Complex Sedimentary and Tectonic Events Captured in Stable Sulfur Isotope Profiles from the IODP Expedition 344

The impact of dynamic sedimentary and tectonic systems on biogeochemical processes, particularly the sulfur cycle, is poorly understood. To better elucidate these relationships, analyses were conducted on sediments collected during Integrated Ocean Drilling Project (IODP) Expedition 344. A primary goal of the CRISP (Costa Rica Seismogenesis Project) expedition is to explore diagenetic processes, e.g. fluid flow; relating to the complex sedimentary and tectonic environments along the Costa Rica margin.

Samples collected from sites U1381C, U1413B, and U1414A record non-steady state conditions in both the solid phase and the pore water profiles, although it is most pronounced in the latter. The penetration depth of pore water sulfate at these sites varies strongly with depth between 100, 15 and several hundreds of meters, respectively. The corresponding hydrogen sulfide concentrations are >400 µM at Holes U1381C, and U1413B while they are <4 µM at Hole U1414A. The measured concentrations of iron sulfides in the sediments indicate that pyrite is the main sulfur-bearing mineral, with concentrations of 2-3 wt. % at sites U1413B and U1414A.

Recorded in the sulfur isotope signal is the likely origin of the heterogeneity between sites. At Site U1414, the ³⁴S isotopically enriched sulfate (δ³⁴S>+60 ‰) is reflected in the δ³⁴S profile of the in situ iron sulfides. We interpret these data as being indicative of fluid flow, potentially along fracture zones, seeps and/or pockmark features seen elsewhere in this region.