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Since the publication in 1600 of the book De Magnete by William Gilbert, and the measurements by magnetic observatories progressively obtained from various parts of the globe, we know that the Earth’s magnetic field is comparable to one that would be created by a bar-magnet placed at the center of the Earth and inclined by some 11° with respect to the axis of rotation.

Age models for new records of the Laschamp and Iceland Basin excursions from the eastern flank of the South Atlantic mid-ocean ridge (44.15°S, 14.22°W) are derived from radiocarbon dates, and from matching sea-surface temperature records to Antarctic (EPICA) air-temperature records from ice cores. The onset of the Laschamp excursion occurred during Antarctic Isotopic Maximum (AIM) 10, consistent with its occurrence during Greenland Interstadial 10. The end of the Laschamp excursion occurred prior to AIM 9 in Greenland Stadial 10. The age model is supported by synchroneity of directional and relative paleointensity manifestations of the Laschamp excursion in the marine core with peaks in EPICA 10 Be and nitrate flux. The Iceland Basin excursion is synchronous with the final phase of the transition from marine isotope stage (MIS) 7a to MIS 6e as recorded in the EPICA δD record. The onset of the Laschamp and Iceland Basin excursions, defined here by component inclinations >−40°, occurred at 41.4 ka and 190.0 ka, and durations are ∼1 kyr and ∼3.5 kyr, respectively, although these estimates depend on the criteria used to define the directional excursions. By comparison with Laschamp and Iceland Basin excursion records from the North Atlantic Ocean, the two excursions are synchronous at centennial timescales between the two hemispheres, based on synchronization of the GICC05 and AICC2012 age models for Greenland and Antarctic ice cores.

Integrated Ocean Drilling Program (IODP) Expedition 303 to the North Atlantic provided 16 records of the Matuyama–Brunhes polarity transition (MBT), based on u-channel and discrete samples, from holes drilled at three sites (Sites U1304, U1305 and U1306) that have mean Brunhes sedimentation rates of 16–18 cm/kyr. The MBT occurs during the transition from marine isotope stage (MIS) 19c to MIS 18e, with mid-point at ∼773 ka, and a transition duration of ∼8 kyr. Combining the new MBT records, including one new record for the top Jaramillo, with previously published North Atlantic MBT records (ODP Sites 983, 984 and 1063) yields a total of more than 20 high-sedimentation-rate polarity transition records. The MBT yields a repetitive pattern of transitional field states as virtual geomagnetic poles (VGPs) move from high southern latitudes to loop over the Pacific, group in NE Asia, and transit into the mid-latitude South Atlantic before reaching high latitudes in the Northern Hemisphere. The VGPs for the top Jaramillo transition feature a loop over the Pacific, then a NE Asia group before transit over the Indian Ocean to high southerly latitudes. The North Atlantic MBT records described here contrast with longitudinally-constrained VGP paths for the MBT, indicating that relatively low sedimentation rate (∼4 cm/kyr) records of the MBT are heavily smoothed by the remanence acquisition process and do not adequately represent the MBT field. The VGPs at the MBT and top Jaramillo, as measured in the North Atlantic, have similarities with excursion (Iceland Basin) VGP paths, and were apparently guided by maxima in downward vertical flux similar to those seen in the modern non-dipole (ND) field, implying longevity in ND features through time.

We present new sedimentological and environmental magnetic records spanning ∼3.2–2.2 Ma, during the intensification of Northern Hemisphere glaciation, from North Atlantic Integrated Ocean Drilling Program Site U1307 on Eirik Drift. Our new datasets and their high-fidelity age control demonstrate that while inland glaciers – and potentially also at times restricted iceberg-calving margins – have likely existed on southern Greenland since at least ∼3.2 Ma, persistent and extensive iceberg-calving glacial margins were only established in this region at 2.72 Ma, ∼300 kyr later than in northeastern and eastern Greenland. Despite a dramatic increase in Greenland-sourced ice-rafted debris deposition on Eirik Drift at this time, contemporaneous changes in the bulk magnetic properties of Site U1307 sediments, and a reduction in sediment accumulation rates, suggest a decrease in the delivery of Greenland-sourced glaciofluvial silt to our study site. We attribute these changes to a shift in depositional regime from bottom-current-dominated to glacial-IRD-dominated between ∼2.9 and 2.7 Ma, in response to a change in the depth of the flow path of the Western Boundary Undercurrent relative to our study site.

We present a 3.2 Myr record of stable isotopes and physical properties at IODP Site U1308 (reoccupation of DSDP Site 609) located within the ice-rafted detritus (IRD) belt of the North Atlantic. We compare the isotope and lithological proxies at Site U1308 with other North Atlantic records (e.g., sites 982, 607/U1313, and U1304) to reconstruct the history of orbital and millennial-scale climate variability during the Quaternary. The Site U1308 record documents a progressive increase in the intensity of Northern Hemisphere glacial–interglacial cycles during the late Pliocene and Quaternary, with mode transitions at ∼ 2.7, 1.5, 0.9, and 0.65 Ma. These transitions mark times of change in the growth and stability of Northern Hemisphere ice sheets. They also coincide with increases in vertical carbon isotope gradients between the intermediate and deep ocean, suggesting changes in deep carbon storage and atmospheric CO2. Orbital and millennial climate variability co-evolved during the Quaternary such that the trend towards larger and thicker ice sheets was accompanied by changes in the style, frequency, and intensity of millennial-scale variability. This co-evolution may be important for explaining the observed patterns of Quaternary climate change.

The Miocene Climatic Optimum (MCO; similar to 16.9 to 14.7Ma) provides an outstanding opportunity to investigate climate-carbon cycle dynamics during a geologically recent interval of global warmth. We present benthic stable oxygen (O-18) and carbon (C-13) isotope records (5-12kyr time resolution) spanning the late early to middle Miocene interval (18 to 13Ma) at Integrated Ocean Drilling Program (IODP) Site U1335 (eastern equatorial Pacific Ocean). The U1335 stable isotope series track the onset and development of the MCO as well as the transitional climatic phase culminating with global cooling and expansion of the East Antarctic Ice Sheet at similar to 13.8Ma. We integrate these new data with published stable isotope, geomagnetic polarity, and X-ray fluorescence (XRF) scanner-derived carbonate records from IODP Sites U1335, U1336, U1337, and U1338 on a consistent, astronomically tuned timescale. Benthic isotope and XRF scanner-derived CaCO3 records depict prominent 100kyr variability with 400kyr cyclicity additionally imprinted on C-13 and CaCO3 records, pointing to a tight coupling between the marine carbon cycle and climate variations. Our intersite comparison further indicates that the lysocline behaved in highly dynamic manner throughout the MCO, with >75% carbonate loss occurring at paleodepths ranging from similar to 3.4 to similar to 4km in the eastern equatorial Pacific Ocean. Carbonate dissolution maxima coincide with warm phases (O-18 minima) and C-13 decreases, implying that climate-carbon cycle feedbacks fundamentally differed from the late Pleistocene glacial-interglacial pattern, where dissolution maxima correspond to C-13 maxima and O-18 minima. Carbonate dissolution cycles during the MCO were, thus, more similar to Paleogene hyperthermal patterns.

Integrated Ocean Drilling Program (IODP) Expedition 303 to the North Atlantic in 2004 recovered a 175-m Quaternary sedimentary section at IODP Site U1307, located on the northern part of the Eirik Drift, south of Greenland. The seafloor at Site U1307, at 2575 m water depth, is situated within the water-depth range of the present-day main axis of the Western Boundary Under Current (WBUC) but lies on the NW side of the drift crest and is partly shielded from it. The uppermost 120 meters of sediment sequence were sampled with u-channels along a composite section which combines the two holes drilled at this site. An age model was obtained by identification of polarity reversals back to the Reunion Subchron ( ∼ 2.1 Ma ) and then refined in the 0–1.5 Ma interval by correlation of the relative paleointensity (RPI) record to the PISO paleointensity stack. The Site U1307 relative paleointensity (RPI) record is compared with the PISO and EPAPIS (Pacific) stacks, neighboring IODP sites from Eirik Drift (Sites U1305 and U1306), and with the Ocean Drilling Program (ODP) Site 984 record that extends beyond PISO. Magnetite grain-size and concentration proxies are compared to Sites U1305 and U1306 records, in the context of the past evolution of the WBUC.

We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first-order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single-domain (SD), pseudo-single-domain (PSD), and multi-domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end-member (EM) FORC diagrams, located at user-defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt-sized MD component, a fine silt-sized PSD component, and a mixed clay-sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA-based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four-EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi-component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.

The Iceland Basin geomagnetic excursion coincided with the marine isotope stage (MIS) 6/7 boundary. The age and duration of the excursion, at seven North Atlantic sites with sufficient isotope data, are estimated by matching marine isotope stage (MIS) 7a–7c to a calibrated template. Two criteria for defining the excursion, virtual geomagnetic pole (VGP) latitudes

In the framework of the PALEOVAN project, a high-resolution paleomagnetic and rock magnetic study was carried out on a 149 m sedimentary sequence recovered from Ahlat Ridge in the deepest part of the Lake Van (Eastern Turkey; ICDP Site 5034-2). High sedimentation rates (average similar to 30 cm/ka) allow high-resolution measurements of natural remanent magnetization (NRM), magnetic susceptibility (kappa), anhysteretic remanence (ARM) and hence of anhysteretic susceptibility (kappa(ARM)) over the last 350 ka. The carrier of the remanence is detrital titanomagnetite largely from volcanic sources, based on the similarity of magnetite grain size of tephra layers and the other lacustrine lithologies observed in a plot of the it versus kappa(ARM). Bulk magnetic parameters often covary with paleoclimatic signals in the Lake Van sediments. A correlation exists between variations of kappa and ARM intensity and glacial-interglacial marine isotopic stages, as well as dust flux and temperatures observed in ice cores from Greenland and Antarctica. The quality of the paleomagnetic record is compromised by weak NRM intensities, as well as by the presence of tephra and turbidites throughout the sequence. Nonetheless, a correlation is observed between the relative paleointensity (RPI) record, based on NRM/ARM, and the calibrated PISO RPI stack, that supports the independently derived age model for the site. (C) 2014 Elsevier Ltd. All rights reserved.

Accurate age control of the late Tortonian to early Messinian (8.3–6.0 Ma) is essential to ascertain the origin of benthic foraminiferal δ 18 O trends and the late Miocene carbon isotope shift (LMCIS), and to examine temporal relationships between the deep-sea, terrasphere and cryosphere. The current Tortonian–Messinian Geological Time Scale (GTS2012) is based on astronomically calibrated Mediterranean sections; however, no comparable non-Mediterranean stratigraphies exist for 8–6 Ma suitable for testing the GTS2012. Here, we present the first high-resolution, astronomically tuned benthic stable isotope stratigraphy (1.5 kyr resolution) and magnetostratigraphy from a single deep-sea location (IODP Site U1337, equatorial Pacific Ocean), which provides unprecedented insight into climate evolution from 8.3–6.0 Ma. The astronomically calibrated magnetostratigraphy provides robust ages, which differ by 2–50 kyr relative to the GTS2012 for polarity Chrons C3An.1n to C4r.1r, and eliminates the exceptionally high South Atlantic spreading rates based on the GTS2012 during Chron C3Bn. We show that the LMCIS was globally synchronous within 2 kyr, and provide astronomically calibrated ages anchored to the GPTS for its onset (7.537 Ma; 50% from base Chron C4n.1n) and termination (6.727 Ma; 11% from base Chron C3An.2n), confirming that the terrestrial C3:C4 shift could not have driven the LMCIS. The benthic records show that the transition into the 41-kyr world, when obliquity strongly influenced climate variability, already occurred at 7.7 Ma and further strengthened at 6.4 Ma. Previously unseen, distinctive, asymmetric saw-tooth patterns in benthic δ 18 O imply that high-latitude forcing played an important role in late Miocene climate dynamics from 7.7–6.9 Ma. This new integrated deep-sea stratigraphy from Site U1337 can act as a new stable isotope and magnetic polarity reference section for the 8.3–6.0 Ma interval.

A new record of the Mono Lake excursion (MLE) is reported from the Summer Lake Basin of Oregon, USA. Sediment magnetic properties indicate magnetite as the magnetization carrier and imply suitability of the sediments as accurate recorders of the magnetic field including relative paleointensity (RPI) variations. The magnitudes and phases of the declination, inclination, and RPI components of the new record correlate well with other coeval but lower resolution records from western North America including records from the Wilson Creek Formation exposed around Mono Lake. The virtual geomagnetic pole (VGP) path of the new record is similar to that from another high-resolution record of the MLE from Ocean Drilling Program (ODP) Site 919 in the Irminger Basin between Iceland and Greenland but different from the VGP path for the Laschamp excursion (LE), including that found lower in the ODP-919 core. Thus, the prominent excursion recorded at Mono Lake, California, is not the LE but rather one that is several thousands of years younger. The MLE VGP path contains clusters, the locations of which coincide with nonaxial dipole features found in the Holocene geomagnetic field. The clusters are occupied in the same time progression by VGPs from Summer Lake and the Irminger Basin, but the phase of occupation is offset, a behavior that suggests time-transgressive decay and return of the principal field components at the beginning and end of the MLE, respectively, leaving the nonaxial dipole features associated with the clusters dominant during the excursion.

Late Oligocene to Early Miocene relative paleointensity (RPI) proxies can be correlated from the equatorial Pacific (IODP Site U1334 and ODP Site 1218) to the South Atlantic (ODP Site 1090). Age models are constrained by magnetic polarity stratigraphy through correlation to a common geomagnetic polarity timescale. The RPI records do not contain significant power at specific (orbital) frequencies, and hence there is no significant coherency between RPI proxies and the normalizers used to construct the proxies, although orbital power is present in some normalizers. There is no obvious control on RPI proxies from mean sedimentation rate within polarity chrons, magnetic grain size proxies or magnetic concentration parameters. The salient test is whether the equatorial Pacific records can be correlated one to another, and to the records from the South Atlantic. All records are dominated by RPI minima at polarity reversals, as expected, although the comparison within polarity chrons is compelling enough to conclude that the intensity of the Earthʼs axial dipole is being recorded. This is supported by the fit of RPI data from Sites U1334 and 1218 after correlation of the two sites using diverse core-scanning data, rather than polarity reversals alone. We do not see a consistent relationship between polarity-chron duration and mean RPI, and no consistent skewness (“saw-tooth” pattern) for RPI within polarity chrons. Stacks of RPI records for 17.5–26.5 Ma include long-term changes in RPI on Myr timescales that are superimposed on RPI minima associated with polarity reversals, and shorter-term variations in RPI with an apparent pacing of ∼ 50 kyr . The equatorial Pacific to South Atlantic correlations indicate that RPI can be used as a (global) stratigraphic tool in pre-Quaternary sediments with typical pelagic sedimentation rates of a few cm/kyr.

[1] Paleomagnetic analysis and radiocarbon dating of an expanded Holocene deep-sea sediment sequence recovered by Integrated Ocean Drilling Program (IODP) Expedition 303 from Labrador Sea Site U1305 (Lat.: 57°28.5 N, Long.: 48°31.8 W, water depth 3459 m) provides insights into mechanisms that drive both paleomagnetic secular variation (PSV) and magnetization acquisition in deep-sea sediments. Seventeen radiocarbon dates on planktonic foraminifera define postglacial (ca. 8 ka) sedimentation rates as ranging from 35 to > 90 cm/kyr. Alternating field (AF) demagnetization of u-channel samples show that these homogeneous sediments preserve a strong, stable, and consistently well-defined component magnetization. Normalized remanence records pass reliability criteria for relative paleointensity (RPI) estimates. Assuming that the age of magnetization is most accurately defined by well dated PSV records with the highest sedimentation rates, allows us to estimate and correct for temporal offsets at Site U1305 interpreted to result from postdepositional remanence acquisition at a depth of ∼20 cm. Comparisons indicate that the northern North Atlantic PSV and RPI records are more consistent with European than North American records, and the evolution of virtual geomagnetic poles (VGP) are temporally and longitudinally similar to global reconstructions, though with much larger latitudinal variation. The largest deviations from a geocentric axial dipole (GAD) are observed during times of the highest intensities, in contrast to the usual assumption. These observations are consistent with the idea that PSV in the North Atlantic and elsewhere during the Holocene results from temporal oscillations of high-latitude flux concentrations at a few recurrent locations.

article i nfo The orogenic belts surrounding the undeformed Adriatic Sea represent the margins of an area known as Adria, the African promontory. We have undertaken a critical appraisal of paleomagnetic data from regions of Adria considered parautochthonous relative to Africa, obtained either from biostratigraphically dated sed- imentary rocks, corrected for inclination shallowing, or from igneous rocks that are regarded as free from any inclination shallowing bias. Paleomagnetic directions were used to calculate paleomagnetic poles for compar- ison with coeval, and inclination flattening-free, paleomagnetic poles from stable Africa. Visual coherence of paleopoles for several time slices from the Early Permian to the Eocene supports the construction of a com- posite apparent polar wander path (APWP) valid for parautochthonous Adria and stable Africa. This compos- ite APWP is compared to previous APWPs, finding good agreement with the global APWP of Kent and Irving (2010). Both APWPs show a remarkable and rapid polar shift of ~40° in the Jurassic that other APWPs tend to underestimate. We interpret this shift to represent a major episode of true polar wander (TPW), from ~183 Ma in the Early Jurassic to ~151 Ma in the Late Jurassic. Using a simple zonal climate model, the drift motion of Adria attached to Africa appears to be consistent with the distribution of Permian-Cretaceous sed- imentary facies on Adria.

Magnetic properties of late Quaternary sediments on the SW Iberian Margin are dominated by bacterial magnetite, observed by transmission electron microscopy (TEM), with contributions from detrital titanomagnetite and hematite. Reactive hematite, together with low organic matter concentrations and the lack of sulfate reduction, lead to dissimilatory iron reduction and availability of Fe(II) for abundant magnetotactic bacteria. Magnetite grain-size proxies ( κ ARM / κ and ARM/IRM) and S-ratios (sensitive to hematite) vary on stadial/interstadial timescales, contain orbital power, and mimic planktic δ 18 O . The detrital/biogenic magnetite ratio and hematite concentration are greater during stadials and glacial isotopic stages, reflecting increased detrital (magnetite) input during times of lowered sea level, coinciding with atmospheric conditions favoring hematitic dust supply. Magnetic susceptibility, on the other hand, has a very different response being sensitive to coarse detrital multidomain (MD) magnetite associated with ice-rafted debris (IRD). High susceptibility and/or magnetic grain-size coarsening, mark Heinrich stadials (HS), particularly HS2, HS3, HS4, HS5, HS6 and HS7, as well as older Heinrich-like detrital layers, indicating the sensitivity of this region to fluctuations in the position of the polar front. Relative paleointensity (RPI) records have well-constrained age models based on planktic δ 18 O correlation to ice-core chronologies, however, they differ from reference records (e.g. PISO) particularly in the vicinity of glacial maxima, mainly due to inefficient normalization of RPI records in intervals of enhanced hematite input.

We present a high-resolution magnetostratigraphy and relative paleointensity (RPI) record derived from the upper 85 m of IODP Site U1336, an Equatorial Pacific early to middle Miocene succession recovered during Expedition 320/321. The magnetostratigraphy is well resolved with reversals typically located to within a few centimeters resulting in a well-constrained age model. The lowest normal polarity interval, from 85 to 74.87 m, is interpreted as the later part of Chron C6n (18.614–19.599 Ma). Thirty-three other magnetozones occur from 74.87 to 0.85 m, which are interpreted to represent the continuous sequence of chrons onset of Chron C5Er (18.748 Ma) to the end of Chron C5An.1n (12.014 Ma). We identify three putative previously-unrecognized subchrons within Chron C5Cn.1n, Chron 5Bn.1r, and C5ABn. Sedimentation rates vary from about 7 to 15 m/Myr with a mean of about 10 m/Myr. We observe rapid, apparent changes in the sedimentation rate at geomagnetic reversals between ∼16 and 19 Ma that indicate a calibration error in geomagnetic polarity timescale (ATNTS2004). The remanence is carried mainly by non-interacting particles of fine-grained magnetite, which have FORC distributions characteristic of biogenic magnetite. Given the relative homogeneity of the remanence carriers throughout the 85-m-thick succession and the fidelity with which the remanence is recorded, we have constructed a relative paleointensity (RPI) record that provides new insights into middle Miocene geomagnetic field behavior. The RPI record indicates a gradual decline in field strength between 18.5 Ma and 14.5 Ma, and indicates no discernible link between RPI and either chron duration or polarity state.

High sediment accumulation rates in lacustrine and shallow-marine archives around Iceland offer the potential to compare high-resolution paleoclimatic reconstructions from terrestrial and marine archives; however, direct comparisons are hampered by difficulties in stratigraphic correlation and in deriving accurate age models for lacustrine archives. Icelandic paleomagnetic secular variation (PSV) has the potential to synchronize these records. Here we compare Holocene PSV from a well-dated marine core on the North Iceland shelf with PSV from two lacustrine archives with comparable sediment-accumulation rates, HVT03–1A, a glacier-dominated lake, and HAK03–1B, in a nonglacial catchment. Geochemically characterized tephra layers combined with unique high-amplitude structures in the PSV records provide secure tie points every ∼200 yr. Once the records are synchronized, the chronology from the marine core can be reliably transferred to the two lacustrine records. The resultant lacustrine age models reveal large changes in sediment accumulation rate at submillennial scales that escape detection in conventional age models with independent dates every ∼1 k.y. Sediment accumulation rate changes occur at similar times in both lakes, despite very different catchment properties. Low and regular accumulation rates during the Holocene thermal maximum suggest regionally stable, vegetated catchments, followed by a stepped landscape destabilization during the transition into neoglaciation, culminating with maximum sedimentation rates during the Little Ice Age. PSV allows synchronization between multiple records from nearby marine and lacustrine archives, providing improved age models and a means of assessing leads and lags between marine and terrestrial environments.

Magnetic parameters are useful for distinguishing North Atlantic Heinrich-like detrital layers from background sediments. Here we compare magnetic properties with XRF scanning data back to 700 ka and 1.3 Ma at IODP Sites U1302–U1303 and U1308, respectively. Multi-domain magnetite, with grain sizes >20 µm, is characteristic of both Ca- and Si-rich detrital layers, as defined by XRF core scanning, confirming the contribution of ice rafting. Reflectance spectra and magnetic parameters distinguish Ca- and Si-rich IRD layers due the presence of high coercivity hematite in Si-rich layers. Heinrich layer 6 (H6) at Site U1302–U1303 is unlike other detrital layers, being marked by a 45-cm thick homogeneous cream-colored clay layer underlain by a thin (5-cm) graded coarse-sand. Comparison of Site U1302/03 and Site U1308 detrital layers implies a dominant Laurentide source for both Ca- and Si-rich detrital layers. At Site U1308, low benthic δ 13 C values during stadials are in-step with magnetic grain-size coarsening associated with Si-rich detrital layers back to 1.3 Ma, indicating a link between deep-sea ventilation and ice rafting. The surface-sediment tan-colored oxic layer (~2 m thick at Site U1308) yields magnetic hysteresis ratios that are offset from the single-domain to multi-domain (SD–MD) magnetite mixing-line in hysteresis-ratio diagrams. This offset is attributed to maghemite grain-coatings, that form on magnetite in surface sediment, and undergo dissolution as they pass through the oxic/anoxic boundary.

Here we report 420?kyr long records of sediment geochemical and color variations from the southwestern Iberian Margin. We synchronized the Iberian Margin sediment record to Antarctic ice cores and speleothem records on millennial time scales and investigated the phase responses relative to orbital forcing of multiple proxy records available from these cores. Iberian Margin sediments contain strong precession power. Sediment “redness” (a* and 570–560?nm) and the ratio of long-chain alcohols to n-alkanes (C26OH/(C26OH?+?C29)) are highly coherent and in-phase with precession. Redder layers and more oxidizing conditions (low alcohol ratio) occur near precession minima (summer insolation maxima). We suggest these proxies respond rapidly to low-latitude insolation forcing by wind-driven processes (e.g., dust transport, upwelling, precipitation). Most Iberian Margin sediment parameters lag obliquity maxima by 7–8?ka, indicating a consistent linear response to insolation forcing at obliquity frequencies driven mainly by high-latitude processes. Although the lengths of the time series are short (420?ka) for detecting 100?kyr eccentricity cycles, the phase relationships support those obtained by Shackleton []. Antarctic temperature and the Iberian Margin alcohol ratios (C26OH/(C26OH?+?C29)) lead eccentricity maxima by 6?kyr, with lower ratios (increased oxygenation) occurring at eccentricity maxima. CO2, CH4, and Iberian SST are nearly in phase with eccentricity, and minimum ice volume (as inferred from Pacific ?18Oseawater) lags eccentricity maxima by 10?kyr. The phase relationships derived in this study continue to support a potential role of the Earth's carbon cycle in contributing to the 100?kyr cycle.

[1] Sediments from the equatorial Pacific Ocean, at the Integrated Ocean Drilling Program sites U1334 and U1335, record reliable magnetic polarity stratigraphies back to ~26.5 Ma (late Oligocene) at sedimentation rates usually in the 5–20 m/Myr range. Putative polarity subchrons that do not appear in current polarity timescales occur within Chrons C5ACr, C5ADn, and C5Bn.1r at Site U1335; and within Chrons C6AAr.2r, C6Br, C7Ar, and C8n.1n at Site U1334. Subchron C5Dr.1n (~17.5 Ma) is recorded at both sites, supporting its apparent recording in the South Atlantic Ocean, and has an estimated duration of ~40 kyr. The Oligocene-Miocene calcareous oozes have magnetizations carried by submicron magnetite, as indicated by thermal demagnetization of magnetic remanences, the anhysteretic remanence to susceptibility ratio, and magnetic hysteresis parameters. Transmission electron microscopy of magnetic separates indicates the presence of low-titanium iron oxide (magnetite) grains with size (50–100 nm) and shape similar to modern and fossil bacterial magnetite, supporting other evidence that biogenic submicron magnetite is the principal remanence carrier in these sediments. In the equatorial Pacific Ocean, low organic-carbon burial arrests microbial pore-water sulfate reduction, thereby aiding preservation of bacterial magnetite.

We present results of a multi-proxy study on marine sediment core JR179-PC466 recovered from the crest of a sediment drift off the West Antarctic Peninsula at approximately 2300 m water depth. The 10.45 m-long core consists dominantly of glaciomarine terrigenous sedi- ments, with only traces of calcium carbonate (,1 wt%). Despite the very low abundance of calcar- eous foraminifera, planktonic shell numbers are sufficient for stable isotope analyses in two-thirds of the samples studied. The core chronology is based on oxygen isotope stratigraphy and corre- lation of its relative palaeomagnetic intensity (RPI) with a stacked reference curve. According to the age model, core PC466 spans the last 75 ka, with average sedimentation rates of between about 4 and 25 cm ka 21 . Planktonic foraminifera abundances fluctuate between 0 and 30 individ- uals per gram throughout the core, with minima observed during Marine Isotope Stage (MIS) 2 (14–29 ka before present, BP) and MIS4 (57–71 ka BP). Planktonic foraminifera are present in the Holocene but more abundant in sediments deposited during MIS3 (29–57 ka BP), owing to less dilution by terrigenous detritus and/or better carbonate preservation. During MIS3, forami- nifera maxima correlate with Antarctic warming events as recorded in the d 18 O signal of the EPICA Dronning Maud Land (EDML) ice core. They indicate higher planktonic foraminifera production and better carbonate preservation west of the Antarctic Peninsula during that time. The abundance of ice-rafted detritus (IRD) in core PC466 increased during the last deglaciation between about 19 and 11 ka BP, when numerous icebergs drifted across the core site, thereby releasing IRD. During this time, sea-level rise destabilized the Antarctic Peninsula (APIS) and West Antarctic (WAIS) ice sheets that had advanced onto the shelf during the sea-level low- stand of the Last Glacial Maximum (LGM; c. 19–23 ka BP). Overall, our results demonstrate that it is possible to establish an age model and reconstruct palaeoceanographical and climatic changes at high temporal resolution from sedimentary sequences recovered at 2300 m water depth from a West Antarctic drift.

The sediment sequence from Integrated Ocean Drilling Program (IODP) Site U1304 (53 degrees 03.40'N, 33 degrees 31.78'W; water depth, 3024 m) on the southern Gardar Drift, North Atlantic Ocean, covers the last similar to 1.8 Myr with a mean sedimentation rate of similar to 15 cm/kyr. At Site U1304, paleomagnetic directional and relative paleointensity (RPI) records have been generated for the last similar to 1.5 Myr, and benthic oxygen isotope data for the last similar to 1 Myr. The age model for Site U1304 was established by matching 6180 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 01304 sediments record the Matuyama/Brunhes boundary (similar to 772 ka), the Jaramillo Subchron (similar to 993-1071 ka) and the Cobb Mountain Subchron (similar to 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 similar to 888 ka, similar to 1124 ka, and similar to 14631 a, 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 01304, did not reveal significant orbital power.

We present the key features of the Valle di Manche section (Calabria, southern Italy) and discuss the pros and cons of this stratigraphic succession as a candidate section for the GSSP of the Ionian Stage (Middle Pleistocene Subseries).

We present palaeomagnetic results from the Oligocene through Miocene part of the Integrated Ocean Drilling Program Site U1333 (1030.996 ' N, 138825.159 ' W), which is located in 4853 m-deep water over seafloor with an estimated crustal age of 46 Ma. Detailed magnetostra- tigraphic investigations are essential to provide a sound age model for the study of the palaeocli- matic and palaeo-oceanographic history of the Cenozoic of the Equatorial Pacific and to improve the database of Pacific magnetostratigraphy. Rock magnetic measurements were carried out at 1 cm resolution on 81 U-channel samples from the spliced section with the goal of extracting a high-resolution record of the magnetostratigraphy. Stepwise demagnetization of the natural rema- nent magnetization yielded a well-defined magnetostratigraphy over a time interval of approxi- mately 10 Ma between the base of Chron C6n (19.722 Ma) and the middle of Chron C11r (.29.9 Ma) and identification of the Oligocene-Miocene transition at the base of Subchron C6Cn.2n. The palaeomagnetic data are characterized by shallow inclinations, and by 1808 alterna- tions in declinations downhole, reflecting magnetic polarity zones. The relatively high temporal resolution allowed for the identification of three possible excursions previously not identified on the geomagnetic polarity time scale, which were recorded in Subchrons C8n.1r and C11n.2n and in Chron C11r.

Inclination patterns of natural remanent magnetization (NRM) in Quaternary sediment cores from the Arctic Ocean have been widely used for stratigraphic correlation and the construction of age models, however, shallow and negative NRM inclinations in sediments deposited during the Brunhes Chron in the Arctic Ocean appear to have a partly diagenetic origin. Rock magnetic and mineralogical studies demonstrate the presence of titanomagnetite and titanomaghemite. Thermal demagnetization of the NRM indicates that shallow and negative inclination components are largely “unblocked” below ∼300 °C, consistent with a titanomaghemite remanence carrier. Following earlier studies on the Mendeleev–Alpha Ridge, shallow and negative NRM inclination intervals in cores from the Lomonosov Ridge and Yermak Plateau are attributed to partial self-reversed chemical remanent magnetization (CRM) carried by titanomaghemite formed during seafloor oxidation of host (detrital) titanomagnetite grains. Distortion of paleomagnetic records due to seafloor maghemitization appears to be especially important in the perennially ice covered western (Mendeleev–Alpha Ridge) and central Arctic Ocean (Lomonosov Ridge) and, to a lesser extent, near the ice edge (Yermak Plateau). On the Yermak Plateau, magnetic grain size parameters mimic the global benthic oxygen isotope record back to at least marine isotope stage 6, implying that magnetic grain size is sensitive to glacial–interglacial changes in bottom-current velocity and/or detrital provenance.

Biostratigraphy based on calcareous nannofossils, integrated by magnetostratigraphic, geochronological and isotopic data, allowed establishing a precise chronological framework for the Pleistocene ...

We report stable isotope, core scanning XRF, and ice-rafted detritus (IRD) data in glacial-aged sediments from piston Core KN166-14-JPC-13 (hereafter referred to as JPC-13) retrieved at 53.1°N 33.5°W on the southern Gardar Drift, North Atlantic. A chronology was established by correlating millennial-scale features in the benthic [delta]18O record to Portuguese Margin Core MD95-2042. Once the alignment of benthic [delta]18O of JPC-13 to MD95-2042 is fixed, the relative timing of proxy variables is used to determine the phasing of changes in IRD and surface-deep hydrography. Each peak in North Atlantic IRD coincided with a decrease in benthic [delta]18O that, in turn, has been linked to warming in Antarctica (Events A1-A7). The IRD pulses are followed shortly thereafter by abrupt decreases in planktonic [delta]18O, indicating warming and increased heat transport to the subpolar North Atlantic associated with Greenland interstadials (GIS) 8, 12,14, and 16-17. Grain-size and elemental (K/Ti) variations indicate that Iceland-Scotland Overflow Water (ISOW) was stronger during these long, warm interstadial periods of Marine Isotope Stage (MIS) 3. The results are consistent with interhemispheric phase relationships inferred from Iberian Margin sediment cores and with methane synchronization of Greenland and Antarctic ice cores.

A recently recovered w6-m long Holocene sedimentary sequence (piston Core 2004-804-650) collected from the Beaufort Sea (western Canadian Arctic) was dated by combining the paleomagnetic secular variation (PSV) and a time-varying spherical harmonic model of the geomagnetic field (CALS7k.2). A u-channel-based paleomagnetic study reveals the presence of a stable, single component of magnetization (MAD

article i nfo Existing correlations of Early Cretaceous nannofossil events to polarity zones in Italian pelagic limestones are the basis for correlation of polarity chrons to geologic stages, and hence calibration of the Early Cretaceous geomagnetic polarity timescale. Here, we present correlations of nannofossil events to polarity chrons for the Late Jurassic and Jurassic/Cretaceous boundary interval from the Southern Alps, making the case for revisions of current geologic timescales in this interval. The Jurassic/Cretaceous and Kimmeridgian/Tithonian boundaries can be usefully defined at the onsets of polarity chrons CM18R and CM22R, respectively. The Oxfordian/Kimmeridgian boundary can be assigned to CM25. An apparent polar wander path (APWP) for the Southern Alps, derived from, and time-calibrated by bio-magnetostratigraphic data, is similar to, and provides a proxy for, the African APWP in the absence of adequate African data of this age. The Southern Alpine APWP is characterized by a cusp (hairpin) at ∼30°N/270°E at a time close to the Oxfordian/ Kimmeridgian boundary (polarity chron CM25, ∼154 Ma), rapid APWP from high latitude (N60°N) in the preceding middle Jurassic (Bajocian-Oxfordian) interval coincident with the onset of Central Atlantic/ Ligurian oceanic rifting, and a return to high latitudes during the Cretaceous. Early Jurassic poles at high latitudes (N60°N) are evident for Sinemurian to Toarcian-Aalenian time (175-195 Ma). An APWP for this time interval with this level of temporal calibration cannot be constructed using data from Africa itself, or from other Atlantic-bordering continents by rotation to African coordinates.

Changes in surface and deep-water hydrography were inferred from variations in stable isotopes and sortable silt mean grain size, respectively, on the southern Gardar Drift in the subpolar North Atlantic. The bathymetric δ13C gradient during the penultimate glaciation was similar to the last glaciation with high- δ13C Glacial North Atlantic Intermediate water above ∼ 2000 m, and low- δ13C water derived from the Southern Ocean below. During Termination II, low-δ13C water was present throughout the water column with minimum values at intermediate depths (∼ 1500–2000 m) and below 3000 m. This pattern continued well into the early part of the Last Interglacial (LIG) period. Sortable silt mean size at 3275 m suggests that deep-water circulation on Gardar Drift was relatively weak during the earliest part of the LIG (128 to 124.5 ka) when planktonic δ18O was at a minimum, reflecting warming and/or reduced salinity. We suggest that low- δ13C water and slow current speed on Gardar Drift during the early part of the LIG was related to increased melt water fluxes to the Nordic Seas during peak boreal summer insolation, which decreased the flux and/or density of overflow to the North Atlantic. The resumption of the typical interglacial pattern of strong, well-ventilated Iceland Scotland Overflow Water was delayed until ∼ 124 ka. These changes may have affected Atlantic Meridional Overturning Circulation.

Sediment cores from the western Arctic Ocean obtained on the 2005 HOTRAX and some earlier expeditions have been analyzed to develop a stratigraphic correlation from the Alaskan Chukchi margin to the Northwind and Mendeleev–Alpha ridges. The correlation was primarily based on terrigenous sediment composition that is not affected by diagenetic processes as strongly as the biogenic component, and paleomagnetic inclination records. Chronostratigraphic control was provided by 14C dating and amino-acid racemization ages, as well as correlation to earlier established Arctic Ocean stratigraphies. Distribution of sedimentary units across the western Arctic indicates that sedimentation rates decrease from tens of centimeters per kyr on the Alaskan margin to a few centimeters on the southern ends of Northwind and Mendeleev ridges and just a few millimeters on the ridges in the interior of the Amerasia basin. This sedimentation pattern suggests that Late Quaternary sediment transport and deposition, except for turbidites at the basin bottom, were generally controlled by ice concentration (and thus melt-out rate) and transportation distance from sources, with local variances related to subsurface currents. In the long term, most sediment was probably delivered to the core sites by icebergs during glacial periods, with a significant contribution from sea ice. During glacial maxima very fine-grained sediment was deposited with sedimentation rates greatly reduced away from the margins to a hiatus of several kyr duration as shown for the Last Glacial Maximum. This sedimentary environment was possibly related to a very solid ice cover and reduced melt-out over a large part of the western Arctic Ocean.

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.

Sediments collected during the Integrated Ocean Drilling Program (IODP) Expedition 303 in the North Atlantic provide records of polarity transitions and geomagnetic excursions at a high resolution. Here we investigate polarity transitions at the upper and lower boundaries of the Jaramillo subchronozone at Sites U1305, U1304, and U1306. The sediments carry strong natural remanent magnetizations (NRM) with median destructive fields consistent with magnetite as the dominant magnetic carrier. Both polarity transitions are characterized by low values of relative paleointensity. The U1305 record of the lower Jaramillo reversal exhibits a marked cluster of virtual geomagnetic poles (VGPs) over southern South America, and a secondary accumulation in the region of NE Asia/North Pacific. The main South American VGP cluster is also visible at Site U1304, which documents a less complex pattern, possibly because of a higher degree of smoothing. Records of the upper Jaramillo polarity transition document a VGP loop over the Americas, followed by north to south motion including a secondary VGP accumulation near India. The similarity between several records of the upper Jaramillo transition obtained at various sites in the North Atlantic is a strong indication that geomagnetic field changes have been faithfully captured. Results suggest that a transverse, possibly dipolar, field component has fluctuated during these polarity reversals, and that these fluctuations combined with a reduced axial dipole component yielded the observed field at the Earth’s surface during the polarity transitions. The lower Jaramillo transition also exhibits VGP clusters in the vicinity of South America and eastern Asia but these clusters are shifted relative to the upper Jaramillo VGP clusters, implying a memory exerted by the upper mantle on polarity transition geometry.

Two piston cores recovered from the Chukchi and the Beaufort seas document Arctic Holocene geomagnetic field behaviour and highlight the potential of secular variation and relative paleointensity as a regional chronostratigraphic tool. Several centennial- to millennial-scale Holocene declination and inclination features can be correlated in both cores, with other high-resolution western North American lacustrine and volcanic paleomagnetic records and with records of changes in Earth’s dipole moment, supporting the geomagnetic origin of these features and implying that they are associated with changes in Earth’s dipole moment.

Orbital cycles with 100 kyr and/or 41 kyr periods, detected in some sedimentary normalized remanence (relative paleointensity) records by power spectral analysis or wavelet analysis, have been attributed either to orbital forcing of the geodynamo, or to lithologic contamination. In this study, local wavelet power spectra (LWPS) with significance tests have been calculated for seven relative paleointensity (RPI) records from different regions of the world. The results indicate that orbital periods (100 kyr and/or 41 kyr) are significant in some RPI records during certain time intervals, and are not significant in others. Time intervals where orbital periods are significant are not consistent among the RPI records, implying that orbital periods in these RPI records may not have a common origin such as orbital forcing on the geodynamo. Cross-wavelet power spectra (|XWT|) and squared wavelet coherence (WTC) between RPI records and orbital parameters further indicate that common power exists at orbital periods but is not significantly coherent, and exhibits variable phase relationships, implying that orbital periods in RPI records are not caused directly by orbital forcing. Similar analyses for RPI records and benthic oxygen isotope records from the same sites show significant coherence and constant in-phase relationships during time intervals where orbital periods were significant in the RPI records, indicating that orbital periods in the RPI records are most likely due to climatic ‘contamination’. Although common power exists at orbital periods for RPI records and their normalizers with significant coherence during certain time intervals, phase relationships imply that ‘contamination’ (at orbital periods) is not directly due to the normalizers. Orbital periods are also significant in the NRM intensity records, and ‘contamination’ in RPI records can be attributed to incomplete normalization of the NRM records. Further tests indicate that ‘contamination’ is apparently not directly related to physical properties such as density or carbonate content, or to the grain size proxy κ ARM / κ . However, WTC between RPI records and the grain size proxy ARM/IRM implies that ARM/IRM does reflect the ‘contamination’ in some RPI records. It appears that orbital periods were introduced into the NRM records (and have not been normalized when calculating RPI records) through magnetite grain size variations reflected in the ARM/IRM grain size proxy. The orbital power in ARM/IRM for some North Atlantic sites is probably derived from bottom-current velocity variations that are orbitally modulated and are related to the vigor of thermohaline circulation and the production of North Atlantic Deep Water (NADW). In the case of ODP Site 983, the orbital power in RPI appears to exhibit a shift from 41-kyr to 100-kyr period at the mid-Pleistocene climate transition (∼750 ka), reinforcing the climatic origin of these orbital periods. RPI records from the Atlantic and Pacific oceans, and RPI records with orbital periods eliminated by band-pass filters, are highly comparable with each other in the time domain, and are coherent and in-phase in time-frequency space, especially at non-orbital periods, indicating that ‘contamination’, although present (at orbital periods) is not debilitating to these RPI records as a global signal that is primarily of geomagnetic origin.

Fuller (Fuller, M., Geomagnetic field intensity, excursions, reversals and the 41,000-yr obliquity signal, Earth Planet. Sci. Lett. 245 (2006) 605–615.) pointed out that, for 9 reversals over the last 3 Myr, reversal age has a non-random relationship to the phase of orbital obliquity. Our analysis, based on Rayleigh tests, indicates that reversals have no preferred phase distribution in the obliquity cycle at the 5% significance level over the last 3 Myr. There is, however, a statistically significant relationship (at the 5% level) between reversal age and the phase of orbital eccentricity for the last 3 Myr, although this relationship breaks down on adding just a few reversals beyond 3 Ma. Over the last 5 Myr, reversals preferentially occurred during decrease of the maximum obliquity envelope although, yet again, the relationship does not hold as additional reversals are added to the analysis, no matter which timescale is tested. The Rayleigh tests are all based on the assumption of no uncertainty in reversal/excursion age, or in orbital solutions. Monte Carlo simulations indicate that reversal/excursion ages would have to be known within 5–10 kyr to resolve a preferred phase in obliquity similar to that advocated by Fuller (Fuller, M., Geomagnetic field intensity, excursions, reversals and the 41,000-yr obliquity signal, Earth Planet. Sci. Lett. 245 (2006) 605–615.) over the last 3 Myr. Reversal/excursion ages would have to be known within ~15 kyr to resolve a preferred phase in orbital eccentricity for reversals over the last 3 Myr, and within ~40 kyr for the last 25 Myr. Comparison of astrochronological reversal timescales indicates that reversal age uncertainties exceed these limits, making it unlikely that a relationship of reversal/excursion age to the phase of obliquity or eccentricity would be resolvable. In the case of the obliquity envelope, the critical levels of reversal age uncertainty (~50 kyr for 0–3 Ma, ~200 kyr for 0–5 Ma, and ~400 kyr for 0–25 Ma) are less stringent. The presence of a significant relationship between reversal age and phase of the obliquity envelope for the last 5 Myr, but not further back in time, implies either larger than expected reversal age uncertainties in pre-Pliocene polarity timescales and a link between reversal age and the obliquity envelope, or, more probably, the fortuitous occurrence of a low probability relationship over the last 5 Ma that has no mechanistic implication.

Ocean Drilling Program (ODP) Site 1092 from the sub-Antarctic South Atlantic produced a clear magnetic stratigraphy for the Late Miocene [Evans H.F., Channell, J.E.T., Upper Miocene Magnetic Stratigraphy from ODP Site 1092 (sub-Antarctic South Atlantic): recognition of cryptochrons in C5n, Geophys. Jour. Int., 153, (2003), 483–496]. Three short intervals of reverse polarity were identified within the long (∼ 1 Myr) normal polarity subchron C5n.2n. These excursion chrons were tentatively correlated to the three “cryptochrons” identified within subchron C5n.2n from NE Pacific marine magnetic anomaly data. New oxygen isotope data from ODP Site 1092 have allowed astronomical calibration of eight polarity chron boundaries in the interval between C4Ar.1n and C5r.1n (9.3–11.2 Ma) as well as calibration of the three excursion chrons identified within C5n.2n. The new polarity chron ages in the C4Ar.1n–C5r.1n interval differ from a current (2004) timescale by up to 48 kyr. The astrochronology yields durations in the 3–4 kyr range for the three excursion chrons in C5n.2n. The relative paleointensity record from Site 1092, and hence the three excursion chrons, can be correlated to the deep-tow magnetic anomaly record from 19°S on the East Pacific Rise (EPR). Based on published correlation of the EPR deep-tow record with the sea-surface magnetic anomaly stack from the NE Pacific, the three excursion chrons do not correspond to the “cryptochrons” recognized in the NE Pacific stack.

The Ocean Drilling Program (ODP) has recovered many long sediment sequences from around the World that contain medium to high-resolution paleomagnetic records of Brunhes age. These records have provided an important new global view of geomagnetic field variability during ‘stable’ magnetic (dipole) polarity that could not be recovered by conventional terrestrial or deep-sea piston coring. The ODP paleomagnetic records of directional variability can be routinely recovered and used regionally as a very highresolution relative chronostratigraphic tool (±200 yr resolution). ODP paleomagnetic records of Brunhes age relative paleointensity have dramatically improved our understanding of global geomagnetic field intensity. These records document the global-scale pattern of paleointensity and demonstrate its use as a high-resolution relative chronostratigraphic tool that under ideal conditions could have a precision of ±500 yr. ODP paleomagnetic records and associated oxygen isotope chronostratigraphies have also greatly improved our understanding of the number and ages of Brunhes geomagnetic field excursions, and their relationship to normal directional secular variation and paleointensity variability. Although significant progress has been made though ODP paleomagnetic studies of Brunhes-aged sediments, it is clear that more work is needed to further resolve and define the detailed global pattern of Brunhes PSV and its relationship to excursions and true polarity reversals.

Component natural remanent magnetizations derived from u-channel and 1-cm3 discrete samples from ODP Site 919 (Irminger Basin) indicate the existence of four intervals of negative inclinations in the upper Brunhes Chronozone. According to the age model based on planktic oxygen isotope data, these “excursional” intervals occur in sediments deposited during the following time intervals: 32–34 ka, 39–41 ka, 180–188 ka and 205–225 ka. These time intervals correspond to polarity excursions detected elsewhere, known as Mono Lake, Laschamp, Iceland Basin and Pringle Falls. The isotope-based age model is supported by the normalized remanence (paleointensity) record that can be correlated to other calibrated paleointensity records for the 0–500 ka interval, such as that from ODP Site 983. For the intervals associated with the Mono Lake and Laschamp excursions, virtual geomagnetic poles (VGPs) reach equatorial latitudes and mid-southerly latitudes, respectively. For intervals associated with the Iceland Basin and Pringle Falls excursions, repeated excursions of VGPs to high southerly latitudes indicate rapid directional swings rather than a single short-lived polarity reversal. The directional instability associated with polarity excursions is not often recorded, probably due to smoothing of the sedimentary record by the process of detrital remanence (DRM) acquisition.

An Oligocene magnetostratigraphy from ODP Sites 1218 and 1219 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 221 discrete samples. U-channel samples were measured at 1 cm intervals and were stepwise demagnetized in alternating fields (AF) up to a maximum peak field of 80 mT. The magnetization directions were determined at 1 cm intervals by principal component analysis of demagnetization steps in the 20 to 60 mT peak field range. A similar treatment was carried out on the discrete samples, which confirmed the results obtained with u-channel measurements. Sites 1218 and 1219 were precisely correlated based on multisensor track, paleontological and shipboard magnetostratigraphic data; this correlation is substantiated by u-channel measurements. Although the magnetostratigraphy obtained from the u-channels is similar to the interpretation deduced from shipboard measurements based on blanket demagnetization at peak AF of 20 mT, the u-channel results are substantially more robust since many interpretative uncertainties are resolved by the stepwise demagnetization and higher stratigraphic resolution. The temporal resolution of u-channel-based magnetic stratigraphy in the Oligocene section of Sites 1218 and 1219 is better than 5 kyr, and it is therefore suitable for detection of brief polarity subchrons. However, in spite of the high resolution, we did not find any reversals corresponding to the numerous cryptochrons identified in this time span by Cande and Kent [S.C. Cande, D.V. Kent, Revised calibration of the geomagnetic polarity time scale for the Late Cretaceous and Cenozoic, J. Geophys. Res. 100 (1995) 6093–6095].

A c. 50 m thick section located in the Crotone Basin (southern Italy) was investigated using oxygen isotopes, pollen and planktonic foraminifera. The section records two complete trans- gressive-regressive cycles mainly driven by glacio-eustasy. Biostratigraphy and oxygen isotope chronology indicate that the section spans from Marine Isotope Stage (MIS) 22 (c. 0.87 Ma) to MIS 18.3 (c. 0.73 Ma), thus straddling the Matuyama-Brunhes (M-B) boundary which occurs in the middle of MIS 19. The rich pollen assemblages provide a unique record of the vegetation in the central Mediterranean during the Early-Middle Pleistocene climatic transition. Interglacials are characterized by a mesothermic vegetation similar to the present day, whereas a rain-demanding conifer forest dominates the glacials of MIS 20 and MIS 18. This is unexpected because it is gener- ally considered that during the Pleistocene, glacials in central Mediterranean were characterized by steppe (arid) conditions. By contrast, arid conditions occur during the deglaciations. These results are inconsistent with the widespread practice of linking glacials with arid conditions in the central Mediterranean during Pliocene and Early Pleistocene times. This study emphasizes the need to establish more accurate land-sea correlation.

Pliocene and Miocene magnetostratigraphy from ODP Site 1218 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 50 discrete samples. U-channel samples were measured at 1 cm intervals and stepwise demagnetized in alternating fields up to a maximum peak field of 80 mT. The component magnetization directions were determined by principal component analysis for demagnetization steps in the 20–60 mT peak field range. A relatively small number of discrete samples were subject to both thermal and alternating field (AF) demagnetization and gave results compatible with u-channel measurements. Magnetostratigraphy from u-channel samples are compared with shipboard data that were based on blanket demagnetization at peak AF fields of 20 mT. U-channel measurements add more detail to the magnetostratigraphic record and allow identification of thin polarity zones especially in the upper part of the section were the sedimentation rates are very low (~2 m/Myr). The component magnetization directions determined from u-channel measurements also gave more reliable and precise estimates of inclination (paleolatitude). The magnetostratigraphy from Site 1218 can be unambiguously correlated with the reference geomagnetic polarity time scale and gives a means of dating the sedimentary sequence. Both Miocene–Pliocene and Oligocene–Miocene stage boundaries were easily identified from the magnetostratigraphic record. Although calculation of paleomagnetic poles is hindered by the low precision of the cores’ azimuthal orientation, the data from both u-channel and discrete samples allow determination of the paleolatitude of the Site through time with good precision. Paleomagnetic data indicate that the paleolatitude of Site 1218 has increased form nearly equatorial latitude in the Oligocene to

SUMMARY Ocean Drilling Program (ODP) sites 984 and 983, located on North Atlantic sediment drifts, provide high-resolution records across the Matuyama‐Brunhes boundary. At ODP site 984 (Bjorn Drift, Iceland Basin), the mean sedimentation rate in the 500‐900 ka interval is ∼12 cm kyr −1 based on an age model derived by matching the planktonic and benthic oxygen isotope records to an Ice Volume Model. The Matuyama‐Brunhes polarity transition at site 984, as defined by virtual geomagnetic polar (VGP) latitudes

At Ocean Drilling Program (ODP) Site 1090 (subantarctic South Atlantic), benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through early Miocene (f24-16 Ma) at a temporal resolution of f5 ky. Over the same interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model, we use the newly derived age of the Oligocene/Miocene (O/M) boundary of 23.0 Ma of Shackleton et al. (Geology 28 (2000) 447), revised to the new astronomical calculation (La2003) of Laskar et al. (Icarus (in press)) to recalculate the spline ages of Cande and Kent (J. Geophys. Res. 100 (1995) 6093). We then tune the Site 1090 y 18 O record to obliquity using La2003. In this manner, we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n through C6Bn from Shackleton et al. (Geology 28 447-450 (2000)) when rescaled to La2003. The results from Site 1090 provide independent evidence for the revised age of the Oligocene/Miocene boundary of 23.0 Ma. For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are generally within one obliquity cycle of those obtained by rescaling the Cande and Kent (J. Geophys. Res. 100 (1995) 6093) interpolation using the new age of the O/M boundary (23.0 Ma) and the

SUMMARY An Upper Miocene magnetic polarity stratigraphy has been developed for the C3r to C5ABn interval of ODP site 1092 from the sub-Antarctic South Atlantic (Evans & Channell 2003). New data from X-ray fluorescence (XRF) scanning of half cores has led to development of a revised composite depth section for the site. This has implications for the magnetostratigraphic interpretation. Using the shipboard-derived composite section, four subchrons were identified within the normal polarity zone correlative to C5n.2n. The revised composite section results in subchrons C5n.2n-2 and C5n.2n-3 of Evans & Channell (2003) becoming a single subchron recorded in two overlapping cores. The revised number of polarity subchrons within C5n.2n is now three, rather than four, which is consistent with the number of ‘cryptochrons’ recognized in marine oceanic anomaly data.