Late Matuyama Research Articles

The Gilsá excursion and the Matuyama/Brunhes transition recorded in40Ar/39Ar dated lavas from Lanai and Maui, Hawaiian Islands

SUMMARY Directional results, Thellier-type palaeointensity determinations and Ar/Ar ages are presented from volcanic units of Pleistocene age from the Hawaiian islands of Maui and Lanai. Twenty-nine non-contiguous lava flows were sampled on Lanai. The lava flows formed during the late Matuyama polarity chron about 1.6 Ma and recorded the Gilsa geomagnetic polarity excursion. The obtained ages of 1.6 Ma for the Lanai lavas are significantly older than ages previously obtained for this island. These new ages are, however, in agreement with the age progression of the Hawaiian hotspot. The palaeomagnetic record over the sampled succession is characterized by reversed and intermediate directions. palaeointensities are generally very low, about half the present-day field intensity, dropping to values of ∼5 μT during transitional field states. Transitional virtual geomagnetic poles (VGPs) from Lanai are situated first near the west coast of South America and then switch to offshore western Australia. Normal polarity is never reached in the sampled succession. Transitional VGPs located close to Australia accompanied by relatively low palaeointensities are found in most records of short geomagnetic excursions during the Matuyama polarity chron (e.g. the Cobb Mountain subchron, the Punaru event and the Matuyama/Brunhes precursor) suggesting very similar physical processes likely related to dipolar dominance during the onset of these excursions and influences of lower mantle heterogeneities. Approximately 800 kyr after the Gilsa excursion, lava flows on Maui recorded the last reversal, the Matuyama/Brunhes transition. Fourteen lava flows were sampled which recorded the pre- and post-transitional behaviour of the Earth's magnetic field. The geomagnetic field intensity is low prior to the reversal, approximately 8 μT, and increases strongly afterwards up to ∼63 μT. These values of about twice the present field intensity of Hawaii after the transition emphasize a strong asymmetry between pre- and post-transitional fields, which is particularly strong in the region of Hawaii.

Carbonate stable isotope signals in the 1-Ma sedimentary record of the HDP-04 drill core from Lake Hovsgol, NW Mongolia

Oxygen and carbon stable isotope ratios in carbonates from the HDP-04 drill core from Lake Hovsgol, NW Mongolia, show an overall covariant relationship suggesting that for the most of the past 1 Ma Hovsgol remained a closed-basin lake. Carbonate δ 18O ratio is responsive to regional climate change: a ca. +1.5‰ basinwide δ 18O shift has occurred with the onset of Bølling–Allerød warming ( sensu lato), followed by a ca. 0.8‰ depletion during the Younger Dryas. The post-glacial δ 18O shift of the same magnitude is recorded in bulk carbonates, shells of two ostracod species and in wet-sieved fine fraction <63 μm. Associated with the lake-level rise and correlative with the post-glacial warming in the northern hemisphere, the observed δ 18O shift is nevertheless positive. This argues against changes in local temperature and hydrology as key driving mechanisms. Most likely, Lake Hovsgol δ 18O reflects a climate-driven shift in the composition of regional precipitation. Tied into a distinct lithologic succession, the radiocarbon-dated late glacial δ 18O shift apparently represents a ‘template’ of the lake's response to glacial–interglacial transitions: a similar pattern of parallel changes in lithology and carbonate stable isotope composition is observed in at least 10 more intervals in the 1-Ma record, including the MIS 20/MIS19 transition at the Brunhes/Matuyama paleomagnetic reversal boundary. The comparison of carbon stable isotope ratios of untreated and in vacuo roasted bulk sediment with those of detrital carbonates suggests that clastic input of carbonates by lake tributaries does not affect the geochemistry of bulk carbonates in the HDP-04 section. The profiles of bulk carbonate δ 18O and δ 13C in the Pleistocene section of the HDP-04 drill core suggest at ca. 15.4 ka, at ca. 100 m below today's level, Lake Hovsgol still stood relatively high as compared with prior extended periods of time during late Matuyama and early Brunhes. Isotopically heavy δ 18O and δ 13C ratios during the mid–late Brunhes, particularly, in carbonate crusts and oolites, are suggestive of past episodes of dramatic evaporative 18O-enrichment of lake waters. Despite the expectation of muted amplitudes of temperature- and precipitation-related isotope signals, the sedimentary record from the sensitive ‘water gauge’ basin of Lake Hovsgol has high potential for providing important constraints on past hydrologic evolution of continental interior Asia during the Pleistocene.

Palaeomagnetic study of Tertiary volcanic domains in Southern Turkey and Neogene anticlockwise rotation of the Arabian Plate

Following collision with Eurasia, the Arabian Shield indenter has continued to deform into the weak Anatolian collisional collage that resulted from subduction of the Neotethyan Ocean. Differential movements have involved rotation and continuing northwards translation, and have been accommodated mainly by slip along major transforms including the northward extension of the Dead Sea Fault Zone (DSFZ) and the East Anatolian Fault Zone (EAFZ). To aid in evaluating post-collisional motions the palaeomagnetism of extensive volcanic domains at the northern margin of the stable shield in southern Turkey is reported here together with the timing of emplacement as constrained by K–Ar study. The age dating results indicate that volcanic activity occurred mainly during mid-late Miocene times corresponding to the final stages of suturing. Volcanic fields in the east of the investigated region are younger and correspond to Neotectonic volcanism in Brunhes and Matuyama chrons. Thermal and alternating field demagnetization of 399 cores from 83 sites in basaltic lavas identifies 29 units of normal and 43 of reversed polarity with 11 sites having transitional or random directions. Volcanic fields west of the Euphrates (Kilis–Gaziantep region) with ages in the range 7.0–20.3 Ma (average 14.9 Ma, SD = 4.3 Ma) have mean remanence D/ I = 353/52° (38 sites, 5.3°). Lava fields east of the Euphrates (Urfa region) are dated 10.4–12.1 Ma and yield a comparable mean remanence D/ I = 350/50° (17 sites, 6.0°). These collections are shown to have properly recorded palaeosecular variation with only minimal inclination shallowing and inferred anticlockwise rotations with respect to Eurasia since mid-Miocene times are 10.9 ± 4.3° and 14.0 ± 5.0° respectively. These contrast with clockwise rotation of 6.3 ± 4.3° derived from late Matuyama–Brunhes epoch volcanic rocks immediately to the north west of the Arabian margin where rotational impingement of the shield indenter into fault blocks within the Karasu Rift at the northern extension of the DSFZ interacts with left lateral motion at a rate of ~ 0.5 cm/year along this zone to produce small scale vertical axis rotations. Results from the volcanic suites sited on the stable shield are compared with other results from the Arabian Plate to conclude that it did not rotate significantly following closure of the Bitlis Suture until Late Miocene–Early Pliocene times since when it has rotated anticlockwise at a rate of ~ 1.0°/Myr. This is comparable to the present day rate of rotation deduced from GPS and correlates with crustal separation and sea floor spreading in the Red Sea. It also temporally links the rotation of the Arabian Plate to the initiation of the intracontinental transforms (North and East Anatolian fault zones) within Anatolia and to the subsequent extrusion of blocks within this accretionary domain by tectonic escape to the west.

Magnetostratigraphy of the Qiliting section (SE China) and its implication for geochronology of the red soil sequences in southern China

SUMMARY The red soil sequences in southern and southeastern China provide important terrestrial archives of palaeoclimate and palaeoenvironment and are important sources of Palaeolithic settlements and, therefore, early human occupation. Here we present detailed palaeomagnetic investigation coupled with mineral magnetic and petrographic analyses of a red soil sequence at Qiliting, southeastern China. The sediments contain both strongly magnetic minerals (such as maghemite and magnetite) and weakly magnetic minerals (such as haematite and possibly goethite). Maghemite and haematite were identified as the main carriers of the natural remnant magnetisations. Only haematite dominates the characteristic remnant magnetisations. Palaeomagnetic findings suggest that the Qiliting sedimentary sequence recorded the early Brunhes Chron and the late Matuyama Chron, including the Jaramillo Subchron. Our finding, coupled with previously published magnetostratigraphy, establishes that the red soil sequences in southern China began to accumulate no later than the Jaramillo age (i.e. ∼1.0 Ma). It is further suggested that the capabilities of recording magnetic polarity stratigraphy in Chinese red soil sequences are variable between areas because of different climate regimes.

A relative paleointensity record of the geomagnetic field since 1.6 Ma from the North Pacific

Abstract A paleomagnetic study was conducted on a sediment core KR0310-PC1 taken from the central North Pacific in order to obtain a relative paleointensity record in the Matuyama chron from this region. The core reached to about 1.6 Ma. The age control is based on the correlation of the S ratio (S −0.1T) variations with a global oxygen-isotope stack. Isothermal remanent magnetization (IRM) was used as the normalizer of the relative paleointensity estimation; anhysteretic remanent magnetization (ARM) was not adopted because ARM is sensitive to magnetostatic interaction among magnetic particles, which is evidenced in these sediments by an inverse correlation between the ratio of ARM to saturation IRM (SIRM) and SIRM without significant magnetic grain-size changes. For the last 350 kyrs, the record of core NGC65, which was obtained at practically the same site as KR0310-PC1 and covers the Brunhes chron (Yamazaki, 1999), was incorporated because the upper part of KR0310-PC1 was physically disturbed. In the record of NGC65/KR0310-PC1, the average paleointensity in the late Matuyama chron is not lower than that during the Brunhes chron, which does not support the conclusion of Valet et al. (2005) based on their Sint-2000 stack. A spectral analysis on the NGC65/KR0310-PC1 paleointensity record shows a power at the ∼100 kyr eccentricity period. The relative paleointensity and magnetic properties of NGC65/KR0310-PC1 were compared with those of MD982185 from the western equatorial Pacific (Yamazaki and Oda, 2002, 2005). The two sites belong to different oceanographic regimes. Coherent variations in the relative paleointensity despite incoherent changes in the magnetic properties suggest that rock-magnetic contamination to the relative paleointensity is small, if any, and the ∼100 kyr period in the relative paleointensity records would reflect the geomagnetic field behavior.

Early and middle Matuyama geomagnetic excursions recorded in the Chinese loess-paleosol sediments

Abstract A detailed paleomagnetic and rock-magnetic investigation on the early and middle Matuyama loess-paleosol sediments has been carried out at the Baoji section, Shaanxi province, southern Chinese Loess Plateau. Our new magnetostatigraphy revises the position of the lower Olduvai boundary from L27 to S26. Seven shortlived geomagnetic excursions, tentatively named as E1, E2, E3, E4, E5, E6, and E7, have been recognized in the L13, S22, L26, L27, S29, and upper and middle parts of L32, respectively. Results of the anisotropy of low-field magnetic susceptibility (AMS) show that the studied loess-paleosol sediments retain the primary sedimentary fabric. Rock magnetic experiments reveal that the sediments from the excursional and polarity transitional intervals have the same magnetic characteristics as those from the surrounding normal and reversed polarity intervals. Assuming a constant accumulation rate between polarity boundaries, these seven excursions are estimated to be at about 1.11 Ma (E1), 1.58 Ma (E2), 1.92 Ma (E3), 2.11 Ma (E4), 2.25 Ma (E5), 2.35 Ma (E6), and 2.42 Ma (E7) Ma. The E1 and E2 in the middle Matuyama Chron can be correlated with the Punaruu and Stage 54 (Gilsa) excursions, respectively. The E4, E5, and E7 in the early Matuyama Chron can be correlated with the Réunion II, Réunion I, and cryptochron C2r.2r-1 (X-subchron), respectively. The E3 in the lower Olduvai subchron and E6 in the early Matuyama Chron have no comparable events. At present they can only be correlated with the anomalous directions observed in the Osaka Bay core (Biswas et al., 1999). Therefore, further investigations are necessary to support their global occurrences. The present result together with the two late Matuyama excursions dated at about 0.89 Ma and 0.92 Ma (Yang et al., 2004) show that the Baoji section yields at least nine Matuyama excursions which, along with the results of the study, suggests that eight excursions occur at 0.9–2.2 Ma (Channell et al., 2002), thereby providing evidence that the short-lived geomagnetic excursions may also be a basic characteristic of the geomagnetic field during the Matuyama Chron.

Magnetostratigraphy of borehole EY02-2 in the southern Yellow Sea and its paleoenvironmental significance

Detailed rock magnetic and paleomagnetic studies have been undertaken on borehole EY02-2 (70m in length) in the southern Yellow Sea (SYS). The main Curie point revealed by magnetic susceptibility-temperature (k-T) curve is 580–600°C indicating magnetite dominance. The hysteresis loop parameters show large variation of magnetic mineral size in different sedimentary contexts: it is larger in subtidal sediment than in terrigenous sediment and even larger than in shallow sea sediment. This trend is correlative with distance to sediment source and dynamic strength. Magnetostratigraphic results show that the M/B polarity boundary (MBPB) is at 63.29m and there are at least 7 polarity transitions (Nr1-7) in Brunhes chron that can be tentatively correlated with 6 named polarity reversals. Three positive polarity reversals occur in late Matuyama chron and the early two may be the record of Kamikatsura happening in 886±3 kaB.P. Magnetic susceptibility (MS) and sediment grain size behave so differently in some sedimentary facies that certain big environmental changes can be clearly revealed. Generally, the MS and grain size of subtidal and terrigenous sediments are larger than shallow sea sediments and MS value around 10×10−5SI and mean grain size of 7ϕ seems to be indicators of shallow sea sediments of deep water depth. However, the frequently used excellent climatic proxies such as MS and grain size in loess and deep sea sediments fail to record such climatic cycles revealed by oxygen isotope in continental sea. The various sediment sources, sedimentation dynamic and their complex changes between glacial and interglacial periods should be the cause of failure.

Evaluation of the Olduvai subchron in the Orce ravine (SE Spain). Implications for Plio-Pleistocene mammal biostratigraphy and the age of Orce archeological sites

The Barranco de Orce (BO) section in the Baza basin (SE Spain) exposes several fossiliferous layers (O-1 to O-7) with Plio–Pleistocene micro- and macromammals. Biostratigraphic and magnetostratigraphic data from this and other sections in the basin have been extensively used to calibrate the Plio–Pleistocene chronology based on mammal biozonations. Because of its stratigraphic and geographic proximity, the BO section has also been used to date the paleontological and archeological sites of Barranco León, Fuentenueva-3 and Venta Micena. This study shows that the BO section crosses a mega-landslide that produces partial repetitions of the sedimentary sequence. The seven fossiliferous layers are actually the repetition of only two (O-6 and O-7) which are found in situ in the upper part of the ravine. New paleomagnetic results demonstrate the presence of Reverse magnetization throughout this section, contradicting the Normal event previously reported and assigned to the ‘Olduvai’ subchron (C2n). Published and new magnetostratigraphic data show that all archeological and paleontological sites in the Orce area are within a Reverse magnetochron, presumably C1r.2r (late Matuyama). The use of BO in the magnetobiostratigraphical calibration of the Pliocene/Pleistocene boundary for western Europe is not advised.

Magnetostratigraphic dating of the Donggutuo and Maliang Paleolithic sites in the Nihewan Basin, North China

A detailed magnetostratigraphic investigation, coupled with rock-magnetic studies, was carried out on a lacustrine sequence in the eastern Nihewan Basin, Northern China, which contains the Donggutuo and Maliang Paleolithic sites. Magnetite and hematite were identified as the main carriers for the characteristic remanent magnetizations. Magnetostratigraphic results show that the lacustrine sequence recorded the late Matuyama and Brunhes chrons. Furthermore, the Maliang artifact layer occurs just below the Brunhes/Matuyama boundary, and the Donggutuo artifact layer is just below the Jaramillo onset. Therefore, the age of the Maliang and Donggutuo artifact layers can be definitely estimated to be about 0.78 myr and 1.1 myr, respectively. These two paleomagnetic ages, coupled with previously obtained paleomagnetic data of the Majuangou, Xiaochangliang, Banshan, Lantian, and Xihoudu Paleolithic sites, suggest an expansion and lengthy flourishing of human groups from northern to north-central China during the entire Early Pleistocene.

MAGNETOSTRATIGRAPHY OF THE HOMO -BEARING PLEISTOCENE DANDIERO BASIN (DANAKIL DEPRESSION, ERITREA)

Four magnetozones have been found in the 530 m thick profile of the Dandiero Group. The lower unit, the Bukra Sand and Gravel, extends in the R1 reversed magnetozone from 150 m below the tephra level which was used as the reference marker between the sampled sections. The normal magnetozone N1 is almost completely covered by the lacustrine and deltaic sediments of the Alat Formation, while the following reversed magnetozone contains both the Wara Sand and Gravel and the lacustrine Goreya Fm. The N2 polarity zone is completely occupied by the Aro Sand. This polarity sequence has been calibrated to the geomagnetic time scale using the Early to Middle Pleistocene age of the associated vertebrate fauna and fission-track dating. The four magnetozones were thus regarded as representing the chrons by which the Pleistocene is correlated with magnetochronology. Their three reversal boundaries provided the dates of 1.07, 0.99 and 0.78 Ma, allowing to determine average sedimentation rates close to 1 m/ky. Cyclostratigraphy of the magnetic signal, analysed by the spectral analysis of the time series across the Jaramillo and late Matuyama chrons, confirmed that value. The evidenced cyclicities were directly related to the alternating lithofacies, and both to the astronomical parameters driving the climate changes during the deposition of the Dandiero group (some five hundred thousand years). The section with the Homo site covers the Jaramillo/Matuyama boundary, and the Homo bed located 2 m below this limit is dated 0.992 Ma.

Evidence for the Kamikatsura and Santa Rosa excursions recorded in eolian deposits from the southern Chinese Loess Plateau

A detailed paleomagnetic investigation of a loess‐paleosol sequence in Baoji, southern Chinese Loess Plateau, reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late Matuyama Chron. Rock magnetic experiments demonstrate that the sediments of the excursion intervals, in loess unit L9, have the same magnetic properties as those from surrounding normal and reversed polarity intervals. Assuming constant sedimentation rate between the Jaramillo‐Matuyama and Matuyama‐Brunhes magnetic polarity boundaries, the midpoint ages and durations for these two excursions are estimated at 0.89 Ma and 16 kyr, and 0.92 Ma and 7 kyr, respectively. In addition, these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr. These estimates are supported by paleoclimatologically inferred age constraints. The two excursions are probably correlated with the Kamikatsura and Santa Rosa excursions, respectively.

Palaeosecular variation in Java and Bawean Islands (Indonesia) during the Brunhes chron

SUMMARY A palaeomagnetic study has been conducted from Indonesian lava flows and dykes. Sampling was concentrated to the Merapi and Merbabu volcanoes (central Java), the Bromo‐Tengger volcanic complex, the Lurus volcano (both east of Java), and the Bawean Island. Numerical dating indicates that the rock units sampled have ages that are well distributed within the Brunhes chron. Only a few reverse polarity flows were probably emplaced during the late Matuyama chron. Alternating field (AF) and thermal demagnetization treatments were routinely used for all sites. Results from both methods were generally in good agreement, demonstrating the reliability and the reproducibility of the palaeomagnetic determinations obtained from 36 independent sites. The large dispersion of the directions and the age distribution indicate that a large spectrum of the secular variation has been sampled. Our best estimate of the mean Brunhes field direction in Java is characterized by a zero declination and an inclination anomaly of −4.7 ◦ (with a 95 per cent confidence angle of 5.3 ◦ ). This value agrees with an axial dipolar field model, or a model to which a persisting zonal quadrupolar term of a few per cent of the geocentric axial dipole is added. This study adds new constraints to recent results which suggested that a large zone of negative inclination anomalies may have persisted in the southwestern Pacific ocean over the past 500 Kyr, and possibly extended toward the eastern Indian ocean. The present results do not support such an extension. They also suggest that the inclination anomaly may have varied over 100 Kyr, and that it was more pronounced in recent times.

Geomagnetic moment instability between 0.6 and 1.3 Ma from cosmonuclide evidence

The reliability of paleomagnetic records as proxies of the geomagnetic field intensity is still a matter of controversy since volcanic materials hardly provide continuous records, and marine sediments are suspected to carry a remanence biased by post‐depositional realignments and/or by overprints. Such long standing debate emphasizes the need for the development of methods independent from paleomagnetism to decipher geomagnetic intensity variations. High resolution measurements of authigenic 10Be/9Be along with a detailed sedimentary record of directional and relative paleointensity variations evidence, over the 0.6–1.3 Ma time interval, frequent and recurrent excursions or short events in the late Matuyama and the early Brunhes epochs, among which two Brunhes‐Matuyama reversal precursors and an intra‐Jaramillo excursion. The results of this study confirm the idea of a highly unstable geomagnetic field as suggested by paleomagnetic evidences.

Development of glacial and interglacial conditions in the Nordic seas between 1.5 and 0.35 Ma

Sedimentological and geochemical proxy records of a deep-sea sediment core from the southern central Nordic seas were used to reconstruct the development of glacial and interglacial conditions during the Early and Middle Pleistocene, i.e., late Matuyama to middle Brunhes Chron (1.5–0.35 Ma). An enhancement of both glacial and interglacial characteristics is observed during early Brunhes oxygen isotope stages (OIS) 16 and 15, respectively. Any intensification of the climatic conditions prior to this, as was previously described for the eastern part of the Nordic seas, is not recognized at our study site. It is further shown that the glacial–interglacial environmental contrasts increased from the early to the middle Bruhnes Chron. Of all glacial periods investigated OIS 12 is characterized by the most severe conditions, showing both maximum input of iceberg-rafted debris (IRD) as well as planktic foraminiferal δ 18O values comparable to those of the Last Glacial Maximum. Among the interglaciations, OIS 11 is by far the longest interval and the first to show fully developed interglacial conditions, i.e., Holocene-like δ 18O values and a minimum of IRD deposition. Hence, our comparison supports bottom water δ 18O studies that have indicated the existence of a gradual intensification of glacial–interglacial climate contrasts during the Middle Pleistocene.

Akhalkalaki: The Taphonomy of An Early Pleistocene Locality in the Republic of Georgia

As part of our investigations into the potential of the Republic of Georgia for providing information on early hominin occupation of Eurasia, we report here on Akhalkalaki, a large late Early Pleistocene locality located along the lower slopes of a Miocene andesitic cone. Originally excavated in the 1950s as a palaeontological site, it was re-opened in the 1990s and stone tools were found associated with the fauna, suggesting that it is also an archaeological occurrence. Excavations in the 1950s and 1990s uncovered thousands of bones of an early Galerian fauna, including the remains of new species of Hippopotamus, Equus , and Canis (Vekua, 1962, 1987) and dominated by the remains of Equus süssenbornensis. We present the stratigraphy of the site, which together with faunal correlations and reversed paleomagnetics indicates an age most likely in the late Matuyama Chron, probably between 980,000 and 780,000 years ago. Taphonomic analysis suggests that the fauna was deposited and buried over a short time period, and was heavily modified by carnivores, but we cannot demonstrate involvement by hominins. Based on evidence of abundant krotovina (animal burrows filled with sediment) and the lack of definitive evidence for hominin modification to the bones, the stone tools at the site may have been mixed in with the older fauna. The taphonomic characteristics of the Akhalkalaki bone assemblage are not readily explained with reference to assemblage formation processes developed with actualistic studies that have been mostly conducted in Africa, including carnivore dens, predator arenas, human hunting and scavenging, mass deaths, or attritional bone deposition. Because of extreme anthropogenic modification of the present environments, the temperate setting, and the presence of mainly extinct taxa, local models based on actualistic studies cannot approximate the mammalian ecology reflected in the Akhalkalaki bone assemblage. A few comparisons are made with preliminary taphonomic observations from Dmanisi, an Early Pleistocene Homo ergaster site not far away.

Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin)

We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9–2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm kyr−1, respectively. For the older part of the record (&gt;1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference δ18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic δ18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30–60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190–1215 (Cobb Mountain Subchron), 1255, 1472–1480, 1567–1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.

Geomagnetic episodes of the last 1.2 Myr recorded in Chinese loess

The Matuyama‐Brunhes (M/B) boundary, termination of Jaramillo normal subchron (J/M) as well as three young geomagnetic excursions has been revealed in the Weinan loess section [Zhu et al., 1994,1998,1999]. Recently, we carried out further paleomagnetic directional studies on the same section and recovered the onset of the Jaramillo normal subchron (M/J) and four geomagnetic excursions, two in the Brunhes and two in the late Matuyama. The fact that all seven excursions found in this section are in loess units and expected excursions in paleosols are not found suggests that short‐lived geomagnetic signals could be only recorded in loess units, and may have been fully smoothed or wiped out in paleosol units by strong pedogenesis. Constrained by a new astronomical timescale (HLD00), ages of the geomagnetic excursions and reversal boundaries have been estimated. We tentatively correlated these excursions with previous results, indicating that a large lock‐in model may not be correct for the whole loess profile, although some apparently big positive age discrepancies were observed for the main boundaries.

Stable Isotopes in Pocket Gopher Teeth as Evidence of a Late Matuyama Climate Shift in the Southern Rocky Mountains

Previous work in the San Luis Valley of south-central Colorado and northern New Mexico at the Hansen Bluff and SAM Cave localities has allowed reconstruction of the paleoclimate in the region during the interval from ∼0.6 myr to ∼2.6 myr. Surface exposures at Hansen Bluff have been correlated to deep-sea oxygen isotope core stages 18–23 and one of the glacial periods contained therein to the oldest “Nebraskan” till in southwestern Iowa. Deposits in SAM Cave correlate to Hansen Bluff on the basis of paleomagnetics, climate interpretation, and microtine rodent biochronology. In this paper, carbon and oxygen stable isotope data of herbivorous rodent teeth are used as indicators of change in the predominance of C3, C4, and CAM plants and of changes in temperature and precipitation. Taken together with other previously analyzed data from these localities, the evidence indicates a shift in the severity of glacial events that is mainly related to the hydrologic characteristics of the climate. This shift may have been caused by broad regional uplift that may have been instrumental in shifting global climate cycles from the 41,000-yr Matuyama cycles to the Brunhes 100,000-yr-dominated cycles.

Relative geomagnetic paleointensity from the Jaramillo Subchron to the Matuyama/Brunhes boundary as recorded in a Mediterranean piston core

Piston core LC07, located west of the Sicily Strait in the Mediterranean Sea, unambiguously records the Matuyama/Brunhes (M/B) and the upper Jaramillo polarity reversals, with similar average sediment accumulation rates (SARs) for the Brunhes Chron (2.29 cm/kyr) and late Matuyama Chron C1r.1r (2.19 cm/kyr). We report a relative paleointensity record for the interval spanning the M/B boundary down into the Jaramillo Subchron, which is unique in the Mediterranean because existing records from this basin cover only the last 80 kyr. The average SAR in core LC07 is used to translate the depth-related paleointensity record to the time domain; the ratio of anhysteretic remanent magnetization to low-field magnetic susceptibility is climatically sensitive and is used to tune the age model. This correlation produces a good fit to the global ice volume model derived for summer insolation at 65°N. With this age model, a paleointensity minimum in association with the M/B boundary has a duration of about 4–5 kyr, while the directional change has a duration of <3 kyr. A second paleointensity minimum of similar duration is found about 16 kyr below the M/B boundary. This feature (precursor or ‘dip’ in the literature) has previously been recognized at the same time interval in many marine records, which reinforces the validity of our age model. Other relative paleointensity minima are found within chron C1r.1r, and, within the uncertainties of the respective age models, these minima coincide with those observed from the few published coeval paleointensity records. In particular, there is good correspondence between the ages of minima at about 0.92 and 0.89 Ma, which probably correlate with two geomagnetic excursions (Santa Rosa and Kamikatsura, respectively) that have been recorded in lava flows and dated using the 40Ar/ 39Ar technique. In contrast, a recently dated excursion at 0.83 Ma from La Palma seems to correspond to a paleointensity maximum. This observation is opposite to that expected and this excursion needs to be confirmed. In contrast to some recently published paleointensity records, spectral analysis of the LC07 record does not reveal identification of significant power at the orbital obliquity frequency.

Records of the Cobb Mountain Subchron from the Bermuda Rise (ODP LEG 172)

Duplicate records of the Cobb Mountain Subchron were obtained from two holes (Holes 1063B at 33°41.204′N, 57°36′W and 1063C at 33°41.181′N, 57°36′W) drilled in a sediment drift at Ocean Drilling Program (ODP) Site 1063 on the northeast Bermuda Rise. We sub-sampled the cores from both holes using U-channels (up to 1.5 m long samples with 2×2 cm cross-section) focusing on continuous sampling through the late Matuyama interval containing the Cobb Mountain Subchron. Results of standard progressive alternating field demagnetization experiments reveal that these sediments generally have a stable characteristic component after removal of a low-coercivity drilling overprint. Additional discrete samples (∼7 cm 3 oriented cubes) used in progressive thermal demagnetization studies display similar behavior. By taking duplicate sets of U-channels, we were able to identify and remove several measurement artifacts, such as edge effects that affect measurements made near the ends of the U-channel samples. The upper normal-to-reverse polarity transition was not obtained because of a coring gap or an interval of coring deformation. After removing measurement artifacts, the records from the two holes were stacked to obtain a composite record. Virtual geomagnetic poles (VGPs) computed from the composite record follow a path through the central Pacific during the lower polarity transition, very similar to VGP paths from Cobb Mountain Subchron records from other sites around the world. Furthermore, the VGPs cluster near southern Africa early in the reversal process and then group in a nearly antipodal patch in the north Pacific late in the reversal. Taken together, these records indicate that the transitional field remained nearly dipolar through the polarity reversal, or at least during the beginning and end of the reversal when the field was dominated by an axial dipole tilted about 20–40° to the spin axis. If a tilted dipole is an inherent part of the initiation and ending of a reversal, it may help explain why many sediment recorders, which are thought to smooth the field, record antipodal VGP paths through many reversals.