Tracer Transport on the Eastern Flank of the Juan De Fuca Ridge

Abstract

Much of the ocean floor is hydrogeologically active, but little is known about the physical properties of oceanic crust. Here is a description of the instrumentation and methodology used to obtain a quantitative assessment of the hydrological properties of the upper ocean crust on the eastern flank of the Juan de Fuca Ridge; preliminary results are also included and discussed. Many tracers were pumped into the upper basement around Hole U1362B during Integrated Ocean Drilling Program (IODP) Expedition 327 as part of a single and cross hole tracer experiment on the eastern flank of the Juan de Fuca Ridge. This paper focuses on the use of fluorescence microspheres as synthetic tracers of hydrologic transport. The aim of this 24-hour pumping experiment was to test a large volume of basement rock around Hole U1362B. This paper reports on the design of the tracer experiment, the methods used to prepare and inject the tracers (using shipboard mud and cement pump systems), and the tools developed to permit shipboard and downhole sampling of injectate fluid. Onshore analysis of the raw injection samples (pre-injection into basement) is essential for interpretation of long-term samples collected from subseafloor borehole observatories (“CORKs”). Borehole samples were continuously collected within a long-term CORK installed in Hole U1362B after the tracer injection was completed. Although the arrival of microspheres at collection site U1301A was not yet detected, the vast decrease in microsphere concentration at injection site U1362B in the 11 months following injection indicate that the microspheres are being transported elsewhere. This paper also addresses some possible explanations for this decrease in concentration and the future research needed to provide a more thorough quantitative assessment of tracer transport behavior in the upper ocean crust