V. SEDIMENTS 



A. Grain Size 



Figure 7 shows the distribution of the median grain size of northern Arabian 

 Sea sediments. Extrapolation of the isopleths was aided by the recorded data and 

 chart notations previously mentioned . 



Along the western and northern margins of the study area, sand-size material is 

 restricted to the inner shelf. To the east, a large area of sand is found on the inner 

 and outer shelf near the mouth of the Indus and also along the outer Indian shelf. 

 The sand-size material of the outer Indian shelf is largely carbonate whereas the 

 wedge of sand at the mouth of the Indus is largely non-carbonate . 



The median grain size of sediments on the continental slope, rise, and adjacent 

 deep-sea floor is most commonly in the silt and clay range. In the samples observed 

 the sand-size fraction was never more than 5% by weight of the total sample . 

 Samples on the ridge are sometimes coarser than sediments of adjacent depths . 



B. Calcium Carbonate Content 



The areal distribution of the percent of calcium carbonate material in northern 

 Arabian Sea sediments is shown in Figure 8. Except for a band of calcium carbonate- 

 rich sediments (up to 86 percent) on the outer shelf, the Indian and eastern Pakistan 

 shelf sediments are remarkably uniform in carbonate content, ranging from 14 to 19 

 percent. No figures are available for shelf sediments along the northern and west- 

 ern margins of the study area . In deeper water, the amount of calcium carbonate 

 ranges from 17 to 51 percent, the lowest values being found in the Gulf of Oman 

 Basin where concentrations of less than 20 percent carbonate are often present . In 

 the Gulf of Oman, carbonate content ranges between 20 and 35 percent of the total 

 sediment. In general, the amount of calcium carbonate in deep-sea sediments in- 

 creases southward . 



C . Organic Carbon Content 



Figure 9 shows the percent organic carbon content of northern Arabian Sea 

 sediments . The organic carbon content ranges from less than 1 to about 6 percent . 

 In a gross way, the organic carbon content is related to grain size; fine sediments 

 contain the most . Further than this, however, the Gulf of Oman shelf and slope 

 sediments contain unusually high organic carbon values, and a large high is present 

 on the slope off the Indus River Delta . 



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