Bioturbation
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environmental information from tracks and burrows. Unlike shells, lebensspuren cannot be transported by currents, but stay in the sediment where they were made. Thus
they indicate in situ conditions, whether it be food supply, stability of the sediment,
wave action, even depth of water.
The tracks and burrows are useful not only for paleoecologic reconstruction but
also for physical geology, and even for tectonics. In the deep sea, for example vertical
burrows do not last if the sediment has a tendency to shear horizontally. Through
systematic box coring of deep-sea carbonates in recent years, it was found that
vertical burrows are extremely abundant in the west equatorial Pacific, in shallow
areas with sandy, stiff sediment. In deep areas, where dissolution of carbonate breaks
down the sand, the sediment loses strength. It can flow when shaken by earthquakes.
There are no vertical burrows here, only horizontal ones.
On land, the field geologist sometimes deals with badly jumbled marine strata - he
may not even be able to tell which side of a layer is up! Burrows, if present, can help
decide the issue. Many burrows are U-shaped. If one finds them as an arch, that is,
upside down, it proves that the layer has been inverted by tectonic forces. Sometimes
there is a question of lacustrine (lake-) versus lagoonal or marine sediments. The
marine environment has large long burrows in contrast to the less diverse and generally smaller ones of lakes.
6.6.3 Preservation. Not all tracks and burrows are equally well preserved, of course.
A delicate surface trail made by a star fish or a mussel has little chance of entering
the record. A 30-cm-deep Arenicola or Mya burrow or especially a 2-m-deep crab
burrow has an excellent chance: we should have to remove a thick layer of sediment
in order to obliterate it. In general, surface tracks can only be reserved by a catastrophic deposition event such as a flood or a turbidity flow. In the case of continuous
accumulation, when sediment builds up gradually and there is a mixed layer from
bioturbation, surface tracks are destroyed while only burrows penetrating the mixed
layer are preserved in the record (Fig. 6.16).
6.7 Bioturbation
6.7.1 Effects of Mixing. Bioturbation prevents the preservation of thin layers, that is,
annual layers, thin turbidite layers, or layers produced by contour currents alike.
Thus, today's well-turbated slope sediments are poorly layered. However, this need
not always have been the case. During times when the ocean was not well oxygenated, burrowing organisms could not as readily do their work of homogenizing the
incoming sporadic supply of sediment. Thus, quite commonly, we see finely layered
slope sediments in the geologic record, deposited in warm oceans with low oxygen
content. (Warm water dissolves much less oxygen than cold water: only since the
Earth has grown ice caps is the deep ocean highly aerated.)
Bioturbation alters and smooths the record. In a finely layered sediment, each
layer has a message about its origin and its environment of deposition. But when the
layers are mixed, the messages are mixed also, and we obtain a signal which is some
177
environmental information from tracks and burrows. Unlike shells, lebensspuren cannot be transported by currents, but stay in the sediment where they were made. Thus
they indicate in situ conditions, whether it be food supply, stability of the sediment,
wave action, even depth of water.
The tracks and burrows are useful not only for paleoecologic reconstruction but
also for physical geology, and even for tectonics. In the deep sea, for example vertical
burrows do not last if the sediment has a tendency to shear horizontally. Through
systematic box coring of deep-sea carbonates in recent years, it was found that
vertical burrows are extremely abundant in the west equatorial Pacific, in shallow
areas with sandy, stiff sediment. In deep areas, where dissolution of carbonate breaks
down the sand, the sediment loses strength. It can flow when shaken by earthquakes.
There are no vertical burrows here, only horizontal ones.
On land, the field geologist sometimes deals with badly jumbled marine strata - he
may not even be able to tell which side of a layer is up! Burrows, if present, can help
decide the issue. Many burrows are U-shaped. If one finds them as an arch, that is,
upside down, it proves that the layer has been inverted by tectonic forces. Sometimes
there is a question of lacustrine (lake-) versus lagoonal or marine sediments. The
marine environment has large long burrows in contrast to the less diverse and generally smaller ones of lakes.
6.6.3 Preservation. Not all tracks and burrows are equally well preserved, of course.
A delicate surface trail made by a star fish or a mussel has little chance of entering
the record. A 30-cm-deep Arenicola or Mya burrow or especially a 2-m-deep crab
burrow has an excellent chance: we should have to remove a thick layer of sediment
in order to obliterate it. In general, surface tracks can only be reserved by a catastrophic deposition event such as a flood or a turbidity flow. In the case of continuous
accumulation, when sediment builds up gradually and there is a mixed layer from
bioturbation, surface tracks are destroyed while only burrows penetrating the mixed
layer are preserved in the record (Fig. 6.16).
6.7 Bioturbation
6.7.1 Effects of Mixing. Bioturbation prevents the preservation of thin layers, that is,
annual layers, thin turbidite layers, or layers produced by contour currents alike.
Thus, today's well-turbated slope sediments are poorly layered. However, this need
not always have been the case. During times when the ocean was not well oxygenated, burrowing organisms could not as readily do their work of homogenizing the
incoming sporadic supply of sediment. Thus, quite commonly, we see finely layered
slope sediments in the geologic record, deposited in warm oceans with low oxygen
content. (Warm water dissolves much less oxygen than cold water: only since the
Earth has grown ice caps is the deep ocean highly aerated.)
Bioturbation alters and smooths the record. In a finely layered sediment, each
layer has a message about its origin and its environment of deposition. But when the
layers are mixed, the messages are mixed also, and we obtain a signal which is some