Bubble sizes in waters of the Atlantic Ocean

Typical size of bubbles obtained from cavitation inception pressure measured in the surface layer of the Atlantic Ocean in situ aboard R/V Professor Vize in 1971 and Nerey in 1973 are reported. These results do not contradict ones of bubble size measurements using optical or acoustical techniques. Variability of bubble size is discovered and described. This variability is related to passing from one geographical region to another (from 68°55'S to 61°52'N), to changes in depth (from 5 to 100 m) and in day time, as well as to spatial fluctuations within an aquatic area. It is suggested that, in addition to wave breaking, there is another source of bubbles at depth 10-20 m that associates with hydrobiological processes.

Pore-water concentration of chloride in sediment cores and equilibrium temperatures and gas bubble analysis at ROV stations from the Vodyanitskii mud volcano

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to 650 mmol L**-1 at 150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at 60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (dD-CH4 = -170.8 per mil (SMOW), d13C-CH4 = -61.0 per mil (V-PDB), d13C-C2H6 = -44.0 per mil (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 x 10**6 mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.

Chemistry of minerals from amphibole-chlorite veins of ODP Site 209-1270

We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low d18O of altered plagiogranite veins (+3.0-4.2 per mil) and adjacent serpentinites (+ 2.6-3.7 per mil). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.

Oxygen and carbon isotope ratios of silicates and carbonates from exhumed peridotites in the Iberian Abyssal Plain

Legs 173 and 149 of the Ocean Drilling Program profiled a zone of exhumed mantle peridotite at the ocean-continent transition (OCT) beneath the Iberia Abyssal Plain. The zone of exhumed peridotite appears to be tens of kilometers wide and is situated between blocks of continental crust and the first products of ocean accretion. Exhumed peridotite is 95-100% serpentinised to probable depths of 2-3 km. Down core oxygen isotope profiles of serpentinised peridotite at Sites 1068 and 1070 (Leg 173) show evidence for two fluid infiltration events. The earlier event involved pervasive infiltration of comparatively warm (>175°C) sea water and accompanied serpentinisation. The later event involved structurally focused infiltration of comparatively cool (650-150°C) sea water and accompanied active mantle exhumation. We therefore conclude that the uppermost mantle was serpentinised before it was exhumed at the Iberian OCT. Implicit to this conclusion is that a sizeable region of serpentinised mantle existed directly beneath thinned but intact continental crust. Serpentinite has comparatively low density, low frictional strength and low permeability. The presence of such a "soft" layer may have localised deformation and consequently promoted detachment-style exhumation of the uppermost mantle. The low permeability of a serpentinite 'cap' layer might help to explain the lack of observed melt at the Iberian OCT.

Calcium concentrations and isotopic composition in carbonates and porefluids from ODP Hole 130-807A

The calcium isotopic compositions (d44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (a_s-f) between calcite and dissolved Ca2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of a_s-f at equilibrium in the marine sedimentary section is 1.0000+/-0.0001, which is significantly different from the value (0.9987+/-0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ~14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in d44Ca of +0.15? for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.