Magnetostratigraphic Investigations Research Articles

Late Cambrian geomagnetic instability after the onset of inner core nucleation

The Ediacaran Period marks a pivotal time in geodynamo evolution when the geomagnetic field is thought to approach the weak state where kinetic energy exceeds magnetic energy, as manifested by an extremely high frequency of polarity reversals, high secular variation, and an ultralow dipole field strength. However, how the geodynamo transitioned from this state into one with more stable field behavior is unknown. Here, we address this issue through a high-resolution magnetostratigraphic investigation of the ~494.5 million-year-old Jiangshanian Global Standard Stratotype and Point (GSSP) section in South China. Our paleomagnetic results document zones with rapid reversals, stable polarity and a ~80 thousand-year-long interval without a geocentric axial dipole field. From these changes, we suggest that for most of the Cambrian, the solid inner core had not yet grown to a size sufficiently large to stabilize the geodynamo. This unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander, supporting instead the rotational stability of the solid Earth during the great radiation of life in the Cambrian.

Revised magnetostratigraphy of the Linxia Basin in the northeast Tibetan Plateau, constrained by micromammalian fossils

The Linxia Basin along the northeastern margin of the Tibetan Plateau has drawn enormous attention due to its rich mammalian fossils and great potential for exploring regional tectonics, paleoenvironmental history, and bio-evolution. However, the geochronology of the sedimentary sequences is in great dispute owing to uncertain lithostratigraphic correlation and poorly calibrated magnetostratigraphy. From 2017 to 2021, we carried out lithostratigraphic correlation of the strata from the southern part of the basin, which is rich in fossils, to the Maogou section in the central basin, and established a solid intra-basinal lithostratigraphic framework with accurate fossil locations. Then, high-resolution magnetostratigraphic investigations were performed on the Maogou section, which comprises, from oldest to youngest, the Jiaozigou, Shangzhuang, Dongxiang, Liushu/Hujialiang, and “Liushu” formations. Magnetostratigraphic results show that the Maogou sedimentary sequence (from Jiaozigou to Liushu/Hujialiang formations) spans from chron C10r to chron C5r.2n, yielding an age of 29–11.6 Ma, with constraints from four mammalian assemblage levels. The revised magnetostratigraphy indicates that the Jiaozigou Formation corresponds to chrons C6Cn.3n–C10r (∼23–29 Ma), with Paraceratherium fauna from the lower Jiaozigou Formation being determined as ∼29 Ma, and that the Hujialiang Formation, containing Platybelodon fauna, corresponds to chrons C5r.2n–C5Ar.2r (∼11.6–12.8 Ma). The revised magnetostratigraphy of the Maogou section has significantly refined the chronology of the Linxia Basin sedimentary sequences and its associated mammalian evolution, tectonism, and climate change in Central Asia.

Intensified Northern Hemisphere Glaciation Facilitates Continuous Accumulation of Late Pliocene Loess on the Western Margin of the Pamir

AbstractThick eolian loess sequences in arid Central Asia (CA) contain a wealth of information on the intensity variations of midlatitude westerlies and the aridification history of the nearby deserts. In this study, detailed lithologic and magnetostratigraphic investigations suggest that loess deposits on the western margin of the Pamir Plateau began to accumulate at around 2.7 Ma, representing the most complete and oldest loess‐paleosol sequence in western CA. The widely distributed continuous loess deposition in the region suggests a rapid desiccation of CA during the late Pliocene. A synthesis of the initial timing and spatial distribution of late Cenozoic eolian deposits on the Eurasian continent further demonstrates that the dramatic increase in global ice volume exerted a dominant role in the expansion of eolian loess deposits in CA, China, and Europe, and middle Pleistocene expansion is a crucial precondition for the establishment of modern distribution patterns of the Eurasian loess belt.

High-resolution magnetostratigraphic records of the pliocene sedimentary successions in Yengisar section, NW China, and its tectonic implications

The Neogene strata upward-coarsening sandstone and conglomerate sequences at the periphery of the Northeastern Pamir record the intense uplift of the paleosurface of the building mountains. To further improve our knowledge of source-sink processes, a detailed magnetostratigraphic investigation was carried out along the Yengisar section, which is located at the southwestern margin of the Tarim Basin. The new high-resolution magnetostratigraphic data revealed that the Artux Formation was deposited from 4.9 Ma to 1.9 Ma with three sedimentation rates changes. The variations in sedimentation rate may be due to the pulsating exhumation of the Western Kunlun Mountain, caused by the northward motion of the Pamir salient. By integrating the evidences from the seismic reflection profile and other magnetostratigraphic investigations in this region, the basal age of the Xiyu Formation and the early stages of growth strata deposition were estimated at ∼1.9 and 1.45 Ma, respectively. Based on the data, we propose that the progradation of the Xiyu Formation and the migration of the deformation front are the two independent responses of the sink region to the uplift events of the source region.

Magnetostratigraphy of the fluvio-lacustrine sequence of core DY-1 in the Datong Basin and its implications for the evolution of the Shanxi Rift System in northern China

The Shanxi Rift System is an intracontinental rift system located between the North China Plain and Ordos Plateau, which was infilled with thousands of meters of Cenozoic fluvio-lacustrine deposits. A comprehensive chronological framework will help better understand the formation and evolutionary process of the rift system. Here, we present a new magnetochronology for the Datong Basin sedimentary sequence in the northern Shanxi Rift System based on high-resolution magnetostratigraphic investigations of the DY-1 core. Correlation of the recognized magnetic polarity sequence of the DY-1 core with the Astronomically Tuned Neogene Time Scale shows that the fluvio-lacustrine sequence in the Datong Basin spans from Chron C3Bn to Chron C1n. The age of the Datong Basin sedimentary sequence can thus be paleomagnetically constrained to an interval from the late Late Miocene to Middle Pleistocene. The basal age of the sediments is ca. 7.0 Ma, which indicates that the initial extension of the northern segment of the Shanxi Rift System was no later than 7.0 Ma. The Datong Basin in the northern Shanxi Rift System has experienced a three-stage evolution: During the initial stage (7.0–4.2 Ma), fluvial environments are dominant. Then during 4.2–1.8 Ma, lacustrine environments are developed due to dip-slip fault activities, as part of the Nihewan paleolake. After 1.8 Ma, the lake gradually shrank. The northeastward progressive growth and expansion of the Tibetan Plateau may have driven the evolution of the Shanxi Rift System.

Magnetostratigraphy and AMS study of a late Cretaceous-Paleogene succession in the eastern Xining basin: constraint on K-Pg boundary tectonic event in NE Tibet Plateau

Cretaceous-Cenozoic basins developed in the NE Tibetan Plateau contain key archives to unravel the growth history of the plateau in response to the India-Eurasia collision. Here we present magnetostratigraphic results of a Late Cretaceous to Paleogene succession of the Zhongba section outcropping at the southern margin of the eastern Xining basin. This succession consists of three lithological units punctuated by two stratigraphic unconformities, which best recorded the deformation history of this foreland basin. Detailed magnetostratigraphic investigation show that the lower terrestrial sedimentary rock unit, the Minhe Group, was deposited in latest Cretaceous in the time span of ~74.5-69.2Ma; the middle unit was deposited in Paleogene in the time span of ~49.3-22Ma; and the upper conglomeratic unit, not dated, possibly was deposited in early Miocene. Accordingly, the K-Pg unconformity, widely observed in the foreland basins of NE Tibet, represents a sedimentary hiatus of ~19.9Ma duration from ~69.2 Ma to ~49.3 Ma, which possibly recorded the far-field response to the tectonic transition from Neo-Tethys oceanic plate subduction to the India-Eurasia collision in southern Tibet. AMS results show a clockwise rotation of the shortening direction from NEN-SWS in Latest-Cretaceous to NE-SW in Paleogene, which support the view that the compressional environment may dominate the NE Tibet in Early-Cenozoic period.

Magnetostratigraphic study of a Late Cretaceous–Paleogene succession in the eastern Xining basin, NE Tibet: Constraint on the timing of major tectonic events in response to the India-Eurasia collision

Abstract Cretaceous-Cenozoic basins developed in the NE Tibetan Plateau contain key archives to unravel the growth history of the plateau in response to the India-Eurasia collision. Here we present magnetostratigraphic results of a Late Cretaceous to Paleogene succession of the Zhongba section outcropping at the southern margin of the eastern Xining basin. This succession consists of three lithological units punctuated by two stratigraphic unconformities, which best recorded the deformation history of this foreland basin. Detailed magnetostratigraphic investigation show that the lower terrestrial sedimentary rock unit, the Minhe Group, was deposited in latest Cretaceous in the time span of ca. 74.5–69.2 Ma; the middle unit was deposited in Paleogene in the time span of ca. 49.3–22 Ma; and the upper conglomeratic unit, not dated, possibly was deposited in early Miocene. Accordingly, the Cretaceous–Paleogene unconformity, widely observed in the foreland basins of NE Tibet, represents a sedimentary hiatus duration of ∼19.9 m.y., from ca. 69.2 Ma to ca. 49.3 Ma, which possibly recorded the far-field response to the tectonic transition from Neo-Tethys oceanic plate subduction to the India-Eurasia collision in southern Tibet. Changes in provenance, sedimentary accumulation rate, and mean susceptibility value at ca. 33–30 Ma, and the total prolate anisotropy of magnetic susceptibility (AMS) ellipsoids and provenance shifting since ca. 23–19 Ma, point to the pulsed growth of West Qinling, and rapid uplift of Laji Shan, respectively, indicating an enhanced effect of the India-Eurasia collision in Oligocene and early Miocene. AMS results show a clockwise rotation of the shortening direction from NEN-SWS in latest Cretaceous to NE-SW in Paleogene.

Towards a high-resolution chronostratigraphy and geochronology for the Pannonian Stage: Significance of the Paks cores (Central Pannonian Basin)

A new stratigraphic standard for the open lacustrine to deltaic Pannonian Stage is emerging from the combined sedimentological, lithostratigraphical, sequence stratigraphical, biostratigraphical, seismic stratigraphical, geochronological, and magnetostratigraphical investigations of 6 long drill cores. These were drilled by Paks II Nuclear Power Plant Plc. as a preparatory step for the construction of a new power plant near the city of Paks, Central Pannonian Basin, between 2015 and 2016. The boreholes are in a distance of 8-12 km from each other, and five of them fully penetrated the local Pannonian sequence in a thickness of 390 to 662 m. Each core includes offshore clay marl deposited far from sediment entry points (Endrőd Fm), heterolithic, sandy siltstones of a <200 m high shelf-margin slope (Algyő Fm), and several stacked deltaic deposits from prodelta silts to sandy mouth bars, heterolithics, lignite and sandy channel-fills of the delta plain (Újfalu Fm). Magnetostratigraphic investigations from two cores and authigenic 10Be/9Be dating from two others were combined by means of seismic correlation between the boreholes, and thus they provide a solid geochronological and chronostratigraphic basis for the interpretation of the sedimentologial and paleontological records of the cores. The continuous representation of the earliest Pannonian (11.6–9.1 Ma) in the cores needs further investigation, as neither magnetostratigraphy nor authigenic 10Be/9Be dating gave reliable age data from the basal, condensed calcareous marls. The 9.1 to 6.5 Ma interval, however, is represented in the cores by various lithologies and abundant and sometimes excellently preserved fossils. In the deltaic succession, 8 sedimentary sequences were correlated between the cores; as their duration is not more than 400 kyr each, they can be regarded as 4th-order sequences. The paleontological record of the cores shows a very good agreement with the formerly established biochronostratigraphical system. The cores provide an insight into the evolution of the sedimentary environment and the biota of Lake Pannon between 9.1 and 6.5 Ma with a so far unprecedented temporal and spatial resolution.

Magnetostratigraphic dating of the hominin occupation of Bailong Cave, central China

Intermontane basins in the southern piedmont of the Qinling Mountains are important sources of information on hominin occupation and settlement, and provide an excellent opportunity to study early human evolution and behavioral adaptation. Here, we present the results of a detailed magnetostratigraphic investigation of the sedimentary sequence of hominin-bearing Bailong Cave in Yunxi Basin, central China. Correlation to the geomagnetic polarity time scale was achieved using previously published biostratigraphy, 26Al/10Be burial dating, and coupled electron spin resonance (ESR) and U-series dating. The Bailong Cave hominin-bearing layer is dated to the early Brunhes Chron, close to the Matuyama-Brunhes geomagnetic reversal at 0.78 Ma. Our findings, coupled with other records, indicate the flourishing of early humans in mainland East Asia during the Mid-Pleistocene climate transition (MPT). This suggests that early humans were adapted to diverse and variable environments over a broad latitudinal range during the MPT, from temperate northern China to subtropical southern China.

Magnetostratigraphic dating of the Shixia red sediments and implications for formation of Nihewan paleo-lake, North China

The sedimentary sequence in Nihewan Basin, northern China, is rich in mammalian fossils and Paleolithic sites. Extensive magnetostratigraphic investigations have been carried out in the Nihewan sediments, but precise age control on the earliest Nihewan red sediments, which have a fluvio-lacustrine origin in the upper part and an eolian origin in the lower part, has remained unavailable. The formation process of Nihewan paleo-lake, therefore, remains unclear. Here we contribute to understanding the age of the early Nihewan reworked and eolian red clays by presenting detailed magnetostratigraphic and rock magnetic results from the Shixia section coupled with geochemical and petrographic analyses. Magnetostratigraphic correlation to the geomagnetic polarity timescale indicates that the Shixia sedimentary sequence recorded part of the Gilbert Chron with the onset of deposition of the Nihewan Formation occurring prior to the Gilbert-Gauss geomagnetic reversal at 3.6 Ma. With tectonic development of a graben basin, early red clay was deposited first in Nihewan Basin during the Early Pliocene before giving way to lacustrine environments in Nihewan paleo-lake. Geochemical and petrographic results indicate a wind-blown origin for the lower red clay deposits, which have been reworked and mixed with conglomerates produced by local tectonic activity to produce the basal Nihewan sediments. Later formation of carbonate nodules has given rise to the sediments that are now exposed in Nihewan Basin.

The middle Burdigalian in the North Alpine Foreland Basin (Bavaria, SE Germany) – a lithostratigraphic, biostratigraphic and magnetostratigraphic re-evaluation

For Oligocene and Miocene sediments of the Central Paratethys realm, regional chronostratigraphic stages have been defined on the basis of characteristic faunal assemblages, often containing abundant endemic elements. Although considerable progress has been made in correlating Paratethyan stages with the Global Time Scale (GTS), the task remains incomplete, especially for the Early Miocene period. The present study focuses on Lower Miocene deposits of the Central Paratethys in southeastern Germany, i.e. the Upper Marine Molasse (OMM) and Upper Brackish Molasse (OBM) of the North Alpine Foreland Basin (NAFB, Molasse Basin). The sediments concerned are assigned to the regional Ottnangian stage of the Central Paratethys, which corresponds to the middle Burdigalian in the GTS. We present a formal lithostratigraphic definition of the Untersimbach and Neuhofen Formations (both OMM), combined with litho-, bio- and magnetostratigraphic investigations, which together provide an improved chronology for the middle Burdigalian interval in the NAFB. The base of the Ottnangian stage is shown to correlate with polarity chron C5En, and we consider an absolute age of around 18.2 Ma to be most plausible. As the OMM in the southeast German part of the NAFB represents a single transgressive-regressive sequence, our new data suggest that the Ottnangian transgression began during chron C5En (similar to 18.2 Ma), whereas the onset of the regressive phase lies within chron C5Dr.2r (similar to 18 Ma). The uppermost marine deposits (top OMM/Glaukonitsande & Blattermergel) and the lowermost brackish sediments (OBM/Oncophora Formation) are correlated with polarity chron C5Dn (similar to 17.4 Ma). Moreover, our results clearly demonstrate that the lower Oncophora Formation (normal polarity) is not time-equivalent to the OBM/Kirchberg Formation in the southwest German sector of the NAFB (largely reverse polarity) as has hitherto been assumed. The lower Oncophora Formation is most probably contemporaneous with the lowermost OBM sediments (Grimmelfingen Formation) in the southwest German Molasse Basin.

A 20–15 ka high-resolution paleomagnetic secular variation record from Black Sea sediments – no evidence for the ‘Hilina Pali excursion’?

A comprehensive magnetostratigraphic investigation on sixteen sediment cores from the southeastern Black Sea yielded a very detailed high-quality paleosecular variation (PSV) record spanning from 20 to 15 ka. The age models are based on radiocarbon dating, stratigraphic correlation, and tephrochronology. Further age constraints were obtained by correlating four meltwater events, described from the western Black Sea, ranging in age from about 17 to 15 ka, with maxima in K/Ti ratios, obtained from X-ray fluorescence (XRF) scanning, and minima in S-ratios, reflecting increased hematite content, in the studied cores. Since the sedimentation rates in the investigated time window are up to 50 cm ka−1, the obtained PSVs records enabled a stacking using 50-yr bins. A directional anomaly at 18.5 ka, associated with pronounced swings in inclination and declination, as well as a low in relative paleointensity (rPI), is probably contemporaneous with the Hilina Pali excursion, originally reported from Hawaiian lava flows. However, virtual geomagnetic poles (VGPs) calculated from Black Sea sediments are not located at latitudes lower than 60°N, which denotes normal, though pronounced secular variations. During the postulated Hilina Pali excursion, the VGPs calculated from Black Sea data migrated clockwise only along the coasts of the Arctic Ocean from NE Canada (20.0 ka), via Alaska (18.6 ka) and NE Siberia (18.0 ka) to Svalbard (17.0 ka), then looping clockwise through the Eastern Arctic Ocean.

Magnetic stratigraphy of the Danube loess: A composite Titel-Stari Slankamen loess section over the last one million years in Vojvodina, Serbia

In Europe, the most complete loess-paleosol sequences over the past million years are preserved in the Serbian part of the Danube River drainage basin. The similarity in stratigraphy and climatic cycles between the Serbian and the Chinese loess sequences suggests that the loess deposits in these two regions could play an important role in the study of intercontinental climatic correlations and linkage. However, previous magnetostratigraphic studies of the Serbian loess, mainly conducted using the alternating field (AF) demagnetization approach, show significant differences with the Chinese loess, especially the stratigraphic positioning of the Matuyama-Brunhes boundary (MBB). Here, we conduct a detailed magnetostratigraphic investigation of a composite Titel-Stari Slankamen loess section in the Vojvodina (northern Serbia), using thermal and hybrid demagnetization methods, aiming to establish new chronological constraints for the Danube Basin loess. Based on the analyses of the mineral magnetic properties, anisotropy of magnetic susceptibility, progressive thermal and hybrid demagnetization, the MBB was determined within S8, broadly consistent with the MBB position seen in the Chinese loess. Present evidence supports the hypothesis that the overall normal polarity in the upper part of L9, with a thickness of 2.1 m, was possibly caused by a combination of the remagnetization of remanence-carrying coarse-grained magnetite during the Brunhes normal chron and bioturbation during the forming of the overlying paleosol S8. Thus, our magnetostratigraphic results provide solid chronological evidence for the coherence of pedostratigraphy and climatostratigraphy for the loess deposits in the Danube Basin and China. The synchronous occurrence of the strongest developed paleosol S5 (corresponding to MISs 15–13) and the least weathered thick loess unit L9 (corresponding to MISs 24–22) in the two regions confirms that extreme climatic events during these two intervals are possibly common features of the climate in the Northern Hemisphere at those times.

Plio-Pleistocene evolution of Bohai Basin (East Asia): demise of Bohai Paleolake and transition to marine environment.

The Bohai Basin was transformed to an inner shelf sea hundreds of thousands years ago. This youngest land-sea transition participated in the significant modification of the distribution of fresh water, sediment fluxes and climate in East Asia, and played an important role in the origin of the Asian marginal seas. Here we present the results of a magnetostratigraphic investigation and propose a conceptual model for the land-sea transition. Our findings indicate that the transition probably started several million years ago, from a fluvial system during the late Miocene and early Pliocene, to a lacustrine environment between the late Pliocene and Middle Pleistocene, and finally to a marine system in the late Pleistocene. Comparison of our results with previous research suggests that the Bohai Paleolake was initiated from the late Pliocene, was fully developed prior to ~1.0 Ma, and terminated around the late Middle Pleistocene. The Miaodao Islands formed the eastern “barrier” of the basin and since the Pliocene or earlier they played a significant role in blocking the lake water and sediments. They deformed from ~1.0 Ma, subsided significantly at ~0.3 Ma and completely by ~0.1 Ma, resulting in the maturation of the basin as an inner shelf sea.

Neogene magnetostratigraphy and rock magnetic study of the Kashi Depression, NW China: Implications to neotectonics in the SW Tianshan Mountains

AbstractThe southwest Tianshan Mountains of China are bordered by the Tarim foreland and comprise an actively deforming segment of the India‐Asia collisional system. We report a detailed magnetostratigraphic study of the Dashankou section in the Kashi Depression of the Tarim Basin to improve the understanding of the history of sedimentation, denudation, and mountain building in this region. The preferred correlation of the succession with the geomagnetic polarity timescale defines a depositional history between 12.4 and 3.0 Ma with a substantial increase in sedimentation rates identified at ~6.7 Ma corresponding to a pulse of rapid uplift in the southwest Tianshan Mountains. Although climatic changes may have modulated the record during Neogene times, they do not appear to have had an important influence on sediment accumulation rates between 7.0 and 2.6 Ma. Magnetic fabrics identify the influence of a regional stress field imparted by ongoing India‐Asia collision in the lower part of the succession contrasting with predominantly sedimentary fabrics in the higher part of the succession. A major clastic influx with a maximum age estimate of ~3.6 Ma comprises the Xiyu conglomerates, and integration with other magnetostratigraphic investigations around the Tianshan demonstrates unambiguously that depositional onset of this coarse clastic episode is diachronous. Hence, the Xiyu Formation cannot be considered as a chronostratigraphic marker related to any specific tectonic or climatic event.

Evolution of the Yellow Sea Warm Current and the Yellow Sea Cold Water Mass since the Middle Pleistocene

The Yellow Sea Warm Current (YSWC) and the Yellow Sea Cold Water Mass (YSCWM) are the most significant hydrological characteristics of the Yellow Sea as they strongly influence the matter transportation, sedimentation, and the benthic and pelagic environments of the region. Previous studies focused mostly on formation and evolution of the YSWC and the YSCWM on shorter time scales (e.g., since the Holocene) and studies on longer time scales are rather sparse. This study conducted a high-resolution magnetostratigraphic investigation on core DLC70-3 (with a water depth of 71.2m) in the northern part of the south Yellow Sea. Results show that the Brunhes/Matuyama polarity reversal boundary is located at a depth of 59.08m. Constrained by both magnetostratigraphy and 14C dating, three sedimentation episodes revealed by the benthic foraminifera content correspond to high sea level periods of marine isotope stage (MIS) 5–MIS9, MIS11–MIS17, and MIS19–MIS21, respectively. Cold-water benthic foraminifera species, total organic carbon, total sulfur, molybdenum element, and long-chain unsaturated alkenones index of the studied sediments show that both the YSWC and the YSCWM have evolved since the middle Pleistocene mainly due to combined effects of fluctuations of sea level and local tectonic subsidence. More specifically, both the YSWC and the YSCWM were intensified during high sea level periods of MIS5e–MIS9, MIS19 and MIS21. In addition, occurrence of the YSWC and the YSCWM are not always coeval as in the MIS5a–5d period, which is characterized by relatively stronger YSCWM but absence of the YSWC. These studies significantly improve our understanding of the evolution mechanism of both the YSWC and the YSCWM in this region.

Oligocene‐Miocene magnetostratigraphy and magnetic anisotropy of the Baxbulak section from the Pamir‐Tian Shan convergence zone

AbstractAs the northernmost part of the Indo‐Eurasian collision belt, the Pamir‐Tian Shan convergence zone (PTCZ) is a strategic location for understanding intracontinental deformation. Here we present a magnetostratigraphic investigation of a continuous section from the Baxbulak region, to better constrain regional tectonic history. Rock magnetic analyses indicate that hematite and magnetite are the main carriers of characteristic remanent magnetization. The resulting polarity sequence allows a distinct correlation to the geomagnetic polarity time scale, showing that the section spans the interval of 29.1–20.7 Ma. Rock magnetic results further suggest that paramagnetic and antiferromagnetic minerals dominantly contribute to anisotropy of magnetic susceptibility (AMS) of the sequence. Thus, the AMS would indicate the preferred orientations of the mineral grains that are sensitive to tectonic strain. At around 26 Ma, the grouped principal minimum perpendicular to the bedding diverts to a girdle distribution in a N‐S direction, demonstrating the overprint of tectonic fabric to previous weakly deformed sedimentary fabric. This would be interpreted as a marked increase in tectonic strain, consistent with various evidence from the Pamir and the neighboring basin that show the Pamir began to migrate northward. Moreover, the coincident changes in distribution of AMS principal axes, in both direction and magnitude, are comparable to the regional counterclockwise rotations observed from paleomagnetic data, likely related to orogenesis.

Paleomagnetic dating of tectonically influenced Plio-Quaternary fan-system deposits from the Apennines (Italy)

<p>The Roveto Valley is a narrow, elongated, NW-trending depression filled with continental Plio-Quaternary deposits that outcrop at different topographic elevations. A morpho-lithostratigraphic succession of the continental deposits has been defined in order to reconstruct the geological Quaternary evolution of the area. These deposits do not contain materials suitable for biostratigraphic dating; therefore, in order to determine their chronology, three different units were sampled for magnetostratigraphic investigations. Paleomagnetic results demonstrated that standard demagnetization techniques are effective in removing secondary remanence components and in isolating the characteristic remanent magnetization, allowing us to determine, for each cycle, whether it was deposited before or after the Brunhes-Matuyama geomagnetic reversal at 781 ka. Preliminary rock magnetic analyses indicated that magnetite is the main magnetic carrier and that hematite, which gives the pink colour to the matrix, is in the superparamagnetic grain-size range and thus does not retain any paleomagnetic remanence.</p>

Magnetostratigraphy, biostratigraphy, and chemostratigraphy of the Pignola-Abriola section: New constraints for the Norian-Rhaetian boundary

A detailed magnetostratigraphic investigation of the Pignola-Abriola section of Norian to Rhaetian age permits the identification of 22 magnetic polarity reversals grouped in 10 magnetozones. We correlate the magnetostratigraphy of the Pignola-Abriola section with the Newark astrochronological polarity time scale (APTS). In total, 19 correlation options were tested, and only one (option 7) yielded a statistically significant correlation that was consistent with the available information on the stratigraphic age of the Newark APTS. After some adjustments to minimize erratic variations in sediment accumulation rates, a final correlation (option 7.1) was used to generate an age model of sedimentation for the Pignola-Abriola section. The Pignola-Abriola section has been correlated with Rhaetian sections from the literature, notably the current global boundary stratotype section and point candidate for the base of the Rhaetian at Steinbergkogel, Austria, where the Norian-Rhaetian boundary is proposed to be placed at a stratigraphic level containing the first appearance datum (FAD) of conodont Misikella posthernsteini , traced on the Newark APTS to ca. 209–210 Ma. Issues regarding the taxonomy of M. posthernsteini , a species characterized by transitional forms with its ancestor Misikella hernsteini , lead us to propose the alternative option of placing the Norian-Rhaetian boundary at a prominent negative δ 13 C org spike observed in the Pignola-Abriola section at meter 44.5, 50 cm below the level containing the FAD of M. posthernsteini sensu stricto and close to the base of radiolarian Proparvicingula moniliformis zone. This level has been magnetostratigraphically correlated to Newark magnetozone E20r.2r at ca. 205.7 Ma. Assuming an age of ca. 201.3 Ma for the Triassic-Jurassic boundary, the Rhaetian Stage would have a duration of ~4.4 m.y.

A Process- and Provenance-Based Attempt to Unravel Inconsistent Radiocarbon Chronologies in Lake Sediments: An Example from Lake Heihai, North Tibetan Plateau (China)

Aquatic macrophytes from a lacustrine environment are highly prone to a reservoir effect, resulting in an overestimation of age. This is often caused by the incorporation of dissolved carbon (CO2 and HCO3–) through photosynthesis from lake waters that have a different 14C activity than the atmosphere. The atmosphere-water disparity is often produced by a mixing of carbon between the water body and its terrestrial surroundings, a process highly prone to temporal variations. Thus, only a comprehensive understanding of the 14C budget over time enables a reliable chronology of lacustrine records. We studied lacustrine sediments from Lake Heihai on the northern Tibetan Plateau with a recent reservoir effect of 6465 ± 75 14C yr as estimated from accelerator mass spectrometry (AMS) dating of three living aquatic plants. Age inversions in a well-laminated composite core from the lake suggest that the reservoir effect markedly changed over the depositional period. In the lower part of the core, an excellent correlation was observed between the allochthonous input of dolomite and the inverse 14C ages, indicating the incorporation of dissolved 14C-dead carbon from a limestone catchment in the plant material. For the upper part of the core, sediment recycling of Holocene high-stand deposits may have further contributed to the reservoir effect. These findings give rise to a reliable process- and provenance-based chronology within a confidence interval supported by 137Cs measurements and magnetostratigraphic investigations. Our results highlight the need to identify the interactions of lakes with their surroundings to estimate reservoir-corrected ages in lacustrine settings.