82
Sources and Composition of Marine Sediments
3.5.4 Clay-Sized sediment is ubiquitous on shelves, slopes, and in the deep sea. Like
the silt, it indicates low energy environments where abundant, because it is easily
picked up and washed away. Exceptions are clays in those high energy environments
where clay supply is extremely high, as off some tropical rivers. Much of the clay, as
mentioned, consists of clay minerals, products of weathering that are brought in by
rivers and winds and are dispersed by waves and currents.
The common clay minerals, or clay mineral groups, are montmorrilonite (or smectite), illite, chlorite, and kaolinite (Appendix A4). Their distribution and paleoclimatic
significance will be discussed in more detail in Chapter 8.
The high surface areas of clay particles give clayey sediments special chemical
properties. For example, they readily adsorb a large variety of substances, and react
with the ions in seawater and in interstitial waters. Thus, during diagenesis, within
buried sediments, new clay minerals can form slowly. Ultimately, this has important
implications for the chemistry of seawater and for geochemistry in general. Also,
clays can readily form aggregates, in seawater.
Clayey sediments in regions of high sedimentation rate are commonly rich in
organic matter, partly because organics cling to the clay during deposition, and partly
because where conditions are quiet enough for the deposition of clay they also are
favorable for the deposition of the fluffY organic particles. Much of the marine clays
may actually be brought to the floor within fecal pellets of organisms filtering the
water. Such clay is associated with food particles. Recent experiments in the Baltic
Sea, with sediment-trapping equipment, in the open ocean off California and elsewhere, suggest that this "fecal pellet transport" mechanism is a significant agent of
sedimentaiton. The association between clay and organic matter is important in petroleum exploration in the search for source rocks.
3.6 Biogenous Sediments
3.6.1 Types of Components. Organisms produce sediments in the form of shells and
other skeletal materials and organic matter. The label "biogenous" generally refers
only to calcareous, siliceous, and phosphatic hard parts (see Table 3.3).
Virtually all of the calcium carbonate deposited on the sea floor belongs in the
category of biogenous sediments. On the shelf it is chiefly the shells and skeletons of
benthic organisms, while on the slope there is an increasing proportion of planktonic
remains away from the coast. In the deep sea, finally, plankton remains (foraminifers,
coccoliths; Appendix A9) are entirely dominant. Opaline skeletons are also important
- again mainly benthic remains (sponges) on certain shelves, and planktonic ones
(diatoms, radiolarians) in slope sediments and in the deep sea. In addition to carbonate and silica, phosphatic particles are produced by organisms. The sedimentation of
such particles plays an important role in the phosphorus budget of the biosphere, and
is geochemically of great interest (see Sect. 1O.3.l). the various other types of hard
parts - strontium-sulfate, manganese-, iron-, and aluminum compounds - are intriguing from an evolutionary standpoint, but do not noticeably contribute to sediment
pattems. Organic matter (organogenic sediment) will be taken up in Chapters 6 and
10, when discussing hydrocarbons.
Sources and Composition of Marine Sediments
3.5.4 Clay-Sized sediment is ubiquitous on shelves, slopes, and in the deep sea. Like
the silt, it indicates low energy environments where abundant, because it is easily
picked up and washed away. Exceptions are clays in those high energy environments
where clay supply is extremely high, as off some tropical rivers. Much of the clay, as
mentioned, consists of clay minerals, products of weathering that are brought in by
rivers and winds and are dispersed by waves and currents.
The common clay minerals, or clay mineral groups, are montmorrilonite (or smectite), illite, chlorite, and kaolinite (Appendix A4). Their distribution and paleoclimatic
significance will be discussed in more detail in Chapter 8.
The high surface areas of clay particles give clayey sediments special chemical
properties. For example, they readily adsorb a large variety of substances, and react
with the ions in seawater and in interstitial waters. Thus, during diagenesis, within
buried sediments, new clay minerals can form slowly. Ultimately, this has important
implications for the chemistry of seawater and for geochemistry in general. Also,
clays can readily form aggregates, in seawater.
Clayey sediments in regions of high sedimentation rate are commonly rich in
organic matter, partly because organics cling to the clay during deposition, and partly
because where conditions are quiet enough for the deposition of clay they also are
favorable for the deposition of the fluffY organic particles. Much of the marine clays
may actually be brought to the floor within fecal pellets of organisms filtering the
water. Such clay is associated with food particles. Recent experiments in the Baltic
Sea, with sediment-trapping equipment, in the open ocean off California and elsewhere, suggest that this "fecal pellet transport" mechanism is a significant agent of
sedimentaiton. The association between clay and organic matter is important in petroleum exploration in the search for source rocks.
3.6 Biogenous Sediments
3.6.1 Types of Components. Organisms produce sediments in the form of shells and
other skeletal materials and organic matter. The label "biogenous" generally refers
only to calcareous, siliceous, and phosphatic hard parts (see Table 3.3).
Virtually all of the calcium carbonate deposited on the sea floor belongs in the
category of biogenous sediments. On the shelf it is chiefly the shells and skeletons of
benthic organisms, while on the slope there is an increasing proportion of planktonic
remains away from the coast. In the deep sea, finally, plankton remains (foraminifers,
coccoliths; Appendix A9) are entirely dominant. Opaline skeletons are also important
- again mainly benthic remains (sponges) on certain shelves, and planktonic ones
(diatoms, radiolarians) in slope sediments and in the deep sea. In addition to carbonate and silica, phosphatic particles are produced by organisms. The sedimentation of
such particles plays an important role in the phosphorus budget of the biosphere, and
is geochemically of great interest (see Sect. 1O.3.l). the various other types of hard
parts - strontium-sulfate, manganese-, iron-, and aluminum compounds - are intriguing from an evolutionary standpoint, but do not noticeably contribute to sediment
pattems. Organic matter (organogenic sediment) will be taken up in Chapters 6 and
10, when discussing hydrocarbons.