U.W. Madison Early Earth & Astrobiology

Radiogenic Isotope Laboratory

It is now clear that there has been a strong feedback loop between the tectonic-magmatic-metamorphic history of the Earth and the origin and evolution of life for perhaps 4 billion years. There are several components to this story that we are currently investigating:

1. Geomicrobiology: When did various metabolic pathways develop on the early Earth?

2. Early Earth Geology: What were the changes in surface conditions on Earth in the Archean?

3. Astrobiology: How was the development of life on Earth related to the history of other planetary bodies, such as Mars?

1. Geomicrobiology:

	Our focus has been on iron-based metabolisms, involving both reduction and oxidation because iron is the most abundant redox-sensitive element in the Solar System. The primordial redox state of Fe on the terrestrial planets was Fe²⁺, and this may have been oxidized through anaerobic photosynthetic Fe oxidation in the absence of an oxygenated atmosphere. The end product of Fe²⁺ oxidation, by biology or through abiologic means, is ferric hydroxide or oxide minerals. We have been studying the pathways involved in producing ferric hydroxide/oxide minerals by biological Fe²⁺ oxidation, as well as abiologic mechanisms. Once produced, these ferric hydroxide/oxide minerals in turn provide excellent energy sources for dissimilatory iron-reducing bacteria (DIRB), which are ubiquitous on the Earth today, and likely reflect a metabolic pathway that was developed in the Archean. We are investigating the interplay between specific DIRB species and the reduced ferrous Fe end products, including aqueous Fe(II), siderite (FeCO₃), and magnetite (Fe₃O₄), largely in collaboration with Prof. Eric Roden in our Department. For example, the SEM image in the upper left is siderite (photo about 10 microns across) that was formed after a DIRB experiment that lasted over one year. In the SEM image in the middle left, large magnetite crystals (~ 10 micron) are seen after 150 days in another DIRB experiment. Identification of the very fine-grained material requires other imaging techniques; the TEM image in the lower left (200 nanometers across) confirms that this is mostly magnetite, although some secondary goethite crystals are also seen. *Finding a biological fingerprint - isotopic compositions:* The minerals produced in these experiments may be preserved in the ancient rock record on Earth or other planetary bodies such as Mars. But how do we tell if such minerals were formed by biologic or abiologic processes? The morphologies of the minerals are generally similar by either process. We have instead focused on the isotopic compositions of the elements, mainly Fe and O in the case of oxides, and additionally C in the case of carbonates. Our current experiments involve both biologic and abiologic formation pathways, and are aimed at determining if there are unique "isotopic biosignatures" for C, O, and Fe isotopes in magnetite and Fe-bearing carbonates. In addition, is now appears that biologically-catalyzed carbonate formation produces chemical compositions in the ternary systems CaCO₃-MgCO₃-FeCO₃ that are not predicted from equilibrium thermodynamics, adding another layer to the biosignature fingerprint. Finally, for Ca- and Mg-bearing carbonates, we will soon be exploring the Ca and Mg isotope variations, which preliminary data suggest may be used as a paleothermometer. It is also possible that there are kinetic isotope fractionations produced dependent upon the pathway, be it biological or abiological. For a review on the geomicrobiology of Fe isotope geochemistry, see this paper.

2. The Early Earth:

We have a large collaborative program on studies of Archean sedimentary sequences that are aimed at understanding the origin and evolution of life and changes in the surface environments of the Earth.

The oldest sedimentary rocks on Earth are approximately 3.8 billion years old and crop out in SW Greenland in the Isua Supracrustal Belt. These include sequences such as banded iron formations (BIFs), which contain iron oxides (e.g., hematite, magnetite) and Fe-bearing carbonates (e.g., siderite). Although these rocks are attractive to study because of their great age, they have been metamorphosed, and so comparative studies of lower-grade BIFs is also important. The photo at left shows the famed Late Archean Brockman Iron Formation of the Pilbara Craton in western Australia (photo by Hiroshi Ohmoto, Penn State). These have been left undisturbed in terms of metamorphism or tectonics for 2.5 billion years (notice they are still flat lying), but they are deeply weathered (notice the red rust color).

The photo on the left shows a fresh drill core of the Late Archean Kuruman Iron Formation (South Africa), which, in weathered outcrop, would look identical to the red beds of the photo above. In the fresh drill core, the pinkish bands are siderite (first to weather), the black bands are magnetite, and the grey layers are primary hematite + chert, reflecting alternating layers (perhaps annual seasons) of oxide deposition and silica deposition. The Fe isotope data suggest that siderite was always precipitated from mid-ocean ridge hydrothermal fluids, and hence is not likely of biological origin. However, the magnetite appears to be a mixture between biological sources of Fe produced by DIRB and mid-ocean ridge hydrothermal fluids. We are in the process of cross-correlating these variations with O isotope studies to see if the biologically-produced magnetite occurs under unique temperature of pore fluid environments.

In addition to work on Archean BIFs, we are also studying other Archean sedimentary sequences, with a particular eye on coupled Fe-S isotope variations, which may constrain the development of dissimilatory iron-reducing and sulfate-reducing metabolisms, respectively.

3. Astrobiology:

The Mars rover Opportunity made several stunning discoveries that provide clear evidence for the presence of liquid water at some time in the past on Mars. These include the occurrence of iron oxide concretions as well as sulfate minerals. In the photo below (credit: Cornell University/NASA), the randomly-oriented features are cavities that reflect dissolution of sulfate minerals from the bedded sedimentary rock. Although many of the sulfate minerals are Ca- and Mg-sulfates, an important discovery was finding the ferric-Fe sulfate mineral jarosite - this mineral is common in acid-sulfate systems on Earth that are produced by bacterial oxidation of pyrite. The rounded blue-colored spheres in the upper part of the photo are iron oxide concretions (false color due to data transmission issues) that also seem likely to have required formation through fluids.

We are in the process of studying the chemical, mineralogical, and isotopic fingerprints of sulfate and iron oxide formation in biologic and abiologic systems. This work includes studies in the laboratory of pyrite oxidation under biologic and abiologic conditions, including investigation of the O, S, and Fe isotope fractionations that are produced. The ultimate goal of this work is to see if we can map our the formation pathways and to determine the isotopic and chemical biosignatures of sulfate minerals that are formed. In addition, we are working on terrestrial field sites, which provides us a real-world comparison with our laboratory studies. These include work on natural acid-sulfate systems, such as at Rio Tinto, Spain, and iron oxide concretions in the Jurassic Navajo Sandstone, Utah.

At Rio Tinto (photo above; credit: Max Coleman, JPL), two end member fluids are produced: a ferric iron- and sulfate-rich red water (main photo above) that reflects near complete oxidation of both Fe and S during pyrite weathering by bacteria, and a green, sulfate-rich but ferric iron-poor water (bottle in inset photo above) that record S oxidation but not Fe oxidation; this later fluid is likely to record an abiologic pathway, where Fe²⁺ oxidation is very slow. This work is in collaboration with NASA-JPL scientists.

We are collaborating with University of Utah researchers on iron oxide concretions that are remarkably similar to those observed by the Mars rover Opportunity. These concretions are contained within the Navajo Sandstone of Utah, which is famous for its spectacular cross-bedding. The concretions can become quite large (see photo below; credit: Marjorie Chan, Univ. Utah), and appear to have formed through re-mobilization of the primary iron oxide cement. Preliminary Fe isotope data suggests that this re-mobilization occurred by dissimilatory iron-reducing bacteria, providing an "isotopic fingerprint" for their origin. In addition, a wide variety of mineralogical, geochemical, and O isotope studies on these sequences is underway.

Finally, searching for evidence of past life on another planetary body requires chemical and isotopic analyses of samples from places like Mars. Although a handful of Martian meteorites exist, and in fact, have formed the basis for proposals that life once existed on Mars, meteorites do not provide samples of the spectacular sedimentary sequences discovered by the Mars rovers. Eventually, NASA intends to send sample return missions to Mars, but on a shorter timeframe, additional robotic missions are planned. It is therefore important to develop methods for in situ chemical and isotopic analysis on Mars, and we have been working with NASA-JPL scientists on such approaches. The miniature mass spectrometer that is required to fit within the strict weight and power constraints of robotic missions is being developed at JPL, and we are working on the sample inlet and purification steps required at U.W. Madison.

Many of these initiatives are being pursued in cooperation with the NASA Astrobiology Institute.

Research Group:

At U.W. Madison:

	Brian Beard
	Heidi Crosby
	Morgan Herrick
	Clark Johnson
	Eric Roden

	Nita Sahai
	John Valley
	Rene Wiesli
	Huifang Xu

At Other Institutions:

	Nik Beukes, Rand Afrikans Univ., South Africa
	Paul Braterman, Univ. N. Texas
	Marjorie Chan, Univ. Utah
	Max Coleman, Jet Propulsion Lab, NASA
	Kase Klein, Univ. New Mexico
	Stephen Moorbath, Oxford Univ., U.K.
	Kenneth Nealson, Univ. Southern Calif.
	Dianne Newman, Caltech
	Hiroshi Ohmoto, Penn. State
	Chris Romanek, Univ. Georgia,
	Mahadeva Sinha, Jet Propulsion Lab, NASA
	Kosei Yamaguchi, IFREE-JAMSTEC, Japan

Selected Publications:

Johnson, CM and Beard, BL (2005) Biogeochemical cycling of iron isotopes. Science 309:1025-1027. [PDF] (835kb).

Crosby, HA, Johnson, CM, Roden, EE, and Beard, BL (2005) Coupled Fe(II)-Fe(III) electron and atom exchange as a mechanism for Fe isotope fractionation during dissimilatory iron oxide reduction. Environ. Sci. Technol. 39:6698-6704 [PDF] (1551kb).

Yamaguchi, KE, Johnson, CM, Beard, BL, and Ohmoto, H (2005) Biogeochemical cycling of iron in the Archean-Paleoproterozoic Earth: Constraints from iron isotope variations in sedimentary rocks from the Kaapvaal and Pilbara Cratons, Spec. Issue on Isotopic Biosignatures, Chem. Geol. 218:135-169. [PDF] (888kb).

Johnson, CM, Roden, EE, Welch, SA, and Beard, BL (2005) Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction. Geochim. Cosmochim. Acta 69: 963-993. Main paper: [PDF] (1004kb). Electronic Appendix EA-1 (Details on Methods): [PDF] (34kb). Electronic Appendix EA-2 (SEM and TEM images of run products): [PDF] (3285kb).

Beard, B and Johnson, C (2004) Chapter 10A: Fe isotope variations in the modern and ancient Earth and other planetary bodies. In “Reviews in Mineralogy and Geochemistry: Geochemistry of Non-Traditional Stable Isotopes”. 55:319-357. [PDF] (810kb).

Johnson, C, Beard, B, Roden, E, Newman, D, and Nealson, K (2004) Chapter 10B: Isotopic constraints on biogeochemical cycling of Fe. In “Reviews in Mineralogy and Geochemistry: Geochemistry of Non-Traditional Stable Isotopes”. 55:359-408. [PDF] (489kb).

Croal, LR, Johnson, CM, Beard, BL, and Newman, DK (2004) Iron isotope fractionation by Fe(II)-oxidizing photoautotrophic bacteria. Geochim. Cosmochim. Acta 68:1227–1242. [PDF] (331kb).

Welch, SA, Beard, BL, Johnson, CM, and Braterman, PS (2003) Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III). Geochim. Cosmochim. Acta 67:4231-4250. [PDF] (358kb).

Beard, BL, Johnson, CM, Von Damm, KL, Poulson, RL (2003) Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans. Geology 31: 629-632. [PDF] (271kb).

Beard, BL, Johnson, CM, Skulan, JL, Nealson, KH, Cox, L, and Sun, H (2003) Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Special issue on Isotopic Record of Microbially Mediated Processes. Chem. Geol. 195:87-117. [PDF] (684kb).

Beard, B.L., Johnson, C.M., Cox, L., Sun, H., and Nealson, K.H. (1999) Iron Isotope Biosignatures: Science, 285, 1889-1892. [PDF] (103kb).

For more information, contact Clark Johnson at clarkj@geology.wisc.edu

Last revised: 10/19/07