Hans-Joachim Wallrabe-Adams

COMPOSITION AND ORIGIN OF VOLCANIC ASH ZONES IN LATE QUATERNARY SEDIMENTS FROM THE REYKJANES RIDGE : EVIDENCE FOR ASH FALLOUT AND ICE-RAFTING

Abstract Seven Quaternary volcanic ash zones including the well-known ash zones AZ I and AZ II have been investigated in five well-dated sediment cores from the Reykjanes Ridge between 59 and 60 °N. The zones were defined on their geochemical composition, morphological variation of the particles and stratigraphic position. The ash zones AZ I and AZ II show a mixture of colorless bubble wall shards and brown blocky glasses, whereas ash zones VZ 1 to 5 are composed of dark to light brown volcanic glasses, ranging in shape and texture from blocky, nonvesicular shards to highly vesicular, pumiceous types. The dark brown glass shards of ash zones VZ 1 to 5 represent subalkalic and low-K subalkalic basaltic series. Geochemical correlation of the ash zones with possible source areas indicates that they all derived from Iceland. The colorless shards in AZ I and AZ II are of rhyolitic composition. The Eastern Volcanic Zone on Iceland is the most important source, with abundant volcaniclastics being produced by subaerial/subglacial eruptions and secondary erosion of volcanic rocks. Explosive volcanism on Iceland is manifested by marine ash layers AZ I and AZ II. Most of the volcaniclastic zones VZ 1 to 5 coincide chronologically with peak abundances of coarse ice-rafted material (IRD) indicating that most of this volcanic glass was incorporated into the Icelandic Ice Sheet and released to the ocean by iceberg calving and subsequent melting. However, two layers in VZ 1 and VZ 5 have no correlatives in the IRD-record, suggesting that they were probably transported by local sediment gravity flows. From this we must infer that most of the ice-rafting episodes bearing volcanic particles as observed in sediments from Reykjanes Ridge, monitor glacier fluctuations on Iceland.

Chemical composition, distribution, and origin of silicic volcanic ash layers in the Greenland-Iceland-Norwegian Sea: explosive volcanism from 10 to 300 ka as recorded in deep-sea sediments

Explosive ocean island volcanism in the Greenland–Iceland–Norwegian Sea (GIN Sea) is indicated by marine tephra layers at 10–300 ka. Peaks of explosive volcanism occurred in oxygen isotope stages 8, 7, 5 and 1. The depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records. Geochemical analyses of the tephra layers show that all originate from Iceland. Here we report the characteristics of tephra from these major Icelandic events in 30 deep-sea cores from the GIN Sea. Our findings provide constraints on the distribution of tephra from the eruption source. For the Vedde Ash (oxygen isotope stage 1) we estimate a minimum fallout area of 2∗105 km2, stretching from central Greenland in the west and southern Sweden in the east, to 71°N in the GIN Sea. The magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events. Oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards.

Hydrodynamic properties and grain-size characteristics of volcaniclastics deposits on the Mid-Atlantic Ridge north of Iceland (Kolbeinsey Ridge)

Surface sediments from a transect across the mid-ocean ridge north of Iceland (Kolbeinsey Ridge) have been analyzed according to their compositional, textural and hydromechanical characteristics. The results were used to reconstruct sediment formation and depositional processes. The ridge sediments are dominated by volcaniclastic particles of hyaloclastic and pyroclastic origin. These particles show a wide variety in size, shape and density. Single-grain settling velocities of the different glass types reveal the suitability of this parameter as a reflector of the particle properties of size, shape and density, which are also known to be relevant to grain transport. Observations concerning different current expositions of central ridge sediments, combined with the parameters of settling velocity distribution, grain-size distribution and sediment particle composition, were applied to distinguish between transport association with rare, easily movable glass shards and poorly sorted sediments in sheltered ponds. A bimodal settling velocity distribution of steep ridge-flank sediments probably indicates the effect of sediment admixture from poorly sorted mass flows. Alternating coarse- and fine-grained layers characterize the transition between ridge-glass sands and the ridge-adjacent plain, which is dominated by slow-settling pelagic material.

Composition and origin of sediments on the mid-oceanic Kolbeinsey Ridge, north of Iceland

In order to characterize the sediments of the South Kolbeinsey Ridge and to determine the influence of morpho- and hydrodynamic conditions in this area, a comprehensive sedimentological investigation was combined with detailed geochemical analyses. Lithological composition and grain size are mainly controlled by the input from two different sources: the submarine, active mid-ocean ridge and Iceland. Coarse-grained volcanic material dominates in the ridge area, whereas fine-grained detritus from Iceland characterizes the adjacent basin. Further, the distribution of these sediments is largely influenced by hydrodynamic conditions and redeposition processes. Strong bottom-water currents indicated by the benthic foraminifer species Cibicides lobatulus prevent extensive deposition of fine-grained material on the top of the ridge. The ridge slope is characterized by redeposited sediments of various grain sizes. Bulk-sediment chemical analyses show element distribution patterns that are strongly correlated with the ratio of volcanic vs. detritic particle composition. This ratio reflects particle input and distribution processes. Two element associations are notable: elements coupled with (1) ridge-derived volcanic particles and (2) the detrital <2 μm fraction which reflects weathered material from Iceland.

Petrology and geotectonic development of the Western Ecuadorian Andes: the Basic Igneous Complex

Abstract The Basic Igneous Complex (BIC) consists of several volcanic formations which form the Ecuadorian Western Cordillera and Coastal Cordillera. The rocks differ significantly from other Cordillerian successions and form an isolated tectnic block. The Upper Cretaceous to Lower Tertiary volcanic series of the Ecuadorian Basic Igneous Complex are of island-arc type according to their petrography and geochemistry. An outer tholeiitic island-arc series (parts of the Pinon Formation) can be distinguished from a central calc-alkalic series, depleted in potassium (Macuchi, Celica, and Silante Formations). The Chontal, Yunguilla, and Zapotillo Formations as well as the upper Cayo Formation (“Guayaquil cherts”) are interpreted to represent back-arc sediments and sediments of small intra-arc basins within a largely submarine island arc. The Cayo Formation consists almost entirely of volcaniclastic and epiclastic rocks deposited on the oceanward side of the island arc, which was situated close to the South American continent. Volcanic activity continued into early Eocene times as revealed by K/Ar-dating of the Cayo and Silante Formations (52 Ma). A small back-arc basin was filled with the sediments of the Yunguilla Formation. Similar to present-day volcanically active island arcs (e.g., Aleutians and Kuriles-Kamchatka) the BIC island arc was mostly underlain by oceanic crust, although parts of it built on continental crust (Celica Formation, southern Ecuador). In the Early Tertiary the island arc was accreted to the South American continent. Cessation of volcanic activity and accretion to the continent may be explained by increased drift velocities of the lithospheric plates accompanied by a change in the geometry and stress in the subduction zone.

Volcaniclastic sediments from mid-oceanic Kolbeinsey Ridge, north of Iceland: Evidence for submarine volcanic fragmentation processes

Volcaniclastic sediments of the active Kolbeinsey Ridge, north of Iceland, reflect changing fragmentation mechanisms due to increasing sea level during the last deglaciation. Glass shards deposited on the western flank of the mid-oceanic ridge before 13.4 ka are vesicular (20%-60%), whereas shards deposited since 13.4 ka are mostly vesicle free and of hyaloclastic origin. The vesicle-rich shards display morphology thought to be atypical for submarine eruptions at this depth (400-500 m). The environmental conditions during the deposition of these layers (regional ice cover, more than 200 km from the nearest subaerial source) preclude a subaerial origin for these shards. We propose an eruption mechanism in which hot vesiculating bombs from a low-energy eruption are transported upward through the water column by convection. These particles reach their volcanic fragmentation depth (VFD) and undergo secondary fragmentation, creating the vesicle-rich shards recovered near the ridge. Rising sea level since the end of the last glaciation was sufficient to prevent the erupted particles from reaching the VFD, thus ending the formation of vesicle-rich shards. This model explains the presence of vesicular shards in sediments of mid-oceanic ridges where no subaerial source can be inferred.

Late Quaternary sedimentation on the Mid-Atlantic Reykjanes Ridge: clay mineral assemblages and depositional environment

Sediment samples from the Mid-Atlantic Reykjanes Ridge (59°N) were taken to get information about sediment genesis and to identify different sources during the late Quaternary. Samples were investigated by X-ray diffraction and grain-size analyses. The clay mineral assemblages in sediments of the Reykjanes Ridge reflect paleoceanographic changes during the late Quaternary. Holocene sediments are characterized by high contents of smectite, mainly of less developed crystallinity. In the spatial distribution of clay minerals high smectite concentrations on the eastern flank and slightly decreasing concentrations on the western flank of the Reykjanes Ridge indicate the action of bottom-water transport. The smectite originates mainly from the volcanogenous Icelandic shelf and reflects the influence of Iceland-Scotland Overflow Water (ISOW). Stratigraphic variability in the clay mineral composition reflects predominantly the influence of different sources, resulting from oceanographic and glacial transport processes. During glacial time sediment transport is due mainly to input by icebergs. Increasing amounts of illite, chlorite, and kaolinite characterize ice-rafted sediments of the “Heinrich layers”. In these sediments smectite crystallinity is well developed. In contrast, several other ice-rafted layers contain smectite with low crystallographic order, similar to smectites of Holocene age. The icelandic source was proved by distinct amounts of basaltic glass in the coarse-grained sediment. At approximately 55 ka increasing amounts of chlorite and kaolinite suggest an enhanced influx of warm North Atlantic surface waters. This hypothesis is supported by a high carbonate shell production at this time. Relative low concentrations and the well-developed crystallinity of smectite minerals characterize the Last Glacial Maximum (LGM; 18–16 ka), indicating a reduced supply of fine icelandic material. Shortly after the LGM, at the beginning of termination IA, a distinct increase in fine-grained quartz (<2µm) and smectite are visible, which are proposed to reflect a supply of fine-grained ice-rafted material. At 13 ka linear increasing smectite concentrations of lower crystallographic order indicate increasing supply of fine-grained material from Iceland, linked to reinitiation of bottom currents of the ISOW. Full reinitiation is indicated at around 10 ka, where a strong increase in smectite of low crystallographic order is detected.

Geochemistry of surface sediments from the mid-oceanic Kolbeinsey Ridge, north of Iceland

In order to assess recent submarine volcanic contributions to the sediments from the active Kolbeinsey Ridge, surface samples were analyzed chemically. The contribution of major and trace elements studied differ within the study area. A statistical analysis of the geochemical variables using factor analysis and cluster method allows to distinguish possible sample groups. Cluster method identifies three distinct sediment groups located in different areas of sedimentation. Group 1 is characterized by highest contents of Fe2O3, V, Co, Ni, Cu and Zn demonstrating the input of volcaniclastic material. Group 2 comprises high values of CaCO3, CaO and Sr representing biogenic carbonate. Group 3 is characterized by the elements K, Rb, Cs, La and Pb indicating the terrigenous component. The absolute percentage of the volcanic, biogenic and terrigenous components in the bulk sediments was calculated by using a normative sediment method. The highest volcanic component (> 60% on a carbonate free basis) is found on the ridge crest. The biogenic component is highest (10–30%) in the eastern part of the Spar Fracture Zone influenced by the East Iceland Current. Samples from the western and southeastern region of the study area contain more than 90% of terrigenous component which appears to be mainly controlled by input of ice-rafted debris.

Evidence for topography- and current-controlled deposition on the Reykjanes Ridge between 59°N and 60°N

Sediment patterns derived from sediment sampling and acoustic subbottom profiling were mapped on the Reykjanes Ridge (North Atlantic) between 59°N and 60°N. Five discrete sediment echo patterns were distinguished and mapped on a regional scale. The prolonged and layered echo facies, which mainly reflect sediment filled basins on the ridge flanks, indicate deposition of predominantly fine-grained sediments deposited by the Iceland-Scotland Overflow Water. A combination of westward flowing currents spilling over the ridge crest due to the Coriolis force together with the existing morphology probably caused the N-S trending facies distribution pattern on the northwest flank. Furthermore, the modern surface sediment distribution is controlled by biological productivity, which is closely related to the mixing zone of cold subpolar surface water masses and the warm North Atlantic Current, and bottom water transport processes. The effect of bottom current transport is reflected in the pattern of settling velocity and sediment grain size. The clay mineral composition indicates that most of the fine-grained material is supplied predominantly from the Icelandic province by the Iceland-Scotland Overflow Water. Erosional processes are concentrated on narrow zones on top of the axial ridges and on the steep flanks of the Catalonia Seamount. Well-sorted foraminiferal sands on these exposed regions are assumed to represent residual sediments.