Early diagenetic processes of saline meromictic Lake Kai-ike, southwest Japan: III. Sulfur speciation and isotopes
Abstract:Lake Kai-ike is a saline meromictic lake located along the coast of Kami-Koshiki Island. The lake is isolated from ocean by a gravel bar, through which seawater infiltrates by tidal pumping. The lake is permanently redox (density)-stratified with a mid-depth development of photic zone anoxia and a dense community of photosynthetic bacteria pinkish “bacterial plate”. The early diagenesis of sulfur in sediments overlain by an anoxic water body was investigated using a sediment core (KAI4) from the lake. We determined abundance of various S-bearing species (i.e., Cr-reducible sulfide (= pyrite S: Spy), acid-volatile sulfide (AVS), sulfate sulfur (SSO4), elemental sulfur (S0), and organic sulfur) by an improved sequential extraction method. Here we focus on drastic and rapid changes on sulfur biogeochemistry found in the uppermost 5cm layer.
With increasing depth, abundance of Spy increased but that of SSO4 and δ34S value of Spy (δ34Spy) decreased. These results suggest progressive formation of bacteriogenic pyrite. The δ34S values of SSO4 (δ34SSO4) ranged from 25.1 ‰ (at sediment surface) to 3.8 ‰ in the uppermost 5 cm layer. This δ34SSO4 decrease in the top 5 cm sediment suggests that SSO4 in the surface sediment inherits SO42- with elevated δ34S values (higher than typical seawater δ34S value of 21‰) in the water column, which is due to extensive bacterial sulfate reduction with preferential removal of low-δ34S sulfur as sulfide. In the lower part of the uppermost 5 cm layer, SO42- formed by oxidation of S0, AVS, and/or Spy with low-δ34S values by SO42--bearing seawater introduced by infiltration through the gravel bar. Increasing δ34Spy values with increasing depth suggest near complete consumption of SO42- by active bacterial sulfate reduction, and this process could be explained by Rayleigh distillation model.
Early diagenesis of sulfur does occur in whole section of 25cm-long KAI4 core that accumulated for the last ~60 years (Yamaguchi et al., 2010; Palaeo3). Geochemical characteristics of sulfur in the uppermost part of Lake Kai-ike sediment were significantly modified during early diagenesis. Such diagenetic modification for sulfur isotopes should be fully taken into account to better reconstruct past anoxic environment such as Cretaceous OAEs and Archean oceans.