may be responsible. The paleomagnetic chronology presented here also demands reconsiderations of the timing, accumulation rate, and origins of the high content of rare-earth elements and yttrium in pelagic clay around Minamitorishima Island. It is also indicated that old oxic pelagic clay can be a faithful paleomagnetic recorder, and success of magnetostratigraphy depends on sedimentation rate and polarity length rather than diagenesis.

Usui, Y., Yamazaki, T. Earth Planets Space 73, 2 (2021). https://doi.org/10.1186/s40623-020-01338-4

How to cite: Usui, Y. and Yamazaki, T.: Magnetostratigraphic evidence for post-depositional distortion of osmium isotopic records in pelagic clay: implications for mineral flux estimates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3695, https://doi.org/10.5194/egusphere-egu21-3695, 2021.

Loess-paleosol sequences (LPSs) are proven valuable archives for continental paleoclimatic reconstructions. However, studied LPSs worldwide, spanning multiple glacial-interglacial cycles, are seldomly sampled and analyzed at a continuous high resolution. Exceptionally, in a quarry setting near the city of Pleven (Bulgaria), a new LPS, with a thickness of 27 m, was continuously sampled at a 2 cm resolution resulting in 1340 bulk-samples. We present herein first rock magnetic results suggesting that the site archives aeolian deposition and soil formation over the last 850 kyrs.  Room temperature bulk mineral magnetic parameters including magnetic susceptibility, hysteresis loop derived parameters, IRM, and ARM (underway) were acquired on all samples. Variations in mineral magnetic data clearly show the alternation of strongly developed paleosols overlying loess units indicative of interglacial and glacial climate cycles. We created a correlative age model by comparing X_ferri/M_s to inverted LR04 benthic oxygen isotope ratios and adjustments undertaken by the Imbrie & Imbrie ice model. This initial correlative age model leads to an assumed continuous dust accumulation for the last 850 kyrs, from MIS 19 to present. In addition to the regionally widely observed L2-tephra, which is observed outcropping along the Pleven LPS, several other sharp spikes in concentration dependent magnetic characteristics suggest that the sedimentary record had preserved also other tephra layers, clearly identified in the magnetic record due to the accomplished high-resolution sampling design. Additional geochemical and mineralogical data are however necessary for an unequivocal source (age) identification of these events. A tentative scheme of a possible correspondence with well dated tephra layers from sedimentary core at Fucino Basin is established. It implies the occurrence of westerly wind directions during the last 850 kyrs in SE Europe. In summary, the Pleven LPS provides new insights into late-Pleistocene climatic regimes, prevailing wind directions and preservation of tephra layers, essential for further correlative terrestrial-aeolian-coupled age models, regional stratigraphic correlations and paleoclimate reconstructions.

How to cite: Laag, C., Jordanova, D., Lagroix, F., Jordanova, N., and Guyodo, Y.: A new reference loess-paleosol archive spanning the last 850 kyrs near Pleven (Bulgaria) – first results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6381, https://doi.org/10.5194/egusphere-egu21-6381, 2021.

The global polarity time scale (GPTS) is relatively unconstrained for the Paleozoic, particularly the Devonian. Constraining the GPTS and reversal frequency for the Devonian is crucial for the understanding of the behaviour of Earth’s magnetic field. Furthermore, construction of a GPTS for the Paleozoic could provide a valuable tool for age determination in other studies. However, most paleomagnetic data from the Devonian is problematic. The data are difficult to interpret and don’t have a single easy to resolve (partial or full) overprint. Paleointensity studies suggest that the field was much weaker than the field of today, which could have been accompanied by many reversals (a hyperreversing field). In order to improve the geomagnetic polarity time scale in the Devonian, and quantify the number of reversals in this time, we sampled three Devonian sections in Germany, Poland and Canada. These sections show evidence that the rocks were not significantly heated, and they show little evidence for remineralisation. This minimises the chance the rocks were remagnetised after the Devonian. Our data show that even when rocks are well qualified to have reliably recorded the Devonian field, the interpretation is not straightforward. We also discuss problems with the Devonian GPTS as presented in the geologic timescale.

How to cite: van der Boon, A., Biggin, A., Thallner, D., Hounslow, M., Nawrocki, J., Wójcik, K., Paszkowski, M., Königshof, P., de Backer, T., Kabanov, P., Gouwy, S., Vandenberg, R., and Bono, R.: Devonian magnetostratigraphy: new data and old problems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9036, https://doi.org/10.5194/egusphere-egu21-9036, 2021.

Magnetostratigraphy has proven to be a powerful and versatile method as well the first line of defence for dating sediments. When properly anchored to the Geomagnetic Polarity Time Scale (GPTS), chron boundaries provide a basis for numerical dating by correlating the local magnetostratigraphy to the GPTS. A caveat and intrinsic limitation when anchoring magnetic stratigraphy to the GPTS is that we deal with essentially a binary code, a sequence of normal and reverse polarity zones. To overcome such limitation biostratigraphy or (ideally) numerical (absolute) age dating is required. Unfortunately, numerical dating of sediments is typically hampered by the lack of amenable minerals for the application of standard methods such as Ar-Ar, requiring thus the use of less conventional methods. Burial dating is possible using methods such as Electron Spin Resonance (ESR) on optically bleached quartz grains. Similar to luminescence, ESR is a paleodosimetric method that provides the time elapsed since the last exposure of quartz grains to natural sun light. Cave sediments are particularly amenable for paleodosimetric methods, as sediments are preserved in the dark and the ESR signal should survive over the geologic history of the deposits. On the down side, we date the moment when the quartz grain enters the karst system, not its deposition. If the transit time is too long, this might be an issue and we would be significantly overestimating the true burial age. Caves at Atapuerca (N Spain) hold the richest Quaternary paleontological record in Eurasia, including fossils and lithic tools. Sediments in these caves have been traditionally dated via magnetostratigraphy by identifying the Matuyama-Brunhes reversal (0.78 Ma) thus providing the Lower to Middle Pleistocene boundary. Nevertheless, the appearance of older sediments in the caves required the combination of paleomagnetism with methods such as ESR to interpret older intra-Matuyama Subchrons. In the deepest levels of the Gran Dolina cave, close to the floor of the cavity, a number of short intervals of normal polarity have been identified in the fluviatile sediments belonging to TD1 unit, which we interpret in terms of Subchrons using ESR ages of quartz grains. We will discuss both paleomagnetic data and interpret the magnetic polarity stratigraphy in the view of the ESR ages obtained from the Multiple Centre (MC) approach. 

How to cite: Pares, J. M., Duval, M., Campaña, I., Bermúdez de Castro, J. M., and Carbonell, E.: New advances on the magnetic chronostratigraphy of cave sediments in Atapuerca Gran Dolina, Spain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9682, https://doi.org/10.5194/egusphere-egu21-9682, 2021.

The Hominin Sites Paleolakes Drilling project (HSPDP) has collected around 2000 meters of drilled cores in lake sediment in Kenya and Ethiopia. All cores were drilled near important sites in human evolution with as main goal to help us better understand the influence of climate change on our evolutionary past.

An important first step in this research is building an age-model for these cores with magnetostratigraphy being important building block. However, building a magnetostratigraphy for the HSPDP cores is not straightforward. Due to the rotational movement of the coring process the azimuthal orientations of the cores is lost. This hinders the construction of magnetostratigraphy based of correctly orientated paleomagnetic samples. For high latitudes a high quality magnetostratigraphy can be reconstructed on the basis of the inclination of the paleomagnetic direction.

However, at low latitudes near the equator the inclination of the (paleo) magnetic field are near zero. As a result a magnetostratigraphy on the basis of inclination alone cannot be made.

In this presentation we discuss two methods that can be used to build a core based magnetostratigraphy at low latitudes. First, the anisotropy of the magnetic susceptibility (AMS) can be used in certain cases to reorientate the paleomagnetic samples by identifying the bedding of the sediments throughout the core.

Second, the present/recent low temperatures –low coercivity (LT/LC) overprint can be used to reorientate the paleomagnetic directions by orientating these LT/LC components towards the north and recalculate the paleomagnetic directions.

Both methods have been used on the ICDP Hominin Sites Paleolakes Drilling Project (HSPDP) cores taken in Ethiopia and Kenia with varied success. Here we will present data of four HSPDP cores as case study to help illustrate the effectiveness of these two methods for building a magnetostratigraphy for low latitude cores.

How to cite: Sier, M. J., Dupont-Nivet, G., Langereis, C., and Cohen, A. and the HSPDP science team: Magnetostratigraphy from the Hominin Sites Paleolakes Drilling Project (HSPDP) drill cores, low latitudes reorientation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12503, https://doi.org/10.5194/egusphere-egu21-12503, 2021.

Magnetostratigraphy is the key to put disparate chronological pieces together in a consistent chronostratigraphic framework. Provided that a long continuous record of reversals can be obtained from the sedimentary record, a correlation with the GPTS may be established. Magnetostratigraphy provides added value to the chronology as long as it keeps certain independence from external age constraints, such as bioevents calibrated elsewhere or radiochronologic data.

An independent correlation is meant to not be anchored to a given chron on the basis of an external age constrain. Our experience recommends that external age constraints are best taken with flexibility, allowing for the searching of a best fit between the magnetic polarity sequence (in meters) and the GPTS (in million-years). This rationale relies on the fact that the Geological Time Scale is the tool that allows earth-scientist of many varied disciplines to understand and discuss about the dimension of time. But the time scale calibration is a task in continuous refinement. As the accuracy and precision of the dating tools increases, our ability to unravel lag times in geological processes increases too. As more refined data is produced, the calibration of the time scale reveals as an ongoing task rather than a final product.

Here we present the case of the Eocene-Oligocene Transition (EOT) as recorded in alluvial-lacustrine sediments of the eastern Ebro Basin. An earlier work provided a magnetostratigraphic correlation that was in agreement with small-mammals biostratigraphic data. A key constraint to this study was the Santpedor locality, which yielded a characteristic post-Grand Coupure small mammal assemblage, then attributed to the lowest Oligocene.

An extended record of the magnetostratigraphy has challenged the earlier correlation and puts forward an alternate scenario that reveals a misfit with earlier and recent biochronological interpretations of the fossil mammal record. The significance of this discrepancy in terms of heterochrony of biostratigraphic events, the punctuated character of faunal replacement across the EOT, and time lags between the marine and continental realms may need to be addressed.

How to cite: Garcés, M., Beamud, E., López-Blanco, M., Gómez, M., Costa, E., Sáez, A., and Cabrera, L.: The record of the Eocene-Oligocene Transition and the “Grande Coupure”. Magnetostratigraphic constraints from the Ebro Basin revisited., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13264, https://doi.org/10.5194/egusphere-egu21-13264, 2021.

A detailed paleomagnetic study of Tarkhanian sediments of Skelya section was carried out with the goal to obtained magnetostratigraphy data. The Skelya section is located on the Azov sea side of Kerch peninsula, Crimea (45^o42′N, 36^o53′E). The Tarkhanian sediments of Skelya section are represented mainly of clays and have a total thickness of ~ 100 m. According to GTS (2012), the Tarkhanian stage of Miocene is related to the lower part of the Langhian of the General Stratigraphic Scale. Standard paleomagnetic measurements have been carried out to investigate magnetic parameters: natural remenent magnetization, magnetic susceptibility, saturation remanent magnetization, anhysteretic   remanent magnetization varied through out the section. The remanent coercitivity force, determined from backfield demagnetization measurements, range between ~34 and 67 mT.   The composition of the ferromagnetic fraction was examined using temperature dependences of saturation remanent magnetic moment. The thermomagnetic analysis showed that the blocking temperatures are  about 320 ^oC and 410-470 ^oC and  greigite and titanomagnetite  are the main carriers of NRM in the section.  The biplot of  IRM_{-100 mT} / SIRM versus  ARM_40mT /SARM showed that the ratios fall down into the field around the titanomagnetite and greigite areas. The pseudo-single domain  state of titanomagnetite and greigite was determined from their Mrs/Ms and Bcr/Bc ratios by Day-plot. Paleomagnetic studies have shown that the interval of the Kuvinian beds in its upper part is composed of sediments of reversal polarity magnetization. The rocks of the Terskian and Argunian beds are characterized by intervals of normal and reversed polarity magnetization. This work was supported by Russian Science Foundation, project № 19-77-10075.

How to cite: Pilipenko, O. and Rostovtseva, Y.: Paleomagnetism and magnetostratigraphy of Tarkhanian sediments of the Eastern Paratethus (Skelya section, Kerch peninsula, Crimea)  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14134, https://doi.org/10.5194/egusphere-egu21-14134, 2021.

The late Pliocene Akchagylian transgression in the Caspian Basin led to a five-fold increase of the Caspian Sea surface water, extending the basin to the vast areas of Central Asia, Caucasian foreland (Kura Basin) and the Russian Plate. It also changed the regional climatic conditions by making the Pliocene glaciation milder. Later, establishment of hydrological connection between the Caspian Sea and the global ocean known as the “Akchagylian flooding” enabled active fauna migrations transforming the paleoecology of the region. Despite a relatively well constrained palaeoenvironmental history, the Akchagylian still lacks a univocal age model and two major age constraints exist - the “long” (3.6-1.8 Ma) and the “short” Akchagylian (2.7-2.1 Ma). In this study, we resolve the age contradictions by magnetostratigraphic and ⁴⁰Ar/³⁹Ar dating of several sections in the Kura Basin. With our new data, we further revise magnetostratigraphy and ⁴⁰Ar/³⁹Ar constraints in 25 sections across the Kura Basin and Turkmenistan. We propose a new unified age model for the Akchagylian Stage: 1. Akchagylian transgression at 2.95±0.02 Ma; 2. Caspian-Arctic connection (2.75–2.45 Ma); 3. “Desalinated” Akchagylian between 2.45-2.13 Ma; 4. Akchagylian-Apsheronian boundary at 2.13 Ma correlated to the Reunion subchron (C2r.1n). Our data shows, that magnetostratigraphy requires a careful assessment of sedimentation rates and support from other proxies such as sedimentology, biostratigraphy and radioisotopic dating. The new ages constrain a much shorter (2.95–2.1 Ma) Akchagylian than in previously mentioned regional geological time scales (3.6–1.8 Ma) and strongly appeal to reconsider the ages of numerous archaeological and mammalian sites in the south Caspian region.

How to cite: Lazarev, S., Kuiper, K., Oms, O., Bukhsianidze, M., Vasilyan, D., Jorissen, E., and Krijgsman, W.: New and revised magnetostratigraphic age constraints on the Akchagylian (late Pliocene) five-fold expansion of the Caspian Sea., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15419, https://doi.org/10.5194/egusphere-egu21-15419, 2021.

Probably the largest negative δ13С anomaly in Earth history called the “Shuram” excursion (SE) had taken place in the Ediacaran period. Determining the duration of SE is needed to resolve its nature and for the stratigraphic correlation of Ediacaran rocks. The cyclostartigraphic method allows to precisely determine the accumulation rates of ancient deposits (the theoretical error is up to 10,000 years), but the testing of the accuracy of the cyclostratigraphic method usually based on biostratigraphy and geochronology meets difficulties for the Precambrian deposits. The reliability of cyclostartigarphic estimates of the SE duration can be determined by the convergence of cyclostratigraphic results obtained from distant sections on different continents and in sections representing different depositional environments. Recently limitations on the SE duration have been obtained in Australia, California, Oman, and China. Recently limitations on the SE duration have been obtained in Australia, California, Oman, and China. Here we present the first cyclostratigraphic estimates of the SE duration from the Zhuya Group of the Patom basin in South Siberia.

Two sections of the Zhuya Group were studied, both recording the decrease of the δ13С values up to -12 ‰  in the nadir point and then increase till -9 ‰. In both sections, the cyclicity of variations in magnetic susceptibility (MS) was studied. The first section (57 m, Nikolskoe Fm.) represents sediments deposited on the slope of the carbonate platform. Spectral analysis of the MS variations revealed peaks above 95% significance level on the period lengths of 11.5, 1.73, 1.04, 0.67, 0.51 m with ratios 1/6.6 /11/17/22.3 respectively. This cyclicity is interpreted as a reflection of orbitally forced climate changes, where the longest-period variations correspond to short eccentricity cycles (100 ky). Then, the studied interval lasted approximately 500 ky, and the duration of the entire Nikolskaya Fm., corresponding to the lower third of the SE, is about 2.5 My.

The second section belongs to the Torgo Fm. in the Berezovskaya depression, which is the epicontinental part of the Patom Basin. MS variations in the studied 14.2 m interval shows significant peaks at period lengths of 2.3, 0.74, 0.51, 0.38, 0.28, 0.27, 0.25, 0.20 m with ratios 1.00/3.13/4.52/6.10/8.03/8.48/9.19/11.52. In this section, we also interpret the longest-period of the MS variations as a reflection of cycles of short eccentricity (100 ka). Then, the duration of the studied interval is 613 ky.  and the SE duration in the whole 200 meters of the Torgo Fm. is estimated as 8.6 My.

The obtained preliminary results are in good agreement with those from Australia (ca. 8 My), Oman (7.7 +/- 0.2 My), North America (8.2 +/- 1.2 My), and China (9.1 +/- 1 My). Thus, the influence of Milankovitch's orbital cycles on the formation of carbonate deposits of the Late Precambrian seems to be quite convincing, and the cyclostratigraphic estimate of the duration of the SE about 10 Ma is more and more reliable.

Research supported by the Russian Science Foundation (project № 20-77-10066)

How to cite: Rud'ko, D., Rud'ko, S., Shatsillo, A., Pokrovskiy, B., Fedyukin, I., Latysheva, I., and Rimskiy, A.: Duration of the Carbon Isotope Excursion in the Zhuya Group (Patom Basin, South Siberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15845, https://doi.org/10.5194/egusphere-egu21-15845, 2021.

The largest megalake in the record formed in Eurasia during the late Miocene, when the epicontinental Paratethys Sea became tectonically-trapped and disconnected from the global ocean. The Paratethys megalake was characterized by several episodes of hydrological instability and partial desiccation, but the chronology, magnitude and impacts of these paleoenvironmental crises are poorly known. The Panagia section on the Taman Peninsula of Russia is the only place known to host a continuous sedimentary record of the late Miocene hydrological crises of Paratethys. Paleomagnetic measurements allow the development of a polarity pattern that can be used to date the regression events. The Panagia polarity pattern consists of 17 polarity intervals, 9 of normal polarity and 8 of reversed polarity, plus 4 additional short-term polarity fluctuations, that are inferred to correspond to the 11-7.5 Ma interval. We identified four major regressions that correlate with aridification events, vegetation changes and faunal turnovers in large parts of Europe. Our paleogeographic reconstructions reveal that Paratethys was profoundly transformed during the regression episodes, losing ~1/3 of the water volume and ~70% of its surface during the most extreme events. The remaining water was stored in a central salt-lake and peripheral desalinated basins while vast regions (up to 1.75 million km2) became emerged land, suitable for the development of forest-steppe landscapes. The dry episodes of the megalake match with climate, food-web and landscape changes throughout Eurasia but the exact triggers and mechanisms remain to be resolved.

How to cite: Krijgsman, W., Palcu, D., Patina, I., Șandric, I., Lazarev, S., Vasiliev, I., and Stoica, M.: Magnetostratigraphic dating of late Miocene megalake regressions in Central Eurasia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15933, https://doi.org/10.5194/egusphere-egu21-15933, 2021.