94 Sources and Composition of Marine Sediments
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9
8
7
6
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4
3
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20 10
20
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0 15 10 5
Air
Waler
Daily
Phylopl an klon mass
Temperalu re ( O C)
Rainfall
(Surface.Waler)
m
(mm)
(ScMmalic)
Fig. 3.14. Annual layers (varves) in the sediment of a bay in the Adriatic Sea (Mljet Island). The
photo shows light and dark laminae. One light-dark pair corresponds to one year. The lower boundary of light layers are generally sharp. they are due to precipitation of carbonate by phytoplankton.which bloom in early summer (record to lhe right) as temperature rises. In fall and winter. rains
bring terrigenous matter which - together with organic detritus - provides for dark colors. The
interpretation of the varves is a complicated matter; recent studies use statistical procedures to
reconstruct climatic conditions in detail. (Photo E. S.)
sediments of Santa Barbara Basin off California using diatoms, dinoflagellates, and
other microfossils (Sect. 7.7.2). Unfortunately, such ideal calendar pages are extremely rare in the marine realm. They can only be preserved where burrowing
organisms cannot live, that is, in the absence of free oxygen above the sedimentwater interface.
It is somewhat hazardous to apply sedimentation rates found for a given type of
sediment to a geologic sequence of such sediments. Episodic or periodic events,
especially erosional events, can change overall rates considerably. In many cases,
sediments of a certain depositional regime are underlain by erosional features of the
same regime: ice-polished rocks under moraines, wind-polished pebble pavements
under dunes, channels under river gravels, lopped-off sea floor beneath graded storm
deposits (shallow water) or turbidites (deep water). Erosion, of course, means loss of
information to the geologists. Missing sections - hiatuses - are erased history, lost
time as it were, whose duration can hardly be estimated. Therefore "accumulation
rates" averaging long time spans and including these losses are smaller than instantaneous sedimentation rates. Of course, the presence of hiatuses suggests we must look
for information about the processes producing them. Also, hiatuses extending over
large regions can be used for the correlation of sediment sequences, especially in
seismic stratigraphy.
In concluding this chapter about sources and composition of marine sediments,
some more esoteric components may be mentioned. So-called "cosmic spherules"
were first described by Murray of the Challenger Expedition. Black magnetic spherical objects up to 0.2 mm in diameter, commonly rich in Fe and Ni, can be isolated
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J
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""' "' "
/
/
?
1
1
1
Mon.
2
1
o
9
8
7
6
5
4
3
2
20 10
20
10
0 15 10 5
Air
Waler
Daily
Phylopl an klon mass
Temperalu re ( O C)
Rainfall
(Surface.Waler)
m
(mm)
(ScMmalic)
Fig. 3.14. Annual layers (varves) in the sediment of a bay in the Adriatic Sea (Mljet Island). The
photo shows light and dark laminae. One light-dark pair corresponds to one year. The lower boundary of light layers are generally sharp. they are due to precipitation of carbonate by phytoplankton.which bloom in early summer (record to lhe right) as temperature rises. In fall and winter. rains
bring terrigenous matter which - together with organic detritus - provides for dark colors. The
interpretation of the varves is a complicated matter; recent studies use statistical procedures to
reconstruct climatic conditions in detail. (Photo E. S.)
sediments of Santa Barbara Basin off California using diatoms, dinoflagellates, and
other microfossils (Sect. 7.7.2). Unfortunately, such ideal calendar pages are extremely rare in the marine realm. They can only be preserved where burrowing
organisms cannot live, that is, in the absence of free oxygen above the sedimentwater interface.
It is somewhat hazardous to apply sedimentation rates found for a given type of
sediment to a geologic sequence of such sediments. Episodic or periodic events,
especially erosional events, can change overall rates considerably. In many cases,
sediments of a certain depositional regime are underlain by erosional features of the
same regime: ice-polished rocks under moraines, wind-polished pebble pavements
under dunes, channels under river gravels, lopped-off sea floor beneath graded storm
deposits (shallow water) or turbidites (deep water). Erosion, of course, means loss of
information to the geologists. Missing sections - hiatuses - are erased history, lost
time as it were, whose duration can hardly be estimated. Therefore "accumulation
rates" averaging long time spans and including these losses are smaller than instantaneous sedimentation rates. Of course, the presence of hiatuses suggests we must look
for information about the processes producing them. Also, hiatuses extending over
large regions can be used for the correlation of sediment sequences, especially in
seismic stratigraphy.
In concluding this chapter about sources and composition of marine sediments,
some more esoteric components may be mentioned. So-called "cosmic spherules"
were first described by Murray of the Challenger Expedition. Black magnetic spherical objects up to 0.2 mm in diameter, commonly rich in Fe and Ni, can be isolated