Interplay of Subduction Tectonics, Sedimentation, and Carbon Cycling

Abstract

Distinct differences were observed in geochemical signatures in sediments from two sites drilled in the upper plate of the Costa Rica margin during Integrated Ocean Drilling Program (IODP) Expedition 334. The upper 80 m at Site U1379, located on the outer shelf, shows pore water non-steady state conditions characteristic of a declining methane flux. These contrast with analyses of the upper sediment layers at the middle slope site (U1378) that reflect steady state conditions. Distinct carbonate-rich horizons up to 11 meters thick were recovered between 63 and 310 meters below seafloor at Site U1379 but were not found at Site U1378. The carbonates and dissolved inorganic carbon from Site U1379 have a depleted carbon stable isotope signal (up to −25‰) that indicates anaerobic methane oxidation. This inference is further supported by distinct δ34S-pyrite and magnetic susceptibility records that reveal fluctuations of the sulfate-methane transition in response to methane flux variations. Tectonic reconstructions of this margin document a marked subsidence event after arrival of the Cocos Ridge, 2.2 ± 0.2 million years ago (Ma), followed by increased sedimentation rates and uplift. As the seafloor at Site U1379 rose from ~2,000 m to the present water depth of ~126 m, the site moved out of the gas hydrate stability zone at ~1.1 Ma, triggering upward methane advection, methane oxidation, and the onset of massive carbonate formation. Younger carbonate occurrences and the non-steady state pore water profiles at Site U1379 reflect continued episodic venting likely modulated by changes in the underlying methane reservoir.