Sedimentology and geochemistry of Oxygen Isotope Stage 3 and Holocene sediments from Laguna Potrok Aike, Patagonia

Seismic reflection studies in the maar lake Laguna Potrok Aike (51°58? S, 70°23? W) revealed an erosional unconformity associated with a sub-aquatic lake-level terrace at a water depth of 30m. Radiocarbon-dated, multi-proxy sediment studies of a piston core from this location indicate that the sediment below this discontinuity has an age of 45kyr BP (Oxygen Isotope Stage 3), and was deposited during an interval of high lake level. In comparison to the Holocene section, geochemical indicators of this older part of the record either point towards a different sediment source or to a different transport mechanism for Oxygen Isotope Stage 3 sediments. Holocene sedimentation started again before 6790cal. yr BP, providing a sediment record of hydrological variability until the present. Geochemical and isotopic data indicate a fluctuating lake level until 5310cal. yr BP. During the late Holocene the lake level shows a receding tendency. Nevertheless, the lake level did not drop below the 30m terrace to create another unconformity. The geochemical characterization of volcanic ashes reveals evidence for previously unknown explosive activity of the Reclús and Mt. Burney volcanoes during Oxygen Isotope Stage 3.

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.

Sedimentology of the Vietnam shelf

On the Vietnam Shelf more than 1000 miles of shallow high-resolution seismics were analyzed to unravel post-glacial evolution in a tropical, siliciclastic environment together with 25 sediment cores from water depths between 21 and 169 m to determine stratigraphy, distribution and style of sedimentation. Fourty-seven samples were dated with the AMS-14C technique. The shelf was grouped into three regions: a southern part, a central part, and a northern part. On the broad Southern Shelf, sedimentation is influenced by the Mekong River, which drains into the SCS in this area. Here, incised valley fills are abundant that were cut into the late Pleistocene land surface by the Paleo-Mekong River during times of sea level lowstand. Those valleys are filled with transgressive deposits. The Holocene sedimentation rate in this low gradient accommodation-dominated depositional system is in the range of 5-10 and 25-40 cm/ky at locations sheltered from currents. The Central Shelf is narrow and the sedimentary strata are conformable. Here, numerous small mountainous rivers reach the SCS and transport large amounts of detrital sediment onto the shelf. Therefore, the Holocene sedimentation rate is high with values of 50-100 cm/ky in this supply-dominated depositional system. The broad Northern Shelf in the vicinity of the Red River Delta shows, as on the Southern Shelf, incised valleys cut into the Pleistocene land surface by paleo river channels. In this accommodation-dominated shelf area, the sedimentation rate is low with values of 5-10 cm/ky. Where applicable, we assigned the sampled deposits to different paleo-facies. The latter are related to certain intervals of water depths at their time of deposition. Comparison with the sea-level curve of (Hanebuth et al., 2000, doi:10.1126/science.288.5468.1033) indicates subsidence on the Central Shelf, which is in agreement with the high sedimentation rates in this area. In contrast, data from the Northern Shelf suggest tectonic uplift that might be related to recent tectonic movements along the Ailao Shan-Red River Fault zone. Data from the Southern Shelf are generally in agreement with the sea-level curve mentioned above.

Stable oxygen isotope and Mg/Ca ratios of planktonic foraminifera from the Holocene and the Last Glacial Maximum

We measured oxygen isotopes and Mg/Ca ratios in the surface-dwelling planktonic foraminifer Globigerinoides ruber (white s.s.) and the thermocline dweller Pulleniatina obliquiloculata to investigate upper ocean spatial variability in the Indo-Pacific Warm Pool (IPWP). We focused on three critical time intervals: the Last Glacial Maximum (LGM; 18-21.5 ka), the early Holocene (8-9 ka), and the late Holocene (0-2 ka). Our records from 24 stations in the South China Sea, Timor Sea, Indonesian seas, and western Pacific indicate overall dry and cool conditions in the IPWP during the LGM with a low thermal gradient between surface and thermocline waters. During the early Holocene, sea surface temperatures increased by ~3°C over the entire region, indicating intensification of the IPWP. However, in the eastern Indian Ocean (Timor Sea), the thermocline gradually shoaled from the LGM to early Holocene, reflecting intensification of the subsurface Indonesian Throughflow (ITF). Increased surface salinity in the South China Sea during the Holocene appears related to northward displacement of the monsoonal rain belt over the Asian continent together with enhanced influx of saltier Pacific surface water through the Luzon Strait and freshwater export through the Java Sea. Opening of the freshwater portal through the Java Sea in the early Holocene led to a change in the vertical structure of the ITF from surface- to thermocline-dominated flow and to substantial freshening of Timor Sea thermocline waters.

AMS radiocarbon dates and pollen analysis of ODP Hole 175-1078C

ODP Site 1078 situated under the coast of Angola provides the first record of the vegetation history for Angola. The upper 11 m of the core covers the past 30 thousand years, which has been analysed palynologically in decadal to centennial resolution. Alkenone sea surface temperature estimates were analysed in centennial resolution. We studied sea surface temperatures and vegetation development during full glacial, deglacial, and interglacial conditions. During the glacial the vegetation in Angola was very open consisting of grass and heath lands, deserts and semi-deserts, which suggests a cool and dry climate. A change to warmer and more humid conditions is indicated by forest expansion starting in step with the earliest temperature rise in Antarctica, 22 thousand years ago. We infer that around the period of Heinrich Event 1, a northward excursion of the Angola Benguela Front and the Congo Air Boundary resulted in cool sea surface temperatures but rain forest remained present in the northern lowlands of Angola. Rain forest and dry forest area increase 15 thousand years ago. During the Holocene, dry forests and Miombo woodlands expanded. Also in Angola globally recognised climate changes at 8 thousand and 4 thousand years ago had an impact on the vegetation. During the past 2 thousand years, savannah vegetation became dominant.

Postglacial deposition of sand in the Eckernfoerder Bay, western Baltic Sea

At the NW-slope of Eckernforder Bay (Western Baltic) between 14 and 21 m water depth 7 sand cores were taken with a vibrocorer. The cores were between 85 and 250 cm long. The sand was analysed for grain size distribution, proportions of organic carbon and carbonate, and contents of microfossils. The radiometric age and stable carbon isotope ratios were determined on organic material from 14 sample. With regard to benthic foraminifera and other microorganisms four different types of depositional conditions could be distinguished: Types 1 and 2: two types of offshore sand areas. Type 3: lagoon and nearshore. Type 4: subaerial or limnic. Using sedimentological and geochemical parameters two formation areas could be distinguished with the aid of a discriminant analysis: offshore (types 1 and 2) and nearshore (types 3 and 4). A juxtaposition of core sections indicated two distinct profiles. Their ages fit into the picture of the assumed postglacial sea-level rise. The lagoon- and nearshore sands are interpreted as the result of sea-level stagnation at 17-18 m below present sea-level. The accumulation rates of the sand in the offshore areas are, with a maximum of 0.15 mm/yr., an order of magnitude smaller than in the mud areas, located several hundred metres away.

Climate proxies in sediment core MD95-2010

High-amplitude, rapid climate fluctuations are common features of glacial times. The prominent changes in air temperature recorded in the Greenland ice cores (Dansgaard et al., 1993, doi:10.1038/339532a0; Grootes et al., 1993 doi:10.1038/366552a0) are coherent with shifts in the magnitude of the northward heat flux carried by the North Atlantic surface ocean (Bond et al., 1993, doi:10.1038/365143a0; Bond and Lotti, 1995, doi:10.1126/science.267.5200.1005); changes in the ocean's thermohaline circulation are a key component in many explanations of this climate flickering (Broecker, 1997, doi:10.1126/science.278.5343.1582). Here we use stable-isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate-change events. Deep water was generated more or less continuously in the Nordic Seas during the latter part of the last glacial period (60 to 10 thousand years ago), but by two different mechanisms. The deep-water formation occurred by convection in the open ocean during warmer periods (interstadials). But during colder phases (stadials), a freshening of the surface ocean reduced or stopped open-ocean convection, and deep-water formation was instead driven by brine-release during sea-ice freezing. These shifting magnitudes and modes nested within the overall continuity of deep-water formation were probably important for the structuring and rapidity of the prevailing climate changes.

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.

Paleomagnetic and radiocarbon-based chronologies of sediment cores MD99-2269 and MD99-2322

We report the intercalibration of paleomagnetic secular variation (PSV) and radiocarbon dates of two expanded postglacial sediment cores from geographically proximal, but oceanographically and sedimentologically contrasting settings. The objective is to improve relative correlation and chronology over what can be achieved with either method alone. Core MD99-2269 was taken from the Húnaflóaáll Trough on the north Iceland shelf. Core MD99-2322 was collected from the Kangerlussuaq Trough on the east Greenland margin. Both cores are well dated, with 27 and 20 accelerator mass spectrometry 14C dates for cores 2269 and 2322, respectively. Paleomagnetic measurements made on u channel samples document a strong, stable, single-component magnetization. The temporal similarities of paleomagnetic inclination and declination records are shown using each core's independent calibrated radiocarbon age model. Comparison of the PSV records reveals that the relative correlation between the two cores could be further improved. Starting in the depth domain, tie points initially based on calibrated 14C dates are either adjusted or added to maximize PSV correlations. Radiocarbon dates from both cores are then combined on a common depth scale resulting from the PSV correlation. Support for the correlation comes from the consistent interweaving of dates, correct alignment of the Saksunarvatn tephra, and the improved correlation of paleoceanographic proxy data (percent carbonate). These results demonstrate that PSV correlation used in conjunction with 14C dates can improve relative correlation and also regional chronologies by allowing dates from various stratigraphic sequences to be combined into a single, higher dating density, age-to-depth model.