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Nature and Distribution of Sediments in the Ocean

Chapter 4

Almost everything that enters the ocean ultimately ends up as sediments on the ocean floor. On the way, particles and dissolved chemicals participate in a variety of complex biological and chemical cycles and interactions that involve some substances at greater rates than others. Recently, deep-sea sediments have become a focus of much research effort, because of the growing need to quantify fluxes of elements within the global carbon (C) cycle. A principal aim is to find out just what happens to the anthropogenic (man-made) CO2 and other gases contributing to global warming (the "greenhouse" effect). If the present trend continues, significant warming, relocation of climatic belts, and sealevel rise will occur. To predict the extent and timing of these changes, it is essential to know more about the rate of increase of greenhouse gases in the atmosphere, fluctuations in oceanic productivity, removal of organic material to the seafloor, exchanges across the air-sea interface, and vertical and horizontal mixing in the oceans.

Sediments are classified according to both size and source:

The size of particles determines how far they can be transported by moving water or air: small grains like clays can reach the middle of the ocean in very slow currents, while pebbles can be moved only by fast moving rivers or waves on beaches. In general larger particles are deposited close to land while the smallest particles dominate sediments in the open ocean.

The composition of sediments identify where they come from (their sources):

Terrigenous sediments come from land — they are the products of erosion and weathering of the continents by wind and rain, glaciers and chemical reactions. Coarse sediments are delivered by avalanche flows (turbidites or underwater avalanches) and ice-rafting, while fine sediments are transported by wind and surface currents.
Mary's Peak was once a submarine volcano, but 45 million years ago it was thrust up onto N. America by subduction. Hence you can see pillow lava flows near top of Mary's Peak, and on top of that are turbidities preserved in sandstone-like layers.

The total river sediment budget is about 1.5 x 1010 tons/year

Biogenous sediments are produced by living creatures (plants and animals) in the ocean.

Pelagic (means open ocean) biogenic sediments are composed of the microscopic remains of predominantly planktonic (drifting) marine organisms that secrete skeletons (tests) of calcium carbonate (CaCO3) - mainly foraminifera and coccolithophores - or silica (SiO2) - mainly radiolaria and diatoms.

Authigenic (also called Hydrogenous) sediments are deposited directly from seawater through chemical reactions. Examples are metalliferous sediments that precipitate from hydrothermal vents (black smokers) and manganese nodules in low sedimentation rate regions.

Volcanogenic sediments come from ash ejected during eruptions, carried by winds and rivers.

Cosmogenous sediments come from outer space, as tiny fragments of meteorites and comets.

Chemical cycles in the oceans

The steady-state ocean (what goes in is removed just as fast) is in chemical equilibrium

Conservative elements are ones whose concentration in seawater is not affected by biogeochemical reactions (e.g., Na), vs non-conservative (C, Ca, Si)

The ocean is mixed about every 500 years (turnover time)

Inputs are: rivers, aeolian (wind), hydrothermal,

Outputs are: sediments, hydrothermal, ultimately subducted

Residence time = total mass of a substance dissloved in the ocean

rate of supply (or removal) of the substance

The biological particle cycle - recycling in the photic zone, transport to seafloor

Gases (N, O, Ar, CO2) enter the oceans through the air-sea interface, and their concentrations are related to temperature, depth, and "age" of water masses

Accumulation of sediments

The geographical separation of calcareous and siliceous sediments is related to the different solubilities of CaCO3 and SiO2 and the chemistry of the water column, as well as the distribution of surface organisms.

Particle settling is governed by Stokes's Law, so biologic "packaging" is important for getting particles to the ocean floor quickly.

Accumulation rates vary tremendously, depending on supply, vertical transport, and preservation: Typical sediment accumulation rates are ~1 cm/1000 yr for the central ocean, abyssal deeps; 1-5 cm/yr for the biogenous ooze dominated regions, and >5 cm/yr for terrigenous deposits along continental margins.

Preservation of pelagic carbonates; CaCO3 is not saturated everywhere in seawater

CO2 (gas) is more soluble in cold water than in warm and increases with pressure (depth) - (that's why sodas fizz when you open the can). Reaction equations in seawater are:

CO2 + H2O = H+ + HCO3- (bicarbonate ion)

HCO3- = H+ + CO3- (carbonate ion)

Dissolution of CaCO3 occurs where increased acidity results from release of H+

CaCO3 + H+ = Ca++ + HCO3-

So, more CaCO3 dissolves when CO2 is high. CO2 is higher in Pacific waters than Atlantic, so CaCO3 should dissolve more readily. Also, because the solubility of CaCO3 in the oceans is depth-dependent, carbonate sediments are preserved in shallower parts of the ocean floor, notably along spreading ridges and seamount chains, but absent from the abyssal plains. CaCO3 is also more soluble in cold water than warm. We use two terms to describe these variations in carbonate preservation:

Lysocline - is the depth at which carbonate skeletal material begins to dissolve.

Carbonate Compensation Depth (CCD) - depth at which the accumulation is ~zero because import rate = dissolution rate.

Preservation of pelagic siliceous remains

Seawater at all depths is undersaturated with respect to SiO2, and preservation on the ocean floor depends on survival during decent (rate) and burial rate (surface productivity).

- CCD is deeper at the equator due to high temperatures
- colder water holds CO2 better, hence is more acidic throughout and no calcarous sediments can survive up there
- siliceous sediments are in areas of high productivity in surface waters (lots of nutrients to feed animals)

A new science is born : Paleoceanography - the remains of organisms can be used to infer past climates.

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