| Stromatolites are commonly defined as laminated organo-sedimentary structures built by the trapping and binding and/or precipitation of minerals via microbial processes. They are thought to represent evidence of some of the oldest life on Earth, and are thus popular targets for geobiologic studies. Despite their high profile in the geobiologic community, the processes that control the different aspects of stromatolite morphology (i.e. form, growth rate, texture, and lamina formation) are poorly understood. If stromatolites are going to be held up as proof of the earliest life on Earth, some of the confusion involved in their study must be resolved. For my thesis, I use Holocene lacustrine stromatolites to address common assumptions about lamination, growth rate, and growth form. Additionally, I report a new stromatolite biosignature based on the detrital magnetic mineral component present in nearly all sedimentary rocks.;Lamination in stromatolites is commonly interpreted to record the periodic response of a microbial community to daily, seasonal, or perhaps yearly environmental forcing. Here, high resolution 14C dating of Holocene stromatolites from Walker Lake, Nevada is used to construct a record of lamination rate over the course of accretion. Laminations formed at a rate of 5.6 +/- 1.6 yrs/lamination at the base of the structure, 2.8 +/- 1.9 yrs/lamination and 1.6 +/- 0.9 yrs/lamination in the middle, and 4.5+/- 0.8 yrs/lamination at the top. Thus, much of the stromatolite grew in response to a forcing with a 4-6 year periodicity. This indicates that lamination formation is likely more closely related to local climate factors (e.g., ENSO) for these stromatolites than microbial metabolism. These results show that generalizations regarding the influence of microbial mats on stromatolite formation should be used with extreme caution.;Typically, stromatolites have a domed morphology that is commonly taken as an indication of microbial photropism (growth towards incident light). The stromatolites of Walker Lake can be found on all sides of boulders, making them an ideal test case for this commonly held assumption. Angle of growth measurements on over 300 stromatolites demonstrate that the structures grew nearly perpendicular to their growth surface, regardless of the initial angle of inclination. Furthermore, there are no significant differences in the distribution of growth angles between north, south, east or west facing samples. This evidence suggests that phototactic growth towards incident light was not a dominant factor in controlling stromatolite morphogenesis, and that the presence of a domed macrostructure cannot be taken as evidence of photosynthetic life.;Given that the results of the lamination and growth direction studies contradict the prevailing theories of stromaotlite morphogenesis, a definitive way to test the biogenicity of stromatolites both ancient and modern is still needed. Here, I report a new biosignature that can be used to test for the presence or absence of a biofilm at the time of formation of a stromatolite. It is hypothesized that the distribution of detrital magnetic grains within a putative microbialite will depend on the presence or absence of "sticky" microbial mats/biofilms. Magnetic grains in an abiotic structure should obey the laws of gravity/angle of repose (swept off peaks, concentrated in lows), while magnetic grains adhered to a biofilm will seem to "defy" the laws of gravity. To test this hypothesis, parallel laboratory experiments were undertaken in which magnetic particles were introduced into a tank that contained either cyanobacterial mats or glass slides (on which carbonate had abiotically precipitated) inclined at a variety of angles. Using the results of the laboratory experiments, a variety of stromatolites of known and unknown biogenicities, ranging in age from near-modern to ancient, were analyzed. The results of these experiments verify the hypothesis that magnetic susceptibility can be used as a biosignature. The sum total of this study sheds light on the processes that form stromatolites, and leads us a step further towards the unambiguous understanding of the bio. |
