Ostracoda fauna of surface sediments from the Persian Gulf

During the "Meteor"-Expedition to the Persian Gulf in March-May 1965, approximately 300 samples were collected. Most of them have been already studied by various authors in sedimentological as well as micropaleontological respects. 49 samples were selected for ostracode studies. These samples are arranged to form a long-axis section ("Laengsprofil"), and 4 shorter cross-profiles, perpendicular to the long-axis profile in the Persian Gulf and Gulf of Oman. 52 species of ostracodes in this area were specifically determined; 39 of them are described under open nomenclature. 13 species are already known from surrounding sea areas: 2 species from the Red Sea; 2 species from the east coast of Africa; 1 species from the Mediterranean Sea; and others from the Indian and Pacific Oceans. 12 species show close relationships to species from the Indopacific Ocean. The ostracode species found in the area can be grouped after the method of BRAUN-BLANQUT into 2 bioassociations. Association 1 with the following 4 characteristic species : Cytherella cf. pulchra, Loxoconcha sp. A, Neomonoceratina sp. A, Alocopocythere reticulata. Association 2 with 1 characteristic species: Ruggieria (Ruggieria) sp. B. The association 1 is widespread in the entire studied area of the Persian Gulf, where it is considered to characterize the shallow water region down to 200 m. The association 2 is restricted to the deeper water below 200 m of the inner part of the Oman Gulf. Only a few species known from the shallow water association of the Persian Gulf are present. Within the two ostracode associations mentioned above 4 zones from the total studied area could be related to the water depth. The zones A-D are characterized more or less readily by the relative abundance of certain species: Zone A : 7-30 m depth, on substrates of poorly coarse-grained clayey marl; Zone B: 30-94 m depth, on substrates of richly coarse-grained calcareous marl; Zone C: 94-1961208 m depth, on substrates of richly coarse-grained calcareous marl; Zone D: 196/208-500 m depth, on substrates of calcareous clay, poor in benthos. The regional and bathymetric distribution of the ostracode fauna in the area studied was compared in relation to 10 environmental factors: water depth, temperature, salinity, water density, O2-concentration, phosphate-silica contents, pH-values, stratification of the water body, water currents and type of sediments. The major environmental factors which appear to control the ostracode distribution are water depth (as a complex factor), O2-concentration and the type of sediment. At 3 stations (GIK01058, GIK01074 and GIK01204) species of the shallow water association were found together with a few bathyal species. These stations are situated at the outer Biaban shelf, in an area where the bottom water of the Persian Gulf flows down the slope towards the Oman Gulf. Several samples of the Zone B in the major part of the Persian Gulf show also a high species diversity containing a high percentage of subfossil ostracode carapaces. It is probable that the recent biocoenosis has been mixed with a late quarternary thanatocoenosis.

Geologic-hydrologic setting and surface sedimentology in the Persian Gulf

1. Morphology and sedimentation The deepest parts of the Persian Gulf lie off the Iranian coast. Several swells separate the Persian Gulf into the Western Basin, the Central Basin and the Strait of Hormuz, which leads without noticeable morphological interruption onto the Biaban Shelf; the latter gradually drops off towards the continental slope, which itself has a strongly subdivided morphology. The sediment distribution in the Western Basin runs parallel to the basin's axis to a depth of 50 -60 m. This is caused by the shallow and uniform slope of the Iranian coast into the Western Basin, by clear exposure of the area to the Shamal-Winds and by tidal currents parallel to the basin's axis. Most other parameters also show isolines parallel to the coast line. Data from the sediment analyses show a net transport which extends out along the Central Swell: coarse fraction > 63 µ, total carbonate content, carbonate in fine fractions < 2 µ, 2-6 µ and 20-63 µ, calcite-aragonite ratios in the fine fractions 2-6 µ and 20-63 µ and quartz-dolomite ratios in fine fraction 2-6 µ. At least the uppermost 10-40 m of this sediment is late Holocene. This implies sedimentation rates of several meters per 1000 years. The slope from the Iranian coast into the Central Basin (max. depth 100 m) is generally steeper, with interspersed islands and flats. Both facts tend to disturb a sediment dustribition parallel to the basin's axis over extensive areas and may preclude any such trend from being detected by the methods and sample net used. The spatial distribution of the coarse fraction, however, seems to indicate sediment transport at greater water depths perpendicular to the basin's long axis and along the steepest gradients well into the Central Basin. The flats of the Central Basin have a sediment cover distinctly different from those of the deeper basin areas. Characteristic parameters are the extremely high percentages of coarse grained sediments, total content of carbonate CO2 over 40, low total organic carbon content, (however values are high if calculated on the basis of the < 63 µ fraction), low total N-content, and low C/N ratios. These characteristics probably result from the absence of any terrigenous material being brought in as well as from exposure to wave action. Finest terrigenous material is deposited in the innermost protected part of the Hormuz Bay. In the deep channel cut into the Biaban Shelf which carries the Persian Gulf out-flow water to the Indian Ocean, no fine grained sediment is deposited as shown by grain size data. 2. Geographic settings and sedimentation Flat lands border the Arabian coast of the Persian Gulf except for the Oman region. The high and steep Zagros Mountains form the Iranian coastline. Flat topography in combination with generally low precipitation precludes fluviatile sediment being added from the South. Inorganic and biogenic carbonates accumulating under low sedimentation rates are dominant on the shallow Arabic Shelf and the slopes into the Western and Central Basins. The fluviatile sediment brought in from the Iranian side, however decisively determine the composition of the Holocene sediment cover in the Persian Gulf and on the Biaban Shelf. Holocene sediments extend 20-30 km seaward into the Western Basin and about 25 km on to the Biaban Shelf. As mentioned before, sedimentation rates are of several meters/1000 years. The rocks exposed in the hinterland influence the sediments. According to our data the Redbeds of the Zagros Mountains determine the colour of the very fine grained sediments near the Iranian Coast of the Persian Gulf. To the West of Hormuz, addition of carbonate minerals is particularly high. Dolomite and protodolomite, deposited only in this area, as well as palygorskite, have proven to be excellent trace minerals. To the East of Hormuz, the supply of terrigenous carbonates is considerably lower. Clay minerals appear to bring in inorganically bound nitrogen thus lowering the C/N ratio in these sediments especially off river mouths. 3. Climate and sedimentation The Persian Gulf is located in a climatically arid region. This directly affects sedimentation through increased wind action and the infrequent but heavy rainfalls which cause flash floods. Such flash floods could be responsible for transporting sedheats into the Central Basin in a direction perpendicular to the Gulf's axis. Eolian influx is difficult to asses from our data; however, it probably is of minor importance from the Iranian side and may add, at the most, a few centimeters of fine sediment per 1000 years. 4. Hydrology and sedimentation High water temperatures favor inorganic carbonate precipitation in southern margin of the Gulf, and probably on the flats, as well as biogenic carbonate production in general. High evaporation plus low water inflow through rivers and precipitation cause a circulation pattern that is typical for epicontinental seas within the arid climate region. Surface water flows in from the adjoining ocean, in this case the Indian Ocean and sinks to the bottom of the Persian Gulf mainly in the northern part of the Western Basin, on the "Mesopotamischer Flachschelf" ard probably in the area of the "Arabischer Flachschelf". This sinking water continually rejuvenates the bottom out-flow water. The inflowing surface water from the Indian Ocean brings organic matter into the Persian Gulf, additional nutrients are added by the "fresh" upwelling waters of the Gulf of Oman. Both nutrients and organic matter diminish very rapidly as the water moves into the Persian Gulf. This depletion of nutrients and organic matter is the reasonfor generally low organic carbon contents of the Persian Gulf sediments. The Central Swell represents a distinct boundary, to the west of which the organic carbon content are lower than to the east when sediment samples of similar grain size distribution are compared. The outflow carries well oxygenated water over the bottom of the Persian Gulf and the resulting oxidation further decreases the content of organic matter. In the Masandam-Channel and in the Biaban-Shelf channel, the outflowing water prevents deposition of fine material and transports sediment particles well beyond the shelf margin. The outflowing water remains at a depth of 200-300 m depending on its density and releases ist suspending sediment load to the ocean floor, irrespectative of the bottom morphology. This is reflected in several parameters in which the sediments from beneath the outflow differ from nearby sediments not affected by the outflowing water. High carbonate content of total samples and of the individual size fraction as well as high aragonite and dolomite contents of individual size fractions characterize the sediment beneath the outflowing water. The tidal currents, which avt more or less parallel to the Gulf's axis, favor mixing of the water masses, they rework sediments at velocities reported here. This fact enlarges to a certain degree the extent of our interfaces which are based on only a few sample points (Persian Gulf and Biaban Shelf one sample per 620 km**2, continental slope one sample per 1000 km**2). The water on the continental slope shows and oxygen minimum at 200-1200 m which favors preservation of organically-bound carbon in the sediment. The low pH-values may even permit dissolution of carbonate minerals.

The Global Plankton Database: an inventory and data from the Former Soviet Union expeditions

At present time, there is a lack of knowledge on the interannual climate-related variability of zooplankton communities of the tropical Atlantic, central Mediterranean Sea, Caspian Sea, and Aral Sea, due to the absence of appropriate databases. In the mid latitudes, the North Atlantic Oscillation (NAO) is the dominant mode of atmospheric fluctuations over eastern North America, the northern Atlantic Ocean and Europe. Therefore, one of the issues that need to be addressed through data synthesis is the evaluation of interannual patterns in species abundance and species diversity over these regions in regard to the NAO. The database has been used to investigate the ecological role of the NAO in interannual variations of mesozooplankton abundance and biomass along the zonal array of the NAO influence. Basic approach to the proposed research involved: (1) development of co-operation between experts and data holders in Ukraine, Russia, Kazakhstan, Azerbaijan, UK, and USA to rescue and compile the oceanographic data sets and release them on CD-ROM, (2) organization and compilation of a database based on FSU cruises to the above regions, (3) analysis of the basin-scale interannual variability of the zooplankton species abundance, biomass, and species diversity.

Layer-by-layer correlation of ODP Leg 117 sites

One hundred thirty-one marker horizons relating to the distinct and traceable layers were described for the Owen Ridge and Oman Margin sites. The correlations incorporated the calculations of true depth, corrected for coring disturbance and gas expansion. Intersite correlation of marker horizons has been improved based on color density data, measured with video densitometer, and oxygen isotope stratigraphic data. Distinct hiatuses were detected by the intersite correlation of the marker horizons in the Owen Ridge. The hiatuses are related to submarine slides induced by increasing gravitational instability for the accumulation of the pelagic sediments on the top of the Owen Ridge. The large amount of sediment supply with variable lithofacies during the glacial stages is represented by layer-bylayer correlation in the Oman Margin. The color density patterns with glacial-interglacial cycles are controlled by the balance of organic carbon content, increasing in the interglacial stages with strong upwelling induced by the southwest monsoon, and flux of terrigenous matter, increasing in the glacial stages. The present distinct climatic cycle relating to the southwest monsoon has been developed since Stage 8, 250 ka. The large amount of sediment supply in the glacial stages can be assumed as fluvial in origin from the humid Arabian Peninsula, relating to the weakened Tropical Easterly Jet, which is induced by the counter-current of the southwest monsoon and maintains the present arid climate in the north Africa and Arabian Peninsula.

(Table 1) Adenosine triphosphate, urea, dissolved organic carbon, nitrogen and methane concentrations in bottom water of the northwestern Indian Ocean

Concentrations of adenosine triphosphate (ATP), urea, and dissolved organic carbon in bottom water are shown to be considerable, sometimes several times higher than in the photic and surface layers of the ocean. Urea and ATP concentrations are inversely proportional. Identified biochemical characteristics of bottom water are of great importance in determining the status of the aquatic environment. The highest life activity (maximum ATP content) in bottom water appeared in the vicinity of faults in rift zones of the ocean, where high gas concentrations were also found. Population of chemoautotrophic microorganisms was clearly present under these conditions. Biochemical investigations provide additional criteria for identifying oil and gas prospects. They are also of definite interest in combination with gasometric determinations, which will undoubtedly give us deeper understanding of processes of formation of oil and gas and will help in finding them.

Amino acids from ODP Holes 117-722B and 177-724C

Sedimentary d15N records are valuable archives of ocean history but they are often modified during early diagenesis. Here we quantify the effect of early diagenetic enrichment on sedimentary N-isotope composition in order to obtain the pristine signal of reactive N assimilated in the euphotic zone. This is possible by using paired data of d15N and amino acid composition of sediment samples, which can be applied to estimate the degree of organic matter degradation. We determined d15N and amino acid composition in coeval sediments from Ocean Drilling Program (ODP) Hole 772 B in the central Arabian Sea and from Hole 724 C situated on the Oman Margin in the western Arabian Sea coastal upwelling area. The records span the last 130 kyr and include two glacial-interglacial cycles. These new data are used in conjunction with data available for surface sediments that cover a wide range of organic matter degradation states, and with other cores from the northern and eastern Arabian Sea to explore spatial variations in the isotopic signal. In order to reconstruct pristine N values we apply the relationship between organic matter degradation and 15N enrichment in surface sediments to correct the core records for early diagenetic enrichment. Reconstructed d15N values suggest a significant role of N2-fixation during glacial stages. An evaluation of two preservation indices based on amino acid composition (Reactivity Index, RI; Jennerjahn and Ittekkot, 1997; and the Degradation Index, DI; Dauwe et al., 1999) in both recent sediments and core samples suggests that the RI is more suitable than the DI in correcting Arabian Sea d15N records for early diagenetic enrichment.

Alkenone variations, total organic carbon and stable oxygen isotope ratios of the western Arabian Sea

We here present records of total organic carbon (TOC) and C37 alkenones, used as indicators for past primary productivity, from the western (WAS) and eastern Arabian Sea (EAS). New data from an open ocean site of the WAS upwelling area are compared with similar records from Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the EAS. These records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing. On the basis of their phase relationship to boreal summer insolation they can be divided into three groups: in the WAS differences between monsoonal proxies (1) and productivity (2) document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds, whereas in the EAS phasing indicates maximum productivity (3) at times of stronger NE monsoon winds associated with precession-related maxima in ice volume.

Planktonic foraminifera in the Arabian Sea

Variations in primary productivity (PP) have been reconstructed in eutrophic, mesotrophic and oligotrophic parts of the Arabian Sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages. Temporal variation in paleoproductivity is most pronounced in the mesotrophic northern (NAST site) and oligotrophic eastern (EAST site) Arabian Sea, and comparatively weak in the western eutrophic GeoB 3011-1 site in the upwelling area off Oman. Higher PP during interglacials (250-320 g C/m**2 year) than during cold stages (210-270 g C/m**2 year) at GeoB 3011-1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities. At NAST, during interglacials, PP is estimated to exceed g C/m**2 year 1, and during glacials to be as low as 140-180 g C/m**2 year. These fluctuations may result from a (1) varying impact of filaments that are associated to the Oman coastal upwelling, and (2) from open-ocean upwelling associated to the Findlater Jet. At EAST, highest productivity of about 380 g C/m**2 year is documented for the transition from isotope stage 5 to 4. We suggest that during isotope stages 2, 4, 5.2, the transition 5/4, and the end of stage 6, deep mixing of surface waters was caused by moderate to strong winter monsoons, and induced an injection of nutrients into the euphotic layer leading to enhanced primary production. The deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep-dwelling planktic foraminiferal species. A high-productivity event in stage 3, displayed by estimated PP values, and by planktic foraminifera and radiolaria flux and accumulation rate, likely resulted from a combination of intensified SW monsoons with moderate to strong NE monsoons. Differential response of Globigerina bulloides, Globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale.

Stable isotope ratios and organic carbon accumulation rates of late Pleistocene sediments of ODP Hole 117-724C

Stable isotope records of coexisting benthic foraminifers Uvigerina spp. and Cibicidoides spp. and planktonic G. ruber (white variety) from Site 724 are used to study the late Pleistocene evolution of surface and intermediate water hydrography (593 m water depth) at the Oman Margin. Glacial-interglacial d18O amplitudes recorded by the benthic foraminifers are reduced when compared to the estimated mean ocean changes of d18Oseawater . Epibenthic d13C remains at its modern level or is increased during glacial times. This implies that Red Sea outflow waters which are enriched in d18Oseawater and d13C (Sum CO2) have been replaced during glacial periods by intermediate waters still positive in d13C (Sum CO2) but more negative in d18Oseawater. Glacial-interglacial amplitudes of the planktonic d18O record exceed those of the mean ocean d18Oseawater variation and imply decreased surface water temperatures (SST) during glacial times. Throughout most of the records these cooling events correlate with enhanced rates of carbon accumulation. However, both negative (colder) SST and positive Corg accumulation rate anomalies do not correlate with potential physical upwelling maxima as inferred from the orbital monsoon index. This is in conflict with the established hypothesis that upwelling in the estern Arabia Sea should be strongest during maxima of the southwest monsoon.

Magnetostratigraphy of ODP Leg 117 sites

Magnetic measurements were made on discrete samples from the Neogene pelagic and hemipelagic sediments recovered during ODP Leg 117. Polarity zones, usually identified for uppermost 200 m, were correlated to the geomagnetic polarity time scale referring to biostratigraphic datums. Quality and resolution of the magnetostratigraphy was partly limited by the weak and relatively soft magnetic character of almost all intervals, and core disturbance by gas expansion at some Oman Margin sites. Clear polarity records of the Brunhes and Matuyama chrons (C1 to C2r) were observed at Sites 724 and 727 on the Oman continental margin. Extended reversal records of the Pliocene were found at Site 722 on the Owen Ridge and Site 728 on the Oman Margin, and correlated to the Gauss to Gilbert chrons (C2A to C3).