Green rust formation controls nutrient availability in a ferruginous water column (original) (raw)
Iron oxide reactivity controls organic matter mineralization in ferruginous sediments
2020
Ferruginous sediments were widespread during the Archaean and Proterozoic Eons, but our knowledge about organic matter mineralization remains mostly conceptual, as analogous modern ferruginous sediments are largely unstudied. In sediments of ferruginous Lake Towuti, Indonesia, methanogenesis dominates organic matter mineralization despite abundant reactive ferric iron phases persisting throughout the core. This implies that ferric iron can be buried over geologic timescales even in the presence of labile organic carbon. Iron reactivity and hence its contribution to organic matter mineralization is highly variable. With negligible methane oxidation, methane may diffuse from the sediment into the water column and reach the atmosphere. We hypothesize that similar conditions prevailed during the Archaean and Proterozoic Eons, and thus, may have contributed to regulating Earth’s early climate.
Geobiology, 2010
The inventories and Fe isotope composition of aqueous Fe(II) and solid-phase Fe compounds were quantified in neutral-pH, chemically precipitated sediments downstream of the Iron Mountain acid mine drainage site in northern California, USA. The sediments contain high concentrations of amorphous Fe(III) oxyhydroxides [Fe(III) am ] that allow dissimilatory iron reduction (DIR) to predominate over Fe-S interactions in Fe redox transformation, as indicated by the very low abundance of Cr(II)-extractable reduced inorganic sulfur compared with dilute HCl-extractable Fe. d 56 Fe values for bulk HCl-and HF-extractable Fe were 0. These near-zero bulk d 56 Fe values, together with the very low abundance of dissolved Fe in the overlying water column, suggest that the pyrite Fe source had near-zero d 56 Fe values, and that complete oxidation of Fe(II) took place prior to deposition of the Fe(III) oxide-rich sediment. Sediment core analyses and incubation experiments demonstrated the production of millimolar quantities of isotopically light (d 56 Fe)1.5 to)0.5&) aqueous Fe(II) coupled to partial reduction of Fe(III) am by DIR. Trends in the Fe isotope composition of solid-associated Fe(II) and residual Fe(III) am are consistent with experiments with synthetic Fe(III) oxides, and collectively suggest an equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III) am of approximately)2&. These Fe(III) oxide-rich sediments provide a model for early diagenetic processes that are likely to have taken place in Archean and Paleoproterozoic marine sediments that served as precursors for banded iron formations. Our results suggest pathways whereby DIR could have led to the formation of large quantities of low-d 56 Fe minerals during BIF genesis.
Geobiology, 2010
The inventories and Fe isotope composition of aqueous Fe(II) and solid-phase Fe compounds were quantified in neutral-pH, chemically precipitated sediments downstream of the Iron Mountain acid mine drainage site in northern California, USA. The sediments contain high concentrations of amorphous Fe(III) oxyhydroxides [Fe(III) am ] that allow dissimilatory iron reduction (DIR) to predominate over Fe-S interactions in Fe redox transformation, as indicated by the very low abundance of Cr(II)-extractable reduced inorganic sulfur compared with dilute HCl-extractable Fe. d 56 Fe values for bulk HCl-and HF-extractable Fe were 0. These near-zero bulk d 56 Fe values, together with the very low abundance of dissolved Fe in the overlying water column, suggest that the pyrite Fe source had near-zero d 56 Fe values, and that complete oxidation of Fe(II) took place prior to deposition of the Fe(III) oxide-rich sediment. Sediment core analyses and incubation experiments demonstrated the production of millimolar quantities of isotopically light (d 56 Fe)1.5 to)0.5&) aqueous Fe(II) coupled to partial reduction of Fe(III) am by DIR. Trends in the Fe isotope composition of solid-associated Fe(II) and residual Fe(III) am are consistent with experiments with synthetic Fe(III) oxides, and collectively suggest an equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III) am of approximately)2&. These Fe(III) oxide-rich sediments provide a model for early diagenetic processes that are likely to have taken place in Archean and Paleoproterozoic marine sediments that served as precursors for banded iron formations. Our results suggest pathways whereby DIR could have led to the formation of large quantities of low-d 56 Fe minerals during BIF genesis.
Geobiology, 2005
During deposition of late Archean-early Palaeoproterozoic Precambrian banded iron formations (BIFs) the downward flux of ferric hydroxide (Fe(OH) 3 ) and phytoplankton biomass should have facilitated microbial Fe(III) reduction. However, quantifying the significance of such a metabolic pathway in the Precambrian is extremely difficult, considering the post-depositional alteration of the rocks and the lack of ideal modern analogues. Consequently, we have very few constraints on the Fe cycle at that time, namely (i) the concentration of dissolved Fe(II) in the ocean waters; (ii) by what mechanisms Fe(II) was oxidized (chemical, photochemical or biological, the latter using either O 2 or light); (iii) where the ferric hydroxide was precipitated (over the shelf vs. open ocean); (iv) the amount of phytoplankton biomass, which relates to the nutrient status of the surface waters; (v) the relative importance of Fe(III) reduction vs. the other types of metabolic pathways utilized by sea floor microbial communities; and (vi) the proportion of primary vs. diagenetic Fe(II) in BIF. Furthermore, although estimates can be made regarding the quantity of reducing equivalents necessary to account for the diagenetic Fe(II) component in Fe-rich BIF layers, those same estimates do not offer any insights into the magnitude of Fe(III) actually generated within the water column, and hence, the efficiency of Fe and C recycling prior to burial. Accordingly, in this study, we have attempted to model the ancient Fe cycle, based simply on conservative experimental rates of photosynthetic Fe(II) oxidation in the euphotic zone. We estimate here that under ideal growth conditions, as much as 70% of the biologically formed Fe(III) could have been recycled back into the water column via fermentation and organic carbon oxidation coupled to microbial Fe(III) reduction. By comparing the potential amount of biomass generated phototrophically with the reducing equivalents required for Fe(III) reduction and magnetite formation, we also hypothesize that another anaerobic metabolic pathway might have been utilized in the surface sediment to oxidize the fermentation by-products. Based on the premise that the deep ocean waters were anoxic, this role could have been fulfilled by methanogens, and maybe even methanotrophs that employed Fe(III) reduction.
Iron isotopes in an Archean ocean analogue
Geochimica et Cosmochimica Acta, 2014
Iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans. Archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the Proterozoic and Phanerozoic. This increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time. We tested these contrasting scenarios through a detailed study of anoxic and ferruginous Lac Pavin (France), which can serve as a modern analogue of the Archean ocean. A depth-profile in the water column of Lac Pavin shows a remarkable increase in dissolved Fe concentration (0.1-1200 lM) and d 56 Fe values (À2.14& to +0.31&) across the oxic-anoxic boundary to the lake bottom. The largest Fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron, leaving the residual Fe enriched in light isotopes. The analysis of four sediment cores collected along a lateral profile (one in the oxic layer, one at the redox boundary, one in the anoxic zone, and one at the bottom of the lake) indicates that bulk sediments, porewaters, and reactive Fe mostly have d 56 Fe values near 0.0 ± 0.2&, similar to detrital iron. In contrast, pyrite d 56 Fe values in sub-chemocline cores (60, 65, and 92 m) are highly variable and show significant deviations from the detrital iron isotope composition (d 56 Fe pyrite between À1.51& and +0.09&; average À0.93&). Importantly, the pyrite d 56 Fe values mirror the d 56 Fe of dissolved iron at the redox boundary-where near quantitative sulfate and sulfide drawdown occurs-suggesting limited iron isotope fractionation during iron sulfide formation. This finding has important implications for the Archean environment. Specifically, this work suggests that in a ferruginous system, most of the Fe isotope variability observed in sedimentary pyrites can be tied to water column cycling-foremost to the oxidation of dissolved ferrous iron. This supports previous suggestions that enhanced iron isotope variability in the Archean may record a unique stage in Earth's history where partial ferrous iron oxidation in upwelling water masses was a common process, probably linked to oxygenic or anoxygenic photosynthesis.
Pelagic photoferrotrophy and iron cycling in a modern ferruginous basin
Scientific Reports, 2015
Iron-rich (ferruginous) ocean chemistry prevailed throughout most of Earth's early history. Before the evolution and proliferation of oxygenic photosynthesis, biological production in the ferruginous oceans was likely driven by photoferrotrophic bacteria that oxidize ferrous iron {Fe(II)} to harness energy from sunlight, and fix inorganic carbon into biomass. Photoferrotrophs may thus have fuelled Earth's early biosphere providing energy to drive microbial growth and evolution over billions of years. Yet, photoferrotrophic activity has remained largely elusive on the modern Earth, leaving models for early biological production untested and imperative ecological context for the evolution of life missing. Here, we show that an active community of pelagic photoferrotrophs comprises up to 30% of the total microbial community in illuminated ferruginous waters of Kabuno Bay (KB), East Africa (DR Congo). These photoferrotrophs produce oxidized iron {Fe(III)} and biomass, and support a diverse pelagic microbial community including heterotrophic Fe(III)-reducers, sulfate reducers, fermenters and methanogens. At modest light levels, rates of photoferrotrophy in KB exceed those predicted for early Earth primary production, and are sufficient to generate Earth's largest sedimentary iron ore deposits. Fe cycling, however, is efficient, and complex microbial community interactions likely regulate Fe(III) and organic matter export from the photic zone. Ferruginous water bodies are rare on the modern Earth, yet they are invaluable natural laboratories for exploring the ecology and biogeochemistry of Fe-rich waters extensible to the ferruginous oceans of the Precambrian Eons 1-4. One modern ferruginous system, Lake Matano (Indonesia) hosts large populations of anoxygenic phototrophic bacteria implicated in photoferrotrophy due to the scarcity of sulfur substrates 4. Low light levels and extremely slow growth rates, however, have precluded the direct measurement of photoferrotrophy in its water column 5. In contrast, recent measurements of Fe-dependent
2010
The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth's surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS).
Low phosphorus concentrations and important ferric hydroxide scavenging in Archean seawater
PNAS Nexus
The availability of nutrients in seawater, such as dissolved phosphorous (P), is thought to have regulated the evolution and activity of microbial life in Earth’s early oceans. Marine concentrations of bioavailable phosphorous spanning the Archean Eon remain debated, with variable estimates indicating either low (0.04 to 0.13 μM P) or high (10 to 100 μM P) dissolved P in seawater. The large uncertainty on these estimates reflects in part a lack of clear proxy signals recorded in sedimentary rocks. Contrary to some recent views, we show here that iron formations (IFs) are reliable recorders of past phosphorous concentrations and preserved a primary seawater signature. Using measured P and iron (Fe) contents in Neoarchean IF from Carajás (Brazil), we demonstrate for the first time a clear partitioning coefficient relationship in the P-Fe systematics of this IF which, in combination with experimental and Archean literature data, permits us to constrain Archean seawater to a mean value ...
The early diagenesis of iron in pelagic sediments: a multidisciplinary approach
Earth and Planetary Science Letters, 1998
Biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial Atlantic are investigated by means of pore water chemistry, chemical leaching experiments and total elemental determinations, color reflectance spectroscopy, rock magnetic measurements and TEM observations of the magnetic fraction. Results indicate that in the presence of nitrate, ascorbate (a weak reducing agent) leachable iron decreased with depth from the sediment surface. Within this upper sediment region, iron assimilation by bacteria is indicated as magnetosomes were found by TEM observations throughout the whole core. Extractions with a strong reducing agent (dithionite), representing iron bound to iron oxides, correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone. The abrupt decrease of red color reflection intensity, relative to the amount of iron oxides below the iron redox boundary, is the result of a decrease in the specific surface area of iron oxyhydroxide=oxides. Magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary, arising from an increase in biogenic magnetite formed by magnetotactic (assimilatory) bacteria, dissolution of very fine-grained magnetite, and the gradual decrease of coarser grained terrigenous (titano-) magnetite with depth. The early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content.