IODP 327 tracer experiment: recovery of microspheres in basaltic crustal fluids from IODP Holes 1362B, 1362A, 1301A, 1301B, and 1026B on the eastern flank of Juan de Fuca Ridge

The basaltic ocean crust is the largest aquifer on Earth but little is known about the actual flow paths and rate of large-scale fluid transport in the ocean crust. Integrated Ocean Drilling Program (IODP) borehole CORK (Circulation Obviation Retrofit Kit) observatories provide invaluable long-term opportunities to study hydrothermal fluid circulating within the basaltic crust (basement). The IODP Expedition 327 tracer experiment was designed to provide fundamental information on rates and directions of transport. Multiple tracers, including fluorescent microspheres to simulate the transportation of particles/microbes, were pumped into upper basement via Hole U1362B on the eastern flank of Juan de Fuca Ridge as part of a single-hole and cross-hole tracer experiment. Fluid was collected by autonomous fluid sampling systems OsmoSamplers and 'GeoMICROBE' sampling sled, and a Mobile Pumping System from four additional boreholes (1362A, 1301A, 1301B and 1026B) located within 2km of Hole 1362B. The Osmosamplers began to collect fluid at ca. 1ml per day from Holes 1026B, U1301A, U1301B, U1362A, and U1362B in July 2010. The 'GeoMICROBE' sampling sled collected 0.5L (unfiltered fluids) and 3-8 L (in situ filtrations) monthly from Holes U1301A (07/27/2010 to 12/24/2010) and U1362B (08/30/2011 to 06/30/2012). The Mobile Pumping System (MPS) collected 1L unfiltered fluid from Holes U1301A, U1362A and U1362B during cruises AT18-07 (July 2011) and AT26-03 (July 2013). This study aims to understand the transportability of microbes in the ocean crust by monitoring the dispersion and recovery of fluorescent microspheres. Fluorescent microspheres pumped into Hole U1362B were 0.49 μm and 1.0 μm bright blue (BB), and 1.1 μm yellow-green (YG) spheres. The initial concentrations of fluorescent microspheres injected into Hole U1362B were 3.5 x 107 1.0μm BB spheres/L, 1.9 x 108 1.1μm YG spheres/L and 2.6 x108 0.49μm BB spheres/L. After 11 months, the concentrations of fluorescent microspheres in Hole U 1362B were 0/L of 1.0um BB microspheres, 8 /L of 1.1μm YG microspheres and 4 /L of 0.49μm BB microspheres. No spheres were detected in samples from Hole U1301 ('GeoMICROBE' sampling sled) and Holes U1301A and 1362B (Mobile Pumping System) . These results show that after 11 months the fluorescent microspheres have dispersed out from Hole U1362B but haven't reached Holes U1301A and U1362A. We are making additional measurements of microsphere concentrations on basement fluid samples collected by OsmoSampler, GeoMicrobe sled and MPS during expedition AT26-03. The rates of microspheres dispersion and transport will be calculated and their implications discussed.