SEARCH FOR EARLY TERRESTRIAL LIFE: TANTALIZING HINTS FROM A TERMINAL PROTEROZOIC PALEOSOL

Soils form on sediments and bedrock exposed to the atmosphere and can record the effects of various processes, including changes in moisture content, temperature, and biological activity, which acted upon soil prior to burial. Although paleosols (ancient soils) can be subject to post-burial alteration, careful analysis of paleosols can yield information about the conditions in which they developed. Previously, Precambrian soils have been studied to determine the oxidation state of the atmosphere. Few recent studies have attempted to investigate the possibility of a microscopic terrestrial biota inhabiting Precambrian paleosols.

A previously unstudied terminal Proterozoic paleosol in a drill core from Elk Point, South Dakota has been analyzed for major oxides, sulfur, organic carbon, and iodine in order to determine if a terrestrial biota was present at this time. The paleosol developed on Precambrian metagabbro basement (~1.7 Ga) and is overlain by the basal Cambrian Mt. Simon Sandstone (0.54 Ga), placing its development chronologically near the metazoan explosion of the early Cambrian.

Major oxide profiles in the paleosol show complete loss of sodium as well as significant losses of calcium, magnesium, and manganese. Phosphorus is absent at the top of the profile, but present at lower depths. Some quantities of iron have been lost in the paleosol. There is no sulfur present, but the paleosol does retain a small amount of organic carbon (up to 0.11%), which has been extracted and analyzed for stable carbon isotopes. Iodine is an element that is closely associated with organic material. Preliminary data suggest that iodine is present in this Precambrian paleosol at variable concentrations. The oxide profiles are consistent with elemental loss through weathering reactions, and the presence of organic carbon and iodine in this paleosol indicates that a terrestrial biota may have been present at the terminal Proterozoic.