Age model of ODP Site 162-983

At ODP Site 983, relative geomagnetic paleointensity and planktic and benthic delta18O records have been acquired for the last 350 kyr. The mean sedimentation rate in this interval is 11.3 cm/kyr. Magnetic properties and hysteresis ratios indicate that pseudo-single domain magnetite is the remanence carrier. Volume susceptibility (kappa), anhysteretic (ARM) and isothermal (IRM) remanence values vary by a factor of 3-4, well within the criteria usually cited for paleointensity studies. Natural remanent magnetization (NRM) is normalized by ARM and IRM to acquire the paleointensity proxy. Arithmetic means of NRM/ARM and NRM/IRM, calculated for five demagnetization steps in the 25-45 mT range, constitute the relative paleointensity estimates. Some paleointensity lows (particularly those at ~40, ~120 and ~188 ka) are associated with directional excursions of the field, especially the event at ~188 ka (referred to here as the Iceland Basin Event) that constitutes a short-lived polarity reversal. For the last 200 kyr, the records can be correlated with other high-resolution paleointensity records such as those from the Labrador Sea, Mediterranean/Somali Basin and Sulu Sea, implying that the millennial scale features are globally synchronous. A labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages.

Natural oxides, elements and remanent magnetization intensities and directions at DSDP Hole 93-603B

Dark green spherules occur in the lower part of a turbidite in Section 603B-22-3, at the 70 cm level. In all probability these spherules originally consisted of massive glass, but now appear to have become completely altered into smectite. The presence of numerous microscopic fissures in the spherules probably mediated in the alteration process. Judging by the presence of similar spherules at the Cretaceous/Tertiary (K/T) boundary in DSDP Hole 390B, the green spherules are thought to represent diagenetically altered impact ejecta from one large or several smaller extraterrestrial objects at the end of the Cretaceous. The presence of anomalously high concentrations of Ni, Co, and As higher up in the turbidite are in agreement with an expected enrichment of these elements in the K/T boundary clay. However, precise Ir analyses are necessary in order to confirm this.

Inclination of the two main components of magnetization and 87Sr/86Sr ratios for dolomites at DSDP Hole 77-536

Site 536 terminated in a shallow-water dolomite of unknown age. Paleomagnetic measurements combined with strontium isotope analyses suggest that the dolomite was deposited in the Middle Jurassic to Early Cretaceous time interval. However, the assumptions required to reach this determination make these results less than conclusive.

Paleomagnetic reconstructions of sediments at ODP Sites 135-840 and 135-841

The geometry of the Tonga Arc implies that it has rotated approximately 17° clockwise away from the Lau Ridge as the Lau Basin formed in between. Questions have arisen about the timing of the opening, whether the arc behaved rigidly, and whether the opening occurred instead from motion of the Lau Ridge, the remanent arc. We undertook to address these questions by taking paleomagnetic samples from sediment cores drilled on the Tonga Arc at Sites 840 and 841, orienting the samples in azimuth, and comparing the paleodeclinations to expected directions. Advanced hydraulic piston corer (APC) cores from Holes 840C and 841A were oriented during drilling with a tool based on a magnetic compass and attached to the core barrel. Samples from Hole 841B were drilled with a rotary core barrel (RCB) and therefore are azimuthally unoriented. They were oriented by identifying faults and dipping beds in the core and aligning them with the same features in the Formation MicroScanner (FMS) wireline logs, which were themselves oriented with a three-axis magnetometer in the FMS tool. The best results came from the APC cores, which yielded a mean pole at -69.0°S, 112.2°E for an age of 4 Ma. This pole implies a declination anomaly of 20.8° ± 12.6° (95% confidence limit), which appears to have occurred by tectonic rotation of the Tonga Arc. This value is almost exactly that expected from the geometry of the arc and implies that it did indeed rotate clockwise as a rigid body. The large uncertainty in azimuth results from core orientation errors, which have an average standard deviation of 18.6°. The youngest cores used to calculate the APC pole contain sediments deposited during Subchron 2A (2.48-3.40 Ma), and their declinations are indistinguishable from the others. This observation suggests that most of the rotation occurred after their deposition; this conclusion must be treated with caution, however, because of the large azimuthal orientation errors. Poles from late and early Miocene sediments of Hole 841B are more difficult to interpret. Samples from this hole are mostly normal in polarity, fail a reversal test, and yield poles that suggest that the normal-polarity directions may be a recent overprint. Late Miocene reversed-polarity samples may be unaffected by this overprint; if so, they imply a declination anomaly of 51.1° ± 11.5°. This observation may indicate that, for older sediments, Tonga forearc rotations are larger than expected.

Natural remanent magnetization (NRM) from ODP Site 1263 covering magnetochron C20r and C21n and high-resolution bulk carbon isotope (d13C) records from Sites 702 and 1263

To explore cause and consequences of past climate change, very accurate age models such as those provided by the astronomical timescale (ATS) are needed. Beyond 40 million years the accuracy of the ATS critically depends on the correctness of orbital models and radioisotopic dating techniques. Discrepancies in the age dating of sedimentary successions and the lack of suitable records spanning the middle Eocene have prevented development of a continuous astronomically calibrated geological timescale for the entire Cenozoic Era. We now solve this problem by constructing an independent astrochronological stratigraphy based on Earth's stable 405 kyr eccentricity cycle between 41 and 48 million years ago (Ma) with new data from deep-sea sedimentary sequences in the South Atlantic Ocean. This new link completes the Paleogene astronomical timescale and confirms the intercalibration of radioisotopic and astronomical dating methods back through the Paleocene-Eocene Thermal Maximum (PETM, 55.930 Ma) and the Cretaceous-Paleogene boundary (66.022 Ma). Coupling of the Paleogene 405 kyr cyclostratigraphic frameworks across the middle Eocene further paves the way for extending the ATS into the Mesozoic.

Low-temperature experiments of sediment core GeoB6229-6 at the South American continental margin off the Rio de la Plata estuary

With various low-temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the Argentine continental slope near the Rio de la Plata estuary, a so far unreported style of partial magnetic self-reversal has been detected. In these sediments the sulphate-methane transition (SMT) zone is situated at depths between 4 and 8 m, where reductive diagenesis severely alters the magnetic mineral assemblage. Throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions. In the SMT zone (titano-)magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium-rich titanohematite, originating from high temperature deuteric oxidation within the volcanic host rocks. These abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy. Warming of field-cooled and zero-field-cooled low-temperature saturation remanence displays magnetic phase transitions of titanium-rich titanohematite below 50 K and the Verwey transition of magnetite. A prominent irreversible decline characterizes zero-field cooling of room temperature saturation remanence. It typically sets out at ~210 K and is most clearly developed in the lower part of the SMT zone, where low-temperature hysteresis measurements identified ~210 K as the blocking temperature range of a titanohematite phase with a Curie temperature of around 240 K. The mechanism responsible for the marked loss of remanence is, therefore, sought in partial magnetic self-reversal by magnetostatic interaction of (titano-)magnetite and titanohematite. When titanohematite becomes ferrimagnetic upon cooling, its spontaneous magnetic moments order antiparallel to the (titano-)magnetite remanence causing an drastic initial decrease of global magnetization. The loss of remanence during subsequent further cooling appears to result from two combined effects (1) magnetic interaction between the two phases by which the (titano-)magnetite domain structure is substantially modified and (2) low-temperature demagnetization of (titano-)magnetite due to decreasing magnetocrystalline anisotropy. The depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments. Typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics. Low-temperature remanence cycling is, therefore, proposed as a diagnostic method to magnetically characterize such alteration (palaeo-)environments.

Paleomagnetic of ODP Leg 133 holes

Paleomagnetic and rock-magnetic analyses from discrete samples of carbonate sites on the Queensland Plateau were used to determine magnetic polarity reversal stratigraphy and the nature of magnetization in these sediments. Magnetic polarity zones were correlated with the geomagnetic polarity time scale in the upper portions of cores at Sites 812 through 814, usually back to a late Pliocene age. Loss of reliable directional data was coincidental with a major decrease in magnetic intensity, below which, no stable polarity zones could be identified. The intensity reduction is either an in-situ alteration of magnetic grains, or an input signal representing progressive increase in the magnetic component of Queensland Plateau sediments. Although not conclusive at this point, the geochemical conditions and differing age of intensity reduction support the former hypothesis. Rock-magnetic analysis of carbonate sediments suggests that ultrafine-grained magnetite or maghemite crystals is an important carrier of remanence and may be biogenic in origin. Application of a recently calibrated anhysteretic remanent magnetization test to assess configuration of single-domain crystal within a natural matrix indicates that cementation (ooze-chalk-limestone) may be important in post-depositional changes affecting magnetostatic grain interaction.