Geochemical dynamics of the Atlantis II Deep (Red Sea): II. Composition of metalliferous sediment pore waters
Geochimica et Cosmochimica Actaa, 64 (23):
3995--4006 (2000)Abstract
The Atlantis II Deep is an axial depression of the Red Sea filled
with highly saline brines and covered by layered metalliferous sediment.
We report new data on the vertical distribution of major salts and
trace metals dissolved in the pore waters of the metalliferous sediments.
We have studied the chemical composition of interstitial waters of
two sediment cores of the western (core 684) and southwestern (core
683) basins. The major dissolved elements are Na and Cl. Their concentrations
are close to those of the brine overlying the sediment. The pore
waters are undersaturated with respect to halite at the in situ conditions
(62 degreesC, 220 bars), but are saturated at the shipboard conditions
(10 degreesC, 1 bar). The salt and water contents of the bulk sediment
show that core 683 contained halite in the solid fraction. A part
of it precipitated after core collection, but most of it was present
in situ. Thermodynamic calculations with a water-rock interaction
model based on Fitter's ion interaction approach reveal that equilibrium
between the pore waters and anhydrite is achieved in sediment layers
for which observations report the presence of this mineral. We used
a transport model, which shows that molecular diffusion can smooth
the profile of dissolved salt and partly erase the pore water record
of past variations of salinity in the lower brine. For example, we
calculated that the pore water record of modern variation of brine
salinity is rapidly smoothed by molecular diffusion. The dissolved
transition metals show large variations with depth in the interstitial
waters. The profiles of core 683 reflect the possible advection of
hydrothermal fluid within the sediment of the southwestern basin.
The distribution of dissolved metals in core 684 is the result of
diagenetic reactions, mainly the reduction of Mn-oxide with dissolved
Fe(II), the recrystallization of primary oxide minerals, and the
precipitation of authigenic Mn-carbonates. Copyright (C) 2000 Elsevier
Science Ltd.