Age models of ODP Leg 175 holes

A multiproxy approach including the use of stable isotopes, magnetic characterization analyses, and organic geochemistry has been adopted to consider factors such as productivity and terrigenous input over the past 1.5 m.y. at two areas off the western coast of Africa. These factors can, in turn, be used to consider variability in ocean circulation and upwelling in addition to changes in climate on the African continent. In particular, studies focused on the influence of glacial-interglacial cycles and evidence for the mid-Pleistocene revolution (MPR), a complex change in climate that occurred at ~1 Ma. A comparison of the records from the two areas drilled during Ocean Drilling Program Leg 175, the Congo Basin, at a latitude of 5°S (Holes 1076A and 1077A), and the Walvis Ridge, at 17°S (Hole 1081A), demonstrates that these sites are affected by different localized factors. The sites in the Congo Basin are strongly influenced by freshwater and sediment from the Congo River, whereas the site at the Walvis Ridge is located in the center of oceanic upwelling and contains a more marine signal. Evidence also suggests that the two sites responded differently to both long- and short-term climatic variations. In particular, the response at the Walvis Ridge to the MPR occurred over an extended period, from 1.1 to 0.8 Ma, and was associated with a change in the dominant source of terrigenous input to the site in conjunction with a change in the productivity signal. In the Congo Basin, the response to the MPR was more rapid, occurring between 0.9 and 0.8 Ma. During this period, the influence of the Congo River became significant. However, productivity records only began to respond toward the end of this interval, at 0.8 Ma.

Age models of northern North Atlantic sediment cores

Benthic foraminiferal stable isotope records from four high-resolution sediment cores, forming a depth transect between 1237 m and 2303 m on the South Iceland Rise, have been used to reconstruct intermediate and deep water paleoceanographic changes in the northern North Atlantic during the last 21 ka (spanning Termination I and the Holocene). Typically, a sampling resolution of ~100 years is attained. Deglacial core chronologies are accurately tied to North Greenland Ice Core Project (NGRIP) ice core records through the correlation of tephra layers and changes in the percent abundance of Neogloboquadrina pachyderma (sinistral) with transitions in NGRIP. The evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic d13C and d18O values, which do not lie on glacial or Holocene water mass mixing lines. These periods correlate with the late Younger Dryas/Early Holocene (11.5-12.2 ka) and Heinrich Stadial 1 (14.7-16.8 ka) during which time freshwater input and sea-ice formation led to brine rejection both locally and as an overflow exported from the Nordic seas into the northern North Atlantic, as earlier reported by Meland et al. (2008). The export of brine with low ?13C values from the Nordic seas complicates traditional interpretations of low d13C values during the deglaciation as incursions of southern sourced water, although the spatial extent of this brine is uncertain. The records also reveal that the onset of the Younger Dryas was accompanied by an abrupt and transient (~200-300 year duration) decrease in the ventilation of the northern North Atlantic. During the Holocene, Iceland-Scotland Overflow Water only reached its modern flow strength and/or depth over the South Iceland Rise by 7-8 ka, in parallel with surface ocean reorganizations and a cessation in deglacial meltwater input to the North Atlantic.

Late Pliocene age models of Agulhas Plateau sediment cores

An ensemble of new, high-resolution records of surface ocean hydrography from the Indian-Atlantic oceanic gateway, south of Africa, demonstrates recurrent and high-amplitude salinity oscillations in the Agulhas Leakage area during the penultimate glacial-interglacial cycle. A series of millennial-scale salinification events, indicating strengthened salt leakage into the South Atlantic, appear to correlate with abrupt changes in the North Atlantic climate and Atlantic Meridional Overturning Circulation (AMOC). This interhemispheric coupling, which plausibly involved changes in the Hadley Cell and midlatitude westerlies that impacted the interocean transport at the tip of Africa, suggests that the Agulhas Leakage acted as a source of negative buoyancy for the perturbed AMOC, possibly aiding its return to full strength. Our finding points to the Indian-to-Atlantic salt transport as a potentially important modulator of the AMOC during the abrupt climate changes of the Late Pleistocene.

Physical models of dilatant normal faulting in jointed cohesive rocks

Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity is not well understood. We present an analogue modeling study using cohesive powder with pre-formed joint sets in the upper layer, varying the angle between joints and a rigid basement fault. We analyze interpreted map-view photographs at maximum displacement for damage zone width, number of connected joints, number of secondary fractures, degree of segmentation and area fraction of massively dilatant fractures. Particle imaging velocimetry helps provide insights on deformation history of the experiments and illustrate the localization pattern of fault segments. Results show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increases, while the area fraction of massively dilatant fractures shows only a minor increase. Models without pre-existing joints show far lower area fractions of massively dilatant fractures while forming distinctly more secondary fractures.

Geochemistry and age models of sediments from the Celtic margin

The sedimentary sections of three cores from the Celtic margin provide high-resolution records of the terrigenous fluxes during the last glacial cycle. A total of 21 14C AMS dates allow us to define age models with a resolution better than 100 yr during critical periods such as Heinrich events 1 and 2. Maximum sedimentary fluxes occurred at the Meriadzek Terrace site during the Last Glacial Maximum (LGM). Detailed X-ray imagery of core MD95-2002 from the Meriadzek Terrace shows no sedimentary structures suggestive of either deposition from high-density turbidity currents or significant erosion. Two paroxysmal terrigenous flux episodes have been identified. The first occurred after the deposition of Heinrich event 2 Canadian ice-rafted debris (IRD) and includes IRD from European sources. We suggest that the second represents an episode of deposition from turbid plumes, which precedes IRD deposition associated with Heinrich event 1. At the end of marine isotopic stage 2 (MIS 2) and the beginning of MIS 1 the highest fluxes are recorded on the Whittard Ridge where they correspond to deposition from turbidity current overflows. Canadian icebergs have rafted debris at the Celtic margin during Heinrich events 1, 2, 4 and 5. The high-resolution records of Heinrich events 1 and 2 show that in both cases the arrival of the Canadian icebergs was preceded by a European ice rafting precursor event, which took place about 1-1.5 kyr before. Two rafting episodes of European IRD also occurred immediately after Heinrich event 2 and just before Heinrich event 1. The terrigenous fluxes recorded in core MD95-2002 during the LGM are the highest reported at hemipelagic sites from the northwestern European margin. The magnitude of the Canadian IRD fluxes at Meriadzek Terrace is similar to those from oceanic sites.

Descriptions of 104 gravity and piston sediment cores from POLARSTERN expedition ARK-VIII/3 to the Arctic Ocean

Sediment descriptions and lithostratigraphy (chapter 6.4) NANSEN BASIN The upperrnost 20-50 cm of sedirnents in the Nansen Basin norrnally cornprise soft dark brown, brown-grayish and brown clay. Except for the toprnost clay, the four piston cores retrieved, contained quite different lithologies: a rnuddy diarnicton with outsized clasts (PS2157-6), sandy-silt beds alternating with clay beds (PS2159-6), and silty clay beds of brownish and grayish colours (PS2161-3). Core PS2208-3 was retrieved frorn a plateau on a searnount. The plateau was serni-encircled by hills. The upper 250 cm of this core cornprise brown and olive brown clays. Below these are several sandlayers and a 74 cm thick unit of a sandy mud with rnud-clasts up to 20 cm in diameter. GAKKEL RIDGE The uppermost 20-50 cm of sediments on the Gakkel Ridge comprise soft dark brown, brown, grayish brown clay. In most of the cores there are two horizons of brown clay separated by olive brown clay. The upper horizon is darker. The older stratigraphy is rather varied. Core PS2165-1 contains several thin gray sandlsilt layers, probably distal turbidites. The sarne is found in Core PS2167-1. This core also has a thick (approx. 2 rn) coarse grained turbidite containing large rnud clasts and basaltic rock fragrnents. The color of the turbiditic layers is dark gray. There are several horizons of hernipelagic sandylsilty clays with quite a variety in colours; black, gray, olive, brown, yellowish brown and reddish. The colour variation rnay be due to hydrotherrnal activity or provenance or a shift in redox potential. Cores PS2168-2 and PS2169-1 have typical sequences of very dark gray sandy mud with sharp lower boundaries grading upwards into olive brown clay. Below the lower boundary is often a thin (1-2 cm) gray clay layer. AMUNDSEN BASIN The giant box cores (GKG) provided in most cases excellently preserved sedirnent surfaces which consisted in the entire Amundsen Basin of dark brown to dark grayish brown silty clay with few dropstones and common calcareous microfossils (foraminifers and calcareous nannofossils). The brown and grayish brown color of the sediment surface is a result of the oxidizing conditions at the seafloor due to the rapid renewal of the bottom water rnasses. Planktic forarninifers and calcareous nannofossils are relatively frequent and well preserved despite the rernote location of the basin and its water depths of >4000 rn. Srnear slide descriptions have shown that the surface sedirnents consist dorninantly of clays to silty rnuds with clay rninerals and quartz as the rnost important constituents. The coarse fractions contained besides planktic and benthic forarninifers and coarse clastic rnaterials, rare bivalves, dropstones and mud clasts. The Station PS2190 at the North Pole is a particular good exarnple of the type of sedirnents deposited at the sea floor surface of the Arnundsen Basin, with hornogenous dark brown soft clay covering a sedirnent sequence of highly variable cornposition. Nurnerous giant box cores also provide insight into the detailed lithostratigraphy of the upperrnost sedirnent layers. Twelve box cores have been collected frorn the Arnundsen Basin. Below the youngest unit of 5-20 crn thick silty clays deposits of variable stratigraphies have been found, rnostly consisting of clays or silty clays. In a few instances turbidites have been observed. Benthic forarninifers have not been found in the surface sedirnents. Other fossils were extrernely rare. Bioturbation is weakly developed on all stations. Benthic anirnals seern to live only in and on the upperrnost 2 cm of the uppermost sediment layer. They cornprise amphipods (on all stations) and holothurians, bryozoans, polychaetes, and porifers at one station each. LOMONOSOV RIDGE Sediments from the Lomonosov Ridge show a variety of colors and textures. Following smear slide analyses they are composed mostly of clay minerals and quartz with mica and feldspars, especially in the siltier and sandier parts. Volcanic glass, microcrystalline carbonate, opaque minerals and green amphibole are occasional accessories. The sediments from the Lomonosov Ridge show a noticeable difference from sediments collected from the surrounding basins. Lomonosov Ridge sediments are richer in silt and sand than basin sediments. Occasional turbidites occur in ridge sediments but these must be of entirely local origin. The ridge sediments include frequent layers of "cottage cheese" texture made up of what appear to be small, angular mud clasts of a variety of colors.

Magnetic extracts of three gravity cores located at the Ceará Rise, the Mid-Atlantic Ridge and the Sierra Leone Rise

The magnetic microparticle and nanoparticle inventories of marine sediments from equatorial Atlantic sites were investigated by scanning and transmission electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation. This information is used to establish source-to-sink relations and to constrain environmental magnetic proxy interpretations for this area. Magnetic extracts were prepared from sediments of three supralysoclinal open ocean gravity cores located at the Ceará Rise (GeoB 1523-1; 3°49.9'N/41°37.3'W), the Mid-Atlantic Ridge (GeoB 4313-2; 4°02.8'N/33°26.3'W), and the Sierra Leone Rise (GeoB 2910-1; 4°50.7'N/21°03.2'W). Sediments from two depths corresponding to marine isotope stages 4 and 5.5 were processed. This selection represents glacial and interglacial conditions of sedimentation for the western, central, and eastern equatorial Atlantic and avoids interferences from subsurface and anoxic processes. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected by scanning and transmission electron microscopy. On basis of these properties, nine particle classes could be defined: detrital magnetite, titanomagnetite (fragmental and euhedral), titanomagnetite-hemoilmentite intergrowths, silicates with magnetic inclusions, microcrystalline hematite, magnetite spherules, bacterial magnetite, goethite needles, and nanoparticle clusters. Each class can be associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large-scale sedimentation patterns are delineated as well: detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid-ocean ridge basalts, and titanomagnetite-hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source-to-sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high-coercive or superparamagnetic properties depending on their partly ultrafine grain sizes. These two magnetic fractions are generally discussed as separate fractions, but we suggest that they could actually be genetically linked.

Age models, iron intensity, magnetic susceptibility records and dry bulk density of sediment cores from around the Canary Islands

A set of 43 sediment cores from around the Canary Islands is used to characterise this region, which intersects meridional climatic regimes and zonal productivity gradients in a high spatial resolution. Using rapid and nondestructive core logging techniques we carried out Fe intensity and magnetic susceptibility (MS) measurements and created a stack on the basis of five stratigraphic reference cores, for which a stratigraphic age model was available from d18O and 14C analyses on planktonic foraminifera. By correlation of the stack with the Fe and MS records of the other cores, we were able to develop age depth models at all investigated sites of the region. We present the bulk sediment accumulation rates (AR) of the Canary Islands region as an indicator of shifts in the upwelling-influenced areas for the Holocene (0-12 ky), the deglaciation (12-18 ky) and the last glacial (18-40 ky). General observations are an enhanced productivity during glacial times with highest values during the deglaciation. The main differences between the analysed time intervals we interpret as result of the sea-level effects, changes in the extent of high productivity areas, and current intensity.