A THERMODYNAMIC MODEL FOR WATER COLUMN PRECIPITATION OF SIDERITE IN THE PLIOPLEISTOCENE BLACK-SEA

It has been hypothesized that finely disseminated, soft, wheat-like gr ains of siderite in the sideritic muds of the Plio-Pleistocene section of the Black Sea are ''sub-aqueous precipitates'' (Hsu and Kelts, 197 8). In an attempt to quantify this hypothesis, we (1) investigated the chemistry, mineralogy, and Isotopic composition of the siderites and associated calcite and (2) developed a thermodynamic model of endogeni c siderite precipitation (that is, directly precipitated from the wate r column) driven by sequential evaporative concentration of acidic, di ssolved organic-carbon, and iron-rich waters. Considering the average composition of the present-day Satilla drainage of the southeastern Un ited States as approximating the composition of the waters that fed th e Neogene Black Sea and allowing this water to evaporate isothermally at 25 degrees C in equilibrium with atmospheric CO2 (PCO2 10(-3.5) atm ), we calculated the aqueous concentrations at which various carbonate phases would precipitate out of the system. It is shown that evaporat ion of this water to roughly one-twelfth of its original volume can re sult in precipitation of the pure endmember siderite alone. Relating o ur geochemical and modeling results to the details of Black Sea climat ostratigraphic and evolutionary history during the times of deposition of the siderite units, we corroborate the qualitative arguments of Hs u and Kelts (1978). We also conclude that climate was the chief forcin g function in transforming episodically the calcite-precipitating eutr ophic water masses of the Plio-Pleistocene basin to one of siderite pr ecipitation. Based on our model calculations of the thickness of sider ite precipitated per cm(2) of basin area, we further predict that the observed laminae/interbeds of siderite could be a composite of several microlayers of siderite and clays.