Notizen: To first order, the estimated thickness Te of the elastic part of the oceanic lithosphere increases linearly with the square root of the age of the lithosphere at the time of loading, Δt. In order to quantify this relationship in the particular case of volcanic loading, a synthesis of Te estimates reported both in this paper and in previous studies is conducted. Excluding the anomalously low estimates from the south-central Pacific, the values are very consistent for the three main oceans and follow the empirical relationship: Te (km) = (2.7 ± 0.15)½Δt (Ma). the relationship is used to predict the age of volcanoes when the age of the crust is known (for the Trindade chain in the south Atlantic Ocean) and to predict the age of the crust when the age of the volcanoes is known (for the Pacific plate east of the Tonga trench). the age estimates are in good agreement with the structural setting.

Notizen: The remote-sensing satellite ERS-1, launched in 1991 to study the Earth's environment, was placed on a geodetic (168-day repeat) orbit between 1994 April and 1995 March to map, through altimetric measurements, the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along-track and cross-track directions. We have analysed the precise altimeter data of the geodetic mission, and, by also using one year of Topex-Poseidon altimeter data, we have computed a global high-resolution mean sea surface. The various steps involved in pre-processing the ERS-1 data consisted of correcting the data for environmental factors, editing, and reducing, through crossover analyses, the radial orbit error, which directly affects sea-surface height measurements. For this purpose, we adjusted sinusoids at 1 and 2 cycle rev−1 along the ERS-1 profiles in order to minimize crossover differences between ERS-1 and yearly averaged Topex-Poseidon profiles. In effect, the orbit of Topex-Poseidon is very accurately determined (within 2–3 cm for the radial component), so Topex-Poseidon altimeter profiles can serve as a reference to reduce the ERS-1 radial orbit error. The ERS-1 residual orbit error was further reduced through a second crossover analysis between all ascending and descending profiles of the geodetic mission. The along-track ERS-1 and Topex-Poseidon data were then interpolated over the whole oceanic domain on a regular grid of 1/16°× 1/16° size. The mapping of the gridded sea-surface heights reveals the very fine structure of the marine geoid, up until now unknown at a global scale. This new data set will be most useful for marine geophysical and tectonic investigations.

Notizen: Laser data analysis on the Lageos satellite for the period 1985–1989 has been conducted to recover temporal variations of the low-degree harmonics of the Earth gravity field, in particular of C̄40, the dynamical flattening, and of C̄30 Temporal variation of these zonal coefficients may represent changes in the Earth inertia tensor, hence mass redistribution inside the solid Earth and the hydrosphere (atmosphere, oceans, ground water and glaciers).No separation has been possible between C̄20 and even zonal harmonics of higher degree (e.g. C̄40) so that the solution represents an effective C̄20. We have not solved for odd zonal harmonics of degree higher than 3, hence the C̄30 solution also represents an effective C̄30.Monthly solutions for the effective C̄20 and C̄30 over 1985–1989 are dominated by a strong seasonal (mostly annual) signal. In 1989, the C̄30 solution shows an unusually large fluctuation. This fluctuation has also been reported by other investigators and is known as the 1989 anomaly’. It may be related to some mismodelled non-gravitational perturbation in the Lageos orbit at this epoch.Spectral analysis of the monthly C̄20 solutions gives annual and semi-annual amplitudes of 1.43 × 10−10 and 0.76 × 10−10 (normalized values) respectively. For C̄30, corresponding amplitudes are 1.95 X 10−10 and 0.33 X 10−10 (normalized values; annual and semi-annual terms). the year 1989 has been excluded from the C̄30 spectral analysis to avoid pollution by the ‘1989 anomaly’.At the annual frequency, most of the observed variations may result from air mass redistribution in the atmosphere. Using global air-pressure data over the same period (1985–1989), we have computed the atmospheric induced C̄20 and C̄30 variations for both inverted and non-inverted-barometer response of the oceans and compared these to the Lageos-derived monthly solutions. Comparison shows better consistency between Lageos and atmospheric C̄20 variations for the non-inverted-barometer response at the seasonal frequency. This result challenges the common assumption that the oceans respond as an inverted barometer to long-period variations in atmospheric pressure. On the other hand, if the inverted-barometer assumption is correct, then most of the annual variations in C̄20 and C̄30 have to be found in other reservoirs. Since the contribution of ground waters and glaciers is known to be small, this leaves us with the oceanic contribution. In addition, errors in modelling annual and semi-annual ocean tides may contribute to the observed seasonal signal. We have subtracted the adjusted annual and semi-annual terms in both Lageos-derived and atmospheric-induced monthly C̄20 and C̄30 solutions. Residuals show short-term fluctuations in both series but the correlation is poor. Removal in the monthly C̄20 solutions of the total atmospheric contribution (assuming non-inverted-barometer response for the oceans) leaves a long-term, interannual fluctuation, reaching a maximum in the years 1987–1988. This interannual signal is possibly associated with the oceanic El NiÑo event which occured at this epoch. A contamination of the 18.yr non-equilibrium ocean tide as well as of