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.

Age determination and carbonate content of sediment core KH-79-3_L3

Laminated sediments deposited under anoxic bottom waters in the Japan Sea during the last glacial maximum (LGM) contain extremely well preserved calcareous microfossils and eolian carbonates. The radiocarbon age-difference between bulk sediment and monospecific planktonic foraminifera in discrete laminae from a core in the southern Japan Sea implies that ~40% of the total carbonates in the sediments at the LGM are of eolian origin. Extrapolation of this result yields a rate of supply of eolian carbonates of ~2800 tons/d to the entire Japan Sea during the LGM. The climatic significance of this flux potentially lies in its broader geographic extension, particularly in the interaction of the carbonate-bearing dust with shallow, corrosive North Pacific waters and with rain in the atmosphere. By increasing the alkalinity of such waters and by enhancing the biological pump the dust flux could have increased CO2 absorption by both the ocean and rain during the LGM.

(Table 2) Details of laminated intervals of ODP Leg 158 holes

Scanning electron microscope (SEM)-based analyses of the laminated diatom oozes encountered during Leg 138 reveal three major laminae types. The first lamina type is composed of multiple layers of ~20-?m-thick diatom mats, which form laminae dominated by assemblages of the pennate diatom, Thalassiothrix longissima. More than one variety/subspecies of T. longissima occurs within these laminae (referred to as the T. longissima Group). The second lamina type is composed of a mixed-assemblage of several species of diatoms (centric and pennate varieties), calcareous nannofossils, and subordinate quantities of radiolarians, silicoflagellates and foraminifers. The third lamina type is dominated by an assemblage of nannofossils and minor amounts of those fossil components mentioned above. This last form of lamination is compositionally similar to the background sediment type, foraminifernannofossil ooze (F-NO). Two lamina associations occur within the laminated intervals; the first comprises alternations of T. longissima Group and mixed-assemblage laminae (average thickness is ~6 mm) and the second is composed of T. longissima and nannofossil-rich laminae (average thickness is ~3.5 mm). The arrangement of laminae probably originates from the deposition of multiple layers of 20-?m-thick mats from one mat-flux episode. The much thinner nannofossil-rich laminae are interpreted to represent periods of more ônormalö deposition between mat-flux episodes. The occurrence of several varieties/subspecies of T. longissima within individual mat layers is consistent with observations of Rhizosolenia diatom mats in the modern world ocean.

(Table T1) Correlation of the postglacial laminated unit between ODP Holes 178-1098A, 178-1098B and 178-1098C

The Antarctic Peninsula region is ideally suited to monitor how global change affects Antarctica because it is one of the most sensitive regions of the continent to rapid climate change. This has been clearly demonstrated by the recent break up of the Larsen A Ice Shelf. Drilling at Ocean Drilling Program Site 1098, Palmer Deep, western Antarctic Peninsula, recovered almost 50 m of sediments that record the paleoceanographic and paleoclimatic history of the region from the last glacial maximum through the rapid climate oscillations of deglaciation into the Holocene. This sedimentary section will provide a wealth of high-resolution paleoenvironmental data from Antarctica that will be useful for climate modelers and paleoceanographers alike. This data report presents the preliminary results of a high-resolution, microscale sediment fabric study of the postglacial sediments from Palmer Deep Site 1098. These sediments have previously been described as being annually laminated; however, this investigation shows that although the interpretation of this sequence as seasonal sediments is most likely correct, there are a number of features that indicate there is strong interannual variability affecting the laminations.

Geochemical records of sediment cores from the Sea of Japan

Intervals of organic C- and carbonate-rich laminated sediments occur in the Sea of Japan with roughly the same frequency as temperature changes observed in Greenland ice cores, providing clear evidence of rapid oceanographic change during the past 36 kyr. Planktonic foraminiferal d18O data suggest that only the laminated sediments deposited during the Last Glacial Maximum (LGM), and perhaps one other interval formed during a period of increased water column stratification. Sedimentary Re and Mo data are consistent with bottom waters that were sulfidic during the LGM and suboxic during other laminated intervals. Results of a numerical model of Corg and Re burial are consistent with a mechanism whereby an increased Corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals. Such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and/or more saline surface waters or by stronger northeasterly monsoonal winds.

Magnetic susceptibility and documentation of sediment cores from the Antarctic Peninsula Pacific margin

The effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the Antarctic Peninsula Pacific margin. Nineteen sediment cores were visually described, logged for magnetic susceptibility, and X-radiographed. About 1000 analyses were performed for grain size, clay minerals and biostratigraphy (foraminifera, nannofossils and diatoms). Four sediment types associated with distinct sedimentary processes are recognised based on textural/compositional analysis. (1) Hemipelagic mud forms the bulk of the interglacial sediment, and accumulated from the pelagic settling of bioclasts and ice-rafted/windtransported detritus. (2) Terrigenous mud forms the bulk of the glacial sediment, and accumulated from a combination of sedimentary processes including turbidity currents, turbid plumes, and bottom current reworking of nepheloid layers. (3) Silty deposits occurring as laminated layers and lenses, represent the lateral spillout of lowdensity turbidity currents. (4) Lastly, glacial/interglacial gravelly mud layers derive from settling of ice-rafted detritus. Five depositional settings are interpreted within sediment Drift 7, each characterised by the dominance/interaction of one or several depositional processes. The repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages (glaciation, glacial, deglaciation, and interglacial), each one characterised by a distinctive clay mineral assemblage and bioclastic content. Variations in clay mineral assemblage within interglacial stage 5 (core SED-06) suggest minor colder climatic fluctuations, possibly correlatable with substages 5a to 5e.

Mid-Pleistocene Mg/Ca, temperature and stable isotopes from Globigerinoides ruber white and Globorotalia inflata of sediment core GeoB3801-6 from the subtropical South Atlantic

The glacial marine isotope stage 14 (MIS 14) appears in many climate records as an unusually warm glacial. During this period an almost monospecific, up to 1.5 m thick, laminated layer of the giant diatom Ethmodiscus rex has been deposited below the South Atlantic Subtropical Gyre. This oligotrophic region is today less favorable for diatom growth with sediments typically consisting of calcareous nannofossil oozes. We have reconstructed temperatures and the stable oxygen isotopic compositions of sea surface and thermocline water (d18Ow) from planktonic foraminiferal (Globigerinoides ruber and Globorotalia inflata) Mg/Ca and stable oxygen isotopes to test whether perturbations in surface ocean conditions contributed to the deposition of the diatom layer at ~530 kyr B.P. Temperatures and d18Ow values reconstructed from this diatom ooze interval are highly variable, with maxima similar to interglacial values. Since the area of the Ethmodiscus oozes resembles the region where Agulhas rings are present, we interpret these hydrographic changes to reflect the varying influence of warm and saline water of Indian Ocean origin that entered the Subtropical Gyre trapped in Agulhas rings. The formation of the Ethmodiscus oozes is associated with a period of maximum Agulhas leakage and a maximum frequency of Agulhas ring formation caused by a termination-type position of the Subtropical Front during the unusual warm MIS 14. The input of silica through the Agulhas rings enabled the shift in primary production from calcareous nannoplankton to diatoms, leading to the deposition of the massive diatom oozes.

Age determination and stable isotope ratios of three sediment cores from the Great Belt

The marine transgression Into the Baltic Sea through the Great Belt took place around 9,370 calibrated C-14-years B.P. The sedimentary sequence from the early brackish phase and the change to marine conditions has been investigated in detail through C-14-datings, and oxygen and carbon isotope measurements, and is interpreted by comparison with modern analogs. The oldest brackish sediments are the strongly laminated clays and silts rich in organic carbon followed by non-laminated heavily bioturbated silts. The bedding and textural characteristics and stable isotope analyses on Ammonia beccarii (dextral) and A. beccarii (sinistral) show that the deposltlonal conditions respond to a change at about 9,100 cal. a B.P. from an unstratified brackish water environment in the initial stage of the Littorina Transgression to a thermohaline layered milieu in the upper unit. The oxygen isotope results indicate that the bottom waters of this latter period had salinities and temperatures comparable to the present day Kiel Bay waters. The isotopic composition of the total organic carbon and the d13C-values of A. beccarii reveal a gradual change from an initially lacustrine/terrestrial provenance toward a brackish/marine dominated depositional environment. A stagnation of the sea level at around 9,100 to 9,400 B.P. is indicated.

Geochemical composition and ages of sediment core BELQ300 obtained from Lake Belau, north Germany

Geochemical and palynological data from an annually laminated core sequence (Lake Belau, Schleswig-Holstein) are interpreted with respect to vegetation and settlement history on the basis of a chronostratigraphical model and archaeological evidence. Most settlement periods indicated by pollen and archaeological data can be geochemically identified in the sediment sequence using tracer elements such as K, Rb, Zr and the K/Zr ratio. Whilst air-borne pollen carry a more regional signal, the sedimentary flux of these trace elements is determined by the allogenic input from the catchment area of the lake and, therefore, provides information about the local history of settlement and agricultural land use in the lake's vicinity. This is exemplified for the period of the middle Neolithic Funnel Beaker Culture ('Iversen landnam'), where a time offset of 250 years between both signals has been detected. In contrast, both geochemical and pollen signals are highly synchronous during the Early Migration Period and the High Medieval Period. Additionally, the Fe/Ca and/or U/Fe ratio may serve as a sensitive tracer for human impact on the trophic state of the lake. The suggested impact of the Romans and the High Medieval civilization can clearly be seen (and quantified) from elevated lead input into Lake Belau sediments at this time. Effects of secular climatic changes on the sedimentary chemistry have not been detected and, if present, seem to have been obliterated by anthropogenic activity.