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.