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Site U13011

Expedition 327 Scientists2

Background and objectives

Four long-term subseafloor borehole observatories (“CORKs”), two of which are located on 3.5–3.6 Ma seafloor in Holes 1026B and 1027C, were installed during Ocean Drilling Program (ODP) Leg 168 (Davis, Fisher, Firth, et al., 1997). Hole 1026B is located above a buried basement high, whereas Hole 1027C is located 2.2 km to the east, where basement is deeper and sediment is thicker. Integrated Ocean Drilling Program (IODP) Expedition 301 returned to this area in 2004, replaced the borehole observatory in Hole 1026B, drilled two additional boreholes, and installed observatories in Holes U1301A and U1301B (Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005). These holes are located 1 km south of Hole 1026B above the same buried basement ridge, 2.4 km from Hole 1027C (Fig. F1). Hole U1301A was drilled through 262 m of sediment and 108 m into upper basement (Fig. F2). Hole U1301B was drilled 35 m from Hole U1301A through 265 m of sediment and 318 m into basement. Instruments were installed in the wellheads of both CORKs and deployed through the inner CORK casing on a Spectra cable. Instruments placed in Hole U1301A were deployed entirely within casing, whereas instruments placed in Hole U1301B were deployed through the CORK casing so that they were hanging in the open hole in basement (Fisher et al., 2005).

The annulus between the 0.41 m (16 inch) and 0.27 m (10¾ inch) casing strings at Site U1301 was supposed to contain a rubber mechanical casing seal near the seafloor. However, this component was not available for use during Expedition 301, and an attempt was made to seal the 0.27 m casing strings at depth with cement. Rubbly basement prevented the cement from sealing between the casing and the borehole wall. Both of these holes functioned as hydrothermal siphons for several years after Expedition 301, drawing cold water into basement at ~5 L/s (Fisher et al., 2008).

Both Site U1301 CORKs were serviced annually between 2004 and 2010. During servicing, pressure data were downloaded from wellhead loggers and seafloor OsmoSamplers were recovered and replaced. The instrument string originally deployed in the CORK in Hole U1301A was retrieved in summer 2008 using flotation and a winch operated from the deck of the R/V Atlantis and was replaced with a new instrument string deployed from the DSRV Alvin. The instrument string recovered from Hole U1301A provided valuable information and samples related to the thermal, chemical, and microbiological state of the borehole and surrounding formation during the first 4 y after installation (Wheat et al., 2010; Orcutt et al., 2011). Thermal data from Hole U1301A showed that the flow of cold water down the hole varied for the first 3 y after the CORK was installed and then reversed in September 2007, with warm formation fluid discharging up the hole and out into the overlying ocean.

Researchers were unable to recover the instrument string deployed in Hole U1301B during a summer 2009 servicing expedition, despite pulling with the surface winch with >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.

Several attempts were made to seal the Site U1301 CORKs using cement delivered with the Alvin in 2006 and 2007, but these attempts were not successful. IODP Expedition 321T filled the reentry cones around both of the Site U1301 CORKs with cement during summer 2009. The cement appears to have stopped flow in Hole U1301B (as discussed later in this chapter), but shimmering water continued to flow from Hole U1301A at the start of Expedition 327.

The primary scientific objectives at Site U1301 were to recover the CORK instrument string deployed in Hole U1301B during Expedition 301 and to deploy a new instrument string that included some combination of thermal sensors, fluid samplers, and microbial growth substrate. The extent of instrumentation to be deployed in Hole U1301B was to depend on the components recovered with the R/V JOIDES Resolution and the amount of space available in the 4½ inch CORK casing. 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 the Plasma line used on the Atlantis in 2009. 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.

1Expedition 327 Scientists, 2011. Site U1301. In Fisher, A.T., Tsuji, T., Petronotis, K., and the Expedition 327 Scientists, Proc. IODP, 327: Tokyo (Integrated Ocean Drilling Program Management International, Inc.).

2Expedition 327 Scientists’ addresses.

Publication: 5 September 2011
MS 327-104