Structure and evolution of the volcanic rift zone at Ponta de São Lourenço,eastern Madeira

Ponta de São Lourenço is the deeply eroded eastern end of Madeira’s east–west trending rift zone, located near the geometric intersection of the Madeira rift axis with that of the Desertas Islands to the southeast. It dominantly consists of basaltic pyroclastic deposits from Strombolian and phreatomagmatic eruptions, lava flows, and a dike swarm. Main differences compared to highly productive rift zones such as in Hawai’i are a lower dike intensity (50–60 dikes/km) and the lack of a shallow magma reservoir or summit caldera. ⁴⁰Ar/³⁹Ar age determinations show that volcanic activity at Ponta de São Lourenço lasted from >5.2 to 4 Ma (early Madeira rift phase) and from 2.4 to 0.9 Ma (late Madeira rift phase), with a hiatus dividing the stratigraphy into lower and upper units. Toward the east, the distribution of eruptive centers becomes diffuse, and the rift axis bends to parallel the Desertas ridge. The bending may have resulted from mutual gravitational influence of the Madeira and Desertas volcanic edifices. We propose that Ponta de São Lourenço represents a type example for the interior of a fading rift arm on oceanic volcanoes, with modern analogues being the terminations of the rift zones at La Palma and El Hierro (Canary Islands). There is no evidence for Ponta de São Lourenço representing a former central volcano that interconnected and fed the Madeira and Desertas rifts. Our results suggest a subdivision of volcanic rift zones into (1) a highly productive endmember characterized by a central volcano with a shallow magma chamber feeding one or more rift arms, and (2) a less productive endmember characterized by rifts fed from deep-seated magma reservoirs rather than from a central volcano, as is the case for Ponta de São Lourenço.     

六元体系Li-Na-K-Mg-Cl-SO4-H2O多温相平衡预测

采用基于扩展UNIQUAC模型的AQSOL027软件针对水盐体系Li-Na-K-Mg-Cl-SO_4-H_2O的六个五元子体系进行相平衡理论计算,首次报道了多个五元子体系在多个温度条件下的相图,将计算相图与已有实验相图以及基于其它热力学模型的计算相图进行了比较。结果表明:AQSOL027在相图计算中具有较好的准确性和可靠性。AQSOL027是一款基于Excel、用于计算复杂多温水—盐体系Li-Na-K-Mg-Ca-H-Cl-(HSO_4/SO_4)-(CO_2/HCO_3/CO_3)-(H_3BO_3/B_4O_7/BO_2)-OH-H_2O的计算机软件,集成了扩展UNIQUAC模型和相平衡算法,该软件可结合SysCAD和Aspen过程模拟器,实现盐田工艺过程的计算机辅助设计。     

An emendation of elastic rebound theory: Main rupture and adjacent belt of right-lateral distortion detected by Viaduct at Kaynaşli, Turkey 12 November 1999 Düzce Earthquake

The fault trace of the 12 November 1999 earthquake in theDüzce-Bolu region in Anatolia crossed the alignment of a 2.4 kmviaduct in Kaynali that had been carefully surveyed. The builders of theviaduct, the ASTALDI-BAYINDIR Co., resurveyed the viaduct after theearthquake. We repeated the survey for approximately one kilometre of theeastern end of the viaduct and obtained essentially identical results. Thoughit was unfortunate that the earthquake damaged the new structure, the piersdid produce a very rare record of ground deformation of an earthquake.In effect, the viaduct was a giant strain gage that yielded reliable data aboutground movement and distortion near a fault. This paper describes thesurvey data and their evaluation leading to convincing evidence that (a) thefault trace must be considered, not as a fault line or plane, but as a faultzone with a finite width and that (b) the structural damage within the zonewas caused, not primarily by ground acceleration, but by ground distortion.Along the right-lateral fault at Kaynali, the fault zone consists ofright-lateral movement at the main trace, a zone of right-lateral distortionnear the trace, bounded by left-lateral distortion. The 12 November 1999event in Turkey, like the ground deformation and fracturing at Landers,California (Johnson et al., 1994, 1996), thus affirmed a forgottenconclusion from the studies by Lawson (1908), Gilbert and Reid (1910)of the 1906 San Francisco earthquake that earthquake ruptures typicallyoccur throughout zones or belts, rather than along linear traces or planes.     

NICMOS imaging search for high-redshift damped Lyα galaxies

We are engaged in a programme of imaging with the STIS and NICMOS (NIC2) instruments aboard the Hubble Space Telescope ( HST ), to search for the galaxy counterparts of 18 high-redshift z >1.75 damped Ly α absorption lines and five Lyman-limit systems seen in the spectra of 16 target quasars. This paper presents the results of the imaging campaign with the NIC2 camera. We describe the steps followed in reducing the data and combining in mosaics, and the methods used for subtracting the image of the quasar in each field, and for constructing error frames that include the systematic errors associated with the PSF subtraction. To identify candidate counterparts, which are either compact or diffuse, we convolved the image and variance frames with circular top-hat filters of diameter 0.45 and 0.90 arcsec respectively, to create frames of summed S /N within the aperture. For each target quasar we provide catalogues listing positions and aperture magnitudes of all sources within a square of side 7.5 arcsec centred on the quasar, detected at S / N >6 . We find a total of 41 candidates, of which three have already been confirmed spectroscopically as the counterparts. We provide the aperture magnitude detection limits as a function of impact parameter, for both detection filters, for each field. The average detection limit for compact (diffuse) sources is H _AB =25.0 (24.4) at an angular separation of 0.56 (0.79) arcsec from the quasar, improving to H _AB =25.5 (24.8) at large angular separations. For the brighter sources we have measured the half-light radius and the n parameter of the best-fitting deconvolved Sersic-law surface brightness profile, and the ellipticity and orientation.     

Geochemical processes and solute transport at the seawater/freshwater interface of a sandy aquifer

Geochemical processes occurring at a seawater/freshwater interface were studied in a shallow coastal siliclastic aquifer containing minor amounts of calcite. Data were collected from 106 piezometers in a 120-m transect from the coastline and landward. In the first 40 m from the coastline, a wedge of saltwater is intruding below the freshwater aquifer. The aquifer is strongly reduced with mineralization of organic matter by methanogenesis in the freshwater aquifer, and sulfate reduction dominating in the most seaward part of the saline aquifer. The spatial separation of cations in the aquifer indicated a slow freshening process where Ca²⁺ from freshwater displaced the marine cations Na⁺ and Mg²⁺ from the exchanger complex. The resulting loss of Ca²⁺ from solution decreases the saturation state for calcite and possibly causes calcite dissolution. A storm-flooding event was recorded where pulses of dense seawater sank through the fresh aquifer. As a result, the terminal electron accepting process switched from methanogenesis to sulfate reduction. The pulses of sinking seawater also triggered cation exchange reactions where Ca²⁺ was expelled from the exchanger by seawater Na⁺ and Mg²⁺. The released Ca²⁺ is being flushed from the aquifer by groundwater flow, and this export of Ca²⁺ will, in the long term, cause decalcification of the sediment. The water composition in the aquifer is in a transient state as the result of various processes that operate on different timescales. Oxidation of organic matter occurs continuously but at a rate decreasing on a geological time scale. The freshening of the aquifer operates on the timescale of a few years. The episodic flooding and sinking of seawater through the aquifer proceeds in the course of days to weeks, but occurs irregularly with years in between.     

Lake metabolism scales with lake morphometry and catchment conditions

We used a comparative data set for 25 lakes in Denmark sampled during summer to explore the influence of lake morphometry, catchment conditions, light availability and nutrient input on lake metabolism. We found that (1) gross primary production (GPP) and community respiration (R) decline with lake area, water depth and drainage ratio, and increase with algal biomass (Chl), dissolved organic carbon (DOC) and total phosphorus (TP); (2) all lakes, especially small with less incident light, and forest lakes with high DOC, have negative net ecosystem production (NEP < 0); (3) daily variability of GPP decreases with lake area and water depth as a consequence of lower input of nutrients and organic matter per unit water volume; (4) the influence of benthic processes on free water metabolic measures declines with increasing lake size; and (5) with increasing lake size, lake metabolism decreases significantly per unit water volume, while depth integrated areal rates remain more constant due to a combination of increased light and nutrient limitation. Overall, these meta-parameters have as many significant but usually weaker relationships to whole-lake and benthic metabolism as have TP, Chl and DOC that are directly linked to photosynthesis and respiration. Combining water depth and Chl to predict GPP, and water depth and DOC to predict R, lead to stronger multiple regression models accounting for 57–63% of the variability of metabolism among the 25 lakes. It is therefore important to consider differences in lake morphometry and catchment conditions when comparing metabolic responses of lakes to human impacts.