Validation of climate model-inferred regional temperature change for late-glacial Europe

Comparisons of climate model hindcasts with independent proxy data are essential for assessing model performance in non-analogue situations. However, standardized palaeoclimate data sets for assessing the spatial pattern of past climatic change across continents are lacking for some of the most dynamic episodes of Earth’s recent past. Here we present a new chironomid-based palaeotemperature dataset designed to assess climate model hindcasts of regional summer temperature change in Europe during the late-glacial and early Holocene. Latitudinal and longitudinal patterns of inferred temperature change are in excellent agreement with simulations by the ECHAM-4 model, implying that atmospheric general circulation models like ECHAM-4 can successfully predict regionally diverging temperature trends in Europe, even when conditions differ significantly from present. However, ECHAM-4 infers larger amplitudes of change and higher temperatures during warm phases than our palaeotemperature estimates, suggesting that this and similar models may overestimate past and potentially also future summer temperature changes in Europe.

Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits

The Quaternary Vakinankaratra volcanic field in the central Madagascar highlands consists of scoria cones, lava flows, tuff rings, and maars. These volcanic landforms are the result of processes triggered by intracontinental rifting and overlie Precambrian basement or Neogene volcanic rocks. Infrared-stimulated luminescence (IRSL) dating was applied to 13 samples taken from phreatomagmatic eruption deposits in the Antsirabe–Betafo region with the aim of constraining the chronology of the volcanic activity. Establishing such a chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption deposits and IRSL ages suggest at least five phreatomagmatic eruption events in Late Pleistocene times. In the Lake Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109 ± 15 and 90 ± 11 ka and is possibly related to an eruption at the Amboniloha volcanic complex to the north. The younger one gives ages between 58 ± 4 and 47 ± 7 ka and is clearly related to the phreatomagmatic eruption that formed Lake Andraikiba. IRSL ages of a similar eruption deposit directly overlying basement laterite in the vicinity of the Fizinana and Ampasamihaiky volcanic complexes yield coherent ages of 68 ± 7 and 65 ± 8 ka. These ages provide the upper age limit for the subsequently developed Iavoko, Antsifotra, and Fizinana scoria cones and their associated lava flows. Two phreatomagmatic deposits, identified near Lake Tritrivakely, yield the youngest IRSL ages in the region, with respective ages of 32 ± 3 and 19 ± 2 ka. The reported K-feldspar IRSL ages are the first recorded numerical ages of phreatomagmatic eruption deposits in Madagascar, and our results confirm the huge potential of this dating approach for reconstructing the volcanic activity of Late Pleistocene to Holocene volcanic provinces.

Late Quaternary history of North Eurasian Norway spruce ( Picea abies ) and Siberian spruce ( Picea obovata ) inferred from macrofossils, pollen and cytoplasmic DNA variation

Aim We used combined palaeobotanical and genetic data to assess whether Norway spruce (Picea abies) and Siberian spruce (Picea obovata), two major components of the Eurasian boreal forests, occupied separate glacial refugia, and to test previous hypotheses on their distinction, geographical delimitation and introgression. Location The range of Norway spruce in northern Europe and Siberian spruce in northern Asia. Methods Pollen data and recently compiled macrofossil records were summarized for the Last Glacial Maximum (LGM), late glacial and Holocene. Genetic variation was assessed in 50 populations using one maternally (mitochondrial nad1) and one paternally (chloroplast trnT–trnL) inherited marker and analysed using spatial analyses of molecular variance (SAMOVA). Results Macrofossils showed that spruce was present in both northern Europe and Siberia at the LGM. Congruent macrofossil and pollen data from the late glacial suggested widespread expansions of spruce in the East European Plain, West Siberian Plain, southern Siberian mountains and the Baikal region. Colonization was largely completed during the early Holocene, except in the formerly glaciated area of northern Europe. Both DNA markers distinguished two highly differentiated groups that correspond to Norway spruce and Siberian spruce and coincide spatially with separate LGM spruce occurrences. The division of the mtDNA variation was geographically well defined and occurred to the east of the Ural Mountains along the Ob River, whereas the cpDNA variation showed widespread admixture. Genetic diversity of both DNA markers was higher in western than in eastern populations. Main conclusions North Eurasian Norway spruce and Siberian spruce are genetically distinct and occupied separate LGM refugia, Norway spruce on the East European Plain and Siberian spruce in southern Siberia, where they were already widespread during the late glacial. They came into contact in the basin of the Ob River and probably hybridized. The lower genetic diversity in the eastern populations may indicate that Siberian spruce suffered more from past climatic fluctuations than Norway spruce.

Rapid responses of high-mountain vegetation to early Holocene environmental changes in the Swiss Alps

1 The Early Holocene sediment of a lake at tree line (Gouillé Rion, 2343 m a.s.l.) in the Swiss Central Alps was sampled for plant macrofossils. Thin (0.5 cm) slices, representing time intervals of c. 50 years each from 11 800 to 7800 cal. year bp, were analysed and the data compared with independent palaeoclimatic proxies to study vegetational responses to environmental change. 2 Alpine plant communities (e.g. with Salix herbacea) were established at 11 600–11 500 cal. year bp, when oxygen-isotope records showed that temperatures increased by c. 3–4 °C within decades. Larix decidua trees reached the site at c. 11 350 cal. year bp, probably in response to further warming by 1–2 °C. Forests dominated by L. decidua persisted until 9600 cal. year bp, when Pinus cembra became more important. 3 The dominance of Larix decidua for two millennia is explained by dry summer conditions, and possibly low winter temperatures, which favoured it over the late-successional Pinus cembra. Environmental conditions were a result of variations in the earth's orbit, leading to a maximum of summer and a minimum of winter solar radiation. Other heliophilous and drought-adapted species, such as Dryas octopetala and Juniperus nana, could persist in the open L. decidua forests, but were out-competed when the shade-tolerant P. cembra expanded. 4 The relative importance of Larix decidua decreased during periods of diminished solar radiation at 11 100, 10 100 and 9400 cal. year bp. Stable concentrations of L. decidua indicate that these percentage oscillations were caused by temporary increases of Pinus cembra, Dryas octopetala and Juniperus nana that can be explained by increases in moisture and/or decreases in summer temperature. 5 The final collapse of Larix decidua at 8400 cal. year bp was possibly related to abrupt climatic cooling as a consequence of a large meltwater input to the North Atlantic. Similarly, the temporary exclusion of Pinus cembra from tree line at 10 600–10 200 cal. year bp may be related to slowing down of thermohaline circulation at 10 700–10 300 cal. year bp. 6 Our results show that tree line vegetation was in dynamic equilibrium with climate, even during periods of extraordinarily rapid climatic change. They also imply that forecasted global warming may trigger rapid upslope movements of the tree line of up to 800 m within a few decades or centuries at most, probably inducing large-scale displacements of plant species as well as irrecoverable biodiversity losses.

Holocene vegetation development in the catchment of Sagistalsee (1935 m asl), a small lake in the Swiss Alps

Pollen and plant macrofossils were analysed at Sägistalsee (1935 m asl), a small lake near timber-line in the Swiss Northern Alps. Open forests with Pinus cembra and Abies alba covered the catchment during the early Holocene (9000–6300 cal. BP), suggesting subcontinental climate conditions. After the expansion of Picea abies between 6300 and 6000 cal. BP the subalpine forest became denser and the tree-line reached its maximum elevation at around 2260 m asl. Charcoal fragments in the macrofossil record indicate the beginning of Late-Neolithic human impact at ca. 4400 cal. BP, followed by a extensive deforestation and lowering of the forest-limit in the catchment of Sägistalsee at 3700 cal. BP (Bronze Age). Continuous human activity, combined with a more oceanic climate during the later Holocene, led to the local extinction of Pinus cembra and Abies alba and favoured the mass expansion of Picea and Alnus viridis in the subalpine area of the Northern Alps. The periods before 6300 and after 3700 cal. BP are characterised by high erosion activity in the lake's catchment, whereas during the phase of dense Picea-Pinus cembra-Abies forests (6300–3700 cal. BP) soils were stable and sediment-accumulation rates in the lake were low. Due to decreasing land-use at higher altitudes during the Roman occupation and the Migration period, forests spread beween ca. 2000 and 1500 cal. BP, before human impact increased again in the early Middle Ages. Recent reforestation due to land-use changes in the 20th century is recorded in the top sediments. Pollen-inferred July temperature and annual precipitation suggest a trend to cooler and more oceanic climate starting at about 5500 cal. BP.

Untangling a Holocene pollen record with forest model simulations and independent climate data

Adaptation potential of forests to rapid climatic changes can be assessed from vegetation dynamics during past climatic changes as preserved in fossil pollen data. However, pollen data reflect the integrated effects of climate and biotic processes, such as establishment, survival, competition, and migration. To disentangle these processes, we compared an annually laminated late Würm and Holocene pollen record from the Central Swiss Plateau with simulations of a dynamic forest patch model. All input data used in the simulations were largely independent from pollen data; i.e. the presented analysis is non-circular. Temperature and precipitation scenarios were based on reconstructions from pollen-independent sources. The earliest arrival times of the species at the study site after the last glacial were inferred from pollen maps. We ran a series of simulations under different combinations of climate and immigration scenarios. In addition, the sensitivity of the simulated presence/absence of four major species to changes in the climate scenario was examined. The pattern of the pollen record could partly be explained by the used climate scenario, mostly by temperature. However, some features, in particular the absence of most species during the late Würm could only be simulated if the winter temperature anomalies of the used scenario were decreased considerably. Consequently, we had to assume in the simulations, that most species immigrated during or after the Younger Dryas (12 000 years BP), Abies and Fagus even later. Given the timing of tree species immigration, the vegetation was in equilibrium with climate during long periods, but responded with lags at the time-scale of centuries to millennia caused by a secondary succession after rapid climatic changes such as at the end of Younger Dryas, or immigration of dominant taxa. Climate influenced the tree taxa both directly and indirectly by changing inter-specific competition. We concluded, that also during the present fast climatic change, species migration might be an important process, particularly if geographic barriers, such as the Alps are in the migrational path.

Vegetation Changes and Timberline Fluctuations in the Central Alps as Indicators of Holocene Climatic Oscillations

Pollen and plant-macrofossil data are presented for two lakes near the timberline in the Italian (Lago Basso, 2250 m) and Swiss Central Alps (Gouille Rion, 2343 m). The reforestation at both sites started at 9700-9500 BP with Pinus cembra, Larbc decidua, and Betula. The timberline reached its highest elevation between 8700 and 5000 BP and retreated after 5000 BP, due to a mid-Holocene climatic change and increasing human impact since about 3500 BP (Bronze Age). The expansion of Picea abies at Lago Basso between ca. 7500 and 6200 BP was probably favored by cold phases accompanied by increased oceanicity, whereas in the area of Gouille Rion, where spruce expanded rather late (between 4500 and 3500 BP), human influence equally might have been important. The mass expansion of Alnus viridis between ca. 5000 and 3500 BP probably can be related to both climatic change and human activity at timberline. During the early and middle Holocene a series of timberline fluctuations is recorded as declines in pollen and macrofossil concentrations of the major tree species, and as increases in nonarboreal pollen in the pollen percentage diagram of Gouille Rion. Most of ·the periods of low timberline can be correlated by radiocarbon dating with climatic changes in the Alps as indicated by glacier ad­ vances in combination with palynological records, solifluction, and dendrocli­ matical data. Lago Basso and Gouille Rion are the only sites in the Alps showing complete palaeobotanical records of cold phases between 10,000 and 2000 BP with very good time control. The altitudinal range of the Holocene treeline fluc­ tuations caused by climate most likely was not more than 100 to 150 m. A possible correlation of a cold period at ca. 7500-6500 BP (Misox oscil­ lation) in the Alps is made with paleoecological data from North America and Scandinavia and a climatic signal in the GRIP ice core from central Greenland 8200 yr ago (ca. 7400 yr uncal. BP).

Towards mapping the late Quaternary vegetation change of Europe

The number of well-dated pollen diagrams in Europe has increased considerably over the last 30 years and many of them have been submitted to the European Pollen Database (EPD). This allows for the construction of increasingly precise maps of Holocene vegetation change across the continent. Chronological information in the EPD has been expressed in uncalibrated radiocarbon years, and most chronologies to date are based on this time scale. Here we present new chronologies for most of the datasets stored in the EPD based on calibrated radiocarbon years. Age information associated with pollen diagrams is often derived from the pollen stratigraphy itself or from other sedimentological information. We reviewed these chronological tie points and assigned uncertainties to them. The steps taken to generate the new chronologies are described and the rationale for a new classification system for age uncertainties is introduced. The resulting chronologies are fit for most continental-scale questions. They may not provide the best age model for particular sites, but may be viewed as general purpose chronologies. Taxonomic particularities of the data stored in the EPD are explained. An example is given of how the database can be queried to select samples with appropriate age control as well as the suitable taxonomic level to answer a specific research question.

Present-day vegetation and the Holocene and recent development of Egelsee-Moor, Salzburg province, Austria

This paper describes the present-day vegetation, stratigraphy and developmental history of the mire of Egelsee-Moor (Salzburg, Austria; 45°45′N, 13°8.5′E, 700 m a.s.l., 15 ha in area) since the early Late Glacial on the basis of 4 transects with 14 trial borings across the peatland. We present a vegetation map of the mire, a longitudinal section through the peat body based on six cores showing the peat types, overview macrofossil diagrams of six cores showing the local mire development and two pollen diagrams covering the Late Glacial and Holocene. The chronology of the diagrams depends on biostratigraphic dating for the Late Glacial and early Holocene and radiocarbon dating for the remaining Holocene. The northern part of the mire originated through terrestrialisation of nutrient-rich, mostly inundated fen and the southern part through paludification of wet soils. The very small lake of today was a reservoir until recently for providing water-power for timber rafting (‘Holztrift’). The mire vegetation today is a complex of forested parts (mainly planted Pinus sylvestris and Thuja occidentalis, but also spontaneous Picea abies, Betula pubescens and Frangula alnus), reed-lands (Phragmites) and litter meadows (Molinietum, Schoenetum, etc.). The central part has hummock-hollow complexes with regionally rare species of transitional mires (Drosera anglica, D. intermedia, Lycopodiella inundata, Scorpidium scorpioides, Sphagnum platyphyllum, S. subnitens). The results indicate that some of the mid-Holocene sediments may have been removed by the timber-rafting practices, and that water extraction from the hydrological catchment since 1967 has resulted in a partial shift of transitional mire to ombrotrophic bog. The latter potentially endangers the regionally rare species and was used as an argument to stop further water extraction.

Mid-Wisconsin to Holocene permafrost and landscape dynamics based on a drained lake basin core from the northern Seward Peninsula, Northwest Alaska

Permafrost-related processes drive regional landscape dynamics in the Arctic terrestrial system. A better understanding of past periods indicative of permafrost degradation and aggradation is important for predicting the future response of Arctic landscapes to climate change. Here, we used a multi-proxy approach to analyze a ~4 m long sediment core from a drained thermokarst lake basin on the northern Seward Peninsula in western Arctic Alaska (USA). Sedimentological, biogeochemistical, geochronological, micropaleontological (ostracoda, testate amoeba) and tephra analyses were used to determine the long-term environmental Early-Wisconsin to Holocene history preserved in our core for Central Beringia. Yedoma accumulation dominated throughout the Early to Late-Wisconsin but was interrupted by wetland formation from 44.5 to 41.5 ka BP. The latter was terminated by deposition of 1 m of volcanic tephra, most likely originating from the South Killeak Maar eruption at about 42 ka BP. Yedoma deposition continued until 22.5 ka BP and was followed by a depositional hiatus in the sediment core between 22.5 and 0.23 ka BP. We interpret this hiatus as due to intense thermokarst activity in the areas surrounding the site, which served as a sediment source during the Late-Wisconsin to Holocene climate transition. The lake forming the modern basin on the upland initiated around 0.23 ka BP, which drained catastrophically in spring 2005. The present study emphasizes that Arctic lake systems and periglacial landscapes are highly dynamic and permafrost formation as well as degradation in Central Beringia was controlled by regional to global climate patterns and as well as by local disturbances.

Distribution of benthos in late Quaternary turbidite layers of sediment cores from the North-West African continental margin

The CaCO3-contents and the fractions > 40 µm have been analysed from 5 kastenloten, one piston core and two kastengreifer taken between Senegal and Cape Verde Islands. Numerous benthonic and planktonic organisms and different terrigenous components have been distinguished. The four cores off Senegal reach middle Wuerm sediments; cores GIK12329-6 and TAG72-1 reach the V-zone and core GIK12331-4 the X-zone (Eem); the two kastengreifer contain sediments of Holocene age. Correlation of the cores has been made. Holocene sedimentation rates decrease from the shallow cores (6-11 cm/1000 years) to the deep-sea (1-2 cm/1000 years). The following climatic variations could be deduced from the sediments off the Senegal: during Holocene climate was in general as today, the Senegal river transporting fine grained material to the sea. The upper Wuerm was arid with no river influence but with red dune sand transported to the continental slope. During middle Wuerm the climate was humid again. The deep-sea cores have been influenced by eolian material from arid regions during glacial and interglacial periods, indicated by relatively high "Wuestenquarz-numbers". However, during Wuerm "Wuestenquarz-numbers" are higher than during Holocene and Eem, indicating that more intensely red coloured sediment was exposed to wind activity on the continent during this period. Varying amounts of terrigenous material and CaCO3-contents indicate varying wind strengths (lower in Holocene and Eem than during Wuerm). The boundary between humid and arid Wuerm climate was at approximately 20 °N. Influence of upwelling is difficult to establish in the sediments off Senegal, because river influence, while increasing fertility also dilutes the diatoms which are typical for upwelling. High amounts of organic carbon, low plankton/benthos ratios of foraminifers and low plankton foraminifer/radiolarian ratios in Holocene sections might be interpreted as influenced by upwelling. Turbidites occur in cores 72 and 31 and at the Holocene/Pleistocene boundary of core GIK12329-6. Their composition indicates provenance from the continental shelf of the Cape Verde Islands for core 31 and the continental shelf and slope off Senegal for core TAG72-1. Volcanic material, rare in the normal pelagic sediment of core GIK12331-4 is more frequent in the turbidites.

Radioisotope stratigraphy, sedimentology and geochemistry of Late Quaternary sediments from the Eastern Arctic Ocean

To reconstruct Recent and past sedimentary environments, marine sediments of Upper Pleistocene and Holocene ages from the eastern Arctic Ocean and especially from the Nansen-Gakkel Ridge (NGR) were investigated by means of radioisotopic, geochemical and sedimentological methods. In combination with mass physical property data and lithological analysis these investigations allow clearly to characterize the depositional environments. Age dating by using the radioisotope 230Th gives evidence that the investigated sediments from the NGR are younger than 250,000 years. Identical lithological sediment sequences within and between sediment cores from the NGR can be related to sedimentary processes which are clearly controlled by palaeoclimate. The sediments consist predominantly of siliciclastic, terrigenous ice-rafted detritus (IRD) deriving from assorted and redeposited sediments from the Siberian shelfs. By their geochemical composition the sediments are similar to mudstone, graywacke and arcose. Sea-ice as well as icebergs play a major roll in marine arctic sedimentation. In the NGR area rapid change in sedimentary conditions can be detected 128,000 years ago. This was due to drastic change in the kind of ice cover, resulting from rapid climatic change within only hundreds of years. So icebergs, deriving mostly from Siberian shelfs, vanished and sea-ice became dominant in the eastern Arctic Ocean. At least three short-period retreats of the shelf ice between 186,000 and 128,000 years are responsible for the change of coarse to fine-grained sediments in the NGR area. These warmer stages lasted between 1,000 and 3,000 years. By monitoring and comparing the distribution patterns of sedimentologic, mass physical and geochemical properties with 230Th ex activity distribution patterns in the sediment cores from the NGR, there is clear evidence that sediment dilution is responsible for high 230Th ex activity variations. Thus sedimentation rate is the controlling factor of 230Th ex activity variations. The 230Th flux density in sediments from the NGR seems to be highly dependent On topographic Position. The distribution patterns of chemical elements in sediment cores are in general governed by lithology. The derivation of a method for dry bulk density determination gave the opportunity to establish a high resolution stratigraphy on sediment cores from the eastern Arctic Ocean, based on 230Thex activity analyses. For the first time sedimentation and accumulation rates were determined for recent sediments in the eastern Arctic Ocean by 230Th ex analyses. Bulk accumulation rates are highly variable in space and time, ranging between 0.2 and 30 g/cm**2/ka. In the sediments from the NGR highly variable accumulation rates are related to the kind of ice cover. There is evidence for hydrothermal input into the sediments of the NGR. Hydrothermal activity probably also influences surficial sediments in the Sofia Basin. High contents of As are typical for surficial sediments from the NGR. In particular SL 370-20 from the bottom of the rift valley has As contents exceeding in parts 300 ppm. Hydrothermal activity can be traced back to at least 130,000 years. Recent to subrecent tectonic activity is documented by the rock debris in KAL 370 from the NGR. In four other sediment cores from the NGR rift valley area tectonically induced movements can be dated to about 130,000 years ago, related most probably to the rapid climate change. Processes of early diagenesis in sediments from the NGR caused the aobilization and redeposition of Fe, Mn and Mo. These diagenetic processes probably took place during the last 130,000 years. In sediment cores from the NGR high amounts of kaolinite are related to coarse grained siliciclastic material, probably indicating reworking and redeposition of siberian sandstones with kaolinitic binding material. In contrast to kaolinite, illite is correlated to total clay and 232Th contents. Aragonite, associated with serpentinites in the rift valley area of the NGR, was precipitated under cold bottom-water conditions. Preliminary data result in a time of formation about 60 - 80 ka ago. Manganese precipitates with high Ni contents, which can be related to the ultrabasic rocks, are of similar age.

Late Neogene carbonate sedimentation and stable isotope record of ODP Site 121-758

The evolution of oceanic and climatic conditions the northeast Indian Ocean during the last 7 m.y. is revealed in the sediments from Site 758. We present detailed and continuous records of d18O and d13C from planktonic foraminifers, weight percent calcium carbonate, weight percent coarse fraction, magnetic susceptibility, and geomagnetic reversals. Sample spacing of the records ranges from 3 to 10 cm and is equivalent to an average time interval of 2000 to 6000 yr. Despite the fact that core recovery ranged between 100% and 105%, recovery gaps as large as 2.7 m occurred at nearly every break between advanced hydraulic piston cores. Approximately 12% of the late Neogene sequence was not recovered in each of the two holes drilled at Site 758. To circumvent the discontinuity introduced by the gaps, a composite depth section was constructed from multiple cores taken from offset holes at Site 758. The resulting composite depth section extends continuously from 0 to 116 mbsf, from the Holocene to the upper Miocene. A detailed chronostratigraphy is based on geomagnetic reversals which extend from the Brunhes Chron to Chron 6, and on d18O stages 1 through 105, which span from 0 to 2.5 Ma. The d18O record is dominated by a ~40-k.y. cycle in the late Pliocene and early Pleistocene, and is followed by a change to a ~100-k.y. cycle in the late Pleistocene. The mid-Pleistocene transition between these two modes of variability occurs between d18O stages 25 and 22 (between 860 and 800 Ka). Thirteen major volcanic ash horizons from the Indonesian arc are observed throughout the sedimentary section and are dated by their relative position within the geomagnetic reversals and the d18O chronostratigraphy. Since 5 Ma, there has been a long-term decline in weight percent CaCO3 and CaCO3 mass accumulation rates, and an associated rise in non-CaCO3 mass accumulation rates. We attribute these changes to a decrease in CaCO3 productivity and an increase in terrigenous sedimentation through enhanced riverine input. Such input may be linked to rapid tectonic uplift of the Himalayas and the Tibetan Plateau via mechanisms such as the intensification of the monsoonal rains, increased fluvial erosion, and regional glaciation. The long-term increase in percent coarse fraction since 5 Ma suggests a gradual increase in CaCO3 preservation. Higher frequency fluctuations in CaCO3 preservation are superimposed on the long-term trend and are related to climate fluctuations. The abrupt drop (-50%) in CaCO3 accumulation at 3.4 Ma signals a dramatic decrease in CaCO3 production that occurred over much of the Indian Ocean.

A time-slice study in Heinrich Stadial 1 and Mid Holocene sediment cores off northern Senegal

Sediment dynamics in limnic, fluvial and marine environments can be assessed by granulometric and rock-magnetic methodologies. While classical grain-size analysis by sieving or settling mainly bears information on composition and transport, the magnetic mineral assemblages reflect to a larger extent the petrology and weathering conditions in the sediment source areas. Here, we combine both methods to investigate Late Quaternary marine sediments from five cores along a transect across the continental slope off Senegal. This region near the modern summer Intertropical Convergence Zone is particularly sensitive to climate change and receives sediments from several aeolian, fluvial and marine sources. From each of the investigated five GeoB sediment cores (494-2956 m water depth) two time slices were processed which represent contrasting climatic conditions: the arid Heinrich Stadial 1 (~ 15 kyr BP) and the humid Mid Holocene (~ 6 kyr BP). Each sediment sample was split into 16 grain-size fractions ranging from 1.6 to 500 µm. Concentration and grain-size indicative magnetic parameters (susceptibility, SIRM, HIRM, ARM and ARM/IRM) were determined at room temperature for each of these fractions. The joint consideration of whole sediment and magnetic mineral grain-size distributions allows to address several important issues: (i) distinction of two aeolian sediment fractions, one carried by the north-easterly trade winds (40-63 µm) and the other by the overlying easterly Harmattan wind (10-20 µm) as well as a fluvial fraction assigned to the Senegal River (< 10 µm); (ii) identification of three terrigenous sediment source areas: southern Sahara and Sahel dust (low fine-grained magnetite amounts and a comparatively high haematite content), dust from Senegalese coastal dunes (intermediate fine-grained magnetite and haematite contents) and soils from the upper reaches of the Senegal River (high fine-grained magnetite content); (iii) detection of partial diagenetic dissolution of fine magnetite particles as a function of organic input and shore distance; (iv) analysis of magnetic properties of marine carbonates dominating the grain-size fractions 63-500 µm.

Response of deep-sea benthic foraminifera to Late Quaternary climate changes, southeast Indian Ocean, offshore Western Australia

The Late Quaternary benthic foraminifera of four deep-sea cores off Western Australia (ODP 122-760A, ODP 122-762B, BMR96GC21 and RC9-150) have been examined for evidence of increased surface productivity to explain the anomalously low sea-surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations. The delta13C trends of Cibicidoides wuellerstorfi, and differences between the delta13C trends of planktics (Globigerinoides sacculifer) and benthics (C. wuellerstorfi) in the four cores indicate that during stage 6 bottom waters were significantly depleted in delta13C, and strong delta13C gradients were established in the water column, while during stage 2 and the Last Glacial Maximum, delta13C trends did not differ greatly from that of the Holocene. Two main assemblages of benthic foraminifera were identified by principal component analyses: one dominated by Uvigerina peregrina, another dominated by U. proboscidea. Abundance of these Uvigerinids, and of taxa preferring an infaunal microhabitat, and of Epistominella exigua and Bulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes, and particularly so during stage 6, while evidence for upwelling during the Last Glacial Maximum is less strong. The Penultimate Glaciation upwellings were established within the areas of low sea-surface paleotemperature indicated by planktic foraminifera. During the Last Interglacial Climax, upwelling appears to have been established in an isolated region offshore from a strengthened Leeuwin Current off North West Cape. Last Glacial Maximum delta13C values of C. wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial-interglacial changes suggesting the development of a deep hydrological front. A similar vertical stratification/bathyal front was also established during the Penultimate Glaciation.