![[Photo]](site-graphics/bob-deathvalley2005.jpg)
| Death Valley, California. (Photo by R. Petterson) |
Science
The present is a guide to the past, albeit an imperfect one, and the past is one of our best maps to the future: these two precepts guide much research in Earth history and Earth system processes, my own included. I am interested in questions like: How do records of biogeochemical processes get preserved in the rock record? What is the record of life from the early Precambrian to the Recent, and how does it reflect global biogeochemical conditions? What sorts of changes in global biogeochemical and climatic conditions have occurred in the past, and what lessons do these past changes hold for the future? The geological record is filled with evidence of alternate states of the Earth system: from the anoxic Earth of the Archean, to the Snowball Earths of the Paleoproterozoic and Neoproterozoic, to the Hothouse Earth of the Mesozoic and early Paleogene, to the Pleistocene Ice Age Earths. Understanding the nature of alternate Earths like these and the transitions between different states is the best way of testing models of Earth's future.
The lens through which I spend most of my time looking is forged of iron. Iron is the fourth-most abundant element in the Earth's crust, after oxygen, silicon, and aluminum. Unlike silicon and aluminum, it is redox-active under surface conditions, which makes it a critical element in Earth surface processes: an element essential for living organisms and a key player in many abiotic reactions as well. Thus, though iron is only one among a large number of elements, techniques that reveal the nature and abundance of iron-bearing minerals have applications in many areas of the Earth sciences, from plate tectonics to geobiology. Because iron-bearing minerals are frequently magnetic, magnetism and related physical techniques for studying iron-bearing natural samples, like ferromagnetic resonance (FMR) spectroscopy, are versatile tools that provide high sensitivity levels while being non-destructive and often relatively rapid.
Preservation of magnetization in carbonate sediments
In carbonate platform sediments, what are the sources of magnetization and what controls their preservation?
Carbonate platforms are a common marine sedimentary environment from the late Archean onward. Carbonate platform sediments therefore form an important archive for sedimentary paleomagnetism. To interpret this archive properly, however, it is necessary to understand the sources of magnetic materials in carbonate sediments and the factors that control their preservation. Moreover, a better understanding of these sources and processes will facilitate interpretation of the paleoenvironmental information carried by the makeup of sedimentary magnetic particles. Toward that end, Adam Maloof (Princeton University) and I are studying cores of carbonate sediments from the Three Creeks Region, Andros Island, the Bahamas. Our approach combines paleomagnetic, rock magnetic, and FMR techniques with sedimentological, geochemical, and electron microscopy analyses.
In general, possible sources of magnetic sediments include: lithogenic particles transported by wind or water, biogenic particles produced either by magnetotactic bacteria or bacteria that produce magnetic minerals extracellularly as a byproduct of iron reduction or iron oxidation, iron oxides and iron sulfides produced by diagenetic processes, and anthropogenic pollutants. On Andros Island, lithogenic input is minimal, while our preliminary FMR and rock magnetic data suggest that the microbial contribution to the magnetization of the sediments is considerable. The strong microbial role is consistent with the presence of layered microbial communities, which frequently grow in the uppermost sediments and would have been even more common in the Precambrian carbonate platforms. Our analyses of the cores will determine how diagenetic processes like redox chemistry and bioturbation disrupt paleomagnetic signals and alter the magnetic mineralogy. These studies will thereby facilitate interpretation of records in carbonate platforms in general.
The Paleoproterozoic Snowball Earth and the Rise of Oxygen
What was the nature of the transition from an anoxic to an oxygenated Earth?
In the Archean eon, three billion years ago, Earth's atmosphere was free of oxygen. This absence is reflected in numerous proxies, particularly the record of mass-independent fractionation of sulfur isotopes. By two billion years ago, in the middle of the Paleoproterozoic era, many proxies suggest that oxygen had built up to a small but significant level in the atmosphere. Aside from the origin of life itself, the transition from the anoxic Archean to the oxic Proterozoic is the most radical change to occur to occur in the history of the Earth system.
Some researchers suggest that this transition occurred gradually, perhaps driven by a slight change in the oxidation state of the mantle or the onset of plate tectonics. Based on a critical analysis of the available data, with a strong dose of caution applied to the use of uniformitarian interpretations in such a radically different world, Joe Kirschvink (Caltech) and I have suggested instead that the transition occurred rapidly. We propose that the Makganyene Snowball Earth event, which occurred at ~2.3 billion years ago, was a direct consequence of the transition. During the Snowball Earth, the presence of which is indicated by glacial deposits with low paleolatitudes from the Transvaal Supergroup in South Africa, the entire planet may have been sheathed in ice for tens of millions of years.
Based on a sample biogeochemical flux model I constructed, if the Archean Earth was kept warm by a methane greenhouse, then the evolution of oxygenic photosynthesis could have triggered a Snowball Earth event on a time scale as short as about a million years. Rather than a gradual transition, the anoxic-to-oxic transition, triggered by a chance evolution occurrence, may have been a catastrophic event: the world's first biologically-caused climate disaster.
This hypothesis is an end-member hypothesis, but it is testable and serves to motivate research in this critical interval in Earth history.
Ferromagnetic resonance and rock magnetic identification of magnetotactic bacteria
How can we identify fossil magnetotactic bacteria?
One of the major focuses of my recent work has been the development of FMR spectroscopy as a tool to supplement more traditional rock magnetic techniques. FMR spectroscopy is a form of microwave spectroscopy based on the resonant absorption of microwaves by electrons precessing in a magnetic field. In magnetic materials, FMR provides a way of assessing the internal fields generated by interparticle interactions and by particle anisotropy. It is thus a macroscopic tool for assessing microscopic particle arrangement and structure.
FMR spectroscopy is a rapid technique that generally requires about five minutes to acquire a single spectrum and is thus extremely useful for high-resolution stratigraphic sampling. Although FMR spectra are generally measured using an electron paramagnetic resonance spectrometer, which employs a fixed frequency microwave source and a variable magnetic field, a group at the Jet Propulsion Laboratory is working on developing a zero-field, variable-frequency, field-ready FMR instrument.
Because FMR is sensitive to microscopic particle arrangement and structure, one potential application is screening for samples likely to contain fossil magnetotactic bacteria. Magnetotactic bacteria are a group of bacteria defined by the common trait of generating intracellular chains of magnetic particles, which passively orient them within the Earth's magnetic field. These particles have narrow distributions of size, shape, and arrangement that increase the efficiency with which the bacteria produce a magnetic moment. This optimization suggests that the chains of magnetic particles provide the bacteria with a selective advantage, likely related to navigating within the sharp chemical gradients in which the bacteria commonly live. These distinctive, optimized traits are all in theory detectable with macroscopic physical techniques.
Using laboratory-grown cultures of magnetotactic bacteria, I have demonstrated that these traits generate distinctive FMR spectra. Mutants grown by Cody Nash (Caltech) have allowed me to untangle the different contributions made to the FMR spectra of bacterial magnetite by the magnetite crystal structure, particle elongation, and chain arrangement. I have also constructed first-order physical models of ferromagnetic resonance that fit many of the features observed well. There remains, however, much physical modeling work to be done by a student with a strong physics focus.
Although FMR spectroscopy is a 60-year-old technique, its application to natural samples is currently in its infancy. At present, therefore, it is still important to use FMR in conjunction with standard rock magnetic techniques. Rock magnetism provides supporting evidence for certain traits that contribute to FMR spectra, such as sample composition, particle size, and three-dimensional interparticle interactions. My work with the mutant bacteria has demonstrated a correlation between these traits as measured by rock magnetism and as extracting from FMR spectra.
FMR spectroscopy is the only technique, however, capable of measuring the distinctive microscopic anisotropy produced by particle elongation and arrangements in chains. Moreover, rock magnetic experiments are considerably slower than FMR measurements, so I therefore use FMR to select representative samples for rock magnetic experiments. The complementary use of FMR and rock magnetism is therefore ideal for high-resolution stratigraphic measurements of magnetic properties.