180
Productivity and Benthic Organisms
where CL is the concentration of the tracer at a distance L above the first occurrence,
Co is the original concentration in the mixed layer, right after introduction of the
tracer, and M is the thickness of the mixed layer. Application of this formula to the
distribution of volcanic ash, observed in cores from the North Atlantic, suggests that
M is in the neighborhood of 6 cm. This estimate agrees well with the mixed-layer
thickness suggested by the 14C-stratigraphy shown in Fig. 6.17, for a core from the
west-equatorial Pacific. It should be noted, however, that radio carbon of the sand
fraction may yield results that differ from those based on bulk ages.
6.8 Limits of Paleoecologic Reconstruction
We have discussed environmental factors, especially physical ones, and we have
looked at the production of lebensspuren and what they may tell us about the environment. Also, we have introduced the problem of bioturbation, the process which alters
and smooths the record. What about the organisms themselves, their interactions,
their reproduction rates, their larval dispersion? How do larvae know where to settle?
How many survive? Who competes with whom? Who eats whom? Which are the
symbiotic relationships, which parasitic? How do they control distributions? And
especially: how much of all this information enters the geologic record?
These problem are constantly encountered when studying communties, here, benthic communities. Already the definition of communities is difficult - the biologists
use "organisms that are often found together". Presumably, their web of interaction
provides the checks and balances which keep a community stable, that is, the same
kinds of organisms in roughly the same proportions live together over extended
periods of time.
The record also has "communties", that is, co-occurring fossils. Not surprisingly,
the difficulties in guessing the interactions of organisms long dead are virtually
insurmountable.
Even seemingly simple questions are hard to answer. For example, how large a
territory does a benthic foraminifer feed on? On the shelf of the Arctic there are up to
50 individuals per square meter of the foraminifer Astrorhiza. Their shells are about
5 mm in diameter; but, when alive, the net of pseudopodia takes up an area 6 cm in
diameter, an area more than 100 times greater than that of the shell. A large part of
the sea floor may be "occupied" by these tiny organisms. How would we know this
if we only had their shells?
It does not help, of course, that only a selected portion of the community is
eventually fossilized. Let us inspect some shallow water communities recognized
especially through studies of Danish biologists (Fig. 6.18). In many Arctic and cooltemperate regions a Venus community lies offshore in 10 to 20 m water depth. Venus,
a bivalve, is a conspicuous member. Others are the star fish Astropecten, the sea
urchin Echinocardium, and the polychaete tube-building worm Pectinaria koreni.
Species may change, but the genera and the structure of the community are similar
everywhere (parallel communities). As geologists, we can expect the preservation of
some of the shells, some of the burrows. Nothing recognizable will be left of the
Productivity and Benthic Organisms
where CL is the concentration of the tracer at a distance L above the first occurrence,
Co is the original concentration in the mixed layer, right after introduction of the
tracer, and M is the thickness of the mixed layer. Application of this formula to the
distribution of volcanic ash, observed in cores from the North Atlantic, suggests that
M is in the neighborhood of 6 cm. This estimate agrees well with the mixed-layer
thickness suggested by the 14C-stratigraphy shown in Fig. 6.17, for a core from the
west-equatorial Pacific. It should be noted, however, that radio carbon of the sand
fraction may yield results that differ from those based on bulk ages.
6.8 Limits of Paleoecologic Reconstruction
We have discussed environmental factors, especially physical ones, and we have
looked at the production of lebensspuren and what they may tell us about the environment. Also, we have introduced the problem of bioturbation, the process which alters
and smooths the record. What about the organisms themselves, their interactions,
their reproduction rates, their larval dispersion? How do larvae know where to settle?
How many survive? Who competes with whom? Who eats whom? Which are the
symbiotic relationships, which parasitic? How do they control distributions? And
especially: how much of all this information enters the geologic record?
These problem are constantly encountered when studying communties, here, benthic communities. Already the definition of communities is difficult - the biologists
use "organisms that are often found together". Presumably, their web of interaction
provides the checks and balances which keep a community stable, that is, the same
kinds of organisms in roughly the same proportions live together over extended
periods of time.
The record also has "communties", that is, co-occurring fossils. Not surprisingly,
the difficulties in guessing the interactions of organisms long dead are virtually
insurmountable.
Even seemingly simple questions are hard to answer. For example, how large a
territory does a benthic foraminifer feed on? On the shelf of the Arctic there are up to
50 individuals per square meter of the foraminifer Astrorhiza. Their shells are about
5 mm in diameter; but, when alive, the net of pseudopodia takes up an area 6 cm in
diameter, an area more than 100 times greater than that of the shell. A large part of
the sea floor may be "occupied" by these tiny organisms. How would we know this
if we only had their shells?
It does not help, of course, that only a selected portion of the community is
eventually fossilized. Let us inspect some shallow water communities recognized
especially through studies of Danish biologists (Fig. 6.18). In many Arctic and cooltemperate regions a Venus community lies offshore in 10 to 20 m water depth. Venus,
a bivalve, is a conspicuous member. Others are the star fish Astropecten, the sea
urchin Echinocardium, and the polychaete tube-building worm Pectinaria koreni.
Species may change, but the genera and the structure of the community are similar
everywhere (parallel communities). As geologists, we can expect the preservation of
some of the shells, some of the burrows. Nothing recognizable will be left of the