Evaluating a primary carbonate pathway for manganese enrichments in reducing environments

Most manganese (Mn) enrichments in the sedimentary rock record are hosted in carbonate minerals, which are assumed to have formed by diagenetic reduction of precursor Mn-oxides, and are considered diagnostic of strongly oxidizing conditions. Here we explore an alternative model where Mn-carbonates form in redox-stratified water columns linked to calcium carbonate dissolution. In ferruginous Brownie Lake in Minnesota, USA, we document Mn-carbonates as an HCl-extractable phase present in sediment traps and in reducing portions of the water column. Mn-carbonate becomes supersaturated in the Brownie Lake chemocline where dissolved oxygen concentrations fall below 5 μM, and Mn-oxide reduction increases the dissolved Mn concentration. Supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline. In the same zone, sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon (DIC) with negative . These observations demonstrate that sedimentary Mn enrichments may 1) develop from primary carbonate phases, and 2) can occur in environments with dissolved oxygen concentrations 200 μM), and where Mn and Fe are partitioned by S cycling, photoferrotrophy, or microaerophilic Fe-oxidation. A shallow lysocline enhances Mn-carbonate production by providing additional DIC and nucleation sites for crystal growth. This carbonate model for Mn-enrichments is expected to be viable in both euxinic and ferruginous environments, and provides a more nuanced view of the relationships between Mn and carbon cycling, with applications throughout the rock record

Limited Zn and Ni mobility during simulated iron formation diagenesis

Iron formations (IF) are iron- and silica-rich chemical precipitates that were deposited during the Precambrian. Several recent studies have demonstrated how the trace metal abundances in IF can be used as proxies for the bioavailability of trace metals in ancient seawater; with the ultimate goal being to understand first-order controls on the composition of the ancient biosphere. However, the utility of IF as proxies depends on the immobilization of trace metals during diagenesis. Here, we assess the mobility of Zn and Ni from ferric oxyhydroxides (ferrihydrite) in the absence and presence of organic matter (glucose) during simulated diagenesis (170 °C, 1.2 kbar); similar to what some Precambrian IF experienced. Quantitative concentration data, coupled with X-ray diffraction analysis and electron microprobe element mapping, demonstrate that both metals are relatively immobile during simulated diagenesis. Additionally, the mechanism for initial Ni sorption is examined using X-ray adsorption spectroscopy. For the initial sorption of trace elements in abiotic ferrihydrite experiments, 93.38% Zn and 65.95% Ni were initially sorbed. In experiments utilizing biogenic ferrihydrite, 97.03% of Zn and 93.38% of Ni were initially sorbed. Following the diagenetic capsule treatments, more than 99% of Zn and more than 91.9% of Ni were retained under the varied conditions considered here. Capsule experiments suggest the strong retention of Zn and Ni following the diagenesis of either abiotic or biogenic ferrihydrite. Overall, our results indicate that paleomarine Zn and Ni concentrations are likely to be faithfully recorded in well-preserved IF deposits

Manganese oxides, Earth surface oxygenation, and the rise of oxygenic photosynthesis

Oxygenic photosynthesis is arguably the most important biological innovation in Earth's history, facilitating the transition to a habitable planet for complex life. Dating the emergence of oxygenic photosynthesis, however, has proven difficult with estimates spanning a billion years. Sedimentary manganese (Mn) enrichments represent a potentially important line of evidence given the high redox potentials necessary to oxidize Mn in natural environments. However, this view has been challenged by abiotic and anaerobic Mn oxidation pathways that decouple Mn enrichments from oxygenation. With these in mind, we review Mn oxidation pathways and Mn enrichments and evaluate their relation to Earth's oxygenation. We argue that despite possible alternative pathways, shallow oxygenated seawater is a prerequisite for generating and, importantly, preserving significant sedimentary Mn enrichments (and associated geochemical signals). This implies that Mn enrichments indeed track Earth's oxygenation and oxygenic photosynthesis emerged 100 s of millions of years prior to irreversible atmospheric oxygenation

Globally asynchronous sulphur isotope signals require re-definition of the Great Oxidation Event

© 2018 The Author(s). The Great Oxidation Event (GOE) has been defined as the time interval when sufficient atmospheric oxygen accumulated to prevent the generation and preservation of mass-independent fractionation of sulphur isotopes (MIF-S) in sedimentary rocks. Existing correlations suggest that the GOE was rapid and globally synchronous. Here we apply sulphur isotope analysis of diagenetic sulphides combined with U-Pb and Re-Os geochronology to document the sulphur cycle evolution in Western Australia spanning the GOE. Our data indicate that, from ~2.45 Gyr to beyond 2.31 Gyr, MIF-S was preserved in sulphides punctuated by several episodes of MIF-S disappearance. These results establish the MIF-S record as asynchronous between South Africa, North America and Australia, argue for regional-scale modulation of MIF-S memory effects due to oxidative weathering after the onset of the GOE, and suggest that the current paradigm of placing the GOE at 2.33-2.32 Ga based on the last occurrence of MIF-S in South Africa should be re-evaluated

High pCO2-induced exopolysaccharide-rich ballasted aggregates of planktonic cyanobacteria could explain Paleoproterozoic carbon burial

The contribution of planktonic cyanobacteria to burial of organic carbon in deep-sea sediments before the emergence of eukaryotic predators ~1.5 Ga has been considered negligible owing to the slow sinking speed of their small cells. However, global, highly positive excursion in carbon isotope values of inorganic carbonates ~2.22-2.06 Ga implies massive organic matter burial that had to be linked to oceanic cyanobacteria. Here to elucidate that link, we experiment with unicellular planktonic cyanobacteria acclimated to high partial CO2 pressure (pCO2) representative of the early Paleoproterozoic. We find that high pCO2 boosts generation of acidic extracellular polysaccharides (EPS) that adsorb Ca and Mg cations, support mineralization, and aggregate cells to form ballasted particles. The down flux of such self-assembled cyanobacterial aggregates would decouple the oxygenic photosynthesis from oxidative respiration at the ocean scale, drive export of organic matter from surface to deep ocean and sustain oxygenation of the planetary surface