Unexpected High Levels of Ozone Measured in Córdoba, Argentina

Results of field measurements carried out from June 15 to December 31, 1995, in Córdoba city (Argentina) are presented. During this field campaign, surface ozone mixing ratios were generally around 30–35 ppb (afternoon peak). However, during the first week of September, days with excessive ozone values close to 100 ppb were found. These elevated ozone concentrations appeared together with high values of NOx, CO, PM10, and an unusual meteorological situation for this time of the year. These results made this episode an interesting one to be studied in more detail. In this work, we used chemical and meteorological data to trace the region from where the assumed precursors were emitted and we identified possible source characteristics.     

Polarimetric receiver in the forty gigahertz band: new instrument for the Q-U-I joint Tenerife experiment

This paper describes the analysis, design and characterization of a polarimetric receiver developed for covering the 35 to 47 GHz frequency band in the new instrument aimed at completing the ground-based Q-U-I Joint Tenerife Experiment. This experiment is designed to measure polarization in the Cosmic Microwave Background. The described high frequency instrument is a HEMT-based array composed of 29 pixels. A thorough analysis of the behaviour of the proposed receiver, based on electronic phase switching, is presented for a noise-like linearly polarized input signal, obtaining simultaneously I, Q and U Stokes parameters of the input signal. Wideband subsystems are designed, assembled and characterized for the polarimeter. Their performances are described showing appropriate results within the 35-to-47 GHz frequency band. Functionality tests are performed at room and cryogenic temperatures with adequate results for both temperature conditions, which validate the receiver concept and performance.     

Late Pleistocene: Holocene record of environmental changes in Lake Zirahuen,Central Mexico

Geochemical data obtained from X-ray fluorescence, physical properties, total organic and inorganic carbon content (TOC/TIC), and diatom analysis from a 6.61-m-long sedimentary sequence near the modern northern shore of Lake Zirahuen (101° 44′ W, 19° 26′ N, 2000 m asl) provide a reconstruction of lacustrine sedimentation during the last approximately 17 cal kyr BP. A time scale is based on ten AMS ¹⁴C dates and by tephra layers from Jorullo (AD 1759-1764) and Paricutin (AD 1943-1952) volcanoes. The multiproxy analyses presented in this study reveal abrupt changes in environmental and climatic conditions. The results are compared to the paleo-record from nearby Lake Patzcuaro. Dry conditions and low lake level are inferred in the late Pleistocene until ca. 15 cal kyr BP, followed by a slight but sustained increase in lake level, as well as a higher productivity, peaking at ca. 12.1 cal kyr BP. This interpretation is consistent with several regional climatic reconstructions in central Mexico, but it is in opposition to record from Lake Patzcuaro. A sediment hiatus bracketed between 12.1 and 7.2 cal kyr BP suggests a drop in lake level in response to a dry early Holocene. A deeper, more eutrophic and turbid lake is recorded after 7.2 cal kyr BP. Lake level at the coring site during the mid Holocene is considered the highest for the past 17 cal kyr BP. The emplacement of the La Magueyera lava flows (LMLF), dated by thermoluminiscence at 6560 ± 950 year, may have reduced basin volume and contributed to the relative deepening of the lake after 7.2 cal kyr BP. The late Holocene (after 3.9 cal kyr BP) climate is characterized by high instability. Extensive erosion, lower lake levels, dry conditions and pulses of high sediment influx due to high rainfall are inferred for this time. Further decrease in lake level and increased erosion are recorded after ca. AD 1050, at the peak of Purepechas occupation (AD 1300–1521), and until the eighteenth century. Few lacustrine records extend back to the late Pleistocene—early Holocene in central Mexico; this paper contributes to the understanding of late Pleistocene-Holocene paleoclimates in this region.     

Rb-Sr dating of alteration events in granitoids

Veins and other effects of low-grade metamorphism are often found in igneous rocks. A method for dating such veins is proposed. It is based on the presence of Ca-bearing, Rb-free minerals in the veins and their aureoles. The method is applicable if five conditions are satisfied. The effects of low-grade metamorphism on Rb-Sr dating of the emplacement (extrusion) age are also studied. Veins and their aureoles in granitoid rocks from Central Sweden are used as a working example. Two groups of vein ages were found, 1600-1500 Ma and 1250-1100 Ma, which coincide with the two known periods of tensional movements in the region. The ratio ⁸⁷Sr/⁸⁶Sr in the aureoles and their neighbourhood at the time of vein formation (Sr(a)) shows several different patterns. One with constant Sr(a), independent of the distance to the vein, indicates complete homogenization at the time of alteration. Another shows an increase in Sr(a) close to the vein, indicating that it was rejuvenated. Furthermore, the Sr ratio might determine whether an alteration effect could be of deuteric origin or not: deuteric effects should be characterized by a Sr(a) value equal to the initial Sr-isotope ratio.