IODP Proceedings    Volume contents     Search
iodp logo

doi:10.2204/iodp.proc.304305.103.2006

Appendix B

Water sampling at bottom of hole

To investigate the physical properties and chemical composition of water in Hole U1309D, two water samples were taken from near the bottom of the hole at ~396 mbsf on 17 January 2005 and at ~1215 mbsf on 16 February.

At the end of Expedition 304, the hole was filled with freshwater to potentially improve resistivity contrast for logging. The pipe was pulled out on 30 December 2004, and the hole was equilibrated for 17 days before the first reentry of Expedition 305. After reentering Hole U1309D, the pipe was lowered without circulation to 5 m above the bottom of the hole (~396 mbsf). Between 0500 and 0715 h on 17 January 2005, the WSTP was deployed. After recovery of the probe, 800 mL of water was drained from the overflow chamber and ~10 mL of water was recovered from the titanium sampling tube within the WSTP. The 10 mL water sample was transferred by the shipboard microbiologist to a sterile container, a 3 mL aliquot was passed to the geochemists, and a 2 mL aliquot was analyzed on board for salinity, alkalinity, and pH. Both samples had a brownish milky color. The water from the overflow container was, therefore, filtered through 0.45 µm cellulose filters, and the filterate was kept for further analyses. After filtration, the sample was split into two 0.5 L polyethylene bottles; one contained 1 mL concentrated HNO3 (trace metal grade) to acidify the sample.

The second water sample was taken after a regular pipe trip on 16 February, 5 m above the bottom of the hole (~1215 mbsf). After recovery of the probe, 300 mL of water was drained from the overflow container and ~15 mL of water was recovered from the sterile titanium sampling tube within the WSTP. The water from the coil was colorless, and the water from the overflow container had a light brownish color. The water from the overflow container was, therefore, filtered over 0.45 µm cellulose filters, and the filterate was kept for further analyses. The 15 mL water sample was transferred by the shipboard microbiologist to a sterile container, and a 5 mL aliquot was handed to the geochemists. A 2 mL aliquot of the overflow container was analyzed onboard for salinity, alkalinity, and pH.

Salinity was measured with a handheld-refractometer (Reichert), and pH and alkalinity were acquired using a autotitrator (Brinkmann). The onboard measurements showed that the borehole water, in both cases, was similar to seawater in salinity and pH (Table AT2). The salinity was measured to be 36‰ in both samples. The pH of the sample from 396 mbsf was 7.7, and the sample from 1215 mbsf showed a slightly lower pH of 7.4. The alkalinity of the water sample from 396 mbsf was 6.1 mm and was very different from seawater. The second water sample from 1215 mbsf had an alkalinity of 2.0 mm, which is close to seawater. Because of problems with the temperature tool and the data recording, the measured temperature profile seems to be unreliable for both sampling runs.

The similarity of the bottom water to seawater argues against a significant influx of formation water. Furthermore, crustal fluids derived from a serpentinization front in peridotite (Früh-Green et al., 2003) would increase the pH. However, a minor influx of formation water can not necessarily be detected using the employed methods. The hole was filled with freshwater (low salinity) and it reequilibrated within 17 days back to seawater salinity. This could be due to the density difference between seawater and freshwater or could be due to convection driven by the geothermal gradient. Most interesting is the high alkalinity of 6.1 mm measured for the water sample from 396 mbsf, taken on 17 January.