Physical and acoustic sediment properties of sediment cores from the eastern equatorial Atlantic

To reconstruct the deep-water circulation for the last 3.5 Ma from deep-sea sediments of the eastern equatorial Atlantic, sea floor morphology, sub-bottom reflectors and the echo character have been mapped on the basis of 3.5 kHz records and sediment cores. Physical properties of sediments and synthetic seismograms derived from them enable us to decipher reflector sequences in environments of pelagic, current-resuspended and turbidity sedimentation. The individual reflectors originate from carbonate dissolution, hiatus, coarse sand layers and interferences. Those which are related to carbonate dissolution and hiatus provide evidence of water-mass boundaries by their distribution. Five phases of different deep-water circulation can be seen in the record of th elast 3.5 Ma, and these are related to climate history: 1. Between 3.7 Ma and 2.2 Ma a strong deep-water circulation indicates a northward flow of bottom water below 4200 m (AABW = Antarctic-Bottom Water) and a southward flow of deep-water above 4200 m (NADW = North-Atlantic Deep Water). 2. Between 1.6 and 1.4 Ma a southward flow of bottom water below 4500 m and a diminished southward flow above 4500 m can be detected. This water-mass geometry can be interpreted by an expansion of the NADW-masses and a displacement of the AABW-masses during the same time. 3. Since 1.4 Ma a northward flow of a bottom-water current developed again. This current flow created a leeside sediment ridge in the southern part of the Kane Gap and furrows in the northern part of it. 4. Between 400,000 and 200,000 yrs B. P. the oceanic and atmospheric circulation increased. The strengthened oceanic circulation caused and increase in carbonate dissolution, which is documented by a traceable reflector from 2800 m to 4500 m water depth. At the same time an increase of the atmospheric circulation caused a drastic rise in the pelagic sediment accumulation (> 100 %) through an intensification of upwelling. This runs parallel with a higher oceanic productivity in the northern equatorial divergence zone and an enhanced supply of fluvial and probably eolian sediments from the Senegal and Guinea. 5. Before 10,000 yrs B. P. an erosive northward flowing bottom-water current prevailed below 4500 m water depth. After 10,000 yrs B.P. the bottom-water flow was sluggish and non erosive.

Age determination of sediment cores C23-81 and EN32-PC6

It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep-sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice sheet.

Age models of sediment cores from the North-West African continental margin

A statistical analysis ol 15 deep sea cores in the eastern North Atlantic off NW Africa revealed the typical fluctuation pattern of distinct species proups as has been described from various parts of the world ocean. Only the "WBF-group" appears to be correlated with global climatic changes, i.e. warmer periods as the Eemian and the Atlanticum. A partly antagonistic "High Productivity group" (HPR-group) is in general not linked with global changes but times of increased fertility in the surface water and the resulting flux of organic matter reaching the bottom. The groups were extracted from cluster analysis of more than 150 surface samples (HPR-group) and a factor analysis of selected cores (WBF-group). In contrast to previous studies the observed fluctuations can not be explained by drastic changes in bottom water masses, but by the pulsation of a distinct "High Productivity Patch" in space and time. At present, this patch is located below the well known upwelling area between 22° and 12° northern latitude. It shifted to the north (up to 27 °N) during the latest glacial period ( 18 ky), indicating an equivalent shift of upwelling productivity caused by advection of nutrient rich upwelling SACW-waters, probably during most of isotopic stages 2 and 3.

Bulk sediment element analysis of sediment cores GeoB10065-9 and GeoB10065-7, offshore northwest Sumba Island, Indonesia

The Australian-Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ~6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ~3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

Rock magnetic records and geochemical methods of sediment cores off NW Africa

Two gravity cores retrieved off NW Africa at the border of arid and subtropical environments (GeoB 13602-1 and GeoB 13601-4) were analyzed to extract records of Late Quaternary climate change and sediment export. We apply End Member (EM) unmixing to 350 acquisition curves of isothermal remanent magnetization (IRM). Our approach enables to discriminate rock magnetic signatures of aeolian and fluvial material, to determine biomineralization and reductive diagenesis. Based on the occurrence of pedogenically formed magnetic minerals in the fluvial and aeolian EMs, we can infer that goethite formed in favor to hematite in more humid climate zones. The diagenetic EM dominates in the lower parts of the cores and within a thin near-surface layer probably representing the modern Fe**2+/Fe**3+ redox boundary. Up to 60% of the IRM signal is allocated to a biogenic EM underlining the importance of bacterial magnetite even in siliciclastic sediments. Magnetosomes are found well preserved over most of the record, indicating suboxic conditions. Temporal variations of the aeolian and fluvial EMs appear to faithfully reproduce and support trends of dry and humid conditions on the continent. The proportion of aeolian to fluvial material was dramatically higher during Heinrich Stadials, especially during Heinrich Stadial 1. Dust export from the Arabian-Asian corridor appears to vary contemporaneous to increased dust fluxes on the continental margin of NW Africa emphasizing that melt-water discharge in the North Atlantic had an enormous impact on atmospheric dynamics.