Next to water quality deterioration, cyanobacteria blooms can affect turnover of aqueous carbon, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). We investigated interactions of these three phases and their stable isotopes in a freshwater pond with periodic cyanobacterial blooms over a period of 23 months. This helped to map turnover and sources of aqueous carbon before, during, and after bloom events. During bloom events POC isotope values (delta C-13(POC)) increased up to -17.4 parts per thousand, after aqueous CO2 (CO2(aq)) . fell below an atmospheric equilibration value of 412 mu atm. Additionally, carbon isotope enrichment between CO(2(aq) )and POC (epsilon(CO2-phyto)) ranged from 2.0 to 21.5%o with lowest fractionations observed at pH values above 8.9. The increase of delta C-13(POC) and decrease of epsilon(CO2-phyto) values at low pCO(2) and high pH was most likely caused by the activation of the carbon concentrating mechanism (CCM). This mechanism correlated with prevalent assimilation of C-13-enriched HCO3-. Surprisingly, CO2(aq) still contributed more than 50% to the POC pool down to pCO(2) values of around 150 mu atm. Only after this threshold the reduced epsilon(CO2-phyto) suggested incorporation of C-13-enriched HCO3-. (C) 2021 Elsevier B.V. All rights reserved.

At present most knowledge on the impact of iron on 18O / 16O ratios (i.e. δ18O) of dissolved oxygen (DO) under circum-neutral conditions stems from experiments carried out under controlled laboratory conditions. These showed that iron oxidation leads to an increase in δ18ODO values. Here we present the first study on effects of elevated Fe(II) concentrations on the δ18ODO in a natural, iron-rich, circum-neutral watercourse. Our results show that iron oxidation was the major factor for rising dissolved oxygen isotope compositions in the first 85 m of the system in the cold season (February) and for the first 15 m during the warm season (May). Further along the course of the stream, the δ18ODO decreased towards values known for atmospheric equilibration around +24.6 ‰ during both seasons. Possible drivers for these changes may be reduced iron oxidation, increased atmospheric exchange and DO production by oxygenic phototrophic algae mats. In the cold season, the δ18ODO values stabilized around atmospheric equilibrium, whereas in the warm season stronger influences by oxygenic photosynthesis caused values down to +21.8 ‰. In the warm season from 145 m downstream of the spring, the δ18ODO increased again until it reached atmospheric equilibrium. This trend can be explained by respiratory consumption of DO combined with a relative decrease in photosynthetic activity and increasing atmospheric influences. Our study shows that dissolved Fe(II) can exert strong effects on the δ18ODO of a natural circum-neutral spring system even under constant supply of atmospheric O2. However, in the presence of active photosynthesis, with supply of O2 to the system, direct effects of Fe oxidation on the δ18ODO value become masked. Nonetheless, critical Fe(II) concentrations may indirectly control DO budgets by enhancing photosynthesis, particularly if cyanobacteria are involved.

The first terrestrial habitats would have developed coincident with the generation of continental crust and the emergence of continents. Associated with this critical Earth-Life transition would have been the initiation of substantial subaerial weathering. Recently, fluvial sediments and nitrogen isotope-based evidence for a terrestrial biosphere have been reported from the 3.2 billion year old Moodies Group. Here we report hafnium isotope values for marine banded iron formations from the same succession. Values observed are amongst the most extreme values reported. These data exhibit a systematic departure from the terrestrial array, and if primary, can be explained by transport and weathering of felsic minerals under subaerial conditions, with values comparable to modern Amazon river waters and acid leachates of granite. These data from marine sediments likely constitute the earliest geochemical evidence of subaerial weathering processes affecting oceanic chemistry. This would imply that continental crust was an ecological niche and source of nutrients by the time they formed, at 3.22 billion years ago at the end of the Paleoarchean, consistent with interpretations of terrestrial sediments from the same succession that indicate microbial life and biogeochemical cycling were present at this time.

RationaleInvestigations of the isotope ratios of dissolved oxygen (δ18ODO) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine δ18ODO values during storage and can lead to a misinterpretation. We sampled an Fe(II)‐rich spring system and measured δ18ODO values at various time intervals in order to determine influences of Fe‐oxidation.MethodsWater samples were collected from an Fe‐rich spring and related stream and the δ18ODO values were measured in fresh, 4‐ and 13‐day‐old samples with an isotope ratio mass spectrometer. Three replicates were measured for each sample with a 1σ of ± 0.2‰. On‐site parameters and Fe(II) contents were also measured over the course of the spring system by multi‐parameter probes and spectrophotometry.ResultsThe δ18ODO values over the course of the spring system in fresh, 4‐ and 13‐day‐old samples revealed differences of up to 8‰. We explain this increase by the consumption of DO by Fe(II)‐oxidation. After a flow length of 85 m the differences in δ18ODO values between fresh and older samples decreased because most of the Fe(II) was consumed.ConclusionsFalse interpretations of δ18ODO values are possible if Fe‐rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.

Biogeochemistry investigates chemical cycles which influence or are influenced by biological activity. Astrobiology studies the origin, evolution and distribution of life in the universe. The biogeochemical Fe cycle has controlled major nutrient cycles such as the C cycle throughout geological time. Iron sulfide minerals may have provided energy and surfaces for the first pioneer organisms on Earth. Banded iron formations document the evolution of oxygenic photosynthesis. To assess the potential habitability of planets other than Earth one looks for water, an energy source and a C source. On Mars, for example, Fe minerals have provided evidence for the past presence of liquid water on its surface and would provide a viable energy source. Here we present Mössbauer spectroscopy investigations of Fe and C cycle interactions in both ancient and modern environments. Experiments to simulate the diagenesis of banded iron formations indicate that the formation of ferrous minerals depends on the amount of biomass buried with ferric precursors rather than on the atmospheric composition at the time of deposition. Mössbauer spectra further reveal the mutual stabilisation of Fe-organic matter complexes against mineral transformation and decay of organic matter into CO2. This corresponds to observations of a 'rusty carbon sink' in modern sediments. The stabilisation of Fe-organic matter complexes may also aid transport of particulate Fe in the water column while having an adverse effect on the bioavailability of Fe. In the modern oxic ocean, Fe is insoluble and particulate Fe represents an important source. Collecting that particulate Fe yields small sample sizes that would pose a challenge for conventional Mössbauer experiments. We demonstrate that the unique properties of the beam used in synchrotron-based Mössbauer applications can be utilized for studying such samples effectively. Reactive Fe species often occur in amorphous or nanoparticulate form in the environment and are therefore difficult to study with standard mineralogical tools. Sequential extraction techniques are commonly used as proxies. We provide an example where Mössbauer spectroscopy can replace sequential extraction techniques where mineralogical information is sought. Where mineral separation is needed, for example in the investigation of Fe or S isotope fractionation, Mössbauer spectroscopy can help to optimize sequential extraction procedures. This can be employed in a large number of investigations of soils and sediments, potentially even for mineral separation to study Fe and S isotope fractionation in samples returned from Mars, which might reveal signatures of biological activity. When looking for the possibility of life outside Earth, Jupiter's icy moon Europa is one of the most exciting places. It may be just in reach for a Mössbauer spectrometer deployed by a future lander to study the red streak mineral deposits on its surface to look for clues about the composition of the ocean hidden under the moon's icy surface.

Post-depositional diagenetic alteration makes the accurate interpretation of key precipitation processes in ancient sediments, such as Precambrian banded iron formations (BIFs), difficult. While microorganisms are proposed as key contributors to BIF deposition, the diagenetic transformation of precursor Fe(III) minerals associated with microbial biomass had not been experimentally tested. We incubated mixtures of ferrihydrite (proxy for biogenic ferric oxyhydroxide minerals) and glucose (proxy for microbial biomass) in gold capsules at 1.2 kbar and 170 degrees C. Both wet chemical analysis and mineralogical methods (microscopy, X-ray diffraction and Mossbauer spectroscopy) were used to analyze the reaction products. Under these conditions, ferrihydrite (Fe-III(OH)(3)) transforms to hematite ((Fe2O3)-O-III), magnetite ((FeFe2O4)-Fe-II-O-III), and siderite ((FeCO3)-C-II). Silica-coated ferrihydrite prepared at conservative Si: Fe ratios (as predicted for the Precambrian oceans) and mixed with glucose yielded hematite and siderite, whereas magnetite could not be identified microscopically. Our results show that electron transfer from organic carbon to Fe(III) minerals during temperature/pressure diagenesis can drive the production of key BIF minerals. Our results also demonstrate that the post-depositional mineralogy of BIF does not directly archive the oceanic or atmospheric conditions present on Earth during their lithification. As a consequence, atmospheric composition regarding concentrations of methane and CO2 during the time of BIF mineral deposition cannot be directly inferred from BIF mineralogical data alone. (C) 2013 Elsevier B. V. All rights reserved.

During deposition of Precambrian iron formation, the combined sedimentation of ferrihydrite and phytoplankton biomass should have facilitated Fe(III) reduction during diagenesis. However, the only evidence for this reaction in iron formations is the iron and carbon isotope values preserved in the authigenic ferrous iron-containing minerals. Here we show experimentally that spheroidal siderite, which is preserved in many iron formation and could have been precursor to rhombohedral or massive siderite, forms by reacting ferrihydrite with glucose (a proxy for microbial biomass) at pressure and temperature conditions typical of diagenesis (170 °C and 1.2 kbar). Depending on the abundance of siderite, we found that it is also possible to draw conclusions about the Fe(III):C ratio of the initial ferrihydrite-biomass sediment. Our results suggest that spherical to rhombohedral siderite structures in deep-water, Fe-oxide iron formation can be used as a biosignature for photoferrotrophy, whereas massive siderite reflects high cyanobacterial biomass loading in highly productive shallow-waters.

Banded iron formations (BIF) represent the largest source of iron in the world. They formed throughout the Precambrian, and today are globally distributed on the remnants of the ancient cratons. The first BIF dates back to at least 3.9–3.8 billion years. Little is known about this early period in earth’s history, in particular about the presence of molecular oxygen, O2, and therefore also about the deposition mechanisms of BIF at that time.