Original paper

Zur Entwicklung der spätquartären Sedimentfazies im Südpolarmeer

[Facies of late Quaternary Sediments of the Antarctic Ocean]

Grobe, Hannes; Fütterer, Dieter K.;Hubberten, Hans W.;Kuhn, Gerhard;Mackensen, Andreas

Kurzfassung

Die Sedimentationsprozesse während der beiden letzten Klimazyklen des Quartär werden aus Fazies vom antarktischen Kontinentalhang rekonstruiert. Die zeitliche Einstufung der Sedimente erfolgt durch Korrelation einer Lithostratigraphie mit einer Sauerstoffisotopenstratigraphie. Diese Lithostratigraphie ermöglicht die Berechnung eines gestapelten sedimentologischen Datensatzes aus elf Kernen eines ausgewählten Gebietes im östlichen Weddellmeer. Das Probenmaterial wird durch weitere Kerne aus dem Kontinentalrandbereich ergänzt. Das aus diesem Datensatz abgeleitete generelle Modell erlaubt Aussagen zur glazialmarinen Sedimentation und zu den spezifischen Paläoumweltbedingungen am antarktischen Kontinentalhang während der letzten 300 000 Jahre. Die Sedimentationsprozesse werden durch ein komplexes Wechselspiel von ozeanographischen und glaziologischen Prozessen kontrolliert, die ihrerseits weitgehend durch das globale Klima gesteuert werden. Die Sedimentation wird vorwiegend durch Eistransport bestimmt und spiegelt Massenbilanz und Verhalten des kontinentalen Eisschildes wider. Die Sedimentationsraten nehmen mit der Entfernung vom Kontinent und innerhalb eines Klimazyklus vom Interglazial zum Glazial ab. Es werden fünf Sedimentfazies dargestellt, deren Bildungsbedingungen spezifischen Ablagerungsräumen zugeordnet werden können. 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.

Abstract

To reveal the late Quaternary paleoenvironmental changes at the Antarctic continental margin, we test a lithostratigraphy, adjusted to a stable isotope record from the eastern Weddell Sea. The stratigraphy is used to produce a stacked sedimentological data set of eleven sediment cores. We derive a general model of glacio-marine sedimentation and paleoenvironmental changes at the East Antarctic continental margin during the last two climatic cycles (300 kyr). The sediments can be classified into five distinct facies and correlated to different paleoenvironments. The sedimentary processes considered include biological productivity, ice-rafting, current transport, and gravitational downslope transport. These processes are controlled by a complex interaction of sea-level changes and paleoceanographic and paleoglacial conditions in response to changes of global climate and local insolation. Sedimentation rates are mainly controlled by ice-rafting which reflects mass balance and behaviour of the Antarctic ice sheet. The sedimentation rates decrease with distance from the continent and from interglacial to glacial. Highest rates being up to five times higher than during glacials occur at the very beginning of interglacials, i. e. of oxygen isotope events 7.5, 5.5, and 1.1. At glacial terminations (isotope events 8.0, 6.0, and 2.0), the Antarctic cryosphere adjusts to new climatic conditions. The sedimentary processes are controlled by the rise of sea level, the destruction of ice shelves, the retreat of seaice and the recommenced feeding of warm North Atlantic Deep Water (NADW) to the Circumpolar Deep Water (CDW). During peak warm interglacial periods (at isotope events 7.5, 7.3, 5.5., and 1.1), the CDW promotes warmer surface waters and thus the retreat of sea-ice which in turn controls the availability of light in surface waters. At distinct climatic thresholds local insolation might also influence sea-ice distribution. Primary productivity and bioturbation increase, the CCD (carbonate compensation depth) rises and carbonate dissolution occurs in slope sediments also in shallow depth. Ice shelves and coastal polynyas favour the formation of very cold and saline Ice Shelf Water (ISW) which contributes to bottom water formation. During the transition from peak warm time to a glacial (isotope stages 7.2-7.0, and 5.4-5.0) the superimposition of both intense ice-rafting and reduced bottom currents produces a typical facies which occurs with a distinct lag in the time of response of specific sedimentary processes to climatic change. With the onset of a glacial (at isotope events 7.0, and 5.0) the Antarctic ice sheet expands due to the lowering of sea-level with the extensive glaciations in the northern Hemisphere. Gravitational sediment transport becomes the most active process, and sediment transfer to the deep sea is provided by turbidity currents through canyon systems. During Antarctic glacial maxima (isotope stages between 7.0-6.0, and 5.0-2.0) the strongly reduced input of NADW into the Southern Ocean favours further advances of the ice shelves far beyond the shelf break and the continous formation of sea ice. Below ice shelves and/or closed sea ice coverage contourites are deposited on the slope.

Keywords

CoresPleistocenepaleooceanographypaleoglaciologycontinental marginsedimentation processeslithofaciesice raftinggrain sizeclay mineralsstable isotopes18Oorganic materialratesglacial-interglacial environmentssea level Weddell S