Implication for the occurrence of low temperature smectite to illite reaction in the Bering Sea slope sediments (IODP Expedition 323)
Thursday, 18 December 2014
The smectite to illite (S-I) reaction has been recognized as a diagenetic process, which typically occurs at relatively high temperature deep sediments over 60°C, impacting on in-situ physical and geochemical conditions. Recent experimental studies showed that anaerobic microbial respiration possibly contributes to promote the S-I reaction at low temperatures by reduction of Fe (III) in smectite (Kim et al. 2004). Nevertheless, the occurrence of low temperature S-I reaction has not been observed in natural marine sediments. We present here an implication for the occurrence of low temperature S-I reaction (<40°C) in the Bering Sea slope sediments, which is based on pore water chemistry and clay mineralogy in sediments drilled down to ~800 m below seafloor (mbsf) at Sites U1343, and U1344 (Bering Sea slope), and U1341 (Bowers Ridge) during the Integrated Ocean Drilling Program Expedition 323. Chemical analyses of pore water from the slope sediments showed that chloride concentrations were slightly decreased from 560 mM near the seafloor to 500 mM at 700 mbsf. Dissolved potassium concentrations decreased from 10 mM to 6 mM. δ18O and δD of pore water increased from 0‰ to 1.5‰ and decreased from –2‰ to –10‰ with increasing depth, respectively. These trends can be attributed to the addition of dehydrated water and the potassium uptake by the S-I reaction. Such trends were not observed from the Bowers Ridge. Illite/smectite mixed layers indicating the intermediate stage of the S-I reaction, were identified only from the slope sediments based on XRD analyses. Concentration ratios of potassium and magnesium in the clay-sized fraction from the slope sediments increased with increasing depth below 300 mbsf, suggesting the potassium uptake into illite by the S-I reaction in-situ. The thermal gradient at Sites U1343 and U1344 was 49.0°C/km and 53.3°C/km, respectively, indicating that the temperature range in the cored sediments is generally lower than 40°C. Based on these evidences, we infer that the low temperature S-I reaction occurs in the Bering Sea slope sediments. A possible explanation for this phenomenon might be that the organic rich continental margin sediments harbors phylogenetically diverse anaerobic microbial communities possibly utilizing Fe(III) and mediating the S-I reaction.