Silica diagenesis leads to dramatic petrophysical variations in the host sediment across the depth of an opal-A to opal-CT transition zone. Predicting the present-day diagenetic status of opal-A to opal-CT transition zones, i.e., active versus fossilized fronts, is essential to constraining the drivers that control abrupt changes in the physical state of sediment. This study assesses whether there are modern signatures of ongoing silica diagenesis in the sediment pore water, and demonstrates the potential for pore-water-chemistry profiles for distinguishing between active opal-CT precipitation and fossil transition zones. Pore-water chemistry, mineralogy, and thermodynamic analyses of the Ocean Drilling Program Wells 794 and 795 indicate that solubility equilibrium has been reached with respect to opal-CT in the transition zones captured by the Neogene biosilica in the Sea of Japan. Even though silica dissolution might be triggering a reverse-weathering process, the equilibrium reached with respect to diagenetic opal strongly suggests that the silica drop across the transition zones is mainly influenced by active opal-A to opal-CT transformation. Owing to abrupt petrophysical variations associated with opal-CT formation, other interstitial profiles—major ions and primary parameters—have been influenced by silica diagenesis. The extremely low silica diffusion fluxes in the sediment, the low permeability of host sediment, and the occurrence of considerable pore-water loss at the depth of the transition zone all support this conclusion that the dissolved species have not been diffused in the sediment at rates comparable to those by pore-water advection. Advection and diffusion, however, appear to have ceased recently because they have failed to smooth the signature of ongoing silica diagenesis. The porosity drop during opal-A to opal-CT diagenesis at Sites 794 and 795 is principally attributed to chemically induced anomalous compaction, causing the sediment framework to lose its strength under fragmentation and extensive opal-A dissolution.

中文翻译：

---

孔隙水化学：跟踪正在进行的二氧化硅成岩作用特征的代理

二氧化硅成岩作用会导致蛋白石从蛋白石A到蛋白石CT过渡带的整个深度发生明显的岩石物性变化。预测当今蛋白石A到蛋白石CT过渡带的成岩状态，即活动带和化石带前沿，对于约束控制沉积物物理状态突变的驱动器至关重要。这项研究评估了沉积物孔隙水中是否存在正在进行的二氧化硅成岩作用的现代特征，并证明了孔隙水化学特征可用于区分活性蛋白石CT沉淀和化石过渡带的潜力。孔隙水化学，矿物学，海洋钻探程序794井和795井的热力学分析表明，在日本海，新近纪生物二氧化硅捕获的过渡带中，相对于蛋白石CT已达到溶解度平衡。尽管二氧化硅的溶解可能会触发逆风化过程，但相对于成岩蛋白石达到的平衡强烈表明，过渡带上的二氧化硅滴主要受活性蛋白石A向蛋白石CT转变的影响。由于与蛋白石CT形成相关的突然的岩石物性变化，二氧化硅的成岩作用影响了其他间隙分布（主要离子和主要参数）。沉积物中二氧化硅的扩散通量极低，宿主沉积物的渗透率低，过渡区深度处大量孔隙水流失的发生都支持这一结论，即溶解物质并未以与孔隙水平流相当的速率扩散到沉积物中。然而，平流和扩散最近似乎已停止，因为它们未能使正在进行的二氧化硅成岩作用的信号平滑。蛋白石A到蛋白石CT成岩过程中，部位794和795的孔隙度下降主要归因于化学诱导的异常压实，导致沉积物骨架在破碎和蛋白石A大量溶解下失去强度。由于它们未能使正在进行的二氧化硅成岩作用的信号变得平滑，因此最近似乎已停止。蛋白石A到蛋白石CT成岩过程中，部位794和795的孔隙度下降主要归因于化学诱导的异常压实，导致沉积物骨架在破碎和蛋白石A大量溶解下失去强度。由于它们未能使正在进行的二氧化硅成岩作用的信号变得平滑，因此最近似乎已停止。蛋白石A到蛋白石CT成岩过程中，部位794和795的孔隙度下降主要归因于化学诱导的异常压实，导致沉积物骨架在破碎和蛋白石A大量溶解下失去强度。