Brunhes-Matuyama Research Articles

Understanding the fluvial capture of the Guadix-Baza Basin in SE Spain through its oldest exorheic deposits

Abstract The fluvial capture of endorheic basins represents a milestone in basin chronology, implying a profound disequilibrium that triggers critical geomorphological, sedimentological, paleogeographic, and even paleoecological transformations. The primary goal of many geomorphological studies is to determine the timing of endorheic-to-exorheic transitions with the objective of unveiling the dynamics that follow the capture event. The age of the Guadix-Baza Basin capture in the Central Betic Cordillera (S Spain) remains a subject of controversy, with proposed estimates ranging from 17 to 600 ka. In this study, we present new 234U/230Th and optically stimulated luminescence ages from exorheic deposits exposed within the basin's main fluvial valley, the Guadiana Menor River. We acquired the oldest numerical age recorded to date for a postcapture deposit within the basin. This age corresponds to a travertine platform formed 240.8 ± 25 ka on a surface level that was already incised into the glacis surface at approximately 250 m. Using these data, we estimate that basin capture took place earlier than ca. 240 ka, plus the time required for the river to incise 250 m to the position of the travertine. Furthermore, the proximity of the Matuyama-Brunhes reversal (781 ka) to the top of the endorheic succession and the ages of the paleontological sites (> ca. 750 ka) throughout the basin suggest that the capture could have occurred earlier than the oldest previously proposed age of 600 ka.

Provenance changes of the Yangtze River Delta sediments since ∼3.6 Ma: Evidence from heavy mineral assemblages and detrital zircon U–Pb ages spectra

The Yangtze River drains from the eastern Tibetan Plateau and provides insights into the evolution of the Asian monsoon and uplift of the Tibetan Plateau. The age and evolution of the Yangtze River have been investigated, yet consensus remains lacking. This study aimed to investigate the sediment source changes of the Yangtze Delta since approximately 3.6 Ma. A new sediment core (YZQK2, 300.36 m) was subjected to high-resolution magnetostratigraphic, zircon U–Pb age spectral, and heavy mineral assemblage analyses. Magnetostratigraphy showed that 79.65, 171.22, and 293.4 m reflected the Matuyama–Brunhes, Gaussian–Matuyama, and Gilbert–Gaussian boundaries, respectively. The bottom age of the core was approximately 3.60 Ma. Between 3.60 and 3.44 Ma, heavy mineral assemblies were characterized as high reductive heavy minerals, pyrite, and siderite with the dominant peak of the detrital zircon U–Pb ages being 400–450 Ma. The sediments were primarily fed by tributaries draining into the middle Yangtze River, which were the Han River oriented in the Tongbai-Dabieshan orogenic belt and Xiang River and Gan River oriented in the Cathaysia Block. Between 3.44 and 2.79 Ma, the reductive heavy minerals, pyrite, and siderite disappeared, while garnet, apatite, sphene, and ilmenite showed a sharp increase. The dominant peak of detrital zircon U–Pb ages exhibited an extremely narrow range of 100–150 Ma. The sediments originated from the rock basin in the lower Yangtze River, the dominant river system, and were sourced locally or regionally from the adjacent Cretaceous granite plutons and volcanic rocks. The zircon age spectrum similar to the mainstream of the Yangtze River, containing amphibole, pyroxene, and magnetite from the upper to the middle and lower reaches of Yangtze River, has formed the Yangtze drainage pattern since 2.79 Ma. The sediment in the middle Yangtze reached the Yangtze Delta no later than 3.6 Ma, while the integration of the upper Yangtze into the Yangtze Delta occurred no earlier than 2.79 Ma. Our results provide novel insights into the established timeline of the Yangtze River.

Global Geomagnetic Field Evolution From 900 to 700 ka Including the Matuyama‐Brunhes Reversal

AbstractPolarity reversals and excursions are the most significant geomagnetic field changes generated in the liquid outer core of the Earth, therefore studying them helps understand geodynamo processes. This study examines the Matuyama‐Brunhes (MB) reversal using a new reconstruction of the global geomagnetic field based on paleomagnetic data, termed Global Geomagnetic Field Model for the MB reversal (GGFMB). GGFMB covers 900–700 ka, including late Matuyama and early Brunhes. This allows us to also investigate the Kamikatsura excursion (ca. 888 ka). The model is based on 38 high‐quality paleomagnetic sediment records with age control mostly independent of the magnetic signal. GGFMB suggests that the MB reversal began about ∼799 ka, when non‐dipole field components increased and the axial dipole component decreased. The transitional fields first appeared on Earth's surface in the high‐latitude southern hemisphere and equatorial regions. The minimum dipole strength was reached around 780 ka and the axial dipole changed sign. After ∼10 Kyr, the field stabilized in the normal polarity of the early Brunhes. The MB reversal lasted ∼29 Kyr (from 799 to 770 ka) and had slower rate of dipole decay than recovery as well as lower dipole moment for several millennia before than after the reversal. According to GGFMB, the dipole moment during the Kamikatsura excursion was approximately half that of the current field and it was a regional excursion observed only over eastern Asia and North America. Our sediment data collection is heavily biased toward the northern hemisphere, thus more southern hemisphere records are needed to demonstrate GGFMB's robustness in this region.

The earliest European Acheulean: new insights into the large shaped tools from the late Early Pleistocene site of Barranc de la Boella (Tarragona, Spain)

Since the oldest known Acheulean lithic techno-typological features in Europe were reported at the site of Barranc de la Boella (Tarragona, Spain), continuous fieldwork has been conducted there in archeological deposits of the late Early Pleistocene age (0.99–0.78 Ma). As a result, excavations in two of the three open-air localities have significantly expanded the collection of lithic and faunal remains, allowing us to make progress in the interpretation of the hominin behaviors in an open-air fluvial-deltaic sedimentary environment. This includes examples of cumulative palimpsests, such as those found at the locality of La Mina, in which hominins only had a minimal role as modifying agents, as well as the extraordinary mammoth butchery site recorded at the Pit 1 locality. The aim of this paper is to present a comprehensive update of the collection of large shaped tools and to assess its significance in the framework of the earliest occurrence of the Acheulean in Europe. This cultural entity is increasingly well-documented for the early Middle Pleistocene, but very little is known about its presence in Europe before the Brunhes–Matuyama boundary. Large shaped tools appear in the three localities explored in the Unit II of Barranc de la Boella, including choppers (unifacial and bifacial) and standard Acheulean forms, such as picks, knives, and cleaver-like forms. Techno-typological and morphometrical analyses revealed a basic heavy-duty component obtained through simple shaping sequences coupled with significantly more elaborate tools produced on various large blanks (cobbles, slabs, or flakes). The complete bifacial and bilateral shapings have yet to be documented, but the present specific tool assemblage attests to the Early Acheulean technological threshold. Hence, the archaeological data from Barranc de la Boella provide insights into the first appearance of the Acheulean technology in Europe and add critical information to the debate on the technological variability of the Early Pleistocene hominin occupation of the continent. The results of this study revealed a technological assemblage unique in the known late Early Pleistocene archeological record from Europe, different from the rest of ancient Acheulean sites in this continent, which are dated at the Middle Pleistocene. This lends support to the hypothesis that Barranc de la Boella may represent a previously unrecognized Early Acheulean dispersion out of Africa connected to its first evidence at the gates of Eurasia, potentially moving over the northern Mediterranean coastal road to reach Western Europe.

Characteristics of the Matuyama‐Brunhes Magnetic Field Reversal Based on a Global Data Compilation

AbstractMagnetic field reversals are irregular events in Earth's history when the geomagnetic field changes its polarity. Reversals are recorded by spot and continuous remanent magnetization data collected from lava flows and marine sediments, respectively. The latest field reversal, the Matuyama‐Brunhes reversal (MBR), is better covered by paleomagnetic data than prior field reversals, hence providing an opportunity to understand the physical mechanisms. Despite the quantity of data, a full understanding of the MBR is still lacking. The evolution of the MBR in time and space is explored in this work by compiling a global set of paleomagnetic data, both from sediments and volcanic rocks, which encompass the period 900–700 ka. After careful evaluation of data and dating quality, regional and global stacks of virtual axial dipole moment (VADM), virtual geomagnetic pole (VGP), and paleosecular variation index (Pi) are constructed from the sediment records using bootstrap resampling. Individual VADMs and VGPs calculated from lavas are compared to these stacks. Four phases of full‐vector field instability are observed in these stacks over the period 800–770 ka. The first three phases, observed at 800–785 ka, reflect a rapid weakening of the field coupled with low VGP latitude, after which the field returned to the reverse polarity of the Matuyama chron. The fourth phase, lasting from 780 to 770 ka, is when the field reversal process completed, such that the field entered the Brunhes normal polarity state. These findings point to a complex reversal process lasting ∼30 Kyr, with the reversal ending at ∼770 ka.

The last 30,000 to 700,000 years ago: Unravelling the timing of human settlement for the Palaeolithic site of Kozarnika

Kozarnika cave is a renowned prehistoric site in the Balkans, which contributes significantly to our understanding of the human past due to its rich assemblages associated with the Lower to Upper Palaeolithic. Various dating methods have been employed to unravel the timing of human occupations in Kozarnika. Radiocarbon dating was used to unfold the time frame for the Kozarnikian lithic tradition uncovered in the Upper Palaeolithic sequence of the cave, and palaeomagnetic dating assigned the Brunhes–Matuyama reversal to the layer beneath the Lower Palaeolithic assemblages.In this study, we employed luminescence-dating methods, including a body of different signals to date sediment layers from the top to the bottom of the sequence covering the period of ca 30 to 700 ka. Our investigations revealed that the Kozarnikian tradition in layers 5a-c falls between 30 and 35 ka. Following that, we suggest that the Middle Palaeolithic period initiated between 250 and 309 ka and lasted until 40–53 ka. More importantly, we have updated the age of the Neanderthal radius discovered in the Mousterian assemblages to 201 ± 17 ka.Our dating resulted in a period of ca 300–700 ka for the Lower Palaeolithic assemblages in the cave. Although this age range fits perfectly with the palaeomagnetic boundary <780 ka established for Kozarnika, the possibility of reaching the threshold of luminescence dating cannot be ruled out. Thus, at this stage, this age range may represent the minimum age for the Lower Palaeolithic.

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka.Graphical

Aeolian imprints of multiple Mediterranean invasions of the Black Sea during Pleistocene

Climate changes determined the repeated connections between the Black Sea, Caspian Sea and Mediterranean Sea. The landlocked anoxic Black Sea basin was exposed to several transgressions throughout Quaternary by the Mediterranean Sea through the Straits of Istanbul (Bosphorus) and by the Caspian Sea through the Manych-Kerch spillway. Sedimentological records of these connections are limited mostly to the marine terrace deposits of Marine Isotope Stage (MIS) 5e while the pre-MIS 5e period remains uncertain due to a lack of robust facies and chronological data from deep-sea sedimentary sequences. Here we discuss the imprints of multiple Mediterranean transgressions during Middle Pleistocene in the Black Sea based on facies analysis and the optical age of coastal carbonate aeolianites. Contrary to today's hydro-climate of the Black Sea, the aeolianites bear witness to the transformation of the Black Sea into a warm inland sea during successive Mediterranean invasions. Prior to the onset of aeolian deposition, paleosols were formed on the Eocene-aged hardened sandy silts, suggesting strongly washed soil. This is evidenced by no calcium carbonate and a high Rb/Sr ratio, with quartz amounting to of 99.8%. According to our OSL ages, carbonates deposited on the shelf plain under higher temperature and increased evaporation conditions in MIS 15 and the later interglacial phases were transported to the coastal sand dunes during the transitional phases of MIS 15–14, MIS 13–12, MIS 11–10 and MIS 9–8. We suggest that the carbonate-rich and ooid-containing aeolianites were repeatedly formed in the multiple Mediterranean transgression stages, beginning with an increasingly severe dry phase following the Brunhes-Matuyama magnetic reversal. • The aeolianite is a robust record for Mediterranean transgressions of the Black Sea. • The ooid-rich aeolianites was repeatedly formed in multiple transgression stages. • The aeolianites bear witness to the transformation of the Black Sea into a warmer sea.

Early–Middle Pleistocene Magnetostratigraphic and Rock Magnetic Records of the Dolynske Section (Lower Danube, Ukraine) and Their Application to the Correlation of Loess–Palaeosol Sequences in Eastern and South-Eastern Europe

We present new palaeomagnetic and rock magnetic results with a stratigraphic interpretation of the late Early–Middle Pleistocene deposits exposed on the left bank of the River Danube at Dolynske, southern Ukraine. A thick succession of water-lain facies is succeeded by reddish-brown clayey soils, topped by a high-resolution loess–palaeosol sequence. These constitute one of the most complete recently discovered palaeoclimate archives in the Lower Danube Basin. The suggested stratigraphy is based on the position of the Matuyama–Brunhes boundary, rock magnetic, palaeopedological and sedimentological proxies, and it is confidently correlated with other loess records in the region (Roksolany and Kurortne), as well as with the marine isotope stratigraphy. The magnetic susceptibility records and palaeosol characteristics at Dolynske show an outstanding pattern that is transitional between eastern and south-eastern European loess records. Our data confirm that the well-developed S4 soil unit in Ukraine, and S5 units in Romania, Bulgaria and Serbia, correlate with the warm MIS 11. Furthermore, we suggest the correlation of rubified S6 palaeosols in Romania and Bulgaria and the V-S7–V-S8 double palaeosol in Serbia with S6 in Ukraine, a strong Mediterranean-type palaeosol which corresponds to MIS 15. Our new results do not support the hypothesis of a large magnetic lock-in depth like that previously interpreted for the Danube loess, and they prove that the Matuyama–Brunhes boundary is located within the palaeosol unit corresponding to MIS 19. The proposed stratigraphic correlation scheme may serve as a potential basis for further regional and global Pleistocene climatic reconstructions.

The Matuyama—Brunhes boundary in the loess-palaeosol sequence of Dolynske, southern Ukraine

We present the results of a palaeomagnetic study of the Early—Middle Pleistocene deposits exposed on the left bank of the River Danube at Dolynske, southern Ukraine. A thick succession of water-lain facies is succeeded by stratigraphically complete loess-palaeosol sequence; these constitute a unique palaeoclimate archive in the southern margin of the East European loess province. The Matuyama—Brunhes boundary (MBB) has been detected at the bottom of the Lower Shyrokyne (S7S3) subunit and not in the Martonosha (S6) unit as previously thought. New data align with previous results from the Roksolany and Vyazivok sections, where the MBB was determined at the same stratigraphical level in the S7S3 soil. In contrast to terrestrial Pleistocene records in China and сentral Europe, where the MBB was regularly determined in a loess layer (representing a cold period), the MBB in the Ukrainian subaerial succession is located in the soil unit (representing a warm period). Furthermore, eight, and not seven, glacial-interglacial cycles are recorded in the Brunhes chron. This may indicate the stratigraphic completeness of the loess-soil succession of Ukraine, which can be compared with the reference global marine and terrestrial palaeoclimatic archives. Further palaeomagnetic studies of loess-palaeosol sequences of other regions of Ukraine will allow revision and correlation of still inconsistent stratigraphic and magnetostratigraphic schemes of the Pleistocene deposits.

Significance of the Ukrainian loess-palaeosol sequences for Pleistocene climate reconstructions: rock magnetic, palaeosol and pollen proxies

Variations of rock magnetic parameters in loess-palaeosol sequences, related to climatic and environmental conditions during their formation, are a powerful tool for palaeoclimate reconstruction. Combined enviromagnetic study of loess deposits in Ukraine and its assessment for the palaeoreconstruction purposes are carried out in the framework of the National Research Foundation of Ukraine project 2020.02/0406 ‘Magnetic proxies of palaeoclimatic changes in the loess-palaeosol sequences of Ukraine’. Environmental/climatic reconstructions of the past are fulfilled using a significant number of palaeoindicators: morphology and lithological properties of palaeosols and loesses, their pollen assemblages and a wide range of magnetic characteristics. In this paper, we present a multi-proxy approach to palaeoenvironmental reconstructions, and introduce preliminary results obtained from magnetic susceptibility of loess-palaeosol sequences in the northern (at Vyazivok), central (Stari Kaydaky) and southern (Roksolany) parts of the Ukrainian loess belt. The amplitudes of palaeoclimate change established using magnetic proxies are well correlated with the lithological, palaeopedological and palynological patterns of the sites, and with the global oxygen-isotope scale (MIS). Ongoing studies of the Stari Kaydaky section confirm the correlation of the Upper Zavadivka (S3) soil unit and Lower Zavadivka (S4) soil unit with MIS 9 and MIS 11, respectively (this was proved earlier at the Vyazivok and Roksolany sites). The underlying Lubny (S5) pedocomplex likely corresponds to MIS 13, and the Martonosha (S6) pedocomplex to MIS 15. Palaeomagnetic investigations at Stari Kaydaky have not so far reached the Lower Shyrokyne unit, in which the Matuyama—Brunhes boundary has been detected at Roksolany and Vyazivok. The Upper Shyrokyne (S7S1) palaeosol unit has normal polarity and is preliminarily correlated with MIS 17.

Rock magnetic and palaeomagnetic studies of loess-palaesol sections — Lower Palaeolithic sites within the Southern Bug Valley (Medzhybizh, Holovchyntsi)

Combined rock magnetic and palaeomagnetic studies of loess-soil sections — Lower Paleolithic sites in the valley of the Southern Bug (Medzhybizh, Holovchintsi) — have been performed for the first time in order to determine the suitability of these objects for palaeomagnetic study and the establishment of magnetostratigraphic markers. Investigated sections by rock magnetic characteristics are closest to the sections of the Volynian Upland, and refer to the intermediate «Chinese» type of formation of magnetic properties, with an admixture of the «Alaskan» mechanism. These sections are characterized by a low concentration of ferrimagnetic material, the destruction of the primary sedimentary magnetic texture, which makes them unsuitable for qualitative magnetostratigraphic studies. According to the data of magnetic-mineralogical analysis, the samples are subdivided into three groups: the first group includes samples with new formation at 300 °C, which is associated with iron hydroxides or the presence of organic matter; in the second group, the thermomagnetic curves are not informative, which makes it difficult to determine the minerals-carriers; in some samples, the magnetization carrier is magnetite (with defects or finely dispersed). A zone of normal polarity, probably the Brunhes chron, has been reliably determined in the upper part (G-S7S1 soil) of the Holovchyntsi section. In the lower part of the M-S4 soil unit (MIS 11) at Medzhybizh, the Unnamed geomagnetic event at 430 ky has been detected. The paleomagnetic veracity of the remaining investigated layers is questioned. The analogies for the archaeological assemblages from the lower layers of Medzhybizh-A and Holovchintsi-1 are seen in archaic industries of mode 1, which in southeastern Europe are dated back from 800 ky and older. However, reliable data on the Matuyama—Brunhes boundary and, correspondingly, data on the &gt;780 ky age of any layers with artifacts in the Medzhybizh and Holovchintsi sections were not obtained by the palaeomagnetic studies.

Disentangling magnetic and environmental signatures of sedimentary 10Be/9Be records

Reconstructions of the global production rate of the cosmogenic isotope 10Be from sedimentary records of authigenic 10Be/9Be ratios have been successfully used to obtain independent estimates of geomagnetic dipole moment variations caused by field excursions or reversals. In this study, we assess the reliability of 10Be/9Be as a proxy for the cosmogenic 10Be production rate by evaluating two potential biasing sources represented by sediment composition and climatic modulation. For this purpose, we compare five high-resolution 10Be/9Be records of the Matuyama-Brunhes (M-B) field reversal from sediment cores of the Indian, West Pacific, and North Atlantic oceans. Significant increase of 10Be/9Be ratios at ∼774 ka is explained in terms of the dominant control of geomagnetic modulation during the M-B reversal. Results do not support the existence of a direct proportionality between measured sedimentary 10Be/9Be ratio and cosmogenic 10Be production rate, as shown by 10Be/9Be records that offset relative to each other during and outside the M-B reversal.Residual differences between offset-corrected rescaled records do not appear to be related to an incomplete correction of variable sediment scavenging efficiencies by 9Be normalization. Instead, these differences can be explained by a common climatic modulation model, assuming a linear relation between 10Be/9Be and the global 10Be production rate with site- and time-dependent additive and multiplicative coefficients. These coefficients are linear functions of a single global climate proxy identified with the benthic δ18O record. Additive coefficients are almost constant in time and can represent up to ∼60% of the average 10Be/9Be value during periods of stable field polarity. Multiplicative coefficients are also site-specific, with mean values representing the bulk scavenging efficiency of the site, and variations about this mean expressing a multiplicative climatic modulation of the 10Be production rate. The amplitude of this modulation amounts to 10–15% of the maximum variations recorded during the M-B reversal and is sufficiently large to mask minor variations of the dipole moment during stable polarity periods. Reconstructions of the geomagnetic dipole intensity can benefit from the information about climatic modulation effects gained with our modelling approach. Best suited sites for magnetic field reconstructions should be characterized by minimal Be-recycling contributions from ancient 10Be reservoirs and minimal climatic modulation, as far as it can be determined from relative comparisons with other records. These conditions are most likely encountered in open basins at sites (1) with < 2.8 km water depth, (2) > 200 km offshore, and (3) located underneath a large current system extending over regions with minimum terrigenous inputs. Scaling all records with respect to a chosen reference enables to produce 10Be/9Be stacks with reduced noise and short-term local environmental effects. Differences between stacks obtained in this manner highlight global climatic effects that need to be considered when generating calibrated reconstructions of the geomagnetic dipole moment.

Formal ratification of the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries of the Quaternary System: the Chiba Section, Japan†

by Yusuke Suganuma, Makoto Okada, Martin J. Head, Koji Kameo, Yuki Haneda, Hiroki Hayashi, Toshiaki Irizuki, Takuya Itaki, Kentaro Izumi, Yoshimi Kubota, Hiroomi Nakazato, Naohisa Nishida, Masaaki Okuda, Yasufumi Satoguchi, Quentin Simon, and Yoshihiro Takeshita. Episodes 2021;44:317-47. https://doi.org/10.18814/epiiugs/2020/020080

Magnetostratigraphy and magnetic susceptibility of the best developed Pleistocene loess-palaeosol sequences of Ukraine: implications for correlation and proposed chronostratigraphic models

We provide a revised magnetostratigraphy and magnetic susceptibility stratigraphy of the most complete and thickest (to nearly 60 m) loess-palaeosol sequences in Ukraine spanning the past 1 My: the Roksolany (Black Sea Lowland) and Vyazivok (Dnieper Lowland) sections. The Matuyama–Brunhes boundary has been detected in both sequences in stratigraphically different palaeosol units according to current regional chronostratigraphic schemes. Hypotheses of a large magnetic lock-in depth at Vyazivok and lithostratigraphic incompleteness at Roksolany do not resolve this inconsistency. Instead, new chronostratigraphic models following the Chinese loess designation system, which are supported by correlation of the magnetic susceptibility records with the marine isotope record and established magnetostratigraphic control points, are proposed. We conclude that the Matuyama–Brunhes reversal in the Roksolany and Vyazivok sections belongs to the same palaeosol unit, the Shyrokyne (according to our nomenclature, the U-S7), which corresponds to MIS 19. This novel interpretation resolves the inconsistency of the stratigraphic position of the Matuyama–Brunhes boundary in Ukrainian loess, ends long-standing debate regarding the chronostratigraphy of the Roksolany section, and allows precise correlation of the most representative loess-palaeosol sequences of Ukraine with those in the Danube Basin and the Chinese Loess Plateau. It is considered that the Roksolany Tephra in MIS 6 loess unit can be related to the L2 Tephra which is widely distributed in southeastern European loess records and lacustrine archives. In the light of our results, the Roksolany sequence may serve as a national lectostratotype of the Middle Zavadivka (U-L4) loess unit corresponding to MIS 10. Additionally, a generalized pedostratigraphic column of the past 1 My for central and southern Ukraine has been constructed and correlated with the Hungarian, Serbian and Chinese loess stratigraphies, as well as with the marine isotope record down to MIS 25.

A full sequence of the Matuyama\u2013Brunhes geomagnetic reversal in the Chiba composite section, Central Japan

Geological records of the Matuyama–Brunhes (M–B) geomagnetic reversal facilitate the development of an age model for sedimentary and volcanic sequences and help decipher the dynamics of the Earth’s magnetic field. However, the structure of the geomagnetic field during the M–B geomagnetic reversal remains controversial due to its complex field behavior. In this study, we conducted paleo- and rock-magnetic analyses of samples from the Chiba composite section (CbCS), a continuous and expanded marine succession in Central Japan, to reconstruct the full sequence of the M–B geomagnetic reversal. We define an average stratigraphic position of the M–B boundary and estimate its age based on three sections in the CbCS and a neighboring drill core, TB-2. The average stratigraphic position of the M–B boundary in the CbCS is established at 1.1 ± 0.3 m above a widespread volcanic ash bed (the Byk-E tephra). Assuming a chronological error associated with orbital tuning of 5 kyr and stratigraphic uncertainty of 0.4 kyr, the M–B boundary in CbCS is at 772.9 ± 5.4 ka (1σ). The virtual geomagnetic pole, which is calculated from the paleomagnetic directions, shows several short fluctuations between 783 and 763 ka, with concomitant decreases in geomagnetic field intensity index. After termination of the field instabilities, the field intensity recovered and became higher than before the M–B boundary, with a stable normal polarity direction. The paleomagnetic records in the CbCS exhibit a field asymmetry between the axial dipole decay and field recovery, providing a full sequence of the M–B reversal, suggesting that the non-axial dipole field dominated several times during periods ca. 20 kyr long across the M–B boundary, due to depletion in the main axial dipole component. Our results provide probably the most detailed sedimentary record of the M–B geomagnetic reversal and offer valuable information to further understand the mechanism and dynamics of geomagnetic reversals.Graphical abstract

A centennial-resolution terrestrial climatostratigraphy and Matuyama\u2013Brunhes transition record from a loess sequence in China

Terrestrial records of the last geomagnetic reversal often have few age constraints. Chronostratigraphy using suborbital-scale paleoceanic events during marine isotope stage 19 may contribute to solving this problem. We applied the method to an 8 m long, high-resolution paleomagnetic record from a loess sequence in China and revealed millennial-to-sub-centennial scale features of the Matuyama–Brunhes (MB) transition. All samples were subjected to progressive thermal demagnetization with 14–15 steps up to 650–680 °C. As a result, 96% of the samples yielded a high-quality remanent magnetization. The MB transition terminated with a 75 cm thick zone with nine polarity flips. The polarity flip zone, dated at about 779–777 ka, began between the warm events “I” and “J” and terminated at the end of the cooling event coincident with the lowest axial-dipole strength interval. Most polarity flips occurred within 70 years. The virtual geomagnetic poles (VGPs) in the upper polarity flip zone clustered in the SW Pacific region, where the MB transitional VGPs from lavas of the Hawaiian and Canary Islands and lacustrine deposits of Java also clustered. These sites were probably dominated by dipolar fields. The absence of transitional fields across polarity flips implies a short time span for averaging fields due to a thin loess-magnetization lock-in zone. The reverse-to-normal polarity reversal dated at about 778 ka in Lingtai occurred at the end of the SW Pacific VGP zone, an important key bed for MB transition stratigraphy. The reversal is a good candidate for the main MB boundary. We found an excursion at about 766 ka spanning about 1 ka.

Kinematics of the virtual geomagnetic poles during Brunhes–Matuyama times

Abstract Twenty magnetostratigraphic profiles from IODP (International Oceanic Drilling Project) sediment cores distributed on the Earth's surface allowed the scatter of virtual geomagnetic poles (VGPs) during Brunhes–Matuyama times to be examined. We identified two groups of recording sites which give different paths for the VGPs during a time interval of about 1.1 myr. Calculations of the VGP velocities and accelerations, as well as the corresponding azimuths, resulted in mean/median values like those observed for recent times. No significant differences were observed during the ‘stable’ and transitional fields. The acceleration azimuths show variations from north–south to east–west depending on the field state: normal/reversed or transitional. Despite the uncertainties in the magnetization of the sediments (overprints and/or low-resolution records), we demonstrate that the use of this database is valid for obtaining kinematic parameters of the geomagnetic field when analysed on a statistical basis.

Recordings of Fast Paleomagnetic Reversals in a 1.2 Ma Greigite‐Rich Sediment Archive From Lake Ohrid, Balkans

AbstractState‐of‐the‐art paleoclimate research strongly depends on the availability of time‐equivalent markers as chronological control to disentangle interrelationships in the climate system from regional to global scale. Geomagnetic reversals are regarded as excellent age constraints because they are global events and independent from climatic conditions. However, spatial variations of timing and internal dynamics of reversals may limit their precision. Our 1.2 Ma high‐resolution (~25 cm/kyr) sediment record from Lake Ohrid is promising to precisely depict the Matuyama‐Brunhes (MB) reversal and the Jaramillo subchron. Two generations of diagenetic ferrimagnetic minerals are present in glacial intervals of the Lake Ohrid record. Early diagenetic greigite acquired a quasi synsedimentary chemical magnetization, while a late diagenetic greigite formation, triggered by the upward diffusion of H2S‐rich waters, obscures the polarity record at the top of the Jaramillo. Interglacial intervals are unaffected by greigite formation, likely due to low iron concentrations. Based on an orbitally tuned age model with tephrostratigraphic markers, the base of Jaramillo can be precisely dated to 1072.4 ka, and the MB reversal to 778.5 ka. Both polarity reversals occurred very rapidly in our record, lasting 2.3 and 1 kyr, respectively. Our results reveal that the dipole component of the Earth's magnetic field fell below the nondipole components only for a short duration in the Mediterranean region. The comparison of the timing of the MB boundary across different archives implies that the onset of the reversal provides a more synchronous age marker compared to often used midpoint ages.

Constraining in situ cosmogenic nuclide paleo-production rates using sequential lava flows during a paleomagnetic field strength low

The geomagnetic field prevents a portion of incoming cosmic rays from reaching Earth’s atmosphere. During magnetic reversals and excursions, the field strength can decrease by up to 90% relative to the modern-day value. During such anomalies, cosmic ray bombardment to Earth’s atmosphere increases as evident from atmospheric 10Be anomalies recorded in sediment and ice cores. However, how the flux of cosmic rays to Earth’s surface varies during such geomagnetic anomalies is not well constrained. We measured fossil cosmogenic 3He in olivine from the tops of two pairs of 40Ar/39Ar age-dated Tahitian lava flows that erupted during the Matuyama-Brunhes reversal precursor event. We corrected these raw values for the diffusive loss of helium caused by heating from the overlying flow with a diffusion model using cooling rates and maximum temperature conditions based on field measurements of active lava flows from Kilauea, Hawaii. We assume the maximum temperature suggested by field measurements and thus present a limiting case for the highest diffusive loss corrections and thus the highest paleo-production rates. Based on paleomagnetic field strength reconstructions and scaling factor models, the upper limits of the corrected in situ3He paleo-production rates (100 ± 23, 144 ± 35 atoms g−1 a−1) are in agreement with those expected during the period of a geomagnetic field strength low when these flow tops were exposed. However, the more plausible contact temperatures (<700°C maximum temperature in diffusion model) are associated with diffusion corrected paleo-production rates lower than those predicted by scalar models. This potential underestimation is likely a function of changes in local non-dipole field components, atmospheric density and/or an overestimation of the dipole field strength reduction during the M-B precursor event.