Geoscience Reference
In-Depth Information
include clay and rock fragments rafted on icebergs which have melted and dropped their
load. Elsewhere, wind-blown dust from deserts and volcanic ash makes up a greater pro-
portion of deep-water sediment, sometimes accompanied by micro-meteorite dust, sharks'
teeth, and even the ear bones of whales.
8. The ocean drilling ship
JOIDES Resolution
.
The derrick towers 60 metres above
the water-line.
Where ocean productivity is high on the surface, there are often also the sunken remains
of various types of plankton. In comparatively shallow water, the calcareous skeletons of
coccolithophores and foraminifera are common, forming a calcareous ooze that may con-
solidate to form chalk or limestone. But the solubility of calcium carbonate increases with
depth and pressure. Somewhere between 3.5 and 4.5 kilometres down in the water, we
reach the so-called carbon compensation depth (CCD), below which the tiny skeletons will
tend to dissolve away. Here, their place can be taken by silicaceous ooze, made from the
tiny silica skeletons of diatoms and radiolarians. Silica too would be soluble, but enough
gets through to form significant deposits in the Southern Ocean and parts of the Indian and
Pacific oceans. In a few places, usually where ocean circulation is restricted, such as in the
Black Sea, the bottom water is anaerobic and black shales are deposited. They are some-
times rich in organic material that is not oxidized or consumed in the anaerobic conditions
and can slowly turn into oil. Occasionally, the anaerobic deposits are more widespread, re-
flecting so-called anoxic events, where changes in ocean circulation prevent oxygen-rich
waters sinking to the ocean floor.
