Mineralogy, lithology and stable isotope composition of diagenetic siliceous rocks from ODP Leg 129 sites

Ocean Drilling Program Leg 129 recovered chert, porcellanite, and radiolarite from Middle Jurassic to lower Miocene strata from the western Pacific that formed by different processes and within distinct host rocks. These cherts and porcellanites formed by (1) replacement of chalk or limestone, (2) silicification and in-situ silica phase-transformation of bedded clay-bearing biosiliceous deposits, (3) high-temperature silicification adjacent to volcanic flows or sills, and (4) silica phase-transformation of mixed biosiliceous-volcaniclastic sediments. Petrologic and O-isotopic studies highlight the key importance of permeability and time in controlling the formation of dense cherts and porcellanites. The formation of dense, vitreous cherts apparently requires the local addition and concentration of silica. The influence of permeability is shown by two examples, in which: (1) fragments of originally identical radiolarite that were differentially isolated from pore-water circulation by cement-filled fractures were silicified to different degrees, and (2) by the development of secondary porosity during the opal-CT to quartz inversion within conditions of negligible permeability. The importance of time is shown by the presence of quartz chert below, but not above, a Paleogene hiatus at Site 802, indicating that between 30 and 52 m.y. was required for the formation of quartz chert within calcareous-siliceous sediments. The oxygen-isotopic composition for all Leg 129 carbonate- and Fe/Mn-oxide-free whole-rock samples of chert and porcellanite range widely from d18O = 27.8 per mil to 39.8 per mil vs. V-SMOW. Opal-CT samples are consistently richer in 18O (34.1 per mil to 39.3 per mil) than quartz subsamples (27.8 per mil to 35.7 per mil). Using the O-isotopic fractionation expression for quartz-water of Knauth and Epstein (1976) and assuming d18Opore water = -1.0 per mil, model temperatures of formation are 7°-26°C for carbonate-replacement quartz cherts, 22°-25°C for bedded quartz cherts, and 32°-34°C for thermal quartz cherts. Large variations in O-isotopic composition exist at the same burial depth between co-existing silica phases in the same sample and within the same phase in adjacent lithologies. For example, quartz has a wide range of isotopic compositions within a single breccia sample; d18O = 33.4 per mil and 28.0 per mil for early and late stages of fracture-filling cementation, and 31.6 per mil and 30.2 per mil for microcrystalline quartz precipitation within enclosed chert and radiolarite fragments. Similarly, opal-CT d101 spacing varies across lithologic or diagenetic boundaries within single samples. Co-occurring opal-CT and chalcedonic quartz in shallowly buried chert and porcellanite from Sites 800 and 801 have an 8.7 per mil difference in d18O, suggesting that pore waters in the Pigafetta Basin underwent a Tertiary shift to strongly 18O-depleted values due to alteration of underlying Aptian to Albian-Cenomanian volcaniclastic deposits after opal-CT precipitation, but prior to precipitation of microfossil-filling chalcedony.

Geochemistry on sediment core Co1014 from Rauer Group, Antarctica

The history of glacial advances and retreats of the East Antarctic ice sheet during the Holocene is not well-known, due to limited field evidence in both the marine and terrestrial realm. A 257-cm-long sediment core was recovered from a marine inlet in the Rauer Group, East Antarctica, 1.8 km in front of the present ice-sheet margin. Radiocarbon dating and lithological characteristics reveal that the core comprises a complete marine record since 4500 yr. A significant ice-sheet expansion beyond present ice margins therefore did not occur during this period.

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.

Composition of sedimemts from the Transpacific profile

The book is devoted to comprehensive study of composition of sediments from the North Pacific Ocean. The sediments have been divided characterized by their lithologic and facial types, grain size composition and mineralogy. Influence of volcanism on formation of mineral and chemical composition of these sediments has been shown. Regularities of distribution of sediment accumulation rates and of a number of chemical elements on the Transpacific profile have been found. Determining role of mechanical fractionation in their localization has been shown.

Uranium and radioactive isotopes in bottom sediments and Fe-Mn nodules and crusts of seas and oceans

The main stages of the sedimentary cycle of uranium in modern marine basins are under consideration in the book. Annually about 18 thousand tons of dissolved and suspended uranium enters the ocean with river runoff. Depending on a type of a marine basin uranium accumulated either in sediments of deep-sea basins, or in sediments of continental shelves and slopes. In the surface layer of marine sediments hydrogenic uranium is predominantly bound with organic matter, and in ocean sediments also with iron, manganese and phosphorus. In diagenetic processes there occurs partial redistribution of uranium in sediments, as well as its concentration in iron-manganese, phosphate and carbonate nodules and biogenic phosphate detritus. Concentration of uranium in marine sediments of various types depending on their composition, as well as on forms of its entering, degree of differentiation and of sedimentation rates, on hydrochemical regime and water circulation, and on intensity of diagenetic processes.

Composition of hydrothermal manifestations in the World Ocean

The monograph summarizes results of studies of hydrothermal fields on the ocean floor, hydrothermal plumes and metalliferous sediments. Hydrothermal ore manifestations formed in different geodynamic settings, with different character of volcanism in different facial conditions of deposition are described. Causes of non-uniformity of hydrothermal system functioning in different parts of the ocean and therefore variability of hydrothermal deposits are under consideration. On the base of found relationships of these irregularities with geodynamics, volcanism and sedimentation a new classification of hydrothermal processes and genetic models of hydrothermal ore formation in the ocean have been created. Regularities of hydrothermal sedimentary material dispersion in bottom waters are discussed.

Late Pleistocene oxygen isotope record of biognic silica

We determined the isotopic composition of oxygen in marine diatoms in eight deep-sea cores recovered from the Atlantic sector of the Southern Ocean. The analytical reproducibility and core-to-core consistency of the isotopic signal suggests that diatom delta18O can be used as a new paleocenographic tool to reconstruct past variations in surface water characteristics and to generate 18O -isotope-based stratigraphy for the Southern Ocean. The data indicate that diatom delta18O reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430 ka. The delta18O analyses of different diatom assemblages reveal that the isotopic signal is free of species effects and that the common Antarctic species have the same water-opal fractionation. The transition from the last glacial maximum (LGM) to the Holocene is fully recorded in high sedimentation rate cores. An 18O enrichment during the LGM, a post-LGM meltwater spike and an input of meltwater during the late Holocene are the main isotopic features observed in down core records. The origin of this meltwater was very likely melting icebergs and/or continental ice or by melting sea ice that had accumulated snow. The most pronounced meltwater effects are recorded in cores that are associated with the Weddel gyre. Our results provide the basis for extending isotope studies to oceanic regions devoid of carbonate; further, isotopic stratigraphies may be constructed for records and regions where they were previously not possible.

On the effects of circulation, sediment resuspension and biological incorporation by diatoms in an ocean model of aluminium, link to model data

The distribution of dissolved aluminium in the West Atlantic Ocean shows a mirror image with that of dissolved silicic acid, hinting at intricate interactions between the ocean cycling of Al and Si. The marine biogeochemistry of Al is of interest because of its potential impact on diatom opal remineralisation, hence Si availability. Furthermore, the dissolved Al concentration at the surface ocean has been used as a tracer for dust input, dust being the most important source of the bio-essential trace element iron to the ocean. Previously, the dissolved concentration of Al was simulated reasonably well with only a dust source, and scavenging by adsorption on settling biogenic debris as the only removal process. Here we explore the impacts of (i) a sediment source of Al in the Northern Hemisphere (especially north of ~ 40° N), (ii) the imposed velocity field, and (iii) biological incorporation of Al on the modelled Al distribution in the ocean. The sediment source clearly improves the model results, and using a different velocity field shows the importance of advection on the simulated Al distribution. Biological incorporation appears to be a potentially important removal process. However, conclusive independent data to constrain the Al / Si incorporation ratio by growing diatoms are missing. Therefore, this study does not provide a definitive answer to the question of the relative importance of Al removal by incorporation compared to removal by adsorptive scavenging.

Dissolved aluminium measured on water bottle samples during POLARSTERN cruise ARK-XXII/2 (SPACE)

Concentrations of dissolved (0.2 µm filtered) aluminium (Al) have been determined for the first time in the Eurasian part of the Arctic Ocean over the entire water column during expedition ARK XXII/2 aboard R.V. Polarstern (2007). An unprecedented number of 666 samples was analysed for 44 stations along 5 ocean transects. Dissolved Al in surface layer water (SLW) was very low, close to 1 nM, with lowest SLW concentrations towards the Canadian part of the Arctic Ocean and higher values adjacent to and in the shelf seas. The low SLW concentrations indicate no or little influence from aeolian dust input. Dissolved Al showed a nutrient-type increase with depth up to 28 nM, but large differences existed between the different deep Arctic basins. The differences in concentrations of Al between water masses and basins could largely be related to the different origins of the water masses. In the SLW and intermediate water layers, Atlantic and Pacific inflows were of importance. Deep shelf convection appeared to influence the Al distribution in the deep Eurasian Basin. The Al distribution of the deep Makarov Basin provides evidence for Eurasian Basin water inflow into the deep Makarov Basin. A strong correlation between Al and Silicon (Si) was observed in all basins. This correlation and the nutrient-like profile indicate a strong biological influence on the cycling and distribution of Al. The biological influence can be direct by the incorporation of Al in biogenic silica, indirect by preferential scavenging of Al onto biogenic siliceous particles, or by a combination of both processes. From the slope of the overall Al-Si relationship in the intermediate water layer (AIDW; ~ 200-2000 m depth), an Al/Si ratio of 2.2 atoms Al per 1000 atoms Si was derived. This ratio is consistent with the range of previously reported Al/Si uptake ratio in biogenic opal frustules of diatoms. In the deepest waters (>2000 m depth) a steeper slope of the Al-Si relationship of 7.4 to 13 atoms Al per 1000 atoms Si likely results from entrainment of cold shelf water into the deep basins, carrying the signal of dissolution of terrigenous particles with a much higher Al:Si ratio of crustal abundance. Only a small enrichment with such crustal Al and Si component may readily account for the higher Al:Si slope in the deepest waters.

Particle fluxes and in-situ particle properties measured with sediment traps and a moored camera system from 2008 - 2010 in region off Cape Blanc / NW-Africa

We compared particle data from a moored video camera system with sediment trap derived fluxes at ~1100 m depth in the highly dynamic coastal upwelling system off Cape Blanc, Mauritania. Between spring 2008 and winter 2010 the trap collected settling particles in 9-day intervals, while the camera recorded in-situ particle abundance and size-distribution every third day. Particle fluxes were highly variable (40-1200 mg m**-2 d**-1) and followed distinct seasonal patterns with peaks during spring, summer and fall. The particle flux patterns from the sediment traps correlated to the total particle volume captured by the video camera, which ranged from1 to 22 mm**3 l**-1. The measured increase in total particle volume during periods of high mass flux appeared to be better related to increases in the particle concentrations, rather than to increased average particle size. We observed events that had similar particle fluxes, but showed clear differences in particle abundance and size-distribution, and vice versa. Such observations can only be explained by shifts in the composition of the settling material, with changes both in particle density and chemical composition. For example, the input of wind-blown dust from the Sahara during September 2009 led to the formation of high numbers of comparably small particles in the water column. This suggests that, besides seasonal changes, the composition of marine particles in one region underlies episodical changes. The time between the appearance of high dust concentrations in the atmosphere and the increase lithogenic flux in the 1100 m deep trap suggested an average settling rate of 200 m d**-1, indicating a close and fast coupling between dust input and sedimentation of the material.

Micropaleontological and geochemical proxy studies of sediment core GeoB7926-2

[1] The low-latitude upwelling regime off the Mauritanian coast in the subtropical NE Atlantic accounts for a significant part of global export production. Although productivity variations in coastal upwelling areas are usually attributed to changes in wind stress and upwelling intensity, productivity dynamics off Mauritania are less straightforward because of the complex atmospheric and hydrographic setting. Here we integrate micropaleontological (diatoms) and geochemical (bulk biogenic sediment components, X-ray fluorescence, and alkenones) proxies to examine on submillennial-to-millennial changes in diatom production that occurred off Mauritania, NW Africa, for the last 25 ka. During the Last Glacial Maximum (LGM, 19.0-23.0 ka B.P.), moderate silicate content of upwelled waters coupled with weakened NE trade winds determined moderate diatom productivity. No significant cooling is observed during the LGM, suggesting that our alkenone-based SST reconstruction represents a local, upwelling-related signal rather than a global insolation related one. Extraordinary increases in diatom and opal concentrations during Heinrich event 1 (H1, 15.5-18.0 ka B.P.) and the Younger Dryas (YD, 13.5-11.5 ka B.P.) are attributed to enhanced upwelling of silica-rich waters and an enlarged upwelling filament, due to more intense NE trade winds. The synchronous increase of CaCO3 and K intensity and the decreased opal and diatoms values mark the occurrence of the Bølling/Allerød (BA, 13.5-15.5 ka B.P.) due to weakened eolian input and more humid conditions on land. Although the high export of diatoms is inextricably linked to upwelling intensity off Mauritania, variability in the nutrient content of the thermocline also plays a decisive role.

Geological, geochemical and biogeochemical data on boundary zones of the Atlantic Ocean

Results of studies in two biogeochemically active zones of the Atlantic Ocean (the Benguela upwelling waters and the region influenced by the Congo River run-off) are reported in the book. A multidisciplinary approach included studies of the major elements of the ocean ecosystem: sea water, plankton, suspended matter, bottom sediments, interstitial waters, aerosols, as well as a wide complex of oceanographic studies carried out under a common program. Such an approach, as well as a use of new methodical solutions led to obtaining principally new information on different aspects of oceanology.