98/03335 Anthropogenic changes of the isotopic composition of carbon in the atmosphere in the area of Upper Silesia: Rakowski, A. Z. et al. Zesz. Nauk. Politech. Slask., Mat.-Fiz., 1996, 79, 143–155. (In Polish)

Abstract

The Mariana forearc is a non-accretionary forearc with numerous seamounts. Dredge hauls from these seamounts are composed primarily of serpentinite, indicating a non-volcanic origin. In addition, dredged carbonate samples reveal (1) the presence of aragonite in water depths greater than the aragonite compensation depth, (2) unusual porosity and cement types, (3) light carbon and heavy oxygen isotopic compositions, and (4) an unusual trace element geochemistry of aragonite and calcite. These mineralogic, petrologic, and geochemical characteristics formed the basis of a prediction that methane-bearing fluids are venting from these seamounts. Subsequent to this prediction, submersible dives on a Mariana forearc seamount found and sampled chimneys seeping cool water that contains dissolved methane. The serpentine substrate beneath the chimneys appears to form from cold gravitational flows of serpentine material.Fluid inclusion contents of chimney samples also show an earlier history of methane, as well as longer chain hydrocarbons, aromatics and acetate ions. Drilling into the flank and the summit of this seamount by the Ocean Drilling Program during Leg 125 confirmed the presence of methane, other light alkanes, and organic acids in the present-day interstitial pore fluid of the cores. The presence of aromatic compounds and organic acids, if derived from the maturation of organic compounds, indicates a thermogenic origin for these volatiles.Strontium isotopic ratios of the aragonitic chimney samples and of the present-day interstitial waters range from present-day seawater values to significantly lower values. The strontium isotopic ratios of the pore waters within the seamount trend toward a relatively less radiogenic composition with increasing depth below the seafloor. This strontium isotopic signature is probably not the consequence of near-surface processes, but more likely a contribution from an igneous source deeper in the lithosphere.The lack of a large accretionary prism in the Mariana forearc removes the complication of fluid contribution from dewatering of the prism by compression during convergence, as in the Oregon-Washington or Barbados margins. The organic compounds in the fluid inclusions and interstitial pore fluids, as well as the isotopic and the geochemical composition of the carbonates and the dissolved constituents, suggest that fluids are derived from dewatering, compression, and dehydration of subducted sediments and the subducting plate. Furthermore, the composition of the fluid is influenced by thermal maturation of organics in subducted sediments as well as by a contribution from a deep igneous source.