Lithium and strontium isotopic compositions of sediments and pore waters at ODP Sites 152-918 and 152-919

The distribution of Li isotopes in pore waters to a depth of 1157 m below seafloor is presented for ODP Sites 918 and 919 in the Irminger Basin, offshore Greenland. Lithium isotope data are accompanied by strontium isotope ratios to decipher diagenetic reactions in the sediments which are characterized by the pervasive presence of volcanic material, as well as by very high accumulation rates in the upper section. The lowering of the 87Sr/86Sr ratio below contemporaneous seawater values indicates several zones of volcanic material alteration. The Li isotope profiles are complex suggesting a variety of exchange reactions with the solid phases. These include cation exchange with NH4+ and mobilization from sediments at depth, in addition to the alteration of volcanic matter. Lithium isotopes are, therefore, a sensitive indicator of sediment-water interaction. d6Li values of pore waters at these two sites vary between -42 and -25?. At shallow depths (<100 mbsf), rapid decreases in the Li concentration, accompanied by a shift to heavier isotopic compositions, indicate uptake of Li into alteration products. A positive anomaly of d6Li observed at both sites is coincident with the NH4+ maximum produced by organic matter decomposition and may be related to ion exchange of Li from the sediments by NH4+. In the lower sediment column at Site 918, dissolved Li increases with depth and is characterized by enrichment of 6Li. The Li isotopic compositions of both the waters and the solid phase suggest that the enrichment of Li in deep interstitial waters is a result of release from pelagic sediments. The significance of sediment diagenesis and adsorption as sinks of oceanic Li is evaluated. The maximum diffusive flux into the sediment due to volcanic matter alteration can be no more than 5% of the combined inputs from rivers and submarine hydrothermal solutions. Adsorption on to sediments can only account for 5-10% of the total inputs from rivers and submarine hot springs.

Holocene sedimentation in the Strait of Otranto, Mediterranean Sea

An extensive radiograph study of 24 undisturbed, up to 206-cm long box and gravity cores from the western part of the Strait of Otranto revealed a great variety of primary bedding structures and secondary burrowing features. The regional distribution of the sediments according to their structural, textural, and compositional properties reflects the major morphologic subdivisions of the strait into shelf, slope, and trough bottom (e.g., the bottom of the northern end of the Corfu-Kephallinia Trough, which extends from the northeastern Ionian Sea into the Strait of Otranto): (1) The Apulian shelf (0 to -170m) is only partly covered by very poorly sorted, muddy sands without layering. These relict(?) sands are rich in organic carbonate debris and contain glauconite and reworked (?Pleistocene) ooids. (2) The slope sediments (-170 to -1,000 m) are poorly sorted, sandy muds with a high degree of burrowing. One core (OT 5) is laminated and shows slump structures. An origin of these slumped sediment masses from older deposits higher on the slope was inferred from their abnormal compaction, color, texture, organic content, and mineral composition. (3) Cores from the northern end of the Corfu-Kephallinia Trough (-980 to -1,060 m) display a few graded sand layers, 2-5 cm (maximum 30 cm) thick with parallel and ripple-cross-laminations, deposited by oceanic bottom or small-scale turbidity currents. They are intercalated with homogeneous lutite. (4) Hemipelagic sediments prevail in the more southerly part of the Corfu-Kephallinia Trough and on the "Apulian-Ionian Ridge", the southern submarine extension of the Apulian Peninsula. Below a core depth of 160 cm, these cores have a laminated ("varved") zone, representing an Early Holocene (Boreal-Atlanticum) "stagnation layer" (14C age approximately 9,000 years). The terrigenous components of the surface sediments as well as those of the deeper sand layers can be derived from the Apulian shelf and the Italian mainland (Cretaceous Apulian Plateau and Gargano Mountains, southern Apennines, volcanic province of the Monte Vulture). Indicated by the heavy mineral glaucophane, a minor proportion of the sedimentary material is probably of Alpine origin. If this portion is considered to be first-cycle clastic material it reaches the Strait of Otranto after a longitudinal transport of 700 km via the Adriatic Sea. The lack of phyllosilicates in the coarse- to medium-grained shelf samples might be explained by the activity of the "Apulian Current" (surface velocities up to 4 knots) which in the past possibly has affected the bottom almost down to depths of the shelf edge. The percentage of planktonic organisms, and also the plankton: benthos ratio in the sediments is a useful indicator for bathymetry (depth zonation).

Petrology and geochemistry of the Izu-Bonin Forearc tephras

Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer additional information about the timing and spatial characteristics of arc volcanism and the evolution of island arcs. Explosive volcanism along the Izu-Bonin Arc, with maxima just before rifting of the arc at ~40 and 5-0 Ma, produced black and white tephras of variable grain sizes and chemical compositions. Most of the tephras belong chemically to low-K and low-alkali tholeiitic rock series with a few tephra of the high-K and alkalic rock series. Most of the tephras (low-K series) were derived from the Izu-Bonin Arc, although a few were produced far to the west of the Izu-Bonin Arc (e.g., from the Ryukyu Arc). Black tephras may have come from nearby sources, such as Aogashima, Sumisu, and Torishima islands. The high-K series of tephras, within the sediments younger than 3 Ma, may reflect thickening of the island-arc crust.

Chemical composition of iron-manganese and phosphate rocks from aseismic ridges of the Indian Ocean and from the continental slope of the Southeast Africa

Study of chemical composition of 26 samples collected at depths from 400 to 1400 m on vertex surfaces of the Southeast Indian Ridge, Mascarene Ridge, Madagascar Ridge, and Mozambique Ridge, as well as on the upper part of the Southeast Africa continental slope showed that the samples represent three groups of rocks: 1) low phosphate or phosphate-free ferromanganese rocks, 2) phosphate ferromanganese rocks 3) phosphorites and phosphatized limestones.

Chert intervals in DSDP and ODP sites (Table 1)

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.

Geochemistry of the volcanic basement of ODP Hole 134-831B

The basement of Bougainville Guyot drilled at Site 831 consists of andesitic hyalobreccias derived from a submarine arc volcano. The volcanic sequence has been dated by K/Ar at approximately 37 Ma. The 121 m of andesitic hyalobreccias drilled in Hole 831B have been divided into five subunits of two types: one appears to be primary, and the other contains evidence of reworking and a subaerial clastic input. Variations are attributed to fluctuations in water depth. The distinctive hyalobreccias consist of andesitic blebs with chilled margins and peripheral fractures set in a chaotic greenish matrix that is mainly altered glass, with crystals similar to those in the blebs or clasts. Their formation is attributed to violent reaction of andesitic magma discharged into seawater, in perhaps the submarine equivalent of fire-fountaining. There was limited reworking by currents and debris flows on the flanks of the submarine volcano. The andesite shows no significant compositional variation in phenocryst phases throughout the drilled sequence and contains phenocrysts of plagioclase (An88-43), clinopyroxene (Ca44Mg46Fe10-Ca41Mg40Fe19), orthopyroxene (Ca4Mg79Fe17-Ca3Mg58Fe39), and titanomagnetite. There is a systematic change in volcanic composition with height in the section, from more mafic andesites at the base, to overlying more acid andesites, and strong evidence exists that magma mixing may have played a significant role in the genesis of these lavas. The andesites have affinities with the low-K arc tholeiite series. Trace element and isotopic systematics for these rocks indicate very minor involvement of a LILE- and 87Sr-enriched slab-derived fluid in their petrogenesis. This accords with the previous suggestion that Bougainville Guyot forms part of an Eocene proto-island arc developed along the southern side of the d'Entrecasteaux Zone, above a southward-dipping subduction zone.

Composition and age of rocks from the King's Trough and Palmer Ridge

This paper presents results of studies of rocks sampled during Cruise 19 of R/V Akademik Mstislav Keldysh with the Mir submersibles in the Atlantic Ocean (slopes of the King's Trough and Palmer Ridge). Based on these materials and published data two stages of magmatism and evolution in the region are distinguished: 1) formation of a mid-ocean ridge in the rift zone (68-32 Ma); 2) development of intraplate volcanism during movement of the plate over a "hot spot" (32-0 Ma).