A thermodynamic model for water column precipitation of siderite in the Plio-Pleistocene Black Sea

Abstract

It has been hypothesized that finely disseminated, soft, wheat-like grains of siderite in the sideritic muds of the Plio­ Pleistocene section of the Black Sea are sub-aqueous precipitates (Hsu and Kelts, 1978). 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 endogenic siderite precipitation (that is, directly precipitated from the water column) driven by sequential evaporative concentration of acidic, dissolved or~anic-carbon, and iron-rich waters. Considering the average com.Position of the present-day Satilla drainage of the southeastern Umted States as approximating the composition of the waters that fed the Neogene Black Sea and allowing this water to evaporate isothermally at 25°C in equilibrium with atinospheric C02 (PC02 = l 0- 3 . 5 attn), we calculated the aqueous concentrations at which various carbonate phase5 would precipitate out of the system. It is shown that evaporation of this water to roughly one-twelfth of its original volume can result in precipitation of the pure endmember siderite alone. Relating our geochemical and modeling results to the details of Black Sea climatostratigraphic and evolutionary history dur­ ing the times of deposition of the siderite units, we corroborate the qualitative arguments of Hsu and Kelts (1978). We also conclude that climate was the chief forcing function in transforming episodically the calcite-precipitating eutrophic water masses of the Plio-Pleistocene basin to one of siderite ptecipitation. Based on our model calculations of the thickness of siderite precipitated per cm2 of basin area, we further predict that the observed laminae/interbeds of siderite could be a composite of several microlayers of siderite and clays.