『Abstract
Two sediment cores retrieved at the northern slope of Sakhalin
Island, Sea of Okhotsk, were analyzed for biogenic opal, organic
carbon, carbonate, sulfur, major element concentrations, mineral
contents, and dissolved substances including nutrients, sulfate,
methane, major cations, humic substances, and total alkalinity.
Down-core trends in mineral abundance suggest that plagioclase
feldspars and other reactive silicate phases (olivine, pyroxene,
volcanic ash) are transformed into smectite in the methanogenic
sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al
in the solid phase decrease with sediment depth indicating a loss
of mobile cations with depth and producing a significant down-core
increase in the chemical index of alteration. Pore waters separated
from the sediment cores are highly enriched in dissolved magnesium,
total alkalinity, humic substances, and boron. The high contents
of dissolved organic carbon in the deeper methanogenic sediment
sections (50-150 mg dm-3) may promote the dissolution
of silicate phases through complexation of Al3+ and
other structure-building cations. A non-steady state transport-reaction
model was developed and applied to evaluate the down-core trends
observed in the solid and dissolved phases. Dissolved Mg and total
alkalinity were used to track the in-situ rates of marine silicate
weathering since thermodynamic equilibrium calculations showed
that these tracers are not affected by ion exchange processes
with sediment surfaces. The modeling showed that silicate weathering
is limited to the deeper methanogenic sediment section whereas
reverse weathering was the dominant process in the overlying surface
sediments. Depth-integrated rates of marine silicate weathering
in methanogenic sediments derived from the model (81.4-99.2 mmol
CO2 m-2 year-1) are
lower than the marine weathering rates calculated from the solid
phase data (198-245 mmol CO2 m-2
year-1) suggesting a decrease in marine weathering
over time. The production of CO2 through
reverse weathering in surface sediments (4.22-15.0 mmol CO2 m-2 year-1) is about one
order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The
evaluation of pore water data from other continental margin sites
shows that silicate weathering is a common process in methanogenic
sediments. The global rate of CO2 consumption
through marine silicate weathering estimated here as 5-20 Tmol
CO2 year-1 is as high as the global
rate of continental silicate weathering.』
1. Introduction
2. Study area
3. Sampling and analytical techniques
4. Numerical modeling procedure
5. Results and discussion
5.1. Solid phase composition
5.2. Pore water composition
5.3. Model results
5.4. Cation exchange
5.5. Marine silicate weathering
5.6. Reverse weathering
5.7. Global rates of marine silicate weathering
5.8. Effects of anoxic sediment diagenesis on marine and global
carbon cycling
6. Conclusions and perspectives
Acknowledgments
Appendix A
References