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
ONE
Evidence
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.
Achievement
Explains how temperature
changes and salinity changes
affect water density and relates
to thermohaline circulation.
Achievement with Merit
Achievement with Excellence
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.
NCEA Level 3 Earth and Space Science 91413 (3.4) – Page 2 of 6
TWO
NØ
N1
N2
A3
A4
M5
M6
E7
E8
No response,
no relevant
evidence.
ONE relevant
idea explained.
Eg, concept of
density affecting
circulation.
The effect of
density OR
salinity on
circulation
stated.
Changes in
density and
salinity
explained but
effect on
circulation
(movement)
unclear.
Explanation of
how
temperature
change and
salinity change
relate to
thermohaline
circulation.
Causes of
change are
explained but
only one
change clearly
linked to
circulation.
Causes of both
changes are
explained and
linked to
circulation.
Interactions
are discussed
but one aspect
may be less
clear. Eg, deep
current.
Full answer.
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
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.
NCEA Level 3 Earth and Space Science 91413 (3.4) – Page 3 of 6
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).
NCEA Level 3 Earth and Space Science 91413 (3.4) – Page 4 of 6
NØ
THREE
N1
N2
A3
A4
M5
M6
E7
E8
No response,
no relevant
evidence.
Both pump
systems are
described but
only one is
clearly
explained.
Both pump
systems are
clearly
explained.
Both pumps
are explained
and the way
they affect
CO2
concentration
is explained.
Both pumps
are explained
and the way
they affect CO2
concentration
to cycle carbon
is explained.
Both pumps
are discussed
in detail
showing the
complexity of
the system
Both pumps
are discussed
in detail
showing the
complexity of
the system and
relating global
cycling of
carbon to the
ocean system.
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.
Explanation of the properties of
each layer. Eg, the mixed layer is
the surface layer; it is mixed by
the wind.
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.
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.
NCEA Level 3 Earth and Space Science 91413 (3.4) – Page 5 of 6
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.
NCEA Level 3 Earth and Space Science 91413 (3.4) – Page 6 of 6
NØ
N1
N2
A3
A4
M5
M6
E7
E8
No response,
no relevant
evidence.
Description of
the properties
of ONE layer is
given.
Description of
the properties
of TWO layers
is given.
Explanation of
the properties
of all TWO
layers is given
Explanation of
the properties
of all THREE
layers is given.
Explanation of
the processes
occurring in
TWO layers is
given.
Explanation of
the processes
occurring in all
THREE layers
is given.
Properties of
TWO layers
are discussed
along with the
reasons for the
separation and
any interaction
that occurs.
Properties of
all THREE
layers are
discussed
along with the
reasons for the
separation and
any interaction
that occurs.