Siliceous microplankton in surface sediments and the water column of the Red Sea

We studied the siliceous microplankton assemblages (mainly diatoms) from plankton tows (mesh size 20 µm) and surface sediment samples collected along a N-S transect in the northern Red Sea (28-21°N). In addition, we analyzed differences/similarities between plankton and sediment assemblages within a brine-filled basin (the southern basin) of the Shaban Deep and compared these assemblages with those from outside the brine. Plankton samples revealed the overwhelming dominance of diatoms over other siliceous groups. Diatoms accounted for ca. 97% of all biosiliceous particles at 120-20 m (vs. 2.9% silicoflagellates and 0.4% radiolarians), and ca. 94% at 200-120 m (vs. 4.5% silicoflagellates and 1.6% radiolarians). In general, a marine, warm-water (tropical/subtropical) diatom assemblage characterizes the plankton samples. Representatives of the Nitzschia bicapitata group are by far the most abundant contributors at both depth intervals (average=43%), ranging from ca. 30% in the North to ca. 60% in the South. Biogenic opal content in non-brine surface sediments is very low, (below 0.2 wt.% SiO2); and concentration of siliceous microorganisms is also low and of the order of 5*10**3-10**4 microorganisms/g dry sediment. Diatoms are the main contributors to the opal signal in the 20-40 µm fraction, while they share dominance with radiolarians in the >40 µm fraction. Total diatom concentrations average 1.2*10**4 valves/g in the 20-40 µm fraction and 4*10**3 valves/g in the >40 µm fraction. Robust taxa of warm water affinity (Alveus marinus, Azpeitia neocrenulata, Azpeitia nodulifera and Roperia tesselata) characterize the surface sediments. In contrast, biogenic opal content in brine surface sediment samples is much higher than in the non-brine samples, ranging from 2.8 to 3.8 wt.% SiO2, and concentration of siliceous microorganisms is 3-4 orders of magnitude higher. In addition here, diatoms dominate the opal signal. The taxa found in these samples are a mixture of non-brine and plankton samples, and fragile forms (e.g., N. bicapitata group, Neodelphineis indica) are well preserved in these sediments. Thus, brine sediments in this region seem to offer a great potential for palaeoenvironmental studies.

Terrestrial organic matter in marine sediments of the Central Equatorial Atlantic

Terrestrial organic matter (OM) in pelagic sediments is discussed with regard to depositional processes and land-sea interactions in the modern and past glacial/interglacial Equatorial Atlantic. Special emphasis is placed on a critical evaluation of different analytical approaches (C/N, Rock-Eval Pyrolysis, stable carbon isotopes, palynology, organic petrology, and selected biomarkers) which are currently used for the qualitative and quantitative assessment of terrigenous organic carbon. If binary mixing equations are used to calculate terrestrial and marine proportions of organic carbon, we consider the definition of endmember values to be most critical since these values may be biased by a great number of independent controls. A combination of geochemical methods including optical studies (organic petrology and palynology) is therefore suggested to evaluate each individual proxy. Organic geochemical analyses performed on sediments from the modern and Late Quaternary Equatorial Atlantic evidence fluctuations in eolian supply of terrigenous OM related to changes in intensity of the trade winds. Quantification of this organic fraction leads to differing proportions depending on the approach applied, i.e. the organic carbon isotopic composition or maceral analyses. Modern distribution of terrigenous OM reveals a decrease in supply towards the basin contributing less than a fifth of the total OM in pelagic areas. Organic geochemical data indicate that sedimentation in the modern northeastern Brasil Basin is affected by lateral advection of reworked OM probably from southern source areas. Glacial/interglacial deposits from the pelagic Equatorial Atlantic (ODP Site 663), covering isotopic stages 12 and 11, reveal that deposition of terrigenous OM was higher under past glacial conditions, in correspondence to generally enhanced dust fluxes. Proportions of terrigenous OM, however, never exceed 50% of the total OM according to maceral analyses. Other estimates, recently proposed by Verardo and Ruddiman (1996), are considered to be too high probably for analytical reasons. Palynological records in the Equatorial Atlantic parallel dust records. Increased portions of grass pollen suggest the admixture of C4-plant material under modern and past glacial conditions. It is therefore assumed, as one possible interpetation, that C4-plant debris has an effect on sedimentary d13Corg and might explain differences between isotopic and microscopic quantitative estimates. Using the difference between these two records, we calculate that maximum supply of C4-material remains below 20% of the total OM for the deep modern and past glacial/interglacial Equatorial Atlantic.

Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf

Based on benthic and planktic foraminifera, Bolboforma, oxygen isotope measurements and seismic data, major changes in Miocene, Pliocene and Pleistocene paleoenvironments on the mid Norwegian shelf are discussed and a possible scenario of the late Cenozoic uplift history is given. The dating of the Neogene sequence has been done using foraminifera and Bolboforma. Four main assemblage zones have been identified with nine distinct subzones. Most of the Miocene sequence is preserved. The lower Miocene sediments contain only siliceous microfossils. A period of high fertility and upwelling in the study area prevailed. The early Miocene-early mid Miocene (15 Ma?) change from a siliceous to a calcareous rich microfauna, dominated by Nonion barleeanum, can be related to increased surface-water circulation due to overflow across the Iceland-Faeroe ridge. During the Miocene the temperature decreased in the study area. Evidence of increased amounts of coarser sediments may suggest that an uplift of the mainland areas occurred during the mid-late Miocene. Lower Pliocene sediments contain a foraminiferal fauna that seems to occur in slightly colder conditions than the late Miocene fauna suggesting a further cooling. Possibly, Arctic waters entered the study area in the early Pliocene. A very marked change in lithology (from compacted claystone to unconsolidated diamicton), fauna (from deep dwelling to shallow dwelling species) and seismic signature (from flat lying reflectors to prograding clinoforms) occurs during the mid?-late Pliocene. A two step cooling trend is indicated by the microfauna of these prograding wedges. (1) The first wedge buildups might have been associated with an uplift of the mainland during the early late Pliocene (mid Pliocene, ca. 4 Ma). However, the age determination is somewhat uncertain and may very well be of late Pliocene age. (2) The second step of wedge buildup is associated with a glacial phase where the dominating microfauna exists of arctic species. Large continental ice sheets might have occurred at this time reaching coastal areas and that possibly many of the geomorphological features such as the strandflat were made during this episode. The Pleistocene epoch is represented by an increased percentage of boreal foraminifera intermingled with high arctic species which indicates that interglacial-glacial cycles prevailed and the dynamics of the glacier system changed.

Sediments in arctic sea ice - entrainment, characterization and quantification

Sediments in Arctic sea ice are important for erosion and redistribution and consequently a factor for the sediment budget of the Arctic Ocean. The processes leading to the incorporation of sediments into the ice are not understood in detail yet. In the present study, experiments on the incorporation of sediments were therefore conducted in ice tanks of The Hamburg Ship Model Basin (HSVA) in winter 1996/1997, These experiments showed that on average 75 % of the artificial sea-ice sediments were located in the brine-channel system. The sediments were scavenged from the water column by frazil ice. Sediments functioning as a nucleus for the formation of frazil ice were less important for the incorporation. Filtration in grease ice during relatively calm hydrodynamic conditions was probably an effective process to enrich sediments in the ice. Wave fields did not play an important role for the incorporation of sediments into the artificial sea ice. During the expedition TRANSDRIFT III (TDIII, October 1995), different types of natural, newly-formed sea ice (grease ice, nilas and young ice) were sampled in the inner Laptev Sea at the time of freeze-up. The incorporation of sediments took place during calm meteorological conditions then. The characteristics of the clay mineral assemblages of these sedirnents served as references for sea-ice sediments which were sampled from first-year drift ice in the outer Laptev Sea and the adjacent Arctic Ocean during the POLARSTERN expedition ARK-XI/1 (July-September 1995). Based on the clay mineral assemblages, probable incorporation areas for the sedirnents in first-year drift ice could be statistically reconstructed in the inner Laptev Sea (eastern, central, and Western Laptev Sea) as well as in adjacent regions. Comparing the amounts of particulate organic carbon (POC) in sea-ice sediments and in surface sediments from the shelves of potential incorporation areas often reveals higher values in sea-ice sediments (TDIII: 3.6 %DM; ARK-XI/1: 2.3 %DM). This enrichment of POC is probably due to the incorporation process into the sea ice, as could be deducted from maceral analysis and Rock-Eval pyrolysis. Both methods were applied in the present study to particulate organic material (POM) from sea-ice sediments for the first time. It was shown that the POM of the sea-ice sediments from the Laptev Sea and the adjacent Arctic Ocean was dominated by reworked, strongly fragmented, allochthonous (terrigenous) material. This terrigenous component accounted for more than 75 % of all counted macerals. The autochthonous (marine) component was also strongly fragmented, and higher in the sediments from newly-formed sea ice (24 % of all counted macerals) as compared to first-year drift ice (17 % of all counted macerals). Average hydroge indices confirmed this pattern and were in the transition zone between kerogen types II and III (TDIII: 275 mg KW/g POC; ARK-XI/1: 200 mg KW/g POC). The sediment loads quantified in natural sea ice (TDIII: 33.6 mg/l, ARK-XI/1: 49.0 mg/l) indicated that sea-ice sediments are an important factor for the sediment budget in the Laptev Sea. In particular during the incorporation phase in autumn and early winter, about 12 % of the sediment load imported annually by rivers into the Laptev Sea can be incorporated into sea ice and redistributed during calm meteorological conditions. Single entrainment events can incorporate about 35 % of the river input into the sea ice (ca. 9 x 10**6 t) and export it via the Transpolar Drift from the Eurasian shelf to the Fram Strait.

Mineral composition of 0.1-0.05 mm grain size fraction from the surface layer of bottom sediments in the West European province of the Atlantic Ocean

Rock material sampled from the Mir manned deep-sea submersibles and by dradges, grabs, and sediment cores over a vast area of the North Atlantic was analyzed to show that this material is of continental origin, unlike original rocks of the ocean floor. It is proved to be related to iceberg rafting during Quaternary glaciations. Independent data on distribution and composition of sandy and silty grains in sediment cores also support this relation to the recent glaciation. New criteria for identification of iceberg rock matter in pelagic sediments are presented on the base of analysis of all available data.

Distribution of species of planktic foraminifera in bottom sediments of the Morocco Basin

Two lithological-stratigraphic intervals have been distinguished in the Morocco basin by a study of lithologic composition of bottom sediments as well as of planktic and bottom foraminifera present in them. Conditions, under which they developed, are discussed.

Carbonate and terrigenous content and chemical composition of sediments in the Central Equatorial Pacific

This study addresses changes in the absolute magnitude and spatial geometry of particle flux and export production in a meridional transect across the central equatorial Pacific Ocean's upwelling system during oxygen isotope Stage 11 and Stage 12 and compares these time periods to the current Holocene interglacial system. Temporal and spatial variability in several chemical proxies of export production, and in particular the distributions of Ba, scavenged Al, and P, are studied in a suite of sediment cores gathered along a cross-equator transect at 5°S, 2°S, 0°, 2°N, and 4°N. Because this latitudinal range preserves strong gradients in biogenic particle flux in the modern equatorial Pacific Ocean, we are able to assess variations in the relative magnitude of export production as well as the meridional width of the equatorial system through the late Quaternary glacial/interglacial cycles. During interglacial oxygen isotope Stage 11 the chemical proxies each indicate lower particle flux and export production than during Stage 12. These records are consistent throughout the transect during this time period, but geographic narrowing (during the interglacial) and widening (during the glacial) of the meridional gradient also occurs. Although carbonate concentration varies dramatically through glacial/interglacial cycles at all latitudes studied, the productivity proxies record only minimal glacial/interglacial change at 5°S and 4°N, indicating that the carbonate minima at these latitudes is controlled dominantly by dissolution rather than production. The chemical data indicate that although the spatial geometry of the system during Stages 11 and 12 indicates maximum productivity at the equator during both glacial and interglacial conditions, the absolute magnitude of export production integrated from 5°S to 4°N during Stage 11 was 25-50% less than during Stage 12, and also was 25-50% less than it is now.

Benthic foraminifers in Mesozoic and Cenozoic sediments of the southwestern Atlantic

Benthic foraminiferal assemblages in Mesozoic and Cenozoic sediments were studied at Sites 511, 512, 513, and 514 drilled during Leg 71 in the southwestern Atlantic on the Maurice Ewing Bank and in the Argentine Basin. Benthic foraminifers in almost all stratigraphic subdivisions of Sites 511 and 512 reflect the gradual subsidence of the Falkland Plateau from shelf depths in the Barremian-Albian, when a semiclosed basin with restricted circulation of water masses and anaerobic conditions existed, to lower bathyal depths in the Late Cretaceous and Cenozoic, with an abrupt acceleration at the boundary of Lower and Upper Cretaceous. The composition, distribution, and preservation of Late Cretaceous assemblages of benthic foraminifers suggest considerable fluctuations of the foraminiferal lysocline and the CCD. This is evidenced by dissolution facies and foraminiferal assemblages in which agglutinated and resistant calcareous forms predominated during high stands of the CCD and by calcareous facies in which rich assemblages of calcareous species predominated during low stands. The highest position of the CCD on the Plateau (less than 1500-2000 m) was in the late Cenomanian, Turonian, and Coniacian. In the Santonian and Campanian the CCD was at depths below 1500-2000 meters. At the end of the Campanian the CCD shifted again to depths comparable with those of Cenomanian and Turonian time. In the latest Campanian and the Maestrichtian the CCD was low and nanno-foraminiferal oozes with a rich assemblage of benthic foraminifers accumulated. Foraminiferal assemblages at Sites 513 and 514 in the Argentine Basin also testify to oceanic subsidence from lower bathyal depths in the Oligocene to abyssal ones at present. This process was complicated by the influence of geographical migrations of the Polar Front caused by extensions of the ice sheet in the Antarctic after the opening of the Drake Passage during the Oligocene. In Mesozoic and Cenozoic deposits of the Falkland Plateau and the Argentine Basin seven assemblages of benthic foraminifers were distinguished by age: early-middle Albian, middle-late Albian, Late Cretaceous (including four groups), middle Eocene, late Eocene-early Miocene, middle-late Miocene, and Pliocene-Quaternary. The Albian assemblages contain many species common to the foraminiferal fauna of the Austral Biogeographical Province. The Late Cretaceous assemblage contains, along with Austral species, species common to foraminifers of North America, Western Europe, the Russian platform, and the south of the U.S.S.R. Deep-sea cosmopolitan species prevail in Cenozoic assemblages.

Cenozoic sediments facies and geochemistry of ODP Hole 103-637A

The late Cenozoic deposits recovered at ODP Site 637 from the Iberian Abyssal Plain near the continental margin off northwestern Spain include three main facies groups. Turbidites are the dominant facies association (two-thirds of the total thickness), followed by pelagites (one-fourth), and subordinate amounts of contourites (one-tenth). Slump deposits occur locally in the upper Miocene and middle Pliocene. Turbidity currents and pelagic settling were the significant sediment depositional processes from the Pliocene to the Pleistocene, whereas bottom currents predominated during the late Miocene. Fine-grained, base-cut-out turbidites, normally starting with the Td division, are the most abundant sequence type. The pelagites include both carbonate-rich pelagic and hemipelagic facies. The two types of contourites, sandy and calcareous-rich or fine-grained terrigenous, record two types of bottom-current processes. The Cenozoic deposits at Site 637 show a general upward transition from contourites in the upper Miocene to turbidites in the Pliocene-Quaternary. The entire section is rhythmically bedded and has a poorly developed cyclic pattern defined by variations in the total carbonate content. The low sedimentation rates also show the same cyclicity, with lower values for the late Miocene and late Pliocene. This evolution reflects the predominant depositional processes and the dissolution of carbonates by a lower CCD during the late Miocene.

Calcareous nannofossil stratigraphy of sediments from ODP Holes 186-1150A and 186-1151C, Japan Trench

To assess the paleoceanographic potential of Leg 186 sediments, we investigated Quaternary calcareous nannofossil flora at Sites 1150 and 1151 in the Japan Trench. Because of the frequent occurrence of barren intervals and the lack of oxygen isotope data, a detailed paleoceanography is not feasible for these cores. We limited our study to the upper 26.07 m of the section from Hole 1150A and the upper 21.01 m of the section from Hole 1151C. The studied samples from Cores 186-1150A-1H through 3H are younger than 0.085 Ma. Core 186-1151C-1H (upper 1.92 meters below seafloor [mbsf]) is younger than 0.085 Ma, and samples between 2H-7, 5-7 cm, and 3H-CC, 5-7 cm, (9.99-21.01 mbsf) are older than 0.245 Ma and younger than 0.408 Ma.