Quantitative coccolithophore analyses in sediment core MD01-2446

Quantitative coccolithophore analyses were performed in core MD01-2446, located in the mid-latitude North Atlantic, to reconstruct climatically induced sea-surface water conditions throughout Marine Isotope Stages (MIS) 14-9. The data are compared to new and available paleoenvironmental proxies from the same site as well as other nearby North Atlantic records that support the coccolithophore signature at glacial-interglacial to millennial climate scale. Total coccolithophore absolute abundance increases during interglacials but abruptly drops during the colder glacial phases and deglaciations. Coccolithophore warm-water taxa (wwt) indicate that MIS11c and MIS9e experienced warmer and more stable conditions throughout the whole photic zone compared to MIS13. MIS11 was a long-lasting warmer and stable interglacial characterized by a climate optimum during MIS11c when a more prominent influence of the subtropical front at the site is inferred. The wwt pattern also suggests distinct interstadial and stadial events lasting about 4-10 kyr. The glacial increases of Gephyrocapsa margereli-G. muellerae 3-4 µm along with higher values of Corg, additionally supported by the total alkenone abundance at Site U1313, indicate more productive surface waters, likely reflecting the migration of the polar front into the mid-latitude North Atlantic. Distinctive peaks of G. margereli-muellerae (> 4 µm), C. pelagicus pelagicus, Neogloboquadrina pachyderma left coiling, and reworked nannofossils, combined with minima in total nannofossil accumulation rate, are tracers of Heinrich-type events during MIS12 and MIS10. Additional Heinrich-type events are suggested during MIS12 and MIS14 based on biotic proxies, and we discuss possible iceberg sources at these times. Our results improve the understanding of mid-Brunhes paleoclimate and the impact on phytoplankton diversity in the mid-latitude North Atlantic region.

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.

Quartz content, accumulation and sedimentation rates at DSDP Hole 91-595A

The quartz contents of sediments from Hole 595A, determined by X-ray diffractometry, serve as an indicator of eolian transport of terrigenous material to the central southern Pacific. The quartz contents are very small and, within limits of analytical resolution, vary only slightly from the Cretaceous to the present. However, the accumulation rate of the eolian quartz does change significantly. The quartz accumulation reflects the changing position of the site with respect to the terrigenous source areas and the variations in wind systems through time.

Manganese concentrations and flux data of sediments from Broken and Ninetyeast Ridge

Decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone (OMZ) in the northwest Indian Ocean. This zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the Arabian coast. Based upon benthic foraminiferal assemblage changes, it has been suggested that this OMZ was even more extensive during the late Miocene-early Pliocene (6.5-3.0 Ma), with a maximum volume and/or intensity at approximately 5.0 Ma. While this inference may contribute to an understanding of the history of northwest Indian Ocean upwelling, corroborating geochemical evidence for this interpretation has heretofore been lacking. Ocean Drilling Program (ODP) sites 752, 754, and 757 on Broken and Ninetyeast ridges are located within central Indian Ocean intermediate water depths (1086-1650 m) but outside the present lateral dimensions of the Indian Ocean OMZ. High-resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of Mn and normalized Mn concentrations between 6.5 and 3.0 Ma that are most pronounced at approximately 5.0 Ma. Because late Miocene-Pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates (0.3-1.3 cm/ky) most likely precluded sedimentary metal oxide diagenesis, we suggest that the observed Mn depletions reflect diminished deposition of reducible Mn oxyhydroxide phases within O2 deficient intermediate waters and that this effect was most intense at approximately 5.0 Ma. This interpretation implies that waters with less than 2.0 mL/L O2 extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages. We further suggest this expanded Indian Ocean OMZ is related to regionally and/or globally increased biological productivity.

Organic matter preservation in sediments of the Peru and Oman continental margin

Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681, 688) and Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quantitative understanding of its accumulation and degradation. Microscopical as well as nanoscopical investigations reveal that the OM in sediments affected by upwelling mechanisms mainly (up to 98%) consists of unstructured (amorphous) organic aggregates without any apparent biological structures. In sediments which are not or to a lesser extent affected by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediments deposited below upwelling cells and 0.2% in sediments outside the upwelling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C/S-ratios of "normal" marine sediments, whereas at the other locations no correlation or even a negative correlation between sulphur and TOC concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulphur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non-metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90-99% of the OM produced in the photic zone was remineralized and 1-3% was consumed by microbial sulphate reduction. The aerobic and anaerobic processes have greatly affected degradation and conservation of OM.

Distribution of organic carbon in surface sediments covered during SONNE cruise SO184

Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (d13Corg) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (Corg) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum d15N values of 3.7 per mil were measured in the northern Mentawai Basin, whereas they varied around 5.4 per mil at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L**1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum Corg accumulation rates (CARs) were measured in the Lombok (10.4 g C m**-2 yr**-1) and northern Mentawai basins (5.2 g C m**-2 yr**-1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1-7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.

Stable-isotope records from Dronning Maud Land, Antarctica

The European Project for Ice Coring in Antarctica (EPICA) includes a comprehensive pre-site survey on the inland ice plateau of Dronning Maud Land. This paper focuses on the investigation of the 18O content of shallow firn and ice cores. These cores were dated by profiles derived from dielectric-profiling and continuous flow analysis measurements. The individual records were stacked in order to obtain composite chronologies of 18O contents and accumulation rates with enhanced signal-to-noise variance ratios.These chronologies document variations in the last 200 and 1000 years.The 18O contents and accumulation rates decreased in the 19th century and increased during the 20th century.Using the empirical relationships between stable isotopes, accumulation rates and the 10m firn temperature, the variation of both parameters can be explained by the same temperature history.But other causes for these variations, such as the build-up of the snow cover, cannot be excluded. A marked feature of the 1000 year chronology occurs during the period AD 1180-1530 when the 18O contents remains below the long-term mean. Cross-correlation analyses between five cores from the Weddell Sea region and Dronning Maud Land show that 18O records can in some periods be positively correlated and in others negatively correlated, indicating a complex climatic history in time and space.

Benthic foraminifera of cores from ODP Leg 117

The relative abundances of benthic foraminifers from the Oman margin have been analyzed from ODP Sites 725 and 726 near the upper boundary of the oxygen-minimum zone (OMZ) and 728 near the lower boundary. The relative abundance pattern of the benthic foraminiferal species in the two shallow sites show synchronous changes, which, together with variations in the faunal composition, may be attributed to changes in the location of the upper boundary of the OMZ during the last 7 million years. At the deeper site, the relative abundance pattern shows considerable variation in the faunal composition during the last 8 million years. The strong dominance of the shallow-water species Ammonia beccarii during the early Pliocene at Site 728 suggests a water depth less than 400 m during the early Pliocene and subsequent subsidence during the middle and late Pliocene to the present > 1400 m water depth.

Thorium and radiocarbon measurements on sediment cores from the Northseast Atlantic

Radiocarbon and 230Thexcess data from six NE Atlantic box cores are considered. The cores form a transect from the Porcupine Abyssal Plain over the East Thulean Rise to the southern end of Feni Drift. The chronology for the cores is established from bulk sediment carbonate radiocarbon data and reveals that sections exhibiting constant accumulation rates can be identified in all the cores, with rates of 3.0-3.5 cm/kyr on the plain through the Holocene and late Holocene rates of 4.3-6.6 cm/kyr elsewhere. Five out of the six cores show accumulations of more 230Thexcess than is produced in the overlying water column, with the greatest inventories (up to 225% of production) in the cores from the rise and drift. A size fraction comparison between two cores from the plain and rise reveals that the higher overall accumulation rates and 230Thexcess inventories in the off-plain cores are due to an increased fine (<5 µm) component fraction, whereas the flux of coarser material is similar to that received on the plain. This suggests that the higher fluxes of materials observed are physically (rather than biogeochemically) driven and also that drift formation has been continuously active in the late Holocene. Sections of all the cores where regular accumulation is defined by the radiocarbon data are modeled first by a linear radiocarbon age/depth model and second by a constant rain (230Thexcess)0 model prorated for the observed core inventories. These modeling approaches yield historical mass accumulation rate estimates which are generally in reasonable agreement (±30%), but the differences observed appear to be well organized in time rather than random.