Compositions of branched glycerol dialkyl glycerol tetraethers (GDGTs) in sediment samples from Bullenmoor peat bog, Northern Germany

Two types of intact branched glycerol dialkyl glycerol tetraethers (GDGTs) were detected in peat bog samples from Bullenmoor, Northern Germany. Glucuronosyl and glucosyl branched GDGTs comprise on average ca. 4% of the microbial intact polar lipids in the anoxic, acidic peat layer ca. 20 cm below the surface of the bog, suggesting an important ecological role for the source microorganisms. No corresponding phospholipids were detected. Notably, glycosidic branched GDGTs are 5-10 times less abundant than their intact isoprenoid counterparts derived from Archaea, while branched GDGT core lipids exceed their isoprenoid analogues by about an order of magnitude. These contrasting relationships may reflect lower standing stocks of the biomass of producers of branched GDGTs, combined with higher population growth rates relative to soil Archaea. Search strategies for the microbial producers of these conspicuous orphan lipids should benefit from the discovery of their intact polar precursors.

Site characteristics, sediment geochemistry and pigment concentration of northern Victoria Land lakes

Antarctic ice-free areas contain lakes and ponds that have interesting limnological features and are of wide global significance as early warning indicators of climatic and environmental change. However, most limnological and paleolimnological studies in continental Antarctica are limited to certain regions. There are several ice-free areas in Victoria Land that have not yet been studied well. There is therefore a need to extend limnological studies in space and time to understand how different geological and climatic features affect the composition and biological activity of freshwater communities. With the aim of contributing to a better limnological characterization of Victoria Land, this paper reports data on sedimentary pigments (used to identify the main algal taxa) obtained through a methodology that is more sensitive and selective than that of previous studies. Analyses were extended to 48 water bodies in ice-free areas with differing lithology, latitude, and altitude, and with different morphometry and physical, chemical, and biological characteristics in order to identify environmental factors affecting the distribution and composition of freshwater autotrophic communities. A wider knowledge of lakes in a limnologically important region of Antarctica was obtained. Cyanophyta was found to be the most important algal group, followed by Chlorophyta and Bacillariophyta, whereas latitude and altitude are the main factors affecting pigment distribution.

Pollen record and age determination of a sediment core from the Kara Sea

AMS-14C dated sediment cores from the Ob and Yenisei estuaries and the adjacent inner Kara Sea were investigated to determine the siliclastic and organic carbon fluxes and their relationship to paleoenvironmental changes. The variability of sediment fluxes during Holocene times is related to the post-glacial sea-level rise and changes in river discharge and coastal erosion input. Whereas during the late/middle Holocene most of the terrigenous sediments were deposited in the estuaries and the areas directly off the estuaries, huge amounts of sediments accumulated on the Kara Sea shelf farther north during the early Holocene before about 9 Cal. kyrs. BP. The maximum accumulation at that time is related to the lowered sea level, increased coastal erosion, and increased river discharge due to the final stage of mountain deglaciation of the Putoran Massif. Increased supply of Yenisei-derived material indicated by peak magnetic susceptibility values probably occurred in climate-related pulses culminating near 11, 10, and 9 Cal. kyrs. BP. As sea level rose, the main Holocene depocenter migrated southward. Based on hydrogen index values and n-alkanes, the organic matter is predominantly of terrigenous origin. Maximum accumulation rates of 1.5 to more than 6 g/cm2/y occurred in the early Holocene sediments, suggesting more humid climatic conditions with an increased vegetation cover in the source area at that time. In general, high organic carbon accumulation rates characterize the estuaries and the inner Kara Sea as important sink for terrigenous organic carbon. A high-resolution record of Holocene variability of magnetic susceptibility (MS) in an AMS14C-dated sediment core from the northern Yenisei estuary may indicate natural variability of Arctic climate change and river discharge on a centennial to millenial time scale. Short-term maxima in MS probably related to warmer climate, enhanced precipitation, intensified weathering/erosion and increased river discharge, display a frequency of about 300 to 700 years.

Mineralogical assemblage in sediment core GeoB12309-5 and in area samples from the Makran subduction zone

In order to study late Holocene changes in sediment supply into the northern Arabian Sea, a 5.3 m long gravity core was investigated by high-resolution geochemical and mineralogical techniques. The sediment core was recovered at a water depth of 956 m from the continental slope off Pakistan and covers a time span of 5 kyr. During the late Holocene source areas delivering material to the sampling site did, however, not change and were active throughout the year.

Variability of the Brazil Current during the late Holocene analysed on two marine sediment cores

Our understanding of the centennial-scale variability of the Brazil Current (BC) during the late Holocene is elusive because of the lack of appropriate records. Here we used the Mg/Ca and oxygen isotopic composition of planktonic foraminifera from two marine sediment cores collected at 27° S and 33° S off southeastern South America to assess the late Holocene variability in the upper water column of the BC. Our results show in phase fluctuations of up to 3 °C in sea surface temperatures (SST), and 0.8 per mil in oxygen isotopic composition of surface sea water, a proxy for relative sea surface salinity (SSS). Time-series analyses of our records indicate a cyclicity with a period of ca. 730 yr. We suggest that the observed cyclicity reflects variability in the strength of the BC associated to changes in the Atlantic meridional overturning circulation (AMOC). Positive (negative) SST and SSS anomalies are related to a strong (weak) BC and a weak (strong) AMOC. Moreover, periods of peak strength in the BC occur synchronously to a weak North Brazil Current, negative SST anomalies in the high latitudes of the North Atlantic, and positive (negative) precipitation anomalies over southeastern South America (equatorial Africa), further corroborating our hypothesis. This study shows a tight coupling between the variability of the BC and the high latitudes of the North Atlantic mediated by the AMOC even under late Holocene boundary conditions.

Stable isotopes and Mg/Ca measurements of two sediment cores in the northeastern tropical Atlantic

Two SST records based on Mg/Ca of G. ruber (pink) from the continental slope off West Africa at 15°N and 12°N shed new light on the thermal bipolar seesaw pattern in the northeastern tropical Atlantic during periods of reduced Atlantic Meridional Overturning Circulation (AMOC) associated with Heinrich stadials H1 to H6. The two records indicate that the latitudinal position of the bipolar seesaw's zero-anomaly line, between cooling in the North and warming in the South, gradually shifted southward from H6 to H1. A conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaw's fulcrum. The conceptual model suggests latitudinal movements of the Intertropical Convergence Zone, driven by a combination of orbital-forced changes in the meridional temperature gradient within the realm of the Hadley cell and the expansion of the Northern Hemisphere cryosphere, as a major factor.

Age model and sediment description of DSDP Hole 90-594, Chatham Ridge east of New Zealand

Detailed sedimentological investigations were performed on sediments from DSDP-Site 594 (Chatham Rise, east of New Zealand) in order to reconstruct the evolution of paleoclimate and paleoceanographic conditions in the Southwest Pacific during the last 6 million years. The results can be summarized as follows: (1) High accumulation rates of biogenic opal and carbonate and the dominance of smectites in the clay fraction suggest increased oceanic productivity and an equable dominantly humid climate during the late Miocene. (2) During Pliocene times, decreasing contents of smectites and increasing feldspar/quartz ratios point to an aridification in the source area of the terrigenous sediments, culmunating near 2.5 Ma. At that time, accumulation rates of terrigenous components distinctly increased probably caused by increased sediment supply due to intensified atmospheric and oceanic circulation, lowered sea level, and decreased vegetation cover. (3) A hiatus (1.45 to 0.73 Ma) suggests intensified intermediate-water circulation. (4) Major glacial/interglacial cycles characterize the upper 0.73 Ma. During glacial times, oceanic productivity and terrigenous sediment supply was distinctly increased because of intensified atmospheric and oceanic circulations and lowered sea level, whereas during interglacials productivity and terrigenous sediment supply were reduced. (5) An increased content of amphibols in the sediments of Site 594 indicates increased volcanic activities during the last 4.25 Ma.

Sea surface temperature reconstruction of sediment profile W8709A-8

Assessment of changes in surface ocean conditions, in particular, sea-surface temperature (SST), is essential to understand long-term changes in climate especially in regions where continental climate is strongly influenced by oceanographic processes. To evaluate changes in SST in the northeast Pacific, we have analyzed long-chain alkenones of prymnesiophyte origin at 38 depths in a piston and associated trigger core collected beneath the contemporary core of the California Current System at 42°N, ~270 km off the coast of Oregon/California. The samples span 30,000 years of deposition at this location. Unsaturation patterns (UK'37) in the alkenone series display a statistically significant difference (p <<0.001) between interglacial (0.44 ± 0.02, n = 11) and glacial (0.29 ± 0.04, n = 20) intervals of the cores. Detailed examination of other compositional features of the C37, C38, C39 alkenone series and a related C36 alkenoate series measured downcore suggests the published UK'37 - temperature calibration (UK'37 = 0.034 * T + 0.039 ) , defined for cultures of a strain of Emiliania huxleyi isolated from the subarctic Pacific, provides best estimates of winter SST at our study site. This inference is purely statistical and does not imply, however, that the phytoplankton source of these biomarkers is most productive in winter or at the ocean surface. The temperature record for UK'37 implies (1) an ~4°C shift occurred in winter SST from ~7.5 ± 1.1°C at the last glacial maximum to ~11.7 ± 0.7°C in the present interglacial period, and (2) this warming trend was confined to the time frame 14-10 Ka within the glacial to interglacial transition period. These conclusions are corroborated entirely by results from an independent SST transformation of radiolarian species assemblage data obtained from the same core materials.

Abundance of organic-walled dinoflagellate cysts and geochemical analysis of sediment core GeoB5546-2

NW African climate shows orbital and millenial-scale variations, which are tightly connected to changes in marine productivity. We present an organic-walled dinoflagellate cyst (dinocyst) record from a sediment core off Cape Yubi at about 27°N in the Canary Basin covering the time period from 47 to 3ka before present (BP). The dinocyst record reflects differences in upwelling intensity and seasonality as well as the influence of fluvial input. Sea-level changes play an important role for the upwelling pattern and productivity signals at the core site. Within the studied time interval, four main phases were distinguished. (1) From 45 to 24ka BP, when sea-level was mostly about 75m lower than today, high relative abundances of cysts of heterotrophic taxa point to enhanced upwelling activity, especially during Heinrich Events, while relatively low dinocyst accumulation rates indicate that filament activity at the core location was strongly reduced. (2) At sea-level lowstand during the LGM to H1, dinocyst accumulation rates suggest that local filament formation was even more inhibited. (3) From the early Holocene to about 8ka BP, extraordinary high accumulation rates of most dinocyst species, especially of Lingulodinium machaerophorum, suggest that nutrient supply via fluvial input increased and rising sea-level promoted filament formation. At the same time, the upwelling season prolongated. (4) A relative increase in cysts of photoautotrophic taxa from about 8ka BP on indicates more stratified conditions while fluvial input decreased. Our study shows that productivity records can be very sensitive to regional features. From the dinocyst data we infer that marine surface productivity off Cape Yubi during glacial times was within the scale of modern times but extremely enhanced during deglaciation.

Clay mineral and stratigraphic record of two sediment cores from the Aegean Sea

Different source areas, oceanography and climate regimes influenced the clay mineral assemblages and grain size distribution of two sediment cores from the North and South Aegean Sea during the last glacial and the Holocene. In the North Aegean Sea, clay mineral composition is mainly controlled by sea level evolution, melting of southeastern European glaciers, and establishment of the connection between the Black Sea and Aegean Sea. The long-term development of clay mineral assemblages in the South Aegean Sea reflects changes in the Nile discharge and African dust input. At this site, the establishment of pluvial conditions in the Nile catchment during the early to middle Holocene resulted in a substantial rise in smectite/illite ratios. In the late Holocene, stepwise aridification of the southern borderlands caused an increase in windblown sediment material and a decrease in Nile suspended material. The clay mineral records exhibit periodic millennial-scale fluctuations. In the North Aegean Sea, the changes are centred at a period of 1.3-1.8 ka and can be attributed to short-term climate and weathering changes in the northern borderlands. The changes in the South Aegean Sea are centred at periods of 3.2-4.3, 1.9-2.4 and 1.3-1.7 ka reflecting short-term changes in wind strength and Northeast African hydrology.

Coccoliths and carbonate grain-size compositions in sediment samples from the Mid-Atlantic Ridge

This study presents a differentiated carbonate budget for marine surface sediments from the Mid-Atlantic Ridge of the South Atlantic, with results based on carbonate grain-size composition. Upon separation into sand, silt, and clay sub-fractions, the silt grain-size distribution was measured using a SediGraph 5100. We found regionally characteristic grain-size distributions with an overall minimum at 8 µm equivalent spherical diameter (ESD). SEM observations reveal that the coarse particles (>8 µm ESD) are attributed to planktic foraminifers and their fragments, and the fine particles (<8 µm ESD) to coccoliths. On the basis of this division, the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated. Foraminifer carbonate dominates the sediments in mesotropic regions whereas coccoliths contribute most carbonate in oligotrophic regions. The grain size of the coccolith share is constant over water depth, indicating a lower susceptibility for carbonate dissolution compared to foraminifers. Finally, the characteristic grain-size distribution in fine silt (<8 µm ESD) is set into context with the coccolith assemblage counted and biometrically measured using a SEM. The coccoliths present in the silt fraction are predominantly large species (length > 4 µm). Smaller species (length < 4 µm) belong to the clay fraction (<2 µm ESD). The average length of most frequent coccolith species is connected to prominent peaks in grain-size distributions (ESD) with a shape factor. The area below Gaussian distributions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates. The quantitative division of carbonate into the fraction produced by coccoliths and that secreted by foraminifers enables a more precise estimate for source/sink relations of consumed and released CO2 in the carbon cycle. The allocation of coccolith length and grain size (ESD) suggests size windows for the separation or accumulation of distinct coccolith species in investigations that depend on non to slightly-mixed signals (e.g., isotopic studies).

Pollen distribution in marine sediment samples along the south-western African coast

The distribution of pollen in marine sediments is used to reconstruct pathways of terrigenous input to the oceans and provides a record of vegetation change on adjacent continents. The wind transport routes of aeolian pollen is comprehensively illustrated by clusters of trajectories. Isobaric, 4-day backward trajectories are calculated using the modelled wind-field of ECHAM3, and are clustered on a seasonal basis to estimate the main pathways of aeolian particles to sites of marine cores in the south-eastern Atlantic. Trajectories and clusters based on the modelled wind-field of the Last Glacial Maximum hardly differ from those of the present-day. Trajectory clusters show three regional, and two seasonal patterns, determining the pathways of aeolian pollen transport into the south-eastern Atlantic ocean. Mainly, transport out of the continent occurs during austral fall and winter, when easterly and south-easterly winds prevail. South of 25°S, winds blow mostly from the west and southwest, and aeolian terrestrial input is very low. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in marine surface sediments and the occurrence of the source plants on the adjacent continent. The northern Angola Basin receives pollen and spores from the Congolian and Zambezian forests mainly through river discharge. The Zambezian vegetation zone is the main source area for wind-blown pollen in sediments of the Angola Basin, while the semi-desert and desert areas are the main sources for pollen in sediments of the Walvis Basin and on the Walvis Ridge. A transect of six marine pollen records along the south-western African coast indicates considerable changes in the vegetation of southern Africa between glacial and interglacial periods. Important changes in the vegetation are the decline of forests in equatorial Africa and the north of southern Africa and a northward shift of winter rain vegetation along the western escarpment.

Osmium isotope composition of leachates and bulk sediment from ODP Holes in the Bengal Fan

Os isotopic compositions and OS and Re concentrations were measured in H2O2-H2SO4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/188Os ratios (Pegram et al., 1994, doi:10.1016/0012-821X(94)90172-4) much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y. Bengal Fan leachate 187Os/188Os ratios increase with increasing depositional age, in contrast to the seawater Os isotopic ratio, which decreases with increasing age. Several lines of evidence suggest that, at the time of sediment burial, the leachate Os compositions most likely reflected the seawater values. Thus, the current divergence is probably the result of post-depositional processes. One such process, in situ radiogenic ingrowth of 187Os, can be excluded because the measured Re concentrations of these sediments are too low. Similarly, since most of the bulk rock Os isotopic ratios were lower than those of the associated leachates, the high leachate 187Os/188Os values cannot be explained by in situ sediment alteration. Instead, it is proposed that the increase with age results from radiogenic OS brought in by thermoconvective circulation from further upslope in the Fan. The ultimate source of this 187Os would then be alteration of radiogenic sediments or post-depositional radioactive decay of Re in sediments rich in organic carbon. Finally, the divergence between the results obtained on Bengal Fan sediments and those obtained in the open ocean (Pegram et al., 1992, doi:10.1016/0012-821X(92)90132-F) by the same leaching technique suggest that Os sediment leachate data must be interpreted with caution.

Late Pliocene age models of Agulhas Plateau sediment cores

An ensemble of new, high-resolution records of surface ocean hydrography from the Indian-Atlantic oceanic gateway, south of Africa, demonstrates recurrent and high-amplitude salinity oscillations in the Agulhas Leakage area during the penultimate glacial-interglacial cycle. A series of millennial-scale salinification events, indicating strengthened salt leakage into the South Atlantic, appear to correlate with abrupt changes in the North Atlantic climate and Atlantic Meridional Overturning Circulation (AMOC). This interhemispheric coupling, which plausibly involved changes in the Hadley Cell and midlatitude westerlies that impacted the interocean transport at the tip of Africa, suggests that the Agulhas Leakage acted as a source of negative buoyancy for the perturbed AMOC, possibly aiding its return to full strength. Our finding points to the Indian-to-Atlantic salt transport as a potentially important modulator of the AMOC during the abrupt climate changes of the Late Pleistocene.