Using a novel approach to precisely measure the sedimentation velocity of a (bio)chemical sediment: cyanobacteria-ferrihydrite aggregates
Banded iron formations (BIF) are iron-rich (~20-40% Fe), siliceous (~40-50% SiO2), and organiclean (<0.5 wt.%) sedimentary deposits that precipitated widely during the late Archean (2.7-2.5 Ga) and Paleoproterozoic (2.5-1.8 Ga). As cyanobacteria evolved, the reaction between dissolved Fe(II) and photosynthetically produced O2 would have became a viable mechanism for Fe(II) oxidation near the surface of the Precambrian oceans. If a biological mechanism was important in the initial process of
... tial process of Fe(II) oxidation in an Archean ocean water column, it is then expected that biomass settled to the seafloor along with the Fe(III) minerals . Yet, BIF contain very little organic carbon (i.e., < 0.5 wt.%; , meaning either that biomass was oxidized in the water column or bottom sediment via the combined metabolic processes of fermentation and chemoheterotrophy (e.g., or that the biomass and ferrihydrite did not associate and settle together . The presence or absence of a strong association of cells with minerals or encrustation in minerals especially influences the rate and extent of sedimentation of the microbial cells, i.e., the biomass. Sedimentation velocity of such chemical sediments are typically calculated using Stokes's law. However, applying it to chemical sediments that form in situ in the water column is not ideal because the particle properties do not fulfil many of the assumptions underpinning the applicability of Stokes' law. As a consequence, it has been difficult to predict the sedimentation rate of BIF because their primary sediments likely comprised aggregates of ferric hydroxides, such as ferrihydrite [Fe(OH)3], and marine bacterial biomass, including cyanobacteria. In this thesis, I used a novel approach that combined geochemical, geophysical and geomicrobiological methods to comprehensively investigate the sedimentation processes, sedimentation velocity, total Fe content and total biomass content of ferrihydrite-Synechococcus aggregates that formed in situ. over a wide range of pH and iii initial Fe(II) concentrations. My results indicate that: (1) Using Stokes' law to measure the sedimentation velocity of chemical sediments is not suitable nor precise; (2) the sedimentation velocity is positively correlated with both pH and initial Fe(II) concentration; (3) it was unlikely that Archean and Paleoproterozoic oceans were at pH 6 due to the extremely slow sedimentation velocity of Fe-cell aggregates such that there would have been no sufficient Fe minerals deposited to form BIF on a massive scale; and (4) the actual Fe:Corg ratio far in the water column must have exceeded the theoretical ratio of 4:1 (as expected from biological oxidation of ferrous iron), thus accounting for the excess Fe(III) deposited to the sediments that later was transformed into hematite with little to no organic carbon preserved. iv Preface This is a paper-based thesis that represents a cumulative work of two papers that focused on cyanobacteria-Fe aggregates, the primary sediments to banded iron formations. As the first author of the following two papers, I was actively involved in all aspects of each paper from developing a new approach, conducting experiments and subsequent analysis, to drafting each paper. This work was conducted under the supervision of Dr. Kurt Konhauser, and with the assistance of Dr.