Brunhes Chron Research Articles

PADM2M: a penalized maximum likelihood model of the 0-2 Ma palaeomagnetic axial dipole moment

SUMMARY We present a new time-varying model for palaeomagnetic axial dipole moment (PADM) for the past 2 Myr and compare it with earlier virtual axial dipole moment (VADM) reconstructions which have been based on stacking and averaging scaled relative palaeointensity records. The PADM is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood (PML) approach to recover a cubic B-spline representation of axial-dipole field variations on million year timescales. The PML method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average VADM reconstructions. We apply the PML method to a set of 96 032 published data (1800 palaeointensities from igneous rocks, 3300 archaeointensities and 86 relative palaeointensity time-series of variable lengths and resolutions). Two models are discussed: PADM2Mp is a trial model based on a subset of the nine longest available sedimentary records; PADM2M uses a comprehensive data set (76 records, 81 446 data; 10 records were eliminated) and is our preferred model. PADM2M has a lower mean than existing VADM reconstructions but shows similarities in long-period variability. Some differences in timing, amplitude and resolution of certain features can be attributed to variations in age assignments. Others result from our more comprehensive data set and a reduction in bias attributable to PML modelling. PADM2M has an average axial dipole moment over 0–2 Ma of 5.3 × 1022 Am2 with a standard deviation of 1.5 × 1022 Am2. The Brunhes chron average (6.2 × 1022 Am2) is higher than for earlier epochs of Matuyama (4.8 × 1022 Am2), as seen in some previous studies. The power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 Myr−1. We see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour.

Magnetostratigraphy of the Apalós Formation (early Pleistocene): Evidence for pulsed uplift of Cyprus

[1] Landscape formation on Cyprus is controlled by processes directly linked to the uplift and unroofing of the Troodos ophiolite complex since mainly early Miocene times. Understanding the island's geology and dating individual tectonic events will help in differentiating between tectonically controlled uplift and eustatic sea level or climatic changes. In order to improve the timing of these events, a magnetostratigraphic study was carried out on two terraces in the Mesaoria Basin of central Cyprus. At Vlokkaria, southwest of Nicosia, an artificial cliff exposes the sedimentary Apalos Formation of early Pleistocene age. A nearby section situated on a terrace, probably postdating the Vlokkaria section, was sampled at Kokkinos. Remanence-carrying minerals are end-member magnetite and maghemite formed by exsolution and oxidation from ophiolitic titanomagnetite. Hematite as a late alteration product is also present. Alternating field demagnetized paleomagnetic samples yield predominantly reversed polarities interpreted to have been acquired as early detrital remanent magnetization during the Matuyama chron. Two zones of normal polarity within the Apalos Formation are interpreted to correlate to the Olduvai and Jaramillo subchrons. Pre-Apalos marine sediments at the bottom of the Vlokkaria section show transitional polarity behavior and therefore might correlate with the onset of the Reunion event. Results from the Kokkinos section are exclusively of normal polarity, which has been acquired during the Brunhes chron. Sedimentation rates derived from these magnetostratigraphic results are in the order of 3–6 cm/kyr with a marked increase to 50 cm/kyr just below the Matuyama-Jaramillo polarity transition, which is interpreted to reflect increased uplift of the source area and supports the hypothesis of pulsed uplift of the Troodos ophiolite complex.

Monospecific planktonic diatom assemblages in the Paleo-Kathmandu Lake during the middle Brunhes Chron: Implications for the paradox of the plankton

The almost monospecific assemblages of planktonic diatoms that occurred in ancient lakes during the middle Brunhes Chron are an important research issue for lake ecosystems because their existence implies that the famous paradox of the plankton was no paradox at that time. To investigate their cause, I examined the assemblage structure and diversity of planktonic diatoms in the Paleo-Kathmandu Lake at Nepal Himalaya over the past 600kyr. The assemblage structure changes frequently after 300ka but remains nearly stable before then. Additionally, planktonic species show a relatively high average diversity after 300ka but show an extremely low average before then. Those abnormal ecological conditions before 300ka reflect the formation of almost monospecific planktonic diatom assemblages of Cyclotella kathmanduensis and Puncticulata versiformis. The assemblage structure and species diversity of the planktonic diatoms were controlled by lake-level fluctuations (especially lake-level declines) as ecological disturbances. After 300ka, fluctuations in lake levels occur in about 20ka intervals, which probably helped to sustain diverse planktonic diatom assemblages. In contrast, much longer intervals occurred between lake-level fluctuations before 300ka, and these were of a frequency that was too low to sustain diverse planktonic diatom assemblages. This may have allowed C. kathmanduensis and P. versiformis to form monospecific assemblages.

Magnetic properties of rocks of the Kapuskasing uplift (Ontario, Canada) and origin of long-wavelength magnetic anomalies

SUMMARY The sources of long wavelength magnetic anomalies (LWA) in the crust are poorly understood. We have measured remanent and induced magnetizations of 210 samples of anorthosite, tonalite and mafic gneiss from the Kapuskasing uplift, an exposed partial crustal crosssection in northwestern Ontario, Canada. Anorthosites have generally high Q ratios of remanent/induced magnetization, in the range 0.3–60, and their natural remanent magnetization (NRM) is resistant to both thermal and alternating-field (AF) demagnetization. However, anorthositeNRMs(0.001–0.3Am −1 )aretooweaktoexplainLWAamplitudes.Maficgneisses also have relatively high Q ratios, peaking in the range 1–10, and the NRM is resistant to thermal and AF demagnetization. NRM and induced magnetization (IM) intensities are in the ranges 0.01–2 and 0.01–0.6 A m −1 , respectively. Tonalites have a bimodal distribution of magnetization. The more strongly magnetic group has both NRM and IM intensities in the range 0.1–5 A m −1 and wide-ranging Q values, from 0.1 to 10 approximately. Some tonalites could be an LWA source, although the long-term stability of their NRMs at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 ◦ C. In general, Q values measured at surface temperatures overestimate remanence at depth. In the deep crust, IM remains more or less constant but remanence decreases both reversibly and irreversibly, leading to Q ratios of <0.2–0.3 for multidomain grains and ≈1–3 for single-domain grains near their blocking temperatures. Thermoviscous magnetization over the Brunhes chron could add substantially to the effective induced magnetization. Typically induced + thermoviscous magnetization in the direction of the present Earth’s field will outweigh remanence in the direction of an ancient field as a source of LWA originating in the deepest crust. Remanence may play a larger role for mid-crustal sources where single-domain grains are well below their blocking temperatures.

Paleomagnetic evidence for multiple late Cenozoic glaciations in the Tintina Trench, west-central Yukon, CanadaThis article is a companion paper to Duk-Rodkin et al., also in this issue.

The Tintina Trench in west-central Yukon has preserved an extensive record of late Cenozoic preglacial, glacial, and interglacial deposits. These deposits comprise multiple sequences of tills, outwash, loesses, and paleosols. The sediments that were laid down directly by ice (tills) are of both local (montane) and regional (Cordilleran) provenance. The Tintina Trench area was impacted repeatedly by montane ice from the southern Ogilvie Mountains to the northwest (2500 m above sea level (asl)), and also repeatedly along its southern extent by Cordilleran ice from the Selwyn Mountains to the east (2759 m asl), the latter forming the continental divide in this region. We report here the magnetostratigraphy of three sections: Rock Creek (64°13′N, 139°07′W), West Fifteenmile River (64°29′N, 139°55′W), and East Fifteenmile River (64°23′N, 139°48′W). The majority of the units identified at these sections record late Pliocene to mid-Pleistocene glaciations, although relatively thin surficial sequences of late middle Pleistocene to late Pleistocene loesses and tills are present as well. Of the 11 units described in the Tintina Trench, seven have normal polarity, three have reversed polarity, and one has an undefined polarity. These units span about 3.0 million years. It appears that most of the polarity chrons and subchrons of the late Cenozoic are present and that the sequence of six reversals record at least 10 glaciations (three in the Brunhes Chron and seven in the Matuyama Chron), and 11 interglaciations (four in the Brunhes Chron and seven in the Matuyama Chron). The interglacials are recorded as either paleosols or unconformities between glacial or loess units having opposite polarity. While not all Matuyama Chron glacial and interglacial cycles recorded in marine isotopic records are seen on land, the terrestrial records found in the Tintina Trench have thus far proven to be the most complete in terms of the polarity record. While no absolute ages were obtained from the sediments in the trench, the extensive polarity sequence constrains the timing of glaciations to a considerably greater degree than was previously possible for this region. The magnetostratigraphy of the trench sites are compared with the glacial, glaciofluvial, and loessic deposits at the nearby Klondike River valley and Fort Selkirk sites, central Yukon, where tephras and basalts provide absolute ages, and stratigraphic units contain an extensive late Cenozoic climate proxy for northwestern North America (eastern Beringia). In this study, we present new paleomagnetic polarity data and establish a magneto-lithostratigraphy describing preglacial, glacial, and interglacial deposits in the Tintina Trench. These deposits are referred to as the West Tintina Trench Allogroup and provide a broad framework for establishing a paleoclimate record for the northern Canadian Cordillera.

Lentic Basommatophora molluscs and hygrophilous land snails as indicators of habitat and climate in the Early-Middle Pleistocene (0.78 Ma) at the site of Gesher Benot Ya‘aqov (GBY), Israel

The lentic Basommatophora molluscs and hygrophilous land snails of the Early-Middle Pleistocene site of Gesher Benot Ya‘aqov (GBY), 0.78 Ma, crossed by the Matuyama–Brunhes chron boundary [MBB] were studied in order to reconstruct their specific habitats and possible reactions to climate change along the site sequence. Samples of equal sizes from 27 of the 46 layers along the 100 k.yr. time-span of the site were examined. About 2000 specimens of 21 lentic and hygrophilous species belonging to five families: Planorbidae (11), Lymnaeidae (6), Acroloxidae (2), Carychiidae (1) and Succineidae (1) were identified. The family with the largest biodiversity is the Planorbidae and of these, the most abundant species include Gyraulus piscinarum (937), Planorbarius corneus (210) and Radix labiata (160). The recent known zoogeographic origin of 81% of the species is Palaearctic and Holarctic. The MBB coincides with remarkable environmental changes reflected in molluscs, other faunal and floral elements and stable isotopes. The Planorbidae and Lymnaeidae reach greater abundance (90% and 80% of their assemblages, respectively) pre-MBB, while Acroloxidae, Succineidae and Carychiidae are more abundant (74%, 64% and 90%) post-MBB. Our data indicate that GBY molluscs show a two-phase pattern (shallow and deep lake) in each of the five defined cycles. Their numbers increase during the shallow water phases, thus the site climate changes from cold and humid in the oldest layers, to dry and cold up to the MBB and few succeeding layers. Between cycles 2 and 3, post-MBB, we see a short period of warm and humid climate that enables the influx of African and Asian elements. At the same time, cold climate species of Euro-Siberian and Palaearctic origin disappear. The succeeding layers indicate a cooler and humid climate. Finally, the sequence indicates deep water in the oldest layers and desiccation towards MBB and deeper water post-MBB.

Paleomagnetic record of the late Pleistocene reef sequence of Tahiti (French Polynesia): A contribution to the chronology of the deposits

Rock- and paleomagnetic investigations were performed on the late Pleistocene reef sequence of Tahiti, and take part of a multidisciplinary study of the Holocene and Pleistocene reef sequences, carried out by the IODP (Integrated Ocean Drilling Program) Expedition 310. The objectives were to characterize the natural remanent magnetization and its mineral carriers, in order to identify high-resolution magnetostratigraphic markers of the late Brunhes chron. A series of 85 specimens subsampled within a 79 m-thick silty-carbonate sequence cored at the Maraa site (SW of Tahiti) display a strong and stable natural remanent magnetization (NRM) carried by titanomagnetite inherited from the basalts which form the bulk of the island. The average negative inclination is assignable to the field generated by the normal polarity axial dipole field at the site latitude. A few specimens (103–119 m below modern sea level) carry a reversed primary magnetization acquired in a weak excursional field. Based on subsidence rate estimates ranging from 0.25 to 0.4 m/kyr, this suggests the identification either the Laschamp excursion (∼ 40 ka) or the Blake event (115–120 ka). Radiometric dates of ∼ 131 to ∼ 134 ka at 117 m below modern sea level confirm that these layers were deposited during the marine isotope stage 5.5 and recorded the direction and intensity anomaly associated with the Blake event. Another sea-level highstand reef unit, occurring deeper in the Pleistocene sequence has seemingly recorded another paleomagnetic intensity anomaly which is not yet definitely identified. Reef systems, which developed around volcanic islands, contain geological records of excursions. But, it is necessary to improve the understanding of NRM acquisition and the role of carbonate diagenesis in order to establish radiometrically dated records of geomagnetic excursions.

Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China

SUMMARY Magnetic polarity stratigraphy has proved to be useful in dating of both marine and terrestrial sedimentary sequences over the world. However, the reliability of magnetostratigraphic results of red soil sequences in subtropical southern China has been found to be variable in different regions. To probe into the capabilities of recording magnetic polarity stratigraphy in the red soils, three red soil sequences have been selected for detailed mineral magnetic, petrographic and/or palaeomagnetic analyses. These include the Xuancheng and Qiliting sequences, which are located in the lower reaches of the Yangtze River and the Damei sequence in the Bose Basin near the Tropic of Cancer. Palaeomagnetic results indicate that the Xuancheng sequence has recorded the Brunhes Chron and the late Matuyama Chron, including a short interval probably representing the Santa Rosa geomagnetic event. The Xuancheng and Qiliting sequences have faithfully recorded the palaeogeomagnetic field behaviour while the Damei sequence has failed. Detailed mineral magnetic and petrographic measurements suggest that four magnetic minerals (magnetite, maghemite, haematite and goethite) are contained in the three studied sequences, but the characteristic remanent magnetization (ChRM) carriers are different. For the Xuancheng and Qiliting samples, the ChRM carriers are magnetite and haematite, which are all of detrital origin. However, the ChRMs of Damei samples are carried by pedogenic haematite with an unblocking temperature (TB) of about 630–640 °C, which has overprinted the primary remanence of the Damei red soil deposits. This pedogenic haematite is mainly produced by chemical weathering, which is commonly intensified by the climate of high temperature and rainfall. It further suggests that climatic conditions have a great effect on the nature of palaeomagnetic signals of the red soil sequences. In addition, the red pigment components in the red soils may be used to indicate the degree of remagnetization.

Geomagnetic paleointensity deduced for the last 300 kyr from Unzen Volcano, Japan, and the dipolar nature of the Iceland Basin excursion

We have conducted paleointensity measurements of lavas and pyroclastic flows from Unzen Volcano, Japan. They have been shown in a previous study to contain a record of Brunhes Chron secular variation, with one flow recording a transitional direction (Senbongi geomagnetic excursion; 191±17ka). Various rock magnetic analyses indicate that the main magnetic carriers of samples are Ti-poor titanomagnetites, which have mixtures of single domain (SD) (and/or pseudo-SD) and multi-domain grain sizes. The low-temperature demagnetization (LTD)-double heating test (DHT) Shaw method yielded 209 successful results from 56 sites, giving 27 acceptable site-mean paleointensities for the past 5–300kyr. When excluding the Senbongi excursion data and two other data points, the data give an average virtual dipole moment (VDM) of 7.69×1022Am2 with a standard deviation of 2.51×1022Am2 (N=24). This is consistent with the selected 80 site-mean Thellier paleointensity data from the latest paleointensity database, with an average VDM of 7.64×1022Am2 and a standard deviation of 2.56×1022Am2. Paleointensity measurements on samples that record the Senbongi excursion, which is claimed to correlate with the Iceland Basin excursion, provide a value of 7.74–9.37μT using the LTD-DHT Shaw method. This corresponds to an average (standard deviation) VDM of 2.19×1022Am2 (0.21×1022Am2). When calibrating sedimentary records that capture the Iceland Basin excursion by averaging the LTD-DHT Shaw VDMs obtained from the pre- and post-excursional period, the records give consistent VDM values with the volcanic (Senbongi) record during the excursion. This indicates that variation of the geomagnetic field direction and intensity during the Iceland Basin excursion can be well represented by a simple dipolar variation.

Origin of apparent magnetic excursions in deep‐sea sediments from Mendeleev‐Alpha Ridge, Arctic Ocean

Arctic deep‐sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick, implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere, and the lack of tight age control usually provides excessive freedom in the labeling of Arctic excursions. Fortuitous variations in sedimentation rate have been invoked to explain the “amplified” excursions. Alternating field demagnetization of natural remanent magnetization (NRM) of sediment cores 08JPC, 10JPC, 11JPC, and 13JPC recovered by the Healy Oden Trans‐Arctic Expedition in August 2005 (HOTRAX05) to the Mendeleev‐Alpha Ridge yields apparent magnetic “excursions” in sediments deposited in the Brunhes Chron. Thermal demagnetization of the NRM, however, implies multiple magnetization components with negative inclination components usually “unblocked” below ∼350°C. Analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another. Magnetic experiments conducted under high and low temperatures show features that are characteristic for (titano)magnetite and titanomaghemite. Presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for X‐ray energy dispersive spectroscopy (EDS) and by high‐resolution X‐ray diffraction (XRD). It is unlikely that anomalously thick intervals of negative inclination in these Brunhes‐aged sediments are caused by unusual behavior of the magnetic field in the Arctic area. We therefore attribute low and negative NRM inclinations in these cores to partially self‐reversed chemical remanent magnetizations, apparently carried by titanomaghemite and acquired during the oxidation of detrital (titano)magnetite grains. The high Ti contents and high oxidation states indicated by EDS and XRD data provide the conditions required for partial self‐reversal by ionic reordering during diagenetic maghemitization, and this process appears to have affected all HOTRAX05 cores collected from the Mendeleev‐Alpha Ridge.

Rock magnetism and palaeomagnetism of the Montalbano Jonico section (Italy): evidence for late diagenetic growth of greigite and implications for magnetostratigraphy

SUMMARY The Montalbano Jonico (MJ) section, cropping out in Southern Italy, represents a potential candidate to define the Lower/Middle Pleistocene boundary and it has been proposed as a suitable Global Stratotype Section and Point (GSSP) of the Ionian Stage (Middle Pleistocene). The MJ section is the only continuous benthic and planktonic δ18O on-land reference in the Mediterranean area for the Mid-Pleistocene transition, spanning an interval between about 1240 and 645 ka. Combined biostratigraphy and sapropel chronology, tephra stratigraphy and complete high-resolution benthic and planktonic foraminiferal stable oxygen isotope records already provide a firm chronostratigraphic framework for the MJ section. However, magnetostratigraphy was still required to precisely locate the Brunhes-Matuyama transition and to mark the GSSP for the Ionian stage. We carried out a palaeomagnetic study of a subsection (Ideale section) of the MJ composite section, sampling 61 oriented cores from 56 stratigraphic levels spread over a ca. 80-m-thick stratigraphic interval that correlates to the oxygen isotopic stage 19 and should therefore include the Brunhes-Matuyama reversal. The palaeomagnetic data indicate a stable and almost single-component natural remanent magnetization (NRM). A characteristic remanent magnetization (ChRM) was clearly identified by stepwise demagnetization of the NRM. The ChRM declination values vary around 0° and the ChRM inclination around the expected value (59°) for a geocentric axial dipole field at the sampling locality. This result indicates that the section has been remagnetized during the Brunhes Chron. A preliminary study of 27 additional not azimuthally oriented hand samples, collected at various levels from other parts of the MJ composite section, indicates that all the samples are of normal polarity and demonstrates that the remagnetization is widespread across the whole exposed stratigraphic sequence. A series of specific rock magnetic techniques were then applied to investigate the nature of the main magnetic carrier in the study sediments, and they suggest that the main magnetic mineral in the MJ section is the iron sulphide greigite (Fe3S4). Scanning electron microscope observations and elemental microanalysis reveal that greigite occurs both as individual euhedral crystals and in iron sulphides aggregates filling voids in the clay matrix. Therefore, we infer that the remagnetization of the section is due to the late-diagenetic growth of greigite under reducing conditions, most likely resulting in the almost complete dissolution of the original magnetic minerals. Iron sulphide formation in the MJ section can be linked to migration of mineralized fluids. Our inferred timing of the remagnetization associated with greigite growth represents the longest remanence acquisition delay documented in greigite-bearing clays of the Italian peninsula so far.

Climato-chronostratigraphic framework of Pleistocene terrestrial and marine deposits of Northern Eurasia, based on pollen, electron spin resonance, and infrared optically stimulated luminescence analyses

Marine and terrestrial records spanning the Brunhes Chron (0-780 000 years) were synthesized using data from palaeoshelf, glacial, periglacial, and extraglacial zones of Northern Eurasia. The chronostratigraphic position of the identified palaeoenvironmental events and respective horizons in the composite chronostratigraphic column were established on the basis of electron spin resonance analysis of subfossil mollusc shells collected from marine, freshwater, and terrestrial deposits. Environ- ment and vegetation evolution during this period is characterized by pollen data from a series of spaced-apart reference sections on the East European Plain. The climate-chronostratigraphic record displays a sequence of eight intervals (the Holocene included) of warm climate and sea level highstand when marine sedimentation occurred on North Eurasian palaeo-shelves, and seven glacial epochs. A clear agreement between long pollen records and reliably dated warm-climate-related deposits was established for at least the last 600 000 years (from marine isotope stages 15 to 1). When integrated, these records have a potential of assigning warm/cold-climate-related deposits to the chronostratigraphically-organized sequence of the middle and late Quaternary palaeoenvironmental events.

Relative geomagnetic paleointensity of the Brunhes Chron and the Matuyama–Brunhes precursor as recorded in sediment core from Wilkes Land Basin (Antarctica)

Paleomagnetic and rock magnetic investigation was performed on the 35-m long MD03-2595 CADO (Coring Adélie Diatom Oozes) piston core recovered on the continental rise of the Wilkes Land Basin (East Antarctica). Analysis of the characteristic remanent magnetization (ChRM) inclination record indicates a normal magnetic polarity for the uppermost 34 m of the sequence and a distinctive abrupt polarity change at the bottom of the core. This polarity change, which spans a 27 cm thick stratigraphic interval, represents a detailed record of the Matuyama–Brunhes (M–B) transition and it is preceded by a sharp oscillation in paleomagnetic directions that may correlate to the M–B precursor event. Paleomagnetic measurements enable reconstruction of geomagnetic relative paleointensity (RPI) variations, and a high-resolution age model was established by correlating the CADO RPI curve to the available global reference RPI stack, indicating that the studied sequence reaches back to ca. 800 ka with an average sedimentation rate of 4.4 cm/ka. Orbital periodicities (100 ka and 41 ka) were found in the ChRM inclination record, and a significant coherence of ChRM inclination and RPI record around 100 ka suggests that long-term geomagnetic secular variation in inclination is controlled by changes in the relative strength of the geocentric axial dipole and persistent non-dipole components. Moreover, even if the relatively homogeneous rock magnetic parameters and lithofacies throughout the recovered sequence indicates a substantial stability of the East Antarctic Ice Sheet during the middle and late Pleistocene, influence of the 100 ka and 41 ka orbital periodicities has been detected in some rock magnetic parameters, indicating subtle variations in the concentration and grain-size of the magnetic minerals linked to orbital forcing of the global climate.

Chronology of the transition from a spreading ridge to an accretional seamount in the Marsili backarc basin (Tyrrhenian Sea)

AbstractInversion of new high‐resolution magnetic data from the Marsili seamount and the surrounding basin in the Tyrrhenian Sea reveals NNE–SSW magnetization stripes ranging from the Matuyama chron to the Brunhes chron, including the short positive Jaramillo subchron. The detailed magnetic chronology shows that from the late Matuyama (1.77 Ma), the average half spreading rate was about 1.5 cm yr−1, with a slight decrease between the Jaramillo and the Brunhes events, when the growth of the volcanic edifice overcame lateral spreading. Analysis of spreading rate and volume of erupted lava indicates that at the beginning of the Jaramillo subchron (1.07 Ma), the Marsili basin evolved from pure horizontal spreading to a superinflated seamount as a consequence of tearing of the Ionian slab. Our data give us a snapshot of the geodynamic transition from an active backarc spreading phase to the vertical accretion of the seafloor because of a radical change in the subduction dynamics.

Determination of neotectonic features of the Karasu Basin (SE Turkey) and their relationship with Quaternary volcanic activity using Landsat ETM+ imagery

The junction between the East Anatolian and the Dead Sea fault zones lies close to an unstable F: Transform fault (FFF) triple junction where the African, Eurasian and Arabian plates meet in south-eastern Turkey. The Karasu Basin is an ephemeral rifted structure located close to the junction of these plate boundaries and is expressed by a range of tectono-morphological features. This study uses remote sensing to define tectonic structures and discriminate volcanic rocks linked to rifting within the basin using Landsat Enhanced Thematic Mapper Plus (ETM+) imagery. Prior to advanced image processing stages, images were corrected to eliminate atmospheric scattering effects and determine the best band combination for multi-spectral processing techniques based on statistical methods. Following pre-processing stages, edge detection filters were applied to derive tectonic structures defining the basin. The Brovey transformation, a statistical data merging method, was used to combine reflective multi-spectral bands with the Landsat ETM+ panchromatic band. This method fuses higher spatial data with data of lower spatial value. The western margin of the Karasu Basin is defined by the left-lateral Amanos Fault Zone with a contemporary motion (c. –0.4 mm a−1) probably accommodating most of the left-lateral strike-slip motion between Arabian and African plates on the northern continuation of the Dead Sea Fault Zone. The rift zone has been the site of extensive recent volcanism concentrated within the Brunhes Chron (<0.78 Ma) and linked to fault block rotations between the intracontinental master faults. The boundaries of this activity are defined here using a regolith mapping technique.

Gold Run tephra: a Middle Pleistocene stratigraphic and paleoenvironmental marker across west-central Yukon Territory, Canada

Gold Run tephra has been found at Thistle Creek, Sixtymile River, and the Klondike goldfields of west-central Yukon, Canada. It is a hornblende-bearing rhyolitic tephra with thicknesses of up to 10 cm at each site, suggesting a widespread distribution across interior Alaska and the Yukon Territory, given the long distance to the nearest volcanic centre. Old Crow, Flat Creek, and TA tephra beds are stratigraphically associated with Gold Run tephra at our study sites and have distinctive compositions. Gold Run tephra is not accommodated by the current classification scheme for late Cenozoic distal tephra beds in Alaska and the Yukon Territory — a scheme based on the physical and chemical attributes — so that its provenance is unknown. An early-Middle Pleistocene age is supported by a glass fission-track age of 0.74 ± 0.06 Ma, a normal remanent magnetic polarity for the enclosing loess, and the presence of a late Irvingtonian faunal assemblage in the associated organic-rich silts at Thistle Creek. Hence, Gold Run tephra was deposited during the very early part of the Brunhes Chron, at which time a shrub tundra environment prevailed across west-central Yukon.

Self-reversal and apparent magnetic excursions in Arctic sediments

The Arctic oceans have been fertile ground for the recording of apparent excursions of the geomagnetic field, implying that the high latitude field had unusual characteristics at least over the last 1–2 Myrs. Alternating field demagnetization of the natural remanent magnetization (NRM) of Core HLY0503-6JPC from the Mendeleev Ridge (Arctic Ocean) implies the presence of primary magnetizations with negative inclination apparently recording excursions in sediments deposited during the Brunhes Chron. Thermal demagnetization, on the other hand, indicates the presence of multiple (often anti-parallel) magnetization components with negative inclination components having blocking temperatures predominantly, but not entirely, below ~ 350 °C. Thermo-magnetic tests, X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the negative inclination components are carried by titanomaghemite, presumably formed by seafloor oxidation of titanomagnetite. The titanomaghemite apparently carries a chemical remanent magnetization (CRM) that is partially self-reversed relative to the detrital remanent magnetization (DRM) carried by the host titanomagnetite. The partial self-reversal could have been accomplished by ionic ordering during oxidation, thereby changing the balance of the magnetic moments in the ferrimagnetic sublattices.

La dorsal NE de Tenerife: hacia un modelo del origen y evolución de los rifts de islas oceánicas

The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evi-dence of the origin and dynamics of these major volcanic features. The rift developed in three successi-ve, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quies-cence or reduced activity: A Miocene stage (7266 ± 156 ka), apparently extending the central Mioceneshield of Tenerife towards the Anaga massif; an Upper Pliocene stage (2710 ± 58 ka) and the lateststage, with the main eruptive phase in the Pleistocene.Detailed geological (GIS) mapping, geomagnetic reversal mapping and stratigraphic correlation, andradioisotopic (K/Ar) dating of volcanic formations allowed the reconstruction of the latest period of riftactivity. In the early phases of this stage the majority of the eruptions grouped tightly along the axis ofthe rift and show reverse polarity (corresponding to the Matuyama chron). Dykes are of normal andreverse polarities. In the final phase of activity, eruptions are more disperse and lavas and dykes areconsistently of normal polarity (Brunhes chron). Volcanic units of normal polarity crossed by dykes ofnormal and reverse polarities yield ages apparently compatible with normal subchrons (M-B Precursorand Jaramillo) in the Upper Matuyama chron. Three lateral collapses successively mass-wasted the rift:The Micheque collapse, completely concealed by subsequent nested volcanism, and the Guimar and LaOrotava collapses, that are only partially filled. Time occurrence of collapses in the NE rift apparentlycoincides with glacial stages, suggesting that giant landslides may be finally triggered by sea level chan-ges during glaciations. Pre-collapse and nested volcanism is predominantly basaltic, except in theMicheque collapse, where magmas evolved towards intermediate and felsic (trachytic) compositions.Rifts in the Canary Islands are long-lasting, recurrent features, probably related to primordial, plume-rela-ted fractures acting throughout the entire growth of the islands. Basaltic volcanism forms the bulk of theislands and rift zones. However, collapses of the flanks of the rifts disrupt their established fissural feedingsystem, frequently favouring magma accumulation and residence at shallow emplacements, leading to diffe-rentiation of magmas, and intermediate to felsic nested eruptions. Rifts and their collapse may therefore act asan important factor in providing petrological variability to oceanic volcanoes. Conversely, the possibility existsthat the presence of important felsic volcanism may indicate lateral collapses in oceanic shields and ridge-likevolcanoes, even if they are concealed by post-collapse volcanism or partially mass-wasted by erosion.

The influence of sea-level changes on tropical coastal lowlands; the Pleistocene Coropina Formation, Suriname

The Pleistocene Coropina Formation largely constitutes the Old Coastal Plain of Suriname. It is exposed fully only in open-pit bauxite mines in the central coastal plain as part of the unconsolidated overburden of Paleocene–Eocene bauxites. This study deals with the stratigraphy, sedimentology and chronology of this formation, and is based on a study in the recently closed Lelydorp-III bauxite mine operated by N.V. BHP Billiton Maatschappij Suriname. The Coropina Formation consists of the Para and Lelydorp Members. We present a detailed lithological subdivision of these members. In the Para Member, four units are discerned which are grouped in two transgressive cycles, both ranging upward from terrestrial towards chenier and coastal mudflat deposits reflecting glacio-eustatic sea-level changes. The sandy sediments represent fluviatile and beach-bar (chenier) deposits, and were supplied by rivers from the Precambrian basement and to a lesser extent by westward longshore coastal drift. Clays, largely derived from the Amazon River and transported alongshore over the shelf, were deposited in extensive coastal mudflats. The Lelydorp Member, also comprising four units, represents a depositional system that is closely comparable to the recent Suriname coastal setting, i.e., a lateral and vertical alternation of mudflat and chenier deposits formed over a period characterised by more or less constant sea level. Palaeomagnetic data indicate a dominantly reversed magnetic polarity in the Para Member, whereas the Lelydorp Member shows a normal magnetic polarity with a minor reversed polarity overprint. The reversed polarities of the Para Member exclude a Brunhes Chron (0.78–0.0 Ma) age, and thus assign it to the Matuyama Chron (2.58–0.78 Ma). This implies that the Coropina Formation is much older than hitherto assumed, and that one or more (long-term) hiatuses are not recognizable in the lithological succession.

Sedimentation, geochemistry and palaeomagnetism of the West Runton Freshwater Bed, Norfolk, England

The sedimentary succession of the West Runton Freshwater Bed (WRFB) is described and interpreted. The sediments represent a fluvial valley floor accumulation that trends west- to east, subparallel to the modern cliffline. Stabilisation of the stream channels was initially followed by deposition of fine-grained silt in standing water, punctuated by intermittent flood events. Decreased flood intensity resulted in vertical accretion of fine-grained, organic, fossiliferous sediments in carbonate-rich spring water.Increased organic deposition produced a transition to black detrital sediment. Periodic flooding continued, but flood frequency decreased in response to reduction in accommodation. Local erosion of channel banks and floors also occurred. The sediments were disturbed by water-release structures and bioturbation, the latter resulting from large mammals locally entering the channel and pools, churning the deposits during walking, bathing and so on. River-flow velocity diminished later as forest vegetation stabilised catchment and valley-floor ground surfaces. Stabilisation of the channel led to deposition from low-velocity flowing and standing water. The uppermost part of the succession indicates vertical accretion, sheet-like deposition. The organic silts were deposited in an anoxic environment, and organic sulphur from rotting organic matter allowed formation of early diagenetic sulphides.Deposition began during the late-glacial (Beestonian Stage) and continued into the first half of a temperate-climate, interglacial event (the Cromerian Stage s.s.). The infill comprises an overall fining-upward sequence, the stream adopted a stable meandering pattern. The WRFB stream showed typical chalk-stream behaviour.The magnetic polarity of the WRFB sediments is normal. The palaeomagnetic data indicates that the sediments are of Brunhes Chron age, and imply that they post-date mid-MIS 19.