Biogeochemical parameters of sea water and bottom sediments near the Curieuse Island (Seychelles Islands)

Biogeochemical reef studies carried out in 1981 and 1984 found low concentration of total natural and anthropogenic hydrocarbons in inshore waters. Detection of lignin in marine and bottom sediments indicates that the land has major effect on makeup of organic matter there. Comparison of compositions of organic matter in sea water, suspended matter and bottom sediments indicated that it was altered rapidly by the reef community. Thus, in the inshore zone of the island, runoff from the land is important in supplying nutrients to the reef ecosystem alongside with transport of nutrients by deep waters. Concentrations of nutri¬ents (N, P) in the inshore zone are higher than in waters of the tropical part of the ocean. Nitrogen is the limiting element in development of phytoplankton in the inshore zone.

(Table 2) pH and concentrations of dissolved and particulate chemical elements in the mixing zone of the Yur'eva River water and the Okhotsk Sea water

Data on behavior of iron, manganese, nickel, copper, and zinc in the zone where acidic volcanic waters of the Yur'eva River (Paramushir Island, Kuril Islands) mix with sea water are presented. Distributions of dissolved and particulate forms of these elements indicate that the mixing zone acts as a pH-based geochemical barrier, at which almost all dissolved iron and smaller amounts of other metals are precipitated. When chemogenic particulate matter formed in the mixing zone enters the open ocean, it can sorb trace elements from sea water.

Stable isotopic composition of bottom water and benthic foraminifera from Hakon Mosby Mud Vulcano

To investigate the use of benthic foraminifera as a means to document ancient methane release, we determined the stable isotopic composition of tests of live (Rose Bengal stained) and dead specimens of epibenthic Fontbotia wuellerstorfi, preferentially used in paleoceanographic reconstructions, and of endobenthic high-latitude Cassidulina neoteretis and Cassidulina reniforme from a cold methane-venting seep off northern Norway. We collected foraminiferal tests from three push cores and nine multiple cores obtained with a remotely operated vehicle and a video-guided multiple corer, respectively. All sampled sites except one control site are situated at the Håkon Mosby mud volcano (HMMV) on the Barents Sea continental slope in 1250 m water depth. At the HMMV in areas densely populated by pogonophoran tube worms, d13C values of cytoplasm-containing epibenthic F. wuellerstorfi are by up to 4.4 per mil lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach d13C values of -7.5 and -5.5 per mil Vienna Pee Dee Belemnite (VPDB), respectively, whereas d13C values of their empty tests are higher by 4 per mil and 3 per mil. However, d13C values of empty tests are never lower than those of stained specimens, although they are still lower than empty tests from the control site. This indicates that authigenic calcite precipitates at or below the sediment surface are not significantly influencing the stable isotopic composition of foraminiferal shells. The comparatively high d13C results rather from upward convection of pore water and fluid mud during active methane venting phases at these sites. These processes mingle tests just recently calcified with older ones secreted at intermittent times of less or no methane discharge. Since cytoplasm-containing specimens of suspension feeder F. wuellerstorfi are almost exclusively found attached to pogonophores, which protrude up to 3 cm above the sediment, and d13C values of bottom-water-dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test d13C values of F. wuellerstorfi are the result of incorporation of heavily 13C-depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water d13CDIC. Alternatively, the pogonophores, which are rooted at depth in the upper sediment column, may serve as a conduit for depleted d13CDIC that ultimately influences the calcification process of F. wuellerstorfi attached to the pogonophoran tube well above the sediment/water interface. The lowest d13C of live specimens of the endobenthic C. neoteretis and C. reniforme are within the range of pore water d13CDIC values, which exceed those that could be due to organic matter decomposition, and thus, in fact, document active methane release in the sediment.

Geochemistry of recent marine sediments, interstitial water, and sea water from the West African continental margin

Carbon dioxide, ammonia, and reactive phosphate in the interstitial water of three sediment cores of the West African continental margin result from oxidation of sedimentary organic matter by bacterial sulfate reduction. The proposed model is a modification of one initially suggested by Richards (1965) for processes in anoxic waters: (CH2O)106 (NH3)8 (H3PO4) (0.7-0.2) + 53 SO4**2- =106 CO2 + 106 H20 + 8 NH3 + (0.7 - 0.2) H3PO4 + 53 S**2- The amount of reduced interstitial sulfate, the carbon-to-nitrogen-to-phosphorus atomic ratio of the sedimentary organic matter, as well as small amounts of carbon dioxide, which precipitated as interstitial calcium carbonate, are included in the general oxidation-reduction reaction. Preferential loss of nitrogen and phosphorus from organic matter close to the surface was recorded in both the interstitial water and sediment composition. It appeared that in deeper sections of the core organic carbon compounds were oxidized which were probably in an even lower oxidation state than that indicated by the proposed model. An estimated 2 % of the amount of organic matter still present was oxidized after it became incorporated into the sediment; whereas sulfide sulfur contents indicate that a much larger percentage (15-20%) seemed to have been subject to bacterial oxidation during the Pleistocene period, when a very thin oxidizing layer on the sediment allowed the above decomposition process to start relatively early favoured by almost fresh organic matter, and by almost unrestricted exchange of sulfate with the overlying water.

3He/4He ratios of pore gases in pore water and chemical composition of sediments at DSDP Sites 87-583 and 87-584

We have measured the 3He/4He and 20Ne/4He ratios of gases dissolved in the pore water in sediments at two sites in the Nankai Trough (Site 583) and the Japan Trench (Site 584). The 3He/4He and 20Ne/4He ratios vary from 0.215 * 10**-6 to 1.23 * 10**-6 and from 50 * 10**-3 to 2700 * 10**-3, respectively. These values can be explained by mixing two components, one from the atmosphere and one with a 3He/4He ratio of (0.2 to 0.3) * 10**-6. The latter component may be derived from the ocean crust near the subduction zone.

(Table 1) Deuterium ratios, chloride content and hydrate volume of pore water and hydrate meltwater from ODP Site 164-996

Site 996 is located above the Blake Diapir where numerous indications of vertical fluid migration and the presence of hydrate existed prior to Ocean Drilling Program (ODP) Leg 164. Direct sampling of hydrates and visual observations of hydrate-filled veins that could be traced 30-40 cm along cores suggest a connection between fluid migration and hydrate formation. The composition of pore water squeezed from sediment cores showed large variations due to melting of hydrate during core recovery and influence of saline water from the evaporitic diapir below. Analysis of water released during hydrate decomposition experiments showed that the recovered hydrates contained significant amounts of pore water. Solutions of the transport equations for deuterium (d2H) and chloride (Cl-) were used to determine maximum (d2H) and minimum (Cl-) in situ concentrations of these species. Minimum in situ concentrations of hydrate were estimated by combining these results with Cl- and d2H values measured on hydrate meltwaters and pore waters obtained by squeezing of sediments, by the means of a method based on analysis of distances in the two-dimensional Cl- d2H space. The computed Cl- and d2H distribution indicates that the minimum hydrate amount solutions are representative of the actual hydrate amount. The highest and mean hydrate concentrations estimates from our model are 31% and 10% of the pore space, respectively. These concentrations agree well with visual core observations, supporting the validity of the model assumptions. The minimum in situ Cl- concentrations were used to constrain the rates of upward fluid migration. Simulation of all available data gave a mean flow rate of 0.35 m/k.y. (range: 0.125-0.5 m/k.y.).