Alkenone data of sediment traps off Cape Blanc, NW Africa

We analysed long-chain alkenones in sinking particles and surface sediments from the filamentous upwelling region off Cape Blanc, NW Africa, to evaluate the transfer of surface water signals into the geological record. Our study is based on time-series sediment trap records from 730 m (1990-1991) to 2195-3562 m depth (1988-1991). Alkenone fluxes showed considerable interannual variations and no consistent seasonality. The average flux of C37 and C38 alkenones to the deep traps was 1.9 µg/m**2/d from March 1988 to October 1990 and sevenfold higher in the subsequent year. Alkenone fluxes to the shallower traps were on average twice as high and showed similar temporal variations. The alkenone unsaturation indices UK'37, UK38Me and UK38Et closely mirrored the seasonal variations in sea-surface temperature (weekly Reynolds SST). Time lags of 10-48 days between the SST and unsaturation maxima suggest particle sinking rates of about 80 and 280 m/d for the periods of low and high alkenone fluxes, respectively. The average flux-weighted UK'37 temperature for the 4-year time series of the deeper traps was 22.1°C, in perfect agreement with the mean weekly SST for the same period. This and the comparison with seasonal temperature variations in the upper 100 m of the water column suggests that UK'37 records principally the yearly average of the mixed-layer temperature in this region. A comparison between the average annual alkenone fluxes to the lower traps (2400 µg/m**2/yr) and into the underlying sediments (4 µg/m**2/yr) suggests that only about 0.2% of the alkenones reaching the deep ocean became preserved in the sediments. The flux-weighted alkenone concentrations also decreased considerably, from 2466 µg/gC in the water column to 62 µg/gC in the surface sediments. Such a low degree of alkenone preservation is typical for slowly accumulating oxygenated sediments. Despite these dramatic diagenetic alkenone losses, the UK'37 ratio was not affected. The average UK'37 value of the sediments (0.796±0.010 or 22.3±0.3°C) was identical within error limits to the 4-year average of the lower traps. The unsaturation indices for C38 alkenones and the ratio between C37 and C38 alkenones also revealed a high degree of stability. Our results do not support the hypothesis that UK'37 is biased towards higher values during oxic diagenesis.

Water chemistry and fluffy layer geochemistry and mineral content at four sites in the western Baltic Sea

The fluffy layer was sampled repeatedly during nine expeditions between October 1996 and December 1998 at four stations situated along a S-N-transect from the Oder Estuary to the Arkona Basin. Geochemical and mineralogical analyses of the fluff show regional differences (trends) in composition, attributed to provenance and to hydrographical conditions along their transport pathways. Temporal variability is very high at the shallow water station of the estuary, and decreases towards the deeper stations in the north. In the shallow water area, intensive resuspension of the fluff due to wind-driven waves and currents leads to an average residence time of only one to two days. Near-bottom lateral transport of the fluff is the main process that transfers the fine grained material, containing both nutrients and contaminants, from the coastal zone into the deeper basins of the Baltic Sea. Seasonal effects (e.g. biogenic production in relation to trace metal variation) are observed at the Tromper Wiek station, where the residence time of the fluffy material is in the scale of seasons. Thus, the fluffy layer offers suitable material for environmental monitoring programs.

Dinoflagellate cyst assemblages, oxygen isotope records of foraminifera, and alkenone and Mg/Ca-based SST estimates for the time interval 3.40-3.18 Ma (Late Pliocene) from five North Atlantic and Caribbean sediment cores

The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Our geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current (NAC) and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the Greenland ice sheet during MIS M2, despite near-modern atmospheric CO2 concentrations. Before and after MIS M2, heat transport via the NAC was crucial in maintaining warm climates comparable to those predicted for the end of this century.

Phytoplankton photosynthetic competence, underway data of SOIREE cruise track 1999-02-08 to 1999-02-24

Active fluorescence (fast repetition rate fluorometry, FRRF) was used to follow the photosynthetic response of the phytoplankton community during the 13-day Southern Ocean Iron RElease Experiment (SOIREE). This in situ iron enrichment was conducted in the polar waters of the Australasian-Pacific sector of the Southern Ocean in February 1999. Iron fertilisation of these high nitrate low chlorophyll (HNLC) waters resulted in an increase in the photosynthetic competence (Fv/Fm) of the resident cells from around 0.20 to greater than 0.60 (i.e. close to the theoretical maximum) by 10/11 days after the first enrichment. Although a significant iron-mediated response in Fv/Fm was detected as early as 24 h after the initial fertilisation, the increase in Fv/Fm to double ambient levels took 6 days. This response was five-fold slower than observed in iron enrichments (in situ and in vitro) in the HNLC waters of the subarctic and equatorial Pacific. Although little is known about the relationship between water temperature and Fv/Fm, it is likely that low water temperatures - and possibly the deep mixed layer - were responsible for this slow response time. During SOIREE, the photosynthetic competence of the resident phytoplankton in iron-enriched waters increased at dissolved iron levels above 0.2 nM, suggesting that iron limitation was alleviated at this concentration. Increases in Fv/Fm of cells within four algal size classes suggested that all taxa displayed a photosynthetic response to iron enrichment. Other physiological proxies of algal iron stress (such as flavodoxin levels in diatoms) exhibited different temporal trends to iron-enrichment than Fv/Fm during the time-course of SOIREE. The relationship between Fv/Fm, algal growth rate and such proxies in Southern Ocean waters is discussed.

Stable oxygen isotope record of Globigerinoides sacculifer and age determination of sediment cores from teh Somali Basin

This study documents the biological signatures impressed upon the sedimentary record underlying both the 5°N upwelling system of the Somali Current and the equatorial area of the Somali Basin out of the upwelling influence. The evolution of these two distinct hydrographic systems is compared for the last 160 kyr. Correspondence and cluster analyses are performed on combined radiolarian and planktonic foraminiferal quantitative data in order to study the changes of the planktonic assemblages through time and space. The Upwelling Radiolarian Index (URI) is used as a productivity proxy. The water temperature and hydrographic structure of the upper water masses appear to be the major factors controlling the distribution patterns of the fauna. The relative abundances of three groups of foraminifera, cold water form (dextral N. pachyderma), mixed layer dwellers (G. trilobus, G. ruber, G. sacculifer, G. conglobatus, and G. glutinata), and thermocline dwellers (G. menardii, G. tumida, N. dutertrei, G. crassaformis, and P. obliquiloculata), follow distinct evolutionary patterns at the two sites during the last 160 kyr. At the equatorial site (core MD 85668), downcore fluctuations in the relative abundances of the three groups are closely related to the glacial/interglacial cyclicity and provide some insights into the interpretation of hydrographic changes. The dominance of the mixed layer foraminifera at the transition intervals between isotope stages 6/5 and 2/1, combined with weak URI values, is thought to reflect the reorganization of the oceanographic circulation. These short-term events (with a duration of < 5000 year) could be related to the rapid inflow of oxygen-depleted water through the Indonesian straits as a result of sea level rise during deglaciation. Underneath the 5°N gyre (core MD 85674), the response to global climatic changes is overprinted by the regional effect of the Somalian upwelling, which has been persistent over the last 160 kyr.

Composition of sedimentary material from the bottom and the area surrounding the Lake Untersee, East Antarctica

This paper presents data on geographic and geologic conditions of modern sedimentation in the Lake Untersee, the largest lake in the East Antarctica. Geochemical and sedimentation data indicate that the leading mechanism supplying aluminosilicate sedimentary material to the surface layer of bottom sediments is seasonal melting of the Anuchin glacier and the mountain glacier on the southeastern part of the valley hosting the lake. Strongly reduced conditions in the lowermost 25 m of the water column in the smaller of two depressions of the lake bottom were favorable for enrichment of the bottom sediments in bacteriogenic organic matter, Mo, Au, and Pd. H2S-contaminated water results to significant enrichment of the sediments only in redox-sensitive elements that are able to migrate in anionic complexes and precipitate (co-precipitate) as sulfides.

The Cenomanian/Turronian Boundary Event at ODP Hole 103-641A

Drilling at ODP Site 641 (on the western margin of Galicia Bank, off northwestern Spain) revealed a thin, but pronounced, interval of black shale and gray-green claystone. Our high-resolution study combines the sedimentology, micropaleontology (palynomorphs and others), organic and inorganic geochemistry, and isotopic values of this layer to demonstrate the distinct nature of the sediment and prove that the sequence represents the local sedimentary expression of the global Cenomanian/Turonian Oceanic Anoxic Event (OAE) of Schlanger and Jenkyns (1976), Arthur and Schlanger (1979), and Jenkyns (1980), also called the Cenomanian/Turonian Boundary Event (CTBE). The most striking evidence is that the strong positive d13C excursion characterizing the CTBE sequences in shallow areas can be traced into a pronounced deep-sea expression, thus providing a good stratigraphic marker for the CTBE in various paleosettings. The isotopic excursion at Site 641 coincides with an extremely enriched trace metal content, with values that were previously unknown for the Cretaceous Atlantic. Similar to other CTBE occurrences, the organic carbon content is high (up to 11%) and the organic matter is of dominantly marine origin (kerogen type II). The bulk mineralogy of the CTBE sediments does not differ significantly from the general trend of Cretaceous North Atlantic sediments (dominance of smectite and zeolite with minor amounts of illite and scattered palygorskite, kaolinite, and chlorite); thus, no evidence for either increased volcanic activity nor a drastic climatic change in the borderlands was found. Results from Site 641 are compared with the CTBE section found at Site 398, DSDP Leg 47B (Vigo Seamount at the southern end of the Galicia Bank).

Geochemistry and mineralogy at DSDP Holes 61-462, 61-462A and 33-315A

Sedimentary rocks of Barremian through early Maestrichtian age recovered on Deep Sea Drilling Project Leg 61 had their principal source in the complex of igneous rocks with which they are interlayered in the Nauru Basin. Relict textures and primary sedimentary structures show these Cretaceous sediments to be of hyaloclastic origin, in part reworked and redeposited by slumps and currents. The dominant composition now is smectite, but locally iron, titanium, and manganese oxides, plagioclase, pyroxene, analcime, clinoptilolite, chalcedonic quartz, cristobalite, amphibole, nontronite, celadonite, and pyrite are also present. The mineral assemblages and the geochemistry reflect the original basaltic composition and its subsequent alteration by one or more processes of submarine weathering, authigenesis, hydrothermal circulation, and contact metamorphism. Hyaloclastitic sandstone, siltstone, and breccia within the sheet flows below 729 meters sub-bottom depth have Barremian fossils, thus establishing the age of the lower, or extrusive, complex of post-ridge-crest volcanism. Similar hyaloclastites between 564 and 729 meters are invaded by hypabyssal sills of the upper igneous complex, and fossil ages of Albian or Cenomanian set an older limit to the age of that second post-ridge-crest episode. Cenomanian to early Campanian sedimentary rocks between 490 and 564 meters have a substantial contribution of clays of submarine-weathered-basalt origin, as well as hydrothermal and pelagic components. The interval of reworked hyaloclastitic siltstone, sandstone, and breccias between 450 and 490 meters is of late Campanian and early Maestrichtian age. These sediments probably formed from glassy basalt that fragmented upon eruption nearby, when sills were being emplaced. In addition to pelagic elements, these Upper Cretaceous volcanogenic sediments include redeposited material of shallow-water origin, apparently derived from the Marshall Islands.

Modern and fossil corals from Palmyra and Christmas islands

The relationship between decadal to centennial changes in ocean circulation and climate is difficult to discern using the sparse and discontinuous instrumental record of climate and, as such, represents a large uncertainty in coupled ocean-atmosphere general circulation models. We present new modern and fossil coral radiocarbon (D14C) records from Palmyra (6°N, 162°W) and Christmas (2°N, 157°W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral D14C measurements from the 10th, 12th-17th, and 20th centuries do not contain significant interannual to decadal-scale variations, despite large changes in coral d18O on these timescales. A centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (~900-1200 AD) to the Little Ice Age (~1500-1800) can be largely explained by changes in the atmospheric D14C, as determined with a box model of Palmyra mixed layer D14C. However, large 12th century depletions in Palmyra coral D14C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the D14C of higher-latitude source waters reaching the equatorial Pacific during this time. SEM photos reveal evidence for minor dissolution and addition of secondary aragonite in the fossil corals, but our results suggest that coral D14C is only compromised after moderate to severe diagenesis for these relatively young fossil corals.

(Table 2) Mineralogy and carbon, oxygen, and hydrogen isotope ratios for secondary mineral samples from igneous rocks from ODP Hole 128-794D

The basaltic rocks of Hole 794D drilled during Leg 128 are strongly altered. Microprobe analyses and XRD spectra on small quantities of matter extracted from thin sections show that primary minerals and glassy zones of the groundmass are totally or partially replaced by clay minerals with chlorite/saponite mixed-layer composition whatever the rock sample considered. This mixed-layer was also identified in veins and vesicles where it crystallizes in spheroidal aggregates. The largest veins and vesicles are filled by a zoned deposit: the chlorite/saponite mixed-layer always occupies the central part and is rimmed by pure saponite. Calcite crystallizes in secondary fractures which crosscut the clayey veins and vesicles. Chemographic analysis based on the M+-4Si-3R2+ projection shows that the chemical composition of the saponite component in the mixed-layer is identical to that of the free saponite. This indicates that the clay mineral crystallization was controlled by the chemical composition of the alteration fluids. From petrographic evidence, it is suggested that both chlorite/saponite mixed-layer and free saponite belong to the same hydrothermal event and are produced by a temperature decrease. This is supported by the stable isotopic data. The isotopic data show very little variation: d18O saponite ranges from 13.1 per mil to 13.5 per mil, and dD saponite from -73.6 per mil to -70.0 per mil. d18O calcite varies from +19.7 per mil to +21.9 per mil vs SMOW and d13C from -3.2 per mil to +0.4 per mil vs. PDB. These values are consistent with seawater alteration of the basalt. The formation of saponite took place at 150°-180°C and the formation of calcite at about 65°C.