2002 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2001 – JUN 2002
Isotopic and Molecular Tracers of Life
Task 1. Stable Isotopic Tracers of Life (Fogel, Scott)
In January, James Scott was appointed a Staff Associate of the Geophysical Laboratory of the Carnegie Institution, at which time he was also named a formal Co-Investigator on our NAI team. Scott has continued his work on stable isotope projects from Lake Tanganyika and other microbial sediments. He collected particulate organic matter from the photic zone of Lake Tanganyika last year in collaboration with a team of biogechemists who measured chemical parameters in the lake. Scott’s goal is to determine whether hydrogen is a significant energy source for microbial growth in this unusual ecosystem.
Albert Colman, a newly appointed postdoctoral scientist, is working on developing O and S isotope tracers of phosphates and sulfates as tracers for microbial processing. He is setting up a lab to carry out this work. Whether this technique will work in a complex sedimentary environment is not known, but if it does, it is noteworthy that phosphates are found in Martian sediments. Oxygen isotopes might be signatures of whether they were cycled by microbes.
Kevin Boyce, an NAI Fellow based primarily at Harvard, has been investigating the carbon isotopic compositions of fossilized plants. He is working toward determining the evolutionary pathway for the structural compounds found in higher plants. This year he completed a comparison between an extant species and one of its ancient relatives. He has obtained additional specimens from the paleobotany department of the Smithsonian Institution.
Postdoctoral scientist Jan Toporski has made initial measurements on the isotopic composition of the Enspel formation, a 25-million-year-old deposit with preserved tissues. He found that the fossils in these rocks contain high amounts of organic nitrogen. The stable nitrogen isotopic compositions of these fossilized plants, animals, and bacteria will be used to determine fossil food webs. Toporski is studying the process of fossilization of bacteria in order to determine whether microbes might be detected isotopically in ancient sediments or Mars rocks.
Task 2. Molecular Spectroscopic Investigations into Ancient Biochemistry (Cody)
The colonization of the continents by vascular plants constitutes one of the more significant events in the development of Earth’s biosphere. It is likely that this event had a significant impact on climate and ocean chemistry. One development that may have facilitated continental colonization was the emergence of lignin biosynthesis. Lignin, a major structural biopolymer in extant plants, serves multiple functions, including strengthening of the cell wall, protection against microbial attack, and enhancing water storage capacity.
Together with Andrew Knoll (Harvard) and postdoctoral scientist Boyce, Cody analyzed organic matter preserved in cell walls of permineralized plant fossils using Scanning Transmission X-ray Microscopy (STXM) and spectroscopy at energies near the 1s absorption edge of carbon. Microchemical analyses were performed directly on cellulose acetate peels of the fossils, preserving information on the anatomical distribution of organic materials. Individual tracheid walls in both Eocene and Early Devonian fossils exhibit spatially distinct chemical zoning inherited from original wall biopolymers and cell wall microstructure. Molecular analysis of sub-micron domains using Carbon X-ray Absorption Near Edge Spectroscopy (C-XANES) documents the differential distribution of hydroxylated aromatic and alcohol (and/or ether) carbon in the inner and outer regions of tracheid walls. This zonation reflects the deposition of lignin and structural polysaccharides in Devonian plants following biochemical and developmental pathways similar to those of living tracheophytes.
Task 3. Life Detection in Extraterrestrial Materials (Steele)
Andrew Steele joined the staff of the Carnegie Institution and the CIW NAI team in October 2001. He has been outfitting a new life-detection laboratory. The facilities in his lab include a clean room, a working microbiology laboratory, an epi-fluorescence microscope facility, a DNA / protein chip maker and reader, a DNA extraction apparatus, real-time and standard polymerase chain reaction (PCR), Temperature Gradient Gel Electrophoresis (TGGE)/ Denaturing Gradient Gel Electrophoresis (DGGE) and SNP apparatus, and portable automatic photometric telescope (ATP) luminometry. Experimentation on fluorescence imaging of bacterial contamination of Martian meteorites has been initiated along with microscopic investigation of Apex chert samples. Initial analyses were completed on specimens collected from the Enspel formation. Steele and Toporski prepared a summary of available probes for use with the Mars Immunoassay Life Detection Instrument (MILDI) Project, for which the Principal Investigator is David S. McKay, NASA Johnson Space Center (JSC). Steele also collaborated with Montana State University colleagues on the testing of 15 antibodies and 10 extraction techniques relevant to microbial life detection. Steele has been a part of the Brasier et al. Team, whose article in Nature on a re-examination of the oldest reported microfossils has thrown open the debate on Earth’s earliest life. Further planned experiments on this topic include isotope measurements, ion microprobe analysis, electron microscopy, and Raman spectroscopy.
Task 4. Mass-Independent Sulfur Isotope Fractionation in Archean Rocks (Rumble)
Research is under way to validate the discovery by Farquhar and others of mass-independent sulfur isotope fractionations in Archean rocks and to extend the range of poly-isotope sulfur analytical data in both geologic time and space. That testing has proceeded along two complementary paths. The first was to analyze aliquots of the Ag2S analyzed by Farquhar et al. These results show agreement within 0.1 for most samples. New analyses of Ag2S prepared from a wide variety of Archean rocks not previously studied show a larger range in ∆33S, from 2 to +7 , than those published by Farquhar et al., but the distribution of anomalies with time is similar. Analyzing aliquots of Ag2S, however, does not test whether the preparation of Ag2S, itself, introduces mass-independent artifacts.
The second pathway to testing validity is to use the capability of laser fluorination to analyze sulfide minerals by direct fluorination with no chemical preparation involved. Analysis by laser fluorination measures isotope heterogeneity directly, whereas preparation of Ag2S homogenizes local heterogeneity. One should expect to see similar patterns in analytical results plotted against geologic age from the two methods, but not necessarily exact duplication. The results of this test are not complete, but some similarity in the patterns of analyses is evident. In situ analyses with an ultraviolet laser (KrF excimer, 248-nm wavelength) show small mass-independent fractionations in pyrites from the age range 1.9 to 2.3 Ga but a large range in ∆33S, from 1.5 to +3.0 , in rocks older than 2.5 Ga. Whole grain analyses of pyrite with an infrared laser (CO2 laser, 10.5-ɭm wavelength) show ∆33S values of 0 to 1 from 2.7 to 3.8 Ga. The most extreme values measured on Ag2S have not yet been found with in situ or whole-grain analysis, but this is at least in part because of the fact that both Ag2S and untreated rock from the same site have not been available. Additional Ag2S and untreated rock samples are being obtained for analysis.
New analytical results on poly-isotopic sulfur demonstrate the possible role of the hydrosphere in transferring an isotopic anomaly of atmospheric origin into the lithosphere. The volcanogenic massive sulfide deposits of the Superior District formed in a 2.7-Ga hydrothermal system located at the sea floor. Mass-independent sulfur isotope anomalies with ∆33S from 0.2 to 1.1 have been found by B. Wing (U. Maryland) in these deposits. A preliminary hypothesis suggests that an atmospheric ∆33S anomaly was transferred to seawater sulfate and subsequently, during seafloor hydrothermal alteration, was incorporated into ore minerals of the district.
The data of Farquhar et al. suggested that a transition from mass-independent to mass-dependent fractionation coincided with the time interval during which there is evidence for the oxygenation of the Earth’s atmosphere. The new data show a sharp break in distribution at 2.3 Ga. Before this time there are large excursions in ∆33S with values as high as +3.5 to +7.0 and as low as 1 to 2 . After this time, the range in ∆33S is less than +/- 0.5 . The new data show ∆33S values of essentially zero between 0.7 and 1.5 Ga. The irregular distribution of values in the Archean may be due to sampling; in localities where more than a few samples have been analyzed, mass-independent anomalies are seen.
PROJECT MEMBERS:George Cody
RELATED OBJECTIVES:Objective 1.0
Determine whether the atmosphere of the early Earth, hydrothermal systems or exogenous matter were significant sources of organic matter.
Replicating, catalytic systems capable of evolution, and construct laboratory models of metabolism in primitive living systems.
Describe the sequences of causes and effects associated with the development of Earth's early biosphere and the global environment.
Define how ecophysiological processes structure microbial communities, influence their adaptation and evolution, and affect their detection on other planets.
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Determine the presence of life's chemical precursors and potential habitats for life in the outer solar system.
Determine (theoretically and empirically) the ultimate outcome of the planet-forming process around other stars, especially the habitable ones.
Determine the resilience of local and global ecosystems through their response to natural and human-induced disturbances.
Understand the human-directed processes by which life can migrate from one world to another.