SUMMARY A 37 m long piston core (KC-O1B) was collected from the Calabrian Ridge in the Ionian Sea. Astronomical calibration of sapropels and sapropelic signals-based on rock-magnetic and geochemical properties-yields a very accurate time-frame for the last 1.1 Myr, with the exception of the interval (65&850 ka) straddling the Brunhes/ Matuyama boundary. We used the oxygen isotope data as an additional constraint on our age model. We arrive at an age of 812 ka for the Brunhes/Matuyama boundary, which indicates that its position is determined by delayed acquisition of NRM by some 40 kyr. The age of the top of Jaramillo subchron (1001+3 ka) is slightly older than that of Shackleton, Berger & Peltier ( 1990); that of its bottom ( 1072 f 5 ka) concurs with their estimate. In the Brunhes Chron, four short reversal excursions (CRO-3) were found and dated (CRO 261 f 3 ka, CR1 318 f 3 ka, CR2 515 f 3 ka, CR3 573 3 ka). In all cases, a highcoercivity reversed component is overlapped to a varying degree with a low-coercivity normal component. Three of the reversal excursions (CR1-3) occur in zones where diagenetic processes were shown to be minimal. The fourth (CRO), in which a very large coercivity overlap occurs, is not well expressed and is found in a zone where some dissolution of magnetic minerals has occurred. The existing literature on reversal excursions is reviewed, and some of the older records with an oxygen isotope or sapropel record have been redated. CRO could correspond to one of the Fram Strait excursions (Nowaczyk et at. 1994). CR3 corresponds to the Emperor, which is shown to be equivalent to the Big Lost reversal excursion. For CR1 and CR2 we propose the names Calabrian Ridge 1 and Calabrian Ridge 2, since correlation with existing reversal excursions is uncertain. The Blake was not detected because extensive diagenesis has occurred in the corresponding interval. All reversal excursions observed in KC-O1B correspond to periods of minimal (relative) palaeointensity in the independently dated record of Valet & Meynadier (1993). This seems to imply that reversal excursions are more likely to occur in periods with a relatively large non-dipole field contribution

https://hal.archives-ouvertes.fr/hal-03357057/document

Magnetostratigraphy and astronomical calibration of the last 1.1 Myr from an eastern Mediterranean piston core and dating of short events in the Brunhes

K-Ar ages and paleomagnetic data for basalt samples from a new core hole (site E) at the Idaho National Engineering Laboratory (INEL) indicate that the age of the reversed polarity event recorded in Snake River Plain lavas is older than 465±50 ka (1000 years before present) reported previously by Champion et al. (1981). Nine basalt flows, eight with normal polarity and one with reversed polarity, were recognized in the site E core hole. The flows above and below the reversed flow have ages of 491±80 ka and 580±93 ka, respectively. The inclination of the paleomagnetic field direction of the reversed flow at site E agrees with the inclination of reversed flows elsewhere at INEL which have an age of 565±14 ka. These reversed flows were previously thought to be correlative with the Emperor event. We suggest that this polarity event is an older event which we name the Big Lost Reversed Polarity Subchronozone and Subchron. A review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the Brunhes and two besides the Jaramillo in the late Matuyama. These 10 short subchrons begin to indicate the many short events that Cox (1968) hypothesized must exist if polarity interval lengths have a Poisson distribution. These events are true subchrons, not excursions, and may or may not have low associated paleointensities, although low field strengths might explain why the reversal process aborts. The mean sustained polarity interval length since late Matuyama Chron time is 90,000 years. The similarity of this number with the 105-year period of the Earth's orbital eccentricity suggests anew that linkage between geomagnetic, paleoclimatic, and possible underlying Earth orbital parameters should be evaluated.

Evidence for a new geomagnetic reversal from Lava flows in Idaho: Discussion of short polarity reversals in the Brunhes and late Matuyama polarity chrons

http://www.gm.univ-montp2.fr/IMG/pdf/Lallemand_Jolivet_1985-86_EPSL.pdf

Japan Sea: a pull-apart basin?

https://people.clas.ufl.edu/jetc/files/lundetal2006.pdf

A summary of Brunhes paleomagnetic field variability recorded in Ocean Drilling Program cores

Sur la direction du champ magnetique terrestre, en France, durant les deux derniers millenaires

Oscillating Geomagnetic Field with a Recurring Reversal Discovered from Lake Biwa

A core 197.2m long was taken from the central region of Lake Biwa. About 10000 serial specimens were prepared from the axis of the core to study palaeomagnetism. They have remanent magnetization stable enough to allow us to decipher the geomagnetic field of the past. The palaeomagnetic results are shown in detail down to 30m from the top of the core. The age of this depth is estimated at about 60000yr B. P. Smoothed curves of magnetic inclination and intensity as functions of depth are obtained by a seven-point moving average, reflecting the variation of the past geomagnetic field.

Palaeomagnetism of a Core from Lake Biwa (I)

AS part of the joint study on the palaeolimnology of Lake Biwa-ko and the Japanese Pleistocene, a core 197.2 m long was taken at a point 65.2 m deep in the central region of the lake. The core, containing about forty thin layers of volcanic ashes, is composed chiefly of gyttja and fine clay and is now being analysed in various ways, for example palaeomagnetically. A continuous series of almost 10,000 oriented specimens (each 2×2×2 cm) was taken along the central axis of the core for palaeomagnetic investigations.

Very Short Period Geomagnetic Excursion 18,000 yr BP

On a Possible Land Block Movement of Hokkaido Relative to the Main Island of Japan

The Median Tectonic Line (MTL) is one of the longest tectonic lines in Japan with the lateral movement In order to investigate the subsurface structure around the MTL, we carried out the magnetotelluric soundings and the geomagnetic depth soundings in the northeastern Shikoku district, southwest Japan We estimated the resistivity model at the depth from the surface to 10 km, which explains the observed apparent resistivities and phases The most remarkable feature of the resistivity model is that the subsurface extension of the MTL has a north dip down to 5 km depth This dipping interface also corresponds to a reflection seismic result The north dipping MTL cannot be created by just a present strike slip motion It requires that the horizontally compressive stress dominated in the early stage of the fault

https://link.springer.com/content/pdf/10.1186%2FBF03352127.pdf

Katsumi Yaskawa

Papers

Oscillating Geomagnetic Field with a Recurring Reversal Discovered from Lake Biwa

Palaeomagnetism of a Core from Lake Biwa (I)

Very Short Period Geomagnetic Excursion 18,000 yr BP

On a Possible Land Block Movement of Hokkaido Relative to the Main Island of Japan

The electrical structure across the Median Tectonic Line in east Shikoku, southwest Japan