(Table 1) Chemical and isotopic composition of pore fluids in cores TGC-1 and TGC-6

The role of sediment diagenesis in the marine cycles of Li and B is poorly understood. Because Li and B are easily mobilized during burial and are consumed in authigenic clay mineral formation, their abundance in marine pore waters varies considerably. Exchange with the overlying ocean through diffusive fluxes should thus be common. Nevertheless, only a minor Li sink associated with the low-temperature alteration of volcanic ash has been observed. We describe a low-temperature diagenetic environment in the Black Sea dominated by the alteration of detrital plagioclase feldspars. Fluids expelled from the Odessa mud volcano in the Sorokin Trough originate from shallow (~100–400 m deep) sediments which are poor in volcanic materials but rich in anorthite. These fluids are depleted in Na+, K+, Li+, B, and 18O and enriched in Ca2+ and Sr2+, indicating that anorthite is dissolving and authigenic clays are forming. Using a simple chemical model, we calculate the pH and the partial pressure of CO2 (PCO2) in fluids associated with this alteration process. Our results show that the pH of these fluids is up to 1.5 pH units lower than in most deep marine sediments and that PCO2 levels are up to several hundred times higher than in the atmosphere. These conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments. We propose that in Black Sea sediments enhanced organic matter preservation favors CO2 production through methanogenesis and results in a low pore water pH, compared to most deep sea sediments. As a result, silicate mineral weathering, which is a sluggish process in most marine diagenetic environments, proceeds rapidly in Black Sea sediments. There is a potential for organic matter-rich continental shelf environments to host this type of diagenesis. Should such environments be widespread, this new Li and B sink could help balance the marine Li and Li isotope budgets but would imply an apparent imbalance in the B cycle.

沉积成岩作用（sediment diagenesis）在锂（Li）和硼（B）海洋循环中的作用尚不清楚。由于Li和B在埋藏过程中易被迁移，并在自生黏土矿物（authigenic clay mineral）形成过程中被消耗，因此它们在海洋孔隙水中的丰度变化很大。通过扩散通量与上覆海洋的交换应较为普遍，然而目前仅观察到与火山灰低温蚀变相关的少量Li汇。我们描述了黑海地区一个以碎屑斜长石（detrital plagioclase feldspar）蚀变为主的低温成岩环境。索罗金海槽（Sorokin Trough）敖德萨泥火山（Odessa mud volcano）排出的流体源自浅部（约100–400米深）沉积物，这些沉积物火山物质匮乏但钙长石（anorthite）含量丰富。这些流体中Na+、K+、Li+、B和18O含量较低，而Ca2+和Sr2+含量较高，表明钙长石正在溶解且自生黏土正在形成。通过简单的化学模型，我们计算了与该蚀变过程相关的流体pH值和二氧化碳分压（PCO2）。结果显示，这些流体的pH值比大多数深海沉积物中的pH值低多达1.5个单位，且PCO2水平比大气中的高数百倍。这些条件与陆地土壤环境中有利于硅酸盐矿物风化的条件相似。我们认为，与大多数深海沉积物相比，黑海沉积物中有机质保存能力的增强有利于通过甲烷生成作用产生CO2，并导致孔隙水pH值较低。因此，在大多数海洋成岩环境中缓慢进行的硅酸盐矿物风化过程，在黑海沉积物中进展迅速。富有机质大陆架环境可能存在此类成岩作用。若此类环境广泛存在，这种新的Li和B汇可能有助于平衡海洋Li及其同位素收支，但会导致B循环出现明显失衡。