Submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins.

Scholz, Florian, Hensen, Christian , Schmidt, Mark and Geersen, Jacob (2013) Submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins. Geochimica et Cosmochimica Acta, 100 . pp. 200-216. DOI 10.1016/j.gca.2012.09.043.

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Supplementary data:


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.

Document Type: Article
Additional Information: WOS:000312102100011
Keywords: Silicate weathering; diagenesis; volcanic ash; clay dehydration; pore water geochemistry; fluid flow; subduction; Chilean convergent Margin; DEEP-SEA SEDIMENTS; GAS HYDRATE; ANAEROBIC OXIDATION; SULFATE REDUCTION; SUBDUCTION ZONE; CENTRAL CHILE; FLUID-FLOW; ACCRETIONARY PRISMS; METHANE OXIDATION; MARINE-SEDIMENTS
Research affiliation: OceanRep > SFB 574 > B5
OceanRep > SFB 574
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 574 > A1
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: Yes
Open Access Journal?: No
Publisher: Elsevier
Projects: FLOWS, SFB574
Contribution Number:
SFB 574
Date Deposited: 28 Nov 2012 08:11
Last Modified: 23 Sep 2019 22:32

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