4.2 Grain size Analysis
For quantitative grain size analysis, samples were prepared using standard procedures as described by Howell et al. [2014]. We put a small amount of sample into a cleaned 50 ml plastic centrifuge tube and added 5–7 ml of sodium phosphate solution. The tube was capped and vortexed to deflocculate clay-sized sediment and separate organic particles. The sample was poured through an 850 µm sieve and funneled into a 15 ml glass test tube. After centrifuging and removing the clear supernatant, 2–3 ml of sodium phosphate and 5 ml of 30% H2O2 were added. Tubes were vortexed again and then put into a hot bath that was heated to 70˚C. This step requires persistent monitoring to prevent loss of reactant by spraying it with acetone until the reaction is stabilized. Reactants then sat overnight to completely oxidize organic matter. Reacted supernatant was removed, and 5 ml of sodium phosphate was added. These treated samples were then rinsed with deionized water, transferred into clean 50 ml plastic centrifuge tubes, and topped with sodium phosphate into a sample solution of up to 40 ml. Samples were vortexed again prior to grain size analysis. Grain size analysis was conducted on a Beckmann Coulter LS13 320 laser diffraction particle size analyzer at LSU. The obscuration of all running samples in the aqueous liquid module (ALM) was between 8–12 %. Result of the analysis are provided in Table 2.