TOPIC 8: BIOGEOCHEMICAL CYCLING
IN THE EARTH SYSTEM
A) Introduction: One of the key processes within the overall Earth System is biogeochemical cycling. This process moves matter among the four major parts of the Earth System - the atmosphere, hydrosphere, lithosphere, and biosphere. This is analogous to the energy cycling that we considered in the last lecture as part of the Earth's heat budget.
Earth biogeochemical cycles include the cycling of chemically neutral material through the rock cycle and hydrologic cycle. They also include the chemical cycling of specific ions such as carbon (C), nitrogen (N), sulfur (S), phosphorus (Ph), and oxygen (O).
B) Rock cycle: In general, the rock cycle is related to plate tectonics and explains how various rocks are formed, altered, and recycled deep in the Earth's interior.
The part of the rock cycle most important to biogeochemical cycling is weathering and erosion of rocks at the Earth's surface.
C) Water Circulation Within the Hydrologic Cycle: The hydrologic cycle is the movement of water in the hydrosphere. The amount of water involved in the hydrologic cycle is estimated to be 1017 gallons per year.
The hydrologic cycle also serves as a critical element in the transfer of energy throughout the Earth System. Water evaporation absorbs energy and condensation releases energy in enormmous quantities.
The hydrologic cycle also transfers significant quantities of solid matter (aersols) which serve as surfaces for condensation of moisture in the atmosphere. One of the most important aersols is sea salt.
Sea salt gets into the air as bubbles of sea water burst at the sea surface and kick some amount of sea water into the air. The water evaporates and leaves behind small crystals of sea salt which are carried away by the winds.
D) Residence Time: A useful indicator of the rate of movement of chemicals (elements) through different cycles is the concept of residence time. Note that this is a steady-state concept (input = output).
The residence times of selected elements in the World Ocean are Na (110-260 my), K (8 my), Mg (15-16 my), Ca (1.2 my), HCO3 (0.1 my).
1. Sea water has much higher concentrations of all major elements than rivers or other sources of sea water.
a. Rivers have mainly Ca, Mg, Na, and HCO3-.
b. Sea water has mainly Na, Mg, Cl, SO4.
2. Removal of elements: Elements are removed by a variety of process, some including biological organisms. Important inorganic removal mechanisms include chemical weathering or precipitation into deep sea sediments.
E) Chemical Nutrients:
1. Nutrients are inorganic compounds that are used by bacteria and plants as the building blocks for organic material.
2. Nutrients include
a. phosphates (PO4-)
b. Nitrates (NO3-)
c. Ammonia (NH3)
d. Silica (SiO2)
e. Iron (Fe+2, Fe+3)
3. How Are Nutrients Used?
a. Photosynthesis is the chemical combination of nutrients to form organic matter using light as the energy source:
6CO2 + 6H2O + light energy = C6H12O6 + O2
Plants that carry out this chemical reaction have the chemical chlorophyll (or some other similar chemical) which transforms light energy into chemical energy.
b. Chemosynthetic organisms also generate carbohydrates, but they use the breakdown of other inorganic chemicals as the energy source for the reaction.
c. The organic matter is then eaten by other life forms that use oxygen to oxidize and breakdown the organic matter to provide energy (respiration).
C6H12O6 + O2 = 6CO2 + 6H2O + chemical energy
4. Nutrient Cycles
a. Unlike conservative dissolved elements in sea water, nutrients are rapidly used and recycled in water. Much organic material is recycled in the surface water but some sinks into deeper water where it is decomposed and adds to the nutrient supply of deep water. In this manner, nutrients are stripped from productive surface waters over time without replenishment.
b. Since nutrients are not in infinite supply, the least common ones required for life processes are called the limiting nutrients. The most common limiting nutrients are phosphate and nitrate.
c. Most nutrient cycles are part of more general elemental cycles, some of which are discussed below
F) The Carbon Cycle: The carbon cycle primarily traces the movement of CO2 (carbon dioxide) and HCO3- (bicarbonate ion). CO2 in the atmosphere (and all other atmospheric gasses) diffuses into water at the atmosphere/hydrosphere boundary. Either in water or in the air, CO2 is used during photoshynthesis and recovered during respiration. The formulas are generally as follows:
6CO2 + 6H2O + light energy = C6H12O6 + O2 (photosynthesis)
C6H12O6 + O2 = 6CO2 + 6H2O + chemical energy (respiration)
Bicarbonate precipitates as CaCO3 in several hydrologic systems (lakes and oceans).
As happens in all biogeochemical cycles, material sinks such as organic material and limestone (calcium carbonate) remove C from the cycle for periods of time between weeks to millions of years.
G) Nitrogen Cycle: Nitrogen is stable as N2 in the atmosphere. In water and sediment, N2 can be transformed into nitrates or ammonia by organic methods.
1. Nitrates (NO3) are formed in the atmosphere by lightning discharges which oxidize atmospheric nitrogen. Some nitrates are released by volcanoes and soil processes.
2. Nitrates enter the ocean from rainfall and river discharge; the nitrate supply is augments by some nitrates which are formed in the ocean surface waters by bacteria.
3. In the surface waters, nitrates (and other nitrogen compounds) are rapidly taken up by diatoms (siliceous plant microorganisms) and other primary producers during photosynthesis. Diatoms are 'phytoplankton' or the 'grass of the oceans'. They live in the sunlit surface oceans and are the base of the food chains in the ocean.
4. Sinking dead organic matter is attacked by bacteria which decompose the organic matter using oxygen and regenerate the nitrates. Some nitrate is also 'de-nitrified' by special bacteria which remove and use the oxygen leaving behind nitrogen gas (N2).
5. Some organic matter passes though the water column and is deposited in bottom sediments. Part of this is reworked by bottom life and returned to bottom water as nitrates. Bottom water nitrates get back to the Earth's surface as part of upwelling where the process starts again.
H) The phosphorus cycle: Phosporus is an elemental nutrient critical for most biological life.
I) Sulfur Cycle: Sulfur is another element that is key to biological life. But sulfur is also a by-product of industrial processes and is the source of acid rain.
J.) Vertical Distribution of Nutrients in the Oceans.
A. The following sketch shows the phosphate distribution in the water column of an equatorial latitude open ocean site.
B. Note the phosphorus content is nearly zero at the surface because all phosphate is quickly used by phytoplankton. Phosphate content quickly rise below the photic zone because respiration continues without photosynthesis. Also decomposition of organic matter adds phosphate back to the water column.
C. At mid latitudes, there is a more marked seasonality of nutrient levels in the surface ocean. During winter time, less light reaches the photic zone and productivity goes down. Nutrient levels thus rise. During spring time, light levels increase and the added available nutrients create 'bloom' conditions.
IV. Dissolved Gases in Sea Water.
A. All gases present in the atmosphere are dissolved in the ocean in proportion to the atmospheric percentages and the solubilities in water.
B. Surface waters are always saturated or even supersaturated in dissolved gases due to wind action and wave stirring.
C. Some gases are used in biological processes and cycle rapidly in the system as we noted for nutrients. Carbon dioxide and oxygen are the two most active gases in biological processes. Nitrogen is also important as part of the overall nitrogen cycle.
D. For all gases, the saturation level increases as water temperature decreases and as pressure increases. In cold polar areas significant amounts of gases are dissolved and transported to the deep ocean basins.
E. The oxygen content of sea water is shown below.
gas content is high at the surface where mixing occurs. At depth, oxygen is used to oxidize organic matter and carbon dioxide is released. This produces an oxygen minimum zone. Gas content increase with depth thereafter due to deep ocean water masses.
Formation of the Solar System
A. Characteristics of the Solar System that any theory of formation must explain.
1. All planets (except Pluto) orbit in a common plane (the ecliptic) that is close to the Sun's equatorial plane.
2. The Sun and planets all orbit in the same direction.
3. The planets have nearly circular orbits showing a regular progression in their orbital periods (Bode's Law).
4. The Sun contains most of the mass of the Solar system (740 times the mass of all the planets combined).
5. The inner (terrestrial) planets are 'rocky' and composed primarily of Fe, Ni, Mg, Al, Si, and O. The terrestrial planets are Mercury, Venus, Earth, and Mars; many satellites, meteors, and asteroids are also composed of 'terrestrial' material.
6. The outer (jovian) planets are largely gaseous and composed primarily of H, He, C, N, O, and Ar. The jovian planets are Jupiter, Saturn, Uranus, Neptune, and Pluto(?).
7. Meteorites have a similar composition to the sun, except for light volatile (gaseous) elements.
B. Prevailing Theory of Formation of the Solar System
1. The Sun and planets formed initially from a common planar spinning disk of dust and gas starting about 5 billion years ago. The disk was primarily formed of the elements H and He with much smaller amounts of heavier elements O, Ne, N, C, Si, Mg, Fe, S, Ar, Al, Ca, Na, Ni, P, Cl, P (in decreasing proportion).
2. The dust and gas cloud condensed due to gravity, and its spin increased much like a skater pulling in his/her arms; the increase in spin also created turbulent eddies.
3. A protosun formed from the largest eddy through further gravitational collapse; the collapse and radioactive decay of selected elements increased internal heat to the point where nuclear fusion was initiated and the star was 'born'. At this time the most volatile elements were blown to the outer parts of the disk by strong solar winds of radiation.
4. The protoplanets were formed by localized gravitational collapse forming 'planetoids' which later agglomerated together to form the protoplanets. Each protoplanet essentially swept clean its own ring within the Solar System. The asteroid belt is perhaps a relict of this period - an orbital region wherein the planetesimals (asteroids) never agglomerated to form a planet. Alternatively, the asteroid belt may be the residual of a planet that disintegrated.
C. Formation and initial evolution of individual planets.
1. Gravitational contraction, radioactive decay (U, K, Th, Al), and heat derived from impacts caused heating of the planets and selected melting of some elements or compounds.
2. Heavier elements (which melted) migrated towards the centers of the terrestrial planets forming three basic layers : core (Fe, Ni), mantle (Fe, Mg, Al, Si), and crust (Si, Al, Mg). Planetary differentiation was mostly complete by about 4 billion years ago.The oldest known rocks at the Earth's surface are about 3.5 GA.
3. After primary planetary differentiation, the
outermost layers were relatively rich in water and gasses. Degassing
of these components created planetary atmospheres and hydrospheres.
The Earth's Initial Atmosphere
Initial composition:
Nitrogen - about 80%
Carbon monoxide/dioxide - 10%
Hydrogen - 10% (lightest component, easiest to lose at top of atmosphere)
Was there oxygen initially?
Initially, all oxygen present in the atmosphere due to either degassing or chemical reactions would have recombined with Hydrogen, metals at the surface, or reduced gases to form water, metal oxides , or more oxidized gasses.
No free oxgen until its rate of addition was faster than rates of processes to remove it through chemical reactions (oxidation).
Atmospheric behavior:
rotation and solar insolation wuold have led to atmospheric circulaiton
lower solar lumination could have created colder conditions
no Troposphere (because of no ozone layer), so vertical circulaiton would have been much different from today.
higher CO/CO2 concentration might have created stronger grenhouse conditions counterbalancing lower solar heating
strong erosion/weathering
When did free oxygen (O2) appear?
banded iron formations (>1.9 Billion Years Ago (BYA)) required anoxic oceans to let ferrous iron (Fe+2) to be soluble and mobile.
detrital uraninites/pyrites (>2.2 BYA) only stable during erosion/weathering under anoxic conditions
paleosol redbeds (<2.2 BYA) - oldest soils have no redbeds (Fe) in them. Probably leached under anoxic conditions.
Why did free oxygen appear?
byproduct of originof life and development of photosynthesis
first definite fossils about 3.5 BYA, mostly bacteria and algae.
took > 1 BY for biota to generate enough oxygen to permit free oxygen to be stable in atmosphere.
Ozone
ozone must have appeared after free oxygen because it forms by photochemical reactions in the atmosphere.
presence of marine fossils in shallow water environments by about 1.9 BYA probably indicates an ozone layer was present by that time.