Search

Knowledge of the evolution of North Atlantic Deep Water (NADW) is key to understanding the past evolution of the climatic system. Here we developed a new rock-magnetic method to determine the constituent magnetic minerals of sediments and report on the evolution of NADW during 2.2–2.9 Ma. We measured isothermal remanence acquisition curves of North Atlantic deep-sea sediments drilled at the Gardar Drift and decomposed the first derivatives of these curves into high-coercivity and low-coercivity components. Residuals of the decomposition were sufficiently small throughout the study interval, confirming that the Gardar Drift sediments represent a mixing of the two end-members. Fractional changes of the high-coercivity component represent variation of the Iceland-Scotland Overflow Water, a branch of NADW formed at the Nordic Seas. The high-coercivity component increased significantly during an interglacial period just after ~2.68 Ma, which suggests that NADW formation in the Nordic Seas abruptly intensified at this time.

At two strain-sensitive wells located close (∼5km) to seismic swarm activities off the Izu Peninsula, Japan, we observed water-level changes that were synchronized with bursts of seismic activity during an active period in October 1996. The water levels started to drop at the onset of each burst, and stopped dropping as it ceased. This is the first hydrological observation in response to bursts in this area. The data suggests the occurrence of some local crustal deformation at the time of the bursts of seismicity.

The water level in an observation well (EDY) has been monitored with high frequency sampling (1/sec). Large changes in the water level are observed in this well, which correspond to dynamic strain changes due to the passage of long period seismic waves. For example, fluctuations of more than 200 cm were observed after a M 8.1 earthquake, 1200 km from the observation site. The response characteristic of the water level to strain change was investigated by comparison with nearby volumetric strain changes: the response curve has a peak of sensitivity and a phase transition at periods of 18∼19 seconds. At the resonant period, the sensitivity is as large as 15 mm per 10−9 strain.

The paleomagnetism of a sediment core (KT89-18, P4), recovered from the sea floor off Southwest Japan, was studied to investigate geomagnetic field variation over the past 35,000 years. Alternating fields demagnetization revealed that the remanent magnetization of the unconsolidated sediments was stable and not affected by any secondary overprints. 14C datings were made on 21 horizons, using a Tandetron accelerator mass spectrometer, providing a detailed time scale to the core. The inclination record agrees with that from Lake Biwa [Kawai et al., 1975], and supports the existence of the geomagnetic excursions which had been found in the Lake Biwa core. The ages of the excursions are estimated to be during ca. 32 ky.B.P. ∼ ca. 28 ky.B.P. and ca. 26 ky.B.P. ∼ ca. 24 ky.B.P.

Locations of the geomagnetic pole over the past 10,000 years have been calculated by averaging the VGP positions obtained from paleomagnetic data. The distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45° and 225° longitude, and westward movement of the pole was predominant throughout this period. The time sequence of the polar motion can be divided into three intervals, the intervals between ca. 10,000 B.P. and ca. 7000 B.P., between ca. 7000 B.P. and ca. 3700 B.P., and between ca. 3700 B.P. and the present. During the period between ca. 7000 and ca. 3700 B.P., the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole. Before and after this period, the movement was very active, fluctuating over 10 degrees. The results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by Merrill and McElhinny [1983].