Geochemistry, mineralogy, and geobiology of subducting sediments at the Costa Rican convergent margin

This four component study of the subducting sediment at the Costa Rican convergent margin brings together geochemical, mineralogical, and geobiological aspects of the shallow subduction zone. As a member of the shipboard scientific party for Leg 205 of the Ocean Drilling Program (ODP), I collected sediment samples from the near trench sedimentary wedge and trench-bound and recently subducted Cocos Plate sedimentary blanket at ODP Sites 1253/1039, 1254/1040, and 1255 offshore Costa Rica. In this dissertation, I preface new data and interpretations with introductory comments, which establish the geological setting and necessary conceptual underpinning for the dissertation research. In Chapter 1, I identify dominant bulk and clay mineral components of the subducting sediment at Costa Rica, and assess how their geochemical budgets respond to diagenesis and dehydration and densification reactions. Chapter 2 provides new data and new interpretations for the subducting sediment geochemistry in order to quantify the subducting flux of elements that are tracers of sediment subduction in volcanic arcs and estimate their recycling efficiency given Costa Rican arc lava compositions. In other words, I use geochemical data to track how a subducted sediment signature survives reaction with and alteration by host rocks and pore fluids in early subduction. In Chapter 3, I report lipid biomarker data for a subset of these deep subseafloor sediments (sampled at depths ranging from ∼150 to ∼440 meters below seafloor), and track how microbial community structure appears to change near the fault interface between the overriding and downgoing plates at Costa Rica. The final chapter examines whether early subduction, with its attendant temperature increase and enhanced pore fluid flow, creates a habitable sub-seafloor environment for microbial life; I discuss contrasting Gibbs Free Energy minimization results for four end-member subduction zones (Costa Rica, Cascadia, Izu-Bonin, and Nankai) with the purpose of identifying potential microbial metabolisms in the deep subseafloor at convergent margins. In all, this dissertation integrates geochemical, mineralogical, and lipid biomarker data with reaction path modeling in order to monitor the biogeochemical evolution of subducting sediments.