Dissolution of Biogenic Silica in Permeable Coastal Sands

Riverine inputs, groundwater efflux, and terrestrial runoff enrich coastal waters with silica. Diatoms, which require silicic acid (Si(OH)4) for frustule formation and cell growth, are therefore predominantly found in these waters. Less than half of the pelagic primary production is consumed in the shallow water column; the remaining fraction settles on the seafloor. However, large fractions of these shelf sediments are composed of permeable relict sands, which do not accumulate organic matter. As a result, the fate of the deposited diatoms and the biogenic silica (bSiO2) is poorly understood, and little is known about the processes that control transport and decomposition of sedimented diatoms in permeable shelf beds. To determine the fate of these deposited diatoms, four study objectives were addressed. These objectives were: I. To determine how diatoms are deposited on permeable sediments and how Si(OH)4 is distributed in upper-sediment layers; II. To determine how flushing of permeable sediments affects degradation and dissolution of diatoms embedded in the sands; III. To determine fluxes of Si(OH)4 from permeable sediments under different filtration conditions; and IV. To assess Si(OH)4 concentration ranges in the water and the pore water in the northern Gulf of Mexico in a time series to determine the coupling between water-column and pore-water concentrations. In the laboratory, two flume experiments were conducted to address the first objective. The first experiment revealed that deposition of algal cells occurred at the bottom of sediment-ripple troughs and the upper slopes of the ripples, while the ripple crest was associated with a deposition minimum. The second experiment demonstrated that the highest Si(OH)4 concentrations were found in the upwelling zone at the downstream slope of the ripple and the ripple crest (~100 µmol Si(OH)4). The upwelling pore-water flows that carry Si(OH)4 upwards focus near the ripple crest and dissolution of diatoms deposited in the ripple slopes add to the Si(OH)4 concentration in this zone. Meanwhile, the lowest concentrations were seen in the downwelling zone (e.g., ripple troughs and lower flanks). Here flume water with relatively low Si(OH)4 concentrations penetrated into the sediment. To address the second objective, the dissolution of the frustules in the permeable sand was quantified in five column-reactor experiments that permitted sediment flushing at well-defined pore-water flow velocities. The column experiments showed that the Si(OH)4 remobilization was higher in the pore space than in reaction columns that were filled with water only, suggesting that the dissolution of diatoms is more rapid in flushed sediments. The measured dissolution-rate constants (k) were 0.0144-0.0150 in columns with sediment versus 0.0070-0.0082 in columns with water only. This finding can be explained by the depression of the diffusive-boundary layer at the surface of the frustule when the diatoms are embedded in the sand. Sediment-water flux measurements with in-situ chamber incubations addressed the third objective and revealed that advective pore-water exchange enhanced Si(OH)4 flux up to a factor of six. Day and night fluxes differed due to the changes in Si(OH)4 uptake during daytime. Fluxes of Si(OH)4 were an order of magnitude higher in the sediments than in the water column at both the Gulf and the Bay sites. To address the fourth objective, monthly field samples were taken on St. George Island at the two field sites. These samplings revealed peaks in benthic primary production in the summer months and maxima in water and sediment silica concentrations in the fall and winter months. However, when nutrients became depleted during summer, an increase in Si(OH)4 concentrations was observed, suggesting that diatom growth was limited by other nutrients than Si(OH)4. The results of this study show that in permeable coastal sediments the dissolution of deposited diatom frustules and the release of the mobilized Si(OH)4 from the sediment are enhanced by the advective-pore water flows caused by bottom flow-topography interaction and emphasize the importance of coastal permeable sediments in the cycling of marine silica.