Record of redox status in laminated sediments from Lake Titicaca: A sulfur K-edge X-ray absorption near edge structure (XANES) study

Laminated sediments are commonly used to examine limnological and ecological processes on short time scales. Here, we examine changes in sulfur speciation of lamination couplets from Lake Titicaca, South America using X-ray absorption near edge structure (XANES) spectroscopy. The dark-colored layer of each lamination couplet contains a significant fraction of reduced organic sulfides, with smaller fractions of iron sulfides and sulfate. In contrast, the light-colored layer of each couplet is primarily composed of sulfate. In all laminae, the sulfate is present as inorganic sulfate substituted in calcite. The differences in speciation likely reflect changes in productivity during sediment deposition. During the warm, productive summer months, micritic calcite precipitation was induced in oxidized surface waters due to pH changes induced by consumption of dissolved CO2-this calcite incorporates dissolved sulfate and forms the light-colored layers. This pulse of productivity creates organic matter that partially decomposes during the austral winter months, consuming oxygen and delivering organic-rich sediment to the lake bottom. This decomposition, and the possible presence of a halocline, kept bottom waters low in oxygen, and lead to the production of organic matter rich in reduced sulfur. These data suggest that these laminations are deposited annually and illustrate the utility of S K-edge XANES for the analysis of laminated sediments.