Abstract
Widespread diagenesis of clay minerals occurs in deeply buried marine sediments under high-temperature and high-pressure conditions. For example, the smectite-to-illite (S-I) transformation has been often observed in sediments at in situ temperatures above ~60°C. However, it remains largely unknown whether such diagenetic processes naturally occur in relatively shallow and low-temperature sediments and, if so, what the consequences are of any related chemical reactions to the geochemical characteristics in the deep biosphere. We evaluated the possibility of naturally occurring S-I transformation at temperatures below 40°C in continental slope sediments of the Bering Sea by examining porewater chemistry, clay mineralogy, and chemical composition of clay minerals measured to ~800 m beneath the seafloor (mbsf) in core samples acquired during Integrated Ocean Drilling Program Expedition 323. In porewater from these cores, chloride concentrations decreased with increasing depth from 560 mM near the seafloor to 500 mM at ~800 mbsf; δ18O increased from 0‰ to 1.5‰; and δD decreased from –1‰ to –9‰. These trends are consistent with the addition of water derived from S-I transformation. The discrete low Cl– spikes observed between ~200 mbsf and ~450 mbsf could be attributed to the dissociation of methane hydrate. X-ray diffraction analysis of the clay-size fraction (<2 µm) showed an increase of illite content in the I/S mixed layer with increasing depth to 150 mbsf. This increase may imply the occurrence of S-I transformation. The decrease of Fe3+/Fe2+ in the clay-size fraction with increasing depth strongly suggests microbial reduction of Fe(III) in clay minerals with burial, which also has the potential to promote the S-I transformation. Our results imply the significant ecological roles on the diagenesis of siliciclastic clay minerals underlying the high-productivity surface seawater at continental margins.
Highlights
The smectite-to-illite (S-I) transformation is a ubiquitous and important feature of post-sedimentary alteration processes
We evaluated the hypothesis that microbial Fe(III) reduction in clay mineral, which can promote SI transformation, and S-I transformation both occur at temperatures below 40◦C in continental slope sediments of the Bering Sea down to ∼800 mbsf by examining porewater chemistry, clay mineralogy, and chemical compositions of clay minerals
Samples were diluted and measured with the Agilent 7700 ICP-MS at the Kochi Core Center (KCC) using In as an internal standard. At both sites on the Bering Sea Slope (U1343 and U1344), the Cl− concentrations in porewater generally decreased with increasing depth from ∼560 to 500 mM, the values varied widely (Figures 2A,B)
Summary
The smectite-to-illite (S-I) transformation is a ubiquitous and important feature of post-sedimentary alteration processes. Smectite is transformed to the more stable clay mineral illite. This process releases silica, interlayer water, and cations associated with K fixation (e.g., Perry and Hower, 1972; Freed and Peacor, 1989; Huang et al, 1993), as indicated in the following reaction (Perry and Hower, 1972): smectite + K+ + Al3+ = illite + silica + H2O.
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