chapter 5 sediments

The roads, buildings, freeways, etc. that we all enjoy.

 Distance from sediment source 

Energy regime 

Nature of the sediment source

– Whatitsmadeof

In general, only the smallest particles make it to the ocean (sand, mud)

Sediments are primarily classified by composition (what makes them) and by grain size (how big are the particles)

1. Terrigenous sediments (derived from erosion of the terra, the Earth) are the most abundant by volume in the ocean (>87% of volume, 45% of ocean floor).

That is why sediments are thickest near the continents (and consist mostly of silt and sand.)

• Sediments are carried from their sources primarily by the activities of currents.

– At beaches, this is primarily by LONGSHORE DRIFT.

– In the deep ocean, this is primarily by turbidity and deep ocean currents.

The energy regime of deep sea currents stays fairly consistent over relatively long time scales.

– Their strengths and directions are impacted by global processes such as climate.

• The energy regime of longshore currents can change seasonally, weekly, daily, or even hourly.

Debris flows off continental shelf into deep water

Turbidity currents provide much of the fine clay that makes it into the deep basins.

sediments formed from remnants of organisms

Shells get broken up at beach and tiny fragments may get transported to the deep ocean

* an ooze is a really gloopy sediment composed of at least 30% biogenous material

are created by the accumulation of skeletal debris

another name for a microscopic shell) are made of either:

silica (SiO2)-------> siliceous ooze or calcite ------->calcareous ooze

Calcareous ooze is NOT found everywhere because deep seawater contains more carbon dioxide (CO2) which makes the water slightly acidic. Below the CCD, calcium carbonate dissolves.

Some continental shelf sediments make it to the continental slopes.

mostly terrigenous sands, silts, clays

Deep ocean (aka pelagic) sediments= Terrigenous clays, biogenous oozes

3. Hydrogenous sediments (produced by water) are precipitated directly from seawater (<1% by volume).

is a chemical process involving the joining of ions in solution to form a compound that becomes a solid particle. Example:

Ca+2 + CO3-2 --> CaCO3 (calcite)*

As seawater evaporates, the dissolved minerals

get more concentrated and begin precipitating.

4. Cosmogenous sediments - interplanetary dust (IPD) and micrometeorites (<<1% by volume typical sediments) (about 15000-30000 metric tons of IPD enters the atmosphere each year)

tiny glassy particles resulting from melting during a meteorite impact.

RARE, but VERY important for evidence of big meteorite (asteroid) collisions with Earth.

• Different environments preserve different sediment types in a variety of fashions.

– Can use the sedimentary record to resolve the types of environments sediments are deposited in

Because the nature of sediments is a function of BOTH the original material they are derived from AND the processes that lead to their deposition, they record evidence of their SOURCES and their DEPOSITIONAL ENVIRONMENTS.

Processes in the past behaved like the processes we see today (Laws of nature do not change; the present is the key to the past.)

Sediments Are Historical Records of Ocean Processes

Historical information (climatic and biological change) Location of natural resources, especially crude oil and

natural gas

Marine sediments are important as historical records and a site of natural resources. Scientists study marine sediments using many different methods, including core samples.

Oldest sediments from Isua, Greenland 3.8 billion year old = 3800 million years

1. Sediments contain a history of the oceans but we must be able to construct a temporal framework by dating the sediments.

•Relative ages (which is older and which is younger) are established by comparing the layering patterns of sediments

-lowest lying layers were deposited before those lying on top

(how many years before present was the sediment deposited) can only be determined with radiometric dating (radioistopes) on volcanic ashes deposited within the sediment Layers

-dating is not performed on the sediment grains themselves

• Stuff that cuts other stuff is younger than what is being cut.

Stuff on bottom in place before stuff on top.

The half-life is the amount of time for 1/2 of the radioactive element to decay to its daughter product.Radioactive elements are unstable and decay: Uranium 238 --> Lead 206 Potassium 40 --> Argon 40 Carbon 14 --> Nitrogen 14

Half lives 4.5 billion years 8.4 billion years 5,700 years

Particles of other rocks!!

NO!!! It will give us only the age of the formation of the rocks from which the particles came.