Petrology and geochemistry of sideromelane glass shards from Pleistocene as layers north and south of Gran Canaria

Major elements, S, F, Cl concentrations and relative proportions of S6+ to total S were analyzed with electron microprobe in sideromelane glass shards from Pleistocene volcaniclastic sediments drilled during ODP Leg 157. Glasses are moderately to strongly evolved and represent a spectrum from alkali basalt, basanite and nephelinite through hawaiite, mugearite and tephrite to phonolitic tephrite. Measured S6+/SumS (0.03±0.98) and calculated Fe2+/Fe3+ (2.5±5.8) ratios in the melt yield preeruptive redox conditions ranging from NNO-1.4 to NNO+2.1. The morphology of the glass shards, variations of S and Cl concentrations (0.010±0.127 wt% S, 0.018±0.129 wt% Cl), calculated preeruptive temperatures (1030±1200 °C) and oxygen fugacities suggest that glasses deposited even within the same ash layers have diverse origin and may have resulted from both submarine and subaerial eruptions. Most vesicle-free glasses are characterized by high concentrations of S and represent undegassed or slightly degassed submarine lavas, whereas vesiculated glasses with low concentrations of S and Cl are strongly degassed and can be ascribed to the eruptions in shallow water or on land. Sideromelane glass shards at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 represent mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses deposited at Site 956 resulted from submarine or explosive eruptions on Tenerife.

(Table 1) Geochemistry of ash layers at DSDP Leg 57 Holes

The study of the main characteristics of ash layers in Leg 57 cores shows that they are suitable for an analysis of the effect on eruptive activity of their distribution. We found (1) sediment recovery good and ash layers numerous; (2) sedimentary environment generally free from terrigenous clastic material; (3) reworking limited; (4) volcanic glass very acidic, ranging from rhyolitic to rhyodacitic composition; and (5) alteration and diagenesis negligible above the lower Miocene. The curves of explosive volcanic activity in Holes 438, 439, and 440 display two stages of high activity: an early one around 16 m.y. and a late one starting 5 m.y. B.P., both stages being separated by an upper Miocene quiescence. Detail in these results is limited by the chemical composition of the glass and accounts only for trends in explosive acid volcanism. Nevertheless, results are roughly in agreement with other data from the Northwest Pacific, although some discrepancies in the correlation of intensity of the episodes occur. The data from Leg 57 support the hypothesis of synchronous pulses in explosive volcanism.

Distribution of benthos in late Quaternary turbidite layers of sediment cores from the North-West African continental margin

The CaCO3-contents and the fractions > 40 µm have been analysed from 5 kastenloten, one piston core and two kastengreifer taken between Senegal and Cape Verde Islands. Numerous benthonic and planktonic organisms and different terrigenous components have been distinguished. The four cores off Senegal reach middle Wuerm sediments; cores GIK12329-6 and TAG72-1 reach the V-zone and core GIK12331-4 the X-zone (Eem); the two kastengreifer contain sediments of Holocene age. Correlation of the cores has been made. Holocene sedimentation rates decrease from the shallow cores (6-11 cm/1000 years) to the deep-sea (1-2 cm/1000 years). The following climatic variations could be deduced from the sediments off the Senegal: during Holocene climate was in general as today, the Senegal river transporting fine grained material to the sea. The upper Wuerm was arid with no river influence but with red dune sand transported to the continental slope. During middle Wuerm the climate was humid again. The deep-sea cores have been influenced by eolian material from arid regions during glacial and interglacial periods, indicated by relatively high "Wuestenquarz-numbers". However, during Wuerm "Wuestenquarz-numbers" are higher than during Holocene and Eem, indicating that more intensely red coloured sediment was exposed to wind activity on the continent during this period. Varying amounts of terrigenous material and CaCO3-contents indicate varying wind strengths (lower in Holocene and Eem than during Wuerm). The boundary between humid and arid Wuerm climate was at approximately 20 °N. Influence of upwelling is difficult to establish in the sediments off Senegal, because river influence, while increasing fertility also dilutes the diatoms which are typical for upwelling. High amounts of organic carbon, low plankton/benthos ratios of foraminifers and low plankton foraminifer/radiolarian ratios in Holocene sections might be interpreted as influenced by upwelling. Turbidites occur in cores 72 and 31 and at the Holocene/Pleistocene boundary of core GIK12329-6. Their composition indicates provenance from the continental shelf of the Cape Verde Islands for core 31 and the continental shelf and slope off Senegal for core TAG72-1. Volcanic material, rare in the normal pelagic sediment of core GIK12331-4 is more frequent in the turbidites.