Stable isotope ratios from foraminfera in Kara Sea sediments

Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.-min.) in the isotope values when measuring several (>=5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of >2per mil of d18O and d13C at low absolute levels (d18O: <3per mil, d13C: <-3per mil) near the river estuaries of Ob and Yenisei and low ranges of not, vert, similar1per mil at higher absolute levels (d18O: 2-5.4per mil, d13C: -3 per mil to -1.5per mil) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8-7 cal. ka BP) of a sediment core north of Yenisei estuary revealed d18O and d13C values which on average are lower by 0.6? in both, d18O and d13C, than in the upper core section (<5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.-min.) in the single-valve d18O and d13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise

Sedimentology and stable oxygen isotope ratios of foraminifera from cores of the Porcupine Seabight, Northeast Atlantic

Large carbonate mound structures have been discovered in the northern Porcupine Seabight (Northeast Atlantic) at depths between 600 and 1000 m. These mounds are associated with the growth of deep-sea corals Lophelia pertusa and Madrepra oculata. In this study, three sediment cores have been analysed. They are from locations close to Propeller Mound, a 150 m high ridge-like feature covered with a cold-water coral ecosystem at its upper flanks. The investigations are concentrated on grain-size analyses, carbon measurements and on the visual description of the cores and computer tomographic images, to evaluate sediment content and structure. The cores portray the depositional history of the past ~31 kyr BP, mainly controlled by sea-level fluctuations and the climate regime with the advance and retreat of the Irish Ice Sheet onto the Irish Mainland Shelf. A first advance of glaciers is indicated by a turbiditic release slightly older than 31 kyr BP, coherent with Heinrich event 3 deposition. During Late Marine Isotope Stage 3 (MIS 3) and MIS 2 shelf erosion prevailed with abundant gravity flows and turbidity currents. A change from glaciomarine to hemipelagic contourite sedimentation during the onset of the Holocene indicates the establishment of the strong, present-day hydrodynamic regime at intermediate depths. The general decrease in accumulation of sediments with decreasing distance towards Propeller Mound suggests that currents (turbidity currents, gravity flows, bottom currents) had a generally stronger impact on the sediment accumulation at the mound base for the past ~31 kyr BP, respectively.

Benthic foraminifera, stable isotope ratios and carbon concentrations of sediment cores from Propeller Mound and Porcupine Srabight

On- and off-mound sediment cores from Propeller Mound (Hovland Mound province, Porcupine Seabight) were analysed to understand better the evolution of a carbonate mound. The evaluation of benthic foraminiferal assemblages from the off-mound position helps to determine the changes of the environmental controls on Propeller Mound in glacial and interglacial times. Two different assemblages describe the Holocene and Marine Isotope Stage (MIS) 2 and late MIS 3 (~31 kyr BP). The different assemblages are related to changes in oceanographic conditions, surface productivity and the waxing and waning of the British Irish Ice Sheet (BIIS) during the last glacial stages. The interglacial assemblage is related to a higher supply of organic material and stronger current intensities in water depth of recent coral growth. During the last glaciation the benthic faunas showed high abundances of cassidulinid species, implying cold bottom waters and a reduced availability of organic matter. High sedimentation rates and the domination of Elphidium excavatum point to shelf erosion related to sea-level lowering (~50 m) and the progradation of the BIIS onto the shelf. A different assemblage described for the on-mound core is dominated by Discanomalina coronata, Gavelinopsis translucens, Planulina ariminensis, Cibicides lobatulus and to a lower degree by Hyrrokkin sarcophaga. These species are only found or show significantly higher relative abundances in on-mound samples and their maximum contribution in the lower part of the record indicates a higher coral growth density on Propeller Mound in an earlier period. They are less abundant during the Holocene, however. This dataset portrays the boundary conditions of the habitable range for the cold-water coral Lophelia pertusa, which dominates the deep-water reefal ecosystem on the upper flanks of Propeller Mound. The growth of this ecosystem occurs during interglacial and interstadial periods, whereas a retreat of corals is documented in the absence of glacial sediments on-mound. Glacial conditions with cold intermediate waters, a weak current regime and high sedimentation rates provide an unfavourable environmental setting for Lophelia corals to grow. A Late Pleistocene decrease is observed in the mound growth for Propeller Mound, which might face its complete burial in the future, as it already happened to the buried mounds of the Magellan Mound province further north.

Bulk geochemical analysis and planktonic foraminifera of sediment core GeoB7112-5

A multivariable approach utilising bulk sediment, planktonic Foraminifera and siliceous phytoplankton has been used to reconstruct rapid variations in palaeoproductivity in the Peru-Chile Current System off northern Chile for the past 19000 cal. yr. During the early deglaciation (19000-16000 cal. yr BP), our data point to strongest upwelling intensity and highest productivity of the past 19 000 cal. yr. The late deglaciation (16000-13000 cal. yr BP) is characterised by a major change in the oceanographic setting, warmer water masses and weaker upwelling at the study site. Lowest productivity and weakest upwelling intensity are observed from the early to the middle Holocene (13000-4000 cal. yr BP), and the beginning of the late Holocene (<4000 cal. yr BP) is marked by increasing productivity, mainly driven by silicate-producing organisms. Changes in the productivity and upwelling intensity in our record may have resulted from a large-scale compression and/or displacement of the South Pacific subtropical gyre during more productive periods, in line with a northward extension of the Antarctic Circumpolar Current and increased advection of Antarctic water masses with the Peru-Chile Current. The corresponding increase in hemispheric thermal gradient and wind stress induced stronger upwelling. During the periods of lower productivity, this scenario probably reversed.

Radiocarbon ages, diatom abundance and laminae description of sediment core MD03-2597

Laminated sediments are unique archives of palaeoenvironmental and palaeoceanographic conditions, recording changes on seasonal and interannual timescales. Diatom-rich laminated marine sediments are examined from Dumont d'Urville Trough, East Antarctic Margin, to determine changes in environmental conditions on the continental shelf from 1136 to 3122 cal. yr BP. Scanning electron microscope backscattered electron imagery (BSEI) and secondary electron imagery are used to analyse diatom assemblages from laminations and to determine interlamina relationships. Diatom observations are quantified with conventional assemblage counts. Laminae are primarily classified according to visually dominant species identified in BSEI and, secondarily, by terrigenous content. Nine lamina types are identified and are characterized by: Hyalochaete Chaetoceros spp. resting spores (CRS); CRS and Fragilariopsis spp.; Fragilariopsis spp.; Corethron pennatum and Rhizosolenia spp.; C. pennatum; Rhizosolenia spp.; mixed diatom assemblage; Stellarima microtrias resting spores (RS), Porosira glacialis RS and Coscinodiscus bouvet; and P. glacialis RS. Formation of each lamina type is controlled by seasonal changes in sea ice cover, nutrient levels and water column stability. Quantitative diatom assemblage analysis revealed that each lamina type is dominated by CRS and Fragilariopsis sea ice taxa, indicating that sea ice cover was extensive and persistent in the late Holocene. However the lamina types indicate that the sea ice regime was not consistent throughout this period, notably that a relatively warmer period, ~3100 to 2500 cal. yr BP, was followed by cooling which resulted in an increase in year round sea ice by ~1100 cal. yr BP.

Pollen records of the Bykovsky Peninsula

New pollen and radiocarbon data from the Bykovsky Peninsula document the Late Pleistocene and Holocene environmental history of the Laptev Sea coast. More than 60 AMS-14C and conventional 14C dates indicate that the deposits accumulated during the last 60,000 radiocarbon yr BP. High concentration of green alga colonies (Pediustrum and Botryococcus) in the investigated sediment show that sedimentation was mostly in shallow water environments. Scarce grass and sedge communities dominated the vegetation 53-60 kyr BP. Climate was cold and dry. Open Poaceae and Cypcraccae associations with Asteraceae, Ranunculaceae, and Cichoriaceac, dominated in the area about 48-42.5 kyr BP. Steppic communities with Artemisia and shrubby tundra communities with Salix and Betula sect. Nanae were also present. Climate was dry, but relatively warm. Vegetation cover became denser about 42.5-33.5 kyr BP, reflecting more favorable climate conditions. Scarce Poaceae communities with some Caryophyllaceae, Asteraceae, Cichoriaceae, and Selaginella rupestris covered the Bykovsky Peninsula area during the Sartan (Late Weichselian) stage about 26-16 kyr BP. Disturbed, uncovered soils were very common in the area. Climate was extremely cold and dry. Poaceae and Cyperaceae associations with Caryophyllaceae, Asteraceae, Cichoriaceae dominated the vegetation in the late Sartan, ca 16-12.2 kyr BP. Climate was significantly warmer than in the early Sartan time. The lee Complex sedimentation was interrupted about 12 kyr BP; most likely it was connected with the beginning of the Allerod warnring. Shrubby (Betula sect. Nanae, Alnusfnuicosa, Salix, Ericales) tundra was widely distributed on the Bykovsky Peninsula during the early-middle Holacene. Climate was most favorable between 8200 and 4500 yr BP. Vegetation became similar to modern after 4500 yr BP, suggesting a deterioration of climate.

Organic geochemistry of two sediment cores from the Kara Sea

Within the Russian-German research project on "Siberian River Run-off (SIRRO)" devoted to the freshwater discharge and its influence on biological, geochemical, and geological processes in the Kara Sea, sedimentological and organic-geochemical investigations were carried-out on two well-dated sediment cores from the Yenisei Estuary area. The main goal of this study was to quantify the terrigenous organic carbon accumulation based on biomarker and bulk accumulation rate data, and its relationship to Yenisei river discharge and climate change through Holocene times. The biomarker data at both cores clearly indicate the predominance of terrigenous organic matter, reaching 70 to 100% and 50 to 80% of the total organic carbon within and directly north of the estuary, respectively. During the last about 9 Cal. kyrs. BP represented in the studied sediment section, siliciclastic sediment and (terrigenous) organic carbon input was strongly influenced by postglacial sea-level rise and climate-related changes in river discharge. The mid-Holocene Climatic Optimum is documented by maximum river discharge between 8.2 and 7.3 Cal. kyrs. BP. During the last 2000 years river discharge probably became reduced, and accumulation of both terrigenous and marine organic carbon increased due to increased coagulation of fine-grained material.

Dome C age scale EDC1 for ice core EDC99

A tentative age scale (EDC1) for the last 45 kyr is established for the new 788 m EPICA Dome C ice core using a simple ice flow model. The age of volcanic eruptions, the end of the Younger Dryas event, and the estimated depth and age of elevated 10Be, about 41 kyr ago were used to calibrate the model parameters. The uncertainty of EDC1 is estimated to ±10 yr for 0 to 700 yr BP, up to ±200 yr back to 10 kyr BP, and up to ±2 kyr back to 41 kyr BP. The age of the air in the bubbles is calculated with a firn densification model. In the Holocene the air is about 2000 yr younger than the ice and about 5500 yr during the last glacial maximum.

Age determinations and sedimentology on cores GIK17940 and 184-1144

Sedimentological, geochemical and paleomagnetic records were employed to reconstruct the history of East Asian Monsoon variability in the South China Sea (SCS) on orbital- and millennial-to-sub-decadal time scales. A detailed magnetostratigraphy for the southern central SCS was established as well as a stable isotope stratigraphy for ODP Site 1144 for the last 1.2 million years in the northern South China Sea. Furthermore a volcanic tephra layer from the southern central SCS could be identified as the Youngest Toba Ash, which thus re-presents an important age marker and was used to reconstruct paleo wind directions during the eruption 74 ka. Special attention was paid to the high- and ultrahigh-frequency variability in the last glacial-interglacial cycle and the Holocene, and to a precise age control of climate changes in general.

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.

Profiles of diatoms, pollen, XRD, XRF and organic composition of two sediment cores (PG2022 and PG2023) from Lake Kyutyunda in Yakutia, NE Siberia, Russia

Although the climate development over the Holocene in the Northern Hemisphere is well known, palaeolimnological climate reconstructions reveal spatiotemporal variability in northern Eurasia. Here we present a multi-proxy study from north-eastern Siberia combining sediment geochemistry, and diatom and pollen data from lake-sediment cores covering the last 38,000 cal. years. Our results show major changes in pyrite content and fragilarioid diatom species distributions, indicating prolonged seasonal lake-ice cover between ~13,500 and ~8,900 cal. years BP and possibly during the 8,200 cal. years BP cold event. A pollen-based climate reconstruction generated a mean July temperature of 17.8°C during the Holocene Thermal Maximum (HTM) between ~8,900 and ~4,500 cal. years BP. Naviculoid diatoms appear in the late Holocene indicating a shortening of the seasonal ice cover that continues today. Our results reveal a strong correlation between the applied terrestrial and aquatic indicators and natural seasonal climate dynamics in the Holocene. Planktonic diatoms show a strong response to changes in the lake ecosystem due to recent climate warming in the Anthropocene. We assess other palaeolimnological studies to infer the spatiotemporal pattern of the HTM and affirm that the timing of its onset, a difference of up to 3,000 years from north to south, can be well explained by climatic teleconnections. The westerlies brought cold air to this part of Siberia until the Laurentide ice-sheet vanished 7,000 years ago. The apparent delayed ending of the HTM in the central Siberian record can be ascribed to the exceedance of ecological thresholds trailing behind increases in winter temperatures and decreases in contrast in insolation between seasons during the mid to late Holocene as well as lacking differentiation between summer and winter trends in paleolimnological reconstructions.

Sedimentology, foraminiferal assemblage, and isotope record of the Agulhas Bank region

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200-250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.

Age determination, sedimentological, biogeochemical, and plant macrofossils analyses of outcrop PG2038-1

Thermokarst lakes are a widespread feature of the Arctic tundra, in which highly dynamic processes are closely connected with current and past climate changes. We investigated late Quaternary sediment dynamics, basin and shoreline evolution, and environmental interrelations of Lake El'gene-Kyuele in the NE Siberian Arctic (latitude 71°17'N, longitude 125°34'E). The water-body displays thaw-lake characteristics cutting into both Pleistocene Ice Complex and Holocene alas sediments. Our methods are based on grain size distribution, mineralogical composition, TOC/N ratio, stable carbon isotopes and the analysis of plant macrofossils from a 3.5-m sediment profile at the modern eastern lake shore. Our results show two main sources for sediments in the lake basin: terrigenous diamicton supplied from thermokarst slopes and the lake shore, and lacustrine detritus that has mainly settled in the deep lake basin. The lake and its adjacent thermokarst basin rapidly expanded during the early Holocene. This climatically warmer than today period was characterized by forest or forest tundra vegetation composed of larches, birch trees and shrubs. Woodlands of both the HTM and the Late Pleistocene were affected by fire, which potentially triggered the initiation of thermokarst processes resulting later in lake formation and expansion. The maximum lake depth at the study site and the lowest limnic bioproductivity occurred during the longest time interval of ~7 ka starting in the Holocene Thermal Maximum and lasting throughout the progressively cooler Neoglacial, whereas partial drainage and an extensive shift of the lake shoreline occurred ~0.9 cal. ka BP. Correspondingly, this study discusses different climatic and environmental drivers for the dynamics of a thermokarst basin.

Late Quateranry dinoflagellate cysts of ODP Site 202-1233

The late Quaternary organic-walled dinoflagellate cyst record of Site 1233 (41°S, offshore Chile) was studied with a ?200 year resolution spanning the last 25,000 years. The study provides the first continuous record of sub-recent and recent dinoflagellate cysts in the Southeast (SE) Pacific. Major changes in the composition of the cyst association, cyst concentration and morphology of Operculodinium centrocarpum reflect changes in sea surface temperature (SST), sea surface salinity (SSS), palaeoproductivity and upwelling intensity. These changes can be associated with latitudinal shifts of the circumpolar frontal systems. The high cyst concentration, high Brigantedinium spp. abundances, low species diversity and the occurrence of certain cold water species are supportive for a 7-10° equatorward shift of the Antarctic Circumpolar Current (ACC) during the coldest phase of the Last Glacial Maximum (LGM) between 25 and 21.1 cal ka BP. Deglacial warming initiated at ~18.6 cal ka BP. Termination I (18.6-11.1 cal ka BP) is interrupted by an unstable period of extreme seasonality, rather than a cooling event, between 14.4 and 13.2 cal ka BP, synchronous with the Antarctic Cold Reversal (ACR). The Holocene Maximum is observed between 11.6 and 9.8 cal ka BP and is typified by the most southward position of the northern margin of the ACC. A cooling phase occurred during the early Holocene (until ~7 cal ka BP) and during the last ~0.8 ka. Our data indicates that the SE Pacific (41°S) climate has been influenced over the whole record by changes in the Southern Hemisphere (SH) high-latitudes, while during the mid to late Holocene, also a tropical forcing mechanism was involved, including the El Niño Southern Oscillation and the variable Hadley cell intensity. Furthermore, this study showed a relationship between the variable morphology of the spines/processes of O. centrocarpum and the combined variation of sea surface salinity and temperature (SSS/SST-ratio).

Diatom biostratigraphy and sea ice concentration reconstruction from core DA06-139G, Vaigat Strait in Disko Bugt, West Greenland

A diatom-based sea-ice concentration (SIC) transfer function is developed using 72 surface samples from west of Greenland and around Iceland, and through comparison with the associated modern SIC. Canonical correspondence analysis on surface sediment diatoms and monthly average of SIC reveals that April SIC is the most important environmental factor controlling the distribution of diatoms in the area, and permits the development of a diatom-based SIC transfer function. The consistency between reconstructed SIC based on diatoms from West Greenland and the instrumental and documentary data during the last ~75 years demonstrates that the diatom-based SIC reconstruction is reliable for studying the palaeoceanography off West Greenland. Relatively warm conditions with strong influence of the Irminger Current (IC) are indicated for the early part of the record (~5000-3860 cal. yr BP), corresponding in time to the latest part of the Holocene Thermal Maximum. The April SIC oscillated around the mean value between 3860 and 1510 cal. yr BP and was above mean afterwards, particularly during the time interval 1510-1120 cal. yr BP and after 650 cal. yr BP, indicating more extensive sea-ice cover in Disko Bugt. A high degree of consistency between the reconstructed April SIC and changes in the diatom species suggests that the sea-ice condition in Disko Bugt is strongly influenced by variations in the relative strength of two components of the West Greenland Current, i.e. the cold East Greenland Current and the relatively warm IC.