Soru

In paleoceanographic reconstructions of Cenozoic surface ocean productivity, the accumulation rate of biogenic opal—primarily composed of the siliceous tests of radiolarians and diatoms—has long served as a primary proxy for silica export fluxes. However, oceanographer Elena Rostova demonstrated that raw biogenic opal accumulation rates in deep-sea sediments often fail to correlate with upper-ocean biological productivity due to differential postsedimentary dissolution. In oligotrophic ocean gyres, where bottom waters are strongly undersaturated with respect to amorphous silica, the dissolution rate of radiolarian tests is dictated primarily by the ratio of surface area to structural mass in individual microskeletal frameworks rather than by ambient water temperature. Specifically, Rostova observed that delicate, highly porous lattice structures undergo rapid dissolution in the upper sediment column, whereas robust, thick-walled polycystine shells survive intact, artificially skewing the fossil assemblage toward heavy-bodied taxa. Furthermore, Rostova identified that trace concentrations of adsorbed aluminum ions within the sediment pore waters can passivate the silica surface, reducing dissolution rates by up to two orders of magnitude even in highly undersaturated bottom waters. Consequently, when assessing past silica productivity, researchers who rely solely on total opal weight without accounting for species-specific skeletal density or pore-water aluminum geochemistry risk misinterpreting sediment layers characterized by low opal mass as periods of diminished surface biological activity when, in reality, intense post-depositional dissolution or insufficient aluminum passivation destroyed the lighter siliceous fraction.

According to the passage, Rostova's research explicitly indicates that the preservation of radiolarian tests in undersaturated deep-sea sediments is directly promoted by which of the following factors?