Early to middle Holocene silicic tephra horizons from the Katla volcanic system,Iceland: new results from the Faroe Islands

Comparatively few Icelandic tephra horizons dated to the early part of the Holocene have so far been detected outside Iceland. Here, I present several tephra horizons that have been recorded in a Holocene peat sequence on the Faroe Islands. Geochemical analyses show that at least two dacitic and one rhyolitic tephra layers were erupted from the Katla volcanic system on southern Iceland between ca. 8000 and 5900 cal. yr BP. The upper two layers can be correlated with the SILK tephras described from southern Iceland, whereas the third, dated to ca. 8000 cal. yr BP, has a geochemistry virtually identical to the rhyolitic component of the Vedde Ash. The results suggest that the Late Weichselian and early Holocene eruption history of the Katla volcano was probably more complex than inferred from Iceland. A new, early Holocene rhyolitic tephra dated to ca. 10 500 cal. yr BP probably originates in the Snæfellsnes volcanic centre in western Iceland. These new findings may play an important role in developing a Holocene tephra framework for northwest Europe. Copyright © 2002 John Wiley & Sons, Ltd.     

Geochemistry,dispersal, volumes and chronology of Holocene silicic tephra layers from the Katla volcanic system,Iceland

At least 12 silicic tephra layers (SILK tephras) erupted between ca. 6600 and ca. 1675 yr BP from the Katla volcanic system, have been identified in southern Iceland. In addition to providing significant new knowledge on the Holocene volcanism of the Katla system which typically produces basaltic tephra, the SILK tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts. With one exception the SILK tephras have a narrow compositional range, with SiO₂ between 63 and 67%. Geochemically they are indistinguishable from ocean transported pumice found on beaches in the North Atlantic region, although they differ significantly from the silicic component of the North Atlantic Ash Zone One (NAAZO). Volumes of airborne SILK tephra range from 0.05 to 0.3 km³. We present new isopach maps of the six largest layers and demonstrate that they originate within the Katla caldera. The apparently stable magma system conditions that produced the SILK tephras may have been established as a consequence of the eruption of the silicic component of NAAZO (ca. 10.3 ka) and disrupted by another large‐scale event, the tenth century ad Eldgjá eruption (ca. 1 ka). Despite the current long repose, silicic activity of this type may occur again in the future, presenting hitherto unknown hazards. Copyright © 2001 John Wiley & Sons, Ltd.     

Possible correlation of Oligocene climate changes with Ethiopian Oligocene ignimbrite eruptions

The Oligocene Ethiopian continental flood basalt province (ca. 29–31 Ma) contains significant silicic pyroclastic rocks (>60,000 km³ constituting up to 20% of the volcanic stratigraphy). Rhyolitic tephras, synchronous with the Ethiopian silicic pyroclastic rocks, are found in Indian Ocean ODP holes 711A. They are geochemically akin to the Ethiopian silicic pyroclastic rocks. This suggests that the Indian Ocean tephras originated from Ethiopian silicic eruptions and represents more distal fallout of this volcanism. The temporal coincidence of the Ethiopian flood volcanism with the Oligocene global cooling event (Oi2?～?30.3 Ma) and the emplacement of the Ethiopian silicic pyroclastic eruptions on a near-global scale strongly suggest that the Ethiopian continental flood basalt province may have contributed or at least accelerated the climate change that was already underway.     

Energy-dispersive X-ray fluorescence analysis as a rapid method for identifying tephras

The use of energy-dispersive X-ray fluorescence analysis (XES) for the routine identification of three tephras (Mazama, Bridge River, Mount St. Helens Yn) commonly found in archeological sites in British Columbia has been investigated. Researchers have often assumed that chemical analysis of bulk samples of glass separates would be hampered by contamination and weathering effects. Our results indicate that XES of bulk glass separates provides a very reliable method for rapidly identifying the three tephras in question, even with a very simple sample preparation. This should enable persons not skilled in geology or in tephrochronology to collect and to identify samples of these tephras. Finally, as a part of the study, similar measurements were made on the separated glass portions of these three tephras and of three others (Glacier Peak B and G, White River) from northwest North America. The results suggest that this method may provide tephrochronologists with a useful additional tool for studying tephras in other regions.     

Iceland's best kept secret

The ‘forgotten fjords’ and ‘deserted inlets’ of NE‐Iceland, in the region between Borgarfjörður Eystri and Loðmundarfjörður, are not only prominent because of their pristine landscape, their alleged elfin settlements, and the puffins that breed in the harbour, but also for their magnificent geology. From a geological point of view, the area may hold Iceland's best kept geological secret. The greater Borgarfjörður Eystri area hosts mountain chains that consist of voluminous and colourful silicic rocks that are concentrated within a surprisingly small area (Fig. 1 ), and that represent the second‐most voluminous occurrence of silicic rocks in the whole of Iceland. In particular, the presence of unusually large volumes of ignimbrite sheets documents extremely violent eruptions during the Neogene, which is atypical for this geotectonic setting. As a group of geoscientists from Uppsala University (Sweden) and the Nordic Volcanological Center (NordVulk, Iceland) we set out to explore this remote place, with the aim of collecting material that may allow us to unravel the petrogenesis of these large volumes of silicic rocks. This effort could provide an answer to a long‐standing petrological dilemma; the question of how silicic continental crust is initially created. Here we document on our geological journey, our field strategy, and describe our field work in the remote valleys of NE‐Iceland.     

Lake Torfadalsvatn: a high resolution record of the North Atlantic ash zone I and the last glacial-interglacial environmental changes in Iceland

High resolution environmental records with a refined chronology are essential to understand, reconstruct and model the climate dynamics of the last glacial-interglacial transition. Sediments from Lake Torfadalsvatn in northern Iceland contain at least four primary volcanic tephras that belong to ash zone I in the North Atlantic deep-sea cores. We chemically define these basaltic/rhyolitic tephras and the high resolution allows us to date them to about 10,800, 10,600, 9300 and 8900 BP. This detailed tephrostratigraphy will act as a refined dating and correlation tool in the North Atlantic region and enable calibration between different absolute chronologies. The pollen stratigraphy of the sediments suggests that by 10,400 ¹⁴C years BP plant colonization of coastal north Iceland had begun. The pollen stratigraphy shows a succession of pioneer plants, from open tundra vegetation towards birch-juniper woodland, which probably also reflects a transition from a cool climate at 10,400 BP to conditions similar to today's sub-polar oceanic climate around 9200 BP. Diatom data largely concur with the climatic information from pollen, indicating gradually increasing productivity in the lake.     

Use of magnetic signatures to correlate tephra layers in Holocene loessial soil profiles from a small region, SE Iceland

Rock magnetic measurements have been applied to two adjacent loessial soil sections from a small region, SE Iceland. The soils are composed of reworked locally derived sediment (including airfall tephra) and also contain several visible discrete airfall tephra layers. The main magnetic minerals in the airfall tephras are ferrimagnetic (e.g. magnetite) with paramagnetic minerals also present. The main magnetic grain sizes in the tephras are pseudo single domain. The results show that individual tephras do not have unique magnetic signatures that can be used for identification and correlation between sedimentary sequences. However, a correlation of tephra layers was achieved through the application of statistical techniques to a comprehensive dataset of magnetic parameters. Similarity coefficients and Euclidian distance measures were used to identify the best correlation between tephra layers in the two soil profiles. The technique works well providing some tephras present within both profiles have been formally identified (e.g. Oraefajokulla ad 1362) using electron microprobe analyses. Given this initial framework, the statistical analyses of the magnetic parameters can help in the identification and correlation of unknown tephras between two soil profiles.     

Rayleigh wave tomography in the North Atlantic: high resolution images of the Iceland, Azores and Eifel mantle plumes

Presented in this paper is a high resolution S_v-wave velocity and azimuthal anisotropy model for the upper mantle beneath the North Atlantic and surrounding region derived from the analysis of about 9000 fundamental and higher-mode Rayleigh waveforms. Much of the dataset comes from global and national digital seismic networks, but to improve the path coverage a number of instruments at coastal sites in northwest Europe, Iceland and eastern Greenland was deployed by us and a number of collaborators. The dense path coverage, the wide azimuthal distribution and the substantial higher-mode content of the dataset, as well as the relatively short path-lengths in the dataset have enabled us to build an upper mantle model with a horizontal resolution of a few hundred kilometers extending to 400 km depth. Low upper mantle velocities exist beneath three major hotspots: Iceland, the Azores and Eifel. The best depth resolution in the model occurs in NW Europe and in this area low S_v-velocities in the vicinity of the Eifel hotspot extend to about 400 km depth. Major negative velocity anomalies exist in the North Atlantic upper mantle beneath both Iceland and the Azores hotspots. Both anomalies are, above 200 km depth, 4–7% slow with respect to PREM and elongated along the mid-Atlantic Ridge. Low velocities extend to the south of Iceland beneath the Reykjanes Ridge where other geophysical and geochemical observations indicate the presence of hot plume material. The low velocities also extend beneath the Kolbeinsey Ridge north of Iceland, where there is also supporting geochemical evidence for the presence of hot plume material. The low-velocity upper mantle beneath the Kolbeinsey Ridge may also be associated with a plume beneath Jan Mayen. The anomaly associated with the Azores extends from about 25°N to 45°N along the ridge axis, which is in agreement with the area influenced by the Azores Plume, predicted from geophysical and geochemical observations. Compared to the anomaly associated with Iceland, the Azores anomaly is elongated further along the ridge, is shallower and decays more rapidly with depth. The fast propagation direction of horizontally propagating S_v-waves in the Atlantic south of Iceland correlates well with the east–west ridge-spreading direction at all depths and changes to a direction close to NS in the vicinity of Iceland.     

Chemistry of miocene plume tholeiites from northwest Iceland

The earliest (Miocene) plateau basalts of northwest Iceland form an olivine tholeiite series with elevated contents of Ti, K, P, Rb, Ba and Sr. They are closely similar to ‘plume’ tholeiites of the Faeroes (Paleocene-Eocene) and the Reykjanes Peninsula, Iceland (Recent) and confirm the Miocene renaissance in northeast Atlantic plume activity previously suggested on geophysical grounds.It is argued that the elevated contents of Ti, K, etc., are due to the ascent in a plume column of high pressure alkalic magmas and their re-equilibration to low pressure olivine tholeiites largely by additional melting at 10–20 km depth and 1150–1300°C.The 1800 m northwest Iceland sequence lacks stratigraphic variation suggesting random extraction from an extensive melt region with a nearly stable range of P-T-chemical conditions.     

Holocene tephra horizons at Klocka Bog,west‐central Sweden: aspects of reproducibility in subarctic peat deposits

This paper presents one of the most extensive Holocene tephra records found to date in Scandinavia. Microtephra horizons originating from Icelandic eruptions were recorded in two ca. 2 m thick peat profiles at Klocka Bog in west‐central Sweden. Five of the microtephra horizons were geochemically correlated to the Askja‐1875, Hekla‐3, Kebister, Hekla‐4 and Lairg A tephras respectively. Radiocarbon‐based dating of these tephras broadly agree with previously published ages from Iceland, Sweden, Germany and the British Isles. The identification of the Lairg A tephra demonstrates a more widespread distribution than previously thought, extending the usefulness of Icelandic Holocene tephrochronology further north into west‐central Scandinavia. Long‐lasting snow cover and seasonal wind distribution in the lower stratosphere are suggested as factors that may be responsible for fragmentary tephra deposition patterns in peat deposits of subarctic Scandinavia. Copyright © 2004 John Wiley & Sons, Ltd.     

The Lateglacial to early Holocene tephrochronological record from Lake Hämelsee,Germany: a key site within the European tephra framework

Here we present the results of a detailed cryptotephra investigation through the Lateglacial to early Holocene transition, from a new sediment core record obtained from Lake Hämelsee, Germany. Two tephra horizons, the Laacher See Tephra (Eifel Volcanic Field) and the Saksunarvatn Ash (Iceland), have been previously described in this partially varved sediment record, indicating the potential of the location as an important Lateglacial tephrochronological site in northwest Europe. We have identified three further tephra horizons, which we correlate to: the c. 12.1 ka BP Vedde Ash (Iceland), the c. 11 ka BP Ulmener Maar tephra (Eifel Volcanic Field) and the c. 10.8 ka BP Askja‐S tephra (Iceland). Three additional cryptotephra deposits have been found (locally named HÄM_T1616, HÄM_T1470 and HÄM_T1456‐1455), which cannot be correlated to any known eruption at present. Geochemical analysis of the deposits suggests that these cryptotephras most likely have an Icelandic origin. Our discoveries provide age constraints for the new sediment records from Lake Hämelsee and enable direct stratigraphical correlations to be made with other tephra‐bearing sites across Europe. The new tephrostratigraphical record, within a partially varved Lateglacial sediment record, highlights the importance of Lake Hämelsee as a key site within the European tephra lattice.     

A tephrochronology for the Lateglacial palynological record of the Endinger Bruch (Vorpommern,north‐east Germany)

The tephrostratigraphy of lake sediments in the Endinger Bruch provides the first robust age model for the Lateglacial palynological records of Vorpommern (north‐east Germany). Cryptotephra investigations revealed six tephra layers within sediments spanning from Open vegetation phase I (～Bølling, ～15 ka) to the Early Holocene Betula/Pinus forest phase (～Pre‐boreal, ～10.5 ka). Four of these layers have been correlated with previously described tephra layers found in sites across Europe. The Laacher See Tephra (Eifel Volcanic Field) is present in very high concentrations within sediments of the Lateglacial Betula (/Pinus) forest phase (～Allerød). The Vedde Ash (Iceland) lies midway through Open vegetation phase III (～Younger Dryas). The Hässeldalen and the Askja tephras (Iceland) lie in the Early Holocene Betula/Pinus forest phase (～Preboreal). These tephra layers have independently derived age estimates, which have been imported into the Endinger Bruch record. Furthermore, the layers facilitate direct correlation of the regional vegetation record with other palaeoenvironmental archives, which contain one or more of the same tephra layers, from Greenland to Southern Europe. In doing this, localized variations are confirmed in some aspects of the pollen stratigraphy; however, transitions between the main vegetation phases appear to occur synchronously (within centennial errors) with the equivalent environmental transitions observed in sites across the European continent. Copyright © 2011 John Wiley & Sons, Ltd.     

The Campanian Ignimbrite (Y5) tephra at Crvena Stijena Rockshelter, Montenegro

Clearly defined distal tephras are rare in rockshelter sediment records. Crvena Stijena, a Palaeolithic site in Montenegro, contains one of the longest (> 20 m) rockshelter sediment records in Europe with deposits ranging in age from Middle Pleistocene to mid-Holocene. A distinctive tephra is clearly exposed within the well stratified record approximately 6.5 m below the present land surface. We present geochemical data to confirm that this tephra is a distal equivalent of the Campanian Ignimbrite deposits and a product of the largest Late Pleistocene eruption in Europe. Originating in the Campanian volcanic province of southwest Italy, this tephra has been independently dated to 39.3 ka. It is a highly significant chronostratigraphic marker for southern Europe. Macrostratigraphic and microstratigraphic observations, allied with detailed particle size data, show that the tephra layer is in a primary depositional context and was transported into the rockshelter by aeolian processes. This site is unique because the tephra forms an abrupt boundary between the Middle and Upper Palaeolithic records. Before they can be used as chronostratigraphic markers in rockshelter and cave-mouth environments, it is essential to establish the stratigraphic integrity of distal tephras and the mechanisms and pathways involved in their transport and deposition.     

The distribution of ash in Icelandic lake sediments and the relative importance of mixing and erosion processes

During the Holocene the volcano Hekla explosively emitted highly silicic tephra on four occasions. The ash was widely dispersed by the wind. Distinctive light-coloured ash layers are now to be seen in the peats of Northern Iceland. Ash from the 1104 AD eruption was carried as far as Scandinavia. The most recent three tephra are preserved in the top 6 m of sediment in Lake Svinavatn. Chemical data from the sediment of Lake Svinavatn, which lies near the north coast of Iceland 170 km from Hekla, reveal the presence of silicic ash above the tephra visible to the naked eye. Unlike the vertical spread of ash in ocean sediment cores which results from biological mixing effects, the upwards spread of ash in the Svinavatn lake sediment cores appears to have been produced by erosion of ash from the lake catchment in the decades following the eruptions. The variations in concentrations of 11 elements, as determined by neutron activation analysis, can be explained by an exponentially decreasing input from catchment erosion. The additional input to each of the three Hekla ash layers was in the region of 3% of the ash which fell on the Svinavatn catchment.     

Crustal thermal state and origin of silicic magma in Iceland: the case of Torfajökull,Ljósufjöll and Snæfellsjökull volcanoes

Pleistocene and Holocene peralkaline rhyolites from Torfajökull (South Iceland Volcanic Zone) and Ljósufjöll central volcanoes and trachytes from Snæfellsjökull (Snæfellsnes Volcanic Zone) allow the assessment of the mechanism for silicic magma genesis as a function of geographical location and crustal geothermal gradient. The low δ¹⁸O (2.4‰) and low Sr concentration (12.2 ppm) measured in Torfajökull rhyolites are best explained by partial melting of hydrated metabasaltic crust followed by major fractionation of feldspar. In contrast, very high ⁸⁷Sr/⁸⁶Sr (0.70473) and low Ba (8.7 ppm) and Sr (1.2 ppm) concentrations measured in Ljósufjöll silicic lavas are best explained by fractional crystallisation and subsequent ⁸⁷Rb decay. Snæfellsjökull trachytes are also generated by fractional crystallisation, with less than 10% crustal assimilation, as inferred from their δ¹⁸O. The fact that silicic magmas within, or close to, the rift zone are principally generated by crustal melting whereas those from off-rift zones are better explained by fractional crystallisation clearly illustrates the controlling influence of the thermal state of the crust on silicic magma genesis in Iceland.     

Is the North Atlantic Oscillation reflected in Scandinavian glacier mass balance records?

The North Atlantic Oscillation (NAO) is one of the modes of climate variability in the North Atlantic region. The atmospheric circulation during the winter season in this region commonly displays a strong meridional (north–south) pressure contrast, with low air pressure (cyclone) centred close to Iceland and high air pressure (anticyclone) near the Azores. This pressure gradient drives the mean surface winds and the mid‐latitude winter storms from west to east across the North Atlantic, bringing mild moist air to northwest Europe. The NAO index is based on the difference of normalised sea‐level pressures (SLP) between Ponta Delgada, Azores and Stykkisholmur, Iceland. The SLP anomalies at these stations are normalised by division of each monthly pressure by the long‐term (1865–1984) standard deviation. Interannual atmospheric climate variability in northwest Europe, especially over Great Britain and western Scandinavia has, during the last decades, been attributed mainly to the NAO, causing variations in the winter weather over the northeast North Atlantic and the adjacent land areas. A comparison between the NAO index and the winter (December–March) precipitation between ad 1864 and 1995 in western Norway shows that these are strongly linked (correlation coefficient 0.77). Variations in the NAO index are also reflected in the mass balance records of glaciers in western Scandinavia. The NAO index is best correlated with mass balance data from maritime glaciers in southern Norway (e.g. Ålfotbreen R² = 0.51). The record of Holocene (last ca. 11 500 cal. yr) glacier variations of maritime glaciers in western Scandinavia is thus a proxy of pre‐instrumental NAO variations. Copyright © 2000 John Wiley & Sons, Ltd.     

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

In 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.     

Quaternary volcanic ash zones on the Iceland Plateau,southern Norwegian Sea

Four Quaternary volcanic ash zones in the southern Norwegian Sea have been investigated in core P57-7 from the Iceland Plateau. Both the geochemical composition and morphological variation of each ash layer have been studied. The four volcanic ash zones appear in the light oxygen isotope stages 1, 5, 7 and 11. The ash zones are composed of transparent platy grains, light brown transparent grains, brown blocky and black blocky grains and white/transparent pumice, and each zone shows a distinct stratigraphic evolution. The geochemical results show a mixture of basaltic and silicic grains in each ash zone, and that each zone contains grains from more than one eruption. The geochemical investigations strongly suggest that all the ashes are derived from Iceland. The youngest ash zone includes two layers, which based on their geochemical composition and stratigraphic position are correlated with the ¹⁴C-dated Vedde Ash (10 600 yr BP) and Saksunarvatn Ash (9 100 yr BP). Possible sources on Iceland for these layers are discussed.     

The palaeomagnetism and geology of the Patreksfjördur-Arnarfjördur region of Northwest Iceland

The headlands between the fjords Arnarfjördur and Patreksfjördur in northwest Iceland, consisting mostly of Upper Tertiary plateau basalt lavas, have been geologically mapped in detail. Magnetic properties of samples from some 50 lava flows and dykes in the region have been measured; the mean palaeomagnetic pole position obtained from forty normally magnetized lavas is at 88°N113°E. The application of the present work to stratigraphic correlation and magnetic anomaly interpretation in northwest Iceland is discussed.     

Fingerprinting the Late Pleistocene tephras of Ciomadul volcano,eastern–central Europe

Late Pleistocene tephras derived by large explosive volcanic eruptions are widespread in the Mediterranean and surrounding areas. They are important isochronous markers in stratigraphic sections and therefore it is important to constrain their sources. We report here tephrochronology results using multiple criteria to characterize the volcanic products of the Late Pleistocene Ciomadul volcano in eastern–central Europe. This dacitic volcano had an explosive eruption stage between 57 and 30 ka. The specific petrological character (ash texture, occurrence of plagioclase and amphibole phenocrysts and their compositions), the high-K calc-alkaline major element composition and particularly the distinct trace element characteristics provide a strong fingerprint of the Ciomadul volcano. This can be used for correlating tephra and cryptotephra occurrences within this timeframe. Remarkably, during this period several volcanic eruptions produced tephras with similar glass major element composition. However, they differ from Ciomadul tephras by glass trace element abundances, ratios of strongly incompatible trace elements and their mineral cargo that serve as discrimination tools. We used (U-Th)/He zircon dates combined with U-Th in situ rim dates along with luminescence and radiocarbon dating to constrain the age of the explosive eruptions of Ciomadul that yielded distal tephra layers but lack of identified proximal deposits.