Lecture 10: Systems & Cycles II

System's Basic Properties

Dynamic equilibrium

  • Systems are constantly changing, interacting, balancing


  • maintain or turn to stable condition
  • Many systems tend to maintain stable internal conditions (resistance)


  • will be affected but will come back
  • Some systems (not all) recover easily from disturbances

Emergent properties

  • System characteristics not evident in individual components on their own

Interconnected Reservoirs of Systems and Cycles

  • A cycle is a system of two or more connected reservoirs, in which material/energy is transferred in a cyclical fashion ("what goes around comes around")
  • A way of understanding and modelling where substances come from, where they go, where they "reside" in the Earth system, and how they are transferred and transformed.
  • Many natural processes are best described as cycles.
  • Matter and energy in systems and cycles obey the rules of thermodynamics but they behave somewhat differently
  • Matter is recycled through closed environmental systems
  • Energy comes into Earth system, flows through, is used and degraded, and then exits the system.
  • Energy cannot be created or destroyed but it can be degraded and transformed.
  • Reservoirs ("pools") can be defined by physical boundaries (like a "holding tank")
  • The ocean
  • An organism
  • A magma chamber under volcano
  • Or by contents (a "mass" of material)


Cycle Portrayal

Visually, graphically, or mathematically

  • Model is when we portray the characteristics and functioning of a cycle or other environmental process
  • Models of natural cycles and other processes can be physical models, landscape drawings, box models, mathematical models


  • Box models give quantitative info about:
  • Reservoirs (boxes)
  • Contents (numbers in the boxes)
  • Transfer processes (arrows)
  • Fluxes (number on arrows)
  • Box models are the first step in developing mathematical and computer models
  • Each process, flux etc. is described by a mathematical equation


Content of a Reservoir

  • A function of concentration and overall size
  • Content (or burden) of a reservoir is equal to:
  • total mass of a substance in reservoir
  • concentration x mass of physical unit

Content=concentration×mass of physical unitContent = concentration \times mass \ of \ physical \ unit 

Example \rightarrow Content of sodium (Na) in seawater

=10.78g/kg (salinity of seawater)×1.41021kg (total mass of ocean) =10.78 g/kg \ (salinity \ of \ seawater) \times 1.4 \cdot 10^{21} kg \ (total \ mass \ of \ ocean)

=15.1×1021 g=15.1 \times 10^{21} \ g

=burden of Na in seawater (as NaCl) = burden \ of \ Na \ in \ seawater \ (as \ NaCl)  

Transfer Processes

  • Mechanisms that cause substances to move from one reservoir to another
  • Matter is Transferred via Cycles
  • Physical, chemical, biological, geological, or a combination


  • Hydrologic cycle (precipitation, runoff etc.)
  • Rock cycle (erosion, sedimentation, etc.)
  • Sodium cycle (sea spray, evaporation, etc.)



  • Amount of material transferred described by mass or volume/per unit of time
  • Fluxes are flows of matter into/out of reservoirs
  • Example \rightarrow Evaporation of water from ocean surface to atmosphere is ~383 x 1018 g/yr H2O










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