nai

Project Progress

Submicroscopic Study of Microbial Fossils in Chert (dm)

Note missing modifier for quartz in several places below

The purpose of this study of microbial fossils is to improve our understanding of microbial biosignatures by developing criteria for biogenicity at the nanometer scale using new methods of electron microscopy. To achieve this goal we have been studying the mineralogy and ultrastructure of well known microfossils from the 2.0-billion-year old Gunflint Iron Formation. Gunflint microfossils were initially investigated by standard methods of petrography and electron microscopy to determine whether submicroscopic features of organically preserved microfossils persisted. Our nanoscale studies have shown that microfossils are comprised of aggregated spheroidal grains (200-500 nm) of _-quartz and brown interstitial kerogen. Analytical transmission electron microscopy (TEM) revealed that quartz spheroids are made up of smaller domains of _ -quartz separated by poorly diffracting silica phases. In contrast, the matrix silica is composed of granulitic, coarse-grained (5-10 micormeter) _-quartz. The kerogen not only provides the optically visible biosignature in these samples, but it exerted a clear influence on the nucleation, growth and recrystallization of the silica, resulting in ultrastructural biosignatures not previously recognized.

Hydrofluoric acid (HF) vapor etching, commonly used to prepare microfossils for scanning electron microscopy (SEM) work, was used to preferentially dissolve the matrix silica of samples, leaving microfossils in positive relief. These etched surfaces were then studied using FESEM. The results of FESEM investigations suggest that morphological variations in HF acid-etched microfossils mainly depend on the distribution of kerogen and not silica phase variations between the microfossils and matrix. The presence of kerogen inhibits removal of the associated spheroidal silica, leaving a distinctive morphological biosignature. Consistent with previous work, our SEM and FESEM studies of silicified microfossils have shown the nanoscale structure of the original matter to have been substantially altered by post-mortem degradation processes.

We abandoned our attempts to develop a new vacuum-based HF etching technique (as originally proposed) because the etched surfaces produced with this technique are inferior to other methods and the temperatures required to achieve good results are high enough to be destructive to samples.