Soil pH regulates the capacity of soils to store and supply nutrients, and thus contributes substantially to controlling productivity in terrestrial ecosystems(1). However, soil pH is not an independent regulator of soil fertility-rather, it is ultimately controlled by environmental forcing. In particular, small changes in water balance cause a steep transition from alkaline to acid soils across natural climate gradients(2,3). Although the processes governing this threshold in soil pH are well understood, the threshold has not been quantified at the global scale, where the influence of climate may be confounded by the effects of topography and mineralogy. Here we evaluate the global relationship between water balance and soil pH by extracting a spatially random sample (n = 20,000) from an extensive compilation of 60,291 soil pH measurements. We show that there is an abrupt transition from alkaline to acid soil pH that occurs at the point where mean annual precipitation begins to exceed mean annual potential evapotranspiration. We evaluate deviations from this global pattern, showing that they may result from seasonality, climate history, erosion and mineralogy. These results demonstrate that climate creates a nonlinear pattern in soil solution chemistry at the global scale; they also reveal conditions under which soils maintain pH out of equilibrium with modern climate.

Cavity ring-down spectrometers have generally been designed to operate under conditions in which the background gas has a constant composition. However, there are a number of observational and experimental situations of interest in which the background gas has a variable composition. In this study, we examine the effect of background gas composition on a cavity ring-down spectrometer that measures delta O-18-H2O and delta H-2-H2O values based on the amplitude of water isotopologue absorption features around 7184 cm(-1) (L2120-i, Picarro, Inc.). For background mixtures balanced with N-2, the apparent delta O-18 values deviate from true values by 0.50 +/- 0.001 parts per thousand O-2%(-1) and -0.57 +/- 0.001 parts per thousand Ar%(-1), and apparent delta H-2 values deviate from true values by 0.26 +/- 0.004 parts per thousand O-2 %(-1) and 0.42 +/- 0.004 parts per thousand Ar %(-1). The artifacts are the result of broadening, narrowing, and shifting of both the target absorption lines and strong neighboring lines. While the background-induced isotopic artifacts can largely be corrected with simple empirical or semimechanistic models, neither type of model is capable of completely correcting the isotopic artifacts to within the inherent instrument precision. The development of strategies for dynamically detecting and accommodating background variation in N-2, O-2, and/or Ar would facilitate the application of cavity ring-down spectrometers to a new class of observations and experiments.

CO CV CM CI Ryugu A Ryugu C 0.0 0.5 1.0 1.5 44/40CaSRM915a (age corrected) The Hayabusa2 spacecraft has returned samples from the Cb-type asteroid (162173) Ryugu to Earth. Previous petrological and chemical analyses support a close link between Ryugu and CI chondrites that are presumed to be chemically the most primitive meteorites with a solar-like composition. However, Ryugu samples are highly enriched in Ca compared to typical CI chondrites. To identify the cause of this discrepancy, here we report stable Ca isotopic data (expressed as delta 44/40CaSRM915a) for returned Ryugu samples collected from two sites. We found that samples from both sites have similar delta 44/40CaSRM915a (0.58 +/- 0.03 parts per thousand and 0.55 +/- 0.08 parts per thousand, 2 s.d.) that fall within the range defined by CIs. This isotopic similarity suggests that the Ca budget of CIs and Ryugu samples is dominated by carbonates, and the variably higher Ca contents in Ryugu samples are due to the abundant carbonates. Precipitation of carbonates on Ryugu likely coincided with a major episode of aqueous activity dated to have occurred similar to 5 Myr after Solar System formation. Based on the pristine Ryugu samples, the average delta 44/40CaSRM915a of the Solar System is defined to be 0.57 +/- 0.04 parts per thousand (2 s.d.).

Differential travel times, commonly used to eliminate unwanted near-source and near-receiver heterogeneity from travel time studies, can be affected by slabs and hypocentral errors when phase pair slownesses differ greatly. In particular, D'' and core structure studies can be biased if differential PKP times are used. Here we present differential PKP travel time measurements having significant azimuthal travel time anomalies consistent both in size and pattern with a near-source slab effect and explore other sources of error in differential times.

On May 25, 1987, at 11:31, a M = 5.8 earthquake occurred at the southern end of Vatnafjoll volcanic ridge in south Iceland. This is the largest event in the south Iceland lowland since a hi = 7.0 earthquake in 1912 which was located approximately 15 km to the west of the Vatnafjoll earthquake. Vatnafjoll is located at the junction of the south Iceland seismic zone, a left lateral transform zone, and the eastern volcanic zone which is a zone of rifting and volcanism. In May and June 1987, several foreshocks and aftershocks were recorded on the local seismic network as well as the mainshock. A clear coseismic step associated with the mainshock was observed at all operating stations of a volumetric strainmeter network in southern Iceland. Steps associated with some foreshocks and aftershocks were also observed at the closest strain stations. Slow strain changes, before and after the mainshock, lasting a few days, were also observed. Forward modeling of the coseismic strainmeter signals of the mainshock suggests a double couple solution where the slip is mostly right lateral strike slip on a subvertical plane with a northerly strike. The solution has a good fit to observations and is in good agreement with interpretation of seismometer data. This solution indicates a stress field similar to that in the south Iceland seismic zone. The slow strain changes, which start about 10 min after the first foreshock, may indicate magma involvement in the process. Changes, associated with an intrusion and pressure release, may affect the strain held and possibly trigger the mainshock. The strainmeter records open up a new view of the seismic strain event as a combination of seismic strain release and a slower process of magma intrusion.

Tilt and strain continuous monitoring started in October 2002 in the Trizonia Island, in the Gulf of Corinth, in order to detect possible strain transients in the rift. The hydrostatic tiltmeters, developed at IPGP, are 15 m long and buried in trenches at a depth of 2.5 m, with a few 10(-9)-radian noise level at short period. The strain is measured by a Sacks-Evertson dilatometer has cemented in a borehole at a depth of 148 m, with a few 10(-10) resolution at short period. A 1 h-lasting, 10(-7)-strain transient has been recorded on the dilatometer, possibly related to a seismic swarm that occurred 15 km away. To cite this article: R Bernard et al., C. R. Geoscience 336 (2004). (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.

We developed a long-term, high-quality seismic ocean floor borehole observatory system, the 'Neath Seafloor Equipment for Recording Earth's Internal Deformation (NEREID). Four NEREID borehole observatories were installed in the Japan Trench off-Sanriku area (JT1, JT2), in the northwestern Pacific Basin (WP2), and in the Philippine Sea (WP1). The borehole sensors are cemented in the borehole to assure good coupling of sensors to the ground as well as to avoid effects of water flow around the sensors, which may have been a problem in previous borehole installations. The NEREID seismic records from two of the observatories (JT1, WP2) were free from long-period noise due to turbulence in the seafloor boundary current or to water flowing around the sensor that is significant on the seafloor. The infra-gravity wave noise clearly observed around 0.01 Hz on the horizontal components was significantly higher in the JT1 seismometer in the sediment because of the low shear modulus of the sediment. Ocean waves of long wavelength cause the infra-gravity wave noise. It is thus necessary to install seismometers in boreholes below the sediments to reduce the infra-gravity wave noise.

This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50 degrees and 60 degrees mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or theological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002, A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 +/- 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes. (c) 2006 Elsevier B.V All rights reserved.

Observations of microbarograph recordings on the island of Montserrat in the Caribbean have shown signals with periods of several minutes and amplitude of approximately 1 mb following explosive eruptions of the Soufriere Hills Volcano. These properties suggest that the explosions are causing the generation of atmospheric internal waves. Here an analysis of wave generation by sudden disturbances in a stratified atmosphere is developed, and the properties of these waves are described and compared with observations. Two types of forcing of the atmosphere by a volcanic explosion are considered: that due to the sudden addition of mass, and separately, of thermal energy. The pattern and distribution of these depend on the nature of the explosion, which has a timescale of the order of 1 min. The theoretical results resemble the effect of 'throwing a stone' into the atmosphere, producing transient waves that radiate radially away from the source. Near the source, these waves have frequency around 0.7-0.8N (where N is the buoyancy frequency) initially, and approach N with decreasing amplitude. The results of forcing due to added mass and thermal forcing are presented and compared for a variety of vertical forcing profiles, and these are compared with observations of surface pressure. The results suggest that forcing due to the injection of mass (mostly solid particles) is the principal factor in forcing the observed internal waves. The addition of thermal energy (heat) produces waves that have frequencies closer to N, and persist for much longer periods than those observed at the stations on Montserrat.

Observations from the island of Montserrat in the Caribbean have shown that volcanic eruptions (particularly explosive ones) can generate internal waves in the atmosphere that can be observed by microbarographs at ground level. It is possible that observations of such waves may give early information about volcanic eruptions when other methods are unavailable (because of bad weather, nocturnal eruptions, and poor visibility or remoteness), if it is possible to interpret them. This paper describes a dynamical model of the forcing of internal waves in which the eruption is modelled as a turbulent plume, forced by a source of buoyancy at ground level that specifies the total height and relevant properties of the eruption. Specifically, the rising plume entrains environmental air from ground level to 70% of its maximum height z(M), and above 0.7z(M) the rising fluid spreads radially. During the eruption, this flow forces horizontal motion in the surrounding fluid that generates internal waves, which may be computed by assuming that this is due to a linear dynamical process. Properties of the resulting waves are described for a variety of parameters that include the strength and height of the eruption, the effect of the tropopause, generation in the stratosphere for large eruptions, and the differing effects of the duration of the eruption. Implications for characterising eruptions from observations of these properties are discussed.