Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p54a..03h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P54A-03
Other
0424 Biosignatures And Proxies, 0790 Weathering (1625, 1886), 1039 Alteration And Weathering Processes (3617), 5225 Early Environment Of Earth, 9621 Cambrian
Scientific paper
Before the advent of plants with spores and body structures that are easily preserved in the fossil record, the evidence for terrestrial ecosystems is limited to observations of their effects on the chemistry of weathering surfaces. Seven paleosols and paleosaprolites from southeast South Dakota and northwest and central Iowa have been analyzed to assess the extent of abiotic weathering and the presence of biosignatures in the Midwestern US during the Middle Cambrian. Evidence for extensive weathering exists on a variety of basement material, from the 1800 Ma Harris granite to the 1100 Ma Keweenawan basalts. All of the weathering profiles are overlain by the Middle Cambrian Mt. Simon Sandstone, placing their most likely time of development shortly before the advent of vascular plants. All weathering horizons show high chemical indices of alteration, typically >85, and considerable leaching of mobile cations, such as Na, Ca, Mg, and Mn, which is consistent with tropical to subtropical weathering conditions expected for central Laurentia during this interval. Organic carbon contents of <0.2 wt% are present in all weathering horizons, indicating the potential presence of a terrestrial biota. Carbon isotope values for this organic matter are ~-25 permil for paleosols in the western portion of the study area, and ~-23 permil for paleosols in the eastern half. These results are consistent with a photosynthetic origin for the organic matter. Organic matter in the overlying sandstone tends to be lighter than the soil organic matter. At most sites, this difference is 1 permil, except at the Elk Point site, where the difference is 27 permil, suggesting methanotrophic influence on this organic matter. A strong biosignature present at most sites is the near-complete loss of P from the surface. P is an important limiting nutrient in the terrestrial environment. Near-complete apatite dissolution at the surface of most of the weathering horizons is best explained by leaching via organic acids, most likely from microbial mats or lichens, the putative Middle Cambrian flora based on previous geochemical and molecular clock studies. Strong leaching of P from paleosol and paleosaprolite surfaces during the Middle Cambrian is in stark contrast to incomplete P loss observed in Mesoproterozoic and Neoproterozoic paleosols by other researchers, indicating that terrestrial ecosystems became widespread and increasingly affected soil chemistry during the Cambrian prior to the earliest fossil evidence of terrestrial plants.
Horodyskyj L. B.
Kump Lee R.
White Timothy S.
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