An Introduction to Environmental Chemistry

Identifying removal mechanisms for a specific component is difficult because
removal processes are usually slow and occur over large areas. Some removal
processes are very slow—operating on geological timescales of thousands or mil-
lions of years—and impossible to measure in the present oceans. The require-
ment to study element cycles on geological timescales is further complicated by
processes like climate change and plate tectonics which affect the geometry of
ocean basins and sealevel. These large-scale geological processes can have sig-
nificant effects on removal processes of major ions from the oceans.
The effects of the geologically recent glacial–interglacial oscillations during
the Quaternary period (the last 2 million years) are particularly relevant. Firstly,
the rapid rise in sealevel over the last 11 000 years, following the melting of polar
ice accumulated during the last glacial period, has flooded former land areas to
create large, shallow, continental shelves, areas of high biological activity and
accumulation of biological sediments (Section 6.2.4). Also, the unconsolidated
glacial sediments which mantle large areas of northern-hemisphere (temperate-
arctic zone) land surfaces are easily eroded. This results in high particulate
concentrations in rivers, which carry material to estuaries and continental
shelves. This enhanced sediment supply results in correspondingly high detrital
sediment–seawater interactions, increasing the importance of removal processes
such as ion exchange.

The Oceans 193

Sources Rivers Atmosphere

Seawater

Hydrothermal

(MOR)

Sinks

(see text)

Fig. 6.7Simple box model summarizing material inputs to seawater. Style of arrow indicates
relative importance of input: bold, high; pecked, low. MOR, mid-ocean ridges.