Age determination and faunal composition of sediments off northern Greenland

We present a record encompassing marine isotope stages 7-1 from a hitherto unexplored and heavily ice-covered area of the Arctic Ocean, the Lomonosov Ridge off the northern Greenland-Canada continental margin, using nannofossil and benthic foraminifera stratigraphy. Planktic foraminifera assemblages are used as a key paleoceanographic proxy, and a surprisingly large variability is found for an interior Arctic Ocean site. Abundant small (63-125 µm) subpolar Turborotalita quinqueloba occur in two sections, possibly representing substages 5e (last interglacial) and 5a (warm interstadial). However, the present-day circulation pattern and the very distant location of high productive regions cannot explain such high abundances of subpolar specimens in the interior, perennially sea ice-covered Arctic Ocean. Hence our proxy record indicates that last interglacial sea ice concentrations were reduced off some areas of northern Greenland-Canada. Whether this was part of a larger regional pattern or it represents the influence of polynya areas with locally increased productivity remains to be solved. With respect to glacial conditions, increased ice-rafted debris (IRD) deposition in the area appears to be associated with glacial stages 6, 4, and late 3. Stage 2 sediments (including the Last Glacial Maximum) are condensed with a sparse IRD content only.

Age model and stable oxygen isotope analyses of sediment core GeoB8507-3

The present study is the first study on the stable oxygen isotope composition of the photosynthetic calcareous-walled dinoflagellate species Thoracosphaera heimii off NW Africa during the last 45,000 yr. T. heimii based temperature estimates of sediment core GeoB 8507-3 were compared with those obtained from the stable oxygen isotopes of the planktic foraminifera Globigerina bulloides and Globigerinoides ruber (pink), and the Mg/Ca ratio of G. ruber (pink). We show that the isotopic composition of T. heimii and the temperature estimates based on the equation for inorganically precipitated calcite provide comparable results to those obtained from G. ruber (pink) isotopes and Mg/Ca ratios with exception of the Early Holocene and the Younger Dryas. The recently proposed palaeotemperature equation of Zonneveld et al. (2007), however, provides unrealistic temperature reconstructions that are about 16 °C lower than those based on planktic foraminifera. Thus, this equation needs to be revised. The difference between T. heimii and G. bulloides isotopic and temperature reconstructions can be ascribed to differences in the ecology of both species, especially with regard to their depth habitat and/or seasonal production in the research area. All temperature proxies suggest comparable conditions during the last glacial and Holocene. Small differences between the reconstructed temperature values of T. heimii and the other proxies can be explained by differences in seasonal production of the individual species. The relatively low temperatures recorded by T. heimii at about 15,000 to 8,000 yr BP are interpreted to reflect an increase in duration and/or intensity of the upwelling in the vicinity of the core site in comparison to the last glacial, with an abrupt and strong decrease of upwelling intensity and/or duration during the Early Holocene and the Younger Dryas.

(Table 1) Age and depth of reflectors of the 3.5-kHz and airgun seismic record and their corresponding depth of ash layers, carbonate peaks, and transition zones from DSDP Hole 83-504B

DSDP cores from areas of low (Site 505) and high heat flow (Site 504 B) near the Costa Rica Rift, together with seismic profiles from the Panama Basin, have been studied to determine the relationship between: (1) carbonate content and physical and acoustic properties; and (2) carbonate content, carbonate diagenesis and acoustic stratigraphy. Except for ash and chert layers, bulk density correlates strongly and linearly with carbonate content. Velocity is uniform downcore and only small variations at a small scale are measured. Thus an abrupt change in carbonate content will cause abrupt changes in acoustic impedance and should cause reflectors that can be detected acoustically. A comparison of seismic profiler reflection records with physical properties, carbonate content and reflection coefficients indicates that the main reflectors can be identified with ash layers, diagenetic boundaries, and carbonate content variations. Diagenesis of carbonate sediments is present at Site 504B in a 260 m-thick ooze-chalk-limestone/chert sequence. These diagenetic sequences occur in areas of higher heat flow (200 mW/m**2). Seismic profiler records can be used to map the extent and depth of these diagenetic boundaries.

Marine ice-rafted debris records and age determinations along the northern Eurasian margin

Ice-rafted debris (IRD) (>2 mm), input in eight sediment cores along the Eurasian continental margin (Arctic Ocean), have been studied over the last two glacial/interglacial cycles. Together with the revised chronologies and new micropaleontological data of two cores from the northern Barents Sea (PS2138) and northeastern Kara Sea (PS2741) spanning Marine Isotope Stages (MIS) 6 to 1, the IRD data give new insights into the glacial history of northern Eurasian ice-sheets over the last 150 ka. The chronologies of the cores are based on stable isotope records, AMS 14C datings, paleomagnetic and biostratigraphic data. Extensive episodes of northern Barents Sea ice-sheet growth, probably to the shelf edge, occurred during the late Weichselian (MIS 2) and the Saalian (MIS 6). Major IRD discharge at the MIS 4/3-transition hints to another severe glaciation, probably onto the outer shelf, during MIS 4. IRD-based instabilities of the marine-based ice margin along the northern Barents Sea between MIS 4 and 2 are similar in timing with North Atlantic Heinrich events and Nordic Seas IRD events, suggesting similar atmospheric cooling over a broad region or linkage of ice-sheet fluctuations through small sea-level events. In the relatively low-precipitation areas of eastern Eurasia, IRD peak values during Termination II and MIS 4/3-transition suggest a Kara Sea ice-sheet advance onto the outer shelf, probably to the shelf edge, during glacial MIS 6 and 4. This suggests that during the initial cooling following the interglacials MIS 5, and possibly MIS 7, the combined effect of sustained inflow of Atlantic water into the Arctic Ocean and penetration of moisture-bearing cyclones into easterly direction supported major ice build-up during Saalian (MIS 6) and Mid-Weichselian (MIS 4) glaciation. IRD peak values in MIS 5 indicate at least two advances of the Severnaya Semlya ice-sheet to the coast line during the Early Weichselian. In contrast, a distinct Kara Sea ice advance during the Late Weichselian (MIS 2) is not documented by the IRD records along the northeastern Kara Sea margin.

Age determination of three sediment cores

A total of five sediment cores from three sites, the Arctic Ocean, the Fram Strait and the Greenland Sea, yielded evidence for geomagnetic reversal excursions and associated strong lows in relative palaeointensity during oxygen isotope stages 2 and 3. A general similarity of the obtained relative palaeointensity curves to reference data can be observed. However, in the very detail, results from this high-resolution study differ from published records in a way that the prominent Laschamp excursion is clearly characterized by a significant field recovery when reaching the steepest negative inclinations, whereas only the N-R and R-N transitions are associated with the lowest values. Two subsequent excursions also reach nearly reversed inclinations but without any field recovery at that state. A total of 41 accelerator mass spectrometry (AMS) 14C ages appeared to allow a better age determination of these three directional excursions and related relative palaeointensity variations. However, although the three sites yielded more or less consistent chronological as well as palaeomagnetic results a comparison to another site, PS2644 in the Iceland Sea, revealed significant divergences in the ages of the geomagnetic field excursions of up to 4 ka even on basis of uncalibrated AMS 14C ages. This shift to older 14C ages cannot be explained by a time-transgressive character of the excursions, because the distance between the sites is small when compared with the size of and the distance to the geodynamo in the Earth's outer core. The most likely explanation is a difference of reservoir ages and/or mixing with old 14C-depleted CO2 from glacier ice expelled from Greenland at site PS2644.

(Table 2) Depth-age control points of ODP Site 177-1090

At Ocean Drilling Program (ODP) Site 1090 on the Agulhas Ridge (subantarctic South Atlantic) benthic foraminiferal stable isotope records span the late Oligocene through the early Miocene (25~16 Ma) at a temporal resolution of ?10 kyr. In the same time interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing secure correlation of the isotope record to the GPTS. On the basis of the isotope-magnetostratigraphic correlation we provide refined age calibration of established oxygen isotope events Mi1 through Mi2 as well as several other distinctive isotope events. Our data suggest that the d18O maximum commonly associated with the Oligocene/Miocene (O/M) boundary falls within C6Cn.2r (23.86 Ma). The d13C maximum coincides, within the temporal resolution of our record, with C6Cn.2n/r boundary and hence to the O/M boundary. Comparison of the stable isotope record from ODP Site 1090 to the orbitally tuned stable isotope record from ODP Site 929 across the O/M boundary shows that variability in the two records is very similar and can be correlated at and below the O/M boundary. Site 1090 stable isotope records also provide the first deep Southern Ocean end-member for reconstructions of circulation patterns and late Oligocene to early Miocene climate change. Comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow. Oxygen isotope gradients between sites suggest that the deep Southern Ocean was cold in comparison to the North Atlantic, Indian, and the Pacific Oceans. Dominance of cold Southern Component Deep Water at Site 1090, at least until 17 Ma, suggests that relatively cold circumpolar climatic conditions prevailed during the late Oligocene and early Miocene. We believe that a relatively cold Southern Ocean reflects unrestricted circumpolar flow through the Drake Passage in agreement with bathymetric reconstructions.

Age models for sediment cores off Peru

Planktonic foraminiferal faunas of the southeast Pacific indicate that sea surface temperatures (SST) have varied by as much as 8-10°C in the Peru Current, and by ?5-7°C along the equator, over the past 150,000 years. Changes in SST at times such as the Last Glacial Maximum reflect incursion of high-latitude species Globorotalia inflata and Neogloboquadrina pachyderma into the eastern boundary current and as far north as the equator. A simple heat budget model of the equatorial Pacific shows that observed changes in Peru Current advection can account for about half of the total variability in equatorial SSTs. The remaining changes in equatorial SST, which are likely related to local changes in upwelling or pycnocline depth, precede changes in polar climates as recorded by d18O. This partitioning of processes in eastern equatorial Pacific SST reveals that net ice-age cooling here reflects first a rapid response of equatorial upwelling to insolation, followed by a later response to changes in the eastern boundary current associated with high-latitude climate (which closely resembles variations in atmospheric CO2 as recorded in the Vostok ice core). Although precise mechanisms responsible for the equatorial upwelling component of climate change remain uncertain, one likely candidate that may operate independently of the ice sheets is insolation-driven changes in El Niño/Southern Oscillation (ENSO) frequency. Early responses of equatorial SST detected both here and elsewhere highlight the sensitivity of tropical systems to small changes in seasonal insolation. The scale of tropical changes we have observed are substantially greater than model predictions, suggesting a need for further quantitative assessment of processes associated with long-term climate change.

Age determinations for ODP Sites in the Norwegian-Greenland Sea

Records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2.6 m.y. of ODP Sites 643 and 644 in order to reconstruct surface and deep water regimes in the Norwegian Sea. Surface water regimes record long lasting moderate glacial conditions during the interval 2.6 1.0 Ma. Small intrusions of Atlantic water episodically penetrated into the Norwegian Sea forming a narrow tongue along the eastern margin, which is documented at Site 644. The polar front was most probably situated between the Site 644 and 643 locations on the outer Voring Plateau during these time intervals. Deep water regimes reflect long-term persistent corrosive bottom waters, most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters. Deep water production in the Norwegian-Greenland Sea may have operated in a different way, e.g. brine formation during winter sea ice growth. Bottom waters were oxygenated throughout the entire period, and deep water was exchanged persistently with the North Atlantic. Increased glacial/interglacial enviromental contrasts are documented, reflecting a strengthening of the Norwegian Current and intensified glaciations on the surrounding land masses during the interval 1.0 0.6 Ma. During this time a major shift in the mode of deep water production occurred. Tile onset of large amplitudes in glacial/interglacial environmental conditions with maximum contrasts in surface water regimes, different modes of deep water production, and intensified exchange with the North Atlantic marks the last 0.6 Ma. A broad development of the Norwegian Current is observed during peak interglacials, while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions.

Age determination of Holozene marine deposites from the western Mauritania (Table 1)

In an area regarded to be very favourable for the study of Holocene sea level changes one or several eustatic (?) oscillations of sea have been found using sedimentological and ecological methods. After a maximum of +3 m during the Nouakchottian stage (= Middle Flandrian or Late Atlantic) about 5500 YBP a drop of sea to -3.5 ± 0.5 m about 4100 YBP is testified by stromatolitic algae indicating the former sea level within the tidal zone with high accuracy. This evidence is supported by the observation of post-Nouakchottian regressive and transgressive geologic sequences, by buried beach deposits and flooded hardgrounds, post-Nouakchottian marine terraces of different height and age, the cutting off of one large and several small bays from the open sea etc. Possibly, one or two smaller oscillations followed between 4000 and 1500 years B. P. The radiocarbon age of the marine shells dated may be partly somewhat too old or too young.

Age dating and isotopic analyses of sediment core RC11-120

Using the concept of 'orbital tuning', a continuous, high-resolution deep-sea chronostratigraphy has been developed spanning the last 300,000 yr. The chronology is developed using a stacked oxygen-isotope stratigraphy and four different orbital tuning approaches, each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data. Each approach yields a separate chronology. The error measured by the standard deviation about the average of these four results (which represents the 'best' chronology) has an average magnitude of only 2500 yr. This small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used. Excellent convergence between chronologies developed using each of five different paleoclimatological indicators (from a single core) is also obtained. The resultant chronology is also insensitive to the specific indicator used. The error associated with each tuning approach is estimated independently and propagated through to the average result. The resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr, in excellent agreement with the 2500-yr estimate. Transfer of the final chronology to the stacked record leads to an estimated error of +/-1500 yr. Thus the final chronology has an average error of +/-5000 yr.

Age determinations and stable isotope records of four sediment profiles off Nova Scotia

Continental margin sediments off Nova Scotia accumulate at high rates (up to 360 cm/kyr) and contain a history of millennial-scale environmental changes which are dominated by the proximity of the Laurentide ice sheet during the latest Quaternary. Using stable isotope ratios of oxygen, accelerator mass spectrometer radiocarbon dating, micropaleontology, and sedimentology, we document these changes in six piston cores ranging in water depth from ab. 450 to ab. 4300 m. We find that maximum d18O in N. pachyderma occurred about 15 ka and preceded the maximum abundance of this species in these cores by ab. 1000 years. Between 13 and 14 ka we find a second peak in abundance of N. pachyderma, minimum d18O, and two pulses of ice rafting. The sediment lithology supports terrestrial studies which indicate that there was a general withdrawal of ice beyond the upper Paleozoic and Mesozoic red beds by 14 ka in southeastern Canada, so the ice rafting events between 13 and 14 ka probably reflect ice stream activity in the St. Lawrence valley. The Younger Dryas event is recognized as a peak in abundance of N. pachyderma and ice rafting (dated as ab. 11.3 ka), but meltwater discharge to the Gulf of St. Lawrence was either too small or occurred over too long a time to leave a distinct d18O minimum off Nova Scotia. At 7.1 ka, in the middle of Holocene warming, we find a third peak in abundance of N. pachyderma and another d18O minimum but no ice rafting. We interpret these data as evidence of a late-occurring meltwater event which, if correct, could have originated in the Great Lakes, in the Labrador-Ungava region, or in both. The final millennial-scale phenomenon off Nova Scotia is the onset of "Neoglaciation," marked by increased ice rafting and increased % N. pachyderma beginning about 5 kyr ago.

Age determination of sediments off western Svalbard

Late Quaternary sediment yields from the Isfjorden drainage area (7327 km**2), a high arctic region on Svalbard characterized by an alpine landscape, have been reconstructed by using seismic stratigraphy supported by sediment core analysis. The sediments that accumulated in the fjord during and since deglaciation can be divided into three stratigraphic units. The volumes of these units were determined and converted into sediment yield rates averaged over the drainage basin. During deglaciation, 13 to 10 ka, the sediment yield was ~860 tons(t)/km**2/yr. In the early Holocene it decreased to 190 t/km**2/yr, and then increased to 390t/km**2/yr during the late Holocene Little Ice Age. When normalized to the approximate glacierized area, these rates correspond to a sediment yield of ~800 t/km**2/yr . Sediment yield from non-glacierized parts of the drainage is estimated to be 35 t/km**2/yr. At times when ice advanced to the shelf edge, sediment was scoured from the fjord and deposited on the outer shelf and in a well-defined deep sea fan. Between 200 ka and 13 ka, 328 km**3 of sediment accumulated here, corresponding to a mean sediment yield rate of 335 t/km**2/yr. This is broadly consistent with calculations based on the above rates of sediment yield in glacierized and non-glacierized areas, and on estimates, based on glacial geology, of the temporal variation in degree of glacierization over the past 200 kyr. These figures indicate that much of the glacigenic sediment on the shelf and slope was eroded from the uplands of Svalbard by small glaciers during interstadials and interglacials. The sediments were temporarily stored in the fjord prior to redeposition on the shelf and slope during ice sheet advance. Taken into consideration, such redisposition of pre-eroded material will reduce estimates of primary ice sheet erosion rate.

(Table 1) Age determination of sediment core AAS-62/2

Variations in the Indian summer monsoon (ISM) intensity during the last 16.7?ka have been studied using organic carbon (Corg), d15N of sedimentary organic matter, CaCO3, sediment texture, d18OC, and Mg/Ca-derived sea surface temperature, d18O of sea water and sea surface salinity, in a 14C-dated sediment core from the eastern Arabian Sea. The d18O in water and planktonic foraminifera shells off the central west coast of India may be controlled by the ISM intensity as this area receives high precipitation and land runoff. Also, the Corg and CaCO3 contents of sediments and d15N of sedimentary organic matter may be linked to ISM-induced productivity and denitrification. The results of the present study reveal that between 16 and 15.2 ka BP, the ISM was weak with minor fluctuations and started intensifying around 15.2 ka BP, at the onset of the Bølling-Ållerød (B-A) event. The B-A event is characterized by higher water column denitrification rates comparable to the present day. The ISM signatures observed in the d18OC record of B-A event compare well with those from Timta cave of the western Himalayas and also the Asian summer monsoon signatures from the Hulu caves in China and warming signatures in Greenland Ice Sheet Project 2 (GISP2) suggesting atmospheric teleconnections through Intertropical Convergence Zone. The boundary between the Younger Dryas and the Holocene is discernible with small episodes of abrupt events of increased ISM intensity. This decrease in d18OC values at ~11.8 ka BP is contemporary with June solar insolation maximum at 30° north and the increase in methane in the GISP2 ice core supporting episodes of warmer climate and increase in ISM intensity. The ISM seems to have been most stable between 7 and 5.6 ka BP. The core exhibits periodicity of 500 years that is comparable to the Atlantic water formation and the Chinese monsoon.