Geology topics
Publications and source records attributed to Miriam Kastner.
No abstract available.
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.
Gas hydrates were recovered during coring by Ocean Drilling Program (ODP) Leg 112 at Sites 685 and 688 on the Peruvian outer continental margin at latitudes of 9° and 11.5°S, where water depths are 5070 and 3820 m, respectively. In addition, nearby Sites 682 and 683 yielded compelling evidence that gas hydrates are present, but gas hydrates were not directly observed there. Anomalous acoustic reflectors, known as bottom-simulating reflectors (BSRs), on marine seismic profiles from the region also provided inferential evidence that gas hydrates are present. Geothermal gradients of about 43 and 49°C/km were calculated on the basis of relations between depths to BSRs, bottom-water temperatures, and the pressure-temperature stability field of gas hydrates. Geochemical studies revealed that methane concentrations increase rapidly with depth after pore-water sulfate concentrations have been depleted. The relationship between methane and sulfate suggests that microbial processes account for the generation of methane, and the relationship between the carbon isotopic composition of methane and dissolved carbon dioxide supports this suggestion. We believe that decreasing chlorinity in pore water from squeezed sediment at the four sites results mainly from the decomposition of gas hydrates and is a dilution artifact observed as a result of the squeezing procedure. Maximum chlorinity values at or near the surface result from excess salt that comes from the formation of gas hydrates composed of freshwater. Record alkalinity attests to the intensity of diagenetic processes and has significant effects on salinity profiles at these sites. Gas hydrates were recovered at 99 and 166 meters below the seafloor (mbsf) at Site 685, and at 141 mbsf at Site 688 in Pleistocene diatomaceous mud. Methane constitutes more than 99% of the hydrocarbon gas mixture in the gas hydrates. The volumetric ratio of methane to water in the sample from Site 685 is 100, indicating that the sampled gas hydrate is either undersaturated with respect to methane or had partially decomposed during core recovery or both. The discovery of gas hydrates in lower slope deposits of the Peruvian outer continental margin extends our knowledge of gas-hydrate formation and occurrence in the Circum-Pacific region.