From photosynthetic bacteria to stromatolites
Microbial communities form complex assemblages that are often found bound together by biologically produced extracellular polymeric substances (EPS) to form a biofilm, and in more favourable conditions, they may form a microbial mat visible to the naked eye. Under such conditions, bacteria exert significant chemical control over their local environment, and can drive the precipitation of different kinds of minerals, primarily carbonates, phosphates, sulfates, sulfides, and metal oxides, in a process called biomineralisation (see Konhauser, 2007). As stromatolites are formed almost exclusively by carbonate biomineralisation (combined with grain trapping), only this biomineralisation mechanism is explained here.
The net result of cyanobacterial calcification is the uptake of HCO3- from water and export of base (OH-), resulting in a net increase in proximal pH that ultimately favours carbonate precipitation (see Figure 2.4; Thompson and Ferris, 1990). Bacteria further promote carbonate precipitation by binding metallic cations (e.g., Ca2+) on their external wall and locally increasing the carbonate saturation state. It was shown that the cyanobacteria generator of ESP precipitate more CaCO3 than other organisms (Pentecost, 1978). At marine pH, the net calcium carbonate precipitation reaction simplifies to: 2HCO3- + Ca2+ → CaCO3 + CO2 + H2O (5).
Photoferrotrophic and phototrophic sulphur bacteria similarly favour the precipitation of 9 carbonate minerals by Ca2+ adsorption and uptake of CO2 (c.f. reactions 1–3; e.g. Hegler et al. 2008). The carbonate produced accumulates as an encrustation on the cell and can lead to a visible build– up of calcite carbonate, and ultimately, to the formation of microbial carbonate structures such as stromatolites. Photosynthetic activity is often associated with daily or seasonal variations, such that precipitation of CaCO3 may be intense in spring and summer, and may wane during winter, perhaps even leading to periods of dissolution (Thompson and Ferris, 1990). To maintain a favourable environment for growth, phototrophic bacteria will continually migrate up the calcite layer (porous or non-porous) that was most recently precipitated, producing the laminated form characteristic of stromatolites. Such migration and upward growth may be modulated by different parameters, including irradiance, tides, changes in salinity, and sedimentation rates (e.g. Chaftez et al., 1991, Konhauser et al. 2001). Heterotrophic organisms are ubiquitous throughout and metabolize the available organic matter, such as dead cells and abandoned EPS. Stromatolites exist in various forms (e.g., millimetre to metre scale, domal, flat, etc), and are the most common type of microbialite. Others include travertine, thrombolites, speleothems, and yet other morphologies that remain poorly classified. In short, stromatolites are directly linked to photosynthetic bacterial activity; this makes them ideally suited for capturing environmental signatures of oxygenic or anoxygenic photosynthesis.
Molybdenum (Mo), transition metal number 42, has seven stable isotopes from mass 92 to 100, namely 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, and 100Mo. Depending on environmental redox conditions, Mo is present in different oxidation states. In oxic conditions (such as in modern seawater), Mo is present in the highly stable molybdate oxyanion form, MoO42-. In water with free HS- (euxinic conditions), sulfidation of MoO42- replaces O with S and forms a variety of thiomolybdate ions with the general formula MoO4-xSx2- (Helz et al., 1996). These properties render Mo highly sensitive to varying redox conditions. Helz et al. (1996) demonstrated total fixation of Mo in marine sediments above a critical H2Saq concentration of 11 μM, a so-called “geochemical switch”. Mo is also an essential micronutrient and is required for many biological activities and metabolic processes (Mendel, 2005). It is one of most abundant transition metals in modern oceans (~100 nM; Collier, 1985), and is well-mixed with a residence time of ~800ky (Emerson and Huested, 1991). Seawater is thus a significant Mo reservoir on the modern Earth. The oxidative weathering of sulfur minerals in the continental crust is an important source of Mo, which is carried to the ocean by riverine transport with typical dissolved concentrations around 5 nM (Figure 2.5; Morford and Emerson, 1999). Mo displays highly mobile behaviour during weathering, erosion, and transport under oxidizing surface conditions (Siebert et al., 2003). Moreover, low-temperature hydrothermal input is around only 10% of the riverine concentration (McManus et al., 2002). There are two major sinks for Mo in the oceans. The largest sink in the modern oceans are manganese (Mn) oxide minerals (and to a lesser extent, Fe oxide minerals), which bind and highly concentrate Mo by adsorption (Barling et al., 2001; Goldberg, 1954). The second major sink is anoxic sediments, which trap the Mo released by after Fe-Mn oxide reduction by the precipitation of MoS2. Its concentration in anoxic sediments is typically around 100 ppm (Scott et al., 2008), whereas it averages around 1.1 ppm in the continental crust (Rudnick and Gao, 2003). Anoxic sediments precipitate more rapidly than Fe-Mn-oxide crusts, such that their expansion can easily perturb the balance of Mo sinks. Finally, while molybdenum may be found in carbonates as a trace constituent, carbonates represent only ≤1% of the total marine Mo sink (Morford and Emerson, 1999; Voegelin et al., 2009).
Isotopic measurement of transition metals by multiple collector plasma source mass spectrometry (MC-ICP-MS) has only been possible late in the last century (Halliday et al., 1995). Its principle is straightforward (Annex 1): ions of the isotopes of interest are formed when the sample is nebulised and carried in the form of a mist into a high-temperature plasma, usually argon gas-based. They are separated from each other as a function of mass in two steps: a) an electric sector that filters based on kinetic energy, and b) a magnetic sector that deflects isotopes based on their mass, where the lighter isotopes are deflected more than the heavy ones. Once the isotopes are separated, an array of multiple collectors captures all of the isotopes simultaneously and the signal is measured.
Mo, a transition metal, is everywhere around us (especially as a dopant in steel). That is why all of the sample preparation for molybdenum isotope measurements should be performed in a clean room with a particulate attention of metal contamination. The sample needs to be in solution to be analysed. Rock samples are first powdered and then the powder is digested to extract the element under investigation. The goal of this research was to study authigenic molybdenum in carbonates and examine its isotopic signature. Accordingly, the digestion would ideally be delicate, e.g. as part of a leaching procedure (e.g., in 5% acetic acid). However, Voegelin et al. (2009) showed that seawater Mo isotope signatures are retrieved from carbonate even with a “pseudo-total” 6N HCl attack. The final solution to be passed through the spectrometer should be concentrated and purified to eliminate potential interferences and remove potential matrix effects. Column chromatography is a robust way to achieve this goal. Its principle is based on the solubilisation of mobile phases and the retention of stationary phases as a function of their affinity to an exchange resin. Such resins are generally composed of polymer beads with functional groups grafted to them. These groups should retain or release ions as a function of solution chemistry. If the mobile ion is positive, the resin undergoes a “cation exchange”, and if the mobile ion is negative, the resin undergoes an “anion exchange”, which has a positive functional groups positive to attract negative mobile ions. With elution, charged elements in the resin are separated due to different migration speeds.
Table of contents :
! THE EARLIEST
1.2 FROM PHOTOSYNTHETIC BACTERIA TO STROMATOLITES
1.3! MOLYBDENUM AS A REDOX PROXY
2.2.! PRINCIPLES OF MO ISOTOPE MEASUREMENT
2.3. METHODS OF THIS STUDY
2.4. MO ISOTOPIC MEASUREMENTS
4.1. MODERN STROMATOLITES
4.2. 2.52 GA STROMATOLITES, GAMOHAAN FORMATION, SOUTH AFRICA
4.3. 2.8 GA STROMATOLITES, STEEP ROCK, CANADA
4.4. 2.96 GA STROMATOLITES, RED LAKE, CANADA
4.5. EVIDENCE FOR OXYGEN AND OXYGENIC PHOTOSYNTHESIS BEFORE THE GOE