Trails and Burrows
175
attain densities of 200 individuals per square meter (Fig. 6.12). Each wonn can ingest
several hundred grams of sediment per day. At high densities, the entire sediment,
down to about 20 cm depth, may be worked over in a matter of weeks by depositfeeding organisms.
Muddy bottoms, then, are largely fecal material, many times recycled. This is also
true for the deep-sea floor, although here the cycling takes thousands of years rather
than months as in the mudflats - a difference of a factor of 10 000. Of course,
sedimentation rates also differ considerably, by a factor of up to 1000. The greater
number of recycles in the shallow water, before burial, reflects the much higher
supply of organic carbon - and hence energy supply - to the coastal sea floor. The
low level of benthic activity in deep-sea sediments and the relative greater importance of the infauna is illustrated by the fact that on more than 100 000 photographs
from 2000 different deep-sea stations, only about 100 visible animals were counted.
If fine sediments are fecal matter, should one not see more evidence of fecal
pellets and fecal strings? Indeed, fecal pellets are extremely abundant, especially
toward the surface of muddy sea floor. Here they alter entirely the hydraulic properties of the sediment, give rise to bacterial growth, and to development of fungal mats
in places. In the case of calcareous mud, the fecal pellets can harden by cementation
and can thus fossilize more readily. They are very abundant in many limestones, as
near-spherical and oval grains of diameters between 0.03 and 0.1 mm, a fact that is
only realized after careful microscopic study.
The biological reworking of soft sediment on the sea floor is a process of global
geochemical significance. Without such bioturbation, the sediment would quickly
disappear from the marine chemical sy stem. Only a thin upper layer would readily
react with the seawater. Through bioturbation, however, a several-centimeter-thick
layer of sediment keeps exchanging matter with seawater, the falling organic material
remains available for some time before burial, the nutrients in it are remobilized and
are given back to the seawater. Thus, the overall fertility of the ocean is closely linked
to bioturbation.
A surprisingly large proportion of Phanerozoic sediments on land originate from
marine muds. Fossil shales - produced from muds - make up about 50 % of the
sedimentary record. Limestones (in large part originating from calcareous muds)
provide 20 %, and sandstones 30 %.
6.6 Trails and Burrows
6.6.1 Trace Fossils. In the foregoing, we have looked at sea floor and organisms with
an appreciation for the biological viewpoint. However, the geologist's ultimate concern is the final record: how is it made? What can it tell us about conditions of the
past? An entire branch of geology - ichnology - has grown from the study of the
tracks, trails, burrows, and other sedimentary disturbances made by organisms. We
next tum to some of the problems arising in the study of such trace fossils. We also
have to ask just how bioturbation disturbs the orderly recording of events in deep-sea
175
attain densities of 200 individuals per square meter (Fig. 6.12). Each wonn can ingest
several hundred grams of sediment per day. At high densities, the entire sediment,
down to about 20 cm depth, may be worked over in a matter of weeks by depositfeeding organisms.
Muddy bottoms, then, are largely fecal material, many times recycled. This is also
true for the deep-sea floor, although here the cycling takes thousands of years rather
than months as in the mudflats - a difference of a factor of 10 000. Of course,
sedimentation rates also differ considerably, by a factor of up to 1000. The greater
number of recycles in the shallow water, before burial, reflects the much higher
supply of organic carbon - and hence energy supply - to the coastal sea floor. The
low level of benthic activity in deep-sea sediments and the relative greater importance of the infauna is illustrated by the fact that on more than 100 000 photographs
from 2000 different deep-sea stations, only about 100 visible animals were counted.
If fine sediments are fecal matter, should one not see more evidence of fecal
pellets and fecal strings? Indeed, fecal pellets are extremely abundant, especially
toward the surface of muddy sea floor. Here they alter entirely the hydraulic properties of the sediment, give rise to bacterial growth, and to development of fungal mats
in places. In the case of calcareous mud, the fecal pellets can harden by cementation
and can thus fossilize more readily. They are very abundant in many limestones, as
near-spherical and oval grains of diameters between 0.03 and 0.1 mm, a fact that is
only realized after careful microscopic study.
The biological reworking of soft sediment on the sea floor is a process of global
geochemical significance. Without such bioturbation, the sediment would quickly
disappear from the marine chemical sy stem. Only a thin upper layer would readily
react with the seawater. Through bioturbation, however, a several-centimeter-thick
layer of sediment keeps exchanging matter with seawater, the falling organic material
remains available for some time before burial, the nutrients in it are remobilized and
are given back to the seawater. Thus, the overall fertility of the ocean is closely linked
to bioturbation.
A surprisingly large proportion of Phanerozoic sediments on land originate from
marine muds. Fossil shales - produced from muds - make up about 50 % of the
sedimentary record. Limestones (in large part originating from calcareous muds)
provide 20 %, and sandstones 30 %.
6.6 Trails and Burrows
6.6.1 Trace Fossils. In the foregoing, we have looked at sea floor and organisms with
an appreciation for the biological viewpoint. However, the geologist's ultimate concern is the final record: how is it made? What can it tell us about conditions of the
past? An entire branch of geology - ichnology - has grown from the study of the
tracks, trails, burrows, and other sedimentary disturbances made by organisms. We
next tum to some of the problems arising in the study of such trace fossils. We also
have to ask just how bioturbation disturbs the orderly recording of events in deep-sea