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.

Major and trace element geochemistry of ODP Leg 178 holes

Geochemical analyses have been performed on sediment samples collected during Ocean Drilling Program Leg 178 from the continental rise and outer continental shelf of the Antarctic Peninsula. A suite of 21 trace elements was measured by neutron activation analysis in 39 sediment samples, and major element oxides were determined in 67 samples by electron microprobe analyses of fused glass beads. These geochemical data, combined with the X-ray diffraction and X-ray fluorescence data from shipboard analyses, provide a reasonable estimate of the mineral and chemical composition of sediments deposited along the western margin of the Antarctic Peninsula.

Nd and Sr isotopic compositions and isotope dilution trace element concentrations from 7 sediment cores along the southeast Greenland margin

The Nd and Sr isotopic compositions of Quaternary glacial and glacimarine siliciclastic sediments deposited along the margin of southeast Greenland were determined to assess the roles of the Greenland, Iceland, and more distal ice sheets in delivering detritus to this portion of the northern North Atlantic. The isotopic compositions of detritus generated by portions of the southern Greenland Ice Sheet were defined through measurements of till and trough mouth fan sediments. Massive diamicts from the Scoresby Sund trough mouth fan show a restricted range of e-Nd (-11.8 to -16.6) and 87Sr/86Sr (0.7192-0.7246) consistent with their derivation from mixtures of sediments derived from Paleoproterozoic and/or Caledonian basement and Tertiary Greenland basalts. Further south at Kangerlussuaq, till isotopic compositions covary with the underlying basement type, with low e-Nd values in the inner fiord (-18.1) reflecting the erosion of the local Precambrian gneisses, but with higher e-Nd values (-2.3 to 2.5) found where the trough crosses East Greenland Tertiary basalts. Fine-grained (< 63 µm) sediments deposited along the southeast Greenland margin also show regular spatial isotopic variations. Ambient sediments and ice-rafted detritus in the southern Irminger Basin trend towards low e-Nd values (to ~ -28) and 87Sr/86Sr ratios (~ 0.711 to ~ 0.715) and are likely derived from proximal Archean gneisses of SE Greenland. Further north in the northern Irminger and Blosseville Basins, sediments trend toward much higher e-Nd (> -4) and low 87Sr/86Sr (< 0.709) reflecting a component derived from the local Iceland volcanic rocks and/or the East Greenland Tertiary basalts. In all three regions, the locally-derived detritus is intermixed with sediment with an intermediate e-Nd value (~ -10) and 87Sr/86Sr (~ 0.718) that was likely delivered by icebergs emanating from the Eurasian Ice Sheets and not from eastern Greenland. Deposition of glacial sediments from both proximal and distal (Eurasian) sources occurred adjacent to SE Greenland throughout the past 50 Ka, with periodic increases in IRD deposition at various times including those of Heinrich events 1, 2 and 4. These results suggest that at least the southern portions of the Greenland Ice Sheet experienced periodic instabilities during the Last Glacial period.

Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf

Based on benthic and planktic foraminifera, Bolboforma, oxygen isotope measurements and seismic data, major changes in Miocene, Pliocene and Pleistocene paleoenvironments on the mid Norwegian shelf are discussed and a possible scenario of the late Cenozoic uplift history is given. The dating of the Neogene sequence has been done using foraminifera and Bolboforma. Four main assemblage zones have been identified with nine distinct subzones. Most of the Miocene sequence is preserved. The lower Miocene sediments contain only siliceous microfossils. A period of high fertility and upwelling in the study area prevailed. The early Miocene-early mid Miocene (15 Ma?) change from a siliceous to a calcareous rich microfauna, dominated by Nonion barleeanum, can be related to increased surface-water circulation due to overflow across the Iceland-Faeroe ridge. During the Miocene the temperature decreased in the study area. Evidence of increased amounts of coarser sediments may suggest that an uplift of the mainland areas occurred during the mid-late Miocene. Lower Pliocene sediments contain a foraminiferal fauna that seems to occur in slightly colder conditions than the late Miocene fauna suggesting a further cooling. Possibly, Arctic waters entered the study area in the early Pliocene. A very marked change in lithology (from compacted claystone to unconsolidated diamicton), fauna (from deep dwelling to shallow dwelling species) and seismic signature (from flat lying reflectors to prograding clinoforms) occurs during the mid?-late Pliocene. A two step cooling trend is indicated by the microfauna of these prograding wedges. (1) The first wedge buildups might have been associated with an uplift of the mainland during the early late Pliocene (mid Pliocene, ca. 4 Ma). However, the age determination is somewhat uncertain and may very well be of late Pliocene age. (2) The second step of wedge buildup is associated with a glacial phase where the dominating microfauna exists of arctic species. Large continental ice sheets might have occurred at this time reaching coastal areas and that possibly many of the geomorphological features such as the strandflat were made during this episode. The Pleistocene epoch is represented by an increased percentage of boreal foraminifera intermingled with high arctic species which indicates that interglacial-glacial cycles prevailed and the dynamics of the glacier system changed.

Ladderane lipid analysis and pore water and sediment chemistry of sediment cores from the continental shelf and slope off northwest Africa

The presence and abundance of anaerobic ammonium-oxidizing (anammox) bacteria was investigated in continental shelf and slope sediments (300-3000 m water depth) off northwest Africa in a combined approach applying quantitative polymerase chain reaction (q-PCR) analysis of anammox-specific 16S rRNA genes and anammox-specific ladderane biomarker lipids. We used the presence of an intact ladderane monoether lipid with a phosphocholine (PC) headgroup as a direct indicator for living anammox bacteria and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane lipids, both intact and core lipids, in agreement with the presence of anammoxspecific 16S rRNA gene copies, indicating that anammox occurs at all sites. Concentrations of ladderane core lipids in core top sediments varied between 0.3 and 97 ng g**-1 sediment, with the highest concentrations detected at the sites located on the shelf at shallower water depths between 300 and 500 m. In contrast, the C20 [3]-ladderane monoether-PC lipid was most abundant in a core top sediment from 1500 m water depth. Both anammox-specific 16S rRNA gene copy numbers and the concentration of the C20 [3]-ladderane monoether-PC lipid increased downcore in sediments located at greater water depths, showing highest concentrations of 1.2 x 10**8 copies g**-1 sediment and 30 pg g**-1 sediment, respectively, at the deepest station of 3000 m water depth. This suggests that the relative abundance of anammox bacteria is higher in sediments at intermediate to deep water depths where carbon mineralization rates are lower but where anammox is probably more important than denitrification.

Climatic records of the late Holocene from sediment core GeoB9504-3 off Senegal

High-resolution climatic records of the late Holocene along the north-west African continental margin are scarce. Here we combine sediment grain size, elemental distribution and mineral assemblage data to trace dust and riverine sources at a shallow-marine sediment depocentre in the vicinity of the Senegal River mouth. The aim is to understand how these terrigenous components reflect climate variability during the late Holocene. Major element contents were measured and mineral identification was performed on three sub-fractions of our sediment core: (i) fluvial material <2 µm, (ii) aeolian material of 18-63 µm and (iii) a sub-fraction of dual-origin material of 2-18 µm. Results show that more than 80% of the total Al and Fe terrigenous bulk content is present in the fluviogenic fraction. In contrast, Ti, K and Si cannot be considered as proxies for one specific source off Senegal. The Al/Ca ratio, recording the continental river runoff, reveals two dry periods from 3010 to 2750 cal a BP and from 1900 to 1000 cal a BP, and two main humid periods from 2750 to 1900 cal a BP and from 1000 to 700 cal a BP. The match between (i) intervals of low river runoff inferred by low Al/Ca values, (ii) reduced river discharge inferred by integrated palynological data from offshore Senegal and (iii) periods of enhanced dune reactivation in Mali confirms this interpretation.

Major components and major and trace elements from ODP Site 175-1085

We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola-Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO3, opal, and Corg. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO2, Al2O3, TiO2, Fe2O3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being 'basaltic' in composition and the other similar to an 'average shale'. The basaltic end-member comprises approximately 10-15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.

Number of living and dead foraminifers in the upprmost layer of bottom sediments from the Pacific shelf of South America and environmental characteristics

After death of benthic and planktic foraminifera their tests intensive dissolve in sediments of the upper sublittoral zone (depth 30-60 m) in the highest productivity area of surface water in the northern Peruvian region. Dissolution of fine pelitic ooze is more intensive than of sandy sediments. Rate of dissolution is lower in the lower sublittoral zone (60-200 m) than in the upper part of the zone. Within the upper bathyal zone (300-500 m) dissolution decreases and results to accumulation of carbonate test in this zone. Benthic tests are more abundant than planktic ones. Very poor species composition and a peculiar set of species are characteristic of foraminiferal assemblages found in the sublittoral and upper bathyal zones along the Peruvian coast.

Sedimentation processes in the Mar del Plata Canyon (Southwest Atlantic) reconstructed from three sediment cores (GeoB13832-2, GeoB13833-2, GeoB13862-1)

The Mar del Plata Canyon is located at the continental margin off northern Argentina in a key intermediate and deep-water oceanographic setting. In this region, strong contour currents shape the continental margin by eroding, transporting and depositing sediments. These currents generate various depositional and erosive features which together are described as a Contourite Depositional System (CDS). The Mar del Plata Canyon intersects the CDS, and does not have any obvious connection to the shelf or to an onshore sediment source. Here we present the sedimentary processes that act in the canyon and show that continuous Holocene sedimentation is related to intermediate-water current activity. The Holocene deposits in the canyon are strongly bioturbated and consist mainly of the terrigenous "sortable silt" fraction (10-63 µm) without primary structures, similarly to drift deposits. We propose that the Mar del Plata Canyon interacts with an intermediate-depth nepheloid layer generated by the northward-flowing Antarctic Intermediate Water (AAIW). This interaction results in rapid and continuous deposition of coarse silt sediments inside the canyon with an average sedimentation rate of 160 cm/kyr during the Holocene. We conclude that the presence of the Mar del Plata Canyon decreases the transport capacity of AAIW, in particular of its deepest portion that is associated with the nepheloid layer, which in turn generates a change in the contourite deposition pattern around the canyon. Since sedimentation processes in the Mar del Plata Canyon indicate a response to changes of AAIW contour-current strength related to Late Glacial/Holocene variability, the sediments deposited within the canyon are a great climate archive for paleoceanographic reconstructions. Moreover, an additional involvement of (hemi) pelagic sediments indicates episodic productivity events in response to changes in upper ocean circulation possibly associated with Holocene changes in intensity of El Niño/Southern Oscillation.