Effects of bioturbation on sediment—Seawater interaction

Abstract

Large gradients of dissolved silica are common in the uppermost layers of abyssal marine sediments. These gradients can be sustained only by an active, continuous supply of dissolved silica from within the sediments. The flux of dissolved silica is approximately equivalent to the total accumulation rate of all siliceous material (including aluminosilicates) in the sediments; if aluminosilicates were the source of dissolved silica they would have to give up nearly all of their silicon. Since marine sediments commonly show no such depletion in silicon with depth, dissolution of aluminosilicates cannot be the primary source of dissolved silica emanating from the sea floor. The main source of this flux must be particulate biogenous silica that is mixed downward into the more inert sedimentary material and dissolves leaving little residue. Biological and physical processes stir deep-sea sediments at rates which can be described by vertical mixing coefficients. These coefficients can be evaluated from microtektite redistribution or from the depth profiles of plutonium in sediments. Measured rates range between 0.5 and 400 cm 2 kyr −1 in the deep sea. By redistributing reactive particles, this vertical mixing influences the chemical gradients in interstitial waters and the chemical fluxes across the sea-floor boundary. Dissolving particles of silica or calcium carbonate will have more effect on pore water concentrations when they are deeper within the sediment. Model calculations show the input flux of biogenous particulate silica to be the primary controlling factor on the flux of dissolved silica and on the interstitial profile of dissolved silica. Bioturbation rates and dissolution rates also have an important effect on interstitial profiles and this effect can be estimated although the actual rates are poorly known. Model calculations indicate that the concentrations of dissolved species in pore water are controlled by a dynamic balance between competing processes. Equilibrium reactions between sediments in the upper few meters of the sediment column and their associated pore water are almost irrelevant in establishing the composition of the pore waters. Silica solubility is only slightly affected by temperature and pressure changes in the deep sea, and abyssal waters are always far from saturation. Accordingly the water composition can have only a minimal effect on dissolution rates of SiO 2. On the other hand, the actual mechanism which prevents dissolution of some siliceous fossils remains unknown. A stagnant boundary layer in water overlying the sea floor will have almost no effect on concentrations or fluxes of silica. Variations in bioturbation rates or in the flux of biogenous particulate silica to the sediment would affect the balance of concentrations in the pore water profile. Such changes over the last few hundred years would be recorded in the composition of pore waters which are associated with sedimentary particles hundreds of thousands of years old. In this sense the “age” of the pore waters is far less than the “age” of the associated solids in the upper meters of the sea floor.