2003 Annual Science Report
Pennsylvania State University Reporting | JUL 2002 – JUN 2003
Evolution of Atmospheric O2, Climate, and the Terrestrial Biosphere - Stewart
Geochemical Tracers in Earth’s Oceans
Precambrian marine sediments record important information about the geochemical and redox evolution of the Earth’s earliest oceans. In collaborative work with former Pennsylvania State Astrobiology Research Center (PSARC) postdoc Michael Bau (now at International University Bremen), we find that the rare Earth element (REE) patterns in ~2.6 Ga carbonate from the Hamersley Basin, western Australia, are remarkably close to those of the present-day oceans, and that Nd isotopes of shallow carbonate are similar to those of the deeper banded iron formations. However, the Sm-Nd system shows evidence of resetting at 2.1 Ga from a regional metamorphic event, so ancient marine carbonate REE patterns must be interpreted with great care. In a parallel study of a present-day terrestrial drainage basin (Owens River system, eastern California), we find that coexisting clastic and chemical sediments derived from the eastern Sierra Nevada range show a small but systematic offset in Nd isotope composition. In this case, selective weathering of different minerals or parent lithologies appears to bias the Nd isotope record preserved in carbonate. This is consistent with our previous Sr isotope work, where we found that the Sr isotopic composition of the dissolved load is heavily weighted toward hydrothermal systems and acid generation by pyrite oxidation. In another study of REE behavior in organic-rich sediments (Permian coals), we find evidence for REE remobilization and incorporation in the organic matrix, although clastic mineral matter exerts the primary control on REE patterns.
Weathering in the Terrestrial Environment
Paleosols (ancient weathering profiles) preserve a record of Earth’s early atmospheric and climatic conditions. A neodymium isotopic investigation of the Archean Steep Rock paleosol, Canada, has allowed us the opportunity to quantify possible alteration effects on weathering REE patterns. We find that the REE patterns are substantially preserved at this ~3 Ga paleosol, despite later metamorphism and K-metasomatism. Graduate student K. Walden is currently applying this technique to the 2.6-2.4 Ga Kalkkloof paleosol, South Africa, which shows significant cerium anomalies (indicating possibly oxidizing conditions).
PROJECT INVESTIGATORS:Brian Stewart
PROJECT MEMBERS:Hiroshi Ohmoto
RELATED OBJECTIVES:Objective 2.1
Earth's early biosphere
Environmental changes and the cycling of elements by the biota, communities, and ecosystems