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doi:10.2204/iodp.proc.338.201.2014

Samples

A total of 49 (20 cm³ plug) samples were chosen from Hole C0021B from 0 to 0.06 mbsf and from 83.6 to 194 mbsf. Coring was not conducted between 0.06 and 83.6 mbsf. Twenty-nine samples were collected from MTDs: 11 from MTD-A and 18 from MTD-B. The remaining 20 samples were selected to establish the background non-MTD particle size distribution. Moisture and density data for Site C0021 reveals a range of grain densities between 2.55 and 2.77 g/cm³ and an average of 2.73 g/cm³ for all 49 samples. Therefore, a density value of 2.7 was used in Eqs. 1 and 2, and the error associated with using this value is <5%.

Sample preparation

Sample preparation followed the ASTM hydrometer method (ASTM, 2007). We briefly summarize the method as follows:

1. Each sample was removed from its sealed plastic shipping bag, weighed, and placed into a 250 mL beaker.
2. Each sample was soaked for 12 h in 125 mL of 5% solution of deflocculant (sodium hexmetaphosphate [Na₆P₆O₁₈]).
3. Each sample was disaggregated by a malt mixer for 2 min and then wet-sieved through a 63 μm screen to separate sand-size particles.
4. The retained sand was dried and weighed for final percent sand calculations.
5. The remaining <63 μm slurry was transferred to a 1000 mL graduated cylinder and filled with deionized water to 1000 mL.
6. One control cylinder was prepared with 5% solution of dispersing agent and deionized water filled to 1000 mL. We measured the hydrometer reading and monitored the temperature. The laboratory temperature control system maintained a stable temperature environment, and thus there was little variation in viscosity. However, if the temperature fluctuated by 1°C, the resulting change in diameter was approximately 1%.

Hydrometer analysis

Before starting each experiment, samples were mixed for 2 min in the graduated cylinder using a plunger. The timer was started when the plunger was removed and the hydrometer was inserted. Hydrometer readings were taken at 15, 30, and 60 s without the hydrometer being removed. After the 60 s reading, the hydrometer was removed and rinsed in deionized water. The hydrometer was then reinserted, and readings were taken at 2, 4, 8, 16, 32 min, and so on until samples crossed the silt/clay boundary (2 μm), which typically occurred after a total settling time of 10–12 h. We recorded hydrometer readings to the nearest 0.1 g/L. An example hydrometer worksheet and graph are shown in Figures F4 and F5. After each sample crossed the silt/clay boundary, it was dried and weighed for a final <62.5 μm dry mass.

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