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:
- The extent and depth of sulfate reduction reflects both
the supply of organic matter and the large range of
sedimentation rates.
- Dissolved silica increases across the SCS at ~1 Ma.
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.
Conclusions
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