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doi:10.2204/iodp.pr.327.2010

Scientific objectives

The primary scientific objectives for Expedition 327 comprised work at Sites U1362 (proposed Sites SR-2A and SR-2B; Fisher et al., 2010), 1027, and U1301. Secondary scientific objectives were achieved at Site U1363 (proposed Sites GRB-1A, GRB-2A, and GRB-3A), adjacent to Grizzly Bare outcrop.

Site U1362

We intended to drill and case Hole U1362A (proposed Site SR-2A), the deepest of the two new holes drilled during Expedition 327, through the sedimentary section and the uppermost 100 m of basement, with coring planned only for ~100–260 m into basement. The final hole depth was to be determined by hole conditions and available time. The operations plan included wireline logging with a single string (to assess lithologic layering and properties and locate suitable locations for setting packers), testing for permeability using a drill string packer, and instrumenting the borehole with a multilevel CORK.

We planned to drill and case Hole U1362B (proposed Site SR-2B) through sediment and the upper ~30 m of basement, followed by ~50 m of basement drilling with no coring or logging. A 24 h pumping and tracer injection experiment was scheduled to be completed before the borehole was instrumented with a CORK to monitor a single basement interval. Both of the Site U1362 CORK designs included instruments to monitor formation fluid pressure and temperature, sample fluids (using downhole and wellhead OsmoSamplers), and provide growth substrate for microbes inhabiting the basement aquifer.

Site U1301

The primary scientific objectives at Site U1301 were to recover the CORK instrument string deployed in Hole U1301B during Expedition 301 and deploy a new instrument string that included some combination of thermal sensors, fluid samplers, and microbial growth substrates. The extent of new instrumentation deployed in Hole U1301B would depend on the amount of instrumentation recovered and the amount of space available in the 4½ inch CORK casing. Researchers were unable to recover the instrument string deployed in Hole U1301B in Summer 2009 from the R/V Atlantis, despite pulling with a surface winch at >5000 lb. Apparently, the instrument package was being held in the open hole below the CORK, probably because the borehole collapsed after the instrument string was originally deployed. We were hopeful that some instruments would be recovered from Hole U1301B because the coring line on the JOIDES Resolution can be pulled with greater force than could the Plasma line used on the portable winch on the Atlantis. We also planned to complete a depth check of open casing in Hole U1301B (unless we were fortunate enough to recover the entire instrument string) and collect thermal data inside the CORK to evaluate the thermal state of the borehole surrounding the CORK installation following Expedition 321T cementing operations.

Site 1027

The primary scientific objectives at Site 1027 were to recover the existing CORK in Hole 1027C, core and deepen the hole by ~40–50 m, run hydrologic tests of the open hole, and deploy a new multilevel CORK for monitoring, sampling, and associated experiments. The CORK in Hole 1027C was installed during Leg 168 and originally contained a data logger, pressure sensors, thermistors at multiple depths, and a fluid sampler. These instruments were retrieved in 1999, and the pressure logging system was replaced. The CORK in Hole 1027C was supposed to be replaced during Expedition 301, but problems setting the CORK in Hole U1301B and a lack of time and materials prevented completion of any Hole 1027C operations during that expedition. Hole 1027C was fully sealed and continued to record formation pressure before, during, and after Expedition 301 (Fisher et al., 2008).

Site U1363

Site U1363 (proposed Sites GRB-1A, GRB-2A, and GRB-3A) was drilled adjacent to the northeastern edge of Grizzly Bare outcrop to test the hypothesis that this outcrop is a site of regional hydrothermal recharge. A short transect of holes extending radially away from Grizzly Bare outcrop was expected to show an initial warming and a loss of oxygen and nitrate in basement fluids. Little change was expected for many of the major ions, even with elevated basement temperatures, because of the slow rate of reaction and the short residence time of formation fluids. The microbial population was expected to initially be predominantly that found in bottom seawater. Farther from Grizzly Bare, as dissolved oxygen and nitrate are depleted, we anticipated a decrease in sulfate as a result of diffusive loss to the overlying sediment during rapid lateral flow in basement. Observable changes were expected in additional minor elements and in the microbial communities seen in sediments immediately above basement.

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