Inorganic chemistry, gas compositions and dissolved organic carbon in fluids from sedimented young basaltic crust on the Juan de Fuca Ridge flanks

Abstract

The permeable upper oceanic basement serves as a plausible habitat for a variety of microbial communities. There is growing evidence suggesting a substantial subseafloor biosphere. Here new time series data are presented on key inorganic species, methane, hydrogen and dissolved organic carbon (DOC) in ridge flank fluids obtained from subseafloor observatory CORKs (Circulation Obviation Retrofit Kits) at Integrated Ocean Drilling Program (IODP) boreholes 1301A and 1026B. These data show that the new sampling methods (Cowen et al., 2012) employed at 1301A result in lower contamination than earlier studies. Furthermore, sample collection methods permitted most chemical analyses to be performed from aliquots of single large volume samples, thereby allowing more direct comparison of the data. The low phosphate concentrations (0.06–0.2μM) suggest that relative to carbon and nitrogen, phosphorus could be a limiting nutrient in the basement biosphere. Coexisting sulfate (17–18mM), hydrogen sulfide (∼0.1μM), hydrogen (0.3–0.7μM) and methane (1.5–2μM) indicates that the basement aquifer at 1301A either draws fluids from multiple flow paths with different redox histories or is a complex environment that is not thermodynamically controlled and may allow co-occurring metabolic pathways including sulfate reduction and methanogenesis. The low DOC concentrations (11–18μM) confirm that ridge flank basement is a net DOC sink and ultimately a net carbon sink. Based on the net amounts of DOC, oxygen, nitrate and sulfate removed (∼30μM, ∼80μM, ∼40μM and ∼10mM, respectively) from entrained bottom seawater, organic carbon may be aerobically or anaerobically oxidized in biotic and/or abiotic processes.