Oxygen and carbon isotopic compositions of foraminifers at DSDP Hole 72-517

Oxygen and carbon isotope analyses were performed on monospecific or mixed-species samples of benthic foraminifers, as well as on the planktonic species Globigerinoides ruber from a 24-m hydraulic piston core raised on the western flank of the Rio Grande Rise, at DSDP Site 517 (30°56.81'S and 38°02.47'W, water depth 2963 m) in the southwestern Atlantic. This site is presently located in the core of North Atlantic Deep Water (NADW). This is the first long isotopic record of Quaternary benthic foraminifers; it displays at least 30 isotopic stages, 25 of them readily correlated with the standard sequence of Pacific Core V28-239. The depths of both the Bruhnes/Matuyama boundary and the Jaramillo Event based on oxygen isotope stratigraphy agree well with paleomagnetic results. Quaternary faunal data from this part of the Atlantic are dated through isotopic stratigraphy and partially contradict data previously published by Williams and Ledbetter (1979). There was a substantial increase in the size of the earth's major ice sheets culminating at Stage 22 and corresponding to a l per mil progressive increase of d18O maximal values. Further, ice volume-induced isotopic changes were not identical for different glacial cycles. Oxygen and carbon isotope analyses of benthic foraminifers show that during Pleistocene glacial episodes, NADW was cooler than today and that Mediterranean outflow might still have contributed to the NADW sources. The comparison of coiling ratio changes of Globorotalia truncatulinoides with planktonic and benthic oxygen isotope records shows that there might have been southward excursions of the Brazil Current during the Pleistocene, perhaps related to Antarctic surface water surges. The question of the location of NADW sources during glacial maxima remains open.

Alkenones in the Mediterranean Sea

Time-series of downward alkenone fluxes have been investigated at 200 m depth over a one year sediment trap experiment, in the Northwestern Mediterranean Sea. Alkenone flux maxima occurred in autumn and to a lesser extent in May, during the spring bloom. Temperature estimates calculated from the UK'37 index revealed that alkenone producers preferentially develop in subsurface waters (at about 50 m) in spring, whereas the autumn alkenone production occurred upper in the water column (around 30 m). Examination of the core-top UK'37 index values at various sites of the Northwestern Mediterranean basin, suggested that the spring bloom period do not significantly imprint the temperatures recorded in the sediments. The sedimentary temperature estimates would rather reflect annually integrated SST, with a major influence of the autumnal post-bloom development of the coccolithophores in the euphotic zone.

Stable oxygen isotope ratios, calcium carbonate and opal content in late Pliocene sediments of DSDP Hole 75-532 in the Southeast Atlantic (Table 1)

Site 532 on the Walvis Ridge was sampled at 4000- to 800-year intervals from 2.24 to 2.60 Ma, spanning the three large glacial advances of the late Pliocene. An age model was created by correlating the oxygen isotope record to Site 607 with linear interpolations between tie-lines. The resultant age model differs from that in the site reports by more than 800,000 years, due to misidentification of a magnetic boundary. Sedimentation rates varied by an order of magnitude at this site, with minimum accumulation during glacial events. Interglacial intervals were charactrized by high marine production and high summer precipitation on land, while glacials had very low production and arid continental climate. During the large glacial events (Stages 96-100) conditions of low production and continental aridity reached their greatest intensity, but there is no evidence of a permanent mode shift in either marine or terrestrial records. Calcite concentration has a strong variation at obliquity frequencies, with maxima during interglacials, but occasionally shows a large amplitude at precessional frequencies as well, so that high concentrations occur in a few glacial intervals. As a result, color variation is not a reliable guide to glacial-scale cycles at this site. Composition of the phytoplankton assemblage is diverse and highly variable, and we have not been able to distinguish a clear indicator of upwelling-related production. Spectral analysis reveals obliquity and precessional signals in the pollen data, while several diatom records contain combination tones, indicating that these data represent a complicated response to both local and high-latitude forcing.

10Be and barium concentrations of sediment core GeoB1008-3

Particle reactive elements are scavenged to a higher degree at ocean margins than in the open ocean due to higher fluxes of biogenic and terrigenous particles. In order to determine the influence of these processes on the depositional fluxes of 10Be and barium we have performed high-resolution measurements on sediment core GeoB1008-3 from the Congo Fan. Because the core is dominated by terrigenous matter supplied by the Congo River, it has a high average mass accumulation rate of 6.5 cm/kyr. Biogenic 10Be and Ba concentrations were calculated from total concentrations by subtracting the terrigenous components of10Be and Ba, which are assumed to be proportional to the flux of Al2O3. The mean Ba/Al weight ratio of the terrigenous component was determined to be 0.0045. The unusualy high terrigenous 10Be concentrations of 9.1 * 10**9 atoms/g Al2O3 are either due to input of particles with high10Be content by the Congo River or due to scavenging of oceanic 10Be by riverine particles. The maxima of biogenic 10Be and Ba concentrations coincide with maxima of the paleoproductivity rates. Time series analysis of the 10Be and of Ba concentration profiles reveals a strong dominance of the precessional period of 24 kyr, which also controls the rates of paleoproductivity in this core. During the maxima of productivity the flux of biogenic Ba is enhanced to a larger extent than that of biogenic 10Be. Applying a model for coastal scavenging, we ascribe the observed higher sensitivity of Ba to biogenic particle fluxes to the fact that the ocean residence time of Ba is approximately 10 times longer than that of 10Be.

Biogeochemical parameters and processes in waters and bottom sediments of the Chukchi Sea

Study of biogeochemical processes in waters and sediments of the Chukchi Sea in August 2004 revealed atypical maxima of biogenic element (N, P, and Si) concentrations and rate of microbial sulfate reduction in the surface layer (0-3 cm) of marine sediments. The C/N/P ratio in organic matter (OM) of this layer does not fit the Redfield-Richards stoichiometric model. Specific features of biogeochemical processes in the sea are likely related to the complex dynamics of water, high primary produc¬tivity (110-1400 mg C/m**2/day), low depth of the basin (<50 m for 60% of the water area), reduced food chain due to low population of zooplankton, high density of zoobenthos (up to 4230 g/m**2), and high activity of microbial processes. Drastic decrease in concentrations of biogenic elements, iodine, total alkalinity, and population of microorganisms beneath the 0-3 cm layer testify to large-scale OM decay at the water-seafloor barrier. Our original experimental data support high annual rate of OM mineralization at the bottom of the Chukchi Sea.

Age control points and sedimentation rates of ODP Hole 145-887B (Table 1)

Hole 887B of the Ocean Drilling Program (ODP) comprises a 44 m (750 kyr) long continuous section recovered from the Patton-Murray Rise, an elevated plateau that is largely isolated from turbidite deposition. The Patton-Murray area is centered under the Alaska Gyre, a region characterized by the domal upwelling of nutrient-rich waters. Marked increases in productivity and rapid settling of biogenic matter are suggested throughout the section by the episodic accumulation of diatomaceous oozes up to ~1 m thick that are accompanied by barium enrichments. Significant delta13Corg maxima in the major diatomaceous bands suggest that mixedlayer [CO2(aq)] may have been drawn down significantly during some of the productivity events. The episodes of enhanced productivity at Site 887 occur synchronously with short-lived minima in planktonic foram delta18O, suggesting a direct link to low salinity, or less likely, warming, events in the Gulf of Alaska. There is no obvious explanation for the events, but they may be related to seasonal incursions of meltwater from Alaska. We speculate that episodic input of meltwater- or dust-borne iron from Asian or Alaskan sources may have promoted the extraordinary diatom production events recorded in the sedimentary record.

Planktic foraminifera isotopes, temperature and d18O of seawater at the central Walvis Ridge for Termination II (MIS 6 to 5), sediment core 64PE-174P13

Salty and warm Indian Ocean waters enter the South Atlantic via the Agulhas leakage, south of Africa. Model simulations and proxy evidence of Agulhas leakage strengthening during glacial terminations led to the hypothesis that it was an important modulator of the Atlantic Ocean circulation. Yet, the fate of the leakage salinity and temperature anomalies remains undocumented beyond the southern tip of Africa. Downstream of the leakage, new paleoceanographic evidence from the central Walvis Ridge (southeast Atlantic) shows that salinity increased at the thermocline, and less so at the surface, during glacial termination II. Thermocline salinity change coincided with higher frequency of Agulhas rings passage at the core location and with salinity maxima in the Agulhas leakage area, suggesting that leakage waters were incorporated in the Atlantic circulation through the thermocline. Hydrographic changes at the Walvis Ridge and in the leakage area display a distinct two-step structure, with a reversal at ca. 134 ka. This matched a wet interlude within the East Asia weak monsoon interval of termination II, and a short-lived North Atlantic warming. Such concurrence points to a Bølling-Allerød-like recovery of the Atlantic circulation amidst termination II, with a northward shift of the Intertropical Convergence Zone and Southern Hemisphere westerlies, and attendant curtailment of the interocean connection south of Africa.

Element concentrations of bulk sediment from the Southern Cape Basin

High-resolution records of Ca and Sr were obtained from shipboard XRF analyses of bulk sediments in five gravity cores from the southern Cape Basin, South Atlantic Ocean. Sr/Ca ratios display regular glacial/interglacial variations of 14-40% and reveal a close correlation with the SPECMAP record, minimum Sr/Ca ratios appearing during glacial (delta18 O) maxima, distinct increases during periods of deglaciation, and highest ratios in interstadials. Shifts in carbonate-producing phytoplankton and/or zooplankton assemblages over glacial/interglacial cycles are suggested to be the main cause for the observed variations in Sr/Ca patterns. Quick assessment of the relationship between Sr/Ca ratios and the SPECMAP record made it possible to easily transfer an age model to the newly collected cores already during the cruise.

Distribution of diatom species as abundance estimates from vertical multinet hauls from 150 m water depth in the Indian Ocean

During the Indian Ocean Expedition of R/V METEOR phytoplankton samples were taken with a multiple closing net (Multinet) at 103 stations. In this material the diatoms were investigated. In all 247 taxa could be identified which belong to 242 species and 5 varieties of formae of 80 genera. Of these 1 variety, 15 pecies, and 3 genera are newly described. New combinations were made for 18 species, and a number of old combinations was reinstated.

Abundance of organic-walled dinoflagellate cysts of sediment core GeoB3603-2

Here, we reconstruct the varying influence of the Agulhas Current (AgC), the South Atlantic Current and the Antarctic Circumpolar Current on the Agulhas Retroflection (AgR) in the eastern South Atlantic Ocean for the last 160,000 years on the basis of the dinoflagellate cysts, pollen and spores present in a sediment core (GeoB 3603-2) from the southeastern Cape Basin offshore South Africa, where the Agulhas Current enters the Atlantic Ocean. Our analyses reveal strong orbital forcing on the heat exchange between the Indian Ocean and the South Atlantic Ocean during the Late Quaternary. Maxima in local productivity appear to be primarily related to a strengthening of the ocean circulation as a result of the high seasonal contrast during precession maxima. During precession minima, seasonal contrast was low and stratified, oligo- to mesotrophic conditions prevailed, notably when these minima coincided with the glacial terminations. The clear presence of periodicities on a sub-Milankovitch scale as well as modulations of the primary frequencies demonstrate that the Agulhas Retroflection furthermore is modulated substantially by complex interactions between the subtropical, 'precession-driven', climate and ocean circulation systems, and the southern, 'obliquity-driven', high latitudes.

(Table 2) Age model and sedimentation rate of sediment core GeoB5559-2

The Canary Basin lies in a region of strong interaction between the atmospheric and ocean circulation systems: Trade winds drive seasonal coastal upwelling and dust storm outbreaks from the neighbouring Sahara desert are the major source of terrigenous sediment. To investigate the forcing mechanisms for dust input and wind strength in the North Canary Basin, the temporal pattern of variability of sedimentological and geochemical proxy records has been analysed in two sediment cores between latitudes 30°30'N and 31°40'N. Spectral analysis of the dust proxy records indicates that insolation changes related to eccentricity and precession are the main periods of temporal variation in the record. Si/Al and grain-size of the terrigenous fraction show an increase in glacial-interglacial transitions while Al concentration and Fe/Al ratio are both in phase with minima in the precessional index. Hence, the results obtained show that the wind strength was intensified at Terminations. At times of maxima of Northern Hemisphere seasonal insolation, when the African monsoon was enhanced, the North Canary Basin also received higher dust input. This result suggests that the moisture brought by the monsoon may have increased the availability of dust in the source region.

Clay mineralogy of South Atlantic surface sediments

Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.

Activity concentrations of radionuclides in 5 short cores taken in 2007-2008 measured by ICP-MS and HPGe gamma spectrometry

Natural radionuclides and man-made 137Cs were analyzed in five short sediment cores taken in northern part of the Gulf of Eilat (Gulf of Aqaba) in order to provide information on sedimentation and mixing rates and sediment sources. The maximum estimates of sedimentation rates based on excess 210Pb were found to vary between 0.105 ± 0.020 and 0.35 ± 0.23 cm · year**-1. Even the lowest estimates are significantly higher than those expected from dust deposition, suggesting other sources and processes being responsible for most of the allochthonous material accumulation, including periodical floods following heavy rain events, internal erosion or triggers, like earthquakes. In 137Cs depth profiles no 1963 related nuclear weapon test maxima were found; instead, the activities decrease monotonically, suggesting that a major process leading to radionuclides' depth distribution might be mixing. The mixing rates calculated from 137Cs, excess 210Pb and excess 228Th reach values up to 2.18 ± 0.69 cm**2 · year**-1.

Sea surface temperatures, alkenones and sedimentation rates at the Alboran basin

Sea surface temperatures (SSTs) recorded by alkenones and oxygen isotopes in the Alboran basin are used here to describe, at an unprecedented fine temporal resolution, the present interglaciation (PIG, initiated at 11.7 ka BP), the last interglaciation (LIG, onset approximately at 129 ka) and respective deglaciations. Similarities and dissimilarities in the progression of these periods are reviewed in comparison with ice cores and stalagmites. Cold spells coeval with the Heinrich events (H) described in the North Atlantic include multi-decadal scale oscillations not previously obvious (up to 4 °C in less than eight centuries within the stadials associated with H1 and H11, ca 133 ka and 17 ka respectively). These abrupt oscillations precede the accumulation of organic rich layers deposited when perihelion moves from alignment with NH spring equinox to the summer solstice, a reference for deglaciations. Events observed during the last deglaciation at 17 ka, 14.8 ka and 11.7 ka are reminiscent of events occurred during the penultimate deglaciation at ca 136 ka, 132 ka and 129 ka, respectively. The SST trend during the PIG is no more than 2 °C (from 20 °C to 18 °C; up to ?0.2 °C/ka). The trend is steeper during the LIG, i.e. up to a 5 °C change from the early interglaciation to immediately before the glacial inception (from 23 °C to 18 °C; up to -0.4 °C/ka). Events are superimposed upon a long term trend towards colder SSTs, beginning with SST maxima followed by temperate periods until perihelion aligned with the NH autumn equinox (before ca 5.3 ka for the PIG and 121 ka for the LIG). A cold spell of around eight centuries at 2.8 ka during the PIG was possibly mimicked during the LIG at ca 118 ka by a SST fall of around 1 °C in a millennium. These events led interglacial SST to stabilise at around 18 °C. The glacial inception, barely evident at the beginning ca 115 ka (North Atlantic event C25, after perihelion passage in the NH winter solstice), culminated with a SST drop of at least 2 °C in two millennia (event C24, ca 111 ka). The Little Ice Age (0.7 ka) also occurred after the latest perihelion passage in the NH winter solstice and could be an example of how a glacial pre-inception event following an interglaciation might be.