If a soil profile pit is left open sometimes, after a few hours,
you will see salts accumulating on the soil profile face—left
behind as moisture evaporates from the exposed surface. The
salts consist largely of chlorides, nitrates, and sulphates of
sodium, potassium, calcium, and magnesium. Graeme
Claridge undertook a detailed study of the salt contents of
Antarctic soils and concluded that the salts are derived primarily from rock weathering but the effect of atmospheric
inputs is evident in the anions with salts at sites nearer the
coast dominated by chlorides. Further inland, on the margins
of the Polar Plateau where the wind is dominated by katabatic flows off the plateau, the salts are dominated by nitrate
which is considered to be derived from the upper atmosphere
circulation system where air flows downwards over the
South Pole.
17.4.2 Physical Properties
a. Introduction
Soils in the Ross Sea region mainly comprise bouldery,
stony, gravelly sands. Some soils, particularly those on
ablation tills (the material that has accumulated on top of an
ablating glacier) may have a higher proportion of fine sand
and silt due to the grinding action of the glacier. Clay contents are often <1% of whole soil. Thus, we infer that most
soils have a very low soil moisture holding capacity and are
quite porous. Soils that have ice-cemented permafrost
effectively have an impermeable layer, due to ice accumulated at the top of the permafrost, that limits downward
movement of water which freezes when it reaches the top of
the permafrost. The physical properties of most interest are
the temperature and moisture content because they impact
strongly on all soil and ecosystem processes.
b. Soil temperature
The soil temperature determines the depth of the active layer,
the availability of liquid moisture, and the conditions for the
survival of soil organisms. As the Ross Sea region is all within
the Antarctic Circle, it experiences darkness for three months
in winter, and daylight 24/hours per day in summer, with a
more usual day/night pattern in the spring and autumn. Thus,
temperatures are cold in winter, gradually warming with the
return of the sun in spring and summer. For example, the
mean annual temperatures range from −18 °C at Marble Point
on the coast to −20 °C in the floor of the Wright Valley, at
about 50 m altitude, and −24 °C at 1700 m altitude on the
slopes of Mt Fleming at the head of the Wright Valley.
Antarctic soils remain at temperatures below 0 °C for
much of the year. However, when the summer sun’s radiant
energy directly warms the dark-coloured soil surface, temperatures may rise to as high as 20 °C even though the air
temperature remains close to zero. Heat is conducted
downwards, warming and thawing the soil, but often the soil
remains frozen just a few centimetres below the soil surface;
thus, there are strong thermal gradients within the soil.
Where there is ice within the soil it takes a lot of energy to
melt the ice (because water has a very high latent heat of
fusion). Thus, the temperature within the soil may remain
near zero for long periods as the soil moisture absorbs the
available energy while ice is melted or sublimated.
There is a lag time between the soil surface reaching
maximum temperature and the temperature rising at depth
within the soil. Also, the extremes of warmth and cold
experienced at the soil surface are gradually dampened with
depth. At a depth of about 15–20 m within the soil, there is
no appreciable change in temperature throughout the year
and this depth is referred to as the depth of zero annual
temperature amplitude. The temperature at that depth is a
good indication of the mean annual temperature.
The maximum depth of thaw (which determines the depth
of the active layer) is generally deeper at lower altitude and
lower latitude. For instance, the mean active layer depth at
1600 m altitude on Mt Fleming is about 6 cm, whereas at
Scott Base, near sea level (and at a similar latitude), it is
about 30 cm. At Cape Hallett, towards the north of the Ross
Sea region, the active layer is about 1 m deep on sites away
from the effects of shading from nearby hills. The active
layer depth also varies from one summer to the next—for
example, the active layer depth at Scott Base between 1999
and 2019 ranged between about 20 and 40 cm. The maximum depth of thaw is greater in moist soils, such as on lake
margins, because the thermal conductivity of moist soil is
greater than that of dry soil. At some sites, meltwater flows
through the soil and effectively conducts heat into the soil,
leading to a deeper active layer.
When there is snow on the soil surface it insulates the
underlying soil from variations in air temperature. In winter,
snow-cover, where present, will protect the soil from
extremely low temperatures. However, snow cover is relatively rare in the McMurdo Dry Valleys such as the Wright
Valley. In winter, cold air tends to pool on the valley floor
and both air and soil surface temperatures gradually drop to
as low as −40 °C. Occasional winter storms bring strong
winds and the air is mixed causing a rapid rise in air temperature, sometimes changing by over 30 °C, in a few hours
with the near-surface soil temperatures also recording a
sharp rise in temperature.
17.4 Key Soil Properties
281
you will see salts accumulating on the soil profile face—left
behind as moisture evaporates from the exposed surface. The
salts consist largely of chlorides, nitrates, and sulphates of
sodium, potassium, calcium, and magnesium. Graeme
Claridge undertook a detailed study of the salt contents of
Antarctic soils and concluded that the salts are derived primarily from rock weathering but the effect of atmospheric
inputs is evident in the anions with salts at sites nearer the
coast dominated by chlorides. Further inland, on the margins
of the Polar Plateau where the wind is dominated by katabatic flows off the plateau, the salts are dominated by nitrate
which is considered to be derived from the upper atmosphere
circulation system where air flows downwards over the
South Pole.
17.4.2 Physical Properties
a. Introduction
Soils in the Ross Sea region mainly comprise bouldery,
stony, gravelly sands. Some soils, particularly those on
ablation tills (the material that has accumulated on top of an
ablating glacier) may have a higher proportion of fine sand
and silt due to the grinding action of the glacier. Clay contents are often <1% of whole soil. Thus, we infer that most
soils have a very low soil moisture holding capacity and are
quite porous. Soils that have ice-cemented permafrost
effectively have an impermeable layer, due to ice accumulated at the top of the permafrost, that limits downward
movement of water which freezes when it reaches the top of
the permafrost. The physical properties of most interest are
the temperature and moisture content because they impact
strongly on all soil and ecosystem processes.
b. Soil temperature
The soil temperature determines the depth of the active layer,
the availability of liquid moisture, and the conditions for the
survival of soil organisms. As the Ross Sea region is all within
the Antarctic Circle, it experiences darkness for three months
in winter, and daylight 24/hours per day in summer, with a
more usual day/night pattern in the spring and autumn. Thus,
temperatures are cold in winter, gradually warming with the
return of the sun in spring and summer. For example, the
mean annual temperatures range from −18 °C at Marble Point
on the coast to −20 °C in the floor of the Wright Valley, at
about 50 m altitude, and −24 °C at 1700 m altitude on the
slopes of Mt Fleming at the head of the Wright Valley.
Antarctic soils remain at temperatures below 0 °C for
much of the year. However, when the summer sun’s radiant
energy directly warms the dark-coloured soil surface, temperatures may rise to as high as 20 °C even though the air
temperature remains close to zero. Heat is conducted
downwards, warming and thawing the soil, but often the soil
remains frozen just a few centimetres below the soil surface;
thus, there are strong thermal gradients within the soil.
Where there is ice within the soil it takes a lot of energy to
melt the ice (because water has a very high latent heat of
fusion). Thus, the temperature within the soil may remain
near zero for long periods as the soil moisture absorbs the
available energy while ice is melted or sublimated.
There is a lag time between the soil surface reaching
maximum temperature and the temperature rising at depth
within the soil. Also, the extremes of warmth and cold
experienced at the soil surface are gradually dampened with
depth. At a depth of about 15–20 m within the soil, there is
no appreciable change in temperature throughout the year
and this depth is referred to as the depth of zero annual
temperature amplitude. The temperature at that depth is a
good indication of the mean annual temperature.
The maximum depth of thaw (which determines the depth
of the active layer) is generally deeper at lower altitude and
lower latitude. For instance, the mean active layer depth at
1600 m altitude on Mt Fleming is about 6 cm, whereas at
Scott Base, near sea level (and at a similar latitude), it is
about 30 cm. At Cape Hallett, towards the north of the Ross
Sea region, the active layer is about 1 m deep on sites away
from the effects of shading from nearby hills. The active
layer depth also varies from one summer to the next—for
example, the active layer depth at Scott Base between 1999
and 2019 ranged between about 20 and 40 cm. The maximum depth of thaw is greater in moist soils, such as on lake
margins, because the thermal conductivity of moist soil is
greater than that of dry soil. At some sites, meltwater flows
through the soil and effectively conducts heat into the soil,
leading to a deeper active layer.
When there is snow on the soil surface it insulates the
underlying soil from variations in air temperature. In winter,
snow-cover, where present, will protect the soil from
extremely low temperatures. However, snow cover is relatively rare in the McMurdo Dry Valleys such as the Wright
Valley. In winter, cold air tends to pool on the valley floor
and both air and soil surface temperatures gradually drop to
as low as −40 °C. Occasional winter storms bring strong
winds and the air is mixed causing a rapid rise in air temperature, sometimes changing by over 30 °C, in a few hours
with the near-surface soil temperatures also recording a
sharp rise in temperature.
17.4 Key Soil Properties
281