Completeness of the rock and fossil record: some estimates using fossil soils

Abstract

Surprisingly, there is a relationship between rates of sediment accumulation and the time spans for which they have been calculated. This relationship can be used to estimate expected rates for specific sedimentary environments and time spans. The most probable completeness of a given sedimentary section at a given short time span can be calculated by the ratio of the measured long-term rate of sediment accumulation to the expected short-term rate. Although the measured time span is usually based on radiometric and paleomagnetic data, the cumulative time of formation estimated from fossil soils in a sequence may also be used to calculate rates and may be useful in comparing the completeness and rate of accumulation of different sequences. By both kinds of estimates, terrestrial sedimentary successions are disappointingly incomplete. Some reasons for incompleteness are illustrated with a simple model of episodic flooding, exceeding a threshold for destruction and sedimentation over a particular kind of vegetation, and thus initiating a new cycle of soil formation. In such a model, rock record is lost to erosion during cutting and filling cycles, to overprinting of weakly developed soils by later, better-developed soils, and to continued development, near steady state, of the soils preserved.

Because fossil soils are evidence of ancient environments and ecosystems independent of the fossil record, they may provide evidence of expected kinds of fossils, such as silica phytoliths, calcareous phytoliths, pollen, leaves, fruits, seeds, charcoal, land snails, coprolites, and bones. The degree to which the kinds of fossils actually found fail to meet these expectations is a crude measure of the completeness of representation of a former ecosystem in the fossil record. Some of the discrepancy between expected and actual occurrence of fossils can be related to the original Eh and pH of a fossil soil, as approximated by the oxidation state of iron in its minerals (for Eh) and by carbonate or zeolite content (for pH). Different kinds of fossils can be envisaged as having a characteristic Eh-pH stability field within which they can be expected to have been preserved if originally present. Even under ideal conditions of preservation, it takes some time for fossils to accumulate in soils to levels at which representative collections can be made. Estimates of this temporal control on preservation can be gained by comparing fossil occurrences with the degree of development of fossil soils. Neither these chemical nor temporal factors account fully for the degree of incompleteness observed because original abundance, trampling, predation and many other factors are also important determinants of fossil occurrence.

These considerations can be used as guidelines for choosing stratigraphic sections appropriate for particular paleobiological and geological problems. For example, a study of speciation of terrestrial vertebrates would best be in a sequence of weakly developed, calcareous fossil soils (Entisols and Inceptisols), of near-uniform texture and yellow to brown color, formed under an extraordinarily high long-term rate of sediment accumulation. On the other hand, a study of coevolution of vertebrates and plants would best be based on a sequence of weakly to moderately developed, calcareous fossil soils of predominantly drab (gray, green, and blue) color, with interbedded carbonaceous shales.