(Tab. XIII.1) TOC content, carbonate content, carbonate mineralogy, percentage of total carbonate, aragonite, low magnesium calcium, and high magnesium calcium, and grain-size contents from samples from the Golfe d'Arguin and from the Western Sahara

Modern carbonate sedimentation takes place on the northern Mauritanian shelf (20°N), where typical tropical components (e.g. hermatypic reefs, calcareous green algae) are absent. Such deposits are reminiscent of extratropical sediment in the geological record. The tropical open shelf of Mauritania is influenced by large siliciclastic dust input and upwelling, highly fertilizing the ocean, as well as strongly limiting the light penetration. In this context, temperature does not appear to be the steering factor of carbonate production. This thesis describes the depositional system of the Golfe d'Arguin off Mauritania and focuses on environmental conditions that control the depositional pattern, in particular carbonate production. The description of this modern analogue provides a tool for paleoenvironmental interpretation of ancient counterparts. The Golfe d'Arguin is a broad shallow shelf comprising extensive shoals (<10 m water depth; i.e. the Banc d'Arguin) on the inner shelf where waters warm up. The sediments collected in water depths between 4 and 600 m are characterized by mixed carbonate and siliciclastic (dust) deposits. They vary from clean coarse-grained, almost pure carbonate loose sediments to siliciclastic-dominated fine-grained sediments. The carbonate content and sediment grain size show a north-south decreasing pattern through the Golfe d'Arguin and are controlled by the hydraulic regime influenced by wind-driven surface currents, swell, and tidal currents. The carbonate grain association is heterozoan. Components include abundant molluscs, foraminifers, and worm tubes, as well as barnacles and echinoderms, elements that are also abundant in extratropical sediments. The spatial distribution of the sedimentary facies of the Golfe d'Arguin does not display a depth zonation but rather a mosaic (i.e. patchy distribution). The depth and climatic signatures of the different sedimentary facies are determined by taxonomic and ecological investigations of the carbonate-secreting biota (molluscs and foraminifers). While certain planktonic foraminifers and molluscs represent upwelling elements, other components (e.g. mollusc and benthic foraminifer taxa) demonstrate the tropical origin of the sediment. The nutrient-rich (and thus also low light-penetration) conditions are reflected in the fact that symbiotic and photosynthetic carbonate-producing organisms (e.g. hermatypic corals) are absent. The Mauritanian deposits represent an environment that is rare in the modern world but might have been more common in the geological past when global temperatures were higher. Taxonomic and ecological studies allow for distinguishing carbonate sediments formed under either tropical high-nutrient or extratropical conditions, thus improving paleoclimate reconstruction.

Radiocarbon dates, grain size measurements and selected foraminifera analysis from sediment cores AvH248260-2 and AvH248260-1, Ameralik Fjord, SW Greenland

Sedimentological and geochemical (XRF) data together with information from diatom and benthic foraminiferal records of a 3.5 m long gravity core from Ameralik Fjord, southern West Greenland, is used for reconstructing late-Holocene environmental changes in this area. The changes are linked to large-scale North Atlantic ocean and climate variability. AMS 14C-dating of benthic foraminifera indicates that the sediment core records the last 4400 years and covers the termination of the Holocene Thermal Maximum (HTM). The late HTM (4.4 3.2 ka BP) is characterized by high accumulation rates of fine (silty) sediments related to strong meltwater discharge from the Inland Ice. The HTM benthic foraminiferal fauna demonstrates the presence of well-ventilated, saline bottom water originating from inflow of subsurface West Greenland Current water of Atlantic (Irminger Sea) origin. The hydrographic conditions were further characterized by limited sea ice probably related to a mild and relatively windy winter climate. After 3.2 ka BP lower fine-grained sedimentation rates, but a larger input from sea-ice rafted or aeolian coarse material prevailed. This can be related to colder atmospheric conditions with a decreased meltwater discharge and more widespread sea-ice cover in the fjord.

(Table 1) Grain size composition of recent sediments from the eastern Barents Sea shelf

Based on materials on geomorphology, hydrology, lithology, and sedimentation dynamics obtained during cruises of the P.P. Shirshov Institute of Oceanology in the Barents Sea, the author prepared a number of charts on content of the main particle size facies in shelf surface sediments, as well as a chart of lithologic types of sediments in the Barents Sea. Factors of sedimentation control and basic features of distribution of sedimentary material over its bottom area are taken into consideration.

Grain size analysis on core plugs of sediment core CRP-1 (Table 1, 2)

Twenty four core samples from CRP-1, seven from Quaternary strata (20-43.55 meters below sea floor or mbsf) and seventeen from early Miocene strata (43.55 to 147.69 mbsf), have been analysed for their grain-size distribution using standard sieve and Sedigraph techniques. The results are in good agreement with estimates of texture made as part of the visual core description for the 1 :20 core logs for CRP-1 (Cape Roberts Science Team, 1998). Interpretation of the analyses presented here takes into account the likely setting of the site in Quaternary times as it is today, with CRP-1 high on the landward flank of a well-defined submarine ridge rising several hundred metres above basins on either side. In contrast, seismic geometries for strata deposited in early Miocene times indicate a generally planar sea floor dipping gently seaward. Fossils from these strata indicate shallow water depths (< 100 m), indicating the possibility that waves and tidal currents may have influenced sea floor sediments. The sediments analysed here are considered in terms of 3 textural facies: diamict, mud (silt and clay) and sand. Most of the Quaternary section but only 30% of the early Miocene section is diamict, a poorly sorted mixture of sand and mud with scattered clasts, indicating little wave or current influence on its texture. Although not definitive, diamict textures and other features suggest that the sediment originated as basal glacial debris but has been subsequently modified by minor winnowing, consistent with the field interpretation of this facies as ice-proximal and distal glaciomarine sediment. Sediments deposited directly from glacier ice appear to be lacking. Mud facies sediments, which comprise only 10% of the Quaternary section but a third of the early Miocene section, were deposited below wave base and largely from suspension, and show features (described elsewhere in this volume) indicative of the influence of both glacial and sediment gravity flow processes. Sand facies sediments have a considerable proportion of mud, normally more than 20%, but a well-sorted fine-very fine sand fraction. In the context of the early Miocene coastal setting we interpret these sediments as shoreface sands close to wave base.

(Table 1) 87Sr/86Sr and Sr/Ca ratios in foraminiferal calcite from DSDP Holes 3-21 and 39-357

The 87Sr/86Sr ratio of ancient seawater, as recorded in marine carbonates, is an important tracer of long-term variations in ocean chemistry (Burke et al., 1982, doi:10.1130/0091-7613(1982)10<516:VOSSTP>2.0.CO;2; Peterman et al., 1970, doi:10.1016/0016-7037(70)90154-7; Dasch and Biscaye, 1971, doi:10.1016/0012-821X(71)90164-6; Veizer and Compston, 1974, doi:10.1016/0016-7037(74)90099-4; Brass, 1976, doi:10.1016/0016-7037(76)90025-9). However, the Sr isotope balance of the oceans has been difficult to constrain; consequently, attempts to evaluate the temporal 87Sr/86Sr changes have been largely qualitative. To constrain the causes of these variations we have measured 87Sr/86Sr ratios in carefully cleaned unrecrystallized foraminifera from DSDP sites 21 and 357. The data presented here have been quantitatively modelled taking advantage of recent advances in understanding of the Sr geochemical cycle. They suggest that whereas hydrothermal fluxes and carbonate recycling are of major importance in defining the marine 87Sr/86Sr ratio, the major control over its variations through the Cenozoic has been changes in the isotope composition of Sr derived from the weathering of silicate rocks.

(Table 1) Cd/Ca and Mn/Ca ratios of benthic foraminifera from DSDP Hole 30-289

The late Miocene carbon shift (~6.2 Myr) -a 0.5-1.0 per mil, d13C decrease in benthic and planktonic foraminifera- has been ascribed to changes in global inventory, deep-ocean circulation, and/or productivity. Cadmium, d13C, and nutrients in the ocean are linked; comparison of d13C and Cd/Ca yields circulation and chemical inventory information not available from either alone. We determined Cd/Ca ratios in late Miocene benthic foraminifera from DSDP Site 289. Results include: (1) late Miocene Pacific Cd/Ca values fall between those of late Quaternary Atlantic and Pacific benthic foraminifera; (2) there are no systematic Cd/Ca offsets between Cibicidoides kullenbergi, Cibicidoides wuellerstorfi and Uvigerina spp.; and (3) there is a very slight Cd/Ca change coincident with d13C. Cd/Ca, slightly higher in younger, isotopically lighter samples, exhibits a smaller increase than predicted if circulation were the primary cause of the carbon shift. The carbon shift may have been due to a long-term shift in the steady-state carbon isotope input or to a change in the sedimentation of organic carbon relative to calcium carbonate.