NCEA Level 3 Earth and Space Science 91413 (3.4) – Page 1 of 6

SAMPLE ASSESSMENT SCHEDULE

Earth and Space Science 91413 (3.4): Demonstrate understanding of processes in the ocean system

Assessment Criteria

Achievement / Achievement with Merit / Achievement with Excellence /
Demonstrate understanding involves explaining processes and the links within the processes in the ocean system. / Demonstrate in-depth understanding involves explaining links between the processes in the ocean system. / Demonstrate comprehensive understanding involves discussing the complexity of the ocean system.

Evidence Statement

Q / Evidence / Achievement / Achievement with Merit / Achievement with Excellence
ONE / The thermohaline circulation uses heat and salinity to alter the density of the water. Warmer water is less dense than cooler water.
Thermohaline circulation is driven by the sun's heat, absorbed by tropical oceans, and impacted by variations in salt content (salinity) in the water. As heat from the tropics is carried by the Gulf Stream into the North Atlantic it is vented into the atmosphere. A deep convection of ocean waters is caused by surface cooling, with the flow of water then sinking to depths and then upwelling back to the surface at lower latitudes.
When water is frozen, salt is excluded and thus when it melts it is salt-free. This factor and the heavier density of salty water are particularly important in polar regions where the convergence of fresh and saline waters influences ocean currents. / Explains how temperature changes and salinity changes affect water density and relates to thermohaline circulation. / Explains what causes salinity and temperature change and how these changes affect circulation (link). / Discusses the causes of, and interactions between, the salinity and temperature and how this drives the circulation of water through the oceans.
NØ
No response, no relevant evidence. / N1
ONE relevant idea explained.
Eg, concept of density affecting circulation. / N2
The effect of density OR salinity on circulation stated. / A3
Changes in density and salinity explained but effect on circulation (movement) unclear. / A4
Explanation of how temperature change and salinity change relate to thermohaline circulation. / M5
Causes of change are explained but only one change clearly linked to circulation. / M6
Causes of both changes are explained and linked to circulation. / E7
Interactions are discussed
but one aspect may be less clear. Eg, deep current. / E8
Full answer.
TWO / Physical pumps
The exchange of carbon between the atmosphere and the ocean takes place in a number of ways. The most important of these mechanisms is through physical mixing of the ocean. It occurs when warm water in oceanic surface currents is carried from low latitudes to high latitudes on Earth and then cooled, making it heavy enough to sink below the surface layer and, in some places, all the way to the bottom.
When seawater is cooled it takes up more carbon dioxide. When cold water returns to the surface and warms up again, it loses carbon dioxide to the atmosphere. In this fashion, vertical circulation makes sure that carbon dioxide is constantly being exchanged between the ocean and the atmosphere.
Thus, vertical circulation acts as an enormous carbon pump, giving the ocean a lot more carbon than if equilibrium with the surface ocean were the only mechanism controlling the sharing of carbon between atmosphere and ocean.
Carbon is found primarily as CO2 in the atmosphere. At the poles the seawater becomes cooler which increases the solubility of the CO2. This means it reacts more with the water to produce carbonic acid. Carbon is also used by marine life.
When oceans become warmer they release CO2 back into the atmosphere. The warmer water cannot hold as much CO2 as it is less soluble.
Biological pumps
The biological pump, in essence, removes carbon dioxide from the surface water of the ocean, changing it into living matter and distributing it to the deeper water layers, where it is out of contact with the atmosphere. Thus, when the ocean shares carbon dioxide with the atmosphere, it does so by not only simply taking on carbon dioxide into solution (physical) but also by incorporating the carbon dioxide into living organisms (biological).
/ Explains biological pumps and physical pumps that affect CO2. / Explanation of how biological and physical pumps cycle carbon in the ocean. / Discussion of how biological and physical pumps cycle carbon in the ocean.
NØ
No response, no relevant evidence. / N1 / N2 / A3
Both pump systems are described but only one is clearly explained. / A4
Both pump systems are clearly explained. / M5
Both pumps are explained and the way they affect CO2 concentration is explained. / M6
Both pumps are explained and the way they affect CO2 concentration to cycle carbon is explained. / E7
Both pumps are discussed in detail showing the complexity of the system / E8
Both pumps are discussed in detail showing the complexity of the system and relating global cycling of carbon to the ocean system.
THREE / Mixed layer: Wind-driven surface currents are restricted mostly to the ocean's uppermost 100 m depending upon the depth of the pycnocline. The thickness of the surface mixed layer is typically 100 m or less.
The strongest currents generally occur in the ocean's surface layer, although some surface currents, such as boundary currents like the Gulf Stream, can be relatively strong to depths of several hundred metres. Surface currents are changeable, continually responding to variations in the wind, precipitation, and heating or cooling. Stirring of surface waters by the wind produces a well-mixed layer of uniform or nearly uniform density.
Pycnocline: is situated between the mixed layer and the deep layer, is where water density increases rapidly with depth because of changes in temperature and/or salinity.
The pycnocline acts as a porous boundary that allows some kinetic energy to penetrate into deep water.
Cold water is denser than warm water and salty water is denser than fresh water. Where a decline in temperature with depth is responsible for the increase in density with depth, the pycnocline is also a thermocline. On the other hand, if an increase in salinity is responsible for the increase in density with depth, the pycnocline is also a halocline.
Typically, the pycnocline extends to a depth of 500 to 1000 m.
Deep layer: The dark, cold deep layer below the pycnocline accounts for most of the ocean's mass.
Within the deep layer, density increases gradually with depth and water moves slowly. In only a few locations (usually near the bottom) are water movements fast enough to be considered currents.
Interactions
The ocean's three-layer structure is an example of how gravity separates a fluid into layers such that the density of each layer is less than the density of the layer below it. More dense fluids sink and less dense fluids rise. The ocean's pycnocline is very stable, thus suppressing mixing between the mixed layer and deep layer: that is, the pycnocline acts as a barrier to vertical motion within the ocean.
The usual stable state of the ocean features a layer of water that is warmest near its interface with the atmosphere (the mixed layer) and the mixed layer overlies water that becomes denser with increasing depth (the pycnocline). Strong storm winds may temporarily disturb this stable stratification bringing colder than usual water to the surface. Once the wind slackens, however, the original layered structure is soon restored. / Explanation of the properties of each layer. Eg, the mixed layer is the surface layer; it is mixed by the wind. / Explanation of the processes occurring within the layers. Eg, the mixed layer is the surface layer. It is stirred by the wind to give a well-mixed relatively uniform layer in spite of variations caused by rain and sunshine. It has the strongest currents. / Discussion of the properties of the layers and how they interact with each other to give the complex ocean system.
NØ
No response, no relevant evidence. / N1
Description of the properties of ONE layer is given. / N2
Description of the properties of TWO layers is given. / A3
Explanation of the properties of all TWO layers is given / A4
Explanation of the properties of all THREE layers is given. / M5
Explanation of the processes occurring in TWO layers is given. / M6
Explanation of the processes occurring in all THREE layers is given. / E7
Properties of TWO layers are discussed along with the reasons for the separation and any interaction that occurs. / E8
Properties of all THREE layers are discussed along with the reasons for the separation and any interaction that occurs.