Investigation of sediment core PS2556 from Bellingshausen Sea, Antarctica

Die Rekonstruktion der glaziomarinen Sedimentationsprozesse am antarktischen Kontinentalrand des westlichen Bellingshausenmeeres erfolgte durch die sedimentologische Auswertung eines 962 cm langen Schwerelotkernes aus 3594 m Wassertiefe. Der Kern wurde während des Fahrtabschnittes ANT-XI/3 mit dem FS "Polarstern" vom Scheitel einer Sediment- "Drift" gezogen. An dem Sedimentkern wurde eine lithologische Beschreibung, sowie sedimentologische Untersuchungen und sedimentphysikalische Messungen durchgeführt. Anhand der Ergebnisse konnten signifikante Änderungen in der Zusammensetzung und Struktur der Sedimente erkannt, und drei Faziestypen unterschieden werden. Die Faziestypen charakterisieren jeweils glaziale oder interglaziale Zeiträume. Der größte Teil der Sedimentabfolge gehört der Laminitfazies an. Dabei handelt es sich um feinlaminierte Sedimentabschnitte, die vorwiegend aus feinkörnigen, terrigenen Komponenten zusammengesetzt sind. In die feinlaminierten Abschnitte sind vereinzelte, wenige Milimeter bis Zentimeter mächtige Siltlagen eingeschaltet. Die biogenen Anteile sind gering, Anzeichen für Bodenleben fehlen völlig. Die Manganfazies wird von authigen gebildeten Mangankonkretionen dominiert, die jeweils diskrete Lagen bilden. Dabei handelt es sich zum einen um Mikromanganknollen und -krusten und zum andern um manganhaltige Gangfüllungen. Biogene und terrigene Anteile sind in diesem Faziestyp unbedeutend. Die Biogenfazies ist von strukturlosen und stark bioturbierten Sedimenten gekennzeichnet. In diesen Sedimentabschnitten ist der hohe Anteil an Eisfracht (IRD) und die erhöhten Gehalte an Kalziumkarbonat und Opal in der Sandfraktion markant. Die stratigraphische Einordnung des Sedimentkernes erfolgte über die von Grobe & Mackensen (1992) entwickelte Lithostratigraphie, mit deren Einheiten die Faziestypen des Sedimentkernes korreliert werden konnten. Dabei ergaben sich zwei mögliche Altersmodelle und ein Basisalter von ca. 250.000 Jahren. Anhand der stratigraphischen Fixpunkte wurden Sedimentationsraten des Gesamtsedimentes und Akkumulationsraten des Kalziumkarbonates, des Biogenopals und des organisch gebundenen Kohlenstoffes berechnet. Dabei wurde gezeigt, daß lediglich das Kalziumkarbonat und der Biogenopal als Anzeiger für biologische Produktion dienen können, wobei Lösungsprozesse in der Wassersäule und im Sediment eine große Rolle spielen. Der Gehalt an organisch gebundenem Kohlenstoff ist in dem Sedimentkern nur erhaltungsbedingt zu erklären. Die Sedimentationsprozesse der einzelnen Faziestypen sind von den Eisverhältnissen, der biologischen Produktion, dem gravitativen Transport und der Umlagerung durch Meeresströmungen abhängig. Die Auswirkung der einzelnen Faktoren ist jeweils unterschiedlich ausgeprägt und wirkt sich spezifisch auf die einzelnen Parameter aus. In den Glazialen hatte ein Vorstoß des Schelfeises über die Schelfkante zur Anlieferung großer Sedimentmassen geführt, die über gravitativen Transport den Kontinentalhang hinunter transportiert wurden. Die Feinfracht wurde über parallel zum Kontinentalhang laufende Konturströme westwärts transportiert und in der Larninitfazies der Driftkörper abgelagert. Am Ende der Glaziale kam es zur Sedimentation der Manganfazies. Die geringen Sedimentationsraten am Kamm der Sedimentdrift kamen aufgrund reduzierter Intensität der Konturströme und fehlender Umlagerung von Schelfsedimenten in Folge rückschreitender Schelfeisrnassen zustande. In den Interglazialen kam es durch den aufsteigenden Meeresspiegel zum Aufschwimmen des Schelfeises. Der damit verbundene Abbau der Eisrnassen über dem Schelf, hatte eine hohe Sedimentation von IRD zur Folge. Mit fortschreitendem Interglazial kam es in Zeiten nur saisonaler Meereisbedeckung zu verstärkter biologischer Produktion und zur Sedimentation biogenen Materials.

Quarternary tephra layers in sediment cores from the Bellingshausen Sea and Scotia Sea

The Southern Ocean is a region of the world's ocean which is fundamental to the generation of cold deep ocean water which drives the global therrno-haline circulation. Previous investigations of deep-sea sediments south of the Polar Front have been significantly constrained by the lack of a suitable correlation and dating technique. In this study, deep-sea sediment cores from the Bellingshausen, Scotia and Weddell seas have been investigated for the presence of tephra layers. The major oxide and trace element composition of glass shards have been used to correlate tephra isochrons over distances in excess of 600 km. The source volcanoes for individual tephra layers have been identified. Atmospheric transport distances greater than 1500 km for >32 pm shards are reported. One megascopic tephra is identified and correlated across 7 sediment drifts on the continental rise in the Bellingshausen Sea. Its occurrence in a sedimentary unit that has been biostratigraphically dated to delta 18O substage 5e identifies it as a key regional marker horizon for that stage. An unusual bimodal megascopic ash layer erupted from Deception Island, South Shetland Islands, has been correlated between 6 sediment cores which form a 600 km NE-SW transect from the central Scotia Sea to Jane Basin. This megascopic ash layer has been 14C dated at c. 10,670 years BP. It represents the last significant input of tephra into the Scotia Sea or Jane Basin from that volcano and forms an important early Holocene marker horizon for the region. Five disseminated tephras can be correlated to varying extents across the central Scotia Sea cores. Together with the megascopic tephra they form a tephrostratigraphic framework that will greatly aid palaeoclimatic, palaeoenvironrnental and palaeoceanographic investigations in the region.

(Tables 1-3) Silicon dioxide, Corg and trace element concentrations in sediments of the outer Eckernfoerder Bay, Western Baltic Sea

The main question, posed in the work scheme before laboratory analysis was started, was expressed as follows: Do marked seasonal fluctuations occur in trace element content of the sediment surface, and what are the probable influences of factors such as changing hydrographical parameters, plankton sequence etc. ? Special attention was paid to elements known as pollutants, for example mercury. Within this framework samples have been analysed for their contents of manganese, iron, zinc, lead, and mercury. The amounts of silica and organically-bound carbon serve in most cases as reference values for the trace element content. On sand temporary conditions of increased C org content raise the concentrations of all determined elements. Especially the values reached for mercury in July are worth nothing. It is concluded that Zn, Pb, and Hg tend to enrich with respect to C org as the decomposition of organic matter progresses. On mud-sand flocculation and precipitation of Mn/Fe-hydroxides probably represent an additional concentrating factor for the other elements as the relationship of the results for zinc and manganese shows. Manganese may indicate a seasonally related concentrating cycle at the sediment surface.

Organic carbon isotope ratios of sea surface sediments and sea-ice from the Amerasian Continental shelf

Organic matter origins are inferred from carbon isotope ratios (delta13C) in recent continental shelf sediments and major rivers from 465 locations from the north Bering-Chukchi-East Siberian-Beaufort Sea, Arctic Amerasia. Generally, there is a cross-shelf increase in delta13C, which is due to progressive increased contribution seaward of marine-derived organic carbon to surface sediments. This conclusion is supported by the correlations between sediment delta13C, OC/N, and delta15N. The sources of total organic carbon (TOC) to the Amerasian margin sediments are primarily from marine water-column phytoplankton and terrigenous C3 plants constituted of tundra taiga and angiosperms. In contrast to more temperate regions, the source of TOC from terrigenous C4 and CAM plants to the study area is probably insignificant because these plants do not exist in the northern high latitudes. The input of carbon to the northern Alaskan shelf sediments from nearshore kelp community (Laminaria solidungula) is generally insignificant as indicated by the absence of high sediment delta13C values (-16.5 to -13.6 per mil) which are typical of the macrophytes. Our study suggests that the isotopic composition of sediment TOC has potential application in reconstructing temporal changes in delivery and accumulation of organic matter resulting from glacial-interglacial changes in sea level and environments. Furthermore, recycling and advection of the extensive deposits of terrestrially derived organic matter from land, or the wide Amerasian margin, could be a mechanism for elevating total CO2 and pCO2 in the Arctic Basin halocline.

Pleistocene sedimentology of ODP Hole 133-820A from the northeastern Australian margin

A principal objective at Site 820, situated on the outer shelf, upper slope of the northeastern Australian continental margin, was to test the relationships between changes in Pleistocene sea level and sedimentary packages produced on a mixed carbonate-siliciclastic continental margin. To this end, we have examined the downcore distribution of grain size, magnetic susceptibility, and calcium-carbonate content throughout Hole 820A and, in particular, the top 35 meters below the seafloor (mbsf). These data are compared with variations in the oxygen-isotope signal defined for the same hole and are interpreted as indicating sea-level oscillations. The distribution of sand, mud, calcium carbonate of the mud fraction and total sample, and magnetic susceptibility during the last 20,000 yr defines the position of a sea-level regression (41,000-18,000 yr B.P.), a lowstand, early (18,000-9,400 yr B.P.) and late transgressions (9400-900 yr B.P.), and a highstand (4900 yr to the present). The regression is seen first in a high-carbonate content peak. Calcium carbonate constituents mainly comprise skeletal carbonate grains, with abundant planktonic and benthic foraminifers, and lime muds. The lowstand is characterized by a maximum abundance of the sand fraction, which contains dominantly skeletal carbonate grains and a minor abundance of lithoclasts. Sand-sized terrigenous sediments are proposed to have bypassed the continental shelf during a lowstand of sea level. Sedimentation rates throughout the regression and lowstand are low (3.0 cm/k.y.). The early transgression, marked by highest values in magnetic susceptibility, displays a rapid increase in sedimentation rate that coincided with an increase in terrigenous mud. Highest sedimentation rates of 82.3 cm/k.y. occurred during the late transgression, with increasing percentages of lime-mud. A decrease in noncarbonate constituents in the mud fraction during the late transgression and highstand of sea level is thought to be the result of restricted inner-shelf sedimentation of terrigenous sediments. The same relationship is also seen in the major sea-level oscillation, which is interpreted as isotope stage 6.

Sea-surface reconstruction of the east-equatorial South Atlantic

In order to reconstruct Late Quatemary variations of surface oceanography in the eastequatorial South Atlantic, time series of sea-surface temperatures (SST) and paleoproductivity were established from cores recovered in the Guinea and Angola Basins, and at the Walvis Ridge. These records, based on sedimentary alkenone and organic carbon concentrations, reveal that during the last 350,000 years surface circulation and productivity changes in the east-equatorial South Atlantic were highiy sensitive to climate forcing at 23- and 100-kyr periodicities. Covarying SST and paleoproductivity changes at the equator and at the Walvis Ridge appear to be driven by variations in zonal trade-wind intensity, which forces intensification or reduction of coastal and equatorial upwelling, as well as enhanced Benguela cold water advection from the South. Phase relationships of precessional variations in the paleoproductivity and SST records from the distinct sites were evaluated with respect to boreal summer insolation over Africa, movements of southem ocean thermal fronts, and changes in global ice volume. The 23-kyr phasing implies a sensitivity of eastem South Atlantic surface water advection and upwelling to West African monsoon intensity and to changes in the position ofthe subtropical high pressure cell over the South Atlantic, both phenomena which modulate zonal strength of southeasterly trades. SST and productivity changes north of 20°S lack significant variance at the 41-kyr periodicity; and at the Walvis Ridge and the equator lead changes in ice volume. This may indicate that obliquity-driven clirnate change, characteristic for northem high latitudes, e.g fluctuations in continental ice masses, did not substantially influence subtropical and tropical surface circulation in the South Atlantic. At the 23-kyr cycle SST and productivity changes in the eastern Angola Basin lag those in the equatorial Atlantic and at the Walvis Ridge by about 3500 years. This lag is explained by variations in cross-equatorial surface water transport and west-east countercurrent retum flow modifying precessional variations of SST and productivity in the eastem Angola Basin relative to those in the mid South Atlantic area under the central field of zonal trade winds. Sea level-related shifts of upwelling cells in phase with global clirnate change may be also recorded in SST and productivity variability along the continental margin off Southwest Africa. They may account for the delay of the paleoceanogreaphic signal from continental margin sites with respect to that from the pelagic sites at the equator and the Walvis Ridge.

Heat flow in the Central Basin of the Indian Ocean and the northern part of the Afanasy Nikitin Rise

Heat flux data obtained during Cruise 20 of R/V Akademik Mstislav Keldysh in the Central Basin of the Indian Ocean and northern part of the Afanasy Nikitin Rise are presented. Thermal conditions on the rise are not associated with an anomalous zone of the large tectonic deformation block north of it. Geothermal data indicate that the Afanasy Nikitin Rise has formed near an ancient spreading axis. Distribution of measured heat flux values indicates an additional source of heat in the Central Basin resulting from dissipative heating of the crust in the two-stage plate tectonics model.

Sea surface fugacity of carbon dioxide measurements in the Pacific ocean

As part of the JGOFS field program, extensive CO2 partial-pressure measurements were made in the atmosphere and in the surface waters of the equatorial Pacific from 1992 to 1999. For the first time, we are able to determine how processes occurring in the western portion of the equatorial Pacific impact the sea-air fluxes of CO2 in the central and eastern regions. These 8 years of data are compared with the decade of the 1980s. Over this period, surface-water pCO2 data indicate significant seasonal and interannual variations. The largest decreases in fluxes were associated with the 1991-94 and 1997-98 El Niño events. The lower sea-air CO2 fluxes during these two El Niño periods were the result of the combined effects of interconnected large-scale and locally forced physical processes: (1) development of a low-salinity surface cap as part of the formation of the warm pool in the western and central equatorial Pacific, (2) deepening of the thermocline by propagating Kelvin waves in the eastern Pacific, and (3) the weakening of the winds in the eastern half of the basin. These processes serve to reduce pCO2 values in the central and eastern equatorial Pacific towards near-equilibrium values at the height of the warm phase of ENSO. In the western equatorial Pacific there is a small but significant increase in seawater pCO2 during strong El Niño events (i.e., 1982-83 and 1997-98) and little or no change during weak El Niño events (1991-94). The net effect of these interannual variations is a lower-than-normal CO2 flux to the atmosphere from the equatorial Pacific during El Niño. The annual average fluxes indicate that during strong El Niños the release to the atmosphere is 0.2-0.4 Pg C/yr compared to 0.8-1.0 Pg C/yr during non-El Niño years.

Abiotic characteristics and diatom abundance and biomass in different sectors of the Argentinien Shelf and antartic waters

A large spatial scale study of the diatom species inhabiting waters from the subantarctic (Argentine shelf) to antarctic was made for the first time in order to understand the relationships between these two regions with regard to the fluctuations in diatom abundances in relation with environmental features, their floristic associations and the effect of the Polar Front as a biogeographic barrier. Species-specific diatom abundance, nutrient and chlorophyll-a concentration were assessed from 64 subsurface oceanographic stations carried out during the austral summer 2002, a period characterized by an anomalous sea-ice coverage corresponding to a ''warm year". Significant relationships of both diatom density and biomass with chlorophyll-a (positive) and water temperature (negative) were found for the study area as a whole. Within the Subantarctic region, diatom density and biomass values were more uniform and significantly (in average: 35 and 11 times) lower than those of the Antarctic region, and did not correlate with chlorophyll-a. In antarctic waters, instead, biomass was directly related with chlorophyll-a, thus confirming the important contribution of diatoms to the Antarctic phytoplanktonic stock. A total of 167 taxa were recorded for the entire study area, with Chaetoceros and Thalassiosira being the best represented genera. Species richness was maximum in subantarctic waters (46; Argentine shelf) and minimum in the Antarctic region (21; Antarctic Peninsula), and showed a significant decrease with latitude. Floristic associations were examined both qualitatively (Jaccard Index) and quantitatively (correlation) by cluster analyses and results allowed differentiating a similar number of associations (12 vs. 13, respectively) and two main groups of stations. In the Drake Passage, the former revealed that the main floristic change was found at the Polar Front, while the latter reflected the Southern ACC Front as a main boundary, and yielded a higher number of isolated sites, most of them located next to different Antarctic islands. Such differences are attributed to the high relative density of Fragilariopsis kerguelensis in Argentine shelf and Drake Passage waters and of Porosira glacialis and species of Chaetoceros and Thalasiosira in the Weddell Sea and near the Antarctic Peninsula. From a total of 84 taxa recorded in antarctic waters, only 17 were found exclusively in this region, and the great majority (67) was also present in subantarctic waters but in extremely low (< 1 cell/l) concentrations, probably as a result of expatriation processes via the ACC-Malvinas Current system. The present results were compared with those of previous studies on the Antarctic region with respect to both diatom associations in regular vs. atypically warm years, and the distribution and abundance of some selected planktonic species reported for surface sediments.

Palynology and dinoflagellate counts of surface sediments, and sea surface conditions, of sites in the North Pacific

Palynological analyses were performed on 53 surface sediment samples from the North Pacific Ocean, including the Bering and Okhotsk Seas (37-64°N, 144°E-148°W), in order to document the relationships between the dinocyst distribution and sea-surface conditions (temperatures, salinities, primary productivity and sea-ice cover). Samples are characterized by concentrations ranging from 18 to 143816 cysts/cm**3 and the occurrence of 32 species. A canonical correspondence analysis (CCA) was carried out to determine the relationship between environmental variables and the distribution of dinocyst taxa. The first and second axes represent, respectively, 47% and 17.8% of the canonical variance. Axis 1 is positively correlated with all parameters except to the sea-ice and primary productivity in August, which are on the negative side. Results indicate that the composition of dinocyst assemblages is mostly controlled by temperature and that all environmental variables are correlated together. The CCA distinguishes 3 groups of dinocysts: the heterotrophic taxa, the genera Impagidinium and Spiniferites as well as the cyst of Pentapharsodinium dalei and Operculodinium centrocarpum. Five assemblage zones can be distinguished: 1) the Okhotsk Sea zone, which is associated to temperate and eutrophic conditions, seasonal upwellings and Amur River discharges. It is characterized by the dominance of O. centrocarpum, Brigantedinium spp. and Islandinium minutum; 2) the Western Subarctic Gyre zone with subpolar and mesotrophic conditions due to the Kamchatka Current and Alaska Stream inflows. Assemblages are dominated by Nematosphaeropsis labyrinthus, Pyxidinopsis reticulata and Brigantedinium spp.; 3) the Bering Sea zone, depicting a subpolar environment, influenced by seasonal upwellings and inputs from the Anadyr and Yukon Rivers. It is characterized by the dominance of I. minutum and Brigantedinium spp.; 4) the Alaska Gyre zone with temperate conditions and nutrient-enriched surface waters, which is dominated by N. labyrinthus and Brigantedinium spp. and 5) the Kuroshio Extension-North Pacific-Subarctic Current zone characterized by a subtropical and oligotrophic environment, which is dominated by O. centrocarpum, N. labyrinthus and warm taxa of the genus Impagidinium. Transfer functions were tested using the modern analog technique (MAT) on the North Pacific Ocean (= 359 sites) and the entire Northern Hemisphere databases ( = 1419 sites). Results confirm that the updated Northern Hemisphere database is suitable for further paleoenvironmental reconstructions, and the best results are obtained for temperatures with an accuracy of +/-1.7 °C.

Sea surface reconstruction of sediment cores off West Spitsbergen

Two sediment cores from the West Spitsbergen area, Euro-Arctic margin, MD99-2304 and MD99-2305, have been investigated for paleoceanographic proxies, including benthic and planktonic foraminifera, benthic foraminiferal stable isotopes and ice rafted debris. Core MD99-2304 is located on the upper continental margin, reflecting variations in the influx of Atlantic Water in the West Spitsbergen Current. Core MD99-2305 is located in Van Mijenfjord, picturing variations in tidewater glacier activity as well as fjord-ocean circulation changes. Surface water warmer than today, was present on the margin as soon as the Van Mijenfjord was deglaciated by 11,200 cal. years BP. Relatively warm water invaded the fjord bottom almost immediately after the deglaciation. A relatively warm early Holocene was followed by an abrupt cooling at 8800 cal. years BP on the continental margin. Another cooling in the fjord record, 8000-4000 cal. years BP, is documented by an increase in ice rafted debris and an increase in benthic foraminiferal delta18O. The IRD-record indicates that central Spitsbergen never was completely deglaciated during the Holocene. Relatively cool and stable conditions similar to the present were established about 4000 cal. years BP.

Sediment facies of the Indian-Pakistan continental margin, Indian Ocean Expedition

The studies described here base mainly on sedimentary material collected during the "Indian Ocean Expedition" of the German research vessel "Meteor" in the region of the Indian-Pakistan continental margin in February and March 1965. Moreover,samples from the mouth of the Indus-River were available, which were collected by the Pakistan fishing vessel "Machhera" in March 1965. Altogether, the following quantities of sedimentary material were collected: 59.73 m piston cores. 54.52 m gravity cores. 33 box grab samples. 68 bottom grab samples Component analyses of the coarse fraction were made of these samples and the sedimentary fabric was examined. Moreover, the CaCO3 and Corg contents were discussed. From these investigations the following history of sedimentation can be derived: Recent sedimentation on the shelf is mainly characterized by hydrodynamic processes and terrigenous supply of material. In the shallow water wave action and currents running parallel to the coast, imply a repeated reworking which induces a sorting of the grains and layering of the sediments as well as a lack of bioturbation. The sedimentation rate is very high here. From the coast-line down to appr. 50 m the sediment becomes progressively finer, the conditions of deposition become less turbulent. On the outer shelf the sediment is again considerably coarser. It contains many relicts of planktonic organisms and it shows traces of burrowing. Indications for redeposition are nearly missing, a considerable part of the fine fraction of the sediments is, however, whirled up and carried away. In wide areas of the outer shelf this stirring has gained such a degree that recent deposits are nearly completely missing. Here, coarse relict sands rich in ooids are exposed, which were formed in very shallow stirred water during the time when the sea reached its lowest level, i.e. at the turn of the Pleistocene to the Holocene. Below the relict sand white, very fine-grained aragonite mud was found at one location (core 228). This aragonite mud was obviously deposited in very calm water of some greater depth, possibly behind a reef barrier. Biochemic carbonate precipitation played an important part in the formation of relict sands and aragonite muds. In postglacial times the relict sands were exposed for long periods to violent wave action and to areal erosion. In the present days they are gradually covered by recent sediments proceeding from the sides. On the continental margin beyond the shelf edge the distribution of the sediments is to a considerable extent determined by the morphology of the sea bottom. The material originating from the continent and/or the shelf, is less transported by action of the water than by the force of gravity. Within the range of the uppermost part of the continental slope recent sedimentation reaches its maximum. Here the fine material is deposited which has been whirled up in the zone of the relict sands. A laminated fine-grained sediment is formed here due to the very high sedimentation rate as well as to the extremely low O2-content in the bottom water, which prevents life on the bottom of the sea and impedes thus also bioturbation. The lamination probaly reflects annual variation in deposition and can be attributed to the rhythm of the monsoon with its effects on the water and the weather conditions. In the lower part of the upper continental slope sediments are to be found which show in varying intensity, intercalations of fine material (silt) from the shelf, in large sections of the core. These fine intercalations of allochthonous material are closely related to the autochthonous normal sediment, so that a great number of small individual depositional processes can be inferred. In general the intercalations are missing in the uppermost part of the cores; in the lower part they can be met in different quantities, and they reach their maximum frequency in the upper part of the lower core section. The depositions described here were designated as turbid layer sediments, since they get their material from turbid layers, which transport components to the continental slope which have been whirled up from the shelf. Turbidites are missing in this zone. Since the whole upper continental slope shows a low oxygen-content of the bottom water the structure of the turbid layer sediments is more or less preserved. The lenticular-phacoidal fine structure does, however, not reflect annual rhythms, but sporadic individual events, as e.g. tsunamis. At the lower part of the continental slope and on the continental rise the majority of turbidites was deposited, which, during glacial times and particularly at the beginning of the post-glacial period, transported material from the zone of relict sands. The Laccadive Ridge represented a natural obstacle for the transport of suspended sediments into the deep sea. Core SIC-181 from the Arabian Basin shows some intercalations of turbidites; their material, however, does not originate from the Indian Shelf, but from the Laccadive Ridge. Within the range of the Indus Cone it is surprising that distinct turbidites are nearly completely missing; on the other hand, turbid layer sediments are to be found. The bottom of the sea is showing still a slight slope here, so that the turbidites funneled through the Canyon of the Swatch probably rush down to greater water depths. Due to the particularly large supply of suspended material by theIndus River the turbid layer sediments show farther extension than in other regions. In general the terrigenous components are concentrated on the Indus Cone. It is within the range of the lower continental slope that the only discovery of a sliding mass (core 186) has been located. It can be assumed that this was set in motion during the Holocene. During the period of time discussed here the following development of kind and intensity of the deposition of allochthonous material can be observed on the Indian-Pakistan continental margin: At the time of the lowest sea level the shelf was only very narrow, and the zone in which bottom currents were able to stir up material by oscillating motion, was considerably confined. The rivers flowed into the sea near to the edge of the shelf. For this reason the percentage of terrigenous material, quartz and mica is higher in the lower part of many cores (e.g. cores 210 and 219) than in the upper part. The transition from glacial to postglacial times caused a series of environmental changes. Among them the rise of the sea level (in the area of investigation appr. 150 m) had the most important influence on the sedimentation process. In connection with this event many river valleys became canyons, which sucked sedimentary material away from the shelf and transported it in form of turbidites into the deep sea. During the rise of the sea level a situation can be expected with a maximum area of the comparatively plane shelf being exposed to wave action. During this time the process of stirring up of sediments and formation of turbid layers will reach a maximum. Accordingly, the formation of turbidites and turbid layer sediments are most frequent at the same time. This happened in general in the older polstglacial period. The present day high water level results in a reduced supply of sediments into the canyons. The stirring up of sediments from the shelf by wave action is restricted to the finest material. The missing of shelf material in the uppermost core sections can thus be explained. The laminated muds reflect these calm sedimentation conditions as well. In the southwestern part of the area of investigation fine volcanic glass was blown in during the Pleistocene, probably from the southeast. It has thus become possible to correlate the cores 181, 182, 202. Eolian dust from the Indian subcontinent represents probably an important component of the deep sea sediments. The chemism of the bottom as well as of the pore water has a considerable influence on the development of the sediments. Of particular importance in this connection is a layer with a minimum content of oxygen in the sea water (200-1500 m), which today touches the upper part of the continental slope. Above and beyond this oxygen minimum layer somewhat higher O2-values are to be observed at the sea bottom. During the Pleistocene the oxygen minimum layer has obviously been locatedin greater depth as is indicated by the facies of laminated mud occuring in the lower part of core 219. The type of bioturbation is mainly determined by the chemism. Moreover, the chemism is responsible for a considerable selective dissolution, either complete or partial, of the sedimentary components. Within the range of the oxygen minimum layer an alkaline milieu is developed at the bottom. This causes a complete or partial dissolution of the siliceous organisms. Here, bioturbation is in general completely missing; sometimes small pyrite-filled burrowing racks are found. In the areas rich in O2 high pH-values result in a partial dissolution of the calcareous shells. Large, non-pyritized burrowing tracks characterize the type of bioturbation in this environment. A study of the "lebensspuren" in the cores supports the assumption that, particularly within the region of the Laccadive Basin, the oxygen content in the bottom sediments was lower than during the Holocene. This may be attributed to a high sedimentation rate and to a lower O2-content of the bottom water. The composition of the allochthonous sedimentary components, detritus and/or volcanic glass may locally change the chemism to a considerable extent for a certain time; under such special circumstances the type of bioturbation and the state of preservation of the components may be different from those of the normal sediment.

(Table 1) Chlorophyll a content and microalgae abundance and biovolume in pack ice in the Bellingshausen Sea

Pack ice in the Bellingshausen Sea contained moderate to high stocks of microalgal biomass (3-10 mg Chl a/m**2) spanning the range of general sea-ice microalgal microhabitats (e.g., bottom, interior and surface) during the International Polar Year (IPY) Sea Ice Mass Balance in the Antarctic (SIMBA) studies. Measurements of irradiance above and beneath the ice as well as optical properties of the microalgae therein demonstrated that absorption of photosynthetically active radiation (PAR) by particulates (microalgae and detritus) had a substantial influence on attenuation of PAR and irradiance transmission in areas with moderate snow covers (0.2-0.3 m) and more moderate effects in areas with low snow cover. Particulates contributed an estimated 25 to 90% of the attenuation coefficients for the first-year sea ice at wavelengths less than 500 nm. Strong ultraviolet radiation (UVR) absorption by particulates was prevalent in the ice habitats where solar radiation was highest - with absorption coefficients by ice algae often being as large as that of the sea ice. Strong UVR-absorption features were associated with an abundance of dinoflagellates and a general lack of diatoms - perhaps suggesting UVR may be influencing the structure of some parts of the sea-ice microbial communities in the pack ice during spring. We also evaluated the time-varying changes in the spectra of under-ice irradiances in the austral spring and showed dynamics associated with changes that could be attributed to coupled changes in the ice thickness (mass balance) and microalgal biomass. All results are indicative of radiation-induced changes in the absorption properties of the pack ice and highlight the non-linear, time-varying, biophysical interactions operating within the Antarctic pack ice ecosystem.