Maps of surface sediment distribution in the northern Wadden Sea, North Sea

From 1978 to 1981, intensive sedimentological investigations were carried out on the Northfrisian intertidal shoals between the small island of Gröde and Nordstrand lsland as a part of an interdisciplinary research projekt. The objective of this sedimentological study was to reveal long and short term tendencies in sedimentation and erosion in the environment. The presented study mainly concentrated on surface mapping in the tidal flats which based on more than 5000 sediment samples. The relative amounts of the grain-size fractions <0.063 mm and >0.125 mm are presented on maps. Predominant sediment typs are well sorted fine sands ("Wattsand") and muddy sands ("Schlicksand"), pure muds covering only small areas. The fine-grained deposits are either found in the lee-side of an island in elongated bays having a low waterdepth during high tide near the shore or near exposed "Klei" outcrops as well as sporadically on the edge of tidal rills. Together with standardized fields observations of biological and physical properties, the maps indicate a slight erosive tendency in large sections of the investigated area.

Sea surface reconstruction for sediments of the California continental margin

Organic matter in sediment samples from three ODP sites (Ocean Drilling Program Leg 167) that form a south-north transect was investigated to reconstruct the paleoclimatic and oceanographic conditions on the California continental margin during the last 160 kyr. Alkenone-derived paleosea surface temperatures (SST) are 3 to 6°C colder in glacial stages and reveal a clear relationship with global climate changes; the differences are greater in the north. Latitudinal SST comparison exhibits water mixing of the colder California Current with warmer waters from the south, particularly in the southern central California borderland area. Organic matter accumulation on the California continental margin indicates an interplay between climatic and atmospheric glacial-interglacial variations and spatially and temporally changing nutrient availability along the California coastline. Climatic and atmospheric dependent circulations apparently caused variations in the intensity of coastal upwelling along the southern central California margin and this suggests, due to the close connection of the California Current to the local wind patterns, that the California Current was weaker during glacial and stronger during interglacial periods.

Nannoplankton of multi-net samples, sediment surface and sediment core samples from the northern Arabian Sea

Recent nannoplankton from the "Meteor"-stations M 242, M 243 and M 245 in the northern Arabian Sea were studied by means of the light and electron microscope, and 19 species were found. The nannoplankton assemblage of the northern Arabian Sea is compared with those of the eastern and western Mediterranean Sea and the Atlantic Ocean. Gephyrocapsa oceanica (Kamptner), Cyclococcolithus leptoporus (Murray & Blackman), Emiliania huxleyi (Lohmann), Helicopontospbaera kamptneri (Hay & Mohler), and Umbilicosphaera mirabilis (Lohmann) are the most common species in the northern Arabian Sea. Reworked nannoplankton and nannoplankton species agglutinated by tintinnids are discussed.

Diatom assamblage in samples from sediment traps across the Pacific Sector of the Southern Ocean

An array of four sediment trap moorings recorded the particulate flux across the Antarctic Circumpolar Current (ACC) at 170 °W, between November 1996 and January 1998, as part of the US JGOFS-Antarctic Environment and Southern Ocean Process Study (AESOPS) program. The trap locations represent sampling within the Polar Frontal Zone, the Antarctic Polar Front, the Antarctic Zone and the Southern Antarctic Zone. Here we report observations from 1000 m below the sea-surface compared to seafloor and surface water distributions. Sub-sample splits from each trap were obtained and total diatom flux and species composition were determined. The diatom fluxes were quantified using both a dilution and a 'spike' method to allow for the rapid repeatability of measurements. Diatom flux was found to be highly seasonal across the ACC particularly at higher latitudes. Marine snow aggregates of intact diatom cells and chains were the major components of the biogenic flux. Siliceous particle size was noted to decrease with increasing latitude, which could be aligned with a shift of the diatom assemblage to small-size species/sea-ice affiliated species. A 'double-structured' diatom flux was recorded at the location of the Antarctic Polar Front trap, with a shift in the diatom assemblage from larger to smaller diatoms in the second flux episode. The sediment trap assemblage shows deviations from the surface water assemblage, while surface sediment samples indicate that significant dissolution occurs after 1000 m and at the sediment-water interface. Estimation of diatom biovolumes across the ACC shows that large diatoms have the potential to greatly impact biogenic fluxes to the ocean interior despite their low fluxes. Small species of the genus Fragilariopsis could potentially export as much Corg as Fragilariopsis kerguelensis near the retreating ice edge. However, their low abundance in the surface sediments also suggests that these diatoms are a shallow export species.

Planktic foraminiferal abundance and stable oxygen isotope ratios of sediment cores from the Southern California Borderlands

A variety of evidence suggests that average sea surface temperatures (SSTs) during the last glacial maximum in the California Borderlands region were significantly colder than during the Holocene. Planktonic foraminiferal delta18O evidence and average SST estimates derived by the modern analog technique indicate that temperatures were 6°-10°C cooler during the last glacial relative to the present. The glacial plankton assemblage is dominated by the planktonic foraminifer Neogloboquadrina pachyderma (sinistral coiling) and the coccolith Coccolithus pelagicus, both of which are currently restricted to subpolar regions of the North Pacific. The glacial-interglacial average SST change determined in this study is considerably larger than the 2°C change estimated by Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) [1981]. We propose that a strengthened California Current flow was associated with the advance of subpolar surface waters into the Borderlands region during the last glacial.

Response of nannoplankton to major changes in sea-surface temperature at the South Chatham Rise, southeastern New Zealand

A high-resolution history of paleoceanographic changes in the subpolar waters of the southern margin of the Subtropical Convergence Zone during the last 130 kyr, is present in foraminiferal assemblages of DSDP Site 594. The foraminifera indicate that sea-surface temperatures during the Last Interglacial Climax were warmer than today, and that between substage 5d through to the end of isotope stage 2, temperatures were mostly cooler than Holocene temperatures. The paleotemperatures suggest that (1) the Subtropical Convergence was located over the site during substage 5e, later moving further north, then moving southwards to near the site during the Holocene, and (2) the Polar Front was positioned over the Site during glacial stages 6, 4, 2 and possibly parts of stage 3. Several major events are indicated by the nannofloral assemblages during these large changes in sea-surface temperature and associated reorganization of ocean circulation. First, the time-progressive trends between E. huxleyi and medium to large Gephyrocupsa are unique to this site, with E. huxleyi dominating over medium Gephyrocupsa during stages 5c-a, middle part of stage 4 and after the middle point of stage 3. This unusual trend may (at least partly) be caused by the shift of the Polar Front across the site. Second, upwelling flora (E. huxleyi and small placoliths) increase in abundance during stages 1, 3 and 5, suggesting that upwelling or disturbance of water stratification took place during the interglacials. Thirdly, there are no significant differences between the distribution patterns of the various morphotypes of medium to large Gephyrocupsu, and the combined value of all medium Gephyrocupsu increases in abundance during glacials (stages 2 and 4 and the end of stage 6), similar to the abundance trends in benthic foraminifera. Finally, subordinate nannofossil taxa also show distinctive climatic trends during the last glacial cycle: (1) Syrucosphaera spp. are present in increased abundance during warmer extremes in climate (substages 5e, 5a, and stage 1); (2) Coccolithus pelagicus and Culcidiscus leptoporus dominate the subordinate nannofossil taxa, and their relative proportions seem to provide a useful paleoceanographic index, with C. pelagicus dominating when the Polar Front Zone is over the site (stages 6, 4 and 2), whilst C. leptoporus is relatively more abundant when the STC is positioned over the site (stages 1 and 5e). Increased abundance of C. pelagicus also can indicate intensified coastal upwelling.

Sea surface temperature estimation for the last 70 ka from IODP Site 306-U1313

During the six Heinrich Events of the last 70 ka episodic calving from the circum-Atlantic ice sheets released large numbers of icebergs into the North Atlantic. These icebergs and associated melt-water flux are hypothesized to have led to a shutdown of Atlantic Meridional Overturning Circulation (AMOC) and severe cooling in large parts of the Northern Hemisphere. However, due to the limited availability of high-resolution records the magnitude sea surface temperature (SST) changes related to the impact of Heinrich Events on the mid-latitude North Atlantic is poorly constrained. Here we present a record of UK37'-based SSTs derived from sediments of Integrated Ocean Drilling Project (IODP) Site U1313, located at the southern end of the ice-rafted debris (IRD)-belt in the mid-latitude North Atlantic (41°N). We demonstrate that all six Heinrich Events are associated with a rapid warming of surface waters by 2 to 4°C in a few thousand years. The presence of IRD leaves no doubt about the simultaneous timing and correlation between rapid surface water warming and Heinrich Events. We argue that this warming in the mid-latitude North Atlantic is related to a northward expansion of the subtropical gyre during Heinrich Events. As a wide-range of studies demonstrated that in the central IRD-belt Heinrich Events are associated with low SSTs, these results thus identify an anti-phased (seesaw) pattern in SSTs during Heinrich Events between the mid-latitude (warm) and northern North Atlantic (cold). This highlights the complex response of surface water characteristics in the North Atlantic to Heinrich Events that is poorly reproduced by fresh water hosing experiments and challenges the widely accepted view that within the IRD-belt of the North Atlantic Heinrich Events coincide with periods of low SSTs.

Age models and sea-surface paleotemperature reconstruction of sediment cores from the eastern equatorial Atlantic

Based on the faunal record of planktonic foraminifers in three long gravity sediment cores from the eastern equatorial Atlantic, the sea-surface temperature history ove the last 750,000 years was studied at a resolution of 3,000 to 10,000 years. Detailed oxygen-isotope and paleomagnetic stratigraphy helped to identify the following major faunal events: Globorotaloides hexagonus and Globorotalia tumida flexuosa became extinct in the eastern tropical Atlantic at the isotope stage 4/5 boundary, now dated at 68,000 years B.P. The persistent occurrence of the pink variety of Globigerinoides ruber started during the late stage 12 at 410,000 years B.P. CARTUNE-age. This datum may provide an easily detectible faunal stratigraphic marker for the mid-Brunhes Chron. The updated scheme of the Ericson zones helped the recognition of a hiatus at the northwestern slope of the Sierra Leone Basin covering oxygen-isotope stages 10 to 12. Classifying the planktonic foraminifer counts into six faunal assemblages, according to the factor analysis derived model of Pflaumann (1985), the tropical and the tropical-upwelling communities account for 57 % at Site 16415, and 86 % at Site 13519, respectively of the variance of the faunal record. A largely continuous paleotemperature record for both winter and summer seasons was obtained from the top of the Sierra Leone Rise with the winter temperatures ranging between 20 and 25 °C, and the summer ones between 24 and 30 °C. The record of cores from greater water depths is frequently interrupted by samples with no-analogue faunal communities and/or poor preservation. Based on the seasonality signal, during cold periods the termal equator shifted to a geographically mnore asymmetrical northern position. Dissolution altering the faunal communities becomes stronger with greater water depth, the estimated mean minimum loss of specimens increases from 70 % to 80 % between 2,860 and 3,850 water depth although some species will be more susceptible than others. Enhanced dissolution occured during stage 4 but also during cold phases in the warm stage 7 and 9. Correlations between the Foraminiferal Dissolution Index and the estimated sea-surface temperatures are significant. Foraminiferal flux rates, negatively correlated to the flux rates of organic carbon and of diatoms, may be a result of enhanced dissolution during cold stages, destroying still more of the faunal signal than indicated by the calculated minimum loss. The fluctuations of the oxygen-isotope curves and the hibernal sea-surfave temperatures are fairly coherent. During warm oxygen-isotope stages the temperature maxima lag often by 5 to 15 ka behind the respective sotope minima. During cold stages, sea-surface temperature changes are partly out of phase and contain additional fluctuations.

Ikaite of sediment cores from the Zaire deep-sea fan

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite delta13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate-methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, [SO4]2- and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.