Physical oceanography and hydrochemistry from the Laptev Sea, Arctic Ocean

Combined d18O/salinity data reveal a distinctive water mass generated during winter sea ice formation which is found predominantly in the coastal polynya region of the southern Laptev Sea. Export of the brine-enriched bottom water shows interannual variability in correlation with atmospheric conditions. Summer anticyclonic circulation is favoring an offshore transport of river water at the surface as well as a pronounced signal of brine-enriched waters at about 50 m water depth at the shelf break. Summer cyclonic atmospheric circulation favors onshore or an eastward, alongshore water transport, and at the shelf break the river water fraction is reduced and the pronounced brine signal is missing, while on the middle Laptev Sea shelf, brine-enriched waters are found in high proportions. Residence times of bottom and subsurface waters on the shelf may thereby vary considerably: an export of shelf waters to the Arctic Ocean halocline might be shut down or strongly reduced during "onshore" cyclonic atmospheric circulation, while with "offshore" anticyclonic atmospheric circulation, brine waters are exported and residence times may be as short as 1 year only.

Methane measurements in the Laptev Sea, Arctic Ocean

In der Wassersäule der Laptew See haben die Bildungs- und Verteilungsbedingungen vielfliltige Ursachen. Für die südliche Lapt ew See konnte eine Methananomalie innerhalb des Lenaausstrorns nachgewiesen werden, die im direkten Zusammenhang mit dem Flußwasser stehen muß. Mit den hohen Konzentrationen am Kontinentalhang ergab sich ein Hinweis auf eventuell vorhandene Gashydrate an der Sole eines zum Hang hin auskeilenden Permafrosts oder auf Gashydrate in den Sedimenten des Kontinentalhangs selbst. Ob es entlang der reliktisch vorhandenen, ehemaligen Flußläufe auf dem Schel f ebenfalls zu Entgasungen kommt, bleibt allerdings weiter unklar, da dieses Phänomen nicht beobachtet wurde oder die Anomalien nicht eindeutig diesem Prozeß zuzuordnen waren. Sicherlich ist die COz-Reduktion im Sediment in der Laptew See eine Hauptquelle für marines, bodennahes Methan. Die Ergebnisse. zeigen, daß dieser Bildungsprozeß vor allem für die küstennahen Bereiche wahrscheinlich ist. Dennoch gibt es auch Bereiche, wo die Zuordnung zu einer expliziten Methanquelle nicht eindeutig ist. Für eine genauere Bewertung der Herkunft der Gase sollten in künftigen Untersuchungen die Methankonzentrationen des Sediments einbezogen werden. Aber auch die Isotopensignaturen des Gases im Sediment können wertvolle Hinweise auf die Genese geben, vor allem wenn die Wasserstoffisotopie mituntersucht wird. Dies erscheint sinnvoll, da sich dur ch leichtes, bodennahes, Methan in der Wassersäule Hinweise auf biogene Bildungen ergaben, dieser Befund könnte durch weitere Untersuchungen präzisiert werden. Dies gilt aber auch für die CH4-Anomalien des OberfIächenwassers. Auch hier ergaben sich durch leicht KohIenstoffsignaturen Hinweise auf biogene in situ-Produktion. Mit detaillierteren Methankonzentrations- und d13C- CH4-Isotopenprofilen der Wassersäule könnte dieser Bildungspfad eindeutiger beschrieben werden. Es konnte ferner gezeigt werden, daß die Lapt ew See während der Sommermonate eine Quelle für atmosphärisches CI L darstellt. Das emittierte Gas geht neben vereinzelten Bodenquellen auch auf in situ-Produktion in der Wassersäule zurück. Abgesehen von der nördlichen Region geht das Methan bodennaher Anomalien innerhalb der Wassersäule sehr schnell zurück und nur ein kleiner Teil gelangt so schließlich in die Atmosphäre. Der während der ARK-XIV Expedition getestete Methansensor hat sich als ungeeignet für den Einsatz gemeinsam mit der CTD erwiesen. Es hat sich gezeigt, daß der Sensor unter diesen Bedingungen nicht genügend Zeit hat, um sein Meßsignal zu stabilisieren. Möglicherweise kann er aber in modifizierter For m und mit einer Kalibration für niedrigere Konzentrationsbereiche als stationäres Meßgerät eingesetzt werden. Für hohe CH4-Konzentrationen, wie man sie an Pockmarks antrifft, ist die Methansensormessung sicherlich auch jetzt schon eine geeignete Methode.

Sulfur compounds in bottom sediments and interstitial waters of the Pacific Ocean to the east of Japan

In near-shore Pacific bottom sediments to the east of Japan unusually high content of free H2S ocurs. H2S resulting from bacterial reduction of sulfates from interstitial waters has a number of derivatives; pyrite dominates among them. Contents of other derivatives of H2S: sulfide sulfur and organic sulfur do not exceed 0,01%, content of organic sulfur does not exceed 0.1%. Due to reduction content of sulfates can reduce to 0,03% S. Capacity of the process of sulfate reduction, estimated by sum of all reduced forms of S - derivatives of H2S, is a function of organic matter content in sediments. Ability of bottom sediments to accumulate free H2S depends on content of reactive forms of Fe. Spatial distribution of reduced forms of S in the studied sediments is considered.

Geochemistry of basement carbonates from ODP Site 801 in the western Pacific Ocean

Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) (Hofmann and White, 1982, doi:10.1016/0012-821X(82)90161-3; Weaver, 1991 doi:10.1016/0012-821X(91)90217-6; Lassiter, and Hauri, 1998, doi:10.1016/S0012-821X(98)00240-4). Such models, however, have remained largely untested against actual subduction zone inputs, due to the scarcity of comprehensive measurements of both radioactive parents (Th and U) and radiogenic daughter (Pb) in altered oceanic crust (AOC). Here, we discuss new, comprehensive measurements of U, Th, and Pb concentrations in the oldest AOC, ODP Site 801, and consider the effect of subducting this crust on the long-term Pb isotope evolution of the mantle. The upper 500 m of AOC at Site 801 shows >4-fold enrichment in U over pristine glass during seafloor alteration, but no net change to Pb or Th. Without subduction zone processing, ancient AOC would evolve to low 208Pb/206Pb compositions unobserved in the modern mantle (Hart and Staudigel, 1989 [Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds. S.R. Hart, L. Gqlen, NATO ASI Series. Series C: Mathematical and Physical Sciences 258, pp. 15-28, D. Reidel Publishing Company, Dordrecht-Boston, 1989]). Subduction, however, drives U-Th-Pb fractionation as AOC dehydrates in the earth's interior. Pacific arcs define mixing trends requiring 8-fold enrichment in Pb over U in AOC-derived fluid. A mass balance across the Mariana subduction zone shows that 44-75% of Pb but <10% of U is lost from AOC to the arc, and a further 10-23% of Pb and 19-40% of U is lost to the back-arc. Pb is lost shallow and U deep from subducted AOC, which may be a consequence of the stability of phases binding these elements during seafloor alteration: U in carbonate and Pb in sulfides. The upper end of these recycling estimates, which reflect maximum arc and back-arc growth rates, remove enough Pb and U from the slab to enable it to evolve rapidly (<<0.5 Ga) to sources suitable to explain the 208Pb/206Pb isotopic array of OIB, although these conditions fail to simultaneously satisfy the 207Pb/206Pb system. Lower growth rates would require additional U loss (29%) at depths beyond the zones of arc and back-arc magmagenesis, which would decrease upper mantle kappa (232Th/238U) over time, consistent with one solution to the "kappa conundrum" (Elliott et al., 1999, doi:10.1016/S0012-821X(99)00077-1). The net effects of alteration (doubling of l [238U/204Pb]) and subduction (doubling of omega [232Th/204Pb]) are sufficient to create the Pb isotopic signatures of oceanic basalts.

Abundance and composition of Fe-Mn nodules and host sediments in the Pacific Ocean

Data on internal structure, distribution, and chemical composition of iron-manganese nodules from the central part of the South Pacific are reported. Nodules with relatively high contents of Fe, Ti, Co, and Pb were found. Formation of these nodules in pelagic regions of the ocean with low sedimentation rates is tentatively ascribed by the authors to leaching of Fe, Mn, and some minor elements during submarine lava outflow and to geochemical mobility of these elements. The role of diagenetic re-distribution of ore elements during formation of nodules is also discussed.

Chemical composition of Fe-Mn nodules and crusts from the Pacific Ocean

The book is devoted to study of diagenetic changes of organic matter and mineral part of sediments and interstitial waters of the Pacific Ocean due to physical-chemical and microbiological processes. Microbiological studies deal with different groups of bacteria. Regularities of quantitative distribution and the role of microorganisms in geochemical processes are under consideration. Geochemical studies highlight redox processes of the early stages of sediment diagenesis, alterations of interstitial waters, regularities of variations in chemical composition of iron-manganese nodules.

Chemical composition of Fe-Mn nodules from the Pacific Ocean

Calcareous and siliceous biogenic components have been studied in deep-water iron-manganese nodules from the northern and southern Pacific Ocean. Calcareous material consists of foraminifera remains, calcareous algae, and coccolithophorids, whereas siliceous material consists of remains of radiolarians and diatoms, as well as sponge spicules. Structures similar in morphology to coccal and filiform bacteria have been found in both outer and inner sections of the nodules indicating that microorganisms may be directly or indirectly involved in their development.