SYNTHESIS (continued)

Interstitial Waters: Silica and Sulfate

The interstitial water profiles measured during Leg 184 reflected sulfate reduction and methanogenesis in the upper sediments. Below the zone of organic matter reduction, interstitial water profiles reflected alteration of volcanic ashes, diagenesis of clays, dissolution of silica, and dissolution/recrystallization of calcite at depth. These data revealed two clear trends in the SCS sediments:

We found that the sulfate gradient (i.e., the decrease in sulfate values from oceanic values, 28.9 mM, to the value of the sulfate plateau) increased linearly with sedimentation rate but not with the TOC concentration. The sulfate gradient is controlled both by the supply of organic matter to be consumed by sulfate reduction and by the extent to which seawater sulfate can continue to diffuse into interstitial water and replenish the sulfate removed by sulfate reduction. Hence, the linear relationship between sulfate gradient and sedimentation rate is established both by the correlation between high LSR and higher TOC flux to the seafloor, and by the length of time that near-surface sediments continue to receive new sulfate from seawater. The extreme sulfate gradient observed at Site 1144 appears to be more a function of high sedimentation rates than the supply of organic matter because the LSR is so high (>62 cm/k.y.) that sediments move out of the diffusional contact with seawater faster than organic matter can be depleted. This rapid removal of organic matter from the zone of sulfate reduction is consistent with the good preservation of organic matter with depth at this site. All other sites follow a more linear relationship between sulfate gradient and both LSR and TOC. Site 1143 has the lowest values observed and is the most distant from continental sources.

All the dissolved silica profiles showed an increase of similar magnitude between ~1 and 0.5 Ma, which corresponds to an increase in the abundance of biogenic silica in the sediments (Fig. 28). This increase in dissolved silica occurs at very different depths at different sites, suggesting that it is not related to diagenetic changes in silica. Instead, the increase in dissolved silica implies that an increase in biogenic silica observed in the sediments is a real change in silica flux to the sediments. This increase is somewhat coincident with increasing sedimentation and organic carbon accumulation rates, which could imply increased preservation related to LSR or overall increased silica productivity. In either case, the increase in silica appears to be a regional change, suggesting climatic or tectonic changes.


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