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Stromatolites as biosignatures and paleoenvironmental records:
experiments with modern mats and examples from the
Eocene Green River Formation
by
Carie M. Frantz
Approved August 16, 2013 by:
Frank A. Corsetti, Committee Chair
Department of Earth Science
University of Southern California
Kenneth H. Nealson
Department of Earth Science
University of Southern California
Moh El-Naggar, Outside Member
Department of Physics
University of Southern California
Object Description
| Title | Stromatolites as biosignatures and paleoenvironmental records: experiments with modern mats and examples from the Eocene Green River Formation |
| Author | Frantz, Carie Marie |
| Author email | cfrantz@usc.edu;cariefrantz@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Geological Sciences |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2013-08-16 |
| Date submitted | 2013-09-13 |
| Date approved | 2013-09-13 |
| Restricted until | 2013-09-13 |
| Date published | 2013-09-13 |
| Advisor (committee chair) | Corsetti, Frank A. |
| Advisor (committee member) |
Nealson, Kenneth H. El-Naggar, Mohamed Y. |
| Abstract | Stromatolites, defined descriptively by Semikhatov et al. (1979) as ""attached, laminated, lifhified, sedimentary growth structures, accretionary away from a point or limited surface of initiation"", are considered important structures for understanding the evolution of life on earth and potentially elsewhere. ❧ In general, Archean and Proterozoic stromatolites are fine-grained whereas most modern marine examples are comparatively coarse-grained. Given that the modern marine forms are commonly studied as analogues to the ancient forms, it is important to understand the processes responsible for the textural differences. Cyanobacteria are typically considered the dominant stromatolite builders through time, but it is well known that many modern marine stromatolites also contain a eukaryotic component (various algae, diatoms, etc.). Thus, we conducted experiments to test the grain trapping and binding capabilities of filamentous cyanobacterial mats (dominated by the trichome-forming Coleofasciculus chtonoplastes) versus algal mats (Chaetomorpha) in order to better understand the grain-size trends in stromatolites through time. The mats were cut into coupons, inclined at angles from 0-75° in saltwater tanks to approximate the angle of lamina observed in typical stromatolites, and grains of various sizes (fine sand, coarse sand, and fine pebbles) were delivered to their surface. We measured both trapping and binding as a function of mat properties such as filament length and mesh density. Experiments were done under very low flow and moderate flow conditions. ❧ The cyanobacterial mats were able to trap fine grains consistently at all angles. At angles beyond the angle of repose, medium and coarse grains were not trapped as efficiently, but some (although very few) coarse grains were trapped even at high angles depending on the maturity of the mat and filament bundle length. Dense algal mats trapped medium and coarse grains efficiently at all angles, but were poor at trapping fine grains. The cyanobacteria bound the grains over time, regardless of angle, over time by physically wrapping them. The algae, in contrast, were unable to bind grains and tended to shed trapped grains over time. When flow was added to the experiment, trapping was significantly reduced in the cyanobacterial experiments, but not the algal experiments. ❧ Our experiments suggest that the presence of grains beyond the abiotic angle of repose can be considered a biosignature in ancient stromatolites where biogenicity is in question. Although we cannot conclude that all fine-grained stromatolites were formed by cyanobacteria, our results suggest that stromatolites where coarse grains are present at high angles at much the same frequency as at low angles (e.g., most modern marine stromatolites) may require a filamentous eukaryotic component in order to efficiently trap coarse grains beyond the angle of repose, and give insight into the evolution of stromatolite microfabrics through time. ❧ In addition to their utility as potential records of the communities that formed biogenic stromatolites, stromatolites are also potentially useful for reconstructing paleoenvironmental conditions. The Eocene Green River Formation (Sweetwater County, Wyoming) represents a system of lakes that covered parts of what is now Wyoming, Colorado, and Utah, and captures the Early Eocene Climatic Optimum (EECO, 52-50 Ma), a period of very high global temperatures representing the warmest period of the Cenozoic Era that is associated with very high levels of atmospheric CO₂. Lakes, especially closed basin lakes, can respond dramatically to environmental change because of their sensitivity to precipitation and evaporation. In this study, I 1) use stromatolites from the Rife Bed of the Green River Formation as fine-scale records of terrestrial paleoenvironmental change during a global hothouse climate, and 2) investigate how the environmental dynamics within the lake system affect the growth of stromatolites. ❧ The stromatolites we studied are composed of branching microdigitate columns laminated on the 10-100 µm scale. Laminae are grouped in cm-scale layers that alternate between micritic, calcite fan, and mixed microstructures. The micrite layers contain evidence for a biogenic origin and are enriched in ¹⁸O, Na, and Mg/Ca. The fan layers, in contrast, appear to have formed abiogenically and are relatively depleted in ¹⁸O, Na, and Mg/Ca compared to the micrite layers. The δ¹³C and δ¹⁸O strongly co-vary, suggesting the stromatolites formed in a closed basin lake (e.g., Talbot, 1990). In addition, clumped isotope analyses provide the first quantitative values for water temperatures in lake water from the Green River Formation. The different microfabrics are associated with significantly different lake water temperatures: ~35°C for micrite and ~28°C for fan layers. ❧ Given that the stromatolites grew in a closed basin lake, we can link changes in the δ¹⁸O (via a Rayleigh distillation model) and sodium ion concentration (assuming it behaves conservatively) to periods of evaporation or recharge, and thus model changes in lake volume/level during stromatolite growth. The two models converge upon similar results that suggest that dramatic lake volume changes occurred many times during the accretion of the ~30 cm thick stromatolite, with lake level rising and falling as much as 8 meters. Because of broad, flat bathymetry of the lake, such lake volume changes would have been accompanied by shoreline shifts on the order of tens of kilometers while the stromatolites were actively growing, throwing the traditional interpretation of stromatolites as shoreline facies into question. As a reality check, we note that the modern Great Salt Lake, a similarly broad and flat lake, experienced similar shoreline shifts over several years in the 1980s. ❧ The calculated lake volume—and consequently lake depth—changes also provide insight into the formation of the stromatolites studied. The micrite microfabric formed when the lake was shallow and warm, whereas the fan microfabric formed in cooler waters when the lake was deeper, possibly below a thermocline. I hypothesize that the alternation between biogenic and abiogenic microfabrics present in the Rife Bed stromatolites are the result of dramatic changes in lake level influencing the microbiology and chemistry of the waters in which the stromatolites were forming, potentially due to the at least intermittent existence of thermal and chemical stratification in the lake. ❧ The isotope and conservative ion lake volume models used with the Rife Bed stromatolites were also applied to stromatolites from the Lower LaClede Bed of the Laney Member of the Green River Formation, which captured major changes in basin hydrology, including transient periods of basin closure during a time when the basin is generally considered to have been balanced-filled. Our high-resolution sampling reveals significant changes in ¹⁸O and elemental composition on the cm-scale, providing a finer scale resolution to the filling and evaporation of Lake Gosuite during the waning Eocene Climatic Optimum, and is complementary to broader scale studies on the hydrology of the basin (e.g., Doebbert et al, 2010). Our results potentially indicate that several filling and evaporation stages representing depth changes of ~3 m occurred over the course of a few cm of stromatolite accretion, potentially indicating the magnitude of short-term climate change during the Eocene Climatic Optimum, the period with the highest temperatures and atmospheric CO₂ levels in the Cenozoic. In addition, periods of basin filling are often marked by sudden changes in stromatolite microfabric. ❧ In summary, this thesis highlights the need for continued experimental study to understand the specific mechanisms responsible for forming the wide range of stromatolite fabrics observed in the fossil record. It also demonstrates the potential usefulness of stromatolites as fine-scale records of the environment in which they formed, as well as of the types of biological communities that contributed (or did not contribute) to their growth. |
| Keyword | stromatolites; microbial mats; paleoenvironment; Early Eocene Climatic Optimum; Green River Formation; Eocene; facies; Lake Gosiute; paleolimnology; cyanobacteria; Proterozoic; paleontology; ichnology; angle of repose; evaporation |
| Language | English |
| Format (imt) | application/pdf |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m |
| Contributing entity | University of Southern California |
| Rights | Frantz, Carie Marie |
| Physical access | The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given. |
| Repository name | University of Southern California Digital Library |
| Repository address | USC Digital Library, University of Southern California, University Park Campus MC 7002, 106 University Village, Los Angeles, California 90089-7002, USA |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-FrantzCari-2038.pdf |
| Archival file | uscthesesreloadpub_Volume7/etd-FrantzCari-2038.pdf |
Description
| Title | Page 1 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | Stromatolites as biosignatures and paleoenvironmental records: experiments with modern mats and examples from the Eocene Green River Formation by Carie M. Frantz Approved August 16, 2013 by: Frank A. Corsetti, Committee Chair Department of Earth Science University of Southern California Kenneth H. Nealson Department of Earth Science University of Southern California Moh El-Naggar, Outside Member Department of Physics University of Southern California |
