Volcanogenic sedimentation in the Iceland Basin: influence of subaerial and subglacial eruptions

Journal of Volcanology and Geothermal Research 83 Ž1998. 47–73

Volcanogenic sedimentation in the Iceland Basin: influence of subaerial and subglacial eruptions Christian Lacasse ) , Steven Carey, Haraldur Sigurdsson Graduate School of Oceanography, UniÕersity of Rhode Island, Narragansett Bay Campus, Narragansett, RI 02882-1197, USA Received 14 April 1997; accepted 17 November 1997

Abstract Cores recovered from the Iceland Basin show evidence of transport and deposition of volcaniclastic sediment from the Eastern Volcanic Zone of Iceland during the Holocene and last glacial period. Three types of deposits have been identified: tephra fall, sediment gravity flows, and bottom-current-controlled deposits. Tephra fall layers contain basaltic glass of composition that suggests Katla volcano as the major source. A chronology of the volcano activity is reconstructed, back to isotopic stage 5d Ž120,000 yr.. Glass chemistry of tephra in sediment gravity flows deposited south of Myrdalsjokull Canyon ¨ indicates a source in the Grımsvotn–Lakagıgar ´ ¨ ´ volcanic system. These volcaniclastic gravity flows were most likely derived from jokulhlaups or large glacial floods, at a time of a more extensive ice cover over the volcanic zone. Deposition of the ¨ sediment gravity flows has created a deep-sea fan south of the canyon. Basalt glass composition, age, and depositional environment suggest that one early Holocene turbidite sequence was derived from a large jokulhlaup of the Grımsvotn ¨ ´ ¨ area. The volcanogenic sediment gravity flows were influenced by a strong contour current, moving across the Katla sediment ridges. The contour current has winnowed the silt fraction and transported it downstream as suspended load. The recovery of numerous silty volcaniclastic layers, enriched in detrital crystals, indicates that they contributed to the sedimentation of contourite drifts. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Iceland Basin; volcanogenic sedimentation; Holocene; Grımsvotn–Lakagıgar; Jokulhlaup; Katla; tephra fall ´ ¨ ´ ¨

1. Introduction Volcanic sediments derived from hotspot volcanism can contribute to the sedimentary record of some ocean basins. Coring and geophysical surveys indicate that basaltic volcaniclastic mass flows Ždebris flows, turbidity currents. are the dominant sedimen)

Corresponding author. Department of Volcanology and Petrology, GEOMAR Research Center for Marine Geosciences, Wischhofstrasse 1-3, 24148 Kiel, Germany. Tel.: q49-4316002643; fax: q49-431-6002978; e-mail: [email protected]

tary process associated with ocean-ridge and hotspot island volcanism Že.g. Hawaii, Iceland, Canary Islands. ŽAumento et al., 1972; Walker et al., 1972; Duffield, 1979; Fornari et al., 1979; Schmincke and von Rad, 1979; Lipman et al., 1988.. Walker et al. Ž1972. suggested that Iceland volcanism has contributed, through these processes, to the growth of the Maury deep-sea fan in the North Atlantic. By comparison, tephra fall layers are of minor volumetric importance, although they are widespread and useful as chronostratigraphic markers ŽSigurdsson and Loebner, 1981..

0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 7 - 0 2 7 3 Ž 9 8 . 0 0 0 1 5 - 8

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C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

The volcanism of Iceland is more diverse than in other regions of the North Atlantic ridge system, as a result of two features: the location of a hotspot near the spreading ridge, and the presence of an ice cover, whose extent has fluctuated over the last 3 million yr. Two types of volcanic zones have been identified, on the basis of crustal and petrologic considerations ŽPalmason, 1971, 1973; Jakobsson, 1979a,b; ´ Beblo and Bjornsson, 1980; Eysteinsson and Her¨ mance, 1985; Floventz, 1980; Gebrande et al., 1980; ´ Oskarsson et al., 1982; Meyer et al., 1985.. The main axial rift zone ŽWestern and Eastern Rift Zones. represents a subaerial extension of the Mid Atlantic Ridge ŽFig. 1.. The main axial rift zone is character-

ized by high heat flow, a thin crust and extensional tectonics, and produces mainly lavas and pyroclastic rocks of olivine tholeiitic and tholeiitic composition. It is partially covered by the three largest ice caps on Iceland ŽVatnajokull, Hofsjokull, Langjokull ¨ ¨ ¨ . ŽFig. 1.. The Snæfellsnes Volcanic Zone ŽSigurdsson, 1970. and the South-Eastern Volcanic Zone ŽSEVZ., are characterized by low heat flow and a thicker crust, with transitional and alkalic basalt volcanism. Ice cover of these flanking zones includes the Myrdalsjokull ice cap and smaller glaciers ŽTindfjal¨ lajokull, Eyjafjallajokull ¨ ¨ ., which overlies part of the SEVZ, and the Snæfellsjokull glacier at the western ¨ tip of the Snæfellsnes Zone ŽFig. 1..

Fig. 1. Simplified geological map Žafter Sædmundsson, 1979. showing the main features of the Icelandic rift system ŽWZ: Western Volcanic Zone; SEZ: South-Eastern Volcanic Zone; RR: Reykjanes Ridge; WRZ: Western Rift Zone; ERZ: Eastern Rift Zone, after Oskarsson et al., 1985.. The present ice cover is also shown Ž1: Vatnajokull; 2: Hofsjokull; 3: Langjokull; 4: Myrdalsjokull; 5: Tindfjallajokull; 6: ¨ ¨ ¨ ¨ ¨ Eyjafjallajokull; 7: Snæfellsjokull ¨ ¨ ..

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49

Fig. 2. Ža. Iceland in the North Atlantic with the studied area in the Iceland Basin. Žb. Location of cores AII94, EW9302 and SU90 in the Iceland Basin. Sites 983 and 984 of ODP Leg 162, and Site 115 of DSDP Leg 12 are also shown. Bathymetry: 100 m contours from the digital Etopo5 data base of seafloor elevations.

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The presence of ice over the volcanic centers has a profound influence on eruption style and on the mode of fragmentation, transport and deposition of pyroclastic material near the vent ŽKjartansson, 1959; Sigvaldason, 1968; Jones, 1969, 1970; Furnes et al., 1980; Werner, 1994; Werner et al., 1996.. Subglacial and subaerial eruptions have been contemporaneous since glaciation first appeared in Iceland between 3 and 4 Ma ŽGeirsdottir 1994.. Glacial ´ and Eirıksson, ´ periods have been identified on basis of the widespread occurrence of subglacial volcanic products Žhyaloclastites and pillow lavas. whereas ice-free periods Žinterglacials. were defined by subaerial lava flows and tephra fall ŽJones, 1969, 1970.. Werner et al. Ž1996. have identified eruptive periods characterized by different physical environments, to account for the evolution of table mountains in Iceland. Icelandic subglacial eruptions often generate voluminous glacial meltwater floods known as jokulhlaups, which result in the episodic discharge of ¨ volcaniclastic sediment or tephra to the coast and into the ocean ŽTomasson, 1974; Jonsson, 1982; ´ ´ Tomasson, 1993; Lacasse et al., 1995, 1996; La´ casse, 1997.. In contrast, tephra from subaerial eruption columns are dispersed by the prevailing atmospheric winds ŽThorarinsson et al., 1960; Thorarinsson, 1967, 1976; Larsen and Thorarinsson, 1977; Thorarinsson, 1979; Sigurdsson and Loebner, 1981.. These important deposits of fine-grained volcaniclastic material of late Pleistocene age can rarely be observed directly on land, as they have generally been reworked or eroded by subsequent glaciations. We present evidence for the influence of Eastern Volcanic Zone subglacial and subaerial eruptions on marine sedimentation in the adjacent Iceland Basin, during the last 130 ka Žisotopic stage 6.. Volcaniclastic layers have been studied in 29 gravity and piston cores recovered from the Iceland Basin, the West and East Katla Ridges and the Bjorn ¨ and Gardar Drifts ŽFig. 2.. A chronostratigraphic framework of the tephra deposits was inferred from downcore measurements of calcium carbonate, oxygen isotopes Ž d 18 O. and magnetic susceptibility ŽShor, 1980; D. Oppo, pers. comm... Results of major-element composition of glass components Žmainly sideromelane. provide new insights on late Pleistocene volcanic activity of the Eastern Volcanic Zone of Iceland, and its influence on marine sedimentation. Chemical cor-

relation with land-based volcanic successions Žtephra and hyaloclastites. indicates that Grımsvotn– ´ ¨ Lakagıgar ´ and Katla volcanic systems were the two major sources of volcaniclastic sediments into the deep-sea. Sedimentology and lithology indicate three dominant types of depositional processes: atmospheric fallout, turbidity currents and transport by bottom currents.

2. Depositional environment 2.1. The West and East Katla Ridges The West and East Katla Ridges ŽAII94 cores. are a pair of sediment highs, striking about 2108, and located on the insular rise south of Iceland ŽShor, 1980. ŽFig. 3.. Their formation has been attributed to the rapid denudation of Iceland during the Neogene, with sediment transport by turbidity currents and subsequent southwestward entrainment by the Iceland–Scotland Overflow Water ŽISOW., that originates from the Iceland–Faeroe Ridge ŽShor, 1980. ŽFig. 3.. A deep and narrow turbidity–current pathway that separates the two ridges, the Myrdalsjokull ¨ Canyon Ž18.58W., originates on the Iceland shelf ŽEgloff and Johnson, 1979; Johnson and Palmason, ´ 1980.. Other morphologic and depositional features on the southern mouth of Myrdalsjokull Canyon ¨ have been distinguished on the basis of acoustically defined sediment types and thickness ŽShor, 1980.. These deep-sea fans and lobes include the Myrdalsjokull Suprafan, the Myrdalsjokull Channel, ¨ ¨ and the South Katla Basin ŽFigure 1.3 in Shor, 1980.. The Reynisdjup Canyon Ž208W., bounds the West Katla Ridge to the west. Its south end is a large abyssal meander, with cut-off channels created by erosive turbidity currents ŽLonsdale and Hollister, 1979.. Both canyons feed the Maury Channel, a major deep-sea channel, that extends over 1000 km from 588N to at least 488N ŽSteele et al., 1962; Ruddiman, 1972.. The accumulation of deposits from the two canyons has led to the formation of the Maury Fan at their junction with the Maury Channel, with sedimentation rates up to 27 cmr1000 yr ŽWalker et al., 1972..

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Fig. 3. The Iceland Basin topography with the high and low areas which control the flow path of the Iceland–Scotland Overflow Water across the basin and the turbidity currents across the continental rise. Bathymetric data Žin meters. from the digital Etopo5 data base of seafloor elevations.

2.2. The Bjorn ¨ and Gardar Drifts Sedimentation in the Iceland Basin is also influenced by bottom currents. Continuing erosion and redeposition along the eastern slope of the Reykjanes Ridge is caused by the North Atlantic Deep Water ŽNADW., a south-flowing bottom Žthermohaline. current. The northeastern branch of the NADW, the Iceland–Scotland Overflow Water ŽISOW., flows over the Iceland–Faeroe sill, into the eastern Atlantic Ocean ŽSwift, 1984; Schmitz and McCartney, 1993. ŽFig. 3.. The accumulation of bottom-current deposits, or contourites, has formed two major sediment drifts ŽEW9302 cores., the Bjorn ¨ and Gardar Drifts. A seismic reflection survey reveals a pattern of sediment cover, with smooth, convex units, and irregular deposits with mudwaves and scours ŽRuddiman, 1972; Manley and Caress, 1994.. Little is known about the extent and sedimentary features of the Bjorn ¨ Drift. Drilling at ODP Site 984 indicates a sediment thickness exceeding 700 m and accumulation rate of 10 cmrkyr ŽJansen et al., 1996.. The Gardar Drift is an elongated, asymmetrical contourite drift, striking 2108, or nearly parallel to the Reykjanes Ridge ŽRuddiman, 1972; McCave et al., 1980; Kidd and Hill, 1987; Manley and Caress,

1994. ŽFig. 3.. It is continuous from the southern slope of Iceland to at least 548N, just north of the Charlie–Gibbs Fracture Zone ŽFig. 2a–b.. Drilling at DSDP Site 611, located at its southern end Ž52850X N, 30818X W., showed that this semi-sinuous drift began to form in Miocene time ŽKidd and Hill, 1987.. Mapping by echosounding shows that mudwaves on the Gardar Drift have an average amplitude of about 15 m and wavelength of 1.6 km ŽManley and Caress, 1994.. Numerous offsets in subbottom reflectors indicate that these mudwaves migrate. Sediment thickness of about 300 m, with accumulation rates exceeding 10 cmrkyr, were found at ODP Site 983 ŽJansen et al., 1996..

3. Volcanism of the Eastern Volcanic Zone 3.1. Petrogenesis of magmas The spreading axis in Iceland has periodically shifted to the east, due to the westward migration of the Mid-Atlantic Ridge rift system over a fixed hotspot ŽOskarsson et al., 1985.. As a result, extinct rift axes were left behind in western Iceland, while new rift zones were initiated as flank zones, likely by

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Fig. 4. The volcanic systems of the Eastern Volcanic Zone Žafter Jakobsson, 1979a., with the present ice cover. Star: location of the 1996 Vatnajokull eruption. Black arrows show the present jokulhlaup routeways Žafter Bjornsson, 1992.. Dashed line: northern limit of the ¨ ¨ ¨ outwash plains. Italic names indicate areas of jokulhlaup deposits on land. Bold names indicate inhabited areas. ¨

rift propagation from the hotspot region. The Eastern Volcanic Zone has propagated south from central Iceland into older lithosphere, forming a flank zone in its southern part, 2–3 m.y. old. Unusual petrological and geochemical diversity is observed in volcanic rocks produced in this tectonic setting ŽMeyer et al., 1985.. Lavas from the nine volcanic systems of the Eastern Volcanic Zone exhibit petrological and geochemical gradients related to their distance from the main rift zone ŽJakobsson, 1979a,b. ŽFig. 4.. South of the main rift zone, the two fissure swarms of Bardarbunga–Veidivotn ´ ¨ and Grımsvotn–Lakagıgar ´ ¨ ´ have erupted tholeiitic Žor low-K sub-alkalic. lavas. Hekla, Vatnafjoll, Tindjoll, ¨ Torfajokull, ¨ ¨ Eyjafjoll ¨ and Katla–Eldgja´ volcanic systems are mainly characterized by transitional Fe–Ti basalts, intermediate and silicic products. At the southern tip of the EVZ, the Vestmannaeyjar volcanic system produces mildly alkalic basalts.

3.2. Composition of basaltic glass Basaltic glasses, derived from the Eastern Volcanic Zone in Iceland, represent volcanic material that can be easily identified in marine sediments because of their geochemical diversity. They can be correlated to individual volcanic system of the volcanic zone. We compiled glass analyses of basaltic postglacial and historical lavas, hyaloclastites and tephra from the major volcanic systems of the Eastern Volcanic Zone ŽMork, ¨ 1982; Devine et al., 1984; Gronvold ¨ and Johannesson, 1984; Meyer et al., 1985; Metrich ´ et al., 1991; Thordarson et al., 1996; Lacasse, 1997.. Compositional variability of basalt glasses along the Eastern Volcanic Zone is illustrated in Fig. 5a–c. The results show a correlation between the Fe–Ti content and alkalinity of basaltic glasses, and a correlation between alkalinity and the distance of the

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Fig. 5. Electron microprobe analyses of basaltic glass from Vestmannaeyjar ŽDevine et al., 1984., Hekla–Vatnafjoll–Tindfjoll ¨ ¨ ŽMeyer et al., 1985., Katla ŽLacasse, 1997., Lakagıgar and ´ ŽDevine et al., 1984; Metrich et al., 1991; Thordarson et al., 1996., Grımsvotn ´ ¨ ŽGronvold ¨ Johannesson, 1984., and Veidivotn ´ ¨ ŽMork, ¨ 1982. volcanic systems. Ža. K 2 O wt.% vs. SiO 2 wt.%, boundary lines between alkalic, sub-alkalic and low-K sub-alkalic basalts after Middlemost Ž1975.; Žb. FeO) wt.% vs. MgO wt.%, with total iron as FeO; Žc. TiO 2 wt.% vs. MgO wt.%.

volcanic centers behind the tip of the propagating rift. 3.3. Subaerial and subglacial Õolcanism All nine volcanic systems of the Eastern Volcanic Zone have erupted lavas or tephra during postglacial time. With the exception of Tindfjoll ¨ volcano, historical eruptions have occurred from the central volcanoes of Hekla, Katla and Grımsvotn ´ ¨ ŽThorarinsson and Sæmundsson, 1979.. The frequency and type of eruptions depend upon a combination of factors including: Ž1. rifting activity, Ž2. structural, thermal and petrochemical characteristics of the volcanic system, and Ž3. the presence of an ice cover. Some 20% of the EVZ is presently ice covered ŽFig. 4.. The interaction of hot magma and ice melts the base of a glacier and results in the accumulation of meltwater within a structural depression in the ice

or the volcanic system. Once a threshold has been reached, a massive, short-lived, discharge of glacial meltwater and sediment occurs, referred to as a ŽTomasson, jokulhlaup 1974; Bjornsson, 1974, 1975; ¨ ´ ¨ Jonsson, 1982; Bjornsson, 1992.. Jokulhlaups consist ´ ¨ ¨ of a hyperconcentrated mixture of meltwater and volcaniclastic sediment, with high flow rate Ž10 4 to 10 6 m3rs. and a sediment load ranging from 1 to 10 kgrm3 ŽBjornsson, 1992.. During historical times ¨ ŽKatla. most have occurred from the Myrdalsjokull ¨ ŽGrımsvotn and Vatnajokull ¨ ´ ¨ . ice caps ŽFig. 4.. Their deposits on the southern coast of Iceland have built extensive flood plains or sandar: Solheimasandur, ´ Skogasandur, Myrdalssandur from Katla volcano, and ´ Skeidararsandur from Grımsvotn ´ ´ ¨ volcano ŽMaizels, . Ž 1989, 1991 Figs. 1 and 4.. The 1996 eruption beneath Vatnajokull glacier, between Bardarbunga ¨ ´ and Grımsvotn ´ ¨ ŽFig. 4., sent about 3.5 km3 of meltwater into a subglacial lake in the Grımsvotn ´ ¨ caldera.

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A month later a large jokulhlaup with discharges ¨ reaching 45,000 m3rs, carried more than 180 million tons of suspended sediment to the North Atlantic ŽEinarsson et al., 1997; A. Snorrason, pers. commun...

4. Methods 4.1. Sediment samples We investigated 29 sediment piston and gravity cores from the Iceland Basin ŽTable 1.. Cores AII94 were collected in the Katla Ridge area, in water depths ranging from 1100 m to 2300 m Ž Atlantis II cruise 94-1. ŽFig. 2b.. Sediment recovery varies from less than 5 m to 11.5 m. Cores include a

northwest–southeast transect from the crest of the East Katla Ridge downslope, and a north–south Žca. 198W. transect, as well as a number of cores sampling the crest of the West Katla Ridge, downslope, the Myrdalsjokull Suprafan and the South Katla Basin ¨ ŽShor, 1980.. We also studied gravity cores and piston cores, which were collected in the Iceland Basin Ž Maurice Ewing cruise EW9302., in water depths of 1100 to 2500 m ŽFig. 2b.. Sediment gravity cores were raised along a north–south Žca. 218W. transect from the continental rise downslope into the Iceland Basin, with a recovery ranging from less than 1 m to 5.3 m. Piston cores were also collected from the Gardar and Bjorn ¨ Drifts, with 12 to 20 m sediment recovery ŽFig. 2b.. These two sites have been recently drilled and cored down to 500 and 260 mbsf during Leg 162 of the Ocean Drilling Program,

Table 1 Location of sediment cores examined for this study Cruise AII94

EW9302

SU90

Core 1PC 2PC 3PC 4PC 5PC 6PC 7PC 8PC 9PC 10PC 11PC 12PC 8JPC 9JPC 12JPC 13GGC 14JPC 15JPC 16JPC 18GGC 21GGC 22GGC 23GGC 25GGC 26GGC 27GGC 28GGC 29GGC 30GGC 32

Latitude Ž8N. X

62834.6 X 62824.0 X 62805.9 X 62818.4 X 62828.5 X 62831.2 X 61844.2 X 61853.2 X 62807.6 X 62809.9 X 61832.0 X 61828.7 X 60848.3 X 60829.3 X 61805.6 X 61805.6 X 61825.1 X 61843.2 X 61856.6 X 60826.8 X 62810.3 X 61825.2 X 61840.3 X 62803.7 X 62819.3 X 62828.1 X 62837.0 X 62836.7 X 62845.0 X 61846 9

Longitude Ž8W. X

18814.0 X 17846.7 X 16837.3 X 17808.6 X 17854.1 X 18805.5 X 18840.9 X 18848.1 X 19802.5 X 19819.8 X 19821.9 X 19824.4 X 25805.2 X 23856.4 X 24816.0 X 24816.0 X 24806.3 X 23858.5 X 23852.6 X 20854.9 X 24826.7 X 21853.5 X 21843.9 X 21828.3 X 21827.4 X 21807.0 X 20838.0 X 20838.2 X 20840.6 X 22825 7

Water depth Žm.

Recovery Žm.

Location

1177 1764 2199 2114 1596 1369 2173 2088 2082 1555 2205 2295 1917 2080 1783 1783 1653 1522 1451 2573 1353 1800 1695 1523 1450 1406 1295 1299 1188 1725

8.45 9.70 10.10 11.70 11.32 11.38 8.14 4.81 7.21 11.28 10.84 11.47 16.38 11.70 16.54 2.04 19.95 15.47 14.91 1.69 1.02 5.22 5.30 5.30 4.31 0.64 3.25 5.22 2.08 11.30

East Katla Ridge

West Katla Ridge

Gardar Drift

Bjorn ¨ Drift

Iceland Basin

Iceland Basin

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at Sites 984 and 983, respectively ŽJansen et al., 1996.. 4.2. Stratigraphy A chronostratigraphic framework of volcaniclastic layers in cores EW9302 was developed on basis of d 18 O isotopes in planktonic foraminifera Ž C. wuellerstorfi ., and the downcore variation of magnetic susceptibility ŽD. Oppo, pers. commun... Time intervals and age boundaries, corresponding to isotopic stages 5a–e, were defined after Martinson et al. Ž1987.. A complete d 18 O profile is available for core EW9302-JPC14, in addition to isotopic stage 5e in cores EW9302-JPC8, -JPC9, -JPC15 and -JPC16 ŽD. Oppo, pers. commun... Oxygen isotope stages 5a to 5d were determined in the same cores, by correlation of the magnetic susceptibility profiles with core EW9302-JPC14. Additional measurements of d 18 O were carried out on N. pachyderma, left and right, and G. bulloides in gravity cores EW9302-28GGC and EW9302-29GGC ŽD. Oppo, pers. commun... Holocene Žpostglacial. and Younger Dryas deposits in cores AII94 were recognized from the downcore variation of calcium carbonate content and planktonic foraminifera assemblages Žafter Shor, 1980., in addition to the occurrence of Ash Zone 1. The Younger Dryas is one of the most dramatic climate change events observed in marine and ice core records. It is characterized by a return to near-glacial conditions that punctuated the last deglaciation.

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sideromelane, phenocrysts and colorless glass. Sideromelane was subdivided in three morphological types: Ža. blocky shards with curvi-planar surfaces, that are mostly vesicle-free, Žb. vesicular, with irregular shapes, and Žc. subrounded, with no vesicles and smooth shape. Colorless silicic glass occurs as platelike or crescent-shaped shards, probably from vesicle walls. Phenocrysts occur within or attached to tachylite and sideromelane. Lithic fragments occur as opaque grains and grains showing microcrystalline texture Žmainly basalt.. Isolated crystals in the sediment were classified as xenocrysts. Biogenic grains were subdivided into foraminifera and siliceous components Ždiatom, radiolarian, spicule.. 4.4. Glass microprobe analysis Major element analysis of volcanic glass Ž) 62 m m. was performed with an automated Cameca electron microprobe, using a beam current of 10 nA, with an accelerating voltage of 15 kV. Between 8 and 16 glass particles were analysed, at a specific depth interval within each volcaniclastic deposit. Na-loss was minimized in rhyolitic glass analyses, by using a beam defocused to 10 m m and by collecting counts at regular intervals Ž2 s. and extrapolating the decay curve back to count initiation ŽNielsen and Sigurdsson, 1981.. Basaltic glass and rhyolitic compositions were calibrated from the standards KN-18 Žcomendite obsidian from Kenya; Nielsen and Sigurdsson, 1981. and VG-2 ŽMORB glass from the Juan de Fuca Ridge., respectively.

4.3. Sedimentology and tephra lithology Four sedimentological types of volcaniclastic deposits occur in cores. They are: Ža. tephra-rich layers, with a high concentration of tephra and a sharp contact at the base; Žb. bioturbated tephra layers; Žc. tephra pods, occurring as isolated patches of tephra; and Žd. dispersed tephra layers, often forming units several tens of centimeter thick within the sediment composed mainly of detrital clay and silt. Maximum grain size at a specific depth within each layer was estimated by averaging the 15 largest tachylite and sideromelane grains. The lithology of volcaniclastic layers was examined using smear slides and grain mounts Ž) 62 micron fraction.. Volcanic particles include tachylite,

5. Results 5.1. Stratigraphy and sedimentation rates 5.1.1. Gardar and Bjorn ¨ Drifts The distribution of tephra layers is shown in Fig. 6. The figure shows the depth intervals corresponding to the isotopic stage 5 boundaries Žca. 71–128 ka. in the Bjorn ¨ and Gardar Drifts. Core EW930214JPC on the Bjorn ¨ Drift ŽSite 984 of ODP Leg 162. has the highest sedimentation rate, averaging 10.8 cmrkyr between stage 6 and present. Isotopic stages 4, 3, 2 and 1 were inferred from correlation with the d 18 O downcore variation of G. Bulloides and N.

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Fig. 6. Stratigraphy and occurrence of volcaniclastic deposits in cores AII94, EW9302 and core SU9032 Žafter Lacasse et al., 1996. in the Iceland Basin. Correlative tephra fall layers and sandy ash turbidites are shown. All the other ash-rich layers are interpreted to be deposits derived from sediment gravity flows andror bottom currents.

pachyderma left in core SU9032, located approximately 100 km northeast ŽLacasse et al., 1996.. The lowest sediment rate, about 4 cmrkyr, was recorded in core EW9302-16JPC, less than 100 km north of core EW9302-14JPC. 5.2. Central Iceland Basin As a result of high accumulation rates on the iceland continental rise, only late glacial and postglacial sediments were recorded in gravity cores EW9302-28GGC through EW9302-30GGC ŽFig. 6.. The location of Ash Zone 1 Ž11,000–11,100 14 C yr BP; Bard et al., 1994., and the oxygen isotopic stratigraphy for N. pachyderma right ŽD. Oppo, pers. comm.. are consistent with the Younger Dryas period, at about 260 cm and 310 cm depth in cores

EW9302-28GGC and EW9302-29GGC, respectively. Mean accumulation rates of about 25–30 cmrkyr are estimated for the Holocene. Correlation of the magnetic susceptibility profiles in cores EW9302-30GGC, EW9302-26GGC and EW930225GGC with that of core EW9302-29GGC indicates lower sedimentation rates Ž10 to 12 cmrkyr., with the Younger Dryas period between 110 and 130 cm depth. A progressive decrease in the amplitude of the magnetic susceptibility profiles is observed southward from core EW9302-30GGC to core EW930222GGC. This gradient in the magnetic susceptibility signal in the Iceland Basin corresponds to decreasing inputs of terrigenous material from Iceland ŽPoutiers and Gonthier, 1978.. Consequently, based on the magnetic concentration of the sediment, no correlation has been attempted for the southernmost cores.

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5.3. Katla ridges Assemblages of planktonic foraminifera and calcium carbonate content were used for recognizing Holocene sediments on the Katla Ridges ŽShor, 1980.. A polar assemblage, dominated by G. pachyderma, was observed in older glacial deposits and used to discriminate Holocene from pre-Holocene deposits ŽFig. 6.. The base of the Holocene section ŽTermination I of Ruddiman and Bowler, 1976. lies at a depth of 310 cm in core AII94-PC3, and the Younger Dryas event at 235 to 295 cm. Based on the occurrence of the same faunal assemblages in core

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AII94-12PC, Termination I Ži.e., the glacial Pleistocene–Holocene transition. lies at about 430 cm depth, indicating accumulation rates in excess of 30 cmrkyr ŽShor, 1980.. Foraminifera in core AII9410PC atop the West Katla Ridge are polar assemblages, at and below 10 cm depth. Calcium carbonate in the same core ranges between 0 and 8%, indicating that sediment corresponding to the Holocene interval is missing. Ash Zone 1 is dispersed over at least 100 cm in cores AII94-PC8 and AII94-PC12, making it difficult to assign a precise age. On the other hand, Ash Zone 2 in core AII94PC12 is dispersed over only a few tens of centimeter

Fig. 7. Photographs of basaltic tephra fall layers. Ža. At 527–529 cm depth in core EW9302-12JPC Žca. 83 kyr BP, isotopic stage 5a.; Žb. at 581–583 cm depth in core EW9302-15JPC Žca. 103 kyr BP, isotopic stage 5c.; Žc. at 232–235 cm and 222–225 cm depth in core AII94-8PC Žearly Holocene..

AII94-PC8

AII94-PC10

AII94-PC12

AII94-PC8

93 229 562 170 195 216 357 429 456 646 659 915

583 685 924 528 668 819 1055 262 263 324 340 715 726 1055 225 235

Depth Žcm .

644 – 648 648 – 663 909 – 916

356 – 359 417 – 481

AL DA DA AL DA

91 – 105 207 – 231 556 – 576 152 – 222

323 – 324 339 – 341 714 – 716 725 – 726.5 1055 – 1055.5 222 – 225 232 – 235

581 – 583 684 – 687 923 – 924 527 – 529 665 – 671 818 – 820 1055 – ? 261 – 263

Depth interval Žcm .

AL AL AL DA

AL AL BA BA BA AL AL

AL AL AL AL BA AL AL AL

Type

4 5 7

3 64

14 24 15 70

1 2 2 1.5 0.5 3 3

2 3 1 2 6 2 ? 2

Thickness Ž cm .

324r375 275r283 363r289 167r171 193r190 170r201 175r170 221r193 157r172

196r195 159r184 186r173 198r201 204r189 495r473 463r447 265r266 322r300 293r263 328r321 374r385 325r315 307r282 150r137 325r327

Max. grain size SiderTach Ž m m.

XX XX XX XX XX XX XX XX X

XXX X

XXX XXX XXX XXX XX XXX XX X XX XX XXX XXX XX XXX XX XX

Tachylite

XXX XX XX

XX

Subrd.

X XX X XX XXX XXX X XX XX XX XX XX

XX XX XX XX XXX X XX X XX XX XX X X XX XXX X

Block.

Sideromelane

X X

X

XX X X X

X X

X X X X X XX XX X X X XX XX XX X X XXX

Vesic.

X X X

XXX X X X X

X

XXX XX

Colorless Glass

X ŽPl,Q . X ŽPl. X ŽPx,Pl

Xenocryst

X ŽPl. X ŽOl,Px,Pl. X ŽPl,Ol,Ox . X ŽOl,Pl,Px . X ŽOx,Pl,Px,Ol. X ŽPl,Ol.

X Ž Ox . X ŽPl,Ox,Px .

X ŽPl,Q . X ŽPl,Cc,Px,Ox . X ŽPl,Q,Pr. X ŽPl,Ol,Q . XX ŽPl,Px,Pr,Q . XX ŽPl,Px,Pr,Q . XXX ŽPl,Ol,Q,Cc,Pr. XX ŽPl,Q,Px,Ze,Pr,Cc . XX ŽPl,Q,Ox .

X Ž Pl,Q,Px,Ol,Cc . X ŽPl,Px .

XX ŽPl,Ol,Px,Ox . X ŽPl. X ŽPl. X ŽPl,Px,Ox . X ŽPl,Ox . X ŽQ,Pl. X ŽPl,Px .

X ŽPl,Ox . X ŽPl,Ol,Ox . X ŽPl,Ox . X ŽPl,Ol,Px,Ox . X ŽPl,Ol. XX ŽPl, Ox, Px . X ŽOl, Pl, Px . X Ž Ol,Px,Pl. X ŽPl,Ol. X ŽPl,Px .

Phenocryst

XX X X X X X X X X X X

XX X X

X X X

Lithic

X X

X X X X

X X

X

X

X

X X X

X

XX X

Foram.

X

X

X

X

X

Silicic biog.

AL: ash-rich layer; BA: bioturbated ash; DA: dispersed ash. Side: sideromelane; Tach: tachylite; Subrd: subrounded; Block: blocky; Vesic: vesicular. XXX: abundant Ž) 50% ., XX: frequent Ž10 – 50% ., X: present Ž- 10% .. Pl: plagioclase, Px: pyroxene, Ol: olivine, Ox: oxide, Pr: prehnite, Cc: calcite, Ep: epidote, Q: quartz, Ze: zeolite, Foram: foraminifera; Silicic biog: silicic biogenic component.

West Katla Ridge

Sandy ash turbidites Central Iceland SU9032 Basin

West Katla Ridge

SU9032

Central Iceland Basin EW9302-28GGC

EW9302-12JPC

EW9302-15JPC EW9302-14JPC

Core

Gardar Drift

Ash fall layers Bjorn ¨ Drift

Region

Table 2 Lithology and grain size of volcaniclastic layers

58 C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

59

Ž909–916 cm depth., dating this level at about 55,000 to 57,500 yr BP, i.e. within isotopic stage 3 ŽLacasse et al., 1996.. 5.4. Sedimentology and lithology of the Õolcaniclastic deposits 5.4.1. Tephra fall layers Tephra occur as discrete horizons, between 1 and 3 cm thick, in the sediment ŽFig. 7.. The layers are black to dark gray and relatively well sorted. Major constituents are sideromelane Žbrown glass. and tachylite Žopaque microcrystalline basaltic fragments., with almost no free crystals ŽTable 2.. The layer display a high peak of magnetic susceptibility, and can be chemically and stratigraphically correlated between the sediment cores ŽFig. 6.. Their widespread dispersal, rare free crystals and good sorting indicate a fallout origin. Other tephra fall layers show various degrees of bioturbation, from discrete layers with basal sharp contact, to moderately to heavily bioturbated tephra layers or tephra pods, with no sharp contacts. As a result of the elevated location of core AII94-10PC, on the crest of the West Katla Ridge, and its proximity to volcanic sources, the tephra fall layers are better preserved. Ash Zone 1 occurs in core EW9302-28GGC as a 2 cm thick tephra fall layer of bimodal composition, basaltic and rhyolitic ŽFig. 6.. Its occurrence as fallout deposit is a rare example of preserved fallout of this Younger Dryas eruption in North Atlantic sediments. This chronostratigraphic marker was found also in other cores, occurring as dispersed glass shards over a several centimeters depth interval ŽFig. 6.. 5.5. Sediment graÕity flow deposits Tephra also occur as layers, up to several tens of centimeters thick, with a sharp base and bioturbated at the top. The layers generally contain silty to sandy parallel laminae, several millimeters to centimeters thick. These sediment units are moderately to poorly sorted, with abundant sideromelane and tachylite clasts, plagioclase and quartz grains, common foraminifera and occasional large bubble-wall glass shards. Erosional base, normal grading and crossstratification are often observed ŽFig. 8.. These sedi-

Fig. 8. Photographs of volcaniclastic sediment gravity flow deposits. Ža. At 152–222 cm depth in core AII94-8PC Žearly Holocene.; Žb. at 644–663 cm depth in core AII94-12PC Žpre-Holocene..

mentary characteristics all indicate deposition by gravity currents or turbidity currents. The deposition of sediment gravity flows, highly concentrated in volcaniclastic material ŽTable 2., contributed to high sedimentation rates. The sediment cores show considerable geographical variation in the occurrence and thickness of the volcaniclastic deposits ŽFig. 6., with accumulation rates during the Holocene ranging from 25–30 cmrkyr on the Iceland continental rise Žcores EW9302-29GGC and EW9302-28GGC. to 10–12 cmrkyr in the central Iceland Basin Žcores EW9302-25GGC and EW9302-26GGC.. A gradient in the input of Holocene and pre-Holocene volcanic material from Iceland is also observed from magnetic susceptibility data. The thicker volcaniclastic

48.00 Ž0.78 . 3.86 Ž0.37 . 12.87 Ž0.15 . 15.23 Ž0.39 . 0.22 Ž0.06 . 4.88 Ž0.08 . 9.68 Ž0.16 . 2.88 Ž0.13 . 0.53 Ž0.05 . 0.28 Ž0.09 . 98.43

SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O 5 Total Age Žka .

48.07 Ž1.08 . 4.06 Ž0.58 . 13.41 Ž0.26 . 14.73 Ž0.57 . 0.26 Ž0.08 . 5.02 Ž0.28 . 9.84 Ž0.35 . 3.01 Ž0.20 . 0.68 Ž0.14 . 0.28 Ž0.07 . 99.36 Early Holocene

SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O 5 Total Age Žka .

EW9302- 14JPC 684 – 687 12

47.88 Ž0.27 . 4.29 Ž0.13 . 13.68 Ž0.07 . 14.17 Ž0.32 . 0.23 Ž0.06 . 4.90 Ž0.08 . 9.66 Ž0.14 . 3.09 Ž0.07 . 0.78 Ž0.04 . 0.34 Ž0.05 . 99.02

AII94-8PC 232 – 235 11 50.12 Ž0.25 . 3.66 Ž0.13 . 12.84 Ž0.13 . 15.16 Ž0.29 . 0.26 Ž0.03 . 4.42 Ž0.10 . 8.94 Ž0.12 . 2.90 Ž0.08 . 0.55 Ž0.04 . 0.37 Ž0.05 . 99.20 ?

AII94-10PC 323 – 324 11

47.88 Ž1.01 . 48.38 Ž0.43 . 3.97 Ž0.41 . 3.79 Ž0.16 . 12.95 Ž0.13 . 12.91 Ž0.14 . 15.27 Ž0.43 . 15.28 Ž0.34 . 0.25 Ž0.04 . 0.28 Ž0.06 . 4.91 Ž0.10 . 4.89 Ž0.10 . 9.78 Ž0.16 . 9.75 Ž0.20 . 2.85 Ž0.19 . 2.91 Ž0.07 . 0.59 Ž0.09 . 0.54 Ž0.04 . 0.29 Ž0.09 . 0.24 Ž0.06 . 98.73 98.97 ca 83 ka ŽIsotopic stage 5a .

EW9302-12JPC 527 – 529 12

N: number of analyses. Number in parentheses: standard deviation of the mean. FeO): total iron as FeO.

AII94-8PC 222 – 225 14

Core: Interval Žcm . : N:

Katla Ridges

SU9032 818 – 820 11

Core: Interval Žcm . : N:

Bjorn ¨ and Gardar Drifts

Table 3 Electron microprobe analyses of glass in tephra fall deposits

EW9302- 15JPC 581 – 583 12

48.98 Ž0.55 . 4.12 Ž0.13 . 13.45 Ž0.09 . 13.57 Ž0.37 . 0.22 Ž0.05 . 4.69 Ž0.10 . 9.58 Ž0.24 . 3.08 Ž0.10 . 0.84 Ž0.07 . 0.35 Ž0.05 . 98.89 ca. 21 ka

AII94-12PC 522 – 523 12

48.97 Ž0.56 . 4.02 Ž0.10 . 13.55 Ž0.15 . 14.01 Ž0.49 . 0.22 Ž0.05 . 4.76 Ž0.13 . 9.52 Ž0.20 . 3.12 Ž0.13 . 0.82 Ž0.08 . 0.40 Ž0.07 . 99.39

AII94-10PC 339 – 341 12

46.89 Ž0.31 . 46.94 Ž0.36 . 4.47 Ž0.24 . 4.35 Ž0.33 . 13.29 Ž0.30 . 13.34 Ž0.11 . 14.93 Ž0.44 . 14.81 Ž0.19 . 0.24 Ž0.06 . 0.22 Ž0.03 . 5.59 Ž0.21 . 5.54 Ž0.12 . 10.68 Ž0.24 . 10.70 Ž0.16 . 2.72 Ž0.13 . 2.72 Ž0.10 . 0.60 Ž0.04 . 0.59 Ž0.06 . 0.24 Ž0.06 . 0.25 Ž0.04 . 99.65 99.44 ca 103 ka ŽIsotopic stage 5c .

EW9302- 14JPC 923 – 924 11

EW9302- 12JPC 665 – 671 10

47.66 Ž1.20 . 4.09 Ž0.49 . 12.85 Ž0.29 . 14.91 Ž0.60 . 0.26 Ž0.07 . 4.90 Ž0.26 . 9.85 Ž0.40 . 2.94 Ž0.17 . 0.62 Ž0.10 . 0.66 Ž0.24 . 98.73 ca. 83 ka

AII94-10PC 714 – 716 16

47.59 Ž0.69 . 4.34 Ž0.14 . 13.39 Ž0.30 . 14.03 Ž0.74 . 0.25 Ž0.06 . 4.92 Ž0.26 . 9.88 Ž0.34 . 3.02 Ž0.15 . 0.76 Ž0.08 . 0.49 Ž0.07 . 98.68 ca. 84 ka

AII94-10PC 725 – 726.5 11

50.14 Ž0.26 . 49.97 Ž0.40 . 3.27 Ž0.12 . 3.39 Ž0.15 . 12.51 Ž0.17 . 12.44 Ž0.10 . 16.01 Ž0.31 . 16.04 Ž0.43 . 0.24 Ž0.04 . 0.29 Ž0.06 . 4.78 Ž0.09 . 4.65 Ž0.09 . 9.01 Ž0.15 . 9.01 Ž0.24 . 2.78 Ž0.15 . 2.71 Ž0.05 . 0.51 Ž0.04 . 0.50 Ž0.03 . 0.31 Ž0.05 . 0.30 Ž0.02 . 99.57 99.29 ca 113 ka ŽIsotopic stage 5d .

SU9032 1055 8

46.34 Ž 0.21 . 4.39 Ž0.16 . 12.99 Ž0.13 . 14.87 Ž0.40 . 0.24 Ž0.06 . 5.42 Ž0.16 . 10.84 Ž0.19 . 2.74 Ž0.07 . 0.52 Ž0.04 . 0.44 Ž0.05 . 98.79 ca. 103 ka

AII94-10PC 1055 – 1055.5 10

60 C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

61

Table 4 Electron microprobe analyses of glass in sediment gravity flow deposits ŽKatla Ridges. Core: AII94-8PC. Interval Žcm.: 152–222. Sampling depth Žcm.: 170 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

49.80 2.99 13.33 14.91 0.21 5.32 10.57 2.54 0.40 0.34 100.42

49.75 3.05 13.03 15.80 0.26 5.19 9.93 2.46 0.45 0.34 100.26

49.82 2.99 12.92 14.24 0.32 5.50 11.03 2.52 0.39 0.26 99.96

49.62 3.89 13.84 13.61 0.26 4.16 9.21 2.99 1.01 0.45 99.03

50.62 2.42 13.69 12.91 0.30 6.17 11.10 2.56 0.34 0.26 100.36

51.11 2.66 13.82 12.93 0.26 5.36 10.52 2.77 0.60 0.34 100.36

49.71 2.91 13.28 14.82 0.22 5.30 9.90 2.68 0.36 0.30 99.48

49.02 4.03 13.42 14.87 0.19 4.45 9.47 3.13 0.98 0.55 100.12

49.51 2.78 13.38 13.49 0.25 5.37 9.93 3.05 0.43 0.32 98.52

48.24 3.76 12.93 15.47 0.23 4.68 9.89 3.07 0.60 0.79 99.66

47.40 3.94 13.81 14.13 0.21 5.51 11.01 2.79 0.52 0.42 99.74

47.69 4.44 13.72 14.75 0.14 5.54 9.55 3.10 0.85 0.66 100.49

48.33 4.51 13.84 14.12 0.12 4.80 9.81 3.05 0.88 0.58 100.03

48.04 4.23 13.04 15.62 0.30 4.82 9.81 3.09 0.61 0.82 100.37

47.49 4.15 13.39 15.04 0.22 5.07 9.90 3.25 0.80 0.42 99.73

47.55 4.30 13.27 15.06 0.22 4.87 9.88 3.30 0.89 0.54 99.34

49.05 3.72 13.14 16.04 0.27 4.97 9.73 2.84 0.54 0.43 100.30

51.39 2.94 13.39 13.89 0.20 4.99 9.75 2.70 0.49 0.30 99.74

49.83 3.82 12.61 15.48 0.33 4.55 8.88 2.67 0.52 0.36 99.05

49.70 4.09 13.18 14.39 0.31 4.31 8.69 3.42 0.79 0.98 99.86

50.00 3.54 13.22 15.08 0.22 5.11 9.68 2.68 0.49 0.35 100.37

51.33 1.60 14.30 11.32 0.15 6.58 11.63 2.29 0.35 0.13 99.68

50.84 2.87 13.89 14.17 0.25 4.94 9.89 2.99 0.67 0.37 100.88

50.42 3.24 13.29 14.88 0.26 5.24 9.97 2.63 0.42 0.24 100.58

50.17 2.64 13.94 13.18 0.29 6.26 11.45 2.47 0.30 0.21 100.90

Core: AII94-8PC. Interval Žcm.: 152–222. Sampling depth Žcm.: 195 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

49.78 3.21 12.98 15.43 0.26 5.28 9.87 2.53 0.41 0.25 100.01

48.36 4.43 13.60 14.40 0.16 4.89 9.54 3.20 0.89 0.66 100.14

50.84 2.66 13.88 13.53 0.19 5.43 10.42 2.82 0.52 0.30 100.59

50.80 2.60 13.64 13.09 0.36 5.29 10.32 2.70 0.43 0.31 99.54

49.88 2.57 13.73 13.43 0.26 6.12 11.05 2.52 0.36 0.28 100.20

50.75 2.77 13.76 13.73 0.28 5.77 10.69 2.48 0.45 0.25 100.94

49.53 2.34 13.58 13.09 0.27 6.13 10.93 2.40 0.34 0.29 98.91

Core: AII94-8PC. Interval Žcm.: 152–222. Sampling depth Žcm.: 216 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

47.81 4.11 13.96 14.75 0.22 4.81 9.85 3.13 0.81 0.61 99.45

50.22 2.17 14.14 12.36 0.21 6.47 11.66 2.33 0.28 0.24 99.84

49.26 3.69 14.28 13.55 0.23 5.21 9.87 3.17 0.88 0.36 100.14

49.27 3.70 14.33 13.77 0.15 5.21 9.96 2.98 0.87 0.37 100.24

50.56 2.44 13.79 13.11 0.26 6.27 11.15 2.49 0.39 0.28 100.46

50.44 2.53 13.92 13.38 0.29 5.92 10.64 2.63 0.42 0.36 100.17

50.03 2.87 13.49 13.98 0.18 5.83 10.74 2.46 0.41 0.29 99.99

Core: AII94-8PC. Interval Žcm.: 356–359. Sampling depth Žcm.: 357 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

50.28 2.76 13.75 13.77 0.22 5.77 10.60 2.64 0.39 0.32 100.51

50.15 2.67 13.62 12.66 0.26 5.88 10.55 2.44 0.37 0.25 98.86

50.10 3.11 13.19 14.73 0.27 5.03 10.00 2.62 0.35 0.28 99.69

50.51 2.72 13.68 13.30 0.24 6.16 11.56 2.51 0.35 0.28 101.30

50.04 3.39 13.36 14.40 0.23 3.72 8.49 3.53 0.99 1.27 99.42

49.63 2.82 13.46 13.61 0.23 6.02 10.96 2.71 0.39 0.28 100.09

50.37 2.78 13.25 14.25 0.25 5.48 10.00 2.63 0.37 0.34 99.73

49.56 2.77 14.25 13.64 0.16 6.02 10.61 2.45 0.42 0.24 100.12

48.05 4.69 13.21 14.79 0.21 4.42 9.26 3.40 0.84 0.80 99.69

C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

62 Table 4 Žcontinued.

Core: AII94-8PC. Interval Žcm.: 417–481. Sampling depth Žcm.: 429 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

49.94 2.43 14.13 12.49 0.18 6.10 10.81 2.44 0.30 0.22 99.04

49.69 3.18 12.91 14.98 0.20 5.17 9.87 2.70 0.35 0.24 99.28

50.37 3.10 13.02 14.32 0.23 5.06 9.82 2.70 0.50 0.30 99.41

49.82 3.56 12.34 16.00 0.23 4.77 10.15 2.50 0.48 0.35 100.22

49.49 2.56 13.55 13.45 0.30 6.05 11.00 2.53 0.30 0.23 99.47

50.10 2.87 13.49 14.16 0.19 5.84 10.54 2.63 0.35 0.29 100.47

50.02 3.21 12.76 14.75 0.12 5.16 9.46 2.53 0.45 0.36 98.82

50.44 3.28 13.00 14.85 0.25 5.20 9.91 2.59 0.42 0.37 100.30

48.53 3.73 12.98 13.94 0.31 4.20 8.57 3.38 0.94 1.39 97.97

49.83 3.19 13.00 14.24 0.34 5.38 10.02 2.49 0.43 0.33 99.25

Core: AII94-8PC. Interval Žcm.: 417–481. Sampling depth Žcm.: 456 SiO 2 49.88 49.86 49.51 50.55 49.48 49.56 TiO 2 2.82 2.40 3.16 2.84 2.39 4.16 Al 2 O 3 13.56 13.85 13.11 13.43 13.98 13.62 FeO) 13.56 13.03 15.12 13.76 15.60 14.63 MnO 0.19 0.24 0.24 0.21 0.14 0.18 MgO 5.89 6.44 5.39 5.60 5.07 4.23 CaO 10.55 11.42 9.99 10.30 9.54 8.76 Na 2 O 2.45 2.30 2.79 2.63 2.70 3.23 K 2O 0.36 0.34 0.38 0.41 0.20 0.82 P2 O5 0.17 0.22 0.32 0.30 0.22 0.70 Total 99.41 100.11 100.01 100.03 99.32 99.89

50.01 2.71 13.66 13.56 0.14 6.20 10.87 2.44 0.28 0.22 100.10

50.36 3.19 13.11 15.07 0.25 5.28 9.57 2.84 0.44 0.29 100.41

50.63 3.41 13.19 15.37 0.20 5.06 9.34 2.71 0.44 0.29 100.64

50.45 3.32 13.02 15.17 0.23 5.24 9.42 2.74 0.47 0.30 100.36

47.44 3.89 13.35 15.02 0.24 5.56 10.72 2.83 0.50 0.33 99.88

48.15 4.32 13.55 14.48 0.20 4.79 9.35 3.01 0.76 0.51 99.11

48.98 3.55 13.62 15.29 0.24 4.48 9.28 3.35 0.64 0.36 99.80

50.14 2.56 13.87 12.75 0.09 6.39 10.84 2.54 0.36 0.19 99.75

50.48 3.58 12.77 14.82 0.33 3.87 8.04 3.23 1.22 0.75 99.08

49.72 2.56 13.96 13.14 0.25 6.52 11.49 2.41 0.35 0.20 100.60

50.78 3.05 12.61 16.02 0.16 4.66 8.74 2.79 0.47 0.31 99.59

50.52 3.18 12.51 15.35 0.17 4.79 8.87 2.79 0.44 0.30 98.93

49.56 2.83 13.04 14.42 0.37 5.97 10.75 2.22 0.43 0.26 99.87

47.44 4.25 13.40 15.52 0.14 5.10 9.93 3.03 0.57 0.39 99.78

47.58 2.69 14.86 12.32 0.25 7.12 11.83 2.41 0.35 0.21 99.62

49.80 2.94 12.85 14.83 0.23 5.04 9.46 2.81 0.38 0.31 98.66

Core: AII94-12PC. Interval Žcm.: 644–648. Sampling depth Žcm.: 646 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

50.08 3.24 13.11 14.44 0.25 5.42 9.95 2.58 0.37 0.27 99.71

48.59 4.54 12.88 14.62 0.17 4.56 9.07 3.18 0.83 0.58 99.03

50.37 2.69 13.75 13.87 0.24 5.92 10.41 2.61 0.29 0.20 100.35

50.60 3.44 12.48 15.93 0.23 4.54 8.72 2.73 0.40 0.36 99.43

50.51 3.23 12.76 16.02 0.20 4.48 8.84 2.61 0.43 0.27 99.36

50.00 3.42 12.68 15.76 0.33 4.97 9.49 2.61 0.44 0.29 100.01

Core: AII94-12PC. Interval Žcm.: 648–663. Sampling depth Žcm.: 659 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

49.91 2.77 13.03 14.22 0.25 5.47 9.85 2.59 0.34 0.22 98.66

50.76 2.68 13.13 14.53 0.25 5.22 9.88 2.67 0.34 0.18 99.65

47.38 3.81 13.84 13.95 0.29 5.78 10.69 2.93 0.57 0.32 99.55

47.41 3.67 14.05 14.19 0.19 5.88 10.75 2.91 0.61 0.42 100.08

50.31 2.75 13.71 12.93 0.16 6.24 11.26 2.54 0.36 0.20 100.46

49.68 2.46 13.47 12.96 0.23 6.24 11.13 2.43 0.28 0.24 99.12

C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

63

Table 4 Žcontinued. Core: All94-12PC. Interval Žcm.: 906–916. Sampling depth Žcm.: 659 SiO 2 TiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O P2 O5 Total

50.01 2.96 13.05 14.25 0.26 5.55 10.08 2.57 0.41 0.32 99.46

51.31 2.88 13.25 14.09 0.31 5.09 9.41 2.80 0.50 0.29 99.93

49.43 2.38 13.83 13.44 0.31 6.29 11.31 2.43 0.25 0.23 99.91

51.63 2.96 13.26 13.59 0.25 4.68 9.33 2.76 0.59 0.21 99.26

47.64 4.02 13.51 15.09 0.21 5.08 9.76 3.04 0.69 0.37 99.41

50.21 2.76 13.24 14.58 0.24 5.24 10.37 2.97 0.38 0.28 100.28

51.30 3.22 13.11 14.26 0.21 4.84 8.86 2.73 0.51 0.30 99.34

49.94 3.31 13.01 14.27 0.32 5.22 10.10 2.56 0.49 0.32 99.56

49.82 3.35 13.20 14.41 0.35 5.30 9.83 2.70 0.40 0.34 99.70

50.51 3.12 11.95 16.12 0.29 4.72 9.28 2.75 0.53 0.32 99.60

FeO): total iron as FeO.

turbidites occur in sediment cores Žcores AII94-8PC and AII94-12PC. south of the Myrdalsjokull Canyon ¨ ŽFigs. 6 and 8., with Holocene accumulation rates exceeding 100 cmrkyr in the South Katla Basin ŽShor, 1980..

5.6. Bottom-current-controlled deposits Visible horizons of silty and sandy tephra, between 0.5 cm and 6 cm thick, occur in sediments of the Bjorn ¨ and Gardar Drifts, with approximately the

Fig. 9. Electron microprobe analyses of glass in tephra fall layers from the Bjorn ¨ and Gardar Drifts. Ža. K 2 O wt.% vs. SiO2 wt.%; boundary line between sub-alkalic and low-K sub-alkalic basalts after Middlemost Ž1975.; Žb. FeO) wt.% vs. MgO wt.%, with total iron as FeO; Žc. TiO 2 wt.% vs. MgO wt.%. Compositional fields of Vestmannaeyjar, Katla, Hekla–Vatnafjoll–Tindfjoll, and ¨ ¨ Grımsvotn–Lakagıgar ´ ¨ ´ Veidivotn ¨ volcanic systems are also shown Žsee Fig. 5 for legend..

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same frequency in core EW9302-12JPC and core EW9302-14JPC Ži.e., about one layer every 14 to 16 kyr for the last 129 ka; Fig. 6.. Most of the tephra layers consist of silty brown glass and crystals that are moderately to well rounded, and some tachylite. They are often characterized by large amount of detrital crystal Žquartz, feldspar. occurring in fine laminations. Good sorting and sharp basal contacts are often observed. They are interpreted to reflect the size-sorting by bottom currents of silty volcanogenic material that might initially derive from fallout or turbidity currents. Occasional grading and thin laminae may reflect the interplay of turbidite and bottom-current Žor contourite. deposits ŽFaugeres ` and Stow, 1993.. These volcaniclastic contourite units are associated with abyssal currents, flowing southwest along the eastern flank of the Reykjanes Ridge ŽFig. 3..

5.7. Source and correlation of marine tephra Two mixed rhyolitic tephra, with minor amount of basaltic glass, Ash Zone 1 and Ash Zone 2, occur in the Younger Dryas and late Pleistocene sediments of cores EW9302 and AII94. The colorless silicic glass shards have compositions consistent with sources in the Eastern Volcanic Zone, either from Katla or Tindfjoll ´ volcanoes ŽFig. 4.. Ash Zone 1 and Ash Zone 2 are valuable chronostratigraphic markers in North Atlantic sediments ŽLacasse et al., 1995, 1996., and have been dated in Greenland ice cores at around 11,980 yr BP and 57,300 yr BP, respectively ŽGronvold et al., 1995; Ram et al., ¨ 1996.. The composition of glass with grain size ) 62 m m in all basaltic layers has been determined ŽTables 3 and 4.. The major element composition of

Fig. 10. Electron microprobe analyses glass in tephra fall layers from the Katla Ridge area. Ža. K 2 O wt.% vs. SiO 2 wt.%, boundary line between sub-alkalic and low-K sub-alkalic basalts after Middlemost Ž1975.; Žb. FeO) wt.% vs. MgO wt.%, with total iron as FeO; Žc. TiO 2 wt.% vs. MgO wt.%. Compositional fields of Vestmannaeyjar, Katla, Hekla–Vatnafjoll–Tindfjoll, ¨ ¨ Grımsvotn–Lakagıgar ´ ¨ ´ and Veidivotn ¨ volcanic systems are also shown Žsee Fig. 5 for legend..

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fresh basaltic glass gives an indication of the degree of homogeneity within each layer, and of the volcanic source of the tephra. The concentration of K 2 O and SiO 2 is effective in differentiating between the basaltic members of the alkalic, transitional and tholeiitic Žlow-K sub-alkalic. series. Elements such as TiO 2 and K 2 O define a specific chemical field for each volcanic system. Tephra layers are divided into three compositional groups: homogeneous, subhomogeneous, and heterogeneous. Homogeneous tephra layers display limited variation of major element composition, and show no fractionation or mixing trend. Subhomogeneous tephra layers display trends in which the observed compositional variation is a consequence of crystal– liquid fractionation processes Žpartial melting or fractional crystallization., or magma mixing. The heterogeneous layers, in contrast, show a wide compositional range, which may indicate multiple sources

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and mechanical mixing of material by bioturbation, gravity or bottom currents. The compositional heterogeneity in these layers shows a large variability, related to the type of deposits. Some heterogeneous layers show no dominant composition that can be attributed to a single source. Other heterogeneous layers display one or two well-defined compositional fields or clusters, which can be correlated to specific sources. 5.8. Homogeneous and subhomogeneous tephra fall layers The homogeneous and subhomogeneous layers ŽTable 3. are considered to be derived from atmospheric fallout. Tephra falling in the Iceland Basin have chemical affinities with the volcanic systems of the Eastern Volcanic Zone ŽFigs. 9 and 10.. Basaltic tephra falls in the Bjorn ¨ and Gardar Drifts Žcores

Fig. 11. Electron microprobe analyses of glass in sediment gravity flow deposits from Katla Ridges, in K 2 O wt.% vs. SiO 2 wt.% diagrams. Boundary line between alkalic, sub-alkalic and low-K sub-alkalic basalts after Middlemost Ž1975.; Ža. Early Holocene units in core AII94-8PC; Žb.Žc. Pre-Holocene units in cores AII94-8PC and AII94-12PC. Compositional fields of Vestmannaeyjar, Katla, HeklaVatnafjoll–Tindfjoll, ¨ ¨ Grımsvotn–Lakagıgar ´ ¨ ´ and Veidivotn ¨ volcanic systems are also shown Žsee Fig. 5 for legend..

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EW9302-JPC and SU9032. have a compositional trend between transitional alkali basalts, enriched in titanium, and sub-alkali basalts ŽFig. 9.. Their basaltic glass composition suggests a chemical affinity with the Katla–Eldgja´ volcanic system for the most Tiand K-rich end members, whereas the relatively low-Ti and low-K sub-alkalic tephra likely originated from the tholeiitic volcanic systems ŽBardar´ bunga–Veidivotn Fig. 4.. ¨ or Grımsvotn–Lakagıgar; ´ ¨ ´ The basaltic tephra falls on the Katla Ridges Žcores AII94-PC. display a different compositional trend ŽFig. 10.. All the tephra layers but one have an affinity with the high-Ti transitional alkali basalts of the Katla volcanic system. They range between two end-members: a transitional alkali basalt, relatively low in potassium, and a more evolved and K-rich transitional alkali basalt. The youngest tephra show the highest potassium and lowest titanium content ŽFig. 10.. A subhomogeneous tephra fall layer of transitional alkali basalt composition occurs at 714–

716 cm in core AII94-10PC. It exhibits a compositional trend of decreasing K and Ti, associated with an increase in silica, while MgO decreases slightly ŽTable 3 and Fig. 10a–c.. A similar trend is displayed in a correlative tephra fall in cores EW930212JPC, EW9302-14JPC, and SU9032 ŽFig. 9.. This chemical trend may indicate magma mixing, rather than crystal–liquid fractionation, as titanium and potassium are incompatible elements. In contrast, the uppermost tephra fall at 323–324 cm depth in core AII94-10PC has affinity with the sub-alkalic basalts of the Eastern Volcanic Zone ŽGrımsvotn–Lakagıgar, ´ ¨ ´ Bardarbunga–Veidivotn ´ ¨ .. 5.9. Heterogeneous and homogeneous graÕity flow deposits Deposits of sediment gravity flows south of the Myrdalsjokull Canyon Žcores AII94-8PC and AII94¨ 12PC., consist of abundant sand-size brown glass

Fig. 12. Electron microprobe analyses of glass in sediment gravity flow deposits from Katla Ridges. Ža. FeO) wt.% vs. MgO wt.%, with total iron as FeO; Žb. TiO 2 wt.% vs. MgO wt.%. Compositional fields of Vestmannaeyjar, Katla, Hekla–Vatnafjoll–Tindfjoll, ¨ ¨ Grımsvotn– ´ ¨ Lakagıgar ´ and Veidivotn ¨ volcanic systems are also shown Žsee Fig. 5 for legend..

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Žsideromelane. of moderately heterogeneous to homogeneous basaltic composition ŽTable 4, Figs. 11 and 12.. The early Holocene unit at 152–222 cm in core AII94-8PC has a glass composition with approximately half transitional alkali basalt affinity and half with sub-alkalic affinity ŽFig. 11a.. These two basaltic glass populations are observed throughout the 70-cm thick unit. The transitional alkali basalt group exhibits a wide compositional range, which overlaps the early Holocene Katla tephra falls found in the same region ŽFig. 11a.. The sub-alkali group lies on the boundary between the sub-alkalic and tholeiitic basalt fields ŽFig. 11a.. It shows affinity with the Grımsvotn–Lakagıgar ´ ¨ ´ volcanic system. Two other sandy tephra turbidites, at 356–359 cm and 417–481 cm in the same core, display less compositional heterogeneity ŽFig. 11b.. Their dominant glass composition is comparable to the sub-alkalic basaltic series of Grımsvotn–Lakagıgar ´ ¨ ´ volcanic system. Rare glasses of transitional basalt composition also occur. Several sandy–silty volcaniclastic layers occur between Termination I and Ash Zone 2 in core AII94-12JPC Ž450–900 cm.. The sandy layers are glass-bearing ŽTable 2., and were likely deposited by sediment gravity flows from the Myrdalsjokull ¨ Canyon. The two sandy glass-rich units at 644–648 cm and 648–663 cm may represent a single sequence. They have the same sub-alkalic affinity as the sediment gravity flow units of core AII94-8PC, with very few glasses that exhibit transitional alkali basalt composition. The majority of the glasses lie on the boundary between sub-alkalic and tholeiitic basalts, and match the Grımsvotn–Lakagıgar ´ ¨ ´ compositional field ŽFig. 11c.. The unit at 909–916 cm contains in addition colorless glass shards Ž- 10%. from the rhyolitic Ash Zone 2.

6. Discussion 6.1. Abundance of the Õolcaniclastic deposits The relative abundance of each volcaniclastic deposit type has been estimated, based on the cumulative thickness of the deposit and the total sediment recovery. In the Bjorn ¨ and Gardar Drifts, tephra fallout and sediment gravity flow deposits constitute less than 0.5% of the sediment, whereas the con-

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tourites account for about 1%. In the central Iceland Basin, the sediment gravity flow units represent at least 3% of the sediment recovered. This is a minimum, as muddy turbidites are not included. Tephra fallout and volcaniclastic contourites total less than 1% in the same region. In the Katla Ridges, at least 10% of the total recovered sediment consists of volcaniclastic turbidites. However, the contribution of sediment gravity flow deposits shows geographical variation, from zero, on top of the West Katla Ridge Žcore AII94-10PC., to about 30%, south of Myrdalsjokull Canyon Žcore AII94-8PC.. The latter ¨ is also a minimum, as only the silty and sandy units were taken into account. Tephra fallout on the Katla Ridges totals less than 0.5% of the sediment, whereas volcaniclastic contourites represent between 1 and 2%. 6.2. Atmospheric dispersal and fallout of tephra The grain size of basaltic tephra falls in the Iceland Basin shows a decrease in maximum diameter of both sideromelane and tachylite away from the inferred volcanic sources ŽFig. 13.. The sideromelane clasts are generally larger than the tachylite clasts in the proximal and distal deposits. This size fractionation between the two basaltic populations may have resulted from a higher settling velocity of tachylite during fallout. Tachylite generally has an equant shape, and contains a significant amount of dense ferromagnesian microlites. Sideromelane grains appear more blocky and consist of essentially microlite-free brown glass. The occurrence of tephra fall layers in the Iceland Basin sediments in addition to glass particles of similar composition, found in the Greenland GRIP ice core ŽGronvold et al., 1995., indicate widespread ¨ dispersal of the tephra dominantly to the southwest. By analogy with present-day wind patterns ŽLacasse, 1997., the dispersal of tephra fall to the southwest in the Iceland Basin indicates unusual wind direction and intensity. Such conditions were also suggested to affect the dispersal of the silicic Ash Zone 2 Žca. 57 ka. in the Irminger Basin, towards southern Greenland, during isotopic stage 3 ŽLacasse et al., in press.. Rawinsonde measurements over Iceland indicate a progressive seasonal change in the direction of

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Fig. 13. Atmospheric transport of basaltic tephra to the Iceland Basin: maximum diameter Žmicron. as function of distance Žkm. from source.

mean tropospheric winds, from dominant southerlies at low elevation to westerlies up to 13.5 km height. Above about 13.5 km a seasonal shift occurs and becomes more significant at higher elevations, with strong westerlies during the fall and winter, to relatively weak easterlies during the spring and summer. No northerlies are observed in the upper troposphere and stratosphere above Iceland. In addition, simulated airborne transport of Icelandic tephra indicates unrealistic eruption column heights Ž) 30 km, Lacasse et al., in press. for the coarsest basaltic tephra falls in cores SU9032 and AII94-12PC ŽFig. 13., and requires more vigorous atmospheric circulation than observed. Because of the limited explosive character of basaltic eruptions, with convective plumes rarely exceeding 15 km height, we suggest that strong winds at low elevation Ži.e., storm systems. or possibly at upper levels Župper troposphere and stratosphere. have brought about atmospheric transport of coarse basalt tephra Ž400 to 500 m m. over distances exceeding 300 km. The hypothesis of transport by ice-rafting of these tephra falls can be ruled out, based on the homogeneous chemical composition

and massive Žnon-dispersed. appearance of the layers ŽTables 2 and 3, Fig. 7.. 6.3. Jokulhlaups and tephra graÕity flow deposits ¨ The early Holocene turbidite unit in core AII948PC ŽFig. 8a. contains abundant basaltic glass ŽTable 2. with the composition of both Katla tephra and Saksunarvatn tephra ŽFig. 11a.. The Saksunarvatn tephra was derived from a major eruption of Grımsvotn subglacial volcano beneath the ´ ¨ Vatnajokull ice cap ŽFig. 4.. This tephra was widely ¨ dispersed by prevailing winds, as far as western Europe ŽMangerud et al., 1986; Sjøholm et al., 1989; Bjork ¨ et al., 1992; Merkt et al., 1993; Birks et al., 1996.. The tephra forms a millimeter-thick layer in the Greenland GRIP ice core, and was dated at 10,180 yr ŽGronvold et al., 1995.. By analogy with ¨ present atmospheric circulation, a minimum column height of 15 km is inferred for this basaltic eruption ŽLacasse et al., submitted.. The Saksunarvatn event most likely generated a large glacial flood. Jokulhlaups generated from Grımsvotn ¨ ´ ¨ caldera dis-

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charge over the Skeidara´ floodplain, to the North Atlantic ocean ŽFig. 4.. The continental shelf and rise near the Skeidararsandur shoreline is cut by the ´ Myrdalsjokull Canyon ŽFig. 14.. Cores AII94-8PC ¨ and AII94-12PC are raised from the Myrdalsjokull ¨ Suprafan and South Katla Basin, fed by this deep-sea channel ŽShor, 1980.. Little is known about behavior of Skeidara´ jokulhlaups flowing into the sea, but ¨ sediment gravity flow would probably transport volcaniclastic material from Grımsvotn ´ ¨ to this deep-sea region.

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Generation of sediment gravity flows derived from jokulhlaups of the Grımsvotn–Lakagıgar volcanic ¨ ´ ¨ ´ system may have been a common feature during the last glacial period, and a major source of volcaniclastic material to the deep-sea fan system south of Myrdalsjokull Canyon ŽFigs. 11 and 12.. Basaltic ¨ glasses of pre-Holocene turbidites in cores AII94-8PC and AII94-12PC show strong geochemical affinity with subalkalic volcanic systems in southern Iceland. The chemical homogeneity of these sediment gravity flow deposits argues for a close relationship in space

Fig. 14. The West and East Katla Ridges with adjacent deep-sea canyons, south of the Eastern Volcanic Zone. Bathymetry after Shor Ž1980. and Lonsdale Žunpublished data.. The Myrdalsjokull Suprafan and South Katla Basin after Shor Ž1980.. Locations of cores AII94-8PC, ¨ AII94-10PC and AII94-12PC are indicated. The Eastern Volcanic Zone with the different volcanic systems and the present ice cover are also shown Žsee Fig. 4 for legend..

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and time with eruptions in the Eastern Volcanic Zone. A more extensive ice cover during the glaciation probably led to more frequent subglacial eruptions from the Lakagıgar–Grımsvotn ´ ´ ¨ volcanic system. Submarine eruptions in shallow environment may also have triggered such sediment gravity flows. Bergh and Sigvaldason Ž1991. suggest that a higher sea level, caused by eustatic change towards the end of a glaciation, may have favored subaqueous eruptions. They have described a Pleistocene subaqueous volcanic sequence, 700 m thick, near Kirkjubæjarklaustur ŽFig. 4., that consists of units of basaltic hyaloclastites and lavas, with interbedded diamictite units. Its emplacement occurred on a submarine shelf, in a drowned valley north of the Myrdalsjokull Canyon ŽFig. 14.. They propose extru¨ sions of basaltic lava from subaquatic fissures, in the general area of the 1783 Laki fissure. The upper and distal parts of the subaqueous flows moved as lowconcentration turbulent suspensions that deposited bedded hyaloclastites ŽFigure 13 in Bergh and Sigvaldason, 1991.. 6.4. Volcaniclastic contourites The Bjorn ¨ and Gardar Drift and Katla Ridges are influenced by bottom currents ŽFig. 3.. These currents have generated a contourite sequence, built up gradually over long period of time. A bottom current, the Iceland–Scotland Overflow Water, flowing across the sediment ridge towards the south–southwest, was measured with velocities exceeding 20 cmrs ŽShor, 1980.. Such strong bottom currents are likely to influence the sediment gravity flow processes described above. Contour currents may winnow the finer turbiditic material downstream, as suspended load.

7. Conclusion Late Quaternary sediments of the Iceland Basin contain numerous volcaniclastic layers of at least three types: tephra fallout, sediment gravity flow deposits Žturbidites. and bottom-current-controlled flow deposits Žcontourites.. Glass chemistry of tephra falls in the Iceland Basin during the last glacial

period, between the Eemian Žisotopic stage 5e. and the Holocene Žisotopic stage 1., indicates that they were erupted from the Eastern Volcanic Zone, primarily from Katla volcano. Eruptions from Katla date back at least to isotopic stage 5c Ž110,000 yr.. Major element composition of basaltic glasses indicates that the chemistry of the magma has been influenced by both crystal–liquid fractionation and magma mixing. The depositional environment south of the Myrdalsjokull Canyon is affected by sediment grav¨ ity flows, rich in volcaniclastic material, that have built up a deep-sea fan system ŽMyrdalsjokull ¨ Suprafan.. Based on geochemical and depositional Žglacier bursts. from the evidence, large jokulhlaups ¨ Grımsvotn–Lakagıgar ´ ¨ ´ volcanic system during the last glacial period, are the principal sources for the volcaniclastic material. In addition, the large Grımsvotn ´ ¨ eruption that produced the Saksunarvatn tephra Žca. 10,180 yr. BP. during the early Holocene also produced a large jokulhlaup that generated turbidity ¨ flows to the Myrdalsjokull Canyon. ¨ A strong bottom current, the Iceland–Scotland Overflow Water ŽISOW., with maximum velocities of 20 cmrs, affected sedimentation on the East Katla Ridge. It has winnowed out the finer turbiditic material Žclay and silt. of sediment gravity flows, transporting it west–southwestward as suspended load, where it contributes to the buildup of the Bjorn ¨ and Gardar Drifts. These silty deposits are enriched in crystals of detrital origin, possibly from ice-rafting.

Acknowledgements This research was funded by the National Science Foundation grant OCE-9402296. We gratefully acknowledge the assistance of James Broda of the WHOI Core Repository. We thank Delia Oppo and Lloyd Keigwin for providing access to data on oxygen isotope and magnetic susceptibility of cores, and Joseph Devine for assistance at the electron microprobe. We thank Dwight Coleman for his assistance in processing Etopo5 bathymetric data, and Haukur Johannesson for providing volcanic glass samples of ´ Saksunarvatn tephra. We thank G. Valentine, N. Riggs and an anonymous reviewer for thorough reviews which greatly improved the manuscript.

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References Aumento, F., Clarke, B.D., Cann, J.R., 1972. Site 114. Initial Rep. Deep Sea Drill. Proj. 12, pp. 313–329. Bard, E., Arnold, M., Mangerud, J., Paterne, M., Labeyrie, L., Dupras, J., Melieres, M.-A., Sønstegaard, E., Duplessy, J.-C., ´ ` 1994. The North Atlantic atmosphere sea surface 14 C gradient during the Younger Dryas climatic event. Earth Planet. Sci. Lett. 126, 275–287. Beblo, M., Bjornsson, A., 1980. A model of electrical resistivity ¨ beneath NE-Iceland, correlation with temperature. J. Geophys. 47, 184–190. Bergh, S.G., Sigvaldason, G.E., 1991. Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf, south Iceland. Bull. Volcanol. 53, 597–611. Birks, H.H., Gulliksen, S., Haflidason, H., Mangerud, J., Possnert, G., 1996. New radiocarbon dates for the Vedde ash and the Saksunarvatn ash from western Norway. Quat. Res. 45, 119– 127. Bjork, O., Haflidason, H., Hallsdottir, M., Ander¨ S., Ingolfsson, ´ ´ son, N.J., 1992. Lake Torfadalsvatn: a high resolution record of the North Atlantic ash zone I and the last glacial–interglacial environmental changes in Iceland. Boreas 21, 1–7. Bjornsson, H., 1974. Explanation of jokulhaups from Grımsvotn ¨ ¨ ´ ¨ Vatnajokull, Iceland. Jokull 24, 1–25. ¨ ¨ Bjornsson, H., 1975. Subglacial water reservoirs, jokulhlups and ¨ ¨ volcanic eruptions. Jokull 25, 1–11. ¨ Bjornsson, H., 1992. Jokulhlaups in Iceland: prediction, character¨ ¨ istics and simulation. Ann. Glaciol. 16, 95–106. Devine, J.D., Sigurdsson, H., Davis, A.N., 1984. Estimates of sulfur and chlorine yield to the atmosphere from the volcanic eruptions and potential climatic effects. J. Geophys. Res. 89, 6309–6325. Duffield, W.A., 1979. Significance of contrasting vesicularity in basalts from DSDP sites 407, 408, and 409 on the west flank of the Reykjanes Ridge. Initial Rep. Deep Sea Drill. Proj. 49, pp. 715–719. Egloff, J., Johnson, G.L., 1979. Erosional and depositional structures of the southwest Iceland insular margin: thirteen geophysical profiles, in geological and geophysical investigations of continental margins. Am. Assoc. Petrol. Geol. Memoir 29, 43–63. Einarsson, P., Brandsdottir, B., Gudmundsson, M.T., Bjornsson, ´ ¨ H., Gronvold, K., 1997. Center of the Iceland hotspot experi¨ ences volcanic unrest. EOS Trans., Am. Geophys. Un. 78 Ž35. 369, 374–375. Eysteinsson, H., Hermance, J.F., 1985. Magnetotelluric measurements across the eastern neovolcanic zone in South Iceland. J. Geophys. Res. 90, 10093–10103. Faugeres, J.-C., Stow, D.A.V., 1993. Bottom-current-controlled ` sedimentation: a synthesis of the contourite problem. Sediment. Geol. 82, 287–297. Floventz, O.G., 1980. Seismic structure of the Icelandic crust ´ above layer three and the relation between body wave velocity and the alteration of the basaltic crust. J. Geophys. 47, 211– 220. Fornari, D.J., Moore, J.G., Calk, L., 1979. A large submarine

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sand-rubble flow on Kilauea volcano, Hawaii. J. Volcanol. Geotherm. Res. 5, 239–256. Furnes, H., Fridleifsson, I.B., Atkins, F.B., 1980. Subglacial volcanics—on the formation of acid Hyaloclastites. J. Volcanol. Geotherm. Res. 8, 95–110. Gebrande, H., Miller, H., Einarsson, P., 1980. Seismic structure of Iceland along RRISP profile I. J. Geophys. 47, 239–249. Geirsdottir, A., Eirıksson, J., 1919. Growth of an intermittent ice ´ ´ sheet in Iceland during the late Pliocene and early Pleistocene. Quat. Res. 42, 115–130. Gronvold, K., Johannesson, H., 1984. Eruption in Grımsvotn ¨ ´ ´ ¨ 1983: course of events and chemical studies of the tephra. Jokull 34, 1–11. ¨ Gronvold, K., Oskarsson, N., Johnsen, S.J., Clausen, H.B., Ham¨ mer, C.U., Bond, G., Bard, E., 1995. Ash layer from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments. Earth Planet. Sci. Lett. 135, 149–155. Jakobsson, S.P., 1979a. Petrology of Recent basalts of the Eastern Volcanic Zone, Iceland. Acta Nat. Isl. 26, 1–103. Jakobsson, S.P., 1979b. Outline of the petrology of Iceland. Jokull ¨ 29, 57–73. Jansen, E., Raymo, M.E., Blum, P., 1996. Proc. ODP, Init. Rep. 162. College Station, TX, Ocean Drilling Program. Jones, J.G., 1969. Intraglacial volcanoes of the Laugarvatn region, south-west Iceland. I. Q. J. Geol. Soc. London 124, 197–211. Jones, J.G., 1970. Intraglacial volcanoes of the Laugarvatn region, south-west Iceland, II. J. Geol. 78, 127–140. Johnson, G.L., Palmason, G., 1980. Observations of the morphol´ ogy and structure of the sea floor south and west of Iceland. J. Geophys. 47, 23–30. Jonsson, J., 1982. Notes on the Katla volcanoglacial debris flows. ´ Jokull 32, 61–68. ¨ Kidd, R.B., Hill, P.R., 1987. Sedimentation on Feni and Gardar sediment drifts. In: Ruddiman, W.F., Kidd, R.B., Thomas, E., et al., Initial Rep. Deep Sea Drill. Proj., 83, pp. 1217–1244. Kjartansson, G., 1959. The Moberg formation, I. In: Thorarinsson, S. ŽEd.., On the Geology and Geomorphology of Iceland. Geogr. Ann. 41, pp. 139–143. Lacasse, C., 1997. Volcaniclastic sedimentation from the Iceland hotspot. Unpublished PhD thesis, University of Rhode Island, 367 pp. Lacasse, C., Sigurdsson, H., Johannesson, H., Paterne, M., Carey, ´ S., 1995. Source of Ash Zone 1 in the North Atlantic. Bull. Volcanol. 57, 18–32. Lacasse, C., Sigurdsson, H., Carey, S., Paterne, M., Guichard, F., 1996. North Atlantic deep-sea sedimentation of late Quaternary tephra from the Iceland hotspot. Mar. Geol. 129 Ž3r4., 207–235. Lacasse, C., Werner, R., Paterne, M., Sigurdsson, H., Carey, S., Pinte, G., in press. Long-range transport of Icelandic tephra to the Irminger Basin, Site 919. In: Saunders, A.D., Larsen, H.C., Clift, P.D., Wise, S.W., Jr. ŽEds.., Proc. ODP, Sci. Results, 152. College Station, TX, Ocean Drilling Program. Lacasse, C., Carey, S., Sigurdsson, H., submitted. Airborne transport of Icelandic tephra. Bull. Volcanol. Larsen, G., Thorarinsson, S., 1977. H 4 and other acid Hekla layers. Jokull 27, 28–46. ¨

72

C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73

Lipman, P.W., Normark, W.R., Moore, J.G., Wilson, J.B., Gutmacher, C.E., 1988. The giant submarine Alika debris slide, Mauna Loa, Hawaii. J. Geophys. Res. 93 ŽB5., 4279–4299. Lonsdale, P., Hollister, C.D., 1979. Cut-offs at an abyssal meander south of Iceland. Geology 7, 597–601. Maizels, J., 1989. Sedimentology and paleohydrology of Holocene flood deposits in front of jokulhlaup glacier, south Iceland. In: ¨ Breven, K., Carling P. ŽEds.., Floods, Hydrological, Sedimentological and Geomorphological Implications. Wiley, New York, pp. 239–252. Maizels, J., 1991. The origin and evolution of Holocene sandur deposits in areas of jokulhlaup drainage, Iceland. In: Maizels, ¨ J., Caseldine C., ŽEds.., Environmental Change in Iceland: Past and Present. Kluwer Academic Publishers, Dordrecht, pp. 267–279. McCave, I.N., Lonsdale, P.F., Hollister, C.D., Gardner, W.D., 1980. Sediment transport over the Hatton and Gardar contourite drifts. J. Sediment. Petrol. 50 Ž4., 1049–1062. Mangerud, J., Furnes, H., Johansen, J., 1986. A 9000-year-old ash ´ bed on the Faroe Islands. Quat. Res. 26, 262–265. Manley, P.L., Caress, D.W., 1994. Mudwaves on the Gardar Sediment Drift, NE Atlantic. Paleoceanography 9 Ž6., 973– 988. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C. Jr., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quat. Res. 27, 1–29. Merkt, J., Muller, H., Knabe, W., Muller, P., Weiser, T., 1993. ¨ ¨ The early Holocene Saksunarvatn tephra found in lake sediments in NW Germany. Boreas 22, 93–100. Metrich, N., Sigurdsson, H., Meyer, P.S., Devine, J.D., 1991. The 1783 Lakagigar eruption in Iceland: geochemistry, CO 2 , and sulfur degassing. Contrib. Miner. Petrol. 107 Ž4., 435–447. Meyer, P.S., Sigurdsson, H., Schilling, J.G., 1985. Petrological and geochemical variations along Iceland’s neovolcanic zones. J. Geophys. Res. 90, 10043–10072. Middlemost, E.A.K., 1975. The basalt clan. Earth Sci. Rev. 11, 337–364. Mork, ¨ M.B.E., 1982. Magma mixing in the post-glacial Veidivotn ¨ fissure eruption, south-east Iceland: a microprobe study of mineral and glass variations. Nord. Volcanol. Inst., Univ. Iceland, Rep. 8205. Nielsen, C.H., Sigurdsson, H., 1981. Quantitative methods for electron microprobe analysis of sodium in natural and synthetic glasses. Am. Miner. 66, 547–552. Oskarsson, N., Sigvaldason, G.E., Steinthorsson, S., 1982. A ´ dynamic model of rift zone petrogenesis and the regional petrology of Iceland. J. Petrol. 23, 28–74. Oskarsson, N., Steinthorsson, S., Sigvaldason, G.E., 1985. Iceland ´ geochemical anomaly: origin, volcanotectonics, chemical fractionation and isotope evolution of the crust. J. Geophys. Res. 90 ŽB12., 10011–10025. Palmason, G., 1971. Crustal structure of Iceland from explosion ´ seismology. Soc. Sci. Isl. 40, 9–187. Palmason, G., 1973. Kinematic and heat flow in a volcanic rift ´ zone, application to Iceland. Geophys. J. R. Astron. Soc. 33, 451–481.

Poutiers, J., Gonthier, E., 1978. Sur la susceptibilite´ magnetique ´ des sediments du bassin d’Islande comme indicateur de la ´ dispersion du materiel ´ volcanoclastique a´ partir de l’Islande et des Faeroe Žmission Faegas II.. Bull. Inst. Geol. ´ Bassin d’Aquitaine, Bordeaux 23, 214–226. Ram, M., Donarummo, J. Jr., Sheridan, M., 1996. Volcanic ash from Icelandic 57,300 ybp eruption found in GISP2 ŽGreenland. ice core. Geophys. Res. Lett. 23 Ž22., 3167–3169. Ruddiman, W.F., 1972. Sediment redistribution on the Reykjanes ridge: seismic evidence. Geol. Soc. Am. Bull. 83, 2039–2062. Ruddiman, W.F., Bowler, F.A., 1976. Early interglacial bottomcurrent sedimentation on the eastern Reykjanes Ridge. Mar. Geol. 21, 191–210. Sædmundsson, K., 1979. Outline of the geology of Iceland. Jokull ¨ 29, 7–28. Schmincke, H.U., von Rad, V., 1979. Neogene evolution of Canary Island volcanism inferred from ash layers and volcaniclastic sandstones of DSDP site 397 Žleg 47A.. Initial Rep. Deep Sea Drill. Proj. 47, pp. 703–725. Schmitz, W.J., McCartney, M.S., 1993. On the North Atlantic circulation. Geophysics 31, 29–49. Shor, A.N., 1980. Bottom currents and abyssal sedimentation processes south of Iceland. PhD thesis, Woods Hole Oceanographic Institution, Unpubl. Sigurdsson, H., 1970. Structural origin and plate tectonics of the Snæfellsnes volcanic zone, western Iceland. Earth Planet. Sci. Lett. 10, 129–135. Sigurdsson, H., Loebner, B., 1981. Deep-sea record of Cenozoic explosive volcanism in the North Atlantic. In: Self, S., Sparks R.S.J. ŽEds.., Tephra Studies. pp. 289–316. Sigvaldason, G.E., 1968. Structure and products of subaquatic volcanoes in Iceland. Contrib. Miner. Petrol. 18, 1–16. Sjøholm, J., Sejrup, H.P., Furnes, H., 1989. Quaternary volcanic ash zones on the Iceland Plateau, southern Norwegian Sea. J. Quat. Sci. 6 Ž2., 159–173. Steele, J.H., Barret, J.R., Worthington, L.V., 1962. Deep currents south of Iceland. Deep-Sea Res. 9, 465–474. Swift, J.H., 1984. The circulation of the Denmark Strait and Iceland–Scotland overflow waters in the North Atlantic. DeepSea Res. 31, 1339–1355. Thorarinsson, S., 1967. The eruption of Hekla in historical times. The eruption of Hekla 1947–1948 I. Soc. Scient. Islendica, 170 pp. Thorarinsson, S., 1976. Gjoskulog ´ ¨ ŽTephra layers.. Samvinnan 70 Ž1., 4–9. Thorarinsson, S., 1979. Tephrochronology and its application in Iceland. Jokull 29, 33–36. ¨ Thorarinsson, S., Sæmundsson, K., 1979. Volcanic activity in historical time. Jokull 29, 29–32. ¨ Thorarinsson, S. et al., 1960. On the geology and geomorphology of Iceland. Geogr. Ann. Stockholm 41, 135–169. Thordarson, Th., Self, S., Oskarsson, N., Hulsebosch, T., 1996. Sulfur, chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki ŽSkaftar ´ Fires. eruption in Iceland. Bull. Volcanol. 58, 205–225. Tomasson, H., 1974. Grımsvatnahlaup 1972, mechanism and sedi´ ´ ment discharge. Jokull 24, 27–39. ¨

C. Lacasse et al.r Journal of Volcanology and Geothermal Research 83 (1998) 47–73 Tomasson, H., 1993. Jokulstıflud votn ´ ¨ ´ ¨ a´ Kili og hamfarahlaup ´ı Hvıta 62 Ž1–2., 77–98. ´ ´ ´ı Arnessyslu. Natturufrædingurinn ´ ´ Walker, G.P.L., Aumento, F., Clarke, B.D., Cann, J.R., Ryall, P.J.C., 1972. Site 115. Deep Sea Drill. Proj. 12, pp. 361–375. Werner, R., 1994. Struktur und Enstehung subglazialerrsublakustrischer Vulkane and Beispiel des Vulkankomplexes Her-

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durbreidrHerdubreidartogl ¨ in Island. Unpublished PhD thesis, University of Kiel. Werner, R., Schmincke, H.-U., Sigvaldason, G., 1996. A new model for the evolution of table mountains: volcanological and petrological evidence from Herdubreid and Herdubreidartogl ¨ volcanoes ŽIceland.. Geol. Rundsch. 85, 390–397.