The chemistry of the Early Earth is widely inferred from the elemental and isotopic compositions of sulfidic sedimentary rocks, which are presumed to have formed globally through the reduction of ...seawater sulfate or locally from hydrothermally supplied sulfide. Here we argue that, in the anoxic Archean oceans, pyrite could form in the absence of ambient sulfate from organic sulfur contained within living cells. Sulfides could be produced through mineralization of reduced sulfur compounds or reduction of organic-sourced sulfite. Reactive transport modeling suggests that, for sulfate concentrations up to tens of micromolar, organic sulfur would have supported 20 to 100% of sedimentary pyrite precipitation and up to 75% of microbial sulfur reduction. The results offer an alternative explanation for the low range of δ
S in Archean sulfides, and raise a possibility that sulfate scarcity delayed the evolution of dissimilatory sulfate reduction until the initial ocean oxygenation around 2.7 Ga.
Remobilization of phosphorus from aquatic sediments has been extensively investigated in systems prone to anoxia, while studies in well-oxygenated systems have been rare. The recycling efficiency of ...P in the offshore sediments in the Great Lakes, in particular, is still poorly known. We investigated phosphorus cycling at 13 locations (26–318 m water depth) in oligotrophic Lake Superior where oxygen penetrates into sediments by 2–12 cm. Vertical distributions of iron and phosphorus were measured in porewater and solid fractions, and transformation rates and vertical fluxes were calculated. Whereas a significant fraction of P is bound to ferric Fe in surface sediments, P effluxes into the water column (2.5–7.0 μmol m-2 d-1) are only weakly affected by iron reduction, because Fe : P ratios in surface sediment are high (∼ 40–80 mol : mol), and P sorption capacity is far from its limit. In contrast to organic rich systems where P effluxes are sensitive to redox conditions, phosphate effluxes in organic-poor well-oxygenated Lake Superior are controlled by the rates of organic phosphorus mineralization, similar to marine sediments. The efficiency of P recycling in Lake Superior sediments, however, is substantially lower than in marine sediments due to different P biogeochemistry. Only ∼ 12% of deposited P is returned to the water column. While burial into sediments is the dominant sink for P in the lake, sediments still contribute up to 40% of total water column P inputs. Similar behavior should be expected in other well-oxygenated freshwater systems, such as other large oligotrophic lakes.
Anoxic and iron-rich (ferruginous) conditions prevailed in the ocean under the low-oxygen atmosphere that occurred through most of the Archean Eon. While euxinic conditions (i.e. anoxic and hydrogen ...sulfide-rich waters) became more common in the Proterozoic, ferruginous conditions persisted in deep waters. Ferruginous ocean regions would have been a major biosphere and Earth surface reservoir through which elements passed through as part of their global biogeochemical cycles. Understanding key biological events, such as the rise of oxygen in the atmosphere, or even the transitions from ferruginous to euxinic or oxic conditions, requires understanding the biogeochemical processes occurring within ferruginous oceans, and their indicators in the rock record. Important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes; their sediments commonly host siderite and Ca-carbonates, which are important Precambrian records of the carbon cycling. Lakes that were ferruginous in the past, or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid-depth oceanic waters during the Proterozoic. Modern ferruginous meromictic lakes, which host diverse anaerobic microbial communities, are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans. Such lakes are believed to be rare, but regional and geological factors indicate they may be more common than previously thought. While physical mixing processes in lakes and oceans are notably different, many chemical and biological processes are similar. The diversity of sizes, stratifications, and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems: near-shore, off-shore, silled basins, or those dominated by terrestrial or hydrothermal element sources. Ferruginous systems, both extant and extinct, lacustrine and marine, host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of Earth’s surface environment.
•Precambrian marine sediments indicate frequent ferruginous conditions with euxinic intervals.•Siderite from ferruginous lakes informs formation pathways in ferruginous oceans.•Ferruginous meromictic lakes are an expected feature of postglacial landscapes.•Ferruginous lakes can be biogeochemical analogues of ferruginous oceans.
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 δ13C. These observations demonstrate that sedimentary Mn enrichments may 1) develop from primary carbonate phases, and 2) can occur in environments with dissolved oxygen concentrations <5 μM. Primary Mn-carbonates are likely to originate in environments with high concentrations of dissolved Mn (>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.
•We document the production and deposition of Mn-carbonate in a ferruginous lake.•Rhodochrosite nucleates on dissolving calcite crystals in Mn-rich waters.•The carbon isotope composition of Mn-carbonates may represent methane oxidation.•Mn-oxide burial is not be required to create a sedimentary Mn enrichment.•Primary Mn-carbonates may accumulate in environments with <5 μM dissolved oxygen.
Sulfate was a trace constituent of Archean seawater Crowe, Sean A.; Paris, Guillaume; Katsev, Sergei ...
Science (American Association for the Advancement of Science),
11/2014, Letnik:
346, Številka:
6210
Journal Article
Recenzirano
In the low-oxygen Archean world (>2400 million years ago), seawater sulfate concentrations were much lower than today, yet open questions frustrate the translation of modern measurements of sulfur ...isotope fractionations into estimates of Archean seawater sulfate concentrations. In the water column of Lake Matano, Indonesia, a low-sulfate analog for the Archean ocean, we find large (>20 per mil) sulfur isotope fractionations between sulfate and sulfide, but the underlying sediment sulfides preserve a muted range of δ
34S values. Using models informed by sulfur cycling in Lake Matano, we infer Archean seawater sulfate concentrations of less than 2.5 micromolar. At these low concentrations, marine sulfate residence times were likely 10³ to 10⁴ years, and sulfate scarcity would have shaped early global biogeochemical cycles, possibly restricting biological productivity in Archean oceans.
To understand the nitrogen (N) cycle in sediments with deep oxygen penetration, we measured pore-water profiles to calculate N fluxes and rates at 13 locations in Lake Superior in water depths ...ranging from 26 to 318 m. Sediments with high oxygen demand, such as in nearshore or high-sedimentation areas, contribute disproportionally to benthic N removal, despite covering only a small portion of the lake floor. These sediments are nitrate sinks (average 0.16 mmol m−2 d−1) and have denitrification rates (average 0.76 mmol m−2 d−1) that are comparable to those in coastal marine sediments. The deeply oxygenated (4 to > 12 cm) offshore sediments are nitrate sources (average 0.26 mmol m−2 d−1) and generate N₂ at lower rates (average 0.10 mmol m−2 d−1). Ammonium is nitrified with high efficiency (90%), and nitrification supports > 50% of denitrification nearshore and ∼ 100% offshore. Oxygen consumption by nitrification accounts for 12% of the total sediment oxygen uptake. About 2% of nitrate reduction is coupled to oxidation of iron, a rarely detected pathway. Our Lake Superior N budget indicates significant contributions from sediment–water exchanges and N₂ production and is closer to balance than previous budgets. Our results reveal that sediment N cycling in large freshwater lakes is similar to that in marine systems. They further suggest that denitrification rates in slowly accumulating, well-oxygenated sediments cannot be described by the same relationship with total oxygen uptake as in high-sedimentation areas; hence, global models should treat abyssal ocean sediments differently than coastal and shelf sediments.
Photoferrotrophs thrive in an Archean Ocean analogue Crowe, Sean A; Jones, CarriAyne; Katsev, Sergei ...
Proceedings of the National Academy of Sciences - PNAS,
10/2008, Letnik:
105, Številka:
41
Journal Article
Recenzirano
Odprti dostop
Considerable discussion surrounds the potential role of anoxygenic phototrophic Fe(II)-oxidizing bacteria in both the genesis of Banded Iron Formations (BIFs) and early marine productivity. However, ...anoxygenic phototrophs have yet to be identified in modern environments with comparable chemistry and physical structure to the ancient Fe(II)-rich (ferruginous) oceans from which BIFs deposited. Lake Matano, Indonesia, the eighth deepest lake in the world, is such an environment. Here, sulfate is scarce (<20 μmol·liter⁻¹), and it is completely removed by sulfate reduction within the deep, Fe(II)-rich chemocline. The sulfide produced is efficiently scavenged by the formation and precipitation of FeS, thereby maintaining very low sulfide concentrations within the chemocline and the deep ferruginous bottom waters. Low productivity in the surface water allows sunlight to penetrate to the >100-m-deep chemocline. Within this sulfide-poor, Fe(II)-rich, illuminated chemocline, we find a populous assemblage of anoxygenic phototrophic green sulfur bacteria (GSB). These GSB represent a large component of the Lake Matano phototrophic community, and bacteriochlorophyll e, a pigment produced by low-light-adapted GSB, is nearly as abundant as chlorophyll a in the lake's euphotic surface waters. The dearth of sulfide in the chemocline requires that the GSB are sustained by phototrophic oxidation of Fe(II), which is in abundant supply. By analogy, we propose that similar microbial communities, including populations of sulfate reducers and photoferrotrophic GSB, likely populated the chemoclines of ancient ferruginous oceans, driving the genesis of BIFs and fueling early marine productivity.
Recent warming of lake Kivu Katsev, Sergei; Aaberg, Arthur A; Crowe, Sean A ...
PloS one,
10/2014, Letnik:
9, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Lake Kivu in East Africa has gained notoriety for its prodigious amounts of dissolved methane and dangers of limnic eruption. Being meromictic, it is also expected to accumulate heat due to rising ...regional air temperatures. To investigate the warming trend and distinguish between atmospheric and geothermal heating sources, we compiled historical temperature data, performed measurements with logging instruments, and simulated heat propagation. We also performed isotopic analyses of water from the lake's main basin and isolated Kabuno Bay. The results reveal that the lake surface is warming at the rate of 0.12°C per decade, which matches the warming rates in other East African lakes. Temperatures increase throughout the entire water column. Though warming is strongest near the surface, warming rates in the deep waters cannot be accounted for solely by propagation of atmospheric heat at presently assumed rates of vertical mixing. Unless the transport rates are significantly higher than presently believed, this indicates significant contributions from subterranean heat sources. Temperature time series in the deep monimolimnion suggest evidence of convection. The progressive deepening of the depth of temperature minimum in the water column is expected to accelerate the warming in deeper waters. The warming trend, however, is unlikely to strongly affect the physical stability of the lake, which depends primarily on salinity gradient.
A diagenetic reaction-transport model was used to simulate how the sediment redox boundary migrates in response to persistent or episodic changes in the deposition flux of degradable organic matter ...and the concentration of oxygen in the overlying bottom water. The position of the redox boundary is represented by the depth of oxygen penetration. The simulations reveal that the position of the redox boundary in organic-poor sediments, such as those in the deep basins of the Arctic Ocean, is highly sensitive to the flux of organic matter: relatively small and/or brief increases in that flux can cause the redox boundary to migrate rapidly from deep within the sediment to within a few centimeters of the sediment-water interface. Reoxidation of the sediment column after such an event can take years. Redox fluctuations can redistribute solid-phase manganese within the sediment column and produce multiple concentration peaks in its depth profile on a decadal time scale. Manganese peaks observed in sediment cores from the deep basins of the Arctic Ocean do not necessarily correspond to the position of the redox boundary during previous climatic periods or reflect historical changes in manganese deposition rates. The model supports the hypothesis that the recent decrease in the Arctic ice cover has increased the flux of organic matter to the seafloor and moved the redox boundary close to the sediment-water interface. The presence of iron sulfides at depths significantly below the bioturbated layer suggests that either the Arctic sediments have been anoxic for millennia, or iron and sulfate are reduced at these depths by dissolved organic matter diffusing downward from the bioturbation zone.
► We applied reaction-transport modeling to analyze the sediment phosphorus (P). ► We studied the effects of aeration and the organic carbon loading on P retention. ► On short time scales the release ...of iron-bound P impacted the P fluxes. ► On long time scales P fluxes followed the sedimentation of organic matter.
Phosphorus (P) releases from lake sediments are controlled in the long term by P burial into the deep sediment and on shorter time scales by the redox conditions at the sediment–water interface. In Lake Sempach (Switzerland), hypolimnetic oxygen concentration was increased by artificial aeration after two decades of nearly anoxic conditions. Using diagenetic reaction-transport modeling and sediment core analysis, we investigated the effects that this change, as well as variations in the organic carbon loadings, had on the long-term mobility of sediment P. During low-oxygen conditions, the reducible iron pool in the sediment was depleted, resulting in the release of previously accumulated P. The remobilization of iron-bound P affected phosphate effluxes from the sediment on the time scale of the sediment iron cycle (several years). On longer time scales, P effluxes followed the sedimentation fluxes of organic matter. Mass balance calculations indicate that, despite the dominance of internal P loading in Lake Sempach, over the long-term phosphorus content in the water column was controlled by the external P inputs. The results suggest that, whereas short-term decreases in sediment P releases may be achieved by preventing sediment anoxia, long-term solutions should involve reductions in the external P inputs.
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