Vertical flux reconstruction for sediment cores off Chile

Biogenic opal and organic carbon vertical rain rates in sediment cores reveal a strong cyclicity in the productivity of the upwelling system off presently arid northern Chile during the last 100,000 years. Changes in productivity are found to be in phase with the precessional cycle (~20,000 years) and with inputs of iron from the continent. During austral summer insolation maxima, increased precipitation and river runoff in the region appear to have brought high inputs of iron, mainly from the Andes, to the coastal ocean enhancing primary productivity there. We interpret our results as providing evidence for iron control of past productivity in this upwelling system and for a tight link between productivity and orbital forcing at midlatitudes.

Radioisotope stratigraphy, sedimentology and geochemistry of Late Quaternary sediments from the Eastern Arctic Ocean

To reconstruct Recent and past sedimentary environments, marine sediments of Upper Pleistocene and Holocene ages from the eastern Arctic Ocean and especially from the Nansen-Gakkel Ridge (NGR) were investigated by means of radioisotopic, geochemical and sedimentological methods. In combination with mass physical property data and lithological analysis these investigations allow clearly to characterize the depositional environments. Age dating by using the radioisotope 230Th gives evidence that the investigated sediments from the NGR are younger than 250,000 years. Identical lithological sediment sequences within and between sediment cores from the NGR can be related to sedimentary processes which are clearly controlled by palaeoclimate. The sediments consist predominantly of siliciclastic, terrigenous ice-rafted detritus (IRD) deriving from assorted and redeposited sediments from the Siberian shelfs. By their geochemical composition the sediments are similar to mudstone, graywacke and arcose. Sea-ice as well as icebergs play a major roll in marine arctic sedimentation. In the NGR area rapid change in sedimentary conditions can be detected 128,000 years ago. This was due to drastic change in the kind of ice cover, resulting from rapid climatic change within only hundreds of years. So icebergs, deriving mostly from Siberian shelfs, vanished and sea-ice became dominant in the eastern Arctic Ocean. At least three short-period retreats of the shelf ice between 186,000 and 128,000 years are responsible for the change of coarse to fine-grained sediments in the NGR area. These warmer stages lasted between 1,000 and 3,000 years. By monitoring and comparing the distribution patterns of sedimentologic, mass physical and geochemical properties with 230Th ex activity distribution patterns in the sediment cores from the NGR, there is clear evidence that sediment dilution is responsible for high 230Th ex activity variations. Thus sedimentation rate is the controlling factor of 230Th ex activity variations. The 230Th flux density in sediments from the NGR seems to be highly dependent On topographic Position. The distribution patterns of chemical elements in sediment cores are in general governed by lithology. The derivation of a method for dry bulk density determination gave the opportunity to establish a high resolution stratigraphy on sediment cores from the eastern Arctic Ocean, based on 230Thex activity analyses. For the first time sedimentation and accumulation rates were determined for recent sediments in the eastern Arctic Ocean by 230Th ex analyses. Bulk accumulation rates are highly variable in space and time, ranging between 0.2 and 30 g/cm**2/ka. In the sediments from the NGR highly variable accumulation rates are related to the kind of ice cover. There is evidence for hydrothermal input into the sediments of the NGR. Hydrothermal activity probably also influences surficial sediments in the Sofia Basin. High contents of As are typical for surficial sediments from the NGR. In particular SL 370-20 from the bottom of the rift valley has As contents exceeding in parts 300 ppm. Hydrothermal activity can be traced back to at least 130,000 years. Recent to subrecent tectonic activity is documented by the rock debris in KAL 370 from the NGR. In four other sediment cores from the NGR rift valley area tectonically induced movements can be dated to about 130,000 years ago, related most probably to the rapid climate change. Processes of early diagenesis in sediments from the NGR caused the aobilization and redeposition of Fe, Mn and Mo. These diagenetic processes probably took place during the last 130,000 years. In sediment cores from the NGR high amounts of kaolinite are related to coarse grained siliciclastic material, probably indicating reworking and redeposition of siberian sandstones with kaolinitic binding material. In contrast to kaolinite, illite is correlated to total clay and 232Th contents. Aragonite, associated with serpentinites in the rift valley area of the NGR, was precipitated under cold bottom-water conditions. Preliminary data result in a time of formation about 60 - 80 ka ago. Manganese precipitates with high Ni contents, which can be related to the ultrabasic rocks, are of similar age.

(Table 2) Annual mass fluxes, 230Th composition and organic geochemistry of sediment trap samples in the South Altantic

The scavenging of 231Pa and 230Th was investigated in the Atlantic Sector of the Southern Ocean by combining results from sediment trap and in situ filtration studies. We present the first high-resolution profile of dissolved 230Th and 231Pa in surface waters across the ACC, showing a dramatic southward increase of both radionuclides around the southern ACC Front at 533S. High dissolved 231Pa/230Th ratios combined with low 230Th/231Pa fractionation factors (F) in these surface waters result in extremely high 231Pa94/230Th94 ratios of material collected in the shallow traps. Particulate 231Pa94/230Th94 ratios in a shallow trap near Bouvet Island increase continuously during the productive period in austral summer, and drop back in the low flux period. This behavior, following the Rayleigh fractionation principle, is interpreted to be due to an increase in the dissolved 231Pa/230Th ratio in the euphotic zone resulting from preferential scavenging of 230Th relative to 231Pa, even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are replenished by upwelling of Circumpolar Deep Water. The high particulate 231Pa94/230Th94 signal is weakened during downward transport of the bloom particles in the water column by incorporation of deep suspended particles, which have a lower 231Pa94/230Th94 ratio. It is shown that under the special hydrographic conditions in the Southern Ocean scavenging from the upper water column significantly influences the budgets of 230Th and 231Pa in the sediment. Nevertheless, the budgets are still made up primarily by scavenging from the large standing stock of deep suspended particles.

Geochemistry of continental slope and deep sea basin sediments off Uruguay

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement (<6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Lead 210 and silicate profiles from sediments of the equatorial Pacific and South Atlantic

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

Geochemical analysis of surface sediments along the Portuguese Margin

Twenty-nine surface samples from the Portuguese shelf, recovered offshore from the mouths of the Ave, Douro, Lis and Mira rivers, were analysed using ICP-OES for selected major and trace elements, after total dissolution. Organic carbon, carbonate content and grain size were also determined. Five evaluation tools have been applied in order to compare the three study areas and to evaluate sediment geochemistry and other sediment compositional variability in the acquired samples: (1) empirical methods based on comparison with standard reference criteria, e.g. the NOAA sediment quality guidelines, (2) normalisation ratios using a grain-size proxy element, (3) "Gradient Method", plotting contaminant vs. organic matter or Al, (4) definition of a regional geochemical baseline from a compiled database, and (5) enrichment factors. The evaluation of element and component associations indicates differences related both to the onshore drainage areas and to the environmental shelf setting. Despite the considerable variability in total metal contents indicated by our results, the sediment metal composition is largely of natural origin. Metal enrichments observed in the Mira area are associated with the drainage of mineralised areas rich in Cu, Pb, Zn, Fe and Mn. The near absence of human impact on shelf sediments, despite the vicinity to urban areas with high industrialisation levels, such as the Ave-Douro and Lis areas, is attributed to effective trapping in the estuaries and coastal zones, as well dilution with less contaminated sediments shelf sediments and removal with fine fractions due to grain-size sorting. The character of the contaminated sediments transported to these shelf areas is further influenced by grain-size sorting as well as by dilution with less contaminated marine sediments. The results obtained individually by the different methods complement each other and allow more specific interpretations.

Sedimentology and Th, Pa, U, and N isotopic composition of core MD84-527

High-resolution records of opal, carbonate, and terrigenous fluxes have been obtained from a high-sedimentation rate core (MD84-527: 43°50'S; 51°19'E; 3269 m) by normalization to 230Th. This method estimates paleofluxes to the seafloor on a point-by-point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents. We also measured sediment delta15N to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations. Our results show that opal accumulation rates on the seafloor during the Holocene and stage 3, based on 14C dating, were respectively tenfold and fivefold higher than the vertical rain rates, At this particular location, changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column. Correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to 230Th disclose important variations in opal rain rates. We found relatively high but variable opal paleoflux during stage 3, with two maxima centered at 36 and 30 kyr B.P., low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early Holocene, We interpret this record as reflecting variations in opal production rates associated with climate-induced latitudinal migration of the southern ocean frontal system. Sediments deposited during periods of high opal paleoflux also have high authigenic U concentrations, suggesting more reducing conditions in the sediment, and high Pa-231/Th-230 ratios, suggesting increased scavenging from the water column. Sediment delta15N is circa 1.5 per mil higher during isotopic stage 2 and deglaciation. The low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion. This suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column. Increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid, leading to lower nitrate supply rates to surface waters.

Inorganic geochemical record of ODP Holes 117-722B and 117-724C

High-resolution sampling from late Pleistocene (last 400 k.y.) sediments of Site 722 (upper 16 m) and Site 724 (upper 70 m), and subsequent inorganic geochemical analysis, has defined the history of productivity in the northwest Arabian Sea. Eolian dust input from the Arabian Peninsula and Somalia is characterized by the record of Ti/Al and Cr/Al. This dust record displays strong precessional periodicity (cycles at 25 k.y.) suggesting the Southwest Monsoon and associated winds play a key role in transporting terrigenous material from the land. High biological productivity results in the accumulation of biogenic CaCO3 and opal in the sediments, the latter having an unexpectedly minor contribution to the total mass flux. Due to dilution of the CaCO3 record by the terrigenous component, the record of biological productivity is best exemplified by Ba. Its record, together with that of other metals recording biological association and redox variability (Cu, Ni, Zn, V, U) clearly identifies the interglacial episodes as being more biologically productive. The striking agreement between Ba and the d18O record in planktonic foraminifers suggests that the supply of nutrients during these periods of high productivity is linked to ocean-wide changes in ocean fertility, and not just local upwelling conditions. High levels of phosphate accumulation in interglacial sediments is attributed to both diagenetic phosphorite formation and biogenic skeletal debris. This study provides a detailed record of productivity variation in the northwest Arabian Sea during the late Pleistocene.