Sands, and Silts, and Muds, Oh My.

This week I realized that I have focused largely on nematodes and other meiofauna, as well as microplastics, but have not brought attention to the environment I work in. For this week's post, I want to therefore talk about marine sediments, and answer some questions I have received about sand, mud, and the ocean floor.

Two major questions for this week: (1) Why is sand different colors? (2) Why do you talk about sand and mud differently? The answer to the first question is a lot simpler than the second, so I will start with what gives sand its color. You may have noticed from traveling to different beaches around the country, world, or even just within your state, that the sand doesn't often look the same. There are many different colors of sand that you may experience, and within a single beach it is unlikely that the sand is uniform in color. Sand's color often comes from two different components, the chemical composition and the extra material. Sand by nature is a mixture of particles, even though many are familiar with sand by its chemical composition (SiO₂), and based on the items in the mixture sand can have various colors. For example, sand that has a lot of copper may be slightly red in color, and sand that has iron ioxide may be tan or brown in color. If you have seen beaches that are mostly white, the white color comes from calcium carbonate, which is the chemical composition of coral skeletons, shells, and other components of marine animals. Extra particles mixed into the sand, like whole or broken shells, pieces of marine plants, fish scales, etc., may also add some color to sands. Furthermore, oil spills that wash ashore can get mixed into beach sand and cause changes in color, though often the color change is not superficial and requires quite some digging to see.

The reason that I talk about sand and mud differently, is because sand and mud ARE different, and perhaps not in the way you might think. Often when people talk about going to the beach or being in the ocean, they talk about all the sediment as sand, which is likely an oversimplification. Sand is actually a size class of sediment particles, rather than a broad term for all sediment found near or in water. In oceanography we classify sediment particles based on the Wentworth scale, which assigns a name and category to particles based on their diameters. By the Wentworth scale, sand is a moderate size, between ~ 2.00 mm and .06 mm, whereas mud is the smallest particle size, at a maximum size of .0039 mm. In my research, I work with muds and sands, and each size class acts differently in the washing process. Sand tends to clog the sieves because it is coarser, and therefore takes longer to process. Muds, however, are finer, and move through the sieves easier, which means I often end up with less sediment retained on the sieve compared to what I started with. If you go back and look, the Panama City Beach sediments are almost all mud, based on visual inspection and data from USGS surveys in the Panama City Beach waterways. The sediments from the Marine Lab are most likely sand or silt (the size class between sand and mud), based on their coarseness. They [the marine lab sediments] take longer to pass through the sieve, longer to wash, and are more prone to clogging.

If you have any more questions about marine sediments, feel free to leave a comment on the blog or use the new contact me page to send me an email with your questions, and I'd be happy to respond directly or make a blog post to answer you. In the coming weeks I will hopefully be highlighting a new step in the microplastics extraction process--the Fenton Reaction.