Geoscience Reference
In-Depth Information
The living ocean
The oceans also contain many chemicals in trace quantities and many of these are nutrients
important for life and hence for ocean productivity. As a result, they are often depleted in
surface waters. Colour scanners flown in space and tuned to the characteristic wavelengths
of pigments such as chlorophyll in phytoplankton can map out the seasonal productive
zones in the ocean. The highest productivity tends to occur in the spring in mid-to-high lat-
itudes where warm water meets cold but nutrient-rich waters. In the 1980s, the late John
Martin of Moss Landing Marine Laboratory in California noticed that blooms of plankton
can arise down-current off volcanic ocean islands. He suggested that iron might be a lim-
iting nutrient in ocean productivity and that the volcanic rock was supplying traces of dis-
solved iron. That has since been confirmed by experiments seeding patches of iron salts in
the South Pacific and also by the observation from sediment cores that ocean productivity
was highest at the onset of glacial conditions, when wind-blown dust was contributing iron
to the ocean. But fertilizing the ocean with iron may not be a cure for the enhanced green-
house effect as most of the carbon dioxide removed by the plankton seems to get recycled
back into solution as they die or are eaten.
Ocean margins
Continents are often fringed by a shallow shelf, little more than 200 metres deep. Geologic-
ally, this is effectively part of the continent not the ocean and, at times of much lower sea
level, parts of it must have been dry land. The continental shelves are often highly product-
ive and support fisheries, or at least they did until over-fishing started to limit the catch.
The organic productivity together with huge quantities of silt, mud, and sand washed by
rivers or blown by wind from the neighbouring land has built up thick sediments. Where
rivers supply these, the dense, sediment-laden water sometimes continues to flow almost
river-like through gorges and over the edge of the continental shelf, sometimes continuing,
as in the case of the Amazon, hundreds of kilometres offshore before dispersing in a delta-
like pattern. In some places, the shelf margins have spectacular underwater scenery of cliffs
and gorges that can only be seen by sonar but are as spectacular as any on land.
The ocean floor
Huge areas of the deep ocean floor are relatively flat and featureless, with little more than
the occasional sea cucumber (actually a type of echinoderm, a relative of starfish) for many
miles. But there are also mountains and canyons. We'll come to the mid-ocean ridges and
trenches later, but there are also many isolated seamounts, sometimes known as guyots,
rising from the ocean floor. Literally like underwater mountains, these are often isolated
