Dinoflagellates of sediment core GeoB1523-1

Large numbers of calcareous dinoflagellate cysts and the vegetative calcareous coccoid species Thoracosphaera heimii are generally found in sediments underlying oligotrophic and/or stratified (sub)surface water environments. It is difficult to distinguish between the relative importance of these two environmental parameters on calcareous cyst and T. heimii distribution as they usually covary, but this information is essential if we want to apply cysts properly in the reconstruction of palaeoenvironments and past surface water hydrography. In the multi-proxy core GeoB 1523-1 from the Ceará Rise region in the western equatorial Atlantic Ocean (covering the past 155 ka), periods of greatest oligotrophy are not synchronous with periods of greatest stratification (Rühlemann et al., 1996, doi:10.1016/S0025-3227(96)00048-5; Mulitza et al., 1997, doi:10.1130/0091-7613(1997)025<0335:PFAROP>2.3.CO;2; 335-338; Mulitza et al., 1998, doi:10.1016/S0012-821X(98)00012-0), giving us the unique opportunity to differentiate between the effects of both parameters on cyst accumulation. The calcareous cyst record of the core reflects prominent increases in accumulation rate of nearly all observed species only during the nutrient-enriched but more stratified isotopic (sub)stages 5.5, 5.3, 5.1 and 1. In this respect, the distribution trends in the core are more similar to those of the eastern equatorial upwelling region (GeoB 1105-4) than they are to those of the oligotrophic north-eastern Brazilian continental slope (GeoB 2204-2), even though temporal changes in bioproductivity are principally in antiphase between the eastern and western equatorial regions. We conclude that stratification of the upper water column and the presence of a well-developed thermocline are probably the more important factors controlling cyst distribution in the equatorial Atlantic, whereas the state of oligotrophy secondarily influences cyst production within a well-stratified environment.

Magnetic extracts of three gravity cores located at the Ceará Rise, the Mid-Atlantic Ridge and the Sierra Leone Rise

The magnetic microparticle and nanoparticle inventories of marine sediments from equatorial Atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation. This information is used to establish source-to-sink relations and to constrain environmental magnetic proxy interpretations for this area. Magnetic extracts were prepared from sediments of three supralysoclinal open ocean gravity cores located at the Ceará Rise (GeoB 1523-1; 3°49.9'N/41°37.3'W), the Mid-Atlantic Ridge (GeoB 4313-2; 4°02.8'N/33°26.3'W), and the Sierra Leone Rise (GeoB 2910-1; 4°50.7'N/21°03.2'W). Sediments from two depths corresponding to marine isotope stages 4 and 5.5 were processed. This selection represents glacial and interglacial conditions of sedimentation for the western, central, and eastern equatorial Atlantic and avoids interferences from subsurface and anoxic processes. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy. On basis of these properties, nine particle classes could be defined: detrital magnetite, titanomagnetite (fragmental and euhedral), titanomagnetite-hemoilmentite intergrowths, silicates with magnetic inclusions, microcrystalline hematite, magnetite spherules, bacterial magnetite, goethite needles, and nanoparticle clusters. Each class can be associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large-scale sedimentation patterns are delineated as well: detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid-ocean ridge basalts, and titanomagnetite-hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source-to-sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high-coercive or superparamagnetic properties depending on their partly ultrafine grain sizes. These two magnetic fractions are generally discussed as separate fractions, but we suggest that they could actually be genetically linked.