OC103 Lesson #10: The Nature & Distribution of Sediments The types of sediments found on the ocean floor vary depending on where they came from and how they got there. This lesson describes the different factors that determine what type of sediment is found on the seafloor in a particular location, and what sediments can tell us about past ocean conditions. Marine Sediments are: Particles of various sizes from a variety of sources that are deposited on the ocean floor. They provide a recording of geologic, oceanographic, biologic, and climatic conditions, including tectonics, evolution, climate, currents, sea level changes, volcanism, and meteorite impacts. Because the deep sea is not subjected to the weathering and erosive forces typically found on land, the oceanic sediment record is remarkably complete compared to the sediment record on land. Close examination of sediments allows us to classify them based upon their size and origin, which in turn tells us about the ocean conditions at the time they were deposited. Classifying Sediments by Size and Origin • By Size o Geologists classify particles based upon their sizes, which have specific definitions, from largest (boulders at >100mm diameter) to smallest (clay at <0.001mm diameter), with cobble, pebble, sand, and silt in between in descending order. Measuring particle size is important because: o Water velocity determines the size of sediment particles that can be transported. Fast-moving rivers can transport large rocks and boulders as well as smaller particles, but slow-moving rivers and ocean currents can only move small particles. When a river reaches the ocean and spreads out, the drop in water velocity means that it can no longer carry its sediment load, so the sediments drop out of suspension and fall to the seafloor, with the smaller sediment particles staying suspended slightly longer, so they are transported slightly farther offshore (see how particle size decreases with distance from shore in figure at right).Classifying Sediments by Origin • By Origin. Where sediments come from are the other key classification criteria o Terrigenous sediments (from land)  River sediment comes from rivers dumping their sediment loads into the oceans. The figure below left (from your textbook) shows how much sediment the various rivers dump into the oceans each year. The amount of sediment is represented by the size of the arrows in the figure, which makes it obvious that the muddy, sediment-laden rivers such as the Ganges and Amazon are major sources of terrigenous sediments to the oceans.  Winds (eolian). Dust that is blown out over the oceans by windstorms settles into the ocean where the wind dies down (as in the NASA photo below center of the west coast of Africa). This is a common type of sediment in parts of the ocean down wind from desert regions such as North Africa and Central Asia.  Glacial (Ice-rafted debris). Glaciers pick up rocks and other sediments as they grind their way across the land, and when the glaciers reach the sea and break up into icebergs, the icebergs carry the sediments with them as they float around on ocean currents. As the icebergs melt they drop their sediment loads (see figure below right). This is a common form of sediments in oceans surrounding Antarctica and heavily glaciated areas of the northern hemisphere, such as Greenland and Southeast Alaska.  Turbidity Currents (submarine mud avalanches). Where terrigenous sediments pile up rapidly along continental margins they can become unstable and collapse and avalanche down the continental slope and into the deep ocean. These avalanches of mud travel at up to 50 mph, and can carve out submarine canyons on the way down the slope (see figure below far right). When the avalanche reaches the foot of the canyon, it spreads out, loses speed, and dumps its sediment load, creating a sediment deposit called a turbidite.Biogenic Sediments o Biogenic sediments (remains of living creatures)  Many single-celled organisms that live near the sea surface secrete shells from chemicals they extract from seawater, and when these organism die, their shells sink to the bottom, contributing to the sediments on the seafloor (only their shells are likely to survive because their soft parts are either eaten by scavengers or decay away). Different types of organisms make their shells from different substances, which is a clue to their origins:  Some types of organisms make calcareous (CaCO3) shells.  Coccolithophores are plants that make calcareous shells.  Foraminifera are animals that make calcareous shells (see photo below left of a microscopic foram shell about 1mm across).  Other types of organisms make siliceous (SiO2) shells.  Diatoms are plants that make siliceous shells (see photo below right of microscopic diatom shells less than 1mm across).  Radiolaria are animals that make siliceous shells.  The accumulation of these tiny shells in the sediments depends on their production rate by the organisms in the overlying ocean and on their preservation rate once they sink to the bottom.  SiO2 is preserved everywhere, it is difficult to dissolve, so it tends to accumulate in the sediments.  But CaCO3 preservation is variable, and depends on the pressure, temperature, and acidity of the seawater at the ocean floor. CaCO3 is preserved in shallow areas of the ocean, but dissolves away in most parts of the deep ocean.  The depth at which CaCO3 shells raining down from the surface waters are dissolved by corrosive deep waters faster than they can accumulate is called the Carbonate Compensation Depth. Where the seafloor is shallow enough to be above the CCD, CaCO3 shells that fall to the bottom can accumulate, but where the seafloor is below the CCD, CaCO3 shells dissolve away faster than they are able to accumulate.  In today’s ocean, the depth of the CCD varies from about 3000 m deep near the poles to about 5000 m deep near the equator.  All of these variables in production and preservation rates mean that accumulation rates of biogenic sediments on the seafloor vary a lot depending on the location and depth.Hydrogenous Sediments o Hydrogenous sediments form directly from seawater. These sediments are formed by chemical elements that precipitate directly from seawater by chemical reactions. The most common examples are: 