we now start our look at biogenous sediment biogenous sediments are derived from the hard parts of living organisms what does that mean think shells or bones or teeth as these organisms die these hard parts will remain and accumulate on the ocean floor but why the hard parts well the soft parts are typically eaten by other things in the ocean and tend to break down a lot faster it's not just limited to smaller organisms large organisms can provide input to biogenous sediments as well this macroscopic biogenous sediment is relatively rare though things like whales the bones of whales could also make up biogenous sediments on the seafloor but it's pretty rare for one think about how many whales are in the ocean right now and then think about how many of them might die every day the numbers are quite low which means that the amount of whale bones falling to the sea floor is pretty small but it's not zero what you see is an image of something called whale fall this is when a whale dies and its carcass settles out onto the sea floor most of the soft parts will be eaten by Deep Sea organisms and the bones and teeth Left Behind become part of the sediment microscopic biogenous sediment is much more common relative to macroscopic most microscopic biogenous sediments are comprised of tests or pills from small dead organisms when you hear the term test again think of shell it's the organism's shell these biogenous sediments can sometimes be referred to as oozes in order for something to be called biogenous ooze it must contain at least 30 percent by weight biogenous test material with the remainder being fine-grained clays and the organisms responsible for contributing to biogenous sediments are algae and protozoans and we're going to discuss some of these later in this video I've mentioned tests and shells already what makes up the test of these organisms and there are two different types that we'll discuss silica in this video and calcium carbonate in the next we'll start with silicious [Music] silica tests are produced by microscopic algae and protozoans called diatoms radiolarians respectively both groups are planktonic organisms meaning that they're free-floating in the ocean they are free-floating in the open water but they don't actively swim diatoms are photosynthetic and therefore located only in the surface waters where light is strong enough to drive photosynthesis and they use Silica to construct their test and these tests can be extremely diverse and actually often beautiful to look at under the microscope radiolarians are also single-celled with silica tests so how do radiolarians differ from diatoms well while diatoms are photosynthetic radiolarians are heterotrophic it means they eat other things in order to acquire energy chiefly bacteria and Plankton in order for a biogenous sediment to be considered salicious ooze it must be comprised of at least 30 percent of the hard parts from silica secreting organisms the ocean is under saturated with silica at all ocean depths so these silicious tests will slowly dissolve over time but if they're slowly dissolving how is it possible to get sediment deposits of silica and it's a good question the basic idea is to generate tests at a rate that is far greater than they can be dissolved this means that environmental conditions must be conducive to growth here's an example of the why diversity but also the beauty of those diatom tests some tests are round some triangular and some are more elongate but they all fit together like the box and lid of a shoebox one half of the test fits snugly inside the other half similar to what you see here we've already mentioned that diatoms are photosynthetic and require sunlight they are also very responsive to the presence of nutrients in the water in fact diatoms are often one of the first groups to respond to nutrient inputs into the water some species can grow dense enough to discolor the water and are referred to as algal blooms diatoms can be found in both fresh and salt water and while some species are planktonic others actually live on the seafloor and called benthic but they live only in Shallow regions where that light can actually penetrate all the way to the bottom so how does a diatom test make it into the sediment when a diatom dies its tests sink in the water column it sometimes makes it to the bottom and other times it dissolves completely before reaching the bottom it could take years for a diatom test to sink to the sea floor is there some way to speed up that sinking process in order to minimize the amount of dissolving yes as we saw in an earlier video it can be eaten when diatoms are eaten by zooplankton the silica test is often excreted back into the water in that form of a fecal pellet and we've seen this picture before but fecal pellets are typically larger than a single diatom and will contain the remains of many additional diatoms these larger pellets will sink to the ocean floor in about 10 to 15 days considerably faster than a single diatom test Sinking by itself silica is an important component of glass production and diatoms are often said to live in glass houses if you imagine a bunch of microscopic crushed glass then you'll start to see why diatoms are used in so many everyday products from filters to abrasives practical Quality Glass even the now retired space shuttles took advantage of the silica tests created by diatoms and last but certainly not least diatomaceous earth is often used as a common natural pesticide are diatoms the only things that create silica tests no if you remember from earlier radiolarians also produce silica tests radiolarians are exclusively found in the ocean and are mostly planktonic they're heterotrophic meaning that they do have to eat other things to obtain energy but they also prefer warm Waters where diatoms will grow warm or cold as a quick recap the ocean is under saturated with silica at all depths so silica will dissolve over time you salicious ooze is commonly associated with areas of high biological productivity at the surface of the ocean how can we measure productivity and deposition in an area by using something called a sediment trap the big yellow cone on the left is an example of a sediment trap the entire structure is lowered to the ocean floor and left there for days to months at a time if you look closely at the bottom you'll see a series of white bottles these are different sample bottles that can be rotated according to a set schedule you can collect sediment for a day a week a month or longer then rotate to the next bottle and keep sampling why the giant cone and this is mainly to increase the amount of sediment captured there likely isn't the large amount of sediment falling in such a short period of time and by increasing the sampling area they can collect more sediment that white baffle looking thing on top is designed to let particles through but reduce the impact of waves or currents flowing into the cone and sweeping those sediments back out and by analyzing the amount and types of sediment the different bottles scientists can investigate changes in the rate of sediment deposition over time in the center picture you see a sediment core and in the image on the right a small sample of the various sediments that can be collected in these things