3 Sources and Composition of Marine Sediments
3.1 The Sediment Cycle
Marine sediments show great variety: there ist the debris from the wearing down of
continents, shells, and organic matter derived from organisms, salts precipitated from
seawater, and volcanic products such as ash and pumice (Fig. 3.1).
Most sediments ultimately derive from the weathering of rocks on land. The
action of ice and water, of hot and cold, breaks up the rocks into small particles and
leaches them, destroying the more soluble minerals. A special type of "weathering"
occurs at the ridge crests and other young volcanic features in the sea: the reactions
of heated seawater with basalt. Such reactions may contribute considerable amounts
of matter to seawater. Just how much is not known, however.
The sea itself takes a toll from the continents: waves and tides can eat into land,
taking debris offshore. The material may stay on the shelf, or it may bypass the shelf
to accumulate on the slope or below.
Sediment which stays on the shelf, of course, essentially stays on the continent.
The other sediment is eventually carried back to the continents or to the mantle by
sea-floor spreading, completing the cycle.
3.2 Sources of Sediment
3.2.1 River Input. The dissolved and particulate load of rivers constitutes the main
source of sediment: the coalescing fans which build the continental slope off California (see Fig. 2.10), for example, are largely riverine mud, with an admixture of shell
and organic matter. These precipitates (calcium carbonate, CaC03; and opal, Si02, x
nH20) also derive to a great extent from river input, namely from the dissolved load
of rivers entering the ocean. We can make a rough estimate of how much material
enters the sea. Deep-sea sedimentation rates are between 1 and 20 mm per 1000
years. Slope sediments accumulate at a rate of up to 100 mm per 1000 years, approximately (Fig. 3.13).
Let us take the value of 100 mm/lOOO years for 10 % of the ocean, and a value of
5 mrn/lOOO years for the rest: the average is near 15 mrn/lOOO years (0.1 x 100 + 0.9
x 5). The ocean floor covers more than twice the area of the land. Hence, the
continents - if they are the ultimate source of all sediment - must wear down at a rate
of near 30 mm/lOOO years. Not a bad guess, actually. Recent estimates of denudation
3.1 The Sediment Cycle
Marine sediments show great variety: there ist the debris from the wearing down of
continents, shells, and organic matter derived from organisms, salts precipitated from
seawater, and volcanic products such as ash and pumice (Fig. 3.1).
Most sediments ultimately derive from the weathering of rocks on land. The
action of ice and water, of hot and cold, breaks up the rocks into small particles and
leaches them, destroying the more soluble minerals. A special type of "weathering"
occurs at the ridge crests and other young volcanic features in the sea: the reactions
of heated seawater with basalt. Such reactions may contribute considerable amounts
of matter to seawater. Just how much is not known, however.
The sea itself takes a toll from the continents: waves and tides can eat into land,
taking debris offshore. The material may stay on the shelf, or it may bypass the shelf
to accumulate on the slope or below.
Sediment which stays on the shelf, of course, essentially stays on the continent.
The other sediment is eventually carried back to the continents or to the mantle by
sea-floor spreading, completing the cycle.
3.2 Sources of Sediment
3.2.1 River Input. The dissolved and particulate load of rivers constitutes the main
source of sediment: the coalescing fans which build the continental slope off California (see Fig. 2.10), for example, are largely riverine mud, with an admixture of shell
and organic matter. These precipitates (calcium carbonate, CaC03; and opal, Si02, x
nH20) also derive to a great extent from river input, namely from the dissolved load
of rivers entering the ocean. We can make a rough estimate of how much material
enters the sea. Deep-sea sedimentation rates are between 1 and 20 mm per 1000
years. Slope sediments accumulate at a rate of up to 100 mm per 1000 years, approximately (Fig. 3.13).
Let us take the value of 100 mm/lOOO years for 10 % of the ocean, and a value of
5 mrn/lOOO years for the rest: the average is near 15 mrn/lOOO years (0.1 x 100 + 0.9
x 5). The ocean floor covers more than twice the area of the land. Hence, the
continents - if they are the ultimate source of all sediment - must wear down at a rate
of near 30 mm/lOOO years. Not a bad guess, actually. Recent estimates of denudation