Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991–1995

The volcano Hekla in south Iceland had its latest eruption in January–March 1991. The eruption was accompanied and followed by considerable seismic activity. This study examines the seismicity in the Hekla region (63°42′–64°18′N, 18°30′–20°12′W) during a period when the high activity related to the eruption had ceased, from July 1991 to October 1995. The aim is to define the level of the normal background seismicity of the area that can be compared to the eruption-related activity. The Hekla Volcano proper was generally aseismic during the study period. The most prominent earthquake cluster is in the neighbouring Torfajökull Volcano. The epicentres are concentrated in the western part of the caldera and west of it. The hypocentres are located at all depths from the surface down to 14?km, with highest activity at 5–12?km. Inside this cluster, in the northwest part of the caldera, is a spherical volume void of earthquakes, approximately 4?km in diameter and centred at 8?km depth. This is interpreted as a cooling magma body. Small, low-frequency events of volcanic origin were occasionally recorded at Torfajökull. This activity has mainly occurred in swarms and was most abundant during the first year of the study period, presumably reflecting some kind of connection to the 1991 Hekla eruption. Our study area also includes the easternmost section of the South Iceland seismic zone, a transform zone characterized by bookshelf faulting on transverse faults. Two lineaments of epicentres were identified, roughly corresponding to mapped faults of the South Iceland seismic zone. The hypocentres are relatively deep, mainly at 6–12?km, matching the general trend of hypocentral depth increasing toward the east. The seismicity is highest in the area of the mapped faults. However, the epicentres extend beyond them and indicate greater width of the South Iceland seismic zone, or 20–30?km rather than approximately 10?km as indicated by the length of the surface faults. The seismicity in the volcanic systems of Hekla and Vatnafjöll shows some characteristics of the South Iceland seismic zone. Epicentres are concentrated into two N–S lineaments, one of which coincides with the location of the 1987 Vatnafjöll earthquake (M_w=5.9), a strike-slip event on a N- to S-trending fault. The hypocentres of the Hekla–Vatnafjöll events are mainly at 8–13?km depth, which indicates a continuation of the depth trend of the earthquakes of the South Iceland seismic zone. The events located at Hekla proper and immediately north of it are all of low-frequency character, which can be held as an indication of volcanic origin. On the other hand, they show clear S arrivals at observing stations like normal high-frequency tectonic earthquakes.     

Organic carbon mobilization in a Bangladesh aquifer explained by seasonal monsoon-driven storativity changes

Currently, the most widely accepted hypothesis to explain high As concentrations in Bangladesh groundwaters is that dissolved organic C (DOC) reduces solid Fe (hydr)oxides and mobilizes sorbed arsenate. The nature of the DOC and its release mechanism are still controversial. Based on weekly to biweekly sampling over the course of one monsoon cycle at six monitoring wells of different depths, it is proposed that storativity changes drive natural DOC release from clay–peat layers to the adjacent aquifers. With a decrease in hydraulic heads during the dry season, total mineralization and DOC concentrations increased. With the onset of the rainy season and an increase in hydraulic heads, release of clay–peat derived components stopped and vertical water displacement due to groundwater recharge from rainwater occurred, causing aquifer flushing and a decrease in total mineralization and DOC concentrations. Total As and DOC concentrations correlated over depth. However, at the depth of maximum concentrations, the As peak was observed during the rainy season. At present, the reason for this inverse seasonal trend between As and DOC is unclear. Higher mineralization or DOC concentrations could lead to increased As sorption or the increased arsenite release is a time-lag abiotic or microbial response to the DOC peak. The vulnerability of the Pleistocene aquifer towards increased As concentrations was found to be much higher than previously assumed. Though sorption capacities were determined to be higher than in the Holocene aquifer, probably due to intact Fe (hydr)oxides, long-term continuous As input from overlying clay and peat layers by the proposed seasonal storativity changes has led to increased aqueous As concentrations of 85 μg/L, considerably higher than drinking water standards. Until now, aquifer and especially aquitard and aquiclude hydraulics have not been considered sufficiently when attempting to explain As mobilization in Bangladesh.     

Earthquake activity related to the 1991 eruption of the Hekla volcano,Iceland

The 1991 eruption of the Hekla volcano started unexpectedly on 17 January. No long-term precursory seismicity was observed. The first related activity was a swarm of small earthquakes that began approximately half an hour before the eruption. Intensive seismicity, both earthquakes and volcanic tremor, accompanied the violent onset of the eruption. Almost 400 events up to M_L magnitude 2.5 were recorded during the first few hours. During the later phases of the eruption, the earthquake activity was modest and the main volcano-related seismic signal was the persistent volcanic tremor. The tremor died away, together with the eruption on 11 March, and Hekla was seismically quiet until the beginning of June 1991, when a sudden swarm of numerous small shallow earthquakes occurred. This activity is atypical for Hekla and is interpreted to be a failed attempt to resume the eruption.     

The Val Gabbro Plutonic Suite: A Sub-volcanic Intrusion Emplaced at the End of Flood Basalt Volcanism on the Kerguelen Archipelago

The age, petrology, major and trace element geochemistry, andSr–Nd–Hf–Pb isotopic geochemistry of basicand felsic rocks from the Val gabbro plutonic suite on the KerguelenArchipelago, Southern Indian Ocean, are used to constrain thetemporal and compositional relationships between sub-volcanicintrusions and flood basalt volcanism during the formation ofa major oceanic island. The 4 km² Val gabbro plutonic suitewas emplaced at 24·25 ± 0·15 Ma (U–Pbzircon) into 25 Ma volcanic rocks of the Southeast Province,locally producing a large zone of overlying basaltic breccia.Cumulate basic–ultrabasic rocks are the dominant lithologyin the intrusion, with horizontally layered peridotites at thebase of the exposed part of the intrusion, overlain by verticallylayered, coarse-grained plagioclase-bearing peridotites, melagabbrosand equigranular gabbros. The intrusion was formed by repeatedinjections of relatively crystal-rich and crystal-poor magmasinto an open-system magma reservoir. Strong geochemical andisotopic similarities between the fine-grained marginal microgabbrosand cross-cutting felsic rocks and the hosting mildly alkalicbasalts and trachytes of the Southeast Province indicate thatthey were derived from similar alkalic basaltic parental magmas,which were dominated by the enriched component of the Kerguelenmantle plume source. At 25 Ma, the change from tholeiitic–transitionalto mildly alkalic basalts marks the terminal stage of floodbasalt volcanism on the Kerguelen Archipelago. This compositionalchange was associated with deeper melting within the Kerguelenplume source, lower extents of melting, a decrease in magmasupply, and the emplacement of high-level intrusions such asthe Val gabbro plutonic suite. KEY WORDS: Kerguelen Archipelago; Val gabbro plutonic suite; oceanic island; gabbros; sub-volcanic intrusion; alkalinity; Sr–Nd–Hf–Pb isotopes     

Conservation Opportunity and Risk Mapping for Carnivores Using Landowner Survey Data from the Greater Yellowstone Ecosystem

Large landscape planning for wildlife corridors often requires inclusion of private lands and willing landowners to establish successful pathways to and from protected core habitats. Using mail survey data, we spatially assessed carnivore occurrences, perceptions of carnivores, and landowner preferences toward conservation planning tools from three communities to quantify conservation opportunity and risk related to carnivore movement on the landscape. The mapping of social data illustrates the importance of understanding individuals for conservation planning. The approach has the potential to identify areas that pose risks or present opportunities for the implementation of on-the-ground conservation actions to facilitate long-term wildlife movement across private lands.     

Combined stomach content and δ13C/δ15N analyses of oilfish,escolar, snake mackerel and lancetfish in the western North Atlantic

Large, mesopelagic teleost fishes have a potentially keystone position in the ecology of the pelagic water column, yet remain relatively unstudied when compared with large, commercially important, epipelagic fishes. Here, the ecological roles of four, large, vertically migrating teleosts were examined. Stomach content analyses were performed on 48 oilfish (Ruvettus pretiosis), 35 escolar (Lepidocybium flavobrunneum), 32 snake mackerel (Gempylus serpens) and seven lancetfish (Alepisaurus spp.) collected from pelagic longline gear in the Western North Atlantic Ocean from 2007 to 2010. Of these specimens, stable carbon and nitrogen isotope analyses were also performed on white dorsal muscle tissue from 33 oilfish, 16 escolar, 27 snake mackerel and seven lancetfish. Based on literature length‐at‐maturity values, all escolar, snake mackerel and lancetfish specimens were mature, while 13 of the 33 oilfish were juveniles. Crustaceans, annelids, salps, cephalopods and teleosts were present in the stomachs and were presumed to be prey items. A Kruskal–Wallis test showed the four species to be isotopically segregated in both δ¹³C and δ¹⁵N. Escolar were the most depleted in δ¹³C, followed by adult oilfish, juvenile oilfish and lancetfish, with snake mackerel the most enriched. The depletion in δ¹³C of adult oilfish and escolar may have been attributable to high C/N values, which were strongly correlated with length in oilfish, weakly correlated with length in escolar and moderately correlated with length in snake mackerel and lancetfish. The high C/N was likely due to the high lipid concentration of these fishes. Other factors that may have contributed to the depletion in δ¹³C may include spawning or a change in carbon source within the ecosystem. Large escolar occupied the highest trophic level (δ¹⁵N = 10.20), followed by snake mackerel (δ¹⁵N = 9.66), adult oilfish (δ¹⁵N = 9.32), lancetfish (δ¹⁵N = 9.05) and juvenile oilfish (δ¹⁵N = 7.83). A marked change in oilfish δ¹³C and C/N at 30–35 cm fork length coincided with a presumed length‐at‐maturity.     

Estimating direct and episodic atmospheric nitrogen deposition to a coastal waterbody

Direct deposition of atmospheric nitrogen to shallow coastal embayments is usually estimated, since insufficient field measurements are available. Using Waquoit Bay (Cape Cod, MA. USA) as a case study, and a recent review of literature, we determined reasonable bounds on wet and dry inputs of inorganic and organic N. Since precipitation and wind vary daily, we explored the potential of episodic events to stimulate phytoplankton blooms. Many coastal waterbodies like Waquoit Bay are small relative to their watersheds. Nevertheless, direct deposition of NH(3), NO(3)(-), and HNO(3) is significant in the loading budget. For Waquoit Bay, direct deposition was calculated to be 7-15.5 kg total N ha(-1)yr(-1), representing 70-150% of the atmospheric N reported to reach the bay via the watershed, and 20-45% of the total N reaching the bay from all land-based sources. Episodic events were estimated to deliver up to 65 mg N m(-2)day(-1), representing a phytoplankton stock of 12.3 mg Chl m(-2), an amount unlikely to stimulate dense blooms in shallow coastal waters in the northeastern United States.     

A new implementation of narrowband interference detection,characterization, and mitigation technique for a software-defined multi-GNSS receiver

Due to the very low power of satellite signals when reaching the earth’s surface, global navigation satellite system receivers are vulnerable to various types of radio frequency interference, and, therefore, countermeasures are necessary. In the case of a narrowband interference (NBI), the adaptive notch filtering technique has been extensively investigated. However, the research on the topic has focused on the adaptation of the notch frequency, but not of the notch width. We present a fully adaptive solution to counter NBI. The technique is capable of detecting and characterizing any number of narrow interfered bands, and then optimizing the mitigation process based on such characterization, namely the estimates of both interference frequency and width. Its full adaptiveness makes it suitable to cope with the unpredictable and diverse nature of unintentional interfering events. In addition to a thorough performance evaluation of the proposed method, which shows its benefits in terms of signal quality improvement, an analysis of the impact of different NBI profiles on GPS L1 C/A and Galileo E1 is also conducted.