Unit 3 Geology of the Ocean Part 3

Unit 3 Geology of the Ocean Part 3 Ocean Sediments Geological Oceanography

Sediments are important to marine scientists 2 l Did you know that ocean sediments hold clues about the earth? l They can provide information on earth’s past climate, the location of oil and natural gas resources, seafloor age, pollution patterns and many other processes l There are many types of sediments and we will learn different ways to classify them

How are sediments classified? l 3 By grain size l By origin – Where do you think sediment comes from?

Where does sediment come from? 4

Lithogenous sediments l come from land – – 5 Rivers, wind, ice and other geologic processes erode and transport rocks and minerals into the sea Also called “terrigenous, ” these particles make up most of the sediment near islands and continents Most prevalent in terms of total volume of sediment Sand is the most easily eroded sediment because it is small and non-cohesive (doesn’t stick to the water)

Distribution: Lithogenous (terrigenous) sediment l l l 6 Lithogenous sediments are generally found close to shore except for clay particles Thus, sediments near the shore tend to be dominated by lithogenous material, while deep-ocean sediment tends to be dominated by biogenic material Lithogenous clay particles are relatively light, so they can be carried further from shore by wind, forming pelagic red clay accumulations.

Distribution: Lithogenous sediment l l 7 Ever wonder why most beaches and immediate shorelines are sandy rather than muddy. This is because tides, waves, and currents strongly control continental shelf sedimentation. Shoreline waves keeps small particles from settling – so those small particles (clay) are carried out to sea. The heavier particles (sand) settles out at the shore.

Can you name some common examples? Lithogenous: Sand Clay Red ? Clay ? Quartz 8 Note: Sand gravel are economically important because they are used to make mild abrasives, used in construction, and used in the production of other materials. Photos: USGS, USDA

Biogenous sediments l Come from organisms – – 9 They come from the shells and exoskeletons of marine organisms They cover most of the area of the sea floor

Distribution: Biogenous sediment l l l 10 The biogenic material in the ocean comes primarily from the breakdown of plankton skeletons Deep ocean sediments are usually high in biogenic material Ooze is deep-ocean sediment that has more than 30% biogenic material – calcareous ooze: composed mostly of calcium carbonate plankton skeletons – siliceous ooze: composed mostly of silica plankton skeletons

Can you name some common examples? Biogenous 11 Plankton ? remnants Cosmogeneous Tektites ? Photos: USGS

Where does sediment come from? l Hydrogenous sediments come from chemical reactions in the water – – – 12 Sources may include minerals or chemicals that dissolve in seawater, river runoff or hydrothermal vent water They make up less than 1% of seafloor sediments and tend to form slowly Include Manganese nodules, calcium carbonates, and evaporates

Cosmogenous sediments l Come from space – – 13 Sources may include meteorites and cosmic dust Least abundant of the four types of marine sediments

Cosmogenous Sediments l l 14 Microtektites are glass particles that form when a large meteorite hits Earth. Upon impact, part of Earth’s crust is melted and hurled into space. They are then re-melted as they reenter Earth’s atmosphere. They form raindrop shapes as they plummet through the air.

Where does sediment come from? l 15 Interesting note: The rock limestone can be either biogenous or hydrogenous. When the remains of organisms with calcium carbonate shells are buried, and hardened they form the biogenous rock limestone. When the shells dissolve into water, which then evaporates, the hydrogenous form of limestone is made.

Sediment study tools and Techniques • • • 16 Clamshell sampler: used when scientists need large sample of the top of sediment Clamshell sampler (grab sampler) is lowered to bottom with jaws open, the jaws close and haul lot of material to surface Limits are: captures only top layers of sediment, does not reveal much about sediment layers

Sediment study tools and Techniques l l 17 Piston corer: an open tube on a cable that is dropped from a ship. Sinks into soil from weight of corer. Corer contains a layered sediment sample. Limits are: samples only down to about 25 meters (82 feet)

Piston corer 18

Sediment study tools and Techniques l 19 To get deeper sediment samples, scientists use drilling equipment on specialized research vessels such as the JOIDES Resolution, a drilling ship that has drilled holes more than 2, 000 meters (6, 500 feet) deep.

Sediment study tools and Techniques 20

Sediment study tools and Techniques 21 l Seismic Refraction: technique using an air gun and a hydrophone l Air gun is lowered over side of ship and floats below water surface. Compressed air in air gun sends sound waves through water that penetrate into seafloor. Sound refracts back to surface and hydrophone records the sound. l Different densities of rocks determines the speed of the refracted sound wave.

Sediment thickness in the deep ocean 22 l Sediment layering is not the same across all of Earth’s ocean l Do you think the Atlantic or Pacific ocean basin has greater sediment thickness? Why?

Let’s look at a global map of ocean thickness 23

The Atlantic Ocean Basin has greater sediment thickness 24 l Sediment thickness on the Pacific Ocean floor is about half that of the Atlantic Ocean. l Rivers flowing into the Atlantic cover more land than those flowing into the Pacific, so they bring more sediment.

Deep Ocean Sediments l l 25 Deep ocean sediment that consist of 30% or more of biogenous sediment is called ooze. Ooze is either calcareous or siliceous depending on the type of plankton remains that make up the ooze.

Calcareous ooze l Made of the remains of plankton that had tests (shells) made of calcium carbonate. Foraminifera - animals 26 Coccolithophore - plants

Siliceous ooze l Made of the remains of plankton that have skeletons made of silica. Diatoms - plants 27 Radiolaria - animals

Carbonate Compensation Depth CCD is the depth at which calcium carbonate dissolves as fast as it accumulates, generally considered to be about 4, 500 meters.

Carbonate Compensation Depth (CCD) u Seawater below about 3, 000 meters contains more carbon dioxide than surface water because carbon dioxide becomes more soluble in colder water and under greater pressure. u When calcareous tests fall to 3, 000 meters they begin to dissolve. u The effects of the water acidity, temperature, and pressure all combine to make calcium carbonate dissolve. u Above the CCD, calcareous oozes are the most abundant biogenous sediment. 29 u Siliceous ooze dissolves much more slowly, therefore it accumulates on the seafloor bottom.

Fecal Pellets l l l 30 Because it takes clay particles take 20 to 50 years to sink to the ocean bottom, you would think that material would drift a long way from where it originates and that the ocean bottom composition would be different from the surface water composition. However, they tend to have the same composition. Why? Large planktonic organisms eat calcareous or siliceous organisms. Then when they eliminate their waste, their fecal pellets (which are dense, heavier, and full of calcareous and siliceous remains) fall to the bottom. The fecal pellets reach the sea bottom in 2 weeks, rather than 20 to 50 years.

Continental Shelf Sediments l l 31 As expected, you find coarse sediment near shore and finer sediment toward the sea. However, something strange happens. Suddenly, you find coarse sediment again which leads you to finer sediment as you go seaward. And then the same thing all over again. Why? Fluctuations in the sea level over time.

Recent and Relict Sediments l l 32 Recent sediments are the sediments that have accumulated since the sea level stabilized. In other words, the sediments accumulated right at the shoreline. Relict sediments are the coarse particle deposits of sediments that are further out on the continental shelf bottom.

Ice Rafting l 33 Glacial sediments are deposited on the sea bottom when rafting ice breaks away from a glacier and floats offshore. As the ice melts, the sediment is released and falls to the bottom and accumulates over time.

Continental Shelf Sedimentation vs. Deep Ocean Sedimentation l l 34 The rate of continental shelf sedimentation is usually faster than the rate of deep ocean sedimentation. Why? Rivers carry large quantities of sediment out to the continental shelf. Deep ocean sediments tend to have proportionately more biogenous sediment than continental shelf sediment. Why? Lithogenous sediment, like sand, tends to settle out before reaching the deep sea.

Mineral Nodules 35

Mineral Nodules l l 36 l The ferromanganese nodules are irregular lumps about the size of potatoes. They consist of iron and manganese and small amounts of cobalt, nickel, chromium, copper molybdenum, and zinc. Nodules are hydrogenous sediments produced by very slow chemical precipitation. Nodules grow at a rate of 1 to 200 mm per million years. Scientists are not sure what causes the chemical precipitation but think maybe some organic (biological) material may be involved. Ferromanganese nodules and phosphorite nodules are found in all of the world’s oceans. Although they contain valuable minerals, at this time it costs more to recover them than the minerals are worth.

Stratigraphy l l l 37 Stratigraphy is the study of sediment layers Deep-sea stratigraphy helps scientists learn about rock composition, microfossils, deposition patterns, age of the sediment layers, and other physical properties. Stratigraphy helps scientists understand changes in the ocean and atmosphere, including previous circulation patterns, former sea levels, and trends in biological productivity.

Paleoceanography l l l 38 Paleoceanography is the study of the prehistoric ocean. Paleoceanographers study chemical ratios and radioactive isotopes in microfossils to obtain evidence about prehistoric ocean conditions. Scientists examining the deep ocean sediments called oozes have been able to accurately estimate prehistoric ocean temperatures and climate conditions.

Sediments as Economic Resources l l l 39 Oil and natural gas are the minerals with the greatest economic value of all minerals mined from the ocean. More than a third of the world’s crude petroleum and a quarter of its natural gas come from sedimentary deposits on the continental shelf. Ocean sediments are economically important if the cost of mining the sediment is less than the economic value of the sediment.