REASSESSING THE BIOGENICITY OF PUTATIVE MICROFOSSILS IN THE 3.5 BILLION YEAR OLD APEX CHERT

Abstract

Scientists generally agree that life first arose on Earth over 3.5 Ga years ago, but the search for evidence of ancient life is a complex and difficult task due to the scarcity of relatively unaltered Archean rocks. The Pilbara craton contains some of the oldest, best exposed, relatively low-metamorphic grade Archean rocks and has therefore been a target of a variety of studies of ancient life. The Apex chert is a unit that has been extensively studied with a variety of analytical techniques primarily because it contains microstructures that were originally described as the oldest microbial fossils nearly 30 years ago. However, researchers have argued over the origin and composition of these microstructures and the debate continues today. The goal of this dissertation is to utilize a suite of samples collected from the Apex chert in 2006 in order to better understand the possible origin and composition of the microstructures, potential evidence for Archean life preserved in the rock record, and to highlight methodological issues that can affect the ability to accurately identify evidence for ancient life. The introduction in Chapter 1 discusses issues with identifying signs of Archean life, introduces Raman spectroscopy, includes a brief introduction of the Apex chert to provide historical and geologic context and outlines the content of this dissertation. Chapter 2 is a literature review of the complex formational and alteration history of the Apex chert to provide geologic context. Chapter 3 documents the identification of hematite pseudofossils in the new samples and shows that carbonaceous material is present in the chert matrix instead of being associated with the microstructures as previously believed. Data quality issues can lead to the misinterpretation of Raman spectroscopic data and the use of multiple types of data collection techniques and analyses that can aid in the identification of materials in geological datasets are presented. Chapter 4 includes an investigation of the structure and context of the previously identified matrix carbonaceous material. Petrographic and Raman spectroscopic data showed the presence of multiple generations of carbon in the Apex chert which raises questions about the degree of thermal alteration of the Apex chert and illustrates the importance of paragenetic context when studying evidence for ancient life such as carbonaceous material or putative microfossils. Chapter 5 illustrates how advanced Raman imaging data collection techniques can be affected by data quality issues such as autofluorescence and can lead to misidentification of materials. In order to address this problem, a software-based analytical method was developed to identify the spatial distribution of autofluorescence and a statistical method was developed to evaluate the quality of a post-acquisition data processing technique that is commonly used to address autofluorescence issues in Raman imaging datasets.