Sea surface reconstruction for sediments of the California continental margin

Organic matter in sediment samples from three ODP sites (Ocean Drilling Program Leg 167) that form a south-north transect was investigated to reconstruct the paleoclimatic and oceanographic conditions on the California continental margin during the last 160 kyr. Alkenone-derived paleosea surface temperatures (SST) are 3 to 6°C colder in glacial stages and reveal a clear relationship with global climate changes; the differences are greater in the north. Latitudinal SST comparison exhibits water mixing of the colder California Current with warmer waters from the south, particularly in the southern central California borderland area. Organic matter accumulation on the California continental margin indicates an interplay between climatic and atmospheric glacial-interglacial variations and spatially and temporally changing nutrient availability along the California coastline. Climatic and atmospheric dependent circulations apparently caused variations in the intensity of coastal upwelling along the southern central California margin and this suggests, due to the close connection of the California Current to the local wind patterns, that the California Current was weaker during glacial and stronger during interglacial periods.

Analysis of Recent benthic foraminiferal assemblages from the oxygen minimum zone of the Pakistan continental margin

Live (Rose Bengal stained) and dead benthic foraminiferal communities (hard-shelled species only) from the Pakistan continental margin oxygen minimum zone (OMZ) have been studied in order to determine the relation between faunal composition and the oxygenation of bottom waters. During R.R.S. Charles Darwin Cruises 145 and 146 (12 March to May 28 2003), 11 multicores were taken on the continental margin off Karachi, Pakistan. Two transects were sampled, constituting a composite bathymetric profile from 136 m (above the OMZ in spring 2003) down to 1870 m water depth. Cores (surface area 25.5 cm2) were processed as follows: for stations situated above, and in the upper part of the OMZ, sediment slices were taken for the 0-0.5 and 0.5-1 cm intervals, and then in 1 cm intervals down to 10 cm. For the lower part of the OMZ, the second centimetre was also sliced in half-centimetre intervals. Each sample was stored in 10 % borax-buffered formalin for further processing. Onshore, the samples were wet sieved over 63 µm, 150 µm and 300 µm sieves and the residues were stained for one week in ethanol with Rose Bengal. After staining, the residue was washed again. The stained faunas were picked wet in three granulometric fractions (63-150 µm, 150-300 µm and >300 µm), down to 10 cm depth. To gain more insight into the population dynamics we investigated the dead (unstained) foraminifera in the 2-3 cm level for the fractions 150-300 µm and >300 µm. The fractions >300 µm and 150-300 µm show nearly the same faunal distribution and therefore the results are presented here for both fractions combined (i.e. the >150 µm fraction). Live foraminiferal densities show a clear maximum in the first half centimetre of the sediment; only few specimens are found down to 4 cm depth. The faunas exhibit a clear zonation across the Pakistan margin OMZ. Down to 500 m water depth, Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata dominate the assemblages. These taxa are largely restricted to the upper cm of the sediment. They are adapted to the very low bottom-water oxygen values (ab. 0.1 ml/l in the OMZ core) and the extremely high input of organic carbon on the upper continental slope. The lower part of the OMZ is characterized by cosmopolitan faunas, containing also some taxa that in other areas have been described in deep infaunal microhabitats.

Distribution of benthic and planktic foraminifera in surface sediments of the Indian and Pakistan continental margin, Arabian Sea

During the Indian Ocean Expedition of the German research vessel "Meteor" and the following cruise with the Pakistani fishing vessel "Machhera" in February and March 1965, sediments were sampled from the shelf, continental slope and the Arabian Basin off Pakistan and India. The biostratigraphic studies are based on sedimentary material from 24 sediment cores up to 480 cm long and 100 grab samples. The faunal residues of the > 160 µ fraction (chiefly foraminifera and pteropods) were determined and counted in order to get an idea of the climatic conditions during the Late Quaternary of this region. Biostratigraphic correlations of these Late Quaternary deposits are only possible if the thanatocoenosis of the surface sediments are well known. The analysis of the benthonic foraminiferal populations resulted in the definition of several foraminiferal facies. The following sequence of forarniniferal facies, named after their most characteristic members, can be distinguished from the shelf to the deep-sea: 1. Ammonia-Florilus facies ; 2. Ammonia-Cancris facies; 3. Cassidulina-Cibicides facies; 4. Uvigerina-Cassidulina facies ; 5. Buliminacea facies ; 6. deepwater facies, partly with Bulimina aculeata or with Nonionidae. On the upper continental slope there is a zone extremely poor in benthonic foraminifera. In this water depth the oxygen minimum layer (0.05-0.02 ml/l) of the water column reaches the slope. Almost no connection can be observed between the living and the dead foraminiferal population of the same sample. The regional distribution of the planktonic foraminifera from plankton tows as well as from the surface sediments shows marked differences in the species composition of faunas from different regions within the area of investigation. That depends on oceanographic conditions such as upwelling, dissolution of carbonate at great depths etc. Based on the results of faunal analysis of samples from the recent sea-floor, a biostratigraphic subdivision of the sediments in the cores was established. The following biostratigraphically defined sections could be distinguished from the top of the sediment cores downwards : 1. Relatively cool climatic conditions are reflected by the foraminifera of the uppermost core sections. 2. The next section is characterized by much warmer conditions (Holocene climatic optimum). The C-14 ages of this interval range from 4000 to 10 000 years B.P. according to different authors. C-14 dates on the material investigated do not give reliable clues. 3. Foraminiferal populations adapted to much colder conditions can be observed in the underlying core section. The boundary between the warm climate reflected by the foraminifera of section 2 and the cold climate (section 3) is relatively sharp. It can be correlated from core to core over the whole area investigated. The cold climate sediments of section 3 are underlain by different cool-, warm- and cold-climate sediments which can only be correlated over very short distances. Since it appears certain that the last really cold conditions ended earlier in the Arabian Sea and its vicinity than in Europe it is recommended not to use the European stratigraphic terms for the Quaternary. Because of the lack of reliable absolute sediment ages for the cores no exact sedimentation rates can be given. According to rough estimates, however, the rates are 1-2 cm/1000 years in the deep basin and up to 40 cm/1000 years on the upper continental slope. Sedimentation rates are always larger near the mouth of the Indus-River than off South India at stations of about the same water depth. Planktonic gastropods (mainly pteropods) cannot be used for biostratigraphic purposes in the region under consideration. All of them seem to be displaced from the shelf. Their distribution there is given in.

Multi-proxy analyses of sediment cores from the Pakistan continental margin

Late Holocene laminated sediments from a core transect centred in the oxygen minimum zone (OMZ) impinging at the continental slope off Pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years. High SW-monsoon-controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon (TOC) and redox-sensitive elements (Ni, V), as well as by a low-diversity, high-abundance benthic foraminiferal Buliminacea association and high abundance of the planktonic species Globigerina bulloides indicative of upwelling conditions. Surface-water productivity was strongest during SW monsoon maxima. Stable OMZ conditions (reflected by laminated sediments) were found also during warm interstadial events (Preboreal, Bølling-Allerød, and Dansgaard-Oeschger events), as well as during peak glacial times (17-22.5 ka, all ages in calendar years). Sediment mass accumulation rates were at a maximum during the Preboreal and Younger Dryas periods due to strong riverine input and mobilisation of fine-grained sediment coinciding with rapid deglacial sea-level rise, whereas eolian input generally decreased from glacial to interglacial times. In contrast, the occurrence of bioturbated intervals from 7 to 10.5 ka (early Holocene), in the Younger Dryas (11.7-13 ka), from 15 to 17 ka (Heinrich event 1) and from 22.5 to 25 ka (Heinrich event 2) suggests completely different conditions of oxygen-rich bottom waters, extremely low mass and organic carbon accumulation rates, a high-diversity benthic fauna, all indicating lowered surface-water productivity. During these intervals the OMZ was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods. The abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the North Atlantic Heinrich events and dust peaks in the Tibetan Loess records. The correlation of the monsoon-driven OMZ variability in the Arabian Sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale.

Pteropod preservation on the Pakistan shelf and continental slope

Fifteen surface sediment samples from the Pakistan shelf and upper continental slope and a Late Quaternary high-sedimentation rate core (573 m water depth, Pakistan continental margin) have been analysed to improve the understanding of the factors influencing pteropod preservation. The aragonite compensation depth (ACD) is located at 250-400 m water depth, which corroborates previous observations of a very shallow ACD in the northern Arabian Sea. With the exception of the Hab transect off Karachi, the ACD coincides with the upper boundary of the OMZ located at 250 m water depth. The shell preservation index of the pteropod Limacina inflata (LDX) was applied on six surface sediment samples showing good to very good preservation (LDX: 2.2 to 1.3). The 30 000 yr long record of sediment core SO90 137KA is characterized by alternations between bioturbated and laminated sediments. Bioturbated sediments occurring in the Early Holocene, Younger Dryas and time-equivalents of Heinrich events contain well to perfectly preserved tests of L. inflata (LDX: 2.1-0.2), whereas only traces of pteropods are found in laminated intervals. The close linkage of pteropod preservation in the surface sediments and in core 137KA to well-oxygenated conditions can be explained by repetitive intermediate water formation in the Arabian Sea down to at least 600 m water depth in times of enhanced NE monsoons during stadials and H-equivalents. Low amounts of pteropods in laminated sediments (interstadials, Late Holocene) and in the present-day oxygen minimum zone (OMZ) indicate a weak NE monsoon, stable OMZ and shallow ACD.

Bulk components and planktonic foraminifera of the Chilean continental margin

A multiparameter investigation including organic carbon, carbonate, opal, and planktic foraminifera was carried out on five sediment cores from the coastal upwelling area between 24°S and 33°S along the Peru-Chile Current to reconstruct the history of the paleoproductivity and its driving mechanisms during the last 40,000 years. Inferred from our data, we conclude that the Antarctic Circumpolar Current as the main nutrient source in this region mainly drives the productivity by its latitudinal shifts associated with climate change. Simplified, its northerly position during the last glacial led to enhanced productivities, and its southerly position during the Holocene caused lower productivities. At 33°S the paleoproductivity was additionally affected by the southern westerlies and records highest levels during the Last Glacial Maximum (LGM). North of 33°S, several factors (e.g., position and strength of the South Pacific anticyclone, wind stress, continental runoff, and El Niño Southern Oscillation events) supplementary influenced upwelling and paleoproductivity, where maximum values occurred prior to the LGM and during the deglaciation.

Geochemistry, clay mineralogy and grain size distribution in various sediment cores drilled during ODP Leg 119 on the Antarctic continental shelf off Prydz Bay

Drilling was undertaken at five sites (739-743) on ODP Leg 119 on a transect across the continental shelf of Prydz Bay, East Antarctica, to elucidate the long-term glacial history of the area and to examine the importance of the area with respect to the development of the East Antarctic ice sheet as a whole. In addition to providing a record of glaciation spanning 36 m.y. or more, Leg 119 has provided information concerning the development of a continental margin under the prolonged influence of a major ice sheet. This has allowed the development of a sedimentary model that may be applicable not only to other parts of the Antarctic continental margin, but also to northern high-latitude continental shelves. The cored glacial sedimentary record in Prydz Bay consists of three major sequences, dominated by diamictite: 1. An upper flat-lying sequence that ranges in thickness from a few meters in inner and western Prydz Bay to nearly 250 m in the outer or eastern parts of the bay. The uppermost few meters consist of Holocene diatom ooze and diatomaceous mud with a minor ice-rafted component overlying diamicton and diamictite of late Miocene to Quaternary age. The diamictite is mainly massive, but stratified varieties and minor mudstone and diatomite also occur. 2. An upper prograding sequence cored at Sites 739 and 743, unconformly below the flat-lying sequence. This consists of a relatively steep (4° inclination) prograding wedge with a number of discrete sedimentary packages. At Sites 739 and 743 the sequence is dominated by massive and stratified diamictite, some of which shows evidence of slumping and minor debris flowage. 3. A lower, more gently inclined, prograding sequence lies unconformably below the flat-lying sequence at Site 742 and the upper prograding sequence at Site 739. This extends to the base of both sites, to 316 and 487 mbsf, respectively. It is dominated by massive, relatively clast-poor diamictite which is kaolinite-rich, light in color, and contains sporadic carbonate-cemented layers. The lower part of Site 742 includes well-stratified diamictites and very poorly sorted mudstones. The base of this site has indications of large-scale soft-sediment deformation and probably represents proximity to the base of the glacial sequence. Facies analysis of the Prydz Bay glacial sequence indicates a range of depositional environments. Massive diamictite is interpreted largely as waterlain till, deposited close to the grounding line of a floating glacier margin, although basal till and debris flow facies are also present. Weakly stratified diamictite is interpreted as having formed close to or under the floating ice margin and influenced by the input of marine diatomaceous sediment (proximal glaciomarine setting). Well-stratified diamictite has a stronger marine input, being more diatom-rich, and probably represents a proximal-distal glaciomarine sediment with the glaciogenic component being supplied by icebergs. Other facies include a variety of mudstones and diatom-rich sediments of marine origin, in which an ice-rafted component is still significant. None of the recovered sediments are devoid of a glacial influence. The overall depositional setting of the prograding sequence is one in which the grounded ice margin is situated close to the shelf edge. Progradation was achieved primarily by deposition of waterlain till. The flat-lying sequence illustrates a complex sequence of advances and retreats across the outer part of the shelf, with intermittent phases of ice loading and erosion. The glacial chronology is based largely on diatom stratigraphy, which has limited resolution. It appears that ice reached the paleoshelf break by earliest Oligocene, suggesting full-scale development of the East Antarctic ice sheet by that time. The ice sheet probably dominated the continental margin for much of Oligocene to middle Miocene time. Retreat, but not total withdrawal of the ice sheet, took place in late Miocene to mid-Pliocene time. The late Pliocene to Pleistocene was characterized by further advances across, and progradation of, the continental shelf. Holocene time has been characterized by reduced glacial conditions and a limited influence of glacial processes on sedimentation.

Mineralogy of sediment cores from the continental slope of Chile

Marine sediment cores from the continental slope off mid-latitude Chile (33°S) were studied with regard to grain-size distributions and clay mineral composition. The data provide a 28,000-yr14C accelerator mass spectrometry-dated record of variations in the terrigenous sediment supply reflecting modifications of weathering conditions and sediment source areas in the continental hinterland. These variations can be interpreted in terms of the paleoclimatic evolution of mid-latitude Chile and are compared to existing terrestrial records. Glacial climates (28,000-18,000 cal yr B.P.) were generally cold-humid with a cold-semiarid interval between 26,000 and 22,000 cal yr B.P. The deglaciation was characterized by a trend toward more arid conditions. During the middle Holocene (8000-4000 cal yr B.P.), comparatively stable climatic conditions prevailed with increased aridity in the Coastal Range. The late Holocene (4000-0 cal yr B.P.) was marked by more variable paleoclimates with generally more humid conditions. Variations of rainfall in mid-latitude Chile are most likely controlled by shifts of the latitudinal position of the Southern Westerlies. Compared to the Holocene, the southern westerly wind belt was located significantly farther north during the last glacial maximum. Less important variations of the latitudinal position of the Southern Westerlies also occurred on shorter time scales.

(Table 1) Age of Cretaceous seamounts in the Western Pacific

Dynamics of the Pacific Plate is recorded in the systematic variation of location and the 40Ar-39Ar age of seamounts in the Western Pacific from 120 to 65 Ma ago. The seamounts are grouped into three linear zones as long as 5000 km. The seamounts become younger in the southeastern direction along the strike of these zones. Correlation between age and location of seamounts allows to divide the history of their formation into three stages. Rate of seamount growth was relatively low (2-4 cm/yr) during the first and the third stages within intervals of 120-90 and 85-65 Ma, whereas during the second stage (90-85 Ma), the seamounts were growing very fast (80-100 cm/yr). In the midst of this stage, at ~87 Ma ago, magmatic activity increased abruptly. Dynamics of seamount building is in good agreement with (1) pulses in development of the Ontong Java, Manihiki, and Caribbean-Colombian oceanic plateaus; (2) age of spreading acceleration in the mid-Cretaceous; and (3) a short period when the Izanagi Plate ceased to exist and the Kula Plate was formed. Variation in seamounts' age and location are in consistence with the hypothesis of diffuse extension of the Pacific Plate in course of its motion with formation of impaired zones of decompression melting. Direction of extension (325°-340° NW) calculated from the strike of seamount zones is consistent with the path of the Pacific Plate (330° NW) in the Late Cretaceous. Immense perioceanic volcanic belts were formed at that time along the margin of the Asian continent. The Okhotsk-Chukchi Peninsula Belt extends at a right angle to the compression vector. Three stages of this belt's evolution are synchronous with the stages of seamount formation in the Pacific Plate. Delay in origination of the East Sikhote-Alin Volcanic Belt and its different orientation were caused by counterclockwise rotation of the vector of convergence of oceanic and continental plates in the mid-Cretaceous. At the same time, i.e. 95-85 Ma ago, volcanic activity embraced the entire continental margin and tin granites were emplaced everywhere in the Eastern Asia. This short episode (90+/-5 Ma) corresponds to the mid-Cretaceous maximum of compression of the continental margin, and its age fits well a culmination in extension of the Pacific Plate.

Early Oligocene continental climate ofthe Paleogene Basin in Hungary and Slovenia

This paper concentrates on the Early Oligocene palaeoclimate of the southern part of Eastern and Central Europe and gives a detailed climatological analysis, combined with leaf-morphological studies and modelling of the palaeoatmospheric CO2 level using stomatal and d13 C data. Climate data are calculated using the Coexistence Approach for Kiscellian floras of the Palaeogene Basin (Hungary and Slovenia) and coeval assemblages from Central and Southeastern Europe. Potential microclimatic or habitat variations are considered using morphometric analysis of fossil leaves from Hungarian, Slovenian and Italian floras. Reconstruction of CO2 is performed by applying a recently introduced mechanistic model. Results of climate analysis indicate distinct latitudinal and longitudinal climate patterns for various variables which agree well with reconstructed palaeogeography and vegetation. Calculated climate variables in general suggest a warm and frost-free climate with low seasonal variation of temperature. A difference in temperature parameters is recorded between localities from Central and Southeastern Europe, manifested mainly in the mean temperature of the coldest month. Results of morphometric analysis suggest microclimatic or habitat difference among studied floras. Extending the scarce information available on atmospheric CO2 levels during the Oligocene, we provide data for a well-defined time-interval. Reconstructed atmospheric CO2 levels agree well with threshold values for Antarctic ice sheet growth suggested by recent modelling studies. The successful application of the mechanistic model for the reconstruction of atmospheric CO2 levels raises new possibitities for future climate inference from macro-flora studies.

Rocks and minerals from the Strakhov Fracture Zone, 4°N Mid-Atlantic Ridge

Geological-geophysical data obtained during Cruises 7, 11, and 12 of R/V Akademic Nikolay Strakhov (1989-1991) within the international project EQUARIDGE in the Strakhov Fracture Zone region (4°N) are presented. The trough of the fracture is interpreted as an open extension joint, a graben produced by stretching along the axis of the Mid-Atlantic Ridge. Bedrock studies showed that typical mid-ocean tholeiitic basalts occur within the narrow (60 nm wide) axial rift zone, whereas igneous rocks not typical for the ocean were found on the eastern and western flank plateaus. This allows to suppose that a reworked relict continental-type basement of pre-Upper Jurassic age possibly exists beneath the flank plateaus, within the segment under discussion. The above data correspond to the hypothesis of E. Bonatti about nonspreading nature of the basement of Mid-Atlantic Ridge within the equatorial segment and the Strakhov Fracture Zone.

Chemical composition of iron-manganese and phosphate rocks from aseismic ridges of the Indian Ocean and from the continental slope of the Southeast Africa

Study of chemical composition of 26 samples collected at depths from 400 to 1400 m on vertex surfaces of the Southeast Indian Ridge, Mascarene Ridge, Madagascar Ridge, and Mozambique Ridge, as well as on the upper part of the Southeast Africa continental slope showed that the samples represent three groups of rocks: 1) low phosphate or phosphate-free ferromanganese rocks, 2) phosphate ferromanganese rocks 3) phosphorites and phosphatized limestones.

Turbidite recognition and radiocarbon ages of marine sediment cores along the continental margin of Chile

Much progress has been made in estimating recurrence intervals of great and giant subduction earthquakes using terrestrial, lacustrine, and marine paleoseismic archives. Recent detailed records suggest these earthquakes may have variable recurrence periods and magnitudes forming supercycles. Understanding seismic supercycles requires long paleoseismic archives that record timing and magnitude of such events. Turbidite paleoseismic archives may potentially extend past earthquake records to the Pleistocene and can thus complement commonly shorter-term terrestrial archives. However, in order to unambiguously establish recurring seismicity as a trigger mechanism for turbidity currents, synchronous deposition of turbidites in widely spaced, isolated depocenters has to be ascertained. Furthermore, characteristics that predispose a seismically active continental margin to turbidite paleoseismology and the correct sample site selection have to be taken into account. Here we analyze 8 marine sediment cores along 950 km of the Chile margin to test for the feasibility of compiling detailed and continuous paleoseismic records based on turbidites. Our results suggest that the deposition of areally widespread, synchronous turbidites triggered by seismicity is largely controlled by sediment supply and, hence, the climatic and geomorphic conditions of the adjacent subaerial setting. The feasibility of compiling a turbidite paleoseismic record depends on the delicate balance between sufficient sediment supply providing material to fail frequently during seismic shaking and sufficiently low sedimentation rates to allow for coeval accumulation of planktonic foraminifera for high-resolution radiocarbon dating. We conclude that offshore northern central Chile (29-32.5°S) Holocene turbidite paleoseismology is not feasible, because sediment supply from the semi-arid mainland is low and almost no Holocene turbidity-current deposits are found in the cores. In contrast, in the humid region between 36 and 38°S frequent Holocene turbidite deposition may generally correspond to paleoseismic events. However, high terrigenous sedimentation rates prevent high-resolution radiocarbon dating. The climatic transition region between 32.5 and 36°S appears to be best suited for turbidite paleoseismology.