Description: Condensed pelagic deposits characterize the uppermost sediment sequence of the Maud Rise in the eastern Weddell Sea. The comparison of subbottom profiling data with the sediments drilled at two ODP sites shows that Lithologic Unit Ib deposited during late Miocene to late Pliocene times is generally characterized by acoustic transparency. A prominent reflector with strongly increased amplitude (called Reflector “Pc”) occurs within this mainly homogeneous diatom ooze unit and is correlated with the porcellanite horizon found at ODP Site 689. As evidenced by sediment findings, Reflector “Pc” marks a distinct stratigraphic layer of early Pliocene age. The development of the Reflector “Pc” and its amplitude of reflection in the echosounding records is most likely caused by the silica cementation of porcellanite within this layer. The analyses of the sediment echosounder profiles from five R.V. Polarstern cruises make it possible to map the regional distribution of Reflector “Pc”. In large areas of the central Maud Rise, a patchy but also continuous distribution of Reflector “Pc” was found. The reflector does not occur in sediments below 3000 meters of water. On average Reflector “Pc” is detected around 11 and 12 meters below seafloor, but depth occurrence is highly variable within the range of 8 m to more than 18 m. The apparent amplitude of Reflector “Pc” is higher in those areas where the diatom ooze Unit Ib reaches greater thicknesses, which may be caused by local enhanced tectonic subsidence. In general a good correlation of Reflector “Pc” amplitude and thickness of lithologic Unit Ib is observed. This may imply that the diatom ooze Unit Ib serves as a silica source, since the extent of silica cementation of the porcellanite layer is controlled by the amount of dissolved silica supply. Other possible interpretations for silica sources include silica-rich interstitial water ascending from deeper sediment levels using tectonic structures for migration.

Description: A porcellanite layer, probably younger than 0.6-0.4 Ma, of a nearly monomineralic composition of opal-CT was sampled on the Southwest Indian Ridge during Polarstern cruise ANT-VI/3. The intense cementation of the rock, together with recent findings by the Ocean Drilling Program (Legs 113 and 120) and the occurrence of a unique older porcellanite from Eltanin Core 47-15, provides evidence of very early silica precipitation in pure diatom oozes of the Southern Ocean. Such porcellanites occur in shallowly buried young sediments and provide a contrast to the established concepts of porcellanite formation.

Description: The formation of marine opal-CT nodules or layers as early diagenetic deposits has been documented only in Antarctic deep-sea sediments. In contrast, porcellanites and cherts in land sections and Deep Sea Drilling Project and Ocean Drilling Program drill sites are usually found in sediment sections of Miocene age and older. During R.V. Polarstem cruises ANT-IX/3 and 4, young porcellanites were recovered for the first time in contact with their host sediment in two cores from the Atlantic sector of the southern ocean. Chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation. In both cores the porcellanites are embedded in sediments rich in opal-A with extremely low levels of detrital minerals, an environment that seems conducive to a rapid transformation of biogenic silica into porcellanites.

Description: Surface sediment samples taken with a vented box corer from the eastern Weddell Sea on four profiles perpendicular to the continental margin have been investigated for their benthic foraminiferal content. The live fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Based on the dead assemblages, potential fossil assemblages were calculated to facilitate the analogy with late Neogene core material. Five distinct live assemblages inhabit the continental margin today. Six dead assemblages and five potential fossil assemblages, respectively, correspond to these biocoenoses.A predominantly calcareous live fauna dominated by Trifarina angulosa is correlated with strong bottom currents and sandy sediments at the shelf break and on the uppermost continental slope. Below this, on the upper slope down to 2000 m water depth, the predominantly calcareous Bulimina aculeata assemblage coincides with the core of warm (〉0°C) Weddell Deep Water and with fine and more organic-rich sediments. These calcareous live assemblages completely change composition during early diagenesis because of calcite dissolution within the uppermost sediment, which depends largely on the grain size distribution of the sediment and the fluxes of organic matter. Therefore, a still calcareous T. angulosa-dominated fossil assemblage indicates the sandy substrates on the shelf break and the upper slope, whereas the deeper slope with hemipelagic calm sedimentation and with high fluxes of organic matter is indicated by Martinottiella nodulosa, the characteristic arenaceous fossil remnant of the former predominantly calcareous live B. aculeata fauna.On a continental terrace between 2500 and 3500 m water depth Cribrostomoides subglobosus dominates the live fauna, but because of rapid disintegration of the empty tests of this agglutinated species a predominantly calcareous fauna characterized by Oridorsalis umbonatus and Epistominella exigua comprises the dead assemblage and the potential fossil assemblage, respectively.On the lower continental slope, between the carbonate lysocline (3500 m) and the carbonate compensation depth (4000 m), tests of Nuttallides umbonifer are the characteristic dead and potential fossil remnants of a former predominantly arenaceous live fauna, which is associated with the lower part of the Antarctic Bottom Water (AABW). This corroborates earlier investigations suggesting a relationship between the carbonate-corrosiveness of water masses and the distribution of N. umbonifer. This is important for inferring paleo-routes and estimates of paleo-production rates of AABW during the Neogene.

Description: Die während der pleistozänen Klimazyklen gebildeten Sedimentfazies am antarktischen Kontinentalhang werden aus Probenmaterial der POLARSTERN-Expeditionen rekonstruiert. Die zeitliche Einstufung der Sedimente erfolgt durch lithologische Korrelation der Kerne untereinander in Kombination mit der globalen Sauerstoffisotopenchronologie. Erstmals wurden an einem umfangreichen Datensatz aus einem ausgewählten Gebiet des östlichen Weddellmeeres gestapelte Parameter berechnet. Die relativen Variationen der Parameterstapel erlauben die Ableitung eines generellen Modells zur glazialmarinen Sedimentation und Faziesgenese unter den spezifischen Paläoumweltbedingungen des Quartär. Das Modell trifft, den paläozeanographischen Einfluß betreffend, zumindest für den atlantischen Teil des Südpolarmeeres zu; die paläoglaziologischen Prozesse gelten überregional, da sie weitgehend von globalen Faktoren wie Klimaveränderungen und Meeresspiegelschwankungen gesteuert werden.Die glazialmarinen Sedimentationsprozesse werden durch ein komplexes Wechselspiel von ozeanographischen und glaziologischen Prozessen kontrolliert, die ihrerseits weitgehend durch das globale Klima und auch lokale Einflüsse (Insolation) gesteuert werden. Die Bildung der Sedimentfazies wird vorwiegend durch Eistransport bestimmt und spiegelt somit Massenbilanz und Verhalten des kontinentalen Eisschildes wider. Weitere faziesrelevante Prozesse sind biologische Produktion, Strömungstransport und gravitativer Hangtransport. Die Sedimentationsraten nehmen mit der Entfernung vom Kontinent und innerhalb eines Klimazyklus vom Interglazial zum Glazial ab. Die Sedimentationsraten sind in dem relativ kurzen Zeitraum zwischen glazialer Termination und Klimaoptimum um ein mehrfaches höher als im Glazial.Aus den untersuchten Ablagerungen können fünf Sedimentfazies mit spezifischen Bildungsbedingungen isoliert werden. In ihrer zeitlichen und räumlichen Verbreitung unterliegen sie den Veränderungen von Meereisverteilung, Meeresspiegel, den Bewegungen der Schelfeiskante, sowie dem Einfluß des Nordatlantischen Tiefenwassers auf die Wassermassen des Südpolarmeeres. Die Meereisdecke beeinflußt über die Lichtlimitierung die Primärproduktion im Oberflächenwasser. Dies führt zu einem Wechsel in der Erhaltung karbonatischer und kieseliger Mikrofossilien; im Interglazial wird Opal erhalten und Karbonat gelöst, während der Übergangszeiten und im Glazial bleibt Karbonat erhalten und Opal wird gelöst.Strömung und Bodenwasserbildung werden durch die Existenz von Küstenpolynyen und von der durch den Meeresspiegel kontrollierten Gesamtfläche der Schelfeisgebiete gesteuert. Der durch Meeresspiegelabsenkung und Ausweitung des Eisschildes initiierte gravitative Hangtransport liefert vorwiegend im Glazial episodisch größere Sedimentmengen zum Hang und, kanalisiert durch Canyons, in die Tiefsee. Während der glazialen Maxima können unter einer vollständig geschlossenen Eisdecke Konturite abgelagert werden.