[1] Variations in Mg/Ca-based sea surface temperature and oxygen isotope ratio (d 18 O) of the surface water in the northern East China Sea (ECS) were reconstructed with high resolution during the last 18 kyr using planktic foraminifera. Millennial-scale variations between warmer, more saline surface water and cooler, less saline surface water were recognized during the early deglacial period and the Holocene, suggesting changes in the mixing ratio between the Kuroshio Water and the Changjiang Diluted Water. Stronger East Asian summer monsoon (EASM) precipitation events in south China are identified at 10.5, 8.8, 7.0, 5.3, 4.7, 2.9, 1.7, and 0.5 ka, based on sea surface salinity (SSS) records of the northern ECS. Weaker EASM precipitation events are also detected at 9. 3, 8.3, 7.3, 6.0, 3.3, 2.3, 0.7, and 0.4 ka during the Holocene. These events agree with the maxima in d18 O records of stalagmites from various parts of the Changjiang (Yangtze) River drainage. This agreement supports that our SSS record properly captures the millennial-scale dry (less EASM precipitation) events over the drainage basin of the Changjiang River during the Holocene. These dry events are also in good agreement with North Atlantic ice-rafted events, suggesting a teleconnection between North Atlantic climate and the EASM during the Holocene.

[1] We have conducted a paleomagnetic study of six sediment cores taken from the equatorial Pacific, three from the West Caroline Basin and three from the Manihiki Plateau, in order to make a relative paleointensity stack during the Matuyama and late Gauss Chrons. The age of the bottom of the cores ranges from 1.2 to 3.0 Ma. The sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity. The age of the cores is controlled by correlating variations in magnetic concentration (magnetic susceptibility and/or anhysteretic remanent magnetization (ARM)) to target oxygen-isotope (d 18 O) curves. All cores from the Manihiki Plateau show an upward decrease of the natural remanent magnetization (NRM) intensity normalized by ARM and isothermal remanent magnetization (IRM) with a decrease in sedimentation rate. Such a trend was removed before being converted to relative paleointensity. This observation implies that a sedimentation-rate change can affect relative paleointensity estimation. Relative paleointensity records from the six cores coincide well with each other within uncertainty of age. We constructed a stacked curve between 0.8 and 3.0 Ma (the equatorial Pacific paleointensity stack EPAPIS-3Ma) after adjusting age; the number of cores stacked is three to four throughout the record. Quasiperiodic paleointensity lows occur in the EPAPIS-3Ma, and corresponding paleointensity lows can be found in previously published records with some shifts in age. At least, parts of the paleointensity minima seem to be accompanied by geomagnetic excursions. The ''asymmetric sawtooth pattern'' of paleointensity variations is not observed at the Matuyama-Gauss boundary and thereafter in our record. A spectral analysis shows that $100 kyr orbital eccentricity frequency may exist in paleointensity variations during the Matuyama and late Gauss Chrons.

A continuous record of the inclination and intensity of Earth's magnetic field, during the past 2.25 million years, was obtained from a marine sediment core of 42 meters in length. This record reveals the presence of 100,000-year periodicity in inclination and intensity, which suggests that the magnetic field is modulated by orbital eccentricity. The correlation between inclination and intensity shifted from antiphase to in-phase, corresponding to a magnetic polarity change from reversed to normal. To explain the observation, we propose a model in which the strength of the geocentric axial dipole field varies with 100,000-year periodicity, whereas persistent nondipole components do not.

Deconvolution of long‐core paleomagnetic data of the Ocean Drilling Program (ODP) was developed based on Bayesian statistics. Samples of deep‐sea sediments obtained by drilling with an advanced piston corer are subjected to pass‐through measurements by a cryogenic magnetometer aboard the ODP's research vessel (Joides Resolution) which provide continuous paleomagnetic records. Deconvolution of the magnetometer output was developed in order to obtain more detailed variations of the magnetization than permitted by the spatial resolution of the pickup coil (approximately 11 cm). The magnetization vector was modeled as a smoothly changing sequence parametrized by L2 norm of second‐order difference and then deconvolved using a matrix calculation as a smoothness constrained least squares method. The optimum smoothness was obtained by minimizing Akaike's Bayesian Information Criterion which is a measure of the logarithm of the likelihood. This deconvolution scheme was applied to pass‐through data artificially produced by convolving synthetic reversal record and adding a Gaussian noise. The amplitude of the noise was estimated and the variations shorter than the spatial resolution of the sensor were obtained. The deconvolution also was applied to the real pass‐through data from Holes 769A and 769B measured at intervals of 5 mm. The deconvolution revealed variations of the magnetizations with a maximum spatial resolution of about 2 cm. The magnetizations after deconvolution were in good agreement with magnetization values measured separately on cube samples taken at intervals of 5 or 10 cm.

[1] We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40 Ar/ 39 Ar ages yielding a weighted mean of 771 ± 8 (1s) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 mT corresponding to virtual dipole moments (VDMs) between 0.9 × 10 22 and 9.6 × 10 22 Am 2 . For the major directional changes, a weak paleointensity of 5 mT is obtained, which gives a VDM of 1.0 × 10 22 Am 2 . For the normal polarity period, paleointensities are 14-21 mT, giving VDMs of 3.5-5.2 × 10 22 Am 2 . For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

No abstract

[1] We have conducted a geophysical survey of the northern Mariana Trough from 19°N to 24°N. The trough evolves southward from incipient rifting to seafloor spreading within this region. This study aims to clarify the location and time of the rifting-to-spreading transition, which was controversial previously, and processes of seafloor spreading after the transition. The new data set includes swath bathymetry with sidescan images and magnetic vector anomaly. The mantle Bouguer gravity anomaly (MBA) was calculated using the free-air gravity anomaly from satellite altimetry. The rifting-to-spreading transition occurs at about 22°N, which is proved by seafloor-spreading fabric in the bathymetry, clear magnetic lineations, and the bull's-eye pattern in MBA. Four ridge segments separated by three nontransform discontinuities are recognized between 19°N and 22°N. The northernmost segment has relatively abundant magma supply compared with the other segments, which is estimated from a larger segment length, shallower axial depths with no rift valley, and lower MBA. The next segment to the south is, on the other hand, a magma-starved segment with a prominent rift valley. Two anomalously deep grabens (called the Central Grabens) formed by amagmatic extension occur near the segment ends. The succession of magma-rich, magma-starved, and normal segments with increasing distance from the volcanic arc is the same as the observation in the Lau Basin reported by Martinez and Taylor spreading center have been significantly larger than the eastern counterpart in general. The asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading.

The development of pass-through superconducting rock magnetometers (SRM) has greatly promoted collection of paleomagnetic data from continuous long-core samples. The output of passthrough measurement is smoothed and distorted due to convolution of magnetization with the magnetometer sensor response. Although several studies could restore high-resolution paleomagnetic signal through deconvolution of pass-through measurement, difficulties in accurately measuring the magnetometer sensor response have hindered the application of deconvolution. We acquired reliable sensor response of an SRM at the Oregon State University based on repeated measurements of a precisely fabricated magnetic point source. In addition, we present an improved deconvolution algorithm based on Akaike's Bayesian Information Criterion (ABIC) minimization, incorporating new parameters to account for errors in sample measurement position and length. The new algorithm was tested using synthetic data constructed by convolving ''true'' paleomagnetic signal containing an ''excursion'' with the sensor response. Realistic noise was added to the synthetic measurement using Monte Carlo method based on measurement noise distribution acquired from 200 repeated measurements of a u-channel sample. Deconvolution of 1000 synthetic measurements with realistic noise closely resembles the ''true'' magnetization, and successfully restored finescale magnetization variations including the ''excursion.'' Our analyses show that inaccuracy in sample measurement position and length significantly affects deconvolution estimation, and can be resolved using the new deconvolution algorithm. Optimized deconvolution of 20 repeated measurements of a u-channel sample yielded highly consistent deconvolution results and estimates of error in sample measurement position and length, demonstrating the reliability of the new deconvolution algorithm for real pass-through measurements.