K-Ar age and chemical composition of volcanites from the Sea of Okhotsk

Volcanogenic rocks from the Sea of Okhotsk are divided into seven age complexes: Late Jurassic, Early Cretaceous, Late Cretaceous, Eocene, Late Oligocene, Late Miocene, and Pliocene-Pleistocene. All these complexes are united into two groups - Late Mesozoic and Cenozoic. Each group reflects a certain stage of development of the Sea of Okhotsk region. Late Mesozoic volcanites build the geological basement of the Sea of Okhotsk, and their petrochemical features are similar to those of the volcanic rocks from the Okhotsk-Chukotka Volcanogen. Pliocene-Pleistocene volcanites reflect stages of tectono-magmatic activity; the latter destroyed the continental margin and produced riftogenic troughs. Geochemical features of volcanites from the Sea of Okhotsk indicate influence of the sialic crust on magma formation and testify formation of the Okhotsk Sea Basin on the destructive margin of the Asian continent.

(Table 1) Light minerals in sand at DSDP Leg 84 holes

The Middle America active continental margin is the best-sampled active plate margin to date, having been drilled during Legs 84, 67, and 66. With nine sites drilled on the continental slope of Guatemala and an additional site drilled on the Costa Rican slope, a summary of slope sediments and sedimentary processes can be made. Sediments are easily subdivided into a thick apron of Neogene and Quaternary volcanically derived hemipelagic and turbidite mud and mudstone and a thinner, more varied assemblage of mostly Paleogene mudstone, radiolarian mudstone, and limestone. This latter assemblage may contain hiatuses or be completely lacking between slope deposits and basement. Cores from the foot of the continental slope (Core 567A-19) consist of Campanian micrite. The pre-Neogene section is much thicker and of more terrigenous provenance beneath the forearc basin landward of the forearc structural high than on the continental slope. Sedimentary processes of the Neogene and Quaternary slope sediments include reworking of hemipelagic and turbidite deposits. Redeposition by slumping, plastic flow, and turbidity current-documentable through benthic foraminiferal analysis-occurs in intracanyon and canyon settings. Erosion by slumping and by turbidity current and deposition of mud or sand in canyons and in local depressions on the continental slope and different rates of sediment accumulation result in dramatic thickness variations of lithologic units over small distances in localized pockets of sand in small filled canyons on the slope or in sediment ponds, and in high-relief basement topography. The age of sediment overlying igneous basement ranges from Cretaceous to Quaternary. Gas hydrate was visible or inferred present at every site drilled during Leg 84. Nevertheless, except for a small amount in the last core, it was not recovered in sufficient quantities to be visible at Site 568, a site specifically chosen for the study of hydrate and located near Site 496, which was abandoned during Leg 67 because of the dangerous abundance of hydrates. The association of hydrate with porous, coarser sediment results in a distribution as localized and unpredictable as the slope sands off Guatemala, which do not occur in beds coherent enough to produce acoustic reflection. Although the normal lithologic section at Sites 567 and 496 limits the volume of sediment that could be part of an accretionary prism offshore Guatemala and the volume of sediment in the Trench axis is not sufficient to argue for significant accumulation of Cocos Plate sediments, the varied lithology and attenuated thickness of pre-Neogene sediment seaward of the forearc structural high do not exclude earlier accretion from the history of the Guatemalan continental margin.

Calcareous nannofossil stratigraphy of ODP Holes 173-1065A and 173-1069A

Upper Jurassic calcareous nannofossil assemblages have been studied from strata cored over basement blocks now buried under the Iberia Abyssal Plain (Ocean Drilling Program Leg 173 Sites 1065 and 1069). The youngest Jurassic assemblages at each site are Tithonian in age, the same as those at nearby Leg 149 Site 901, an age that predates the breakup of the Iberia continental margin. The paucity of the assemblages, the prevalence of coccospheres, and the relatively high organic contents of the fine clastic sediments in which they occur are characteristic of a restricted interior basin that had little communication with the open ocean. During the major rifting episode (a Berriasian event), the Jurassic sequences were dispersed along with their underlying blocks of presumed continental crust across the ocean-continent transition of the Iberia Abyssal Plain, probably as a result of detachment faulting.

Jelly biomass sinking speed reveals a fast carbon export mechanism

Particulate matter export fuels benthic ecosystems in continental margins and the deep sea, removing carbon from the upper ocean. Gelatinous zooplankton biomass provides a fast carbon vector that has been poorly studied. Observational data of a large-scale benthic trawling survey from 1994 to 2005 provided a unique opportunity to quantify jelly-carbon along an entire continental margin in the Mediterranean Sea and to assess potential links with biological and physical variables. Biomass depositions were sampled in shelves, slopes and canyons with peaks above 1000 carcasses per trawl, translating to standing stock values between 0.3 and 1.4 mg C m2 after trawling and integrating between 30,000 and 175,000 m2 of seabed. The benthopelagic jelly-carbon spatial distribution from the shelf to the canyons may be explained by atmospheric forcing related with NAO events and dense shelf water cascading, which are both known from the open Mediterranean. Over the decadal scale, we show that the jelly-carbon depositions temporal variability paralleled hydroclimate modifications, and that the enhanced jelly-carbon deposits are connected to a temperature-driven system where chlorophyll plays a minor role. Our results highlight the importance of gelatinous groups as indicators of large-scale ecosystem change, where jelly-carbon depositions play an important role in carbon and energy transport to benthic systems.

Chemistry of interstitial waters of ODP Leg 112 samples

Two distinct hydrogeochemical regimes currently dominate the Peruvian continental margin. One, in shallower water (150-450 m) shelf to upper-slope regions, is characterized by interstitial waters with strong positive chloride gradients with depth. The maximum measured value of 1043 mM chloride at Site 680 at ITS corresponds to a degree of seawater evaporation of ~2 times. Major ion chemistry and strontioum isotopic composition of the interstitial waters suggest that a subsurface brine that has a marine origin and is of pre-early Miocene "age," profoundly influences the chemistry and diagenesis of this shelf environment. Site 684 at ~9°S must be closest to the source of this brine, which becomes diluted with seawater and/or interstitial water as it flows southward toward Site 686 at ~13?S (and probably beyond) at a rate of approximately 3 to 4 cm/yr, since early Miocene time. The other regime, in deep water (3000-5000 m) middle to lower-slope regions, is characterized by interstitial waters with steep negative and nonsteady-state chloride gradients with depth. The minimum measured value of 454 mM chloride, at Site 683 at ITS, corresponds to ~20% dilution of seawater chloride The most probably sources of these low-chloride fluids are gas hydrate dissociation and mineral (particularly clay) dehydration reactions. Fluid advection is consistent with (1) the extent of dilution shown in the chloride profiles, (2) the striking nonsteady-state depth profiles of chlorides at Sites 683 and 688 and of 87Sr/86Sr ratios at Site 685, and (3) the temperatures resulting from an average geothermal gradient of 50°C/km and required for clay mineral dehydration reactions. Strontium isotope data reveal two separate fluid regimes in this slope region: a more northerly one at Sites 683 and 685 that is influenced by fluids with a radiogenic continental strontium signature, and a southerly one at Sites 682 and 688 that is influenced by fluids with a nonradiogenic oceanic signatures. Stratigraphically controlled fluid migration seems to prevail in this margin. Because of its special tectonic setting, Site 679 at ITS is geochemically distinct. The interstitial waters are characterized by seawater chloride concentrations to -200 mbsf and deeper by a significantly lower chloride concentration of about two-thirds of the value in seawater, suggesting mixing with a meteoric water source. Regardless of the hydrogeochemical regime, the chemistry and isotopic compositions of the interstitial waters at all sites are markedly modified by diagenesis, particularly by calcite and dolomite crystallization.