Chemistry of Modern Soils as a Guide to Interpreting Precambrian Paleosols

Precambrian paleosols are potentially one of our best sources of information about the composition of the atmosphere early in earth's history. To be used this way, however, an alteration profile must be shown to be an in situ product of vadose processes. Data from modern soils show that the ratios of less mobile elements such as Zr, Ti, and Al can be used as a preliminary screen to test for the in situ nature of a profile. If the Ti/Zr ratio departs from that of the host rock by more than 40%, or the Ti/Al ratio departs by more than 50% for moderate degrees of weathering, then a significant allochthonous contribution to the profile is indicated. This technique correctly separates in-place from allochthonous horizons when applied to independent data from two modern profiles. A review of published Precambrian profiles identifies two often-cited examples, Denison and Pronto, as suspect based on these criteria. In the interpretation of paleoatmospheres, it is also necessary to show that Fe is controlled not by diagenetic but by pedogenic processes. Comparison of the distribution of Fe to that of Ti and Zr shows that Fe is generally leached from confirmed paleosols older than 2.2 Ga, but is retained in modern soils and in younger paleosols. Reducing conditions developed during diagenesis can, however, produce a removal of Fe in modern alteration profiles that is very difficult to distinguish from removal by soil processes under a low-oxygen atmosphere. Diagenetic and pedogenic Fe removal can possibly be distinguished by the presence of a vertical zonation with a lower zone of Fe enrichment in the pedogenic case.