Investigation 3

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Research Investigation 3: Testing covariations of biosignatures and paleoenvironmental indicators in natural terrestrial systems

The comprehensive experimental program that underlies Investigation 2 will provide the interpretive framework for understanding the isotopic, chemical, and mineralogical record of life and environments that supported life. In Investigation 3, we will test covariations among these parameters, including multiple isotopic systems in natural terrestrial environments from the earliest rock record 3.8 b.y. ago to present. It is likely that the natural observations in Investigation 3 will in turn feedback into Investigation 2 as our understanding increases regarding the processes that fractionate stable isotopes in biologic and abiologic systems. However, Investigation 3 is intended to go beyond an experiment-nature feedback loop, and this work will provide strong constraints on the evolution of surface environments on Earth and the role of life in determining the nature of those environments. We have designed Investigation 3 to complement other studies of the terrestrial record that are being pursued by other NAI groups, and we have established additional collaborative ties within NAI in our research plan.


Field photo of the 2.5 b.y. old Dales Gorge banded iron formation (BIF) in NW Australia.

Our objectives in Investigation 3 are to:

  • Field test models for sulfur and iron oxidation at Rio Tinto, Spain, and determine the pathways involved in formation of Fe, Ca, and Mg sulfates in such systems and their isotopic fingerprints;
  • Determine the relative roles of abiologic and biologic processes that formed iron oxide concretions in the Navajo Sandstone, a Utah analog for iron concretions discovered on Meridiani Planum, Mars;
  • Study the origin of ferric iron oxides in paleo-weathering horizons and oolitic banded iron formations (BIFs) of 3.5 to 2.5 Ga age, which will constrain the nature of oxidants on the early Earth;
  • Investigate the evolution of sulfate- and Fe-reducing bacteria in early Archean sedimentary sulfides and magnetite deposits; Determine the genesis of early Archean chert layers that contain low- 13C /12C carbon;
  • Understand the oxidative pathways involved in ferric iron oxide formation in key BIFs that formed between 3.8 and 2.5 b.y. ago, as well as determine the role of Fe(III)-reducing bacteria in formation of magnetite and siderite from the same BIFs.

Recent work has laid the foundations for understanding the systematic isotopic behavior of the processes operating in the Rio Tinto, as related to both S and Fe oxidation, as well as production of sulfate and iron oxide minerals. The important discovery of iron oxide spherules by the Mars rover Opportunity, which apparently have weathered out of sedimentary rocks has greatly increased interest in studies of terrestrial analogs. The textural relations of these iron oxide spherules suggest in situ formation, possibly by flowing groundwaters, and similar iron oxide concretions in the Jurassic Navajo Sandstone of Utah will be studied to gain insights into the processes and conditions that may have produced the iron oxide spherules on Mars. In addition, we will investigate the possibility that iron oxide concretions may form through oxidation of sulfides, which are common in C-rich sedimentary rocks. Our focus on the Archean terrestrial record is aimed at finding new approaches to studying long-standing and continued controversies on the origin and evolution of various metabolic pathways on Earth, including photosynthetic pathways that determined the rise of atmospheric O2. We have chosen several key sedimentary sequences in the terrestrial Archean rock record from three continents (Greenland, Africa, and Australia), and we plan work on samples obtained from the NAI-sponsored Archean Biosphere Drilling Program in the Pilbara craton of Western Australia.

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