Cobb Mountain Subchron Research Articles

Earliest evidence of human occupations and technological complexity above the 45th North parallel in Western Europe. The site of Lunery-Rosieres la-Terre-des-Sablons (France, 1.1 Ma)

The site of LuneryRosieres la-Terre-des-Sablons (Lunery, Cher, France) comprises early evidence of human occupation in mid-latitudes in Western Europe. It demonstrates hominin presence in the Loire River Basin during the Early Pleistocene at the transition between an interglacial stage and the beginning of the following glacial stage. Three archaeological levels sandwiched and associated with two diamicton levels deposited on the downcutting river floor indicate repeated temporary occupations. Lithic material yields evidence of simple and more complex core technologies on local Jurassic siliceous rocks and Oligocene millstone. Hominins availed of natural stone morphologies to produce flakes with limited preparation. Some cores show centripetal management and a partially prepared striking platform. The mean ESR age of 1175 ka ± 98 ka obtained on fluvial sediments overlying the archaeological levels could correspond to the transition between marine isotopic stages (MIS) 37 and 36, during the normal Cobb Mountain subchron, and in particular at the beginning of MIS 36. The Lunery site shows that hominins were capable of adapting to early glacial environmental conditions and adopting appropriate strategies for settling in mid-latitude zones. These areas cannot be considered as inhospitable at that time as Lunery lies at some distance from the forming ice cap.

New chronological constraints for the lowermost stratigraphic unit of Atapuerca Gran Dolina (Burgos, N Spain)

We present the results of a combined Electron Spin Resonance (ESR) and Luminescence dating study for the lowermost stratigraphic unit (TD1) of the palaeoanthropological site of Atapuerca Gran Dolina, Spain. Ten samples collected through the Main Section were dated using either the Multiple Centre (MC) ESR approach or the single-grain thermally transferred optically stimulated luminescence (SG TT-OSL) technique, both of which were applied to quartz grains. The two methods yield consistent ages, providing a robust chronostratigraphic framework for the TD1 deposits and enabling improved correlation of the magnetostratigraphic succession (comprising three intervals of normal polarity; N1 to N3, from bottom to top) with the Geomagnetic Polarity Time Scale. Specifically, the results provide a late Early Pleistocene chronology for TD1, and clearly position the deposits in the post-Olduvai time range (<1.77 Ma). The final age estimates range from 1.30 ± 0.14 Ma (TT-OSL) close to the bottom of the section to a mean ESR age of 1.05 ± 0.12 Ma (1 s.d.) for three samples from the N3 interval towards the top. Consequently, the N3 interval may confidently be correlated to the Jaramillo Subchron, while we propose a correlation of N1 and N2 with the Cobb Mountain subchron and Punaruu excursion, respectively; although we cannot exclude that these two normal intervals may correspond to other, unknown, geomagnetic events given the existing uncertainties associated with the ESR and luminescence ages. From a methodological point of view, the ESR MC approach shows that the Titanium signal provides the most reliable burial ages for TD1. In contrast, the Aluminium centre yields systematically overestimated age estimates, as a result of incomplete resetting of the signal before sediment deposition. The SG TT-OSL ages presented here are amongst the oldest published so far, and highlight the potential of the high temperature SG TT-OSL 290 approach for obtaining finite ages exceeding 1 Ma at Atapuerca. Finally, these new dating results provide key insights into the rates and nature of sedimentary processes at Gran Dolina, modifying long-standing interpretations of the cave formation and infilling dynamics. An age of at least 1.4 Ma may be inferred for the base of the sedimentary infill at the Main Section, providing minimum age constraint for the formation of the cavity and, by extension, for the intermediate level of the Atapuerca karst. The opening of the cave, which pre-dates human occupation, most likely occurred close to or slightly after 0.9 Ma. This suggests that both TD4 and TD6 archaeological levels were deposited within a relatively short time range of <100 kyr. The so-called autochthonous fluvial deposits from TD1 unit, or at least those dated in the present study, were also clearly primarily derived from outside the karst system, and were not reworked from older deposits within the karst. They experienced sufficient daylight exposure and minimum transport times within the karst system to enable consistency between the ESR and TT-OSL burial ages and the independent magnetostratigraphic evidence. Our results highlight the need to reconsider the terminology traditionally employed to differentiate the two main phases of Gran Dolina's sedimentary infill. New numerical age results for stratigraphic unit TD1 of Atapuerca Gran Dolina, Spain The presence of Jaramillo magnetic Subchron (1.07-0.99 Ma) is confirmed The base of the sedimentary infill post-dates Oduvai Subchron (<1.77 Ma) Cave opening occurred ca. 0.9 Ma ago, suggesting that TD4-6 units deposited in < 100 kyr TD1 fluvial deposits do not originate from within the karst

Integrated Pliocene-Pleistocene magnetostratigraphy and tephrostratigraphy of deep-sea sediments at IODP Site U1424 (Yamato Basin, Japan Sea)

Sediments from the semi-enclosed Japan Sea are sensitive to paleoclimatic perturbations and they offer great opportunities for many regional and global paleoceanographic and paleoclimatic studies. These studies often require a robust chronology. However, due to rare preservation of calcareous microfossils and drastic changes in surface water salinity during glacial lowstands, the construction of a traditional oxygen isotope stratigraphy for Japan Sea sediments is often difficult. Here, we use sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 346 Site U1424 to build an integrated Pliocene-Pleistocene reference magnetostratigraphy and tephrostratigraphy for sedimentary sequences from the region. Rock magnetic experiments indicate that magnetic remanence of Site U1424 sediments are carried primarily by (titano)magnetite with small contributions from high coercivity minerals (e.g., hematite) and possibly iron sulphides (pyrrhotite and/or greigite). Dark-colored sediments appear to contain less (titano)magnetite probably due to reductive diagenesis under euxinic conditions. Natural remanent magnetization (NRM) of u-channel samples covering a continuous ~ 167.6 m sediment sequence at Site U1424 was repeatedly measured at 1 cm intervals before and after stepwise demagnetization. Despite lower NRM intensity in dark-colored sediments, NRM directional data from both dark- and light-colored sediments are considered suitable for the construction of magnetostratigraphy. Site U1424 sediments clearly recorded the majority of the polarity chrons and subchrons within the last ~ 4.89 Myr, with the Cobb Mountain subchron, the end of Kaena subchron, and the onset of Nunivak subchron less well preserved. Sixteen tephra layers from the site were sampled for chemical composition analyses and the results were correlated to reference tephrostratigraphy of the region. Ages of the identified tephras are consistent with and can be well integrated with the magnetostratigraphy. The resulting age model suggests that sedimentation rates at Site U1424 range between ~ 1.7 and 7.6 cm/kyr with an average of ~ 3.3 cm/kyr. The acquired magnetostratigraphy and tephrostratigraphy at Site U1424 provide a reference chronology that can be correlated with and transferred to other sediment sequences in the region to study paleoceanographic and paleoclimatic changes of the region as well as their links to other regional and global changes.Graphical abstract

Cobb Mountain Subchron recorded at IODP Site U1306 (Eirik Drift, off SE Greenland)

The Cobb Mountain Subchron (CMS) is recorded over ∼5 m of core at ∼180 metres composite depth in the sediment sequence recovered at IODP Site U1306, in an interval where sedimentation rates are estimated at ∼15 cm kyr−1. The relatively high-resolution record of the CMS was acquired from u-channel samples from multiple core sections from four holes drilled at the site. Large-scale swings in magnetization directions at the onset of the subchron lead to virtual geomagnetic poles (VGPs) looping over the Indian Ocean, followed by a looping transit northward through the central Pacific Ocean, a ∼10 kyr (∼1.7 m) interval of VGPs at high northern latitudes particularly in the North Atlantic and NW Pacific Oceans, followed by an abrupt normal to reverse transition as VGPs track southward through Africa. Although the VGP paths are intricate and complex, they can be compared with other CMS VGP paths from Pacific and Atlantic sites, including ODP Sites 983 and 984. The age model for Site U1306 places the CMS in the 1178–1215 ka interval (∼37 kyr duration) which bridges marine isotope stages 35–36.

Quaternary magnetic and oxygen isotope stratigraphy in diatom-rich sediments of the southern Gardar Drift (IODP Site U1304, North Atlantic)

The sediment sequence from Integrated Ocean Drilling Program (IODP) Site U1304 (53°03.40′N, 33°31.78′W; water depth, 3024 m) on the southern Gardar Drift, North Atlantic Ocean, covers the last ∼1.8 Myr with a mean sedimentation rate of ∼15 cm/kyr. At Site U1304, paleomagnetic directional and relative paleointensity (RPI) records have been generated for the last ∼1.5 Myr, and benthic oxygen isotope data for the last ∼1 Myr. The age model for Site U1304 was established by matching δ18O and RPI data to calibrated reference records. Prominent intervals of diatom mats at Site U1304 are associated with the latter stages of interglacial marine isotope stages (MIS) 9, 11, 13, 15, 21, 27 and 35, and with the weak glacial of MIS 14. The deposition of diatom mats, and associated diluted magnetic mineral concentrations, leads to weak magnetizations and susceptibilities in these intervals. Comparison of histograms of paleomagnetic directions and RPI from diatom-rich sediments and from surrounding silty clays indicates that, although results from diatom-rich sediments are more scattered, the occurrence of diatom mats does not appreciably distort the paleomagnetic directional and RPI records. Site U1304 sediments record the Matuyama/Brunhes boundary (∼772 ka), the Jaramillo Subchron (∼993–1071 ka) and the Cobb Mountain Subchron (∼1193–1219 ka), as well as three apparent geomagnetic excursions in the Matuyama Chron, interpreted as the “Kamikatsura/Santa Rosa”, “Punaruu”, and “Gardar” excursions, at ∼888 ka, ∼1124 ka, and ∼1463 ka, respectively. The Site U1304 RPI record can be correlated with the PISO-1500 RPI stack and with other high-resolution RPI records from the North Atlantic Ocean. Wavelet analyses performed on the Site U1304 RPI record and a new high-resolution North Atlantic paleointensity stack for the last 1500 kyr (HINAPIS-1500), comprising Ocean Drilling Program (ODP) Sites 983, 984 and IODP Sites U1306 and U1304, did not reveal significant orbital power.

Dating the Homo erectus bearing travertine from Kocabaş (Denizli, Turkey) at at least 1.1 Ma

Since its discovery within a travertine quarry, the fragmentary cranium of the only known Turkish Homo erectus, the Kocabaş hominid, has led to conflicting biochronological estimations. First estimated to be ∼500 ka old, the partial skull presents a combination of archaic and evolved features that puts it as an intermediate specimen between the Dmanisi fossils (Homo georgicus) and the Chinese Zhoukoudian skulls (Homo erectus) respectively dated to 1.8 to ∼0.8 Ma. Here we present a multidisciplinary study combining sedimentological, paleontological and paleoanthropological observations together with cosmogenic nuclide concentration and paleomagnetic measurements to provide an absolute chronological framework for the Upper fossiliferous Travertine unit where the Kocabaş hominid and fauna were discovered. The 26Al/10Be burial ages determined on pebbles from conglomeratic levels framing the Upper fossiliferous Travertine unit, which exhibits an inverse polarity, constrains its deposition to before the Cobb Mountain sub-chron, that is between 1.22 and ∼1.5 Ma. The alternative match of the normal polarity recorded above the travertine with the Jaramillo subchron (lower limit 1.07 Ma) may also be marginally compatible with cosmogenic nuclides interpretation, thus the proposed minimum age of 1.1 Ma for the end of massive travertine deposition. The actual age of the fossils is likely to be in the 1.1–1.3 Ma range. This absolute date is in close agreement with the paleoanthropological conclusions based on morphometric comparisons implying that Kocabaş hominid belongs to the Homo erectus s.l. group that includes Chinese and African fossils, and is different from Middle and Upper Pleistocene specimens. Furthermore, this date is confirmed by the large mammal assemblage, typical of the late Villafranchian. Because it attests to the antiquity of human occupation of the Anatolian Peninsula and one of the waves of settlements out of Africa, this work challenges the current knowledge of the Homo erectus dispersal over Eurasia.

40Ar/39Ar chronology and paleomagnetism of Quaternary basaltic lavas from the Perşani Mountains (East Carpathians)

Quaternary volcanism in the Perşani Mountains forms an Na-alkali basaltic province inside the bend area of the Carpathians in the southeastern part of Europe. Previous K–Ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles, which were tentatively associated with the Cobb Mountain subchron and a Brunhes chron excursion. We report a new paleomagnetic and rock-magnetic study coupled with 40Ar/39Ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism. Of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity, 19 are normal polarity and 16 have transitional polarity. 40Ar/39Ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142±41 and 800±25ka, four of the normally magnetized lavas erupted at 1060±10, 1062±24, 684±21, and 683±28ka, and two transitionally magnetized lavas formed at 1221±11 and 799±21ka. Both the new 40Ar/39Ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the Jaramillo and Brunhes chrons. This results shows that the last phases of alkalic and calc-alkaline magmatism in the South-East Carpathians were contemporaneous. The age of the older transitionally magnetized lava flow is within error of recent unspiked K–Ar and astrochronologic ages for the reversal that defines the onset of the Cobb Mountain normal polarity subchron. The age of the younger transitional lava is similar to that of an excursion that preceded the Matuyama–Brunhes polarity reversal and which has come to be known as the Matuyama–Brunhes precursor. Omitting the excursion data, the dispersion of the virtual geomagnetic poles (around 19°) is larger than the expected value around 45°N from the global compilation, but closer to the value obtained only from the Time Averaged geomagnetic Field Initiative studies.

Age intercalibration of 40Ar/39Ar sanidine and chemically distinct U/Pb zircon populations from the Alder Creek Rhyolite Quaternary geochronology standard

We report results from a 40Ar/39Ar sanidine and CA-TIMS 238U/206Pb zircon dating study of eruption and crystal residence timescales of the Alder Creek Rhyolite (ACR), California, extruded during the Cobb Mountain normal-polarity subchron (C1r.2n). A 40Ar/39Ar ACR sanidine date of 1.1850±0.0016Ma (2σ external uncertainty), determined relative to the astronomically dated A1 tephra sanidine, is interpreted as the ACR eruption age. This age is supported by CA-TIMS U–Pb zircon dating, guided by LA-ICPMS trace element analyses, titanium-in-zircon (TiZR) thermometry, and cathodoluminescence (CL) imaging. Using these data, two compositionally distinct zircon populations were revealed. “Pre-ACR” Group B zircon exhibit oscillatory zoning, large positive Ce and negative Eu anomalies, high incompatible trace element contents, TiZR temperatures of 650°C–750°C, and Th/U disequilibrium corrected 238U/206Pb dates of 1.38–1.24Ma. These crystals are interpreted as antecrysts inherited from earlier intrusives in the Geysers–Cobb Mountain magma source region. “ACR-related” Group A zircon, present as discrete grains and overgrowths on Group B zircon, display less intense CL with diffuse zoning, and have less pronounced positive Ce and negative Eu anomalies, lower incompatible trace element contents, higher TiZR temperatures that range up to 840°C, and significantly younger dates. The youngest Group A dates yield a weighted mean of 1.1978±0.0046Ma (2σ, including systematic uncertainties) that is interpreted as the mean age of zircon crystallization prior to eruption. The 13±5ka offset between the 40Ar/39Ar and 238U/206Pb dates can be attributed to zircon magma residence time. Recognition of a young population of ACR zircon is consistent with the 40Ar/39Ar eruption age, which coincides with the astronomical age estimate for the Cobb Mountain subchron determined by correlating the oxygen isotope record of the giant piston core MD972143 to the La93(1,1) orbital solution. On the basis of independent radio-isotopic and orbital forcing results, we propose the refined age of 1.1850±0.0016Ma (2σ external uncertainty) for the Quaternary ACR 40Ar/39Ar sanidine standard.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores Archipelago-North Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3°N, 160.7°E, K= 33 and A95= 3.4°) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 ± 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNW-SSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNW-SSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNW-ESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

A well‐established Early–Middle Pleistocene marine sequence on south‐east Zakynthos island, western Greece: Magneto‐biostratigraphic constraints and palaeoclimatic implications

AbstractThe chronostratigraphy of a long, onshore Early–Middle Pleistocene marine sedimentary sequence on the south‐east part of Zakynthos island, Greece, is presented. Correlation of the succession with the isotope record of Ocean Drilling Program Site 963 reveals the combined influence of tectonics and eustacy in this area. The sequence is divided into three formations by two main unconformities that apparently relate to sea‐level lowstands associated with two major northern hemisphere glaciations, those of marine isotope stages (MIS) 22 and 12. The Zakynthos sequence in many ways is comparable with the Italian Valle di Manche section. Magnetostratigraphic and rock magnetic analyses, supported by biostratigraphy, document the position of the Matuyama/Brunhes Chron boundary (0.77 Ma), the top and base of the Jaramillo Subchron (0.99–1.07 Ma), the Cobb Mountain Subchron (1.173–1.185 Ma) and the top of the Olduvai Subchron (1.78 Ma). The underlying strata are constrained exclusively by detailed nannofossil biostratigraphy extending at least to the lowermost Pleistocene at around 2.54 Ma and therefore certainly incorporating the base of the Olduvai Subchron (1.95 Ma) and possibly the Gauss/Matuyama Chron boundary (2.58 Ma). In addition, a remarkable increase in sedimentation rate (from 3.2 and 28 cm ka−1 to 167 cm ka−1) and hence resolution above the Matuyama/Brunhes boundary (Middle Pleistocene) reveals one short‐lived magnetic excursion, possibly 17a (0.66 Ma), within the normal polarity Brunhes Chron. Copyright © 2011 John Wiley &amp; Sons, Ltd.

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.

Age calibrated relative paleointensity for the last 1.5 Myr at IODP Site U1308 (North Atlantic)

Integrated Ocean Drilling Program (IODP) Site U1308 (central North Atlantic) records paleomagnetic directional and relative paleointensity (RPI) variations for the last 1.5 Myr, in 110 m of the sediment sequence at a mean sedimentation rate of 7.3 cm/kyr. A detailed benthic oxygen isotope record was combined with RPI to produce an integrated, high-resolution magneto-isotopic stratigraphy for Site U1308. Apart from the well-known polarity reversals in this interval, the Punaruu excursion is recorded at 1092 ka and the Cobb Mountain Subchron in the 1182–1208 ka interval. The paleointensity proxies are determined as slopes of NRM versus ARM and NRM versus ARMAQ (ARM acquisition) with linear correlation coefficients to monitor the quality of the linear fit. The RPI record for Site U1308 is compared with the three other paleointensity records (one from the Western Equatorial Pacific and two from the North Atlantic) that cover the same time interval and have accompanying oxygen isotope records. The Match protocol of Lisiecki and Lisiecki (2002) is used to optimize the correlation of paleointensity records. Beginning with the original (published) age models for each record, the Match routine is used to optimize the RPI correlations to Site U1308, with checks to ensure compatibility with oxygen isotope records. Squared wavelet coherence (WTC) indicates significant improvement in RPI (and oxygen isotope) correlations after matching each RPI record to Site U1308, particularly for periods > 10 kyr. The level of coherence for the Atlantic RPI records and the lower resolution Pacific record implies synchronous global variability (at scales > 10 kyr) that can be attributed to the axial dipole geomagnetic field.

Vegetation cycles in a disturbed sequence around the Cobb-Mountain subchron in Catalonia (Spain)

A 52-m-long lacustrine sequence was recovered from the basin of Bobila-Ordis, near Banyoles (N–E Spain). The presence of Early Biharian rodent teeth (Early Pleistocene) and of a c. 9-ka-long palaeomagnetic reversal (Cobb-Mountain subchron) suggests an age centred on 1.2 Ma, making this sequence one of the very few well-dated terrestrial sequences of that age in Europe. The first 22.5 m (with an interglacial character) are very homogenous owing to sedimentation affected by underwater springs. In the middle part of the sequence, palynological analyses, supported by sediment visual description, ostracod and mollusc assemblages, allow the reconstruction of one glacial-interglacial cycle, with vegetation succession. A second incomplete climatic cycle is recorded in the top part, within a shallower lake. These brief interruptions in the two climatic cycles are possibly linked to lake bank collapse caused by Hippopotamus amphibius or faulting linked to karst. The succession is likely to correlate to MIS 36-33. The Bobila-Ordis lacustrine series (including two other nested lakes) covers altogether some sections of four glacial and four interglacial periods, not all contiguous.

Constraints on the Pleistocene chronology of sediments from the Lomonosov Ridge

Despite its importance in the global climate system, age‐calibrated marine geologic records reflecting the evolution of glacial cycles through the Pleistocene are largely absent from the central Arctic Ocean. This is especially true for sediments older than 200 ka. Three sites cored during the Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), provide a 27 m continuous sedimentary section from the Lomonosov Ridge in the central Arctic Ocean. Two key biostratigraphic datums and constraints from the magnetic inclination data are used to anchor the chronology of these sediments back to the base of the Cobb Mountain subchron (1215 ka). Beyond 1215 ka, two best fitting geomagnetic models are used to investigate the nature of cyclostratigraphic change. Within this chronology we show that bulk and mineral magnetic properties of the sediments vary on predicted Milankovitch frequencies. These cyclic variations record “glacial” and “interglacial” modes of sediment deposition on the Lomonosov Ridge as evident in studies of ice‐rafted debris and stable isotopic and faunal assemblages for the last two glacial cycles and were used to tune the age model. Potential errors, which largely arise from uncertainties in the nature of downhole paleomagnetic variability, and the choice of a tuning target are handled by defining an error envelope that is based on the best fitting cyclostratigraphic and geomagnetic solutions.

Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: A prolific distal record of Taupo Volcanic Zone volcanism

Coastal sections in the Auckland region reveal highly carbonaceous and/or highly weathered clay‐dominated cover‐bed successions with numerous discrete distal volcanic ash (tephra) layers, fluvially reworked siliciclastic (tephric) deposits, and two widely distributed pyroclastic density current (PDC) deposits generated from explosive silicic volcanism within the Taupo Volcanic Zone (TVZ). The younger of the two PDC deposits (informally named Waiuku tephra) is glass‐isothermal plateau fission‐track (ITPFT) dated at 1.00 ± 0.03 Ma and occurs in a normal polarity interval interpreted as the Jaramillo Subchron. Waiuku tephra is correlated with Unit E sourced from the Mangakino Volcanic Centre of the TVZ. Waiuku tephra can be subdivided into two distinctive units enabling unequivocal field correlation: a lower stratified unit (dominantly pyroclastic surge with fall component) and an upper massive to weakly stratified unit (pyroclastic flow). At many sites in south Auckland, Waiuku tephra retains basal “surge‐like” beds (<1.4 m thickness). This provides clear evidence for primary emplacement and is an exceptional feature considering the c. 200 km this PDC has travelled from its TVZ source area. However, at many other Auckland sites, Waiuku tephra displays transitional sedimentary characteristics indicating lateral transformation from hot, gas‐supported flow/surge into water‐supported mass flow and hyperconcentrated flow (HCF) deposits. The older PDC deposit is dated at 1.21 ± 0.09 Ma, is enveloped by tephras that are ITPFT‐dated at 1.14 ± 0.06 Ma (above) and 1.21 ± 0.06 Ma (below), respectively, and occurs below a short normal polarity interval (Cobb Mountain Subchron) at c. 1.19 Ma. This PDC deposit, correlated with Ongatiti Ignimbrite sourced from the Mangakino Volcanic Centre of TVZ, has laterally transformed from a gas‐supported, fine‐grained pyroclastic flow deposit at Oruarangi, Port Waikato, into a water‐supported volcaniclastic mass flow deposit farther north at Glenbrook Beach. The occurrence of Ongatiti Ignimbrite in Auckland significantly extends its northward distribution. Large numbers of post‐ and pre‐Ongatiti rhyolitic tephra layers, ranging in age from c. 1.31 to 0.53 Ma, are also recognised in the region, with some up to 0.5 m in compacted fallout thickness. Although some tephras can be attributed to known TVZ eruptions (e.g., Ahuroa/Unit D), many have yet to be identified in proximal source areas and remain uncorrelated. However, some can be reliably correlated to tephra layers occurring in marine to nearshore sequences of Wanganui Basin and deep‐sea cores retrieved east of North Island. The identification of previously unrecognised mid‐Pleistocene TVZ‐sourced tephra deposits in the Auckland region, and their correlation to the offshore marine record, represent an advance in the construction of a higher resolution history for the TVZ where, close to eruptive source, the record is fragmentary and obscured by deep burial, or erosion, or both.

Paleomagnetism and 40Ar/39Ar Chronology of Lavas from Meseta del Lago Buenos Aires, Patagonia

As part of a larger project investigating paleosecular variation in South America, 36 paleomagnetic and geochronology sites were sampled on Meseta del Lago Buenos Aires (47°S, 289°E), 200 km east of the active plate boundary in southern Patagonia. Basaltic lavas sampled range from late Miocene to late Pleistocene yet most of the individual lavas are younger than 3.3 Ma. Thirty‐eight isochron ages determined via 40Ar/39Ar incremental‐heating experiments frame a detailed stratigraphy. The isochron ages range from 66.9 ± 4.1 ka for scoria cones and a youthful lava flow in the Rio Pinturas valley to 10.12 ± 1.35 Ma for thick plateau‐forming lavas exposed along the SE edge of the Meseta. The last 3.3 myr is characterized by seven episodes of volcanism at ca. 3.2–3.0 Ma, 2.4 Ma, 1.7 Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka, and &lt;110 ka. The bulk of lavas forming the surface of the Meseta erupted in the last 1.7 my. All sites have stable magnetization, and after step demagnetization using either thermal or alternating field techniques, yield characteristic directions held by magnetite and/or titanomagnetite. Ten sites have distinct transitional directions (defined by pole latitudes &lt;55°), and associated ages indicate possible connections to known reversals within the Matuyama Chron, including the onset of the Jaramillo subchron (1.016 ± 0.01 Ma), the Cobb Mountain subchron (1.25 ± 0.03 Ma), the Ontong‐Java 1 event (1.37 ± 0.03 Ma), and the termination of the Olduvai subchron (1.72 ± 0.02 Ma). Remaining sites are divided into normal (14 sites) and reversed polarity (12 sites). At 95% confidence mean normal and reversed directions overlap. The mean direction for 26 sites is I = −63.0°, D = 3.4°, α95 = 5.4°, which is indistinguishable from the expected geocentric axial dipole. Paleosecular variation, measured by the dispersion of virtual geomagnetic poles about the rotation axis, is higher than expected at 20.0°. Model G, using data for the past 5 Myr, predicts a dispersion of 17°. This discrepancy may be due to true dispersion of the field in southern South America or it may be an artifact of inadequate sampling.

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.

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.

Comment on the Lau Basin Cobb Mountain records by Abrahamsen and Sager

The Cobb Mountain Subchron is one of the few short polarity intervals for which transition records have been obtained both from a wide geographical distribution and from different types of paleomagnetic recorders. These records exhibit VGP paths that are remarkably similar. Given the wide geographic distribution of these sites, the similarity in VGP paths is most simply interpreted as the presence of very large scale, if not dipolar, symmetries in the transitional fields. Only the records from western California and the Lau Basin exhibit VGP paths that are markedly different from the others. New dates for the Punaruu lavas suggest that they recorded a new geomagnetic cryptochron rather than the Cobb Mountain [Singer, B.S., Hoffman, K.A., Chauvin, A., Coe, R.S., 1999. Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new 40 Ar/ 39 Ar timescale of reversals and events. J. Geophys. Res. 104, 679–693]. This new age indicates a revised correlation of some of the other California records whose ages agree better with the Punaruu cryptochron. A re-examination of the Lau Basin records reveals that much of the difference between these and the other records results from a bias introduced when the paleomagnetic data were smoothed. Reanalysis of these records shows that the Lau Basin VGP paths are very similar to those obtained from western Pacific, and the North Atlantic.

Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone, New Zealand

Large volume (100–1000 km3), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of 40Ar/39Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession.