Banded Iron Formations (BIFs): characteristics, modes of occurrence, and geological significance
Matthew V. Kohlbecker
Precambrian Banded Iron Formations (BIFs) constitute a complicated field of study in the geological sciences that has infrequently if at all been simplified and summarized for the understanding of the casually-interested student of geology. BIFs are classified as part of a larger group of rocks containing at least 15% iron in the form of Fe''' or Fe'' called Iron-Bearing Sedimentary rocks (Boggs, 1995). More precisely, BIFs are laterally extensive, marine-derived sedimentary deposits consisting of alternating iron-rich (dominated by magnetite, siderite, and hematite) and iron-poor layers (dominantly silica dioxide). REE studies reveal that the Fe'' is hydrothermally derived (Beukes and Klein, 1992b). As non-glacial BIFs are restricted to Archean (2.6 - 1.8 Ga) depositional environments (Simonson and Hassler, 1996) and their origins have been relatively obscure (Button, 1982), there has been considerable debate as to the conditions under which they form. The purpose of this paper is to briefly review and summarize the different theories concerning the origin of BIFs. Many scientists argue that BIFs originated in an upwelling continental shelf setting (Button et al., 1982) and can possibly be tied to geologic phenomenon such as sea transgression (Simonson and Hassler, 1996). Other scientists counter that BIFs are deep water deposits and are derived from the combination of two sedimentary sources: a deep sea iron reservoir and oxygen input from organic activity nearshore, and the fluctuations in the relative amounts that each source contributes provides different BIF types (Beukes and Klein, 1992). Finally, Cloud (1973, 1983) argues that low Proterozoic oxygen levels would have allowed ferrous iron to be transported in solution to marine environments where periods of higher oxygen levels would have caused the Fe'' to be precipitated as Fe'''. In addition to the problem of iron origin is a problem concerning the origin of the interbedded silica, which this paper will also examine through the research of Ewers (1983). BIFs, then, constitute a complicated area of study that would benefit from a synthesizing summary.
Evolution does not lead to us: an argument for environmental
variability as the determiner of ultimate morphology
The nature of life on other planets has long been a popular topic for discussion among professional scientists and the general public alike. Many science fiction writers have argued that life on other planets would assume a completely different form and composition than that observed on earth, whereas traditional thought would have argued that evolution is an ever-perfecting process, forever fine-tuning defects and progressively marching towards a perfect being. Despite being passed off by some as an issue for mere speculation, this debate has its roots deep in the theory of evolution. In his book, Wonderful Life, Stephen Jay Gould (1989) argues that evolution is dictated by chance, that there is no perfect being towards which evolution is striving, and, therefore, if we were to go to another planet, we would find completely different life forms unclassifiable in our Linnean system. Gould only hints, however, at a major point made by Darwin (1859) and others: life is dependent on the environment in which it spawns. In this paper, I argue that (i) life is dependent on the environment from which it spawns, (ii) environments are onstructed during a highly complex series of events with numerous variables involved that produce uncertain, virtually unrepeatable results, (iii) because of this uncertainty, environments are not constant across planetary systems, and (iv) these differences do not constrain the appearance of life; rather, they produce completely different life forms. Essentially, if we were to travel to another planet and discover life, that life would be mysterious and drastically different from that which we observe on earth because the environment that created that life would be drastically different, both chemically and physically, from that which we have on earth.