(Table 1) Light minerals in sand at DSDP Leg 84 holes

The Middle America active continental margin is the best-sampled active plate margin to date, having been drilled during Legs 84, 67, and 66. With nine sites drilled on the continental slope of Guatemala and an additional site drilled on the Costa Rican slope, a summary of slope sediments and sedimentary processes can be made. Sediments are easily subdivided into a thick apron of Neogene and Quaternary volcanically derived hemipelagic and turbidite mud and mudstone and a thinner, more varied assemblage of mostly Paleogene mudstone, radiolarian mudstone, and limestone. This latter assemblage may contain hiatuses or be completely lacking between slope deposits and basement. Cores from the foot of the continental slope (Core 567A-19) consist of Campanian micrite. The pre-Neogene section is much thicker and of more terrigenous provenance beneath the forearc basin landward of the forearc structural high than on the continental slope. Sedimentary processes of the Neogene and Quaternary slope sediments include reworking of hemipelagic and turbidite deposits. Redeposition by slumping, plastic flow, and turbidity current-documentable through benthic foraminiferal analysis-occurs in intracanyon and canyon settings. Erosion by slumping and by turbidity current and deposition of mud or sand in canyons and in local depressions on the continental slope and different rates of sediment accumulation result in dramatic thickness variations of lithologic units over small distances in localized pockets of sand in small filled canyons on the slope or in sediment ponds, and in high-relief basement topography. The age of sediment overlying igneous basement ranges from Cretaceous to Quaternary. Gas hydrate was visible or inferred present at every site drilled during Leg 84. Nevertheless, except for a small amount in the last core, it was not recovered in sufficient quantities to be visible at Site 568, a site specifically chosen for the study of hydrate and located near Site 496, which was abandoned during Leg 67 because of the dangerous abundance of hydrates. The association of hydrate with porous, coarser sediment results in a distribution as localized and unpredictable as the slope sands off Guatemala, which do not occur in beds coherent enough to produce acoustic reflection. Although the normal lithologic section at Sites 567 and 496 limits the volume of sediment that could be part of an accretionary prism offshore Guatemala and the volume of sediment in the Trench axis is not sufficient to argue for significant accumulation of Cocos Plate sediments, the varied lithology and attenuated thickness of pre-Neogene sediment seaward of the forearc structural high do not exclude earlier accretion from the history of the Guatemalan continental margin.

Tephrostratigraphy and radiometric dating of authigenic carbonates

At convergent margins, fluids rise through the forearc in response to consolidation of the upper plate and dewatering of the subducting plate, and produce various cold-seep-related features on the seafloor (mud diapirs, mud mounds). At the Central American forearc, authigenic carbonates precipitated from rising fluids within such structures during active venting while typical mixed-mud sediments were ejected onto the surrounding seafloor where they became intercalated with normal pelagic background sediments, indicating that mud mounds evolved unsteadily through alternating active and inactive phases. Intercalated regional ash layers from Plinian eruptions at the Central American volcanic arc provide time marks that constrain the ages of mud ejection activity. U/Th dating of drill core samples of authigenic carbonate caps of mud mounds yields ages agreeing well with those constrained by ash layers and showing that carbonate caps grow inward rather than outward during active venting. Both dating approaches show that offshore Nicaragua and Costa Rica (1) active and inactive phases can occur simultaneously at neighboring mounds, (2) mounds along the forearc have individual histories of activity, but there are distinct time intervals when nearly all mounds have been active or inactive, (3) lifetimes of mounds reach several hundred thousand years, and (4) highly active periods last 10-50 k.y. with intervening periods of >10 k.y. of relative quiescence.

Organic geochemistry of DSDP Site 467

Results of the analyses of twenty-three samples from the Middle Miocene to Lower Pliocene strata from DSDP Site 467, offshore California, are presented. The analyses were performed with the aim of determining the origin of the organic matter, the stratigraphic section's hydrocarbon generation potential and extent of organic diagenesis. Organic carbon contents are an order of magnitude greater than those typically found in deep sea sediments, suggesting an anoxic depositional environment and elevated levels of primary productivity. Hydrocarbon generation potentials are above average for most samples. The results of elemental analyses indicate that the kerogens are primarily composed of type II organic matter and are thermally immature. Analysis of the bitumen fractions confirms that the samples are immature. In cores from 541 to 614 meters, the gas chromatograms of the C15+ non-aromatic hydrocarbon fractions are dominated by a single peak which was identified as 17*(H), 18*(H), 21beta(H)-28, 30-bisnorhopane. This interval is the same area in which the highest degrees of anoxia are observed as reflected by the lowest pristane/phytane ratios. This correlation may have some implications with regard to the origin of the bisnorhopane and its possible use as an indicator of anoxic depositional conditions within thermally immature sediments.

Stable isotopes, Mg/Ca ratios and sea surface temperatures on foraminifera from sediment cores off equatorial Peru during the last ~17kyr

The complex deglacial to Holocene oceanographic development in the Gulf of Guayaquil (Eastern Equatorial Pacific) is reconstructed for sea surface and subsurface ocean levels from (isotope) geochemical proxies based on marine sediment cores. At sea surface, southern sourced Cold Coastal Water and tropical Equatorial Surface Water/Tropical Surface Water are intimately related. In particular since ~10 ka, independent sea surface temperature proxies capturing different seasons emphasize the growing seasonal contrast in the Gulf of Guayaquil, which is in contrast to ocean areas further offshore. Cold Coastal Water became rapidly present in the Gulf of Guayaquil during the austral winter season in line with the strengthening of the Southeast Trades, while coastal upwelling off Peru gradually intensified and expanded northward in response to a seasonally changing atmospheric circulation pattern affecting the core locations intensively since 4 ka BP. Equatorial Surface Water, instead, was displaced and Tropical Surface Water moved northward together with the Equatorial Front. At subsurface, the presence of Equatorial Under Current-sourced Equatorial Subsurface Water was continuously growing, prominently since ~10-8 ka B.P. During Heinrich Stadial 1 and large parts of the Bølling/Allerød, and similarly during short Holocene time intervals at ~5.1-4 ka B.P. and ~1.5-0.5 ka B.P., the admixture of Equatorial Subsurface Water was reduced in response to both short-term weakening of Equatorial Under Current strength from the northwest and emplacement by tropical Equatorial Surface Water, considerably warming the uppermost ocean layers.

Terrigenous sedimentation of sediment core GeoB6007-2

Variations in deposition of terrigenous fine sediments and their grain-size distributions from a high-resolution marine sediment record offshore northwest Africa (30°51.0'N; 10°16.1'W) document climate changes on the African continent during the Holocene. End-member grain-size distributions of the terrigenous silt fraction, which are related to fluvial and aeolian dust transport, indicate millennial-scale variability in the dominant transport processes at the investigation site off northwest Africa as well as recurring periods of dry conditions in northwest Africa during the Holocene. The terrigenous record from the subtropical North Atlantic reflects generally humid conditions before the Younger Dryas, during the early to mid-Holocene, as well as after 1.3 kyr BP. By contrast, continental runoff was reduced and arid conditions were prevalent at the beginning of the Younger Dryas and during the mid- and late Holocene. A comparison with high- and low-latitude Holocene climate records reveals a strong link between northwest African climate and Northern Hemisphere atmospheric circulation throughout the Holocene. Due to its proximal position, close to an ephemeral river system draining the Atlas Mountains as well as the adjacent Saharan desert, this detailed marine sediment record, which has a temporal resolution between 15 and 120 years, is ideally suited to enhance our understanding of ocean-continent-atmosphere interactions in African climates and the hydrological cycle of northern Africa after the last deglaciation.

Early Oligocene dinoflagellate cysts from the Upper Rhine Graben

Using principal component analysis and cyst diversity and equity trends, we can recognize four distinct dinoflagellate cyst (dinocyst) assemblages from four Rupelian (Early Oligocene) cores in the Mainz Embayment of the northern Rhine Graben (SW Germany). These assemblages are the Spiniferites ramosus (PC1), Thalassiphora pelagica (PC2), Homotryblium tenuispinosum (PC3), and Vozzhennikovia spinula (PC4) assemblages. The four cores provide an onshore-offshore transect in the Mainz Embayment. The H. tenuispinosum assemblage shows high factor loadings in proximal to intermediate cores, which is interpreted to reflect temporary high-salinity conditions. Mean dinocyst diversity and equity increase with distance from the Mid-Rupelian shoreline, indicating increasingly stable paleoenvironmental conditions towards the center of the Mainz Embayment. Within individual cores, changes in dinocyst assemblages through time are related to paleoenvironmental and paleoclimatological changes. The three proximal to intermediate cores show dominance of the H. tenuispinosum assemblage repeatedly alternating with high factor loadings of the T. pelagica assemblage. In both cases, dinocyst diversity and equity tend to be reduced. Highest factor loadings of the S. ramosus assemblage occur in intervals where neither of the above assemblages is dominant and tend to coincide with dinocyst diversity and equity maxima. We interpret this distribution pattern to denote different paleoceanographic conditions, reflecting drier and more humid phases in the Early Oligocene of Central Europe. During relatively dry periods, increased salinity conditions prevailed in proximal to intermediate settings of the Mainz Embayment, as reflected by the dominance of the H. tenuispinosum assemblage. During more humid periods, increased runoff led to higher nutrient availability and the formation of a pycnocline separating slightly less saline surface waters from higher saline deeper waters, thus impeding vertical circulation. These environmental conditions are documented in high loadings of the T. pelagica assemblage which is indicative of increased eutrophication and/or oxygen-depleted bottom waters. Transitions between drier and more humid periods, i.e. episodes of normal marine conditions, are characterized by high loadings predominantly of the S. ramosus assemblage as well as increased dinocyst diversity and equity values. We propose that the alternations between drier and more humid phases may be related to variations in the ocean-atmosphere moisture flux from the North Atlantic into Central Europe bearing a high-latitude climate signal.

Living benthic foraminiferal assemblages in surface samples of the Arabian Sea

The distribution, biomass, and diversity of living (Rose Bengal stained) deep-sea benthic foraminifera (>30 µm) were investigated with multicorer samples from seven stations in the Arabian Sea during the intermonsoonal periods in March and in September/October, 1995. Water depths of the stations ranged between 1916 and 4425 m. The distribution of benthic foraminifera was compared with dissolved oxygen, % organic carbon, % calcium carbonate, ammonium, % silica, chloroplastic pigment equivalents, sand content, pore water content of the sediment, and organic carbon flux to explain the foraminiferal patterns and depositional environments. A total of six species-communities comprising 178 living species were identified by principal component analysis. The seasonal comparison shows that at the western stations foraminiferal abundance and biomass were higher during the Spring Intermonsoon than during the Fall Intermonsoon. The regional comparison indicates a distinct gradient in abundance, biomass, and diversity from west to east, and for biomass from north to south. Highest values are recorded in the western part of the Arabian Sea, where the influence of coastal and offshore upwelling are responsible for high carbon fluxes. Estimated total biomass of living benthic foraminifera integrated for the upper 5 cm of the sediment ranged between 11 mg Corg m**-2 at the southern station and 420 mg Corg m**-2 at the western station. Foraminifera in the size range from 30 to 125 ?m, the so-called microforaminifera, contributed between 20 and 65% to the abundance, but only 3% to 28% to the biomass of the fauna. Highest values were found in the central and southern Arabian Sea, indicating their importance in oligotrophic deep-sea areas. The overall abundance of benthic foraminifera is positively correlated with oxygen content and pore volume, and partly with carbon content and chloroplastic pigment equivalents of the sediment. The distributional patterns of the communities seem to be controlled by sand fraction, dissolved oxygen, calcium carbonate and organic carbon content of the sediment, but the critical variables are of different significance for each community.

Calcareous dinoflagellate cysts analysis of sediment cores from the Equatorial South Atlantic Ocean

An overview is presented of the current state of knowledge on paleo-ecological aspects of calcareous dinoflagellate resting cysts. Apart from literature-based information, a discussion of new results is also provided from Equatorial Atlantic surface plankton samples, surface sediment samples and Late Quaternary sediments from two gravity cores. With the aid of redundancy analysis statistics, variations in the calcareous cyst content of both cores are correlated to variations in total organic carbon (TOC). On a global scale, the calcareous cyst distribution in bottom sediments varies with latitude and inshore-offshore gradients. In the Equatorial Atlantic Ocean, enhanced calcareous cyst production can be observed in regions and time intervals with stratified, oligotrophic conditions in the upper water masses.