Northwest Iceland Research Articles

A detailed palaeomagnetic study of the oldest (≈15 Myr) lava sequences in Northwest Iceland

SUMMARY We have determined the directions of primary remanence in over 400 lava flows in the northwestern peninsula of Iceland (near 66.1°N, 23.3°W). These lavas were sampled in 16 profiles spread across 75 km, at the stratigraphic level of the oldest (approximately 15 Myr in age) of several distinctive lignite-bearing sediments in the peninsula. We find that the pattern of polarity reversals in the lava pile is broadly similar in these profiles but it is generally not possible to trace short polarity zones and excursions over more than a few kilometres laterally. Our results, which also include measurements on the lignite sediments at three sites, are consistent with previous suggestions that these sediment beds represent a time gap of the order of 0.2 Myr between the lavas above and below. The mean virtual geomagnetic pole of this collection is ‘far-sided’ by approximately 7°, and the scatter of individual poles is greater than that in palaeomagnetic surveys on younger lava series in Iceland.

Late Quaternary Ice Cap Extent and Deglaciation, Húnaflóaáll, Northwest Iceland: Evidence from Marine Cores

The extent of the Last Glacial Maximum (LGM) and the timing of deglaciation around the Northwest Peninsula of Iceland are poorly understood. To provide information on these issues, we report sedimentological, foraminiferal, isotopic, and chronological data from marine piston cores B997-322PC, -323PC, -326PC1, and -326PC2 from a transect along Reykjafjardaáll/Húnaflóaáll, a large trough that extends from the north Iceland coast toward the shelf break, ca. 66.8°N and 20°W. Cores B997-322PC, -323PC, and -326PC1 recovered diamictons (stiff, pebbly muds) overlain by fine-grained, postglacial muds, with intermittent occurrences of iceberg-rafted clasts and volcanic shards. No glacial con-structional features (moraines) were noted on the seismic profiles. In the outermost cores (B997-322PC and 323PC) the diamictons contained small but persistent numbers of foraminifera which gave dates between 25 and 44 ka B.P.; foraminifera are absent in the diamicton at the base of core B997-326PC1. 14C dates immediately above the diamictons have ages of ca. 13 ka B.P., which we take to represent the date of deglaciation of Húnaflóaáll and the withdrawal of the ice margin to the adjacent Strandir coast. Foraminifera are used to construct a bio- and isotope stratigraphy of cores B997-322PC and 323PC. In the diamictons, the fauna is dominated by cold-water benthic foraminifera including Elphidium excavatum and Cassidulina reniforme; Cassidulina neoteretis is also a persistent component. In B997-322PC and -323PC the deglacial sequence is interrupted by an interval of IRD deposition, a dramatic reduction in foraminiferal numbers, and heavy δ18O values; we equate this interval with the Younger Dryas cold event. Alternative working hypotheses are advanced for the origin of the diamictons and their importance to mapping the LGM ice extent. The 12-ky hiatus between the diamictons and the overlying marine muds indicates that the sequence is not conformable. The diamictons probably represent glacial-marine sediments overrun by late Weichselian ice streaming down the trough. The absence of a thick, deglacial sequence of glacial-marine sediments indicates a dramatic retreat of the ice cap to the present coast by ∼13 ka B.P.

Late-Holocene paleoenvironments of northwest Iceland from lake sediments

Frozen sediment cores from two lakes in northwest Iceland, Vatnsdalsvatn and Thiðriksvallavatn, provide a nearly 1000-year, decadal-scale record of past environmental changes, based on environmental magnetism, inductively coupled plasma optical emission spectroscopy geochemistry, carbon content, and C/N analyses. Accelerator mass spectrometry 14C and 210Pb results provide chronologies for both sites, covering the late 20th century to 960 BP at Vatnsdalsvatn and to 480 BP at Thiðriksvallavatn. These analyses reveal higher levels of sedimentary carbon during the Little Ice Age, between 600 and 50 BP, at Vatnsdalsvatn. C/N analyses suggest that some of this carbon enrichment came from terrestrial plants. Higher erosion rates during the Little Ice Age may also have increased the influx of limiting nutrients, enhancing algal productivity. Erosion is the dominant mechanism for sedimentary changes at both sites and seems to occur at predictable intervals. Geochemical indicators of erosion oscillate with a distinct 80–100 yr periodicity for nine centuries at Vatnsdalsvatn and for four centuries at Thiðriksvallavatn. These erosion cycles are consistent with long-term stability in the positive mode of the North Atlantic Oscillation but may also be attributed to local climatic effects associated with Great Salinity Anomalies.

CAN SURVEILLANCE RADAR BE USED TO MONITOR THE FORAGING DISTRIBUTION OF COLONIALLY BREEDING ALCIDS?

Abstract We used surveillance radar to track flocks of colonially breeding alcids at Latrabjarg, northwest Iceland, that were returning from their feeding grounds. We compared the flight directions of incoming flocks during the chick-rearing period with alcid densities at sea during both the incubation and chick-rearing periods. The direction of incoming flocks was significantly related to alcid distribution at sea during the chick-rearing period. At-sea alcid counts suggested a temporal change in alcid use of foraging areas within a breeding season. Surveillance radar appears practical for monitoring preferred foraging areas of alcids within seasons and between years. Further, radar can provide an overall picture of alcid distribution near colonies, complementary to studies on foraging flights of individual birds. ?Pueden Utilizarse Radares de Vigilancia para Monitorear la Distribucion de Alcidos que se Reproducen en Colonias? Resumen. En Latrabjarg, noroeste de Islandia, utilizamos radares de vigilancia...

Distribution, sediment magnetism and geochemistry of the Saksunarvatn (10 180 ± 60 cal. yr BP) tephra in marine, lake, and terrestrial sediments, northwest Iceland

AbstractIn 1997, seismic surveys in the troughs off northwest and north Iceland indicated the presence of a major, regional sub‐bottom reflector that can be traced over large areas of the shelf. Cores taken in 1997, and later in 1999 on the IMAGES V cruise, penetrated through the reflector. In core MD99‐2269 in Húnaflóaáll, this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the North Atlantic‐wide Saksunarvatn tephra. We trace this tephra throughout northwest Iceland in a series of marine and lake cores, as well as in terrestrial sediments; it forms a layer 1 to 25 cm thick of fine‐ to medium‐grained basaltic volcanic shards. The base of the tephra unit is always sharp but visual inspection and other measurements (carbonate and total organic carbon weight %) indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking. Off northwest Iceland the Saksunarvatn tephra has distinct sediment magnetic properties. This is evident as a dramatic reduction in magnetic susceptibility, an increase in the frequency dependant magnetic susceptibility and ‘hard’ magnetisation in a −0.1T IRM backfield. Geochemical analyses from 11 sites indicate a tholeiitic basalt composition, similar to the geochemistry of a tephra found in the Greenland ice‐core that dates to 10 180 ± 60 cal. yr BP, and which was correlated with the 9000 14C yr BP Saksunarvatn tephra. We present accelerator mass spectrometry 14C dates from the marine sites, which indicate that the ocean reservoir correction is close to ca. 400 yr at 9000 14C yr BP off northwest Iceland. Copyright © 2002 John Wiley & Sons, Ltd.

A 36 Ky record of iceberg rafting and sedimentation from north-west Iceland

Evidence from north-west Iceland's shelf and fjords is used to develop a scenario for environmental change during the last 36 cal Ky. The retreat history of the Iceland Ice Cap during the last deglaciation is delineated through lithofacies studies, carbon analyses and magnetic susceptibility, and studies of ice-rafted debris (IRD) in sediment cores. Sedimentological data from lake Efstadalsvatn, Vestfirdir peninsula, trace the glacier retreat on land. In two of the high resolution shelf cores we detect near continuous IRD accumulation from 36 to 11 cal Kya. However, IRD is absent in the cores from ca. 22 to 19 cal Kya, possibly indicating more extensive landfast sea ice conditions. All cores show intensified IRD during the Younger Dryas chronozone; the fjord cores show a continuous IRD record until 10 cal Kya. Magnetic susceptibility and carbon analyses from Efstadalsvatn reveal the disappearance of local ice in the basin just before 10.5 cal Kya. No IRD was detected in the sediment cores during 10 to O4 cal Kya. Some indication of cooling occurs between 4 and 3 cal Kya, with a fresh input of IRD in fjord cores after 1 cal Kya.

Late Quaternary ice extent and glacial history from the Dj�p�ll trough, off Vestfirdir peninsula, north-west Iceland: a stacked 36 cal. Ky environmental record

Djupall is a O90 km long by 15 km wide trough which extends from Isafjardardjup to the shelf break above Blosseville Basin, north of the Denmark Strait. We present 3.5 kHz seismic profiles from this trough and data from cores collected in 1996 (JM96-1232 and ?1234) and five cores collected on cruise B997. We pay particular attention to B997-338 as this core recovered sediments ranging in age between 12 and 36 cal. Ky BP. This is the first such record from the Iceland continental shelf. Dating control is provided by AMS 14C dates and the occurrence of the Saksunarvatn tephra. X-radiographs of the cores enable us to quantify the input of iceberg-rafted detritus (IRD) and to describe the lithofacies. The sediment matrix is fine-grained and might represent either rain-out of suspended sediment plumes or distal turbidites. IRD is present from ca. 12 cal. Ky BP throughout the next 24 cal. Ky with some IRD-free intervals. Using sediment magnetic properties, sampled at 1 cm (o100 yrs/sample) resolution, we provide a stacked environmental record which includes marine isotope stages 1, 2 and part of 3. The sediment magnetic properties kARM and IRM(60), and carbonate and TOC, show multi-millennia quasi-periodic cycles, but there are no obvious events coeval with the North Atlantic Heinrich events. Our data indicate that at the Last Glacial Maximum on the Vestfirdir peninsula (VP), north-west Iceland, ice did not reach the shelf break, but was probably grounded near the mouth of Isafjardardjup. A rapid increase in the rate of sediment accumulation suggests that deglaciation of the VP occurred mainly between 11 and 15 cal. Ky BP.

A 36 Ky record of iceberg rafting and sedimentation from north-west Iceland

Evidence from north-west Iceland's shelf and fjords is used to develop a scenario for environmental change during the last 36 cal Ky. The retreat history of the Iceland Ice Cap during the last deglaciation is delineated through lithofacies studies, carbon analyses and magnetic susceptibility, and studies of ice-rafted debris (IRD) in sediment cores. Sedimentological data from lake Efstadalsvatn, Vestfirdir peninsula, trace the glacier retreat on land. In two of the high resolution shelf cores we detect near continuous IRD accumulation from 36 to 11 cal Kya. However, IRD is absent in the cores from ca. 22 to 19 cal Kya, possibly indicating more extensive landfast sea ice conditions. All cores show intensified IRD during the Younger Dryas chronozone; the fjord cores show a continuous IRD record until 10 cal Kya. Magnetic susceptibility and carbon analyses from Efstadalsvatn reveal the disappearance of local ice in the basin just before 10.5 cal Kya. No IRD was detected in the sediment cores during 10 to Ø4 cal Kya. Some indication of cooling occurs between 4 and 3 cal Kya, with a fresh input of IRD in fjord cores after 1 cal Kya.

Late Quaternary ice extent and glacial history from the Djúpáll trough, off Vestfirdir peninsula, north-west Iceland: a stacked 36 cal. Ky environmental record

Djúpáll is a Ø90 km long by 15 km wide trough which extends from Ísafjardardjúp to the shelf break above Blosseville Basin, north of the Denmark Strait. We present 3.5 kHz seismic profiles from this trough and data from cores collected in 1996 (JM96-1232 and ?1234) and five cores collected on cruise B997. We pay particular attention to B997-338 as this core recovered sediments ranging in age between 12 and 36 cal. Ky BP. This is the first such record from the Iceland continental shelf. Dating control is provided by AMS 14C dates and the occurrence of the Saksunarvatn tephra. X-radiographs of the cores enable us to quantify the input of iceberg-rafted detritus (IRD) and to describe the lithofacies. The sediment matrix is fine-grained and might represent either rain-out of suspended sediment plumes or distal turbidites. IRD is present from ca. 12 cal. Ky BP throughout the next 24 cal. Ky with some IRD-free intervals. Using sediment magnetic properties, sampled at 1 cm (ø100 yrs/sample) resolution, we provide a stacked environmental record which includes marine isotope stages 1, 2 and part of 3. The sediment magnetic properties kARM and IRM(60), and carbonate and TOC, show multi-millennia quasi-periodic cycles, but there are no obvious events coeval with the North Atlantic Heinrich events. Our data indicate that at the Last Glacial Maximum on the Vestfirdir peninsula (VP), north-west Iceland, ice did not reach the shelf break, but was probably grounded near the mouth of Ísafjardardjúp. A rapid increase in the rate of sediment accumulation suggests that deglaciation of the VP occurred mainly between 11 and 15 cal. Ky BP.

Time Allocation and Foraging Pattern of Chick-Rearing Razorbills in Northwest Iceland

AbstractA newly modified, bird-borne data-logger was used to investigate the flight activity and diving behavior of foraging Razorbills (Alca torda) breeding at Latrabjarg, northwest Iceland. The recorders allowed us to determine with high time-resolution all relevant movements in 47 complete foraging trips of 29 different Razorbills during two successive breeding seasons, 1997 and 1998. In 1998, direction recorders equipped with a compass recorded 19 foraging routes of 10 different Razorbills. The trips consisted of a number of flights interrupted by short series of diving bouts by which the birds seemingly explored areas for prey availability. Foraging occurred at sites where Razorbills stopped, dived, and rested for a long time before returning to the nest by a single flight or a sequence of a few flights. The majority of dive profiles (n = 17 095 dives) consisted of nonstop downward and upward movements to depths rarely greater than 35 m, and never exceeding 41 m. There was considerable diurnal variation in foraging behavior, expressed by more frequent and shallower dives around midnight. The Razorbill's diving pattern turns out to be very different from that reported for Thick-billed Murres (Uria lomvia) breeding in the same colony. Considerable differences in duration of trips, average number of dives, and time allocation were observed between the two breeding seasons, suggesting differences in foraging conditions for the birds.Asignación de Tiempo y Patrones de Forrajeo durante la Crianza de los Pichones en Alca torda en el Noroeste de IslandiaResumen. Un archivador de datos recientemente modificado que se acopla a las aves fue utilizado para investigar la actividad de vuelo y el comportamiento de buceo de individuos reproductivos de Alca torda en Latrabjarg, noroeste de Islandia. Los equipos nos permitieron determinar con una alta resolución de tiempo todos los movimientos relevantes en 47 viajes de forrajeo de 29 individuos diferentes durante dos temporadas reproductivas sucesivas (1997 y 1998). En 1998, equipos registradores de dirección equipados con una brújula, registraron 19 rutas de forrajeo de 10 individuos diferentes. Los viajes consistieron en un número de vuelos interrumpidos por pequeñas series de sesiones de buceo por medio de las cuales las aves aparentemente investigaban la disponibilidad de presas en distintas áreas. El forrajeo tuvo lugar en sitios donde las aves se detuvieron, bucearon y descansaron por un período prolongado antes de regresar al nido en un solo vuelo o en una secuencia de pocos vuelos. La mayoría de perfiles de buceo (n = 17 095 inmersiones) consistieron en movimientos ininterrumpidos hacia abajo y hacia arriba a profundidades rara vez mayores de 35 m y nunca superiores a 41 m. Hubo una variación considerable en el comportamiento de forrajeo a lo largo del día, con inmersiones más frecuentes y menos profundas alrededor de la medianoche. Los patrones de buceo de A. torda resultan ser muy diferentes de los que se han reportado para individuos de Uria lomvia que se reproducen en la misma colonia. Se observaron diferencias considerables en la duración de los viajes, el número promedio de inmersiones y la asignación del tiempo entre las dos épocas reproductivas, lo que sugiere que existieron diferencias en las condiciones de forrajeo para las aves.

TIME ALLOCATION AND FORAGING PATTERN OF CHICK-REARING RAZORBILLS IN NORTHWEST ICELAND

A newly modified, bird-borne data-logger was used to investigate the flight activity and diving behavior of foraging Razorbills (Alca torda) breeding at Latrabjarg, northwest Iceland. The recorders allowed us to determine with high time-resolution all relevant movements in 47 complete foraging trips of 29 different Razorbills during two successive breeding seasons, 1997 and 1998. In 1998, direction recorders equipped with a compass recorded 19 foraging routes of 10 different Razorbills. The trips consisted of a number of flights interrupted by short series of diving bouts by which the birds seemingly explored areas for prey availability. Foraging occurred at sites where Razorbills stopped, dived, and rested for a long time before returning to the nest by a single flight or a sequence of a few flights. The majority of dive profiles (n = 17 095 dives) consisted of nonstop downward and upward movements to depths rarely greater than 35 m, and never exceeding 41 m. There was considerable diurnal variation in foraging behavior, expressed by more frequent and shallower dives around midnight. The Razorbill's diving pattern turns out to be very different from that reported for Thick-billed Murres (Uria lomvia) breeding in the same colony. Considerable differences in duration of trips, average number of dives, and time allocation were observed between the two breeding seasons, suggesting differences in foraging conditions for the birds. Asignación de Tiempo y Patrones de Forrajeo durante la Crianza de los Pichones en Alca torda en el Noroeste de Islandia Resumen. Un archivador de datos recientemente modificado que se acopla a las aves fue utilizado para investigar la actividad de vuelo y el comportamiento de buceo de individuos reproductivos de Alca torda en Latrabjarg, noroeste de Islandia. Los equipos nos permitieron determinar con una alta resolución de tiempo todos los movimientos relevantes en 47 viajes de forrajeo de 29 individuos diferentes durante dos temporadas reproductivas sucesivas (1997 y 1998). En 1998, equipos registradores de dirección equipados con una brújula, registraron 19 rutas de forrajeo de 10 individuos diferentes. Los viajes consistieron en un número de vuelos interrumpidos por pequeñas series de sesiones de buceo por medio de las cuales las aves aparentemente investigaban la disponibilidad de presas en distintas áreas. El forrajeo tuvo lugar en sitios donde las aves se detuvieron, bucearon y descansaron por un período prolongado antes de regresar al nido en un solo vuelo o en una secuencia de pocos vuelos. La mayoría de perfiles de buceo (n = 17 095 inmersiones) consistieron en movimientos ininterrumpidos hacia abajo y hacia arriba a profundidades rara vez mayores de 35 m y nunca superiores a 41 m. Hubo una variación considerable en el comportamiento de forrajeo a lo largo del día, con inmersiones más frecuentes y menos profundas alrededor de la medianoche. Los patrones de buceo de A. torda resultan ser muy diferentes de los que se han reportado para individuos de Uria lomvia que se reproducen en la misma colonia. Se observaron diferencias considerables en la duración de los viajes, el número promedio de inmersiones y la asignación del tiempo entre las dos épocas reproductivas, lo que sugiere que existieron diferencias en las condiciones de forrajeo para las aves.

New Holocene tephras and a proxy climate record from a blanket mire in northern Skye, Scotland

AbstractFour Holocene tephras of Icelandic origin have been identified and geochemically characterised from a water shedding blanket peat sequence on the Trotternish ridge, Isle of Skye, Scotland. Geochemical characterisation of the shards propose the Glen Garry tephra to be present, a tephra layer of Hekla origin incorporating shards from Hekla‐4, as well as two new tephras dated by interpolation from a radiocarbon based chronology to ca. 830 cal. yr BP and ca. 2340 cal. yr BP. The new historic tephra has an ambiguous geochemistry and therefore has not been correlated with other known Icelandic historic tephras. The new prehistoric tephra is suggested as originating from the Snæfellsjökull volcano in northwest Iceland and forms an important stratigraphical marker in this Holocene sequence. A proxy climate record has been derived from humification analyses of the peat, which compares well with other regional palaeoclimatic reconstructions, as well as enabling correlations based on tephra geochemical linkages between sites and climatic records at precise times in the past. Copyright © 2001 John Wiley & Sons, Ltd.

The proposed open-pit protection of Bolungarvík, Iceland

AbstractSnow avalanches threaten a large part of the residential area of Bolungarvik, a town of 1100 inhabitants located in the Vestfirðir peninsula, northwest Iceland. Methods for the complete protection of all the houses were requested by the Ministry of Environment. As conventional deflecting and catching dams were not feasible due to avalanche velocities of > 40 m s–1 in the actual defence area, a huge open pit built into the lower mountainside was proposed. The pit is designed to dissipate the energy of avalanches of any expected size. It is 1000 m long, and the upper and lower pit walls are 30–40 and 15–20 m high, respectively. The downhill side of the pit is increased to 30 m by a nearly vertical, 10–15 m high dam. The bottom is 25–40 m wide, and the effective cross-sections are 700–2300 m2. The total storage capacity is approximately 106 m3. Cost-benefit analyses indicate that the development cost of the pit protection is favourable compared to the value of the houses as well as alternative safety solutions.

Slushflow Hazard and Mitigation in Vesturbyggd, Northwest Iceland

Slushflows – flowing mixtures of water and snow – threaten several residential areas within Vesturbyggð, a municipality of 1250 people located in the Vestfirdir peninsula in Northwest Iceland. An overview of the problem is given and particular attention is drawn to the catastrophic events of January 22nd 1983 in Patreksfjörður, when two slushflows damaged or destroyed 20 houses, killing four of the twelve people caught in the them. As the runout zones of the slushflows are densely populated the local authority requested an appraisal study for slushflow defences. For the main hazard zone below Geirseyrargil the work included an evaluation of potential starting zones, hazard areas, mitigative measures and cost estimates compared with the value of the involved properties. Slushflow activity in Vesturbyggð and other parts of Iceland is associated with cyclonic activity during winter as well as spring break-up periods. Most commonly, slushflows are released in drainage channels and streams, but they may also start from inclined depressions. Snow-embanked, water-saturated snowfields are other potential starting zones. Whether a snowpack will reach a critical stability during rain and snowmelt depends on the complex interaction between geomorphic factors, snowpack properties and the relative rate of formation and discharge of free water in the snowpack in potential starting zones. By analysis of meteorological records from the autumn of 1982 and January of 1983 the snowpack development prior to the events and stability of the snowpack at the start of the critical weather period in January 1983 are estimated.

Extreme 3He/ 4He ratios in northwest Iceland: constraining the common component in mantle plumes

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. 3He/ 4He ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently ˜37 R A (R A = air 3He/ 4He) — significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189–218]). The Sr, Nd and Pb isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517–520]) — in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275–24300]). This observation implies that Selardalur may represent the best estimate available to date of the He–Sr–Nd–Pb isotopic composition of the 5th mantle component common to many mantle plumes.

A Diagnostic Study of the Flateyri Avalanche Cyclone, 24–26 October 1995, Using Potential Vorticity Inversion

Abstract The evolution of a deep North Atlantic cyclone, which caused devastating avalanches in northwest (NW) Iceland in October 1995, was investigated. As the main tool for this investigation, potential vorticity analysis was used. This allows the quantification and comparison of the roles of different processes that contribute to the cyclone deepening at different stages. Interpretation of potential vorticity inversions and isentropic air trajectories yields the following picture of the cyclone development. The thermal field over the North Atlantic had acquired strong west–east gradients due to a combination of advection of cold air southeastward from a cold cyclonic gyre south of Iceland and advection of warm air northward on the westward flank of a warm anticyclonic ridge over central Europe. A low-level baroclinic wave forming just south of Ireland was rapidly reinforced due to interaction with a descending, high-value, upper-level potential vorticity anomaly and was isentropically advected from the...

Longitudinal and interhemispheric variations of auroral ionospheric electrodynamics in a realistic geomagnetic field

We investigate the influences of nondipolar features of the geomagnetic field (field strength, orientation, and magnetic coordinate distortion) on auroral ionospheric electrodynamics. We present a conceptual model in which both the electric potential and the distribution functions of precipitating auroral particles are invariant in the magnetic latitude/magnetic local time reference frame, and we explore the predictions of this model concerning longitudinal (or universal time) and interhemispheric (north/south) variations of auroral electrodynamic parameters at ±68° magnetic latitude for a fixed magnetic local time and solar zenith angle. The conductances, electric fields and currents, ion drift velocity components, and Joule heating tend to have two minima and maxima with respect to longitude in the northern hemisphere but have a single minimum and maximum in the southern hemisphere. Particularly large variations are found for the field‐aligned current density (maximum/minimum = 1.76) and the regional Joule heating (maximum/minimum = 1.88), with maxima near northwest Iceland and northwest Alaska, and minima over north‐central Siberia and Hudson Bay. The variations of field‐aligned current intensity may imply a tendency for more frequent field‐aligned electron acceleration and thus brighter aurora near northwest Iceland and northwest Alaska than elsewhere. The longitudinal variations of Joule heating may contribute significantly to universal time variations of total hemispheric Joule heating. Whether or not the conceptual model is correct, at least some of the various electrodynamic parameters must have longitudinal and hemispheric variations of the general magnitudes the model predicts.

Fault systems and paleostresses in the Vestfirdir Peninsula. Relationships with the tertiary paleo-rifts of Skagi and Snaefells (Northwest Iceland)

The Vestfirdir Peninsula of northwestern Iceland mainly consists of tholeiitic lava flows, 8–14 Ma old, gently dipping to the southeast. A detailed study of strike-slip and normal faulting allowed identification of two main paleostress regimes. Two sets of normal faults were recognized. The largest set trends ENE-WSW to NNE-SSW; the minor set trends NW-SE to WNW-ESE. Concerning the major extension, the reconstruction of paleostress trajectories shows a gradual change in trend from ESE-WNW, in the northern half of the peninsula, to NNW-SSE to the south. The minor extension also shows a gradual change from NNE-SSW to ENE-WSW trends, from north to south. The nearly constant perpendicularity between the major and minor trends of extension is accounted for by permutation of stress axes within the general pattern of extension related to oceanic rifting. The progressive azimuthal change of the major extension trend, from northeast to southwest across the peninsula, is interpreted as the expression of a change in trend of the extinct Skagi-Snaefells rift, a structure that cannot be reconstructed directly through geological mapping. The average trends of extension in the southern Vestfirdir Peninsula, N150 °E, suggest a N60 °E trend for the ancient Snaefells rift segment. Likewise, the N100–110 °E trends of extension in northern Vestfirdir suggest a N10–20°E trend for the Skagi paleo-rift.

Fault systems and paleostresses in the Vestfirdir Peninsula. Relationships with the Tertiary paleo-rifts of Skagi and Snaefells (Northwest Iceland)

The Vestfirdir Peninsula of northwestern Iceland mainly consists of tholeiitic lava flows, 8–14 Ma old, gently dipping to the southeast. A detailed study of strike-slip and normal faulting allowed identification of two main paleostress regimes. Two sets of normal faults were recognized. The largest set trends ENE-WSW to NNE-SSW; the minor set trends NW-SE to WNW-ESE. Concerning the major extension, the reconstruction of paleostress trajectories shows a gradual change in trend from ESE-WNW, in the northern half of the peninsula, to NNW-SSE to the south. The minor extension also shows a gradual change from NNE-SSW to ENE-WSW trends, from north to south. The nearly constant perpendicularity between the major and minor trends of extension is accounted for by permutation of stress axes within the general pattern of extension related to oceanic rifting. The progressive azimuthal change of the major extension trend, from northeast to southwest across the peninsula, is interpreted as the expression of a change in trend of the extinct Skagi-Snaefells rift, a structure that cannot be reconstructed directly through geological mapping. The average trends of extension in the southern Vestfirdir Peninsula, N150°E, suggest a N60°E trend for the ancient Snaefells rift segment. Likewise, the N100–110°E trends of extension in northern Vestfirdir suggest a N10–20°E trend for the Skagi paleo-rift., © Elsevier, Paris

Rift relocation — A geochemical and geochronological investigation of a palaeo-rift in northwest Iceland

A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreading axis in response to westward migration of the plate boundary relative to the plume centre. Two major former rifts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spreading system, lava flows from which can be traced some 100 km along a SW-NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a laterite-lignite horizon representing a maximum 200 k.y. hiatus in the lava succession. Lavas below the unconformity dip northwest towards the older axis from which they were erupted, whereas lavas above the unconformity dip southeast towards their source in the younger Snaefellsnes axis. Thus, two nearly complete rift relocation cycles are preserved in western Iceland, each lasting about 8 m.y. as measured between rift extinction events, and for around 12 m.y. from initial propagation to extinction. In this paper we present major- and trace-element analyses, Sr, Nd and Pb isotope data, and 40Ar/ 39Ar dates on basalt samples from above and below the unconformity in northwest Iceland. The Icelandic Tertiary and Quaternary plateau basalts are remarkably homogeneous in composition, in contrast to the much more diverse compositions found in the presently active rift zone. However, basaltic lava flows beneath the unconformity in northwest Iceland show a wider range of incompatible element and radiogenic isotope ratios than do the younger plateau basalts. At least two mantle components, one depleted and the other less depleted with respect to bulk Earth, are required to explain the composition of post-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwest palaeo-rift, however, contains a significant North Atlantic N-MORB component, suggesting that depleted upper mantle can influence the composition of Icelandic basalt in a dying rift that is too far from the plume centre to be dominated by plume mantle. This may account for the periods of low magma productivity represented by troughs between the V-shaped ridges on the Reykjanes Ridge. We suggest that temporal variation in the composition of Icelandic basalt is better explained by crustal accretion and rift relocation processes than by variations in plume composition and temperature.