| 其他摘要 | Using the Constant Addition System, the coprecipitation of several trace elements with calcite or aragonite in seawater was simulated, with concerning on the migration, transformation and reallocation of these target elements in such water-solid interactions. These target elements include B, Na, Mg, Mn, Co, Ni, Cu, Zn, Cd, As, Pb, U, Th, Y and REEs. Based on quantitative descriptions of their partitioning between CaCO3 and seawater, we studied their behaviors to experimental conditions, with attempts to extract some potential proxies for paleoceanography. Basic parameters of experiments, namely [H+], [Ca2+] and total alkalinity were first measured by pH measurements and high precision titrations. Subsequently key factors of the carbonic system were calculated and the kinetic expressions of calcite or aragonite precipitation in seawater at 5, 15, 25℃ and pCO2=0.0015atm. and 0.0030atm. were built. Our results presented that: 1) Under any given condition, the precipitation rates of calcite or aragonite have excellent correlativity with CaCO3 saturation states of seawater, which can be expressed Log R=k*Log(Ω-1)+b; 2) The pCO2 can greatly change the CaCO3 saturation states of solutions and consequently precipitation rates, but it has no influence to the kinetic expressions; 3) The kinetic expressions of calcite or aragonite at different temperature are quite different, proving that relative processes are thermodynamic. With purposed additions of target elements into solutions, their coprecipitation with calcite or aragonite were simulated. And their contents in steady solutions and overgrowths of calcite or aragonite were analyzed by ICP-MS, via pretreatments of direct dissolution and dilution or off-line chelation. For individual element and experiment, its coprecipitation behavior was evaluated by Parition Coefficient. In experiments, experimental conditions such as seed materials (calcite and aragonite), temperature, pCO2, saturation states (2-12, counted by calcite), precipitation rates and initial concentrations of target elements were varied to get the response of partition coefficients to such conditions. Hence, their behavior were distinguished and classified as follows:1) in calcite, Mn, Co, Ni, Cu, Pb, Cd, Th and Mg belong to compatible elements; 2) Mn, Co, Ni, Cd belong to imcompatible elements in aragonite. Different from these metals, B, As and U participate the coprecipitation by forms of legands- BO33-, AsO33- and UO22+, among which BO33- and AsO33- occupy the position of CO32- while UO22+ occupies the position Ca2+ in lattices. With an extraction of these primitive results, several conclusions were drawn. The kinetics of calcite and aragonite precipitation, temperature, ionic radii, crystal structure and solution chemistry were assumed to be key factors for the coprecipitation of trace elements with calcite or aragonite. After that, we attempted to develop different coprecipitation mechanisms for these target elements. At last, we compared our laboratory results with those of field work from literatures. Several potential proxies for paleoceanography were withdrawn, namely: 1) U/Ca, Cd/Ca in aragonite and U/Ca in calcite for CO32- concentrations; 2) B/Ca and As/Ca in aragonite for seawater pH; 3) Mn/Ca in calcite for redox conditions; 4) Co/Ca, U/Ca in calcite and Cu/Ca, Pb/Ca in aragonite for seawater chemistry. 5) Yttrium and rare earth seawater chemistry in calcite for seawater chemistry. Also there are several combinations of elements such as Mn-Co-Ni and U-B etc. In addition, we point out some limiting factors which were neglected by present trace-element proxies for paleoceanography and paleoenvironment. |
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