『Abstract
In order to investigate how submarine weathering processes may
affect the water balance of sediments at convergent plate margins,
six sediment cores were retrieved off Central Chile at water depth
between 〜800 and 4000 m. The sediment solid phase was analyzed
for its major element composition and the pore fluids were analyzed
for dissolved sulfate, sulfide, total alkalinity, major cations,
chloride, bromide, iodide, hydrocarbons as well as the carbon
isotopic composition of methane.
Because of negligible weathering on land, surface sediments off
Central Chile are rich in reactive silicate minerals and have
a bulk composition similar to volcanic rocks in the adjacent Andes.
Deep-sourced fluxes of alkalinity, cations and chloride indicate
that silicate minerals are subject to weathering in the forearc
during burial. Comparison of deep-sourced signals with data from
nearby Ocean Drilling Program sites reveals two different types
of weathering processes: In shallow (tens of meters), methanic
sediments of slope basins with high organic carbon burial rates,
reactive silicate minerals undergo incongruent dissolution through
reaction with CO2 from methanogenesis. At
greater burial depth (hundreds of meters), silicate weathering
is dominated by authigenic smectite formation. This process is
accompanied by uptake of water into the clay interlayers thus
leading to elevated salinities in the surrounding pore water.
Deep-seated smectite formation is more widespread than shallow
silicate dissolution, as it is independent from the availability
of CO2 from methanogenesis. Although solute transport is not focused
enough to form cold seeps in the proper sense, tectonically induced,
diffuse fluid flow transfers the deep-seated signal of smectite
formation into the shallow sediments.
The temperature-controlled conversion of smectite to illite is
considered the most important dehydration process in marine forearc
environments (depth of kilometers). However, in agreement with
other studies at active margins (e.g. Aleutians, Cascadia, Nankai
Trough) and despite ubiquitous evidence for smectite formation,
little evidence for seafloor seepage of dehydration fluids could
be found off Central Chile. We argue that the circular process
of pore water uptake during smectite formation and release upon
illitization implies a balanced freshwater budget and therefore
a rather limited potential for net pore water freshening on a
margin-wide scale. According to this rationale, pore water freshening
at seafloor seeps preferentially occurs at lower latitudes (Central
America, Barbados, Mediterranean Ridge) where terrestrial weathering
is more intense thus leading to external (i.e. detrital) smectite
and thus freshwater inputs to the subduction system.』
1. Introduction
2. Geological setting
3. Material and methods
3.1. Sampling strategy
3.2. Pore water recovery and chemical analyses
4. Results
4.1. Pore water chemistry
5. Discussion
5.1. Identification of deep-sourced alkalinity and calcium
fluxes
5.2. Controls on submarine weathering: silicate dissolution versus
smectite formation
5.3. Transport of deep-seated weathering signals into the shallow
sediments
5.4. Evidence for pore water freshening and relation to submarine
weathering
6. Summary and implications
Acknowledgments
Appendix A. Supplementary data
References