I. Introduction

A. If you've ever used a chalkboard in school then you've had your hands covered with marine sediments from the deep sea; or should I say, former marine sediments.  Chalk is made up of countless tiny particles known as coccoliths. These particles accumulate on the ocean floor and may eventualy be consolidated to form the soft sedimentary rock called "chalk".

 B. The thickest accumulations of sediments in the world are in the oceans.  All but 8% of the world's sediment is in the ocean in piles up to 9 km thick.  The thickest accumulation are on the continental slopes and rises.

C. These sediments on the continental margins form mostly from the weathering and erosion of igneous, sedimentary and metamorphic rocks exposed at the Earth's surface (TERRIGENOUS = LITHOGENOUS). The rocks that make up the crust of the Earth usually form at different conditions than those at which they may eventually find themselves after tectonic activity and mountain building.  Therefore, they tend to breakdown under these new conditions to form sediments that are stable at Earth surface conditions.

D. But other marine sediments form from accumlations of the remains of dead marine organisms (BIOGENOUS), from extraterrestrial particles (COSMOGENOUS) and from chemical reactions that cause solids to precipitate from sea water (HYDROGENOUS).   

E. Besides their origin, sediments also differ as to texture (Texture -  the size, shape and sorting of grains in the sediment), color, and composition. We will also talk about these aspects of marine sediments and finish up with a discussion of the distribution of sediments in the sea.

 

II. Sediment Transport

 

A.  All terrigenous sediments must be transported from land to be deposited in the ocean and most biogenic sediments are produced by organisms which live near the sea surface and must therefore undergo transport to reach the sea floor.  Only remains of bottom-dwelling organisms and hydrogenous sediments may be formed where they are deposited, and even these may be transported by bottom currents. THEREFORE, SEDIMENT TRANSPORTATION IS CRITICAL IN DETERMINING THE TYPE OF SEDIMENT OCCURRING IN A PARTICULAR AREA.

 

B. Effect of grain size

 

1. The rate and manner in which sediment particles are transported to the sea, and the rate at which they sink to the sea floor is controlled by their size,  so it is important to classify sediments according to particle grain size.  Also particle size tells us something about how much energy was necessary to carry that particle and, therefore, tells us something about the environment in which the particle was transported and deposited.  FAST-MOVING WATER = HIGH ENERGY = LARGER PARTICLES.

 

     a. The most commonly used scheme for classifying      sediments according to grain size is shown on p.

     82 of your textbooks. Sediment grains vary from      submicroscopic (1/4000mm) to boulder-sized (>256 mm in      diameter).

     b. We'll only talk about gravel, sand and mud- sized

     grains:

           1) Gravel    > 2mm

 

          2) Sand 1/16 mm < X < 2mm

 

           3) Mud  <  1/16 mm

           

          4) Grains larger than sand-size do occur along

            some high energy, rocky coasts, but the other

            size categories are by far more abundant.

 

2. In general, big particles sink faster than small ones.

     a. Really large particles such as large gravel sink so      rapidly that they are rarely transported in suspension       but mainly by being bounced and dragged along the      bottom.

     b. Smaller grains can be kept in suspension by tur-

     bulent water motion.  Once, the level of turbulence,

     and usually the water velocity drops, they too begin

     to settle to the sea floor.

 

C. Transport mechanisms

 

1. Rivers  -  Most terrigenous sediment (85%) is transported to the sea by rivers, although the quantity of sediment carried by different rivers varies tremendously.

 

     a. The amount of sediment transported depends largely

     on topography and climate.

 

           1) Climate controls the relative importance of

           physical versus chemical weathering, and the

           types of vegetation present.  It also controls

           the amount of water available to transport the

           sediment.

     b. At the current time in Earth history, due to

     retreating glaciers and the resultant rising sea level

     most rivers deliver their sediment to estuaries where

     it is often trapped.  However, during times of low

     sea level stands, huge volumes of sediment are de-

     livered to the continental margins.

 

2. Ice  -  Nearly 10% of terrigenous sediments are transported to the ocean in ice.

     a. Ice bergs do the work and the process is called

     ice-rafting.

           1) These types of sediments are poorly sorted

           (i.e., showing a lot variation in grain size)

           and the particles are angular.

          2) In other oceanographic environments physical

           processes can sort sediments according to grain size. 

           This is very typical of beaches where continuous wave

           action sorts sediment into   different size  fractions in different regions of the beach.Wave energy also smooths and rounds the surfaces of grains by abrasion and by breaking off of rough edges.

3. Wind  --  A little less than 3% of terrigenous sediment is transported to the deep sea as wind-blown (aeolian ) dust, however in some portions of the deep sea the sediment is dominated by such terrigenous sediment.

 

      a. Especially in dry regions (30oN and 30oS latitude)

      with persistent wind patterns (Trade Winds) aeolian dust is important.

           1) Arabia, Austrialia and North Africa are impor-

           tant sources of such dust.

           2) The high altitude atmospheric circulation (Jet

           Stream) is also important in transporting this

           dust and influencing its distribution. (Particles

           < 10 microns).

 

4. Biological rafting

           a. Kelp in hold-fast

           b. Animals that swallow sediment

 

5. Transport from sea surface to sea floor

 

     a. Most of the terrigenous particles that arrive at the sea surface destined to become sea floor sediments are very fine-grained.  The skeletons of marine organisms responsible for biogenic sediments are also mostly very small in size. As a result, these particles should take months or even years to settle down through the water column to the deep sea floor.During the long period of time required for sinking, ocean currents could be expected to redistribute the particles over hughe areas of the sea.  The resulting pattern of sediment distribution on the se floor should bear little resemblance to the pattern of sediment particles delivered to or produced in surface waters.  However, just the opposite is true, the dist;ribution patterns of sediments on the sea floor closely resemble thepatterns of distribution of particles at the se surface.

 

           1)Aeolian dust is found downwind of arid regions.

           2) Biogenic sediment particles tend to occur on

           the sea floor directly under the areas where the

           the contributing organisms are found in large

           numbers.

 

     b. BIOPACKAGING is responsible for this correspondence

 

           Filter-feeders ingest small particles and package

           them into their feces.  These fecal pellets are

           large enough to sink much more rapidly to the

           sea floor.  Studies have shown that virtually

           the coccoliths I mentioned before were delivered

           to the sea floor packaged in fecal pellets.

 

6. Turbidity Currents  -- Not all terrigenous sediments that reach the sea floor sink from the sea surface. Huge quantities of sediment are carried along the bottom in turbid suspension by underwater avalanches known as turbidity currents.

 

     a. Imagine looking up and seeing a cloud of mud, sand,      and gravel hundreds of meters high moving toward you      at 55 km/hr (>30mph). No one has ever actually seen  a

     major turbidity current but there is ample evidence that they occur.

 

     b. In November of 1929, a large earthquake occurred

     off of the Grand Banks of Newfoundland, Canada. 

     Several undersea telegraph cables linking Europe and

     NOrth America cross this area.  At the time the quake

     occurred a few cables broke immediately, and it was

     assumed that they had been cut by the quake.  However,

     an additional 23 cables broke during the 12 hours

     after the quake.  Each of these later breaks was

     progressively deeper and further from the epicenter. This

     mystery was finally solved in 1952 when oceanographers

     found evidence linking the later cable breaks to the quake

     via turbidity currents.

 

III. Sediment Sources

 

A. Already talked about terrigenous or land-derived sediments.  The majority of oceanic sediments are of this type because of the severe conditions encountered on the continents exposed to the ravages of temperature and the atmosphere.

       1. Due to their proximity to the sediment source the

       continental margins have the thickest and most             rapidly accumulating piles of terrigenous sediments.

 

       2. Some very fine-grained terrigenous sediments          (clays) can be carried by wind or water into the           abyssal  regions of the oceans.

 

       3. They make up about 20% of oceanic sediments.

 

B. Biogenic sediments - Sediments in which the grains are formed by the action of a living organism. Shells, tests, and other hard parts secreted by organisms that fall to the bottom of the ocean and slowly accumulate.  When the biogenic component makes up more than 30% of the sediment the sediment is called an ooze.  Oozes composed of the hard parts of various organisms occur in the deep ocean.  They are not very abundant on the continental margins due to dilution by terrigenous sediments. Oozes dominate 62% of deep ocean.    

 

C. Hydrogenous sediments - sediments formed by chemical precipitation of the components dissolved in seawater.

Very minor component.

     1. Evaporites = salt crystals that form when seawater

     evaporates.  Can contribute to very thick piles of

     sedimentary rocks in arid, shallow-water marine

     environments (i.e., Persian Gulf, Red Sea, Mediterranean

     Sea).

           a. At one time the Mediterranean Sea was a desert

           b. Halite, gypsum and calcite  (whiting)

     2. Metal sulfide deposits at mid-ocean ridges.

     3. Manganese nodules which may one day be mined for Cr,

     Mn, etc.

     4. Phosphorites

     5. Some clay minerals around mid-ocean ridges.

D. Cosmogenous - extraterrestrially-derived sediments.

  Very, very minor from meteoritic debris.

 

IV. Distribution of sediments

 

  A. Continental shelf sediments

 

       1. Dominated by terrigenous input.

 

       2. Much of the sediment that reached the shelves in                times of lower sea levels is now accumulating in      drowned river systems called estuaries.  The Pamlico-     Albemarle  Rivers and Sounds are examples of estuarine      systems.

 

       3. In many places large areas of the shelves were

       exposed during times of lower sea levels and have,

       therefore, been subjected to other than the normal

       submarine processes.

 

       4. Biogenic carbonate sediments dominate in regions

        where terrigenous input (silica sand, silt and

        clay) is minimal such as central and southern

        Florida. Also within 30 degrees of the Equator

        where coral reefs are abundant they contribute

        extensive deposits of debris to shelf sediments

        and beaches.  Also in a few places algae

       accumulate extensively in mats trapping sediment

       grains & form large deposits of sediments.

     

B. Continental slope and rise sediments - again mostly terrigenous sediments transported from the shelf

  1. The infamous turbidite deposits of size sorted sand, silt and clay.  Fast-moving, sediment-laden masses of water from the continental shelf roar down submarine canyons and slopes to deposit thick accumulations of graded beds on continental rises -- often in the form of alluvial fans. 

 

C. Deep ocean sediments - Here we begin to see a much larger contribution from biogenic sediments. 

  1. In fact, one of the two main contributors to deep ocean sediments are the tests of microorganisms that settle to the bottom of the sea when the organisms floating in the surface waters die.  When these tests make up greater than 30% of the sediment it is called an ooze.

 

 2. Biogenic oozes accumulate very slowly in the deep ocean.  This is because the surface waters of the central oceans are very poor in the nutrients (mostly land-derived), such as nitrogen and phosporus, that are required by the surface sea creatures.  Therefore these waters are inhabited by only small populations which contribute very slowly to the development of the deep ocean sediment accumulation.  Also in some regions of the oceans the tests of these organisms redissolve before they reach the bottom.  In these regions the sediments are dominated by abyssal clays.

 

  3. There are two major types of Oozes - siliceous and calcareous.

 

       a. Siliceous - i.e. SiO2 oozes are made up of the

       tests of floating (planktonic) organisms that

       extract silica from seawater to make their hard

       parts.  The most abundant of these are the diatoms     

      (plants) and the radiolarians (animals).

 

         1)Nowhere in the oceans does silica precipitate     

           spontaneously without the intervention of an organism.

          Therefore, the tendency for silica is to dissolve     

           everywhere it occurs in the oceans.  So, the only     

           regions  in which siliceous oozes are abundant are in

          regions where the nutrient supply is so large that

          diatom and radiolarian tests accumulate faster than 

         the seawater can redissolve them after death.

 

       These regions are along the Equator in the central

       Pacific and in high latitudes near Antarctica.  The

       high dilution by terrigenous sediment input and ex-

       tensive ice cover in northern latitudes inhibiting

       high biological productivity limit siliceous ooze

       accumulation in northern latitudes.

 

         2) Dominate about 14% of deep ocean.

 

       b. Calcareous oozes - CaCO3 made up of the tests of

       floating (planktonic) organisms that extract CaCO3

      from seawater to make their hard parts.

      Coccolithophores (plants) and foraminifera (animals)

 

            1) CaCO3 precipitates spontaneously in some           

             oceanic regions without the intervention of an

            organism (WHITING). In warm tropical surface

             waters CaCO3 does not  readily dissolve.

             However, in colder deeper waters the presence

             of increased amounts of CO2 in the water

             enhances the dissolution of CaCO3 causing

            the breakdown of calcareous tests.

 

  CaCO3 (s) + H2O (l) + CO2 (g) =  Ca2+ (aq) + 2 HCO3- (aq)

 

       The carbon dioxide and water combine to form

       carbonic acid which dissolves the  CaCO3.  As we

       will see when we talk about the distribution of

       water masses in the deep oceans, the deeper water

       masses form at the surface in cold climates at high

       latitudes and sink towards the bottom where they

       remain for most of their residence in the oceans.

       So, at high latitudes CaCO3 dissolves at all water

       depths. At lower latitudes CaCO3 dissolves at depths

       in the ocean where it encounters these CO2-rich

       water masses.  The depth below which calcareous

       skeletons dissolve as fast as they accumulate is

       called

 

       THE CALCIUM CARBONATE COMPENSATION DEPTH (CCD)

 

        In warm latitudes the CCD occurs at 4-5

        kilometers. Therefore, calcareous oozes will be

        found only at depths less than 4-5 kilometers.

        Where the bottom of the ocean is deeper than 4-5

        kilometers calcareous tests will not accumulate.

        Calcareous oozes, therefore, are found mostly on

        the oceanic ridges and plateaus.

 

  4. The other major contributor to deep oceanic sediments are clay minerals.  The so-called abyssal or pelagic clays are extremely fine-grained particles that have remained in suspension for great distances from the continents.

 

  5. Two other components of deep ocean sediments are very minor but may someday be commercially very important.

       a. Manganese nodules and  Metal sulfide deposits

 

  6. General distribution of deep ocean sediments

 

       a. Age and thickness of sediment increases away from

       ridges.  Also far from the ridges the ocean is

       closer to sources of terrigenous sediments.

 

       b. Terrigenous sediments dominate on the continental

       margins and in the highest latitudes where ice cover          

       restricts biological productivity.

 

       c. Siliceous oozes dominate in highly productive

       waters near the Equator in the central Pacific and

       north of Antarctica between 50 and 65o S.

 

       d. Carbonate oozes dominate in temperate and

       tropical climates at depths less than 4-5 km.

 

       e. Abyssal clays dominate deeper oceanic regions.

 

V. Collection of samples from the ocean

  A. Most sediment samples are retrieved from the ocean floor from a ship floating in the overlying surface waters.  Samples have been collected from all depths up to thousands of feet.    

  B. Dredges or grab samplers sample the surface sediments

  C. Corers sample a vertical section of the subsurface sediment without distorting the layering.

       Piston corers = retrieve cores from deeper sediments

       Box corers

       Gravity corers

  D. Drilling ships = highly specialized ships that can maintain very precise positions in deep water without the need to anchor.  Special engines maintain position.  Can drill in very deep water and retrieve undisturbed cores of a few kilometers thickness.  GLOMAR CHALLENGER JOIDES RESOLUTION.

       The Deep Sea Drilling Project (DSDP) was a major drilling project undertaken by the USA to investigate the sediments and rocks of the ocean basins and to uncover its history.