The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Quebec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

Roots of salt marsh grasses contribute to soil building but also affect decomposition by releasing bioavailable carbon exudates and oxygen. Disentangling exudate and oxygen effects on decomposition is difficult in the field but essential for marsh carbon models and predicting the impacts of global change disturbances. We tested how pulsed, simulated exudates affect soil metabolism under oxic and anoxic conditions, and whether carbon and oxygen availability facilitate mineralization of existing organic matter (i.e., priming). We conducted a laboratory experiment in flow-through reactors, adding carbon pulses weekly for 84 days and then following starvation under low carbon conditions. Oxygen consumption and sulfide production were inhibited under anoxic and oxic conditions and slowed by 21 +/- 10% and 55 +/- 8%, respectively, between 1- and 5- days following exudate pulses. Respiration rates immediately following and between pulses increased over time, suggesting that microbes capitalize on and may acclimate to patchy resources. Starvation caused oxygen consumption and sulfide production to fall 28% and 78% in oxic and anoxic treatments. Smaller decreases in oxygen consumption following pulses could suggest greater access to secondary carbon sources and that sulfate reducers were more reliant on exudates. Soil organic carbon was not the likely secondary source because porewater dissolved inorganic carbon delta C-13 values did not change during transit through the reactors, despite a similar to 26 parts per thousand difference between the supplied seawater and marsh soil. Interpretation of oxygen consumption rates is complicated by non-respiratory oxidation of reduced inorganic compounds and possibly significant lithoautotrophy. Exudate pulses elicited rapid and ephemeral respiratory responses, particularly under anoxia, but non-respiratory oxidation of reduced compounds obscured the impact of oxygen availability in our experimental system. Despite this, greater aerobic respiration rates suggest that oxygen availability has more potential to regulate carbon mineralization in coastal wetlands than root exudates.

Microplastics are small (< 5 mm) synthetic polymers that are a contaminant of emerging concern and can be difficult to identify due to their diversity in size, shape and composition. The hot needle test, or hot point test, helps researchers identify suspected microplastics under optical microscopy by probing their physical melt or deformation behavior, and is a low-cost and practical method for widespread use. However, to our knowledge the accuracy of this test has not been fully evaluated. We noted that articles commonly referenced by researchers for the hot needle method do not have a detailed description nor evaluation of the method accuracy. To address this knowledge gap, we took a mixed methods approach to describe the conditions under which the hot point test performs accurately, including a systematic literature review, reporting of the response of known fibers to a hot point, and evaluation of method performance by researchers in both controlled and environmental samples. In a single-blind trial of researchers applying different hot point conditions to a set of synthetic, semi-synthetic and natural fibers, synthetic and some natural fibers were correctly identified > 70% of the time. While cotton and semi-synthetic fiber results were less consistently identified (< 65% correct), this was improved (82–100% correct) in a second trial when clearer, updated guidance was given regarding the difference between a “pass” and “fail” response, showing the potential for the hot needle test to help analysts avoid false positives. Cellulose acetate from cigarette filters was the most challenging to identify because although this material may melt, response of individual fibers to heat varies and can be difficult to observe for smaller microfibers. Reported confirmation rates by spectroscopy of suspected microplastics that pass the hot needle test vary widely in the literature. Using detailed hot needle test criteria, > 90% of microplastics that we selected from environmental samples (water, sediment) were confirmed by Raman microscopy. It is recommended that researchers assess their hot needle test methods against known standards of both target microplastics and background materials like natural fibers, report the response criteria used in their studies and optimally include spectroscopic verification of results for higher confidence.

Humic substances, a component of terrestrial dissolved organic matter (tDOM), contribute to dissolved organic matter (DOM) and chromophoric DOM (CDOM) in coastal waters, and have significant impacts on biogeochemistry. There are concerns in recent years over browning effects in surface waters due to increasing tDOM inputs, and their negative impacts on aquatic ecosystems, but relatively little work has been published on estuaries and coastal waters. Photodegradation could be a significant sink for tDOM in coastal environments, but the rates and efficiencies are poorly constrained. We conducted large‐scale DOM photodegradation experiments in mesocosms amended with humic substances and nutrients in the Gulf of Finland to investigate the potential of photochemistry to remove added tDOM and the interactions of DOM photochemistry with eutrophication. The added tDOM was photodegraded rapidly, as CDOM absorption decreased and spectral slopes increased with increasing photons absorbed in laboratory experiments. The in situ DOM optical properties became similar among the control, humic‐ and humic+nutrients‐amended mesocosm samples toward the end of the amendment experiment, indicating degradation of the excess CDOM/DOM through processes including photodegradation. Nutrient additions did not significantly influence the effects of added humic substances on CDOM optical property changes, but induced changes in DOM removal.

Gelatinous zooplankton play a crucial role in pelagic marine food webs, however, due to methodological challenges and persistent misconceptions of their importance, the trophic role of gelatinous zooplankton remains poorly investigated. This is particularly true for small gelatinous zooplankton including the marine pelagic tunicate, Dolioletta gegenbauri . D. gegenbauri and other doliolid species occur persistently on wide subtropical shelves where they often produce massive blooms in association with shelf upwelling conditions. As efficient filter feeders and prodigious producers of relatively low‐density organic‐rich aggregates, doliolids are understood to contribute significantly to shelf production, pelagic ecology, and pelagic–benthic coupling. Utilizing molecular gut content analysis and stable isotope analysis approaches, the trophic interactions of doliolids were explored during bloom and non‐bloom conditions on the South Atlantic Bight continental shelf in the Western North Atlantic. Based on molecular gut content analysis, relative ingestion selectivity varied with D. gegenbauri life stage. At all life stages, doliolids ingested a wide range of prey types and sizes, but exhibited selectivity for larger prey types including diatoms, ciliates, and metazoans. Experimental growth studies confirmed that metazoan prey were ingested, but indicated that they were not digested and assimilated. Stable isotopic composition (δ 13 C and δ 15 N) of wild‐caught doliolids, during bloom and non‐bloom conditions, were most consistent with a detrital‐supplemented diet. These observations suggest that the feeding ecology of D. gegenbauri is more complex than previously reported, and have strong and unusual linkages to the microbial food web.

Despite extensive research into the transport and fate of oceanic microplastics (MP, <5mm in size), there is comparatively little focus on river systems considered to be pathways for these contaminants. The Savannah River, forming the border between Georgia and South Carolina, provides a unique location to study MP pollution along a variably industrialized river system terminating in the Atlantic Ocean. We investigated spatial variations in MP concentrations along the Savannah River to better understand their transport and deposition in rural to highly developed fluvial systems. Samples of riverbank sediment and suspended particles captured by a <80μm plankton net were collected along a 115 km reach of the river extending from just below the Strom Thurmond Dam to 25km downstream of Augusta, GA. Laboratory MP separation followed NOAA guidelines with a heavy liquid float-sink separation technique and wet peroxide oxidation treatment. Visually identified MPs were counted and photographed using a stereomicroscope; a subset of particles from each sample were examined using a Horiba XploRa Plus confocal microscope system. Average MP concentrations were measured at 3.1 (range: 1.5-4.6) particles/cubic meter in water and 16.8 (range: 6.2-27.4) particles/kg sediment and primarily composed of polyester fibers and polypropylene pellets. Comparison of MP concentrations between sediment samples from the upper bank and water margin suggests that MP particle deposition is dependent on river stage. Preliminary results further indicate that there is no observable relationship between increasing drainage area and MP concentration, suggesting that concentration may be dependent on localized anthropogenic sources rather than cumulative upstream contributions. Measured concentrations of MP in bank sediment in the upper reaches of the Savannah River are an order-of magnitude less than published concentrations at the river's mouth collected over the same sampled cross-sectional area, suggesting tidal action exerts a significant control on MP pollution in coastal and near coastal areas. Future work will focus on quantifying the predicted role of tidally dominated systems in concentrating microplastics around river mouths and identifying river reaches with highly concentrated MP particles for targeted remediation.

Eutrophication has been linked to increased greenhouse gas emissions from inland waters. Phytoplankton blooms in Lake Erie have increased since the 1990s, although its greenhouse gas emissions are not well characterized. We measured CH4 and N2O concentrations and diffusive fluxes in four seasons around the entire lake, and CO2 fluxes in one summer season. Lake Erie is a source of CH4 all year across the lake, concentrated in spring and summer in the Western Basin. Methane emissions ranged from 0.03 to 14.87 mg C m(-2) d(-1). Methane is predominantly biogenic, and natural gas leaks are an insignificant source. While Lake Erie is an overall N2O source, it is an N2O sink in winter and occasionally during summer. Emissions of N2O ranged from -0.08 to 1.22 mg N m(-2) d(-1). We also measured CO2 fluxes in summer only, when Lake Erie is a small atmospheric CO2 sink. While areal fluxes of CH4 and N2O are similar to those observed elsewhere, total fluxes from Lake Erie are higher due to its surface area. Lake Erie emits similar to 6300 (+/- 19%) metric tons of CH4-C yr(-1) and similar to 600 (+/- 37%) metric tons N2O-N yr(-1): almost 500,000 metric tons CO2-eq yr(-1) total. This is the first comprehensive dataset of CH4 and N2O concentrations and diffusive emissions in a very large lake. More measurements and monitoring are needed to determine whether increased eutrophication in the Great Lakes is tied to increased emissions of these powerful climate forcers in a possible positive feedback to climate warming. (C) 2020 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Eutrophication is linked to greenhouse gas emissions from inland waters. Phytoplankton blooms in Lake Erie, one of Earth’s largest lakes, have increased with nutrient runoff linked to climate warming, although greenhouse gas emissions from this or other large eutrophic lakes are not well characterized. We measured greenhouse gases around Lake Erie in all four seasons and found that CH4 and N2O emissions have increased 10 times or more with re-eutrophication, especially during and after phytoplankton blooms. Lake Erie is a positive source of CH4 throughout the entire year and around the entire lake, with the highest emissions in spring and summer near the mouth of the Maumee River. While Lake Erie is an overall N2O source, it is an N2O sink in winter throughout the lake and in some locations during large phytoplankton blooms. We estimate that Lake Erie emits ~6300 metric tons of CH4-C yr−1 (± 19%) and ~600 metric tons N2O-N yr−1 (± 37%): almost 500,000 metric tons CO2-eq yr−1 total. These results highlight the gravity of eutrophication-related increases in large lake GHG emissions: an overlooked, but potentially major feedback to global climate change.

The US National Oceanic and Atmospheric Administration (NOAA) defines microplastics as any type of plastic fragment that measures less than 5mm in length.Plastics have always been notorious for their damaging effect on the environment, not least because of how slowly they degrade, so improving understanding of microplastics pollution in the environment -particularly in aquatic and marine ecosystems -is essential.Given the small size of microplastics, they can be ingested by animals at the bottom of the marine food chain, causing unknown damage to the ecosystems they form part of.

A portion of the charcoal and soot produced during combustion processes on land (e.g., wildfire, burning of fossil fuels) enters aquatic systems as dissolved black carbon (DBC). In terms of mass flux, rivers are the main identified source of DBC to the oceans. Since DBC is believed to be representative of the refractory carbon pool, constraining sources of marine DBC is key to understanding the long-term persistence of carbon in our global oceans. Here, we use compound-specific stable carbon isotopes (δ13C) to reveal that DBC in the oceans is ~6‰ enriched in 13C compared to DBC exported by major rivers. This isotopic discrepancy indicates most riverine DBC is sequestered and/or rapidly degraded before it reaches the open ocean. Thus, we suggest that oceanic DBC does not predominantly originate from rivers and instead may be derived from another source with an isotopic signature similar to that of marine phytoplankton.

We examined the stable carbon isotopic composition of remineralized organic carbon (δ13COCx) in the northern Gulf of Mexico (nGoM) using incubations (sediment and water) and a three‐end‐member mixing model. δ13COCx in incubating sediments was −18.1‰ ± 1.3‰, and δ13COCx in incubating near‐surface and near‐bottom waters varied with salinity, ranging from −30.4‰ to −16.2‰ from brackish water to full‐strength Gulf water. The average δ13COCx was −18.6‰ ± 1.8‰ at salinity >23. A three‐end‐member mixing model based on a multiyear data set collected in previous summer hypoxia cruises (2011, 2012, 2014, 2015, and 2016) suggested that δ13COCx in near‐bottom waters across the nGoM (5–50 m) was −18.1‰ ± 0.6‰. The close agreement of δ13COCx obtained from the three independent approaches, that is, incubations of water column, surface sediments, and mixing model, suggests that 13C‐enriched organic matter of marine origin played the dominant role in near‐bottom water and benthic oxygen consumption in the nGoM shelf in summer.

Southeastern salt marshes are important repositories of sediment and carbon, and their formation is heavily dependent on deposition and accumulation of inorganic sediment. This study examined Groves Creek marsh near Savannah, GA, a typical Spartina alterniflora salt marsh of the southeastern US. Analyses were focused on the character, deposition and accumulation of material within the marsh on daily, monthly, decadal and centennial timescales, to determine the dominant factors in material supply and redistribution, and on its stratigraphy to determine the 1,000-year history of Groves Creek salt marsh development. Modern processes create gradients in grain size, which shows little variation from the tidal channel flanks up to mean sea level, and which coarsens with distance into the marsh from mean sea level to mean high water. This unexpected result suggests that, although floc transport is an important mechanism of sediment supply near the channel margins, energetic events must supply coarser materials to the marsh platform, where they are not readily removed by typical energy regimes. Daily deposition can approach ~3 g/cm 2 year; however, centennial accumulation rates are orders of magnitude lower (0.11±0.05 g/cm 2 year) and are similar to those present over the past 300 years (0.05–0.2 g/cm 2 year), indicating that much of the daily deposition is remobilized. Stable isotopic δ 13 C (average –18.7‰) and δ 15 N (average 5.7‰) values most likely indicate a large contribution from S. alterniflora as a carbon source throughout the marsh, although heavier δ 15 N on the channel flanks suggest that benthic algae may be locally important. Geologic, geochemical and microfossil evidence suggests that depositional conditions in the Groves Creek marsh have changed significantly over the past ~1,000 years, creating a distinct fining-upward sequence. This sequence preserves the signature (from bottom to top) of subtidal flats grading to intertidal sandflats, an erosional lag created by a migrating tidal channel, point bar deposits, channel fill deposits, and a capping salt marsh. A significant change in environmental conditions occurred between 360–500 cal years BP, which changed the character of geological, geochemical and microfossil components in the marsh, and, because this observation is robust throughout the southeastern US, hints at a major change in estuarine and coastal systems at this time.

Interactions between organic matter and mineral matrices are critical to the preservation of soil and sediment organic matter. In addition to clay minerals, Fe(III) oxides particles have recently been shown to be responsible for the protection and burial of a large fraction of sedimentary organic carbon (OC). Through a combination of synchrotron X-ray techniques and high-resolution images of intact sediment particles, we assessed the mechanism of interaction between OC and iron, as well as the composition of organic matter co-localized with ferric iron. We present scanning transmission x-ray microscopy images at the Fe L 3 and C K 1 edges showing that the organic matter co-localized with Fe(III) consists primarily of C=C, C=O and C-OH functional groups. Coupling the co-localization results to iron K-edge X-ray absorption spectroscopy fitting results allowed to quantify the relative contribution of OC-complexed Fe to the total sediment iron and reactive iron pools, showing that 25–62% of total reactive iron is directly associated to OC through inner-sphere complexation in coastal sediments, as much as four times more than in low OC deep sea sediments. Direct inner-sphere complexation between OC and iron oxides (Fe-O-C) is responsible for transferring a large quantity of reduced OC to the sedimentary sink, which could otherwise be oxidized back to CO 2 .

The questions that chemical oceanographers prioritize over the coming decades, and the methods we use to address these questions, will define our field's contribution to 21st century science. In recognition of this, the U.S. National Science Foundation and National Oceanic and Atmospheric Administration galvanized a community effort (the Chemical Oceanography MEeting: A BOttom-up Approach to Research Directions, or COME ABOARD) to synthesize bottom-up perspectives on selected areas of research in Chemical Oceanography. Representing only a small subset of the community, COME ABOARD participants did not attempt to identify targeted research directions for the field. Instead, we focused on how best to foster diverse research in Chemical Oceanography, placing emphasis on the following themes: strengthening our core chemical skillset; expanding our tools through collaboration with chemists, engineers, and computer scientists; considering new roles for large programs; enhancing interface research through interdisciplinary collaboration; and expanding ocean literacy by engaging with the public. For each theme, COME ABOARD participants reflected on the present state of Chemical Oceanography, where the community hopes to go and why, and actionable pathways to get there. A unifying concept among the discussions was that dissimilar funding structures and metrics of success may be required to accommodate the various levels of readiness and stages of knowledge development found throughout our community. In addition to the science, participants of the concurrent Dissertations Symposium in Chemical Oceanography (DISCO) XXV, a meeting of recent and forthcoming Ph.D. graduates in Chemical Oceanography, provided perspectives on how our field could show leadership in addressing long-standing diversity and early career challenges that are pervasive throughout science. Here we summarize the COME ABOARD Meeting discussions, providing a synthesis of reflections and perspectives on the field.

Soluble manganese in the intermediate +III oxidation state (Mn3+) is a newly identified oxidant in anoxic environments, whereas acetate is a naturally abundant substrate that fuels microbial activity. Microbial populations coupling anaerobic acetate oxidation to Mn3+ reduction, however, have yet to be identified. We isolated a Shewanella strain capable of oxidizing acetate anaerobically with Mn3+ as the electron acceptor, and confirmed this phenotype in other strains. This metabolic connection between acetate and soluble Mn3+ represents a new biogeochemical link between carbon and manganese cycles. Genomic analyses uncovered four distinct genes that allow for pathway variations in the complete dehydrogenase-driven TCA cycle that could support anaerobic acetate oxidation coupled to metal reduction in Shewanella and other Gammaproteobacteria. An oxygen-tolerant TCA cycle supporting anaerobic manganese reduction is thus a new connection in the manganese-driven carbon cycle, and a new variable for models that use manganese as a proxy to infer oxygenation events on early Earth.

Black carbon (BC) is derived from the burning of biomass, a considerable portion of organic matter across terrestrial and marine ecosystems, and a major contributor to global carbon cycles. The benzenepolycarboxylic acid method, which converts condensed aromatic BC structures to molecular markers (BPCAs), has been widely adopted for environmental BC assessments. We present a novel analytical method for the online separation, quantification, and determination of compound-specific stable carbon isotopes of individual BPCAs via high performance liquid chromatography-stable carbon isotope ratio mass spectrometry (HPLC-C-IRMS). To assess delta C-13 values of environmental BC, the HPLC-C-IRMS method was applied to a variety of samples (soil, char, aerosol, and dissolved organic matter). The method reliably measured amounts and delta C-13 values of benzenepentacarboxylic acid (B5CA) and mellitic acid (B6CA) over a range of 8.3-130 mu mol-C of organic matter oxidized. BPCA-specific delta C-13 values deviated from bulk OC delta C-13 values for different sample types. However, the stable carbon isotopic composition of B5CA and B6CA was found to be generally correlated with that of bulk organic carbon. The stable isotopic compositions of environmental samples covered a wide dynamic range, indicating BC-specific delta C-13 values will be useful for tracking BC sources and processing. For instance, the BPCAs derived from marine dissolved organic matter (DOM) were enriched in C-13 relative to BPCAs from riverine DOM, suggesting rivers are not the only source of dissolved black carbon to the oceans.

Anthropogenic atmospheric loading of CO2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effect on carbon cycling are still not well understood. Here, we investigated how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. This helps to remove CO2 from the atmosphere, and may also have fisheries implications.

Photochemical reactions in natural waters are a sink for dissolved organic carbon (DOC) and a source of dissolved inorganic carbon (DIC). Although considered significant to the carbon budget of the oceans, DIC photoproduction rates remain poorly constrained due to the analytical challenge involved in accurately measuring very low production rates (likely sub-μMh−1 for blue ocean waters) relative to high background DIC concentrations (~2mM). In an attempt to overcome this analytical limitation, almost all previous DIC photoproduction studies in marine systems have relied on the stripping of background DIC prior to irradiation, with unknown consequences for the integrity of the DOC pool and its photoreactivity, and none have resulted in a satisfactory determination of photoproduced DIC in open ocean waters. Here, we use small additions of NaH13CO3 (<10% of background DIC concentrations) to achieve moderate DI13C isotope enrichment (MoDIE) of river, estuarine, and seawater samples. We then quantify the shift in δ13C of DIC in irradiated samples via liquid chromatography - isotope ratio mass spectrometry (LC-IRMS) and use these shifts to calculate photoproduced DIC. MoDIE provides a new method to more precisely (±56 to ±340nM DIC) and more rapidly quantify DIC photoproduction compared to previous methods. MoDIE was evaluated by determining initial DIC photoproduction rates, along with associated broadband photochemical efficiency data (ranging from 144 to 280μmol DIC per mol photons absorbed), in riverine to offshore waters, and produced the earliest time point and most precise measurement of DIC photoproduction in unmodified, low-CDOM, blue water (ag(325)=0.30m−1) reported to date. In addition to measuring small DIC changes from photochemistry, the use of MoDIE could provide a major advance for other ocean carbon cycling studies, including examination of oceanic respiration, carbonate mineral precipitation/dissolution, and assessment of ocean acidification.

Reactive Fe(III) minerals can influence methane (CH 4 ) emissions by inhibiting microbial methanogenesis or by stimulating anaerobic CH 4 oxidation. The balance between Fe(III) reduction, methanogenesis, and methane oxidation in ferruginous Archean and Paleoproterozoic oceans would have controlled CH 4 fluxes to the atmosphere, thereby regulating the capacity for CH 4 to warm the early Earth under the Faint Young Sun. We studied CH 4 and Fe cycling in anoxic incubations of ferruginous sediment from the ancient ocean analogue Lake Matano, Indonesia over three successive transfers (500 days total). Iron reduction, methanogenesis, methane oxidation, and microbial taxonomy were monitored in treatments amended with ferrihydrite or goethite. After three dilutions, Fe(III) reduction persisted only in bottles with ferrihydrite. Enhanced CH 4 production was observed in the presence of goethite, highlighting the potential for reactive Fe(III)-oxides to inhibit methanogenesis. Supplementing the media with hydrogen, nickel and selenium did not stimulate methanogenesis. There was limited evidence for Fe(III)-dependent CH 4 oxidation, although some incubations displayed CH 4 -stimulated Fe(III)-reduction. 16S rRNA profiles continuously changed over the course of enrichment, with ultimate dominance of unclassified members of the order Desulfuromonadales in all treatments. Microbial diversity decreased markedly over the course of incubation, with subtle differences between ferrihydrite and goethite amendments. These results suggest that Fe(III)-oxide mineralogy and availability of electron donors could have led to spatial separation of Fe(III)-reducing and methanogenic microbial communities in ferruginous marine sediments, potentially explaining the persistence of CH 4 as a greenhouse gas throughout the first half of Earth history.