Grain Size Assignment (1)

Grain Size Assignment Overview You will use gravimetric analysis to measure the grain size distribution of a marine sediment. The graduated cylinder contains sediment, 50 mL of dispersing agent, and distilled water. The total volume is 1 L. The total sediment concentration for your sample should be written on the graduated cylinder. Step 1. Collect and record the gravimetric data. You will record your data in the formatted Excel data sheet named “grain_size_data” that you should download from Moodle. Record the sediment concentration on the first tab in cell J1. The sediment concentration is labelled on the graduated cylinder. To begin the experiment, thoroughly stir the sediment slurry in the cylinder with a stirrer for 1 minute. Record the start time in the excel data table from Moodle when you stop stirring. Gently lower the hydrometer into the suspension. Take the first density reading 30 seconds after the start time when stirring stopped. The lines on the hydrometer are marked every g/L. Every 10 g/L interval is labelled with a number; intermittent “5” values have a longer line. You can estimate to the nearest g/L or 0.5 g/L- whichever you are most comfortable with. Subsequent measurements should be taken at 1, 2, 3, 5, 10, 15, 20, 30, and 60 minutes. Make sure you are on the first tab named “Change with time.” DO NOT enter any data on the “Size distribution” tab. Enter the results in the R h column (Column C) of the table (on the “Change with time” tab). Step 3. Computation. The Excel data sheet has been set to calculate several variables automatically. The automatic computations are made using these formulas:  Column D: The effective depth H r is the depth of the center of buoyancy of the hydrometer in cm below the surface. The formula obtained from calibration is H r = 15.99 − 0.184 × R h .  Column E: The average sinking velocity of particle of density R h is the effective depth H r divided by elapsed time t, e xpressed as the sinking velocity ( v ) in mm/s.  Column G: The percentage of total sediment that has settled is given by P = 100 − [ 100 × R h / ( total concentration ) ] . You will need to enter the particle diameter yourself. Read the particle diameter from Figure 9, which shows the sinking velocity obtained from Stokes’ law at room temperature as a function of particle diameter D . To determine particle diameter take the sinking velocity from the Excel spreadsheet and find it on the y axis. Follow that value across the graph until it intersects the

blue line. Follow the intercept down to the x axis to determine the grain size. Note that both axes are logarithmic, with equally spaced increments represented by powers of ten rather than a linear scale (e.g. the difference between 1 and 100 is the same as the difference between 0.1 and 1 on a logarithmic scale). Record each value in Column F in the table on the “Change with time” tab in the spreadsheet. Step 4. Graph the data. Use the data to create two scatter plots on the “Change with time” tab in the spreadsheet: 1. Sinking velocity as a function of time 2. Particle diameter as a function of time Double check your graph using the graphing checklist from the first lab to ensure your graph is complete and of high quality. Use the graphs to answer these questions: Question 1. In your own words describe how the sinking velocity varies with time. Why? Question 2. How does the diameter of sediment in suspension change with time? Why?

highlighted in color to make it easier to imagine where the trend line intersects with the size cutoffs. Don’t forget to subtract. You can double check your answers by making sure the three percentages add up to 100%. Record your percentages below and answer the final question: % sand = % silt = % clay = Question 3. Based on the grain size distribution, explain what type of environment it is from (near shore or deep sea)? Explain your answer. Step 6. Save and upload your files. Upload the Word document with your answers embedded and your Excel spreadsheet containing your graphs.