Masaru Kono

Effect of the inner core on the numerical solution of the magnetohydrodynamic dynamo

Abstract We report two simulation results of the magnetohydrodynamic dynamo applied to rapidly rotating spherical systems using fully nonlinear equations under Boussinesq approximation. Calculations were carried out under the same parameter conditions but for a spherical shell and a sphere. We assume that a uniform internal heat source distributed in the whole sphere drives the convection and dynamo and that the physical properties of the inner core are identical to those of the fluid outer core except for its rigidity. This treatment enables us to compare two cases under the same condition, except the existence of the inner core. Magnetic field is effectively generated by strong velocity shear and helicity of the fluid near the top (and bottom) boundaries. A stable axial dipole field develops in the case of the spherical shell because of the steady field generation at both the outer and inner boundaries, while the magnetic field in the sphere fluctuates with time from lack of the bottom boundary before it reaches the dipole dominant state at last. This result suggests that the Earths magnetic field may be stabilized as the inner core grows, even though the total energy input is the same. This study provides a first step to interpret the paleointensity data from the Archaean when there was a transition due to the growth of the inner core.

Roles of the Cyclic Electron Flow Around PSI (CEF-PSI) and O2-Dependent Alternative Pathways in Regulation of the Photosynthetic Electron Flow in Short-Term Fluctuating Light in Arabidopsis thaliana

To assess the roles of the cyclic electron flow around PSI (CEF-PSI) and O₂-dependent alternative pathways including the water-water cycle in fluctuating light, we grew the wild type and pgr5 mutant of Arabidopsis thaliana in constant light, and measured Chl fluorescence and P700 parameters in their leaves in the fluctuating light alternating between 240 (HL) and 30 µmol photons m⁻² s⁻² (LL) every 2 min. At 20% O₂, the photochemical quantum yield of PSII decreased, in particular in the pgr5 plants, soon after the start of the fluctuating light treatment. PSI of the pgr5 plants was markedly photoinhibited by this treatment for 42 min. Slight PSI photoinhibition was also observed in the wild type. We measured energy sharing between PSII and PSI and estimated the PSI and PSII electron transport rates (ETRs). pgr5 showed larger energy allocation to PSI. In contrast to the wild type, the ratio of the PSI to the PSII ETR in pgr5 was higher in LL but lower in HL at 20% O₂ due to PSI acceptor-side limitation. At 2.7% or 0% O₂, the CEF-PSI of the pgr5 plants was enhanced, the acceptor-side limitation of PSI electron flow was released and PSI photoinhibition was not observed. The results suggest that the light fluctuation is a potent stress to PSI and that the CEF-PSI is essential to protect PSI from this stress.

Long-term and short-term responses of the photosynthetic electron transport to fluctuating light

Light energy absorbed by chloroplasts drives photosynthesis. When absorbed light is in excess, the thermal dissipation systems of excess energy are induced and the photosynthetic electron flow is regulated, both contributing to suppression of reactive oxygen species production and photodamages. Various regulation mechanisms of the photosynthetic electron flow and energy dissipation systems have been revealed. However, most of such knowledge has been obtained by the experiments conducted under controlled conditions with constant light, whereas natural light condition is drastically fluctuated. To understand photosynthesis in nature, we need to clarify not only the mechanisms that raise photosynthetic efficiency but those for photoprotection in fluctuating light. Although these mechanisms appear to be well balanced, regulatory mechanisms achieving the balance is little understood. Recently, some pioneering studies have provided new insight into the regulatory mechanisms in fluctuating light. In this review, firstly, the possible mechanisms involved in regulation of the photosynthetic electron flow in fluctuating light are presented. Next, we introduce some recent studies focusing on the photosynthetic electron flow in fluctuating light. Finally, we discuss how plants effectively cope with fluctuating light showing our recent results.

Age and magnetism of lavas in Jökuldalur area, Eastern Iceland: Gilsá event revisited

Abstract We present results of paleomagnetic measurements and K–Ar age determinations of 38 lava flows collected in five separate sections in the Jokuldalur area of Eastern Iceland, including one section previously studied by Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444]. These sites are close to the type locality of the normal “Gilsa event” in the Matuyama chron first identified by McDougall and Wensink [McDougall, I., Wensink, H., 1966. Paleomagnetism and geochronology of the Pliocene–Pleistocene lavas in Iceland. Earth Planet. Sci. Lett. 1, 232–236]. Using new experimental results as well as field observations, we could correlate the sequence of lava flows in the Jokuldalur area spanning the time interval between 1.8 and 0.5 Ma. The magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for Brunhes–Matuyama ages given by Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095] based on marine magnetic anomalies, except that the Gilsa event needs to be added. Existence of Olduvai age lavas under Gilsa was inferred but not certain. Our results are fairly in good agreement with the former studies by Wensink [Wensink, H., 1964a. Secular variation of Earth magnetism in Plio–Pleistocene basalts of eastern Iceland. Geol. Mijnbouw 43, 403–413; Wensink, H., 1964b. Paleomagnetic stratigraphy of younger basalts and intercalated Plio–Pleistocene tillites in Iceland. Geol. Rund. 54, 364–384] and Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444], but the assignment to the polarity zones is different because of the newly obtained K–Ar ages. Based on the present study, we propose that the Gilsa event is a short normal subchron in the Matuyama chron distinct from and above the more well-established Oldvai subchron.

Paleomagnetism of late Mesozoic rocks from northeastern China: the role of the Tan-Lu fault in the North China Block

Paleomagnetic studies were performed on Jurassic, Cretaceous and Tertiary rocks sampled from the Qitaihe area in Heilongjiang and Benxi area in Liaoning Provinces, northeast China. Both locations are near the Tancheng-Lujiang (Tan-Lu) fault system; Benxi is close to but on the eastern side of the fault while Qitahe lies between two major branches of the northwestern extension of this fault. In Mesozoic rocks, secondary magnetization in the present field direction was observed, but it was possible to retrieve the primary components by taking the high-temperature portion of the demagnetizing spectra. The Mesozoic poles thus obtained, especially those for the Cretaceous, deviate from the paleomagnetic poles of similar ages from the central part of the North China Block (NCB), Siberian Block or South China Block (SCB). Although the distances to the poles (flattening) are quite similar, the Benxi pole suggests a small clockwise rotation with respect to the central NCB poles, while the the Qitaihe poles indicate a much larger rotation in the opposite direction. It is shown that the deviation of the Benxi pole is similar to that observed for the Korean Peninsula and Shangdong Province, which all lie to the east of the Tan-Lu fault in the NCB. The Qitaihe pole position is quite different from the poles either west or east of the Tan-Lu fault. From these observations, it is concluded that a left-lateral strike-slip movement at the Tan-Lu fault system since the Cretaceous is the cause of systematic deviation in the position of the poles obtained from east of the fault including the Benxi area, while anomalous direction of Qitaihe rocks may represent a small scale rotation within the Tan-Lu fault system. The estimation of the movement along the Tan-Lu fault depends on which branch of the fault system is considered most active. If the main branch is assumed to be the place of slip, the movement can be represented by an Euler pole which lies to the south of Honshu Island (20°N, 150°E), with an estimated total displacement of 800 km since the Cretaceous.

Paleointensities for 10–22 ka from volcanic rocks in Japan and New Zealand

Abstract Eight volcanic rocks from Japan and New Zealand in the age range 10–50 ka B.P. were studied using the Thellier method paleointensity experiment and yielded five successful results for 10–22 ka. Samples from the Shikotsu, Kuttara and Daisen volcanoes in Japan were taken from pyroclastic flows (both welded and non-welded) and a pumice fall deposit. One of the two New Zealand rocks is a rhyolite lava from the Taupo Volcanic Zone. Four sites out of seven located on rocks that are not usually subjected to Thelliers experiment yielded successful results, which indicates that volcanic products such as pyroclastic flows and rhyolite lavas are as good a material as basalt and andesite lavas for application of the paleointensity experiment. One paleointensity from New Zealand at 9.9 ka is very large, at about 100 μT, and provides support for the paleointensity high found in Europe and Japan [1,2]. The other four paleointensities for 14–22 ka are smaller than the present-day value, and this agrees with the idea of a broad paleointensity minimum for the period 10–50 ka that was suggested by McElhinny and Senanayake [3]. It is also noted that the variation curve of absolute paleointensity from the volcanic rocks is reasonably similar to the relative paleointensity curves from marine and lake sediments which were summarized by Tauxe [4].

Dynamo simulation and palaeosecular variation models

In this paper we examine the simulation results of a fully nonlinear, three–dimensional dynamo and obtain inferences useful in the study of present and past geomagnetic field. This approach has importance because of the limitation in the available data of the real magnetic field: the present field is known with a high accuracy, but the time covered is only a small fraction of the time constant inherent to the geodynamo, while palaeomagnetic data provide data for a long time–span, but they are of poor quality and are distributed quite irregularly both in space and time. Thus, we compare what we see from the external field of the dynamo model with the features established or conjectured for the present or palaeomagnetic fields. We show that some of the statistical properties of the magnetic field generated by dynamo models compare well with those of the real magnetic field: dominance of the axial dipole, similar power in each degree of the harmonic at the core surface, nearly normal distribution of Gauss coefficients, and the lack of correlation among their variations. Differences were observed in other features, such as drift in the azimuthal direction and concentration of magnetic flux in small patches at the core surface. They can be attributed to either the shortness of the observational period, or to the difference in the resolution of the models, which suppresses small–scale features far below the level of the observation.

Spherical harmonic analysis of paleomagnetic data: The case of linear mapping

We consider the problem of describing the ancient magnetic field as completely as possible using paleomagnetic data. One of the best ways to do this is to represent the field by the use of Gauss coefficients. Unlike ordinary cases, however, spherical harmonic analysis of paleomagnetic data is a complicated problem because (1) the data cannot be considered as contemporaneous and (2) they are mostly available only in the form of directions. If the mapping between the model parameters (Gauss coefficients) and the data is linear (e.g., the three components of the field), there is one-to-one correspondence between the means of the parameters and the means of the data. Conventional least squares techniques can be applied to the means as the relations between the data and parameters are the same as in the instantaneous case. One-to-one correspondence also exists between the variances of the parameters and the variances of the data. However, the functions appearing in these relations are not orthogonal to each other, and the matrix to be inverted is quite ill conditioned. For magnetic field directions that are nonlinearly related to Gauss coefficients, one-to-one correspondence is completely absent between the means (variances) of the parameters and those of the data. This means that inversion becomes quite complicated because the mean of such data contains information on both the mean and fluctuations of the parameters. We applied inversion to the paleointensity data of the past 5 Myr and obtained the means and variances of Gauss coefficients for that period. The means are well determined and show that the time-averaged field is dominated by the axial dipole component. It is more axisymmetric than the recent field, suggesting that equatorial dipole components are largely averaged out as assumed by paleomagnetic dipole hypothesis. Inversion of the variances gave less well determined results. Among the variances of Gauss coefficients that are significantly different from zero that of the axial dipole is much larger than those of other coefficients, indicating that the observed variation in paleointensity is largely due to the fluctuation of the magnitude of the dipole moment.

PALEOSECULAR VARIATION OF FIELD INTENSITIES AND DIPOLE MOMENTS

Abstract In order to use paleointensity data for paleosecular variation studies, we first obtain analytical for the intensity of the geomagnetic field and the corresponding dipole moment in terms of Gauss coefficients. Next, statistical parameters for these quantities are derived under the assumption that Gauss coefficients vary randomly with time and that the axial dipole component is much larger than all the other Gauss coefficients. Using these expressions, we construct a ‘homogeneous background model’ for paleosecular variation, in which Gauss coefficients other than the axial dipole term are zero mean random variables, with variances decaying exponentially with the degree of the harmonic. We compare this model with the present field and the data from the last 10 Ma collected in the paleointensity database. Although we have to make certain allowances, because some of the paleointensity data may not be characteristic of the time-averaged paleomagnetic field, we obtain a broad agreement between the observation and a slightly modified form of the homogeneous background model. From this match, we conclude that the amplitude of the non-dipole components may be about twice that predicted from the present field, and that the contribution from the equatorial dipole is very small. The present analysis suggests that a more rigorous selection criteria should be employed to secure the reliability of data and also that the data set should more adequately sample the time interval before we can obtain a more detailed knowledge of the structures of the ancient magnetic field.

Paleointensity database provides new resource

A global paleointensity database constructed from all published data based on volcanic rocks in geological time older than 0.03 Ma is now available and ready for use. It can be obtained from the authors as text files on a 3.5″ diskette in MS-DOS or Macintosh format or via e-mail. It includes not only major parameters, but also minor details that can be used to assess data quality. Each datum consists of reference, latitude and longitude of the site, age and its error, experimental method, site identification, site mean paleointensity and its error, number of samples, virtual dipole moment (VDM), polarity, inclination, and declination. The database contains a total of 1123 flow mean data retrieved from 83 original papers, which were published before or during 1991.