Fh. Lu et al., S and O (SO4) isotopes, simultaneous modeling, and environmental significance of the Nijar messinian gypsum, Spain, GEOCH COS A, 65(18), 2001, pp. 3081-3092
The Messinian evaporites in Nijar Basin, southeastern Spain, consist of coa
rse-grained primary selenites and laminated fine-grained gypsums. These gyp
sums, which are similar to those deposited in other Mediterranean Messinian
basins, have rather variable delta S-34 and delta O-18 (SO4) (+21 to +24 p
arts per thousand for delta S-34 + 10 + 17 parts per thousand for delta O-1
8) that are generally greater than the normal Tertiary marine values (21-22
parts per thousand for delta S-34 and 13 parts per thousand for delta O-18
). Moreover, the variation of delta O-18 (SO4) is larger than that of delta
S-34. Continental input and reservoir effect did not play important roles
for these isotope variations because these factors should have resulted in
lower delta S-34 and delta O-18 (SO4) than the normal marine values. Redox
reactions of sulfur species in brine should have been responsible for the i
sotope variations.
To interpret the variable delta S-34 and delta O-18 (SO4), we quantitativel
y modeled delta S-34 and delta O-18 (SO4) of dissolved sulfate during redox
reactions in two marine evaporative settings: in sediment pores in a shall
ow-water pan, and in a free-brine column in a deep-water basin. Modeled del
ta S-34 and delta O-18 of dissolved sulfate during reduction of sulfate yie
ld small variations, especially for O isotopes in a shallow-water pan. Howe
ver, modeled delta S-34 and delta O-18 of dissolved sulfate during reductio
n and reoxidation cycling processes in a deep-water basin show a 514 progre
ssive increase of up to 10 parts per thousand for both delta S-34 and delta
O-18 (SO4) above their normal marine values. Importantly, the modeled path
ways for the deep-water basin setting are consistent with the Nijar data, w
hereas those for pore waters in shallow-water pans are a poor fit to the Ni
jar data. The modeled results consistent with trace elements, Sr isotope da
ta, and selenite fabrics suggest that these Yesares selenites have formed i
n deep marine brines.
The modeling results also demonstrate that delta O-18 (SO4) can have larger
variation than delta S-34 because the incorporation of water oxygen and pr
obably dissolved free oxygen increases delta O-18 of the reoxidized sulfate
, whereas reoxidation of sulfide to sulfate decreases the total delta S-34.
(SO4). The modeled results could provide important geological implications
for isotope and environment interpretations for ancient evaporites. For ex
ample, redox reactions of sulfur species during sulfate precipitation could
modify normal marine delta S-34 and delta O-18(SO4) more efficiently than
freshwater contributions and reservoir effects. Therefore, to establish del
ta S-34 and delta O-18 (SO4) age curves, redox processes for ancient evapor
ites should be considered. Copyright (C) 2001 Elsevier Science Ltd.