Early diagenetic processes of saline meromictic Lake Kai-ike, southwest Japan: III. Sulfur speciation and isotopes

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: S_py), acid-volatile sulfide (AVS), sulfate sulfur (S_SO4), 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 S_py increased but that of S_SO4 and δ³⁴S value of S_py (δ³⁴S_py) decreased. These results suggest progressive formation of bacteriogenic pyrite. The δ³⁴S values of S_SO4 (δ³⁴S_SO4) ranged from 25.1 ‰ (at sediment surface) to 3.8 ‰ in the uppermost 5 cm layer. This δ³⁴S_SO4 decrease in the top 5 cm sediment suggests that S_SO4 in the surface sediment inherits SO₄^2- 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, SO₄^2- formed by oxidation of S⁰, AVS, and/or S_py with low-δ³⁴S values by SO₄^2--bearing seawater introduced by infiltration through the gravel bar. Increasing δ³⁴S_py values with increasing depth suggest near complete consumption of SO₄^2- 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.