1996. Fossilization processes in siliceous thermal springs: Trends in preservation along thermal gradients. pp. 150â173. In Gregory Bock and Jamie Goode (ed) ...
Workshop on Early Mars (1997)
3031.pdf
FOSSILIZATION PROCESSES IN MODERN THERMAL SPRINGS: CLUES FOR ASSESSING THE BIOGENICITY OF ANCIENT HYDROTHERMAL DEPOSITS. Jack D. Farmer(1) Sherry Cady(1) David J. Des Marais(1) and Malcolm Walter(2) (1) NASA Ames Research Center, MS 239-4, Moffett Field, CA 94035-1000; (2) School of Earth Sciences, Macquarie University, North Ryde, NSW, Australia. Hydrothermal systems have been cited as important targets in the search for evidence of an ancient biosphere on Mars (1,2). Indeed, such environments appear to have been widespread on Mars earlier in its history (3), while subsurface hydrothermal systems may have provided clement environments for life throughout the subsequent history of planet. Comparative studies of modern and ancient hydrothermal systems on Earth have the potential to provide important constraints on longterm evolutionary trends in hydrothermal ecosystems (4). And the results of such studies also assist in refining strategies to explore Mars for a fossil record of ancient hydrothermal life. To create a comparative framework for interpreting the fossil record of ancient hydrothermal deposits on the Earth, and possibly Mars, we have carried out parallel studies of the microbial biosedimentology, taphonomy and geochemistry of active hydrothermal environments in Yellowstone National Park. One goal of the research is the development of highly integrated biosedimentological and paleontological models for siliceous (5,6), carbonate (7) and Fe-oxideprecipitating springs (8, and Farmer et al., work in progress). In this report we emphasize fossilization processes in subaerial siliceous thermal springs over a broad range of temperatures. Although such studies are primarily intended to provide a basis for evaluating the microbial contributions to the fossil record of ancient hydrothermal deposits on Earth, they also have implications for the biogenicity of suspect “nanofossils” in Martian meteorite ALH84001 (9). The microbial contributions to sedimentary fabric in subaerial hydrothermal deposits take on importance where population growth rates keep pace with, or exceed rates of inorganic mineral precipitation, thus allowing for the development of continuous biofilms or mats. The microorganisms of mineralizing thermal springs are typically entombed while they are still viable. But the precise modes of preservation reflect a balance between rates of organic matter degradation, primary mineral
precipitation and secondary infilling during early diagenesis. At the cellular level, the biological information of siliceous thermal spring deposits results from the encrustation of individual cells and filaments primarily within undermat environments. Subaerial sinters are initially quite porous and permeable, and at temperatures higher than about 20–30°C, organic materials are rapidly degraded prior to the infilling (cementation) of sinter frameworks. This appears to explain why organically-preserved microfossils are rare in subRecent and ancient siliceous sinters (Walter et al. 1996) and why observed values for total organic carbon in ancient siliceous sinters are very low (Des Marais, unpublished observations). In ancient sinter deposits, the diagenetic transformation of primary amorphous silica to secondary quartz usually results in the loss of all cellular level information. However, taxes within microbial communities often lead to clumping of cells and/or preferred filament orientations that together define higher order composite mat fabrics (e.g. network, coniform, and palisade). For the photoautotrophic cyanobacterial commnunties that dominate subaerial systems below 73°C, such composite (community-level) fabrics frequently survive diagenesis, providing important clues for biogenicity in ancient deposits (10). Similarly, at temperatures >90°C, siliceous spring communities are dominated by filamentous hyperthermophiles that form biofilms covering most near-vent surfaces. Biofilms localize silica nucleation and appear to contribute significantly to the morphogenesis of various types of geyserite deposits (11). Although certain aspects of the laminated microstructure of geyserites may reflect biogenesis, cellular level preservation is extensively overprinted and lost during infilling, recrystallization and the ordering of amorphous silica during early diagenesis (6). REFERENCES: [1] Walter, M. R. and D. J. Des Marais. 1993. Preservation of biological information in thermal spring deposits: Developing a strategy for the search for a fossil record on Mars. Icarus 101: 129–143. [2] Farmer, J., and D. Des Marais. 1994.
Workshop on Early Mars (1997)
3031.pdf Biogenicity of Ancient Hydrothermal Deposits. J. D. Farmer et al.
Exopaleontology and the search for a fossil record on Mars. Lunar Planetary Science 25: 367–368. [3] Farmer, J.D. 1996. Hydrothermal Processes on Mars: An Assessment of Present Evidence. pp. 273– 299. In Gregory Bock and Jamie Goode (ed) Evolution of Hydrothermal Ecosystems on Earth (and Mars?). John Wiley and Sons. 334 p. [4] Walter, M.R. 1996. Ancient hydrothermal ecosystems on Earth: A new paleobiological frontier. pp. 112–127. In Gregory Bock and Jamie Goode (ed) Evolution of Hydrothermal Ecosystems on Earth (and Mars?). John Wiley and Sons. 334 p. [5] Farmer, J.D., Sherry Cady and David Des Marais. 1995. Fossilization processes in thermal springs. Geological Society of America, Abstracts With Programs, 27(6): 305. [6] Cady, S.L. and J.D. Farmer. 1996. Fossilization processes in siliceous thermal springs: Trends in preservation along thermal gradients. pp. 150–173. In Gregory Bock and Jamie Goode (ed) Evolution of Hydrothermal Ecosystems on Earth (and Mars?). Wiley and Sons. 334 p. [7] Farmer, J. D. and D. J. Des Marais. 1994. Biological versus inorganic processes in stromatolite morphogenesis: Observations from mineralizing
systems. pp. 61–68. In L.J. Stal and P. Caumette (eds.). Microbial Mats: Structure, Development and Environmental Significance. NATO ASI Series in Ecological Sciences, Springer Verlag. 8. Agresti, D.G., T.J. Wdowiak, M.L. Wade, L.P. Armendarez and J.D. Farmer. 1995. A Mossbauer investigation of hot spring iron deposits. Lunar Planetary Science 26: 7–8. [9] McKay, D.S., Gibson, E.K, ThomasKeprta, K.L., Vali, H., Romanek, C.S., Clemett, S.J., Chillier, X.D.F., Maechling, C.R. and Zare, R.N. 1996. Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001. Science 273, 924–930. [10] Walter, M.R., D.J. Des Marais, J.D. Farmer, and N.W. Hinman. 1996. Paleobiology of mid-Paleozoic thermal spring deposits in the Drummond Basin, Queensland, Australia. Palaios 11(6): 497–518. [11] Cady, Sherry, J. D. Farmer, David Des Marais and David Blake. 1995. Columnar and spicular geyserites from Yellowstone National Park, WY: Scanning and transmission electron microsopy evidence for biogenicity. Geological Society of America, Abstracts With Programs, 27(6): 305.
