近平衡状态海水中方解石和文石沉淀速率及动力学研究

Precipitation rates and kinetics of calcite and aragonite in seawater near equilibrium

利用"freedrift"开放反应系统,研究人工海水中近沉淀平衡状态时二氧化碳分压(p(CO2))的变化对方解石、文石沉淀速率及其动力学方程的影响。反应在恒定的温度(25.0℃±0.2℃)、p(CO2)环境下进行,通过实验得到了不同p(CO2)环境下,方解石和文石的沉淀速率及动力学方程。研究发现:(1)晶体类型和碳酸盐碱度相同时,p(CO2)越低沉淀速率越大;饱和度(Ω)和p(CO2)相同时,方解石沉淀速率低于文石,且p(CO2)越高,沉淀速率越低。(2)对于方解石,当p(CO2)介于3 050×10-6和3 200×10-6之间,1.5<Ω<3.1时,反应级数n=2.4;当p(CO2)≈130×10-6,1.2<Ω<3.0时,n=2.6;对于文石,当p(CO2)≈2 300×10-6,1.1<Ω<2.0时,n=3.2;当p(CO2)≈320×10-6,1.1<Ω<1.9时,n=2.0。本研究结果加深了对不同沉积环境下方解石和文石沉淀机制的理解。此外,不同方解石饱和度(Ωc)条件下方解石和文石沉淀速率的研究为探讨为何现代浅海碳酸盐沉积物以文石为主、而海底沉积物中的自生碳酸钙矿物主要为方解石的现象提供了一种较好的解释方案。

The influence of carbon doxide partial pressure（p（CO2）） in seawater on the precipitation rates and kinetics of calcite and aragonite was studied using ＂freedrift＂ open reaction system at constant temperature（25.0℃±0.2℃） and p（CO2）.The precipitation rates were measured under different p（CO2）.We found that：（1） the precipitation rate of aragonite was higher than that of calcite,and the lower the p（CO2）was,the higher the precipitation rate was at the same saturation state（Ω） and constant p（CO2）;（2） as for calcite,the reaction order was 2.4 when p（CO2） was fluctuating between 3 050×10-6～3 200×10-6 and 1.5Ω3.1,and was 2.6 when p（CO2）≈130×10-6 and 1.2Ω3.0;as for aragonite,the reaction order was 3.2 when p（CO2）≈2 300×10-6 and 1.1Ω 2.0,and was 2.0 when p（CO2）≈320×10-6 and 1.1Ω1.9.Results of this study have deepened the understanding of the precipitation mechanisms of calcite and aragonite under different sedimentary conditions.In addition,the different precipitation rates of calcite and aragonite under different calcite saturation（Ωc） provide an explanation of why aragonite dominates the carbonate sediments in modern shallow sea while the main authigenic carbonate mineral in marine sediments is calcite.