Beschreibung: The Foundation Seamounts form a 1400 km-long chain on the Pacific plate from 32 °S, 127 °W to the Pacific-Antarctic spreading axis at 38 °S, 111 °W. Previously only known from sparse single-beam echosoundings and satellite altimetry, we present here the first multibeam bathymetric survey and geological sampling results. We confirm that the submarine topography correlates with the altimetry, and that the chain is volcanic rather than tectonic or microcontinental in origin. The chain can be divided up morphologically and geochemically into three section: (1) west of 125 °W large flat-topped volcanoes composed of incompatible-element depleted lavas ( ≈ 1) of a near-ridge origin with little or no plume influence, (2) between 125 and 115 °W true intraplate volcanoes with incompatible element enrichment ( 〉 1.9) generated over the Foundation plume, (3) east of 115 °W E-W-trending volcanic ridges with compositions ( 2.0-0.3) suggestive of interaction between the plume and the Pacific-Antarctic spreading axis. On the spreading axis moderate incompatible element enrichments ( ≈0.8, cf. ≈ 0.3 outside the Foundation area) also suggest plume influence. It appears that the activity of the Foundation plume in the last few million years has (1) significantly waned and (2) become wholly channeled towards the spreading axis. The Foundation plume may be in the process of “dying”.

Beschreibung: The Easter microplate-Crough Seamount region located between 25° S–116° W and 25° S–122° W consists of a chain of seamounts forming isolated volcanoes and elongated (100–200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065°), to the present day general spreading direction (N 100°) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26°30′ S–115° W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges (〈 2000 m high) bounding the Southwest Rift of the microplate consists of fault scarps where pillow lava, dolerite, and metabasalts are exposed. The degree of rock alternation inferred from palagonitization of glassy margins suggests that the volcanic ridges are as old as the shallow ridges bounding the Southwest Rift of the microplate. The volcanics found on the various structures west of the microplate consist of depleted (K/Ti 〈 0.1), transitional (K/Ti = 0.11−0.25) and enriched (K/Ti 〉 0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element ratios normalized to chondrite values [(Ce/Yb)N = 1−2.5}, {(La/Sm)N = 0.4−1.2} and {(Zr/Y)N = 0.7−2.5} of the basalts are also similar to present day volcanism found in the Easter microplate. The volcanics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e. Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation (≈ 5 Ma). It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25° S–118° W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Crough seamount (〈 800 m depth) was formed by earlier (7–10 Ma) hotspot magmatic activity which also created Easter Island.

Beschreibung: The SO-84 cruise from Las Palmas to Cape Town studied the underwater volcanism associated with the St Helena hotspot. Wide-spaced grid mapping of a region south and west of St Helena island identified three previously unknown hotspot volcanoes (which we named Josephine 16°27'S-9°W, Jade 16°13'S-7°46'W and Benjamin 16°12'S-8°31'W) and several older, probably near-ridge-generated seamounts. The newly discovered hotspot seamounts, and the previously-known seamounts of Bagration, Bonaparte and Kutuzov were mapped and sampled. Onboard geochemical studies confirm that they all show trace-element (Rb, Nb, Y, Zr, Sr) enriched, hotspot magma-type compositions. Morphological studies and the state of the samples collected from the seamounts (all have at least 1 mm-thick Mn crusts) suggest that none of the volcanoes is presently active, and that the St Helena hotspot is probably extinct at the present time. Josephine appears to be the youngest volcano, and one dredge here yielded fossil hydrothermal material (Fe-hydroxides). To test the hypothesis that the extinction of the hotspot is due to its magmas being channelled to the Mid-Atlantic Ridge, several dredges were taken on the ridge opposite St Helena. Furthermore, in the proposed influence zone of the St Helena plume head, Grattan seamount (9°44'S-12°48'W) was also sampled to examine whether its composition has been affected by the St Helena plume. During the transit from Las Palmas to St Helena, several sediment cores were taken in 3000 m water depth close to the ridge axis for palaeoclimatological work.

Beschreibung: The Red Sea is a very young ocean, and is one of the most interesting areas on Earth (ocean in statu nascendi). It is the only ocean where hydrothermal activity associated with ore formation occurs in a sterile environment (anoxic, hot, saline). In addition, its geographical position means that it is predestined to record the monsoonal history of the region in detailed sedimentary sequences. The major aim of the present project is to investigate the dynamics of hydrothermal systems in selected Deeps (Atlantis-II, Discovery, Kebrit, Al Wajh), Additional palaeoceanographic and microbiological questions should also be addressed. Specific aims are: 1. To study the hydrographic changes in individual Deeps (hydrothermal region Atlantis-II) and to investigate the causes of the temperature increase in the last few years (increased heat flow - higher temperature of the brine supply - higher brine flow rates?). 2.a. To document the influence of the hydrothermal systems on the sedimentary organic matter in the Deeps. In particular, the thermogenic production and migration of hydrocarbons in the sediments will be studied. The complex formation mechanisms (bacterial, thermogenic) of short-chain hydrocarbons (trace gases) will also be examined, 2.b. in addition, the polar and macromolecular fraction in samples from the various deeps will be studied in order to elucidate the formation, structure and source of the macromolecular oil fraction. 3. To clarify the palaeoceanographic conditions, sea-level changes and the climatic history (relationship of the circulation system and nutrient supply to the monsoon) of the southern Red Sea. 4. To separate microorganisms from the brines and to characterise them in terms of their metabolic physiology and ecology, and to describe their taxonomy.