Spatial distribution of isotopic compositions of Globigerina bulloides and Neogloboquadrina pachyderma (dex.) and the faunal assemblages of planktic foraminifera in surface sediment samples along the Chilean continental slope between 23°S and 44°S

We report on the spatial distribution of isotopic compositions of the two planktic foraminifera species Globigerina bulloides and Neogloboquadrina pachyderma (dex.), and the faunal assemblages of planktic foraminifera in 91 surface sediment samples along the Chilean continental slope between 23°S and 44°S. Both d13C and d18O data of N. pachyderma (dex.) show little variability in the study area. North of 39°S, the isotopic values of N. pachyderma (dex.) are heavier than those of G. bulloides, whereas south of 39°S, this relation inverses. This is indicative for a change from a well-mixed, deep thermocline caused by coastal upwelling north of 39°S to well-stratified water masses in a non-upwelling environment south of 39°S. In addition, the faunal composition of planktic foraminifera marks this change by transition from an upwelling assemblage north of 39°S to a high-nutrient-non-upwelling assemblage south of 39°S, which is characterized by decreased contributions of upwelling indicators such as G. bulloides, N. pachyderma (sin.), and Globigerinita glutinata. In general, we can conclude that food and light rather than temperature are the primary control of the planktic foraminiferal assemblage between 23°S and 44°S off Chile. Our data point to higher marine productivity at upwelling centers north of 25°S and at 30-33°S. South of 39°S, significant supply of nutrients by fluvial input most likely boosts the productivity.

Age of samples from the sedimentary cover of the Primorye continental slope and from the submarine Bogorov Rise

Results of a complex study of the sedimentary cover (continuous seismic profiling and diatom analysis) in the northeastern Sea of Japan including the Bogorov Rise an adjacent part of the Japan Basin and the continental slope, are presented. Two varied-age complexes were distinguished in the sedimentary cover of the continental slope of Primorye: Middle Miocene and Late Miocene - Pleistocene. These complexes formed in a stable tectonic setting with no significant vertical movements. A depression in the acoustic basement is located along the continental slope and it is divided from the Japan Basin by a group of volcanic structures, the most uplifted part of which forms the Bogorov Rise. The depression probably formed before Middle Miocene. In Middle Miocene the Bogorov Rise was already at depths close to modern ones. In the sedimentary cover near the Bogorov Rise buried zones were found. Probably they were channels for gas transportation in pre-Pleistocene. Deformations of sediments that occurred in the beginning of Pleistocene are established in the basin.

Multiproxy sedimentation patterns of sediment cores from the continental margin off northeast Brazil

Tropical regions have been reported to play a key role in climate dynamics. To date, however, there are uncertainties in the timing and the amplitude of the response of tropical ecosystems to millennial-scale climate change. We present evidence of an asynchrony between terrestrial and marine signals of climate change during Heinrich events preserved in marine sediment cores from the Brazilian continental margin. The inferred time lag of about 1000 to 2000 years is much larger than the ecological response to recent climate change and appears to be related to the nature of hydrological changes.

Volcanic ash from the Peru continental margin

During Leg 112 off Peru, volcanic material was recorded from middle Eocene to Holocene time. The petrographical and chemical composition of tephra is consistent with an origin from the Andean volcanic arc. The amount and thickness of ash layers provide valuable evidence for explosive volcanic episodicity. The first indication of volcanism was found in mid-Eocene sediments. Three volcanic pulses date from Miocene time. Two intense episodes took place in upper Pliocene and from Pleistocene to Holocene time. Pliocene-Pleistocene tephra are restricted to the southern upper-slope and shelf sites, indicating a removal of the volcanic arc and the extinction of the northern Peru volcanoes. The Cenozoic tectonic phases of the Andean margin may be correlated with the Leg 112 volcanic records. The explosive supply of evolved magmatic products succeeded the Incaic and Quechua tectonic phases. Acidic glasses are related to both andesitic and shoshonitic series. The calc-alkaline factor (CAF) of these glasses exhibited moderate magmatic variations during middle and late Miocene time. A dramatic change occurred during the Pliocene-Pleistocene, reflected in a strong CAF increase and the appearance of potassium-rich evolved shoshonitic glasses. This took place when the Nazca Ridge subduction began. This change in the magma genesis and/or differentiation conditions is probably related to thickening of the upper continental plate and to a new configuration of the Benioff Zone.

Magnetic and gravity measurements on North-South profiles across the Grosse Meteor Bank, North Atlantic

During the first section of the "Meteor" cruise No. 2 a profile was run from the Azores to the south across the flanks of the Mid-Atlantic Ridge with a chain of seamounts. The profile extended between the Cruiser (living) and the Hyeres seamounts, which, according to our soundings, form a connected massif, and across the centre of the Grosse Meteor Bank (30°N, 28.5 °W). These seamounts rise from a depth of more than 4000 m up till close to the surface of the sea forming there a large almost flat plateau. In the case of the Grosse Meteor Bank, this plateau has a N-S extension of approx. 30 nautical miles and an E-W extension of approx. 20 nautical miles and reaches a height of 275 m in water depth. The gravity measurements yielded a density of the topographic masses of 2.6 g/cm**3 for the Grosse Meteor Bank. Magnitude and shape of the measured free-air anomaly are very well shown in a model computation with this density. The theoretical gravity effects of the seismically detected swell of cristalline rock and of the Moho depression (mountain root) are not indicated by the observational data. It can, therefore, be assumed that the latter two neutralize each other. It seems, accordingly, that there is no local isostatic compensation of the topographic masses. Hence, the density of 2.6 g/cm**3 obtained would be about the true density of rock. In connection with the mean velocity of P waves (Aric et al., 1968) obtained by seismic refraction methods it must be concluded that the material of the 1200-4000 m thick surface layer of the Grosse Meteor Bank consists of consolidated sediments. This finding is supported by the total intensity of the Earth's magnetic field over the Grosse Meteor Bank. On the assumption of a homogeneous magnetization in the direction of the present Earth's field, the computed anomaly of the massif deviates considerably from the measured anomaly while the magnetic field of the seismically detected crystalline body is capable of interpreting the observed data. Deviating from the prevailing interpretation of the seamounts' plateau as a volcanic cone with submarine abrasion, the Grosse Meteor Bank and the seamounts in the vicinity are assumed to be of continental origin. The questions whether these seamounts submerged later on or whether the sealevel has risen subsequently are, therefore, largely nonexistent.

Age determination of sediment cores from the Shelf Edge off Mauritania, Northwest Africa

The impact of late glacial changes on the sedimentary record was investigated in two long vibracores, collected from the shelf edge off Mauritania, northwest Africa. Lithology and radiocarbon dates indicate that the sedimentary sequences were mainly controlled by sea-level changes on the shelf. The upper Pleistocene sequence is characterized by deposition in coastal environments, while the Holocene sequence represents deposition in shelf environments. During low sea level, much sediment was supplied to the present outer shelf, and the data imply an average accumulation rate of up to 43.0 cm/1000 yrs during the late Pleistocene, which is substantially higher than the Holocene rate. Shelf sediments were continuously reworked and redistributed on a regional scale during falling and rising sea level. The presence of reworked material results in radiocarbon ages which are too old. The mollusc. Venus striatula, which presently is found north of, but not along, the Mauritanian coast, occurs in the upper Pleistocene sequence, suggesting cooler water conditions in the shelf during late glacial times. This species probably migrated to the south during late glacial times, following the southward extension of the cold Canary Current. Radiocarbon dates of the shells broadly coincide with a lowstand of sea level over this part of the continental shelf.

Magnetic stratigraphy of ODP Leg 133 sites

This study is a synthesis of paleomagnetic and mineral magnetic results for Sites 819 through 823 of Ocean Drilling Program (ODP) Leg 133, which lie on a transect from the outer edge of the Great Barrier Reef (GBR) down the continental slope to the bottom of the Queensland Trough. Because of viscous remagnetization and pervasive overprinting, few reversal boundaries can be identified in these extremely high-resolution Quaternary sequences. Some of the magnetic instability, and the differences in the quality of the paleomagnetic signal among sites, can be explained in terms of the dissolution of primary iron oxides in the high near-surface geochemical gradients. Well-defined changes in magnetic properties, notably susceptibility, reflect responses to glacio-eustatic sea-level fluctuations and changes in slope sedimentation processes resulting from formation of the GBR. Susceptibility can be used to correlate between adjacent holes at a given site to an accuracy of about 20 cm. Among-site correlation of susceptibility is also possible for certain parts of the sequences and permits (tentative) extension of the reversal chronology. The reversal boundaries that can be identified are generally compatible with the calcareous nannofossil biostratigraphy and demonstrate a high level of biostratigraphic consistency among sites. A revised chronology based on an optimum match with the susceptibility stratigraphy is presented. Throughout most of the sequences there is a strong inverse correlation both between magnetic susceptibility and calcium carbonate content, and between susceptibility and d18O. In the upper, post-GBR, sections a more complicated type of magnetic response occurs during glacial maxima and subsequent transgressions, resulting in a positive correlation between susceptibility and d18O. Prior to and during formation of the outer-reef barrier, the sediments have relatively uniform magnetic properties showing multidomain behavior and displaying cyclic variations in susceptibility related to sea-level change. The susceptibility oscillations are controlled more by carbonate dilution than by variation in terrigenous influx. Establishment of the outer reef between 1.01 and 0.76 Ma restricted the supply of sediment to the slope, causing a four-fold reduction in sedimentation rates and a transition from prograding to aggrading seismic geometries (see other chapters in this volume). The Brunhes/Matuyama boundary and the end of the transition period mark a change to lower and more subdued susceptibility oscillations with higher carbonate contents. The major change in magnetic properties comes at about 0.4 Ma in the aggrading sequence, which contains prominent sharp susceptibility peaks associated with glacial cycles, with distinctive single-domain magnetite and mixed single-domain/superparamagnetic characteristics. Bacterial magnetite has been found in the sediments, particularly where there are high susceptibility peaks, but its importance has not yet been assessed. A possible explanation for the characteristic pattern of magnetic properties in the post-GBR glacial cycles can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop when the shelf becomes exposed at low sea-level.

Live benthic foraminifera in surface sediments of the Indo-Pakistan continental margin

The Indo-Pakistan Continental Margin represents an extreme habitat for benthic foraminifera since (1) high fluxes of organic matter offer a high food supply, (2) an intensified oxygen minimum Zone (OMZ) develops from the base of the euphotic Zone to water depths over 1000 m and (3) the monsoon causes seasonal oscillations within the biogeochemical cycle. At three stations from the uppermost (233 m), the central (658 m) and the deeper part (902 m) of the OMZ, living benthic foraminiferal assemblages were analyzed within the uppermost 10 cm of the sediment column. The ecologic structure of foraminiferal faunas is characterized by high abundances at the sediment surface and a rapid decrease within the uppermost 2 cm of the sediment column. Despite dysoxic to suboxic bottom-water conditions, stained benthic foraminifera occurred in all cores down to the base of the sampled interval. High surface abundances, a high dominance by few endobenthic calcareous taxa and a low diversity, which may result from specific physiological adaptations to almost anoxic conditions and the absence of predators, are recognized in the central part of the OMZ. The upper and lower margins of the OMZ are characterized by higher diversities and lower abundances. The shallowest part of the OMZ is dominated by calcareous foraminifera, whereas agglutinated species are the most common taxa in the deeper part. Comparisons with previous studies show that benthic foraminiferal assemblages, that are influenced by seasonal oscillations controlling food supply and/or the availability of oxygen, show variations in faunal density and species composition. Since there is strong evidence that oxygen is not a limiting factor for some taxa, it seems more likely that the distribution pattern of benthic foraminifera is preferentially controlled by trophic conditions.