Lab 3 GEOL 1350_V2

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Dec 6, 2023

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GEOL1350 Name: ___________________________ Lab Exercise 3 Sediments of the Sea Floor Purpose : To understand the marine and terrestrial processes that govern the distribution of sediments on the sea floor. Materials needed : Colored pencils. Information: The sediments that cover the ocean floor consist of rock, mineral, and organic particles that have been transported by wind, water, or ice. A very small contribution comes from outer space in the form of meteorites. Marine sediments may be classified into five categories based on the origin of the particles: Lithogenous (Terrigenous) sediments : Fine to coarse grains produced by the weathering and erosion of rocks on land. These sediments are introduced to the ocean by rivers, ice, or wind. Biogenesis sediments : Fine to coarse grains that are derived from the shells and skeletons of organisms. These sediments (often called oozes ) are classified as calcareous or siliceous depending on whether they are composed of either calcium carbonate (CaCO 3 ) or silica (SiO 2 ). The primary calcareous (CaCO 3 ) organisms are foraminifera, coccolithophores, and pteropods, while the primary siliceous (SiO 2 ) organisms are diatoms and radiolaria. Hydrogenous (authigenic) sediments : Particles that are precipitated by chemical or biochemical reactions in the seawater or near the sea floor. Manganese and phosphate nodules are common examples. Volcanogenic sediments : Particles that are ejected from volcanoes, and transported to the ocean by wind or rivers. Distinguished from terrigenous sediments due to the distinct source. Cosmogenic sediments : Particles that originate from outer space and commonly tend to be mixed with terrigenous and biogenic sediments in trace quantities. Controls on sediment distribution : The primary controls on the distribution of terrigenous sediments on the ocean floor are particle size and energy at the site of deposition. The grain-size of sediments is determined using the Wentworth grain-size scale (Table 1). The Hjulström diagram (Figure 1) shows the average water velocities required to transport, erode, or deposit a particle. Volcanogenic sediments are a special type of terrigenous sediment. They are governed by the same distribution controls, and most often accumulate near their volcanic source. The distribution of biogenic sediments depends on the biological productivity in the surface waters, dilution on the bottom with other sediments, and for calcareous sediments, dissolution by acidic bottom waters. The carbonate compensation depth , or CCD is the depth below which calcareous particles are dissolved due to an increase in bottom water acidity. In the Atlantic Ocean, this depth is 4500 m, and in the Pacific, its 3500 m. Authigenic sediments occur where sedimentation rates are very low. This limits their development to the deepest and most central portions of the ocean basins. In the same way, cosmogenic sediments are most prominent where sedimentation is slow, and dilution by other sediments is minimal. 1

Questions to Answer 1. (20 points) Use Table 1 to find out the size definition for gravel and for clay, and note your answer in the space below. Using Figure 1, estimate the current velocity required to erode gravel and to erode clay. Given the great difference in their size, why are their erosion velocities similar? What about deposition of gravel vs. clay? 2. a) (20 points) Use the core descriptions (Table 2) and the map of the hypothetical ocean basins (Figure 2) for this question. Plot the data for the percentage calcium carbonate versus depth for cores 1 through 16 on the graph below and number the cores. Use a point for each sample and connect the points for each ocean. If there is no value given, do not plot. b) (10 points) Why is there a sharp reduction in calcium carbonate below 4000m in Ocean A? 2

c) (10 points) Provide an explanation for the difference in depth at which the sharp reductions in calcium carbonate occur in Ocean A and in Ocean B. 3. (20 points) On the graph below, plot a depth profile of each line of cores in Pacific-like Ocean A and in Atlantic-like Ocean B . On the profile, use colored pencils to indicate the dominant sediment type in each core. Use red for clays, blue for biogenic oozes, green for sand, silt, or terrigenous clays, and black for volcanic rock or basalt. 4. (10 points) What happens to calcareous sediments if the ocean become more acidic due to CO 2 uptake by the ocean? 5. (10 points) Core 5 describes 3 meters of red clay over 3 meters of calcareous ooze. How is it possible to get calcareous ooze at that depth and why is it then covered with red clay? (Hint: think about seafloor spreading and what happens as crust moves away from the ridge). 3

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