Age determination of sediment cores off western Spitsbergen

The Late Weichselian glacial history of the continental shelf off western Spitsbergen is discussed, based on acoustic sub-bottom records and sediment cores. The outer part of Isfjorden and the inner shelf to the west of this fjord are characterized by a thin veneer (10-20 m) of glacigenic sediments and absence of ice-marginal features. Towards the outer shelf the sediment thickness increases significantly, and exceeds 500 m at the shelf edge. Possible moraine complexes were identified in this outer part. Sediment cores from the inner shelf sampled a firm diamicton, interpreted as till, beneath soft glaciomarine sediments. Radiocarbon dates on shells from the clay resting directly on the till, suggest an age of around 12,500 yrs B.P. for the base of the marine sequence. We argue that grounded ice covered the sites shortly before. In contrast to suggestions that the fjords and coast were partly ice free during the Late Weischselian, we conclude that the ice must have reached out onto the continental shelf.

(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.

Neogene diatoms from the western Antarctic Peninsula

During Ocean Drilling Program Leg 178 we cored nine sites on the continental rise (Sites 1095, 1096, and 1101), continental shelf (Sites 1097, 1100, 1102, and 1103), and in an inner shelf basin, Palmer Deep (Sites 1098 and 1099), along the Pacific margin of the Antarctic Peninsula. Fossil diatoms are a key group that provides age constraint for these shelf site sediments to allow reconstruction of Antarctic Peninsula glacial history. This paper provides the systematic paleontology of diatoms applied in biostratigraphic and paleoceanographic studies and includes a total of 33 plates. Taxonomic confusion in previous reports, including biostratigraphically useful species such as Thalassiosira inura and Thalassiosira complicata, is discussed. These systematics and taxonomic discussions help to provide a reference for Neogene diatoms in the Southern Ocean.

Composition of bottom sediments from the Isfjord, Western Svalbard

Lithology, heavy mineral associations, and chemical composition of bottom sediments studied in two gravity cores from Isfjord, Western Spitsbergen (Svalbard) accompanied by high-frequency seismic records, provide a new insight on provenance and glaciomarine sedimentation in the fjord from the last deglaciation through Holocene.

Pollen records from Elbe-Weser Dreieck

A long-running interdisciplinary research project on the development of landscape, prehistoric habitation and the history of vegetation within a "siedlungskammer" (limited habitation areal from neolithic to modern times has been carried out in the NW German lowlands, The siedlungskammer Flögeln is situated between the rivers Weser and EIbe and comprises about 23.5 km^2. It is an isolated pleistocene area surrounded by bogs, the soils consisting mainly of poor sands. In this siedlungskammer large-seale archaeological excavations and mappings have been performed, parallel to pedological, historical and above all pollen analytical investigations. The aim of the project is to record the individual phases in time, to delimit the respective settlement areas and to reconstruct the conditions of life and economy for each time period. A dense network of 10 pollen diagrams has been constructed. Several of them derive from the marginal area and from the centre of the large raised bog north of the siedlungskammer. These diagrams reflect the history of vegetation and habitation of a large region; due to the large pollen source area the habitation phases in the diagrams are poorly defined. Even in the utmost marginal diagram of this woodless bog, a great village with adjoining fields, situated only 100 m away from it, is registered with only low values of anthropogenic indicators. In contrast to this, the numerous pollen diagrams from kettle-hole bogs inside the siedlungskammer yield an exact picture of the habitation of the siedlungskammer and their individual parts. Early traces of habitation can be identified in the pollen diagram soon after the elm decline (around 5190 BP). Some time later in the middle neolithic period there follows a marked habitation phase, which starts between 4500 and 4400 BP and reflects the immigration of the trichterbecher culture. It corresponds to the landnam phase of Iversen in Denmark and begins with a sharp decline of the pollen curves of lime and oak, followed by the increase of anthropogenic indicators pointing to arable and pastural farming. High values of wild grasses and Calluna witness extensive forest grazing. This middle to late neolithic habitation is also registered archaeologically by settlements and numerous graves. After low human activity during Bronze Age and Older Iron Age times the archaeological and pollen analytical records of Roman and Migration periods is again very strong. This is followed by a gap in habitation during the 6th and 7th centuries and afterwards in the western part of the siedlungskammer from about 700 AD until the 14th century by the activity of the medieval village of Dalem, that was also excavated and whose fields were recorded by phosphate mapping to a size of 117 hectares. This medieval settlement phase is marked by much cereal cultivation (mainly rye). The dense network of pollen diagrams offers an opportunity to register the dispersion of the anthropogenic indicators from the areas of settlement to different distances and thus to obtain quantitative clues for the assessment of these anthropogenic indicators in pollen diagrams. In fig. 4 the reflection of the neolithic culture in the kettle-hole bogs and the large raised bog is shown in 3 phases: a) pre landnam, b) TRB-landnam, c) post landnam. Among arboreal pollen the reaction of Quercus is sharp close to the settlement but is not found at more distant profiles, whilst in contrast to this Tilia shows a significant decline even far away from the settlements. The record of most anthropogenic indicators outside the habitation area is very low, in particular cereal pollen is poorly dispersed; much more certain as an indicator for habitation (also for arable farming!) is Plantago lanceolata. A strong increase of wild grasses (partly Calluna aswell) some distance from the habitation areas indicates far reaching forest grazing. Fig. 5 illustrates the reflection of the anthropogenie indicators from the medieval village Dalem. In this instance the field area could be mapped exactly using phosphate investigations, and it has been possible to indicate the precise distances of the profile sites from the medieval fields. Here also, there is a clear correlation between decreasing anthropogenic indicators and increasing distance. In a kettle-hole bog (FLH) a distance of 3000 m away this marked settlement phase is not registered. The contrast between the pollen diagrams SWK and FLH (fig. 2 + 3, enclosure), illustrates the strong differences between diagrams from kettlehole bogs close to and distant from the settlements, for the neolithic as well as for the medieval period. On the basis of the examples presented here, implications concerning the interpretation of pollen diagrams with respect to habitation phases are discussed.

Seawater Cd reconstruction for the western subtropical South Atlantic

Benthic foraminiferal Cd/Ca from an intermediate depth, western South Atlantic core documents the history of southward penetration of North Atlantic Intermediate Water (NAIW). Cd seawater estimates (CdW) for the last glacial are consistent with the production of NAIW and its export into the South Atlantic. At ~14.5 ka concurrently with the onset of the Bølling-Allerød to Younger Dryas cooling, the NAIW contribution to the South Atlantic began to decrease, marking the transition from a glacial circulation pattern to a Younger Dryas circulation. High CdW in both the deep North Atlantic and the intermediate South Atlantic imply reduced export of deep and intermediate water during the Younger Dryas and a significant decrease in northward oceanic heat transport. A modern circulation was achieved at ~9 ka, concurrently with the establishment of Holocene warmth in the North Atlantic region, further supporting a close linkage between deepwater variability and North Atlantic climate.

Jurassic sedimentation history at DSDP Hole 76-534A

Site 534 reflects a complex interplay of global, basinal, and local influences on sedimentation during the Callovian and Late Jurassic. Rifting and rapid subsidence of the continental margins of the North Atlantic-Tethys seaway occurred during the late Early Jurassic (Sinemurian-Pliensbachian), but rapid spreading between the North American margin (Blake Spur Ridge and magnetic lineation) and the northwest African margin did not commence until the Bathonian or earliest Callovian. Site 534, drilled on marine magnetic anomaly "M-28" of Bryan et al. (1980), was initially about 150 km from either continental margin. The ?middle Callovian basal sediments are dusky red silty marl. Callovian transgression led to active carbonate platforms on the margin, recorded at Site 534 as a rise in the CCD (carbonate compensation depth), then arrival of lime-rich turbidites from the Blake Plateau platform across the Blake Spur Ridge. The host pelagic sediment is greenish black, organic-rich, radiolarian-rich, silty claystone. Hydrothermal activity on the nearby spreading ridge enriched this lower unit in metals. In the Oxfordian, the input of terrestrial silt rapidly diminished; radiolarians or other bioclasts were not preserved. The dark variegated claystone has fine-grained marl and reddish claystone turbidite beds. The late Callovian-Oxfordian Western Tethys has radiolarian chert deposition, marine hiatuses, or organic-rich sediments. The Kimmeridgian and Tithonian had a stable or receding sea level. Near the end of the Jurassic many of the carbonate platforms of the margins were buried beneath prograding fan or alluvial deposits. Carbonate deposition shifted to the deep sea. Site 534 records the deepening of the CCD and ACD (aragonite compensation depth) during the Kimmeridgian and early Tithonian, then a rise of the ACD in the middle Tithonian. Similar trends occurred throughout the Western Tethys-Atlantic. High nannofossil productivity of the seaway led to deposition of very widespread white micritic limestone in the late Tithonian-Berriasian. The underlying sediment had a slower deposition rate of carbonate, therefore its higher clay and associated Fe content produced a red marl. A short sea-level incursion occurred on the Atlantic margins during the Kimmeridgian and is reflected in the Site 534 greenish gray marl unit by numerous turbidite beds of shallow-water carbonates.

Geochemistry of samples from Valtournenche, Western Italian Alps

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic history, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallization at HT conditions; ages scatter from 263-294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Stable isotopes, core logging data, relative paleointensity, dry bulk density, biogenic opal, CN-data (TC, TOC, CaCO3, TN), element concentrations, and coarse fraction of three sediment cores from the western Bering Sea

We used piston cores recovered in the western Bering Sea to reconstruct millennial-scale changes in marine productivity and terrigenous matter supply over the past ~180 kyr. Based on a geochemical multi-proxy approach, our results indicate closely interacting processes controlling marine productivity and terrigenous matter supply comparable to the situation in the Okhotsk Sea. Overall, terrigenous inputs were high, whereas export production was low. Minor increases in marine productivity occurred during intervals of Marine Isotope Stage 5 and interstadials, but pronounced maxima were recorded during interglacials and Termination I. The terrigenous material is suggested to be derived from continental sources on the eastern Bering Sea shelf and to be subsequently transported via sea ice, which is likely to drive changes in surface productivity, terrigenous inputs, and upper-ocean stratification. From our results we propose glacial, deglacial, and interglacial scenarios for environmental change in the Bering Sea. These changes seem to be primarily controlled by insolation and sea-level forcing which affect the strength of atmospheric pressure systems and sea-ice growth. The opening history of the Bering Strait is considered to have had an additional impact. High-resolution core logging data (color b*, XRF scans) strongly correspond to the Dansgaard-Oeschger climate variability registered in the NGRIP ice core and support an atmospheric coupling mechanism of Northern Hemisphere climates.

Silt grain-size analysis of ODP holes from the western equatorial Atlantic

Grain-size records of the terrigenous and calcareous silt fraction, preservation of planktic foraminifera, and benthic foraminiferal stable-isotope data (delta13C, delta18O values of C. wuellerstorfi) at ODP Site 927 on the Ceará Rise (5°27.7'N, 44°28.8'W), are used to reconstruct variations in the history of bottom current strength, ventilation, and carbonate corrosiveness of deep waters during the time interval from 0.8 to 0.3 Ma. Glacial periods are characterized by generally smaller mean sizes of the terrigenous sortable silt fraction (mean(SS)), lower delta13C values, and poorer preservation of planktic foraminifera compared to interglacials. This indicates lower bottom current speeds, larger nutrient contents and more corrosive deep water. By contrast, larger mean(SS) sizes, higher delta13C values, and well preserved planktic foraminifera indicate strong circulation and a well ventilated deep-water mass during interglacials. The observed changes are most likely related to the weakening and strengthening of circulation of Lower North Atlantic Deep Water (LNADW). Cross-spectral analysis between the mean(SS) and benthic delta18O records reveals that minima in mean(SS) occur about 7.6 k.y. after the maximum in ice volume. This indicates a considerable lag time between ice-shield induced changes in LNADW production and subsequent changes in the velocity of LNADW flow in the western equatorial Atlantic. Striking changes in bottom current speed occur regularly during glacial to interglacial transitions. Extremely fine mean(SS) minima point to an almost complete shutdown of bottom current vigor in response to a cessation of LNADW production caused by an enhanced melt water release during the initial phases of deglaciation. However, each of the fine minima extremes is followed by a rapid shift to very high mean(SS) values that indicate strong bottom currents, and hence, vigorous LNADW flow during the early interglacials. After the onset of glacial Stage 12, generally poorer carbonate preservation and higher variability is registered. This coincides with a global decrease in carbonate preservation during the mid-Brunhes (mid-Brunhes dissolution event). Detailed grain-size analysis of the calcareous fine fraction (<63 µm) revealed a considerable reduction of particles in the fraction from 7 to 63 µm during periods of enhanced dissolution. This indicates a preferential dissolution of larger planktic foraminiferal fragments which leads to an enrichment of coccoliths in the calcareous fine fraction.