SW Iceland Research Articles

Temperature reconstructions for SW and N Iceland waters over the last 10 cal ka based on δ18O records from planktic and benthic Foraminifera

We have obtained δ 18O carb data from planktic ( N. pachyderma (s.)) and benthic foraminifera (C. lobatulus, M. barleeanus) from eight cores between 40 and 500 m water depth along the SW (core B997–347) and N Iceland margins (cores B997-321, -324, -327, -328, -329, -330, and -332) that span the last 10 cal ka. Sampling resolution varies between centuries to millennial scales. Over this time range, changes in δ 18O carb in foraminifera are the product of changes in the global ice volume, and the temperature and salinity of the water at the level in which the foraminifera live. In order to evaluate the relative roles of these variables we investigate: (1) the salinity ( S‰) and δ 18O water values for present-day waters off Iceland; and (2) the present-day relationships between δ 18O carb (or calcite) values of foraminifera and water column temperature and salinity data. We estimate the average error in the temperature estimate to be ∼±0.3 °C based on a Monto Carlo simulation of the various error terms. We argue that around Iceland the variations in δ 18O carb of both benthic and planktic species over the last 10 cal ka are principally controlled by temperature variations while salinity changes prove less important. Spatial and temporal changes are examined for both bottom water and 50 m water temperature reconstructions based on six and four cores, respectively. Reconstructed temperatures for sites around SW-N Iceland indicate that surface and seafloor water temperatures were warmest in the early Holocene (10–6 cal ka). Beginning at 6 cal ka, surface and seafloor temperatures began to cool in N Iceland and to become less stratified in SW Iceland. Over the last 5000 yrs, sea surface temperature variations have been ⩽1 °C for 50% of the observations with a maximum range of ∼2.5 °C. A profound decrease in temperature of ∼1.5 °C is recorded in our fjord site (B997-328) during the Little Ice Age. An abrupt, earlier cold event is well expressed in at least three N Iceland cores around 5 cal ka.

The deep geothermal structure of the Mid-Atlantic Ridge deduced from MT data in SW Iceland

Iceland is very active tectonically as it is crossed by the Mid-Atlantic Ridge and its associated rift zones and transform faults. The high-temperature geothermal systems are located within the neo-volcanic zone. A detailed comparison of the main features of the resistivity models and well data in exploited geothermal fields has shown that the resistivity structure of Iceland is mainly controlled by alteration mineralogy. In areas where the geothermal circulation and related alteration take place at depths of more than 1.5 km, the investigation depth of the DC and TEM methods is inadequate and the MT method appears to be the most suitable survey method. MT soundings were carried out to determine the deep structure between two neighboring Quaternary geothermal fields: the Hengill volcanic complex and the Brennisteinsfjoll geothermal system, both known as high-temperature systems. MT data were analyzed and modeled using 1D and 2D inversion schemes. Our model of electrical conductivity can be related to secondary mineralization from geothermal fluids. At shallow depths, the resistivity model obtained from the MT data is consistent with the general geoelectrical models of high-temperature geothermal systems in Iceland, as revealed by shallow DC and TEM surveys. The current MT results reveal the presence of an outcropping resistive layer, identified as the typical unaltered porous basalt of the upper crust. This layer is underlain by a highly conductive cap resolved as the smectite–zeolite zone. Below this cap a less conductive zone is identified as the epidote–chlorite zone. A highly conductive material has been recognized in the middle of the profile, at about 5 km depth, and has been interpreted as cooling partial melt representing the main heat source of the geothermal system. This conductor may be connected to the shallow structure through a vertical fault zone located close to the southern edge of the profile.

A statistical evaluation of a ‘stress-forecast’ earthquake

SUMMARY The goodness of fit for competing statistical models with different numbers of degrees of freedom cannot be assessed solely by the residual sum of squares, because more complex models will naturally have lower residuals. A standard approach to hypothesis testing for large data sets is to use the objective Bayesian information criterion (BIC), which penalizes models with larger numbers of free parameters appropriately. We apply this method to the analysis of time delays from data on seismic shear-wave splitting in SW Iceland. The same data set has previously been used to estimate the time at which stress-modified micro-cracking reaches an inferred state of fracture criticality. The method does not forecast the location of the event, however, the time and magnitude were consistent with the actual occurrence of an M = 5 earthquake in the region. The forecast was based on a multi-line model with 17 degrees of freedom (five straight lines, each with a start and end point, plus the variance, with four endpoints being fixed by the occurrence of a main shock), and a signal-to-noise ratio near unity. The BIC is used to assess this forecast in comparison with competing curve fits for Poisson, multiline, sinusoidal, or polynomial (truncated Taylor expansion) hypotheses. The null hypothesis of random occurrence can only be rejected formally for the sinusoidal model, implying cyclical recurrence with a period of 134.6 days. All other models we consider have a lower BIC. We also analyse the selected portion of the data set used to make the forecast of fracture criticality using Gaussian statistics. The 95 per cent confidence intervals on the predicted main shock magnitude range between magnitudes of 3.9 and 6.7. The time range indicated by the same confidence limits starts 42 days before the actual event; a clear end cannot be located. The relation between predicted magnitude and waiting time is not significantly different from that inferred from the background Gutenberg‐Richter frequency‐magnitude relation within the model uncertainties. Thus, it is not possible, based on the data, to formally reject the hypothesis that the magnitude 5 event was part of the normal background seismicity.

Response to ‘A statistical evaluation of a ‘stress-forecast’ earthquake' by T. Seher & I. G. Main

SUMMARY Seher & Main claim to make a statistical evaluation of our successful stress-forecast of the time and magnitude of an earthquake in SW Iceland. A statistical evaluation of the data would be valuable, but unfortunately Seher & Main cannot do this as techniques have not yet been developed for the statistical analysis of discontinuous time-series within an overall continuous data set. Consequently, Seher & Main devise their own models of continuous time-series that are fundamentally different from those of our hypothesis. Seher & Main also briefly suggest several other concepts, which are serious misunderstandings of the stress-forecasting process. Consequently, their results and conclusions are unfortunately irrelevant to evaluating either the hypothesis or the stress-forecast.

Surface effects of triggered fault slip on Reykjanes Peninsula, SW Iceland

Three large earthquakes ( M w>4.5) were triggered within 5 min, 85 km west of a M w 6.5 earthquake in the South Iceland Seismic Zone (SISZ). We report on surface effects of these triggered earthquakes, which include fresh rupture, widespread rockfall, disrupted rockslides and block slides. Field data confirm that the earthquakes occurred along N-striking right-lateral strike-slip faults. Field data also support the conclusion from modeling of InSAR data that deformation from the second triggered event was more significant than for the other two. A major hydrological effect was the draining of water through an open fissure on a lake bed, lowering the lake level by greater than 4 m. Field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone.

Fracture populations on the Reykjanes Peninsula, Iceland: Comparison with experimental clay models of oblique rifting

We have used high‐resolution scanned air photos and field measurements to analyze fracture population systematics for the Reykjanes fissure swarm in order to determine the effect of rift obliquity on the evolution of fracture populations on the Reykjanes Peninsula, SW Iceland. The peninsula is oriented approximately 30° oblique to the direction of plate motion. Data show significant differences between the strike, length, and degree of development of fractures along the margin of the fissure swarm and those in the center of the zone of active volcanism. Along the margin, fractures are long, highly segmented extension fractures, and normal faults with offset on the order of several meters and a predominant strike 20° oblique to the trend of the plate boundary (rift axis). In the center of the zone of active volcanism, fractures are generally shorter, straighter, fewer in number, and strike approximately perpendicular to the direction of maximum horizontal extension and parallel to the trend of eruptive fissures. Right‐lateral oblique‐slip faults striking approximately perpendicular to the spreading direction are present in the center of the rift zone. Scaled experimental models of oblique extension predict a significant change in fracture strike at the rift margins due to the presence of a secondary stress field; on the Reykjanes Peninsula, this corresponds to the boundary between thicker and thinner brittle crust. The models confirm that right‐lateral oblique‐ or strike‐slip faults accommodate significant left‐lateral shear where deformation is distributed in a rift zone.

A four-year study of shear-wave splitting in Iceland: 2. Temporal changes before earthquakes and volcanic eruptions

Abstract This chapter reports temporal variations in the time-delays between split shear-waves before both earthquakes and volcanic eruptions in Iceland. The hypothesis is that during a build-up of stress, crack distributions in a large volume surrounding the immediate source zone are modified until the level of cracking reaches fracture criticality, when shear strength is lost, rocks fracture, and earthquakes, or some types of eruptions occur. In one two-year period, when volcanic and magmatic activity appeared to be low, changes in shear-wave splitting in SW Iceland were observed routinely before earthquakes with magnitudes between M 3.5 and M 5.1. Assuming a linear relationship between earthquake magnitude and the rate of increasing crack aspect-ratio in this comparatively narrow amplitude range, the time and magnitude of a M 5 earthquake was successfully stress-forecast. These results confirm a new understanding of pre-fracturing deformation of in situ rock that has implications over a wide range of situations where the crust undergoes changes at low levels of deformation below those at which rocks fracture. Potential applications of this new understanding include monitoring hydrocarbon production, as well as stress-forecasting earthquakes and some volcanic eruptions.

Three‐dimensional surface motion maps estimated from combined interferometric synthetic aperture radar and GPS data

We provide a technique to efficiently produce high‐resolution three‐dimensional surface motion maps by combining information about the motion of the Earth's surface from interferometric observations of synthetic aperture radar images and repeated Global Positioning System (GPS) geodetic measurements. Unwrapped interferograms, showing pixel‐wise change in range from ground to satellite, and sparse values of three‐dimensional movements are required as input. The problem of finding the full three‐dimensional motion field is separated into two two‐dimensional problems. Initially, the vertical component of the deformation field and its horizontal component in the look direction of the satellite are found. Later, the look direction component is resolved into north and east components. Initial values for the motion fields are assigned to each pixel of interferograms from interpolation of available GPS observations. These values are then updated and optimized by comparison with the interferograms and the GPS observations. An additional constraint is an assumption of a smoothly varying motion field. Markov random field‐based regularization and simulated annealing algorithm are used for the optimization. The technique is applied to create surface motion maps for the Reykjanes Peninsula, SW Iceland.

Life cycles and transmission patterns of seabird digeneans in SW Iceland

Flukes (digeneans) are common parasites of seabirds in Arctic and subarctic regions. This study examined the distribution of such parasites in 1998 and 1999 in the intertidal zones of two study areas in SW Iceland (Skerjafjördur and Grindavik) by investigating seven species of littoral snail, Littorina saxatilis, L. obtusata, L. fabalis, Onoba aculeus, Hydrobia ventrosa, Epheria vincta and Nucella lapillus , which are known to be first intermediate hosts of many species of digenean. A total of 2556 snails was collected and the intramolluscan stages of 23 seabird digenean species were identified. The local occurrence and abundance of final hosts, such as gulls, eiders and waders, largely explain prevalence differences in the study areas. Seabird digeneans with life cycles involving two intermediate hosts and one/two free-living larvae are more frequent in the Icelandic localities studied than in coastal areas of northern Europe (Barents Sea, White Sea, northern Norway). This indicates that the environmental conditions in the coastal ecosystem of SW Iceland are favourable for the transmission of complex life cycles. Also, the abundance and diversity of both potential final hosts (birds) and second intermediate hosts (littoral invertebrates and fishes) are considered to contribute to the richness of the digenean fauna of SW Iceland.

Surface effects of faulting and deformation resulting from magma accumulation at the Hengill triple junction, SW Iceland, 1994–1998

The Hengill triple junction, SW Iceland, is subjected to both tectonic extension and shear, causing seismicity related to strike-slip and normal faulting. Between 1994 and 1998, the area experienced episodic swarms of enhanced seismicity culminating in a M L=5.1 earthquake on June 4, 1998 and a M L=5 earthquake on November 13, 1998. Geodetic measurements, using Global Positioning System (GPS), leveling and Synthetic Aperture Radar Interferometry (InSAR) detected maximum uplift of 2 cm/yr and expansion between the Hrómundartindur and Grensdalur volcanic systems. A number of faults in the area generated meter-scale surface breaks. Geographic Information System (GIS) software has been used to integrate structural, field and geophysical data to determine how the crust failed, and to evaluate how much of the recent activity focused on zones of pre-existing weaknesses in the crust. Field data show that most surface effects can be attributed to the June 4, 1998 earthquake and have occurred along or adjacent to old faults. Surface effects consist of open gashes in soil, shattering of lava flows, rockfall along scarps and within old fractures, loosened push-up structures and landslides. Seismicity in 1994–1998 was distributed asymmetrically about the center of uplift, with larger events migrating toward the main fault of the June 4, 1998 earthquake. Surface effects are most extensive in the area of greatest structural complexity, where N- and E-trending structures related to the transform boundary intersect NE-trending structures related to the rift zone. InSAR, GPS, and field observations have been used in an attempt to constrain slip along the trace of the fault that failed on June 4, 1998. Geophysical and field data are consistent with an interpretation of distributed slip along a segmented right-lateral strike-slip fault, with slip decreasing southward along the fault plane. We suggest a right step or right bend between fault segments to explain local deformation near the fault.

Hydrothermal alteration of plagioclase and growth of secondary feldspar in the Hengill Volcanic Centre, SW Iceland

Dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the Hengill volcanic centre, in SW Iceland was studied by microprobe analysis of cuttings from two ca. 2000 m deep geothermal wells. Well NG-7 in Nesjavellir represents a geothermal system in a rift zone where the intensity of young, insignificantly altered intrusions increases with depth. Well KhG-1 in Kolviðarhóll represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher. This marginal well represents altered basaltic crust in an early retrograde state. The secondary plagioclase in both wells is mainly oligoclase, occurring in association with K-feldspar and chlorite±actinolite. The texture of this assemblage depends on the lithology and intensity of alteration. In Nesjavellir (NG-7) the composition of secondary albite-oligoclase is correlated with the host-rock composition. This connection is not apparent in more intensely altered samples from Kolviðarhóll (KhG-1). The influence of temperature on composition of secondary Na-feldspar is unclear in both wells although Ca is expected to increase with temperature. Any temperature dependence may be suppressed by the influence of rock composition in Nesjavellir and by retrograde conditions at Kolviðarhóll. The absence of clear compositional gradients between igneous plagioclase and secondary feldspar and between Na-feldspar and K-feldspar suggests that secondary feldspars formed by dissolution precipitation reactions.

Accumulation in East Greenland Fjords and on the Continental Shelves Adjacent to the Denmark Strait over the Last Century Based on 210Pb Geochronology

Sediment (SAR) and mass (MAR) accumulation rates for the last 100 years are derived from 210Pb profiles of box cores collected in three fjords and on the continental shelf in the Kangerlussuaq region, East Greenland, and on the SW Iceland continental shelf. The MAR is lower on East Greenland (0.09 g/cm²·yr) than on SW Iceland (0.14 g/cm²·yr) because of differences in the biogenic component of the sediments, although SAR is similar. The differences in accumulation rates of the three East Greenland fjords are due to their dominant sediment sources and basin areas. Kangerlussuaq Fjord, the largest fjord, is dominated by iceberg sediment input and has a MAR of 0.43 g/cm²·yr at 50 km from the fjord head. Miki Fjord, the smallest fjord, is dominated by glaciofluvial sediment input with a MAR of 0.29 g/cm²·yr of sediment at 10 km from the fjord head. Nansen Fjord is intermediate in size, with an iceberg-dominated sediment supply; MAR is 1.01 g/cm²·yr near the fjord head and 0.17 g/cm²·yr at 20 km from the fjord head. 137Cs is used as a secondary indicator of sediment source, and the high activity level of 137Cs in Miki Fjord indicates glaciofluvial sediment sources. Zones of constant activity in some of the box cores are attributed to either bioturbation in the surface mixed layer or turbidites. We would be hesitant to extend the recent 210Pb-based accumulation rates to estimate the Holocene accumulation rates.

Crustal deformation at the oblique spreading Reykjanes Peninsula, SW Iceland: GPS measurements from 1993 to 1998

In 1993 and 1998 a 38‐point GPS network was surveyed on the Reykjanes Peninsula, SW Iceland. According to the NUVEL‐1A plate motion model the spreading rate of the North American and Eurasian plates in SW Iceland is 18.9±0.5 mm/yr toward N102.7°±1.1°E, highly oblique to the plate boundary. Instead of oblique spreading, the measurements indicate left‐lateral shear strain accumulation parallel to the Reykjanes Peninsula seismic zone (∼N76°E) at the rate of about εyx ≈ −0.2 μstrain/yr (tensor shear strain). Subsidence is generally observed toward the seismic zone. A local maximum subsidence of 60 mm was measured in the Svartsengi geothermal area. Subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage. Expansion was observed in the area of the Hengill triple junction. This is probably a result of magma accumulation beneath! mount Hrómundartindur, as previous seismic and geodetic measurements have indicated. Using a simple screw dislocation model, we fit the majority of the data. Assuming a left‐lateral shear zone at depth along the seismic zone, we estimate locking depth of ∼6.5 km and deep slip rate of ∼16.5 mm/yr. The maximum left‐lateral displacement predicted by the screw dislocation model, 11.85±0.06 mm/yr, is consistent with the observed value of 11.9±0.5 mm/yr. If the Hengill area is excluded, little extension is observed across the peninsula. The discrepancy between the NUVEL‐1A oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period.

Digeneans from intertidal molluscs of SW Iceland.

The fauna of digenean daughter-sporocysts, rediae, cercariae and metacercariae infecting molluscs Littorina spp., Onoba aculeus. Nucella lapillus and Epheria vincta has been studied in the Skerjafjordur and Grindavik regions of SW Iceland. In total, intramolluscan stages of 19 digenean species were recorded; 14 of them are new for Iceland and one of them, a microphallid named Cercaria islandica I, was unknown. A description of this new microphallid cercaria is provided. In addition, the identification and separation of the intramolluscan stages of some microphallid, renicolid and echinostomatid species are discussed. Consideration is given to difficulties encountered when identifying digenean species found on the coasts of European countries. Problems have arisen largely because larval and adult stages have been described and named independently and in isolation, synonyms are common and many "definitive" descriptions are inadequate or incomplete.

Glacioisostacy controls chemical and isotopic characteristics of tholeiites from the Reykjanes Peninsula, SW Iceland

We present chemical and isotopic data from Reykjanes Peninsula rift zone tholeiites which show dramatic changes during the past 20 kyr, effectively defining the 9–13 kyr BP period of fluctuations in glacioisostacy associated with the end of the Weichselian glaciation and the Younger Dryas readvance. During this period primitive lavas were erupted with `depleted' characteristics; low Nb/Zr, high 143Nd/ 144Nd, low 87Sr/ 86Sr, and low 206Pb/ 204Pb. We propose that the clustered eruption of these chemically and isotopically anomalous lavas is primarily a reflection of reduced crustal residence times for the parental magmas. These `depleted' magmas largely escaped crustal processes such as mixing, assimilation and fractional crystallisation which would overprint their distinctive chemical and isotopic characteristics. There is some evidence that the northern rift zone may also have been similarly affected during this period of rapid glacioisostatic readjustment. The parental melts to these depleted glacioisostatic lavas may only be produced during fluctuations in glaciation, or could be a normal component in all Iceland magmas.

Crustal Processes: Major Controls on Reykjanes Peninsula Lava Chemistry, SW Iceland

Three hundred stratigraphically constrained samples from the Reykjanes Peninsula, SW Iceland, provide the basis for this study. This area is an elevated section of mid-ocean ridge influenced by the Iceland Plume. Selected chemical, Sr, Nd and laser-assisted fluorination oxygen isotope data are presented. The dataset is subdivided into groups based on criteria which are independent of degree of fractionation and petrography. Two of these groups, Depleted and Stapafell, include high-MgO aphyric samples with Δ18Oolivine values in equilibrium with normal peridotite mantle. Depleted group samples have high 143Nd/144Nd, low Nb/Zr and low incompatible element abundances compared with the dataset as a whole, the reverse of the Stapafell group. The majority of the remaining samples have radiogenic isotope ratios, and incompatible element concentrations and ratios intermediate between the Depleted and Stapafell groups. Some samples, however, define a range in 87Sr/86Sr and δ18Oolivine at constant 143Nd/144Nd, and others possess positive Sr anomalies when normalized to primitive mantle values. We explore the possibility that these and other chemical characteristics have been produced by shallow crustal processes, including assimilation of xenocrysts, cumulates and hydrothermally modified crust. We conclude that although these processes are important, the major crustal process acting to modify characteristics indicative of mantle heterogeneity is magma mixing. Chemical variation previously thought to be a consequence of dynamic melting is more readily explained by magma mixing.

Crustal Processes: Major Controls on Reykjanes Peninsula Lava Chemistry,SW Iceland

Crustal Processes: Major Controls on Reykjanes Peninsula Lava Chemistry,SW Iceland

Chemical Denudation Rates of a Basaltic Catchment, SW Iceland

Chemical Denudation Rates of a Basaltic Catchment, SW Iceland

The 1994–1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in a zone of horizontal shear stress

Since July 1994 an unusually persistent swarm of earthquakes (M<4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hrómundartindur volcanic system. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at 6.5±3 km depth beneath the center of the volcanic system. The system is within the stress field of the south Iceland transform zone, and the majority of the recorded earthquakes represent strike‐slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modeled as a point source in an elastic half‐space, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 μstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.

Fluctuations of the weichselian ice sheet in SW Iceland: A glaciomarine sequence from sudurnes, seltjarnarnes

Sedimentological and palaeontological studies in Reykjavík, SW Iceland, show that the glaciation history during the Late Pleistocene was characterized by a fluctuating glacier flowing WNW from Reykjanes Peninsula. New 14C dates show that marine deposition took place in coastal regions during at least three interstadials, corresponding to the Ålesund, the Bølling, and the Allerød in Scandinavia. The oldest of these interstadials is dated to ca. 28,000 14C years BP and represents the oldest Weichselian interstadial registered in Iceland up to now. Recent studies in Gardabær, SW Iceland, show marine transgression during the Bølling. Earlier work in Fossvogur, Reykjavík, indicates that the Allerød-Younger Dryas boundary did not coincide with a marked change in the water masses in SW Iceland. Sedimentological studies reveal a relatively high energy environment with near-shore and glaciomarine deposition and a period of glacial expansion during the Younger Dryas. The macrofauna, foraminifera, and ostracod faunas from sediments dated to the late Younger Dryas on Sudumes have been analysed and indicate low arctic to high boreal conditions. The Sudumes data show that the Younger Dryas glaciation in the Reykjav $ ́ area took place in a glaciomarine environment. The youngest dated sediments of the Sudurnes sequence probably postdate this glacier expansion.