Lunar and Planetary Science XXXII (2001) 1438.pdf HEMATITE MINERALIZED BACTERIAL REMNANTS: IMPLICATIONS FOR MARTIAN HEMATITE DEPOSITS. RACHEL T. SCHELBLE1, FRANCES WESTALL2, CARLTON C. ALLEN3, ADRIAN J. BREARLEY1, 1DEPARTMENT OF EARTH AND PLANETARY SCIENCES ([email protected]), UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, NM, 87131; 2LUNAR AND PLANETARY INSTITUTE, HOUSTON, TX 77058; 3NASA JOHNSON SPACE CENTER, HOUSTON, TX 77058. INTRODUCTION the Fe-rich stromatolitic layers consist of numerous 1-2 The 2.0 Ga iron-bearing Gunflint Formation was mm hematite crystallites. Larger, hematite-rich spheres, deposited during a transitionary period in Earth history 5-10 mm in diameter, were observed in the cherty when a stable oxic atmosphere was being established. interlayers of the stromatolite. Also seen were 1-2 mm The Animikie Group, which comprises both the Rove wide, 50-200 mm long, hollow filamentous structures in and Gunflint Formations, extends NE-SW for the chert. These structures, which were outlined by approximately 175 km from Thunder Bay, Ontario to hematite, are interpreted as fossil bacteria. northern Minnesota [1]. The thickness of the Gunflint An unetched and an etched rock chip were analyzed ranges from 100 to 180 m, with an average thickness of by SEM. Probable and potential microfossils were about 122 m [2]. more easily identified by SEM in the etched sample. Metamorphism of the Gunflint Formation increases Chains of 1-2 mm diameter spheres (Fig. 1), many of gradationally from east to west. These rocks are which were partially collapsed, were interpreted as essentially undeformed and exhibit a low metamorphic representing the remains of (degraded) coccoidal grade ranging from diagenetic to subgreenschist, with bacteria. This interpretation is based on their maximum temperatures that ranged from 120 to 150 morphological and colonial characteristics, the presence degrees C [3]. of remnant carbon (detected by EDS analysis) in their structure and their association with copious quantities of SAMPLES permineralized and hematite mineralized polymeric Samples from two localities of the Gunflint Iron substances. Some of the hematite crystallites seen in Formation have been examined. Rock chips and thin thin section may represent the hematite mineralized sections from the Mink Mountain and Schreiber Beach fossil coccoids. The larger, 5-10 mm diameter, localities are the focus of this study. hematite-rich spheres seen in thin section present both Mink Mountain samples (PPRG 336) were collected turgid and collapsed shapes in SEM. The majority of about 7.3 km west of Mackies, Ontario, at the western them show no evidence of internal structure, although end of Gunflint exposures [4]. As documented, they are some contain amorphous clumps of hematite-rich approximately 2.0 Ga in age, and are from the Upper material. Increased carbon is present in both the spheres Cherty unit of the Gunflint Formation. and the internal clumps when compared to the The Schreiber Beach locality samples (PPRG 1289) background. On the basis of these observations we were collected in an area about 6.5 km west of interpret the spheres as possible colonial cyanobacterial Schreiber, along the north shore of Lake Superior in sheaths, some of which contain degraded remnants of Southern Ontario, Canada [5]. They are documented to the original cells. be from the lower algal cycle of the Gunflint Formation The permineralized and hematite-mineralized and to be about 2.0 Ga in age. filaments identified in thin section are readily visible with the SEM (Fig. 2). They form localized, loose, METHODS criss-crossing networks on the biofilm surface. The A combination of methods was used to examine the collapsed nature of the filaments is due to the fact that Gunflint samples including optical, scanning, and they were mineralized after partial degradation of the binocular microscopy, as well as electron microprobe original organism. This phenomenon is further analysis and powder x-ray diffraction. Rock chips were evidence for their biogenicity. etched in fumes of hydrofluoric acid (HF) for 15 to 30 All the above structures are embedded in copious minutes, thoroughly rinsed with deionized water, and quantities of a smooth to granular iron-mineralized and then coated with platinum or gold for SEM observation permineralized (silica impregnated) film, which we with a JEOL 6340 FEG-SEM and attached Link energy interpret as fossilized polymeric substances (FPS). dispersive X-ray analysis system (EDS). Polymeric substances are secreted by bacteria in stressed environments, and help them stabilize their RESULTS AND INTERPRETATION surroundings and trap nutrients [6]. The Mink Mountain sample consists of alternating Samples from the Mink Mountain locality display bands of hematite and chert (quartz), each millimeters crystals having the morphology of magnetite with a thick, forming a macroscopic, domal, stromatolitic replacement by hematite (c.f. [5]). The majority of the structure. Thin section optical microscopy showed that Lunar and Planetary Science XXXII (2001) 1438.pdf SCHELBLE, ET AL: HEMATITE MINERALIZED BACTERIAL REMNANTS Fe-oxides in the Schreiber Beach samples exhibit a Thus, the Martian hematite deposits would be an poor, generally amorphous crystallinity. Some of the excellent site to look for past life on Mars. euhedral hematite-replaced magnetite crystals contain carbonaceous inclusions, the nature of which is not yet REFERENCES understood. [1] Floran, R.J. and Papike, J.J. (1978) Journal of Petrology, 19, 215- Fossil microorganisms from the Schreiber Beach 288. [2] Goodwin, A.M. (1956) Economic Geology, 51, 565-595. [3] Klein, C.K. (1983) in Iron Formations: Facts and Problems, 417-471. Locality showed a wider range of preservation than [4] Schopf, J.W., ed. (1983) Earth’s Earliest Biosphere: Its Origins Mink Mountain locality fossils. Fossil structures were and Evolution, 367 p. [5] Schopf, J.W., and Klein, C.K., eds. (1992) confirmed using both optical and scanning electron The Proterozoic Biosphere, 1,348 p. [6] Westall, F., et al. (2000) JGR, m 105, 24,511-24,527. [7] Christiansen, P.R., et al. (2000) JGR, 105, microscopy. Filamentous microorganisms (1-5 m 9,623-9,642. width, 50-180 mm long) (Fig. 3) were observed throughout the sample in a wide range of preservational states. 5-10 mm diameter sheaths of colonial cyanobacteria are the most common spherical remnant identified. The degraded sheaths, in both SEM and in thin section, exhibit a central clump that may be the degraded remains of the cells once living inside. The microfossils were identified in both the hematite-rich and chert-rich bands of the banded iron stromatolite. As with the Mink Mountain sample, the fossilized polymeric substances proved to be the most common biogenic structure preserved. DISCUSSION AND CONCLUSIONS Figure 1. SEM view of partially degraded, hematite mineralized This study shows that bacterial structures can be coccoidal bacteria embedded in fossilized polymeric substances (Mink preserved in Fe-oxides. Bacterial remnants from iron- Mountain locality). [5500x mag.; 1 mm scale bar] rich regions of the Gunflint Iron Formation show a wide range of preservational states, and it is likely that degradation of the structures occurred before fossilization of the microorganisms. The abundance and ubiquity of fossilized polymeric substances in these samples suggests that a possible relationship between bacteria, Fe-oxides, and these substances may exist. Further, there may be a strong relationship between microbial mats and the precipitation of iron oxides, leading to the genesis of banded iron formations. MARS This study has shown that bacteria can be preserved Figure 2. SEM view (backscatter) of a deflated, hematite-mineralized by Fe-oxides for extended periods of time. Although filament (Mink Mountain locality). [2700x mag.; 10 mm scale bar] banded iron formations have not so far been recognized on Mars, hematite deposits have been observed. Christensen, et al. [7] cite five possibilities for the origin of the hematite deposits: · Direct precipitation from standing, oxygenated iron-rich water · Precipitation from iron-rich hydrothermal fluid · Low-temperature dissolution and precipitation through mobile groundwater leaching · Surface weatherings and coatings · Thermal oxidation of magnetite-rich lavas If bacteria did exist on Mars, their preservation by Fe-oxides in any of these potential settings is possible. Figure 3. Thin section view of a hematite-mineralized filament (Schreiber Beach locality). [50x mag.; 40 mm scale bar].