Paleoanthropology: Dating the Cave Deposits of South Africa
Both the South African sites of Sterkfontein and Makapansgat are comprised of cave deposits with sediments that are calcified as cave breccia. This includes the South African sites of Kromdraai and Swartkrans, as well. The provenience of a fossil or artifact is extremely important in that it can establish relative age. In many cases, following the geologic principle of superposition, a paleoanthropologist can determine the relative age of a fossil or artifact based on the strata in which it is found in comparison to other strata. It is crucial to realize, however, superposition can only determine a relative age, not an absolute age, because the time between the formation of strata can vary immensely. In the previously mentioned South African sites, fossils and artifacts are dated by other means. Here, anthropologists must rely on biostratigraphy. Biostratigraphy is a relative dating technique and, by definition, is the "study of fossils in their stratigraphic context." According to the principle of biostratigraphy, "fossils can be dated relative to their stratigraphic correlation with other fossils of known age" (Park, 2008).
One reason anthropologists must rely on biostratigraphy at the sites of South Africa is because "none of these sites contain rocks suitable for precise radiometric dating" (Conroy, 2005). There is also a fair amount of sediment mixing due to past mining activities (oddly enough, something that led to the discovery of these sites to begin with). Finally, South African sites have undergone an intense array of geologic changes. It is not simply a matter of finding a fossil or artifact in provenience and stating a temporal affiliation. These sites have experienced exposure to the surface. For example:
Swartkrans Cave in South Africa was initially formed by the solution of dolomite in ground water. [...] As the water level in the region dropped, the cavern filled with air, and rainwater caused joints or other natural planes of weakness in the rock to form in the dolomite overlying the cave. Eventually, one or more of the joints broke through the surface, thereby providing a direct passageway between the cave and the surface. (Conroy, 2005).
In turn, varying sediment, fossils, and artifacts have washed (or are sometimes dropped by predators) into these sites and are now intermixed and encased in breccia. Superposition obviously becomes a problem in such cases because finds are not necessarily deposited in situ.
A perfect example of biostratigraphic dating can be seen when we examine the studies of Lee Berger (2002). In the American Journal of Physical Anthropology, Berger et. al. examined the results of other studies in regard to the age of Australopithecus at Sterkfontein. Undoubtedly, the strength of the argument proposed by Berger et. al. relies mostly on the evidence that "some mammalian forms once thought to be confined temporally to the Pliocene, and thus of value in establishing palentological ages, are now known from the Pleistocene" (p. 193). In other words, the appearance of the mentioned mammalian forms in the same strata as an australopithecine does not necessarily state a Pliocene affiliation, as earlier supposed. Additionally, as stated in the research, the Sterkfontein site does not follow the principle of superposition. There are "areas of collapse and reworking where mixing of older and younger fossils is evident" (p. 193). As a whole, Berger et. al. have examined the paleontological data collected thus far and presented an alternative hypothesis in terms of biostratigraphy and relative dating of the Sterkfontein deposit:
Our interpretation of the fauna, the archeometric results, and the magnetostratigraphy of Sterkfontein indicate that it is unlikely that any Members yet described from Sterkfontein are in excess of 3.04 Ma in age. [...] Our results suggest that Australopithecus africanus should not be considered as a temporal contemporary of Australopithecus afarensis, Australopithecus bahrelghazali, and Kenyanthropus platyops. (p. 192).
Their observations are certainly worth consideration. In terms of the cave deposits of South Africa, they show the difficulty in determining the relative ages of fossils at these sites. The same remains true for stone tools affiliated, or not affiliated, with fossils discovered in South African cave deposits. Finds can be deposited in these caves a number of ways and, due to this, fossils or artifacts can be intermixed with others in the same strata but still not be situated in the same geologic time.
Berger, L., Lacruz, R., & de Rulter, D. (2002). Breief Communication: Revised Age Estimates of Australopithecus-Bearing Deposits at Sterkfontein, South Africa. American Journal of Physical Anthropology, 119, 192-197.
Conroy, G. (2005). Reconstructing Human Origins. New York: Norton.
Park, M. (2008). Biological Anthropology. New York: McGraw-Hill.