Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal

The natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has a direction opposite to the present geomagnetic field. The self-reversing mechanism can be re-activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field. Laboratory-produced thermoremanent magnetization is dominated by the same self-reversal process in fields up to several mT. Microchemical, optical and Curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite-ulvöspinel and hematite-ilmenite series with average compositions of Fe_2.73Ti_0.27O₄ and Fe_1.38Ti_0.62O₃, respectively. In analogy with the magnetic behaviour of synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films, the self-reversal can probably be interpreted in terms of an exchange field acting between a Ti-poor canted antiferromagnetic and a Ti-rich ferrimagnetic phase in the hemoilmenite grains.     

Eruptive response of oceanic islands to giant landslides: New insights from the geomorphologic evolution of the Teide-Pico Viejo volcanic complex (Tenerife, Canary)

Large sector collapses are a major component of oceanic islands evolution. Here we show that voluminous events such as the Icod landslide on Tenerife (Canary Islands) cause dramatic changes on the magma feeding system and control the subsequent volcanic and geomorphologic evolution of the eruptive complex over a period of more than 150 kyr. Instantaneous unloading by the Icod landslide is marked by the development of a large phonolitic explosive eruption dated at 175 ± 3 ka and interpreted as reflecting the immediate emptying of a shallow pre-existing magma chamber. Geochronological, geomorphological and geochemical analyses, carried out on the post-landslide volcanic succession sampled in a 4.4 km-long underground water-recovery gallery, provide further evidence for an enhanced extrusion of primitive lavas starting in the 10 kyr time interval following the failure. Rapid construction (< 40 kyr) of a thick basaltic volcano in the landslide scar at high eruptive rates (up to 8 km³ kyr⁻¹) increased the lithostatic pressure which then favored the intermittent storage of basic magma under the edifice. This resulted in more episodic construction evidenced by a significant decrease in output rates and the increasing occurrence of lavas with intermediate composition from 117 ± 7 to 52 ± 7 ka. An apparent volcanic gap is observed between 52 ± 7 and 18 ± 1 ka, after which highly differentiated lavas have been dominantly erupted. We propose that part of the gap can be explained by the individualization of a shallow magma reservoir a few kilometers below the base of the Teide volcano. During recent periods, vertical and lateral extrusions of trachytic and phonolitic viscous bodies from this storage area contributed to increase the slope of the main edifice up to 35°, overall favoring its present-day instability.     

Sedimentation Patterns of Photosynthetic Bacteria Based on Pigment Markers in Meromictic Lake La Cruz (Spain): Paleolimnological Implications

Sedimentation fluxes of photosynthetic pigments and their derivatives, focused on the anoxygenic bacteria populations, were determined in meromictic Lake La Cruz over a three-year period using sediment traps. Bacterial settling occurs in pulses in response to lake dynamics. Two main annual events affect particle settling in Lake La Cruz: (1) Following an event of massive calcium precipitation in summer that caused water whiting, a maximum flux of chlorophyll a and derivatives towards the sediment was registered. Although whiting is accompanied by massive phytoplankton biomass sinking, it was not followed by concomitant photosynthetic bacterial sinking. (2) In contrast, the disruption of the chemocline and the contact of bacterial populations with oxygenated waters during autumn overturn provoked a destruction of the sulfur bacterial plate, causing mass mortality, registered in the settling material as photosynthetic pigments and their derivatives from phototrophic bacteria. Among the bacteriochlorophyll a oxidation products, bacterioviridin (bvir) was detected in high relative abundance in the settled material collected by sediment traps. The much increased flux of bvir after thermal overturn, associated with mass mortality of the purple sulfur bacterium, Amoebobacter purpureus, confirms the suitability of this molecule as a useful paleoindicator of the degree of oxygen exposure experienced by the purple sulfur bacteria population.     

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.     

Validation and comparison of Advanced Differential Interferometry Techniques: Murcia metropolitan area case study

This paper is focused on the analysis of the performance of Stable Point Network (SPN) and Coherent Pixel Technique (CPT), which are Advanced Differential Interferometry Techniques (A-DInSAR) that estimate, among other results, mean deformation velocity maps of the ground surface and displacement time series from a SAR dataset. The test site is the metropolitan area of the city of Murcia (Spain) where a moderate slow subsidence induced by the overexploitation of aquifers is present. SAR data acquired between July 1995 and August 2005 from ERS and ENVISAT sensors have been processed by the SPN and CPT techniques and compared with in situ instrumental measurements assumed as reference. Experimental results have shown that both SPN and CPT techniques provide estimates of the deformation evolution in time with an absolute difference below 6 mm consistently in all comparisons: SPN vs extensometer, CPT vs extensometer and SPN vs CPT. The proposed validation and comparison experiment between both A-DInSAR techniques has been useful to observe their differences and complementarities.     

A paleomagnetic and rock-magnetic study of a neogene lava flow sequence in La Gomera (Canary Islands,Spain)

We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main intermediate Curie-temperature phase (T_C = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (T_C = 120 to 200°C) and an intermediate Curie-temperature phase (T_C = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse polarity. The mean direction of the whole sequence is D = 188.2°, I =−35.4° (k = 46.9; α₉₅ = 6.4°) and the calculated paleomagnetic pole yields a longitude λ= 150.7° and a latitude ϕ= 78.8° (k = 59.4; A₉₅ = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter S_B = 5.9 with an upper confidence limit S_up = 8.0 and a lower confidence limit S_low = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied lava flows were probably emitted in a relatively short time interval.     

On climate variability in Northeast of Brazil

The time-series (annual period and dry- and wet-season) of eight climatic variables were analysed to ascertain the existence of climate variability in Northeast of Brazil. Results indicated generally increasing trends in most of these variables (statistically significant at p<0.01 or <0.05) by Mann–Kendall test. However, relative humidity and rainfall presented decreasing behavior. The study showed that most of the stations studied are going through a process of environmental dryness. The results also suggest that the historical trends may be related to climate variability in Northeast of Brazil, which affects both semi-arid and coastal part of the region.     

Sedimentary record of anthropogenic metal inputs in the Tagus prodelta (Portugal)

Favourable oceanographic and environmental conditions allow the formation of a fine-grained deposit (Tagus prodelta) located at the mouth of the Tagus River. This fine-grained deposit results from the sink and accumulation of terrestrial and marine-derived materials. Three short sediment cores collected in the Tagus prodelta were investigated through the variability in grain-size, major and trace elements, C_org, N_tot, δ¹³C and ²¹⁰Pb dating to characterise the historical development of trace metal contamination. Historical trends indicated significant anthropogenic enrichments for Hg, Pb, Zn, Cu, Sb and Sn since the 1930s. Hg presents the highest level of anthropogenic enrichment (ca. EF_Hg=20) in cores 3576 (PO287-26-1B) and 3579 (PO287-27-1B) collected closer to the shore. Despite these elements were derived from distinct industrial sources, all of them presented similar temporal trends, which points to the importance of estuarine mixing processes originated by the tidal regime and wind before transfer to the adjoining coastal areas. The cores were characterised by smoothness of down-core variations and river flood events occurred in the last decades were not registered. Furthermore, the improvement of effluents treatment together with the closing of some industrial point sources in the last two decades is not evidenced in recent sediment composition of the Tagus prodelta. Besides sediment reworking in the upper sediment layers, sediment mixing inside the estuary may explain minor abrupt alterations with the depth.