Marine transgressions or microbial precipitation: What was the controlling factor of continental margin iron formation deposition?

Marine transgressions or microbial precipitation: What was the controlling factor of continental margin iron formation deposition? Introduction Iron formations, which can be found around the world today, have great importance as sources of iron and manganese ore, clues about the evolution of life on Earth, and even hints at possible life on Mars (Weber et al., 2006). Precambrian iron formations contain the bulk of minable iron ore around the world, including both " natural " (60-70% iron) and taconite (15-30% iron) ores (Ramanaidou and Wells, 2014). Iron ores have helped advance human societies since prehistory, arguably making iron one of the most essential natural resources on the planet for humans (Klein, 2005). As paleoenvironmental indicators, iron formations record marine chemical conditions and information about the structure of continental margins in deep time. Of particular significance is the time period around 2.4 Ga, when the " Great Oxidation Event " (GOE) occurred and the earth's atmosphere became oxic (Eigenbrode & Freeman 2006). The period from ~3.5-1.8 Ga was the most prolific for iron formation deposition, leaving a record of conditions before, during, and after the GOE (Klein, 2005). These deposits then are clearly of great importance in understanding the evolution of life on Earth, and it has been suggested that iron formations may hold clues about possible microbial life on Mars (Weber et al, 2006). Accordingly, geologists and paleontologists have taken a keen interest in iron formations and much has been learned about their age (Klein, 2005, and references therein), structure (e. 1996) in the past few decades. But despite these great efforts by researchers worldwide, there are many questions yet to be answered about the process of deposition of iron formations. Roepke 2 However, two distinctions should first be noted regarding the naming convention of iron formations. First, there are two depositional setting that are included under the name " iron formation: " exhalative iron formations, which are now mainly located in metamorphosed Archean greenstone belts (Pufahl et al, 2014), and continental margin iron formations, which are generally agreed to have formed on ancient continental shelves (e. This paper is focused only on continental margin iron formations, as they are where the majority of the research and literature about iron formations has been concentrated. Another distinction in iron formation terminology is granular iron formations (GIFs) versus banded iron formations (BIFs) (see Fig. 1 for example images). This classification is separate …