# Lecture 10: Systems & Cycles II

### System's Basic Properties

#### Dynamic equilibrium

• Systems are constantly changing, interacting, balancing

#### Homeostasis

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

#### Resilience

• 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

#### Quantitatively

• 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

﻿$Content = concentration \times mass \ of \ physical \ unit$﻿

Example ﻿$\rightarrow$﻿ Content of sodium (Na) in seawater

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

﻿$=15.1 \times 10^{21} \ g$﻿

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

### 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

#### Examples

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

### Flux

• 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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