MICROBIAL EFFECTS ON EARLY DIAGENETIC SI CYCLING IN DEEP-SEA AND DELTAIC SEDIMENTS

Bacterial activity affects oceanic Si cycling through accelerating silica remineralization. Si cycling in marine sediments during early diagenesis is thought to primarily controlled by abiotic processes and to our knowledge there has been no described role for bacteria.

To investigate the potential biologic controls on silica cycling in marine sediments we conducted 1 – 2 week incubations utilizing frozen samples from the Congo deep sea fan, and both frozen and freshly collected, unamended, Mississippi river plume (MRP) sediments. Two sets of seawater slurries (solid solution ratio (SSR) range: 0.1-7.9 g l^-1) were run with replication, where one set was an abiotic control (addition of biocide) and the other a biologically active treatment. For MRP experiments, incubations had the radiotracer ³²Si for measurement of dissolved Si adsorption and desorption.

All slurries (biol. active, abiotic) with low SSR had higher Si release rates compared to slurries with high SSR. In Congo samples, Si release rates in biol. active samples varied non-linearly from 1.43- 0.53 μmole Si g^-1 d^-1 (SSR from 0.6 to 4.5 g l^-1). Similarly, in two separate experiments with freshly collected and biol. active MRP samples, Si release rates were 4-0.7 μmole Si g^-1 d^-1 (SSR from 0.1 to 7.9 g l^-1).

The abiotic incubations yielded higher Si release rates compared to the biol. active treatments with both frozen Congo and fresh, unamended, MRP sediments. Cross-plots of Si release rates between the two treatments show that biol. active sediments have lower Si release rates by: 13% (Congo) , 12% - 31% (two MRP experiments) compared to the abiotic controls. We believe the lower Si release rates in biol. active incubations are partially due to rapid Si re-precipitation/re-uptake reactions of dissolved Si. Preliminary data from MRP sediment with added ³²Si confirm active Si-uptake processes that can remove dissolved Si at a rate of 0.14 to 0.22 μmole Si g^-1 d^-1 (for SSD 4.5 to 0.3 g l^-1 , respectively). Adsorption onto particles accounts for >10% of the Si uptake.

These results clearly point to rapid and significant microbial effects on early diagenetic cycling in diverse marine depositional environments with significant implications for the oceanic Si cycle and the cycles of other relevant elements.