Stable isotope composition of carbonates in the Tyrrhenian Sea

The Messinian evaporitic succession recovered at ODP Sites 652, 653, and 654 in the Tyrrhenian Sea was generated under various environmental conditions which ranged from brackish to hypersaline, as deduced from the sedimentary facies and stable isotope compositions of the carbonate and sulfate deposits. Water in the basins had to be shallow to undergo such rapid and large geochemical variations. The marine influence was omnipresent in the basin at least during the deposition of sulfate evaporites; seawater or marine brines might have been supplied either by direct input into evaporitic lagoons as at Sites 653 and 654, or by subterraneous infiltration in marginal areas as at Site 652. Episodes of severe dilution by continental waters occurred frequently throughout Messinian times in the more basinal areas at Sites 653 and 654, while a fresh water body was standing permanently at Site 652. The high heat flow present at Site 652 was responsible for a major late authigenesis of iron-rich dolomites, which was initiated during the subsidence of the basin and ended before Pliocene.

Tritium concentrations of hydrography in the North Atlantic around 40°N

We report, numerically and in graphical form, meaured tritium concentrations from five hydrographic stations in the North Atlantic. Fairly homogeneous concentrations are observed in a surface layer typically 400 m deep. In the thermocline, concentrations decrease steadily down to the sigma-theta = 27.3 density horizon, and are more variable further down. The tritium in the lower part of the thermocline originates from the Subarctic Intermediate Water and the Mediterranean Water. There is a relative tritium maximum associated with the Mediterranean Water on the easternmost station of the section. In the deep water (sigma-theta > 27.8), concentrations east of the Midatlantic Ridge are close to the limit of detection down to 2500m, and undetectable further down, while west of the ridge tritium is found throughout the water column. The deep water tritium is associated with the deep-water advective cores of Arctic origin. The present tritium data can serve as northern boundary values in attempts to use tritium in studies of the North Atlantic main thermocline dynamics. The present data together with data from the literature point to a general division of the North Atlantic main thermocline into two layers separated by an isopycnal surface near sigma-theta = 27.3.

Isotopic composition of various forms of sulfur in barite crusts from the Red Sea rift

Barite crusts were formed by an intermittent hydrothermal vent with output temperature from 85 to 465°C. Principal sources of supply of sulfate sulfur are sea water, evaporites, and tholeiitic basalts of the Red Sea rift. Sulfides and sulfates were formed in conditions of isotope disequilibrium with respect to sulfur because rate of precipitation of sulfur compounds from hydrothermal solution was high compared with rate of isotope exchange.