ALBERT

Description: Geothermal waters with maximum temperatures of 103°C emanate from two sublacustrine hot spring areas which are located in the northem part of Lake Tanganyika (East African Riftsystem). The hydrothermalism leads to the formation of crust- and stockwork-like massive sulfide bodies on the lake bottom to a maximum water depth of 46 m. These geothermal vent areas were investigated and sampled during the German-French TANGANYDRO-campaign in 1991. The aim of this work is to characterize the mineralogy and geochemistry of these sulfides and to reconstruct their genesis. Mineralogical methods that have been used inc1ude scatter electron microscopy (SEM), polarization microscopy, and X-ray diffraction analysis. The geochemical methods inc1ude electron microprobe analysis (EMP) and inductively coupled plasma mass spectrometry (ICP-MS) for the major and trace elements and sulfur isotope measurements in order to determine the d34S-values of the samples. The samples consist exc1usively of iron sulfides. The dominant minerals are marcasite and melnicovite with subordinate pyrite. All samples show collophorm textures indicating their origin from gel-like precursors. They are characterized by high contents of As (up to 3.4 wt%), Sb (up to 0.6 wt%) and Tl (up to 2.6 wt%). Low d34S-values in the range of -11.6 %0 to +2.4 %0 (rel. PDB) indicate bacterial sulfur fractionation. The crystallization of the gel-like precursor leads to the formation of marcasite and pyrite with melnicovite as a transitional phase. Pyrite is formed by the replacement of either melnicovite or marcasite. This mechanism accords to previously postulated models for the formation of iron sulfides in low temperature (〈100°C) hydrothermal systems. A significant sulfur isotope fractionation (increase of d34S) has been observed during the replace- ment of melnicovite by the mature phases marcasite and pyrite. Biogenie impact on the sulfide formation is indicated by low Co/Ni-ratios (〈1), the negative d34S-values and the occurrence of framboidal pyrite. The metabolism of sulfur oxidizing and sulphate reducing bacteria at the wall rock of the vents and in the spring waters is suggested to actively influence the setting of specific pH-values required for the formation of either marcasite or pyrite. The altemating pyrite-marcasite layers are the result of fluctuations in the productivity of those bacteria, which may depend on seasonal variations or changing nutrient support.

Description: Based on foraminiferal transfer-functions, the distribution patterns of early Holocene sea- surface temperatures (SST) were studied, using the information from 154 deep-sea sediment cores (92 Atlantic, 62 Indian Ocean and Western Pacific). For our reconstruction, we employed a uniform high-resolution, AMS 14C-calibrated d18O-chronology, converted to a calendar timescale, and the new SIMMAX-Transfer-Technique in the Atlantic Oceans (Pflaumann et al. in press). The short-term SST fluctuations during the last 30,000 years are not directly related to the relatively slow changes in insolation during this period, reaching maximum seasonal deviations from modern values at approximaterly 11,000 years B.P. Although seasonal changes in solar radiation must have triggered global warming to the modern, interglacial mode, there is little evidence for linear warming and heat transport by ocean currents. The SIMMAX-temperature estimates indicate an early and rapid warming in the Equatorial Atlantic, as well as in the eastern North Atlantic, where modern SSTs were reached for a short time between 20,000 to 16,000 kalendar-years B.P. On a core transect crossing the Island-Faroer Ridge, the history of high-latitude warming along the eastern margins of the big North Atlantic gyres was reconstructed. Prior to the Younger Dryas cold interval (12,000 kalendar years), SSTs of the Norwegian Greenland Sea were still at glacial levels. After the Younger Dryas, there was a rapid inflow of warm Atlantic surface waters into the Norwegian-Greenland basins. In the northern Indian Ocean, the SST-patterns were totally different from the Atlantic during the last 20,000 years. Temperature variations did not exeed 2-3°C in the open ocean. During the Last Glacial Maximum (18,000 years B.P.), temperatures were higher than today whereas they were lowest during the early Holocene. This was caused by changes in the monsoon-induced oceanic upwelling intensity. At this time trade winds off Northwest Africa were also stronger, related to the stronger seasonal constrasts in insolation. Perhaps, the atmospheric circulation was generally enhanced at 10,000 years B.P. High-resolution SST-records from the southern Ocean (Pichon et al. 1992) indicate a slight asymmetry between the two Hemispheres. At 10,000 years B.P, SSTs were 1-2°C higher than today in the southern Indian Ocean. At the same time, somewhat colder SSTs imply still cool, boreal conditions in the middle and high latitudes of the northern hemisphere. Although SSTs of both seasons are only little different from the modern patterns, differences in the direction and strength of the major ocean currents are indicated by internally consistent positive and negative temperature anomaly fields. They were found in both, in the lower and in the high latitudes. The distribution of the anomalies in the North Atlantic further suggests, that the remnants of the ice shields still had a strong impact on the SST distribution. The particulary stronger insolation in the high northern latitudes during summers had nearly no influence. Finally, many details in the SST fluctuations and in the distribution of temperature anomalies imply a more dynamic surface circulation than today which may be the most characteristic difference between the early Holocene and modern surface ocean.

Description: Upper Pliocene and Pleistocene abundance fluctuations of the radiolarian Cycladophora davisiana (Ehrenberg) davisiana (Petrushevskaya) are documented from North Atlantic (Site 609) and Labrador Sea (Site 646B) to provide the first long-term correlation of its abundance fluctuations to oxygen isotope stages 1-114. Also examined are temporal and regional fluctuations in abundances C. d. davisiana and the global dispersal routes of the species. The first occurrence of C. d. davisiana in the eastern North Atlantic Ocean (Site 609) occurred between 2.586 and 2.435 Ma (oxygen isotope stages 109.66-102.19). During the early Matuyama Chron, prior to oxygen isotope stage 63, C. d. davisiana abundances were less than 1% and never greater than 12%, while abundances of greater than 5% are found in stages 65.71-73, 74, and 83-84. The initial major abundance peak (35.7%) of C. d. davisiana was noted near the stage 63/62 boundary. Abundance peaks of greater than 15%, between oxygen isotope stages 35 and 63, are limited to stages 63.02, 58.07, 55.07-54.26, and 50.76-50.22. These represent the only such abundance peaks detected during the first c. 1.5 million years of the species within the North Atlantic. The character of C. d. davisiana abundance fluctuations in Site 609 changes after oxygen isotope stage 35; average abundances are greater (7.7% vs. 4.3%) and abundance maxima of more than 15% are more frequent. Many, but not all, peak abundances of C. d. davisiana occur in glacial stages (e.g., 8, 14, 18, 20, 26, 30, 34, 50, 54, and 58). Increased abundances of the species are also noted in weak interglacial stages (e.g., stages 3, 23, 39, and 41), and significant cool periods of robust interglacial periods (e.g., late stage 11). Sample spacing is adequate in some stages to note some rapid changes in abundance near stage transitions (e.g., stages 4/5, 25/26, 62/63). The sample density in Holes 609 and 611 and the upper portion of 646B is sufficient to detect a synchroneity of many abundance maxima and minima among sites. Some abundance peaks are undetected in one or more of the two holes, warranting further sampling to obtain a more accurate record of regional abundance fluctuations. Prior to stage 36, few ages of Hole 611 peaks are the same as those in the more precisely dated Hole 609. The highest abundances of C. d. davisiana were noted in Labrador Sea Hole 646B where the earliest known occurrence of the species is documented (3.08-2.99 Ma). C. d. davisiana is inferred to have evolved in the Labrador Sea (or Arctic), and migrated next through the Arctic into the North Pacific (2.62-2.64 Ma, stage 114) before migrating into the Norwegian Sea (2.63-2.53 Ma) and North Atlantic (2.59-2.44 Ma, stages 109-102). Additional migration of C. d. dauisiana into the southern South Atlantic (Site 704) occurred much later (2.06 Ma, stage 83).