This study was primarily conducted to determine if a 105ºC drying temperature had overestimated the shipboard water content and porosity values of sediment from Holes 991A, 995A, and 996E during Leg 164. Water contents were determined at sea by drying metal beakers filled with sediment in a convection oven at 105ºC for 24 to 36 hr (Paull, Matsumoto, Wallace, et al., 1996). Those data, in conjunction with the measurement of mass and volume of the dried sediment, were used to calculate downhole porosity (volume of voids/total sample volume) profiles. The porosity values, in turn, were used in a number of other studies. For example, they set boundaries on the amount of gas hydrate that was present in Pressurized Coring System (PCS) samples (Dickens et al., Chap. 11, this volume). This re-examination of shipboard porosity was undertaken after it was suggested by some investigators that a potential existed for a gross overestimation of water content caused by the oven-drying process. The data presented here, determined from samples dried at different temperatures, can also be used in a comparison with more direct measurements of porosity by other methods (e.g., mercury injection).
The effect of drying temperature on water content has been examined previously for a number of soils (Lambe, 1951), but not for modern deep-sea marine sediment. Brown and Ransom (1996) proposed that drying smectite-containing sediment at a high temperature can drive off interlayer water and thereby significantly overestimate water content and porosity. This is increasingly important for deeper sub-bottom sediments where bound water can comprise a majority of the sample's moisture content.
1Paull,
C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP,
Sci. Results, 164: College Station, TX (Ocean Drilling Program).
2U.S. Geological Survey, 384 Woods
Hole Rd., Woods Hole, MA 02543, U.S.A. bwinters@usgs.gov
Date of
initial receipt: 15 April 1998
Date of acceptance: 25 September 1998
Ms 164SR-240