Assessing the Preservation Potential of Biogenic Features in Pre-Neogene Tufas and Travertines – Applications to Exobiology
If evidence for life on Mars is found, it may come from Martian paleo-groundwater deposits (tufas and travertines). Tufas and travertines uniquely preserve evidence of life from both surface and subsurface environments. What is known about tufa and travertine is biased towards Holocene examples. Most geologically older examples remain poorly studied, possibly due to poor preservation potential, or from being overlooked or misclassified. This study examines preservation potential of lithologic and biogenic features in pre-Neogene paleo-groundwater deposits, focusing on biogenic microstructures detected using SEM, EDS, and petrography. This study compares pre-Neogene deposits from the Eocene Chadron Formation (Badlands of South Dakota), the Jurassic Shuttle Meadow Formation (Hartford Basin of Connecticut), and Triassic Mercia Mudstone Group (south Wales, United Kingdom) to Neogene tufa and travertine (San Ysidro Quadrangle, central New Mexico). The goal is to discern trends in biogenic microstructure preservation potential through geologic time. Dominant microstructures observed of probable biogenic origin include clotted micrite, pseudo-stromatolitic features (e.g., shrub-and-ray dendrites), microcolonial fungal sacs, coccoliths, and extracellular polymeric substances (EPS). Neogene samples include pisolites, coated grains, lithoclasts, encapsulated algae, shrub-and-ray dendrites, EPS, and calcite ice. Eocene samples include pisolites, coated grains, lithoclasts, shrub-and-ray dendrites, macrofossils (molluscs, ostracodes, and vertebrate bones), charophyte stems and gyrogonites, microcolonial fungal sacs, coccoliths. Jurassic samples include pisolites, shrub-and-ray dendrites, EPS, calcite ice, calcite microspherules associated with algal and/or bacterial growth, and Fe-Mn oxide star structures. Triassic samples are dolomitized, but show "ghosts" of pisoids and Fe oxide star structures. In summary, paleo-groundwater deposits on Earth contain distinctive features observable through the Late Triassic. Some features are inorganic or biologically mediated, such as pisolites, lithoclasts, and dendrites. Others are clearly biogenic. Eocene paleogroundwater deposits contain oncoids, coated grains, EPS, macrofossils (molluscs, ostracods, and vertebrates), and microfossils (coccoliths and microcolonial fungal sacs). Deposits as old as the Jurassic contain Fe- oxide star structures and spherules. Triassic deposits contain Fe- oxide star structures. The quality of preservation of these features does not substantially decline in samples as old as the Late Triassic, suggesting that tufas and travertines uniquely preserve evidence of life from surface, subsurface, and potentially ancient environments.
School:Bowling Green State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:tufa travertine exobiology neogene paleogene jurassic triassic sem
Date of Publication:01/01/2005