**3. Materials and methods**

The vast tidal flat area of the Ariake Sea, which is 40 % of the total tidal area of Japan, is mainly muddy with high water content. The percentage of clay content is much higher than the sand or silt. To evaluate the temperature variation in different depths of the tidal mud, 5 numbers of thermocouples (Tokyo sokki kenkyojo Co., Ltd. Model no. N004853) were installed at 0.10 m, 0.20 m,0.50 m, 1.0 m and 2.0 m depth which were connected with data logger (TDS- 530) to store the continuous hourly data of the temperature at Higashiyoka tidal flat mud. The sensors were placed about 20 m away from the shore line. The data loggers were kept in a watertight box and put in a small ship which was tied with some anchor and moved upward and downward during the high tide and ebb tide, respectively. Every two days the automatically stored data was collected from the data logger in the ship. This field investigation was carried out from 1st April, 2006 to 8th April, 2006 at Higashiyoka tidal flat.

In order to measure the seasonal temperature variation, the data were collected from both Iida and Higashiyoka tidal flat, 20 m away from the shore line during the ebb tide once in every month. By inserting the thermocouple (3 m long and 0.96 cm diameter) vertically into the tidal flat upto 3.0 m depth and at each 0.10 m interval the data was measured. The thermocouple was connected with a battery and a digital display. The temperature data was displayed directly in degree celcius. The mud samples from tidal flat were collected during the ebb tide and about 20 m distance from the shore line. The sample was then sliced into specified layers in the laboratory to measure various properties in each layer. The sulfide content was measured following the standard method prescribed by the Japan fisheries resource conservation association. The instrument which was used to measure the sulfide content is the GASTEC 201L/H which was also used by Wu et al. (2003) to determine the sulfide content of the marine sediments.

In-situ samples were collected by inserting vertically a thin wall steel tube sampler with a diameter of 0.07 m and a length of 0.90 m at five sites. For sample collection from tidal flat region an amphibious ship was used. The mud samples from tidal flat were collected during the ebb tide and about 40 m distance from the shore line. For sample collection from inside the sea, a ship was used. The ship was stopped in the predetermined location which was fixed by the global positioning system (GPS). The diver dived into the sea and collected the mud samples by inserting the steel tube into the sea bed floor and capped the two openings of the tube. The sample was then sliced into 0.05 m layers in the laboratory to measure the thermal properties in each layer.

The thermal properties analyzer KD2 Decagon Devices, Inc. was used to measure the thermal properties. Thermal conductivity and thermal diffusivity were measured directly from the thermal properties analyzer.


Table 1. Basic physicochemical properties of the samples

In order to measure the seasonal temperature variation, the data were collected from both Iida and Higashiyoka tidal flat, 20 m away from the shore line during the ebb tide once in every month. By inserting the thermocouple (3 m long and 0.96 cm diameter) vertically into the tidal flat upto 3.0 m depth and at each 0.10 m interval the data was measured. The thermocouple was connected with a battery and a digital display. The temperature data was displayed directly in degree celcius. The mud samples from tidal flat were collected during the ebb tide and about 20 m distance from the shore line. The sample was then sliced into specified layers in the laboratory to measure various properties in each layer. The sulfide content was measured following the standard method prescribed by the Japan fisheries resource conservation association. The instrument which was used to measure the sulfide content is the GASTEC 201L/H which was also used by Wu et al. (2003) to determine the

In-situ samples were collected by inserting vertically a thin wall steel tube sampler with a diameter of 0.07 m and a length of 0.90 m at five sites. For sample collection from tidal flat region an amphibious ship was used. The mud samples from tidal flat were collected during the ebb tide and about 40 m distance from the shore line. For sample collection from inside the sea, a ship was used. The ship was stopped in the predetermined location which was fixed by the global positioning system (GPS). The diver dived into the sea and collected the mud samples by inserting the steel tube into the sea bed floor and capped the two openings of the tube. The sample was then sliced into 0.05 m layers in the laboratory to measure the

The thermal properties analyzer KD2 Decagon Devices, Inc. was used to measure the thermal properties. Thermal conductivity and thermal diffusivity were measured directly

Parameters S1 S2 S3 S4 S5 Density (x10-3 kg m-3) 2.71 2.69 2.68 2.69 2.64 Water content (%) 168 235 160 239 253 Liquid limit *w*L(%) 130 150 107 149 142 Plasticity index *I*p 73 87 67 89 88 Ignition loss (%) 11.9 13.3 14.4 12.6 13.7 pH 8.03 7.92 7.60 7.53 7.59 ORP (mV) -40.7 -121.4 128 130 46.38

(x10-3kg kg-1 dry-mud) 0.16 0.42 0.14 0.30 0.49 Salinity(kg m-3) 17 16 20 21 22

Sand 9 7 11 6 6 Silt 36 30 49 46 45 Clay 55 63 36 47 47

Table 1. Basic physicochemical properties of the samples

sulfide content of the marine sediments.

thermal properties in each layer.

Physicochemical

Acid volatile sulphide

Grain size analysis(%)

from the thermal properties analyzer.

The volumetric heat capacity was calculated by the relation: Volumetric heat capacity = Thermal conductivity/thermal diffusivity.
