The Role of Diatom Frustule Morphology in the Formation of Reverse Weathering Products

The transport of dissolved silicic acid (dSi) and burial of biogenic silica (bSi) is dominated by processes occurring in land-sea interface systems; thus, these systems are important for the global Si budget. Previous work reporting the bSi content of low latitude coastal sediments has underestimated the amount of this material by failing to include authigenically altered bSi produced from reverse weathering. In the last decade this process has been shown to be an important sink for Si and other elements in these systems; however, the mechanisms regulating this process are still poorly understood. Here we report the preliminary results of ongoing laboratory batch experiments (5-12 months), examining the formation of authigenic products using three morphologically distinct diatom genera (Thalassiosira, Navicula, Chaetoceros) as a bSi matrix. As with previous studies, diatoms are separated from coastal clay (<2 μm) suspensions by 12-14 kDa dialysis membranes. Clay suspensions were made from northern Gulf of Mexico (GoM) sediments, collected on the 20 m isobaths, near the outflow of the Mobile Bay freshwater plume. All materials were suspended in HEPES buffered GoM seawater and placed in a rotating wheel (2 rpm) to keep particles in suspension. Preliminary results confirm increases in bSi dissolution between genera. Additionally, the presence of sediment suspensions also reduces the bSi dissolution rate relative to controls with no sediments. Fluid chemistries are monitored over the course of the experiment and results from initial solid phase analyses after five-month incubations are presented.