During Leg 205 of the Ocean Drilling Program (ODP) two boreholes were drilled into active hydrologic formations on the Costa Rica margin west of the Nicoya Peninsula. One borehole penetrated through the overriding plate into the décollement at ODP Site 1255. The other borehole penetrated through the subducting sediment section and plate into permeable igneous basement at ODP Site 1253. These two boreholes were sealed and instrumented with a borehole observatory (CORK), allowing pressure, temperature, fluid flow velocity, and fluid chemical composition to be measured within the formation. The 1.5-yr pressure and two-year temperature, fluid velocity, and fluid chemical composition records collected to date have provided a basic knowledge of formation properties, although fluids within the boreholes had yet to reach steady state with the surrounding formations. The data also provide evidence that tectonic forcing related to subduction results in measurable transients in pressure, temperature, fluid velocity, and fluid composition within the décollement. While the initial data provide a baseline for approaching several important scientific and technical questions, they have raised new key questions that will be addressed from additional continuous borehole data. The investigators will a 6-day submersible operation to retrieve instruments and stored data that will provide a continuous record of formation temperature, pressure, fluid flow rate, and chemical composition for an additional five years, from the time of the last visit in 2004 to 2009. This project will result in an extremely valuable long-term synchronous record of hydrologic, geochemical, and geodynamic activity at this subduction zone, and establish a technical and scientific foundation for future borehole studies in a broad range of tectonically and hydrologically active settings.
Objectives and Operations Understanding active fluid flow in convergent margins is important because it has a profound effect on the shallow thermal structure and hydration state of the downgoing plate, physical properties of the subduction interface, and the transport of elements and volatiles to the oceans, the volcanic arc, and the deeper mantle. In order to address these questions two hydrological stations, CORKs, were established and instrumented for high-resolution long-term monitoring of pressure, temperature, fluid chemistry, and flow-rates. The CORKs were deployed at two distinct active hydrologic systems, discovered by drilling during ODP Expedition 170, at the Costa Rica subduction zone offshore the Nicoya Peninsula; the CORKs were installed and instrumented successfully during ODP Expedition 205. The two targeted active hydrologic systems are situated in a fractured igneous formation on the incoming plate (Site 1253) and in the décollement zone (Site 1255) (Morris, Villinger, and Klaus et al., 2003). After the first 16 months of synchronous continuous recordings of formation temperature, pressure, fluid flow rate, and chemistry, the pressure data successfully downloaded and the initial fluid samplers, flow meters, and temperature loggers were recovered. To continue the monitoring, new downhole OsmoSamplers and flow meters for 4 additional years (2004-2009) of monitoring were deployed. The key objectives of the new continuous monitoring were (1) to elucidate the short-term dynamic tectonic processes, constrain the pressure state and fluid composition within the décollement at Site 1255, and primarily to capture transients in chemistry, temperature, pressure, and flow rates; and (2) at Site 1253 to constrain the role of the basement as a path for fluid flow from distant igneous outcrops and/or from the consolidating sediment section arcward of the deformation front. Additional OsmoSamplers were deployed in ‘milk crates’ on the seafloor and the top of the CORK at 1253, in order to monitor the conditions at the seafloor. When we returned the pressure and temperature data from both borehole sites were successfully downloaded; unfortunately, this time the fluids were recovered only from ODP Site 1253. The problems with the recovery of the fluid samplers at Site 1255 were reported to NSF by Goeff Wheat. A new set of OsmoSamplers for ~ 10 years, were successfully deployed at Site 1253. The seafloor and top of CORK 1253 fluid samplers were as well successfully recovered Main Findings Shipboard, we subdivided the Teflon coils recovered from the borehole at Site 1253. Except for salinity all chemical and isotopic data were acquired in our shore-based laboratories. The strontium isotope ratios (87Sr/86Sr) and nitrate concentrations in the formation fluids recovered from the upper igneous basement at Site 1253 were of special interest, because the average 87Sr/86Sr ratio of 0.709531 ± 0.000016 obtained in the 2004-2009 deployment was more radiogenic than modern seawater value (0.709173). The average value obtained in the subsequent deployment was practically identical, the difference in the 87Sr/86Sr between the two monitoring periods was only 0.01%. This significant result indicates that a portion of the fluid sampled in the upper basement at Site 1253, is from sediment-fluid interactions deeper within the subduction zone, acward. This suggests that the permeable upper basement serves as an additional pathway for fluids being expelled from the Costa Rica forearc, having important implications for subduction zone hydrogeology. The Nitrate concentrations were also determined in the Site 1253 formation fluids; the concentrations vary largely from below detection to 5.60 mM. The presence of nitrate at elevated concentrations in the upper oceanic basement fluid corroborates the inference from heat flow measurements, fluid chemistry, and pressure measurements that the upper basement is highly permeable enabling rapid hydrothermal flow within the sediment-bound oceanic crust and efficient advective exchange with the ocean. The rapid flow of contemporaneous seawater provides nitrate, a key nutrient, for the deep biosphere residing in the basement and the overlying sediments. The seafloor and top of the CORK Osmo-Sampler fluids were analyzed for the nutrients nitrate, phosphate and silica. The key objective was to help to understand and constrain the ‘surprising’ fluctuations in temperature and pressure at the seafloor, observed, by Davis, Thompson and Villinger (Davis and Villinger, 2006) at ~4km water depth. Their data indicate the presence of episodic flows, auto-suspending turbidity currents initiated by tidal re-suspension of sediments along the shoaling trench floor, southeast of Site 1253. Because nutrient concentrations are not conservative with water depth, their concentrations should help to determine the depths from which the turbidites originated. The results suggest that pore fluids and suspended sediment comprise a portion of the flow at Site 1253, and that tidally induced turbidity currents may be common in the well-mixed regions of the deep ocean.
