Rethinking the Ancient Sulfur Cycle Fike, David A; Bradley, Alexander S; Rose, Catherine V
Annual review of earth and planetary sciences,
05/2015, Letnik:
43, Številka:
1
Journal Article
Recenzirano
The sulfur biogeochemical cycle integrates the metabolic activity of multiple microbial pathways (e.g., sulfate reduction, disproportionation, and sulfide oxidation) along with abiotic reactions and ...geological processes that cycle sulfur through various reservoirs. The sulfur cycle impacts the global carbon cycle and climate primarily through the remineralization of organic carbon. Over geological timescales, cycling of sulfur is closely tied to the redox state of Earth's exosphere through the burial of oxidized (sulfate) and reduced (sulfide) sulfur species in marine sediments. Biological sulfur cycling is associated with isotopic fractionations that can be used to trace the fluxes through various metabolic pathways. The resulting isotopic data provide insights into sulfur cycling in both modern and ancient environments via isotopic signatures in sedimentary sulfate and sulfide phases. Here, we review the deep-time δ
34
S record of marine sulfates and sulfides in light of recent advances in understanding how isotopic signatures are generated by microbial activity, how these signatures are encoded in marine sediments, and how they may be altered following deposition. The resulting picture shows a sulfur cycle intimately coupled to ambient carbon cycling, where sulfur isotopic records preserved in sedimentary rocks are critically dependent on sedimentological and geochemical conditions (e.g., iron availability) during deposition.
Abstract
Sulfur cycling is ubiquitous in sedimentary environments, where it mediates organic carbon remineralization, impacting both local and global redox budgets, and leaving an imprint in pyrite ...sulfur isotope ratios (δ
34
S
pyr
). It is unclear to what extent stratigraphic δ
34
S
pyr
variations reflect local aspects of the depositional environment or microbial activity versus global sulfur-cycle variations. Here, we couple carbon-nitrogen-sulfur concentrations and stable isotopes to identify clear influences on δ
34
S
pyr
of local environmental changes along the Peru margin. Stratigraphically coherent glacial-interglacial δ
34
S
pyr
fluctuations (>30‰) were mediated by Oxygen Minimum Zone intensification/expansion and local enhancement of organic matter deposition. The higher resulting microbial sulfate reduction rates led to more effective drawdown and
34
S-enrichment of residual porewater sulfate and sulfide produced from it, some of which is preserved in pyrite. We identify organic carbon loading as a major influence on δ
34
S
pyr
, adding to the growing body of evidence highlighting the local controls on these records.
The sulfur biogeochemical cycle plays a key role in regulating Earth’s surface redox through diverse abiotic and biological reactions that have distinctive stable isotopic fractionations. As such, ...variations in the sulfur isotopic composition (δ34S) of sedimentary sulfate and sulfide phases over Earth history can be used to infer substantive changes to the Earth’s surface environment, including the rise of atmospheric oxygen. Such inferences assume that individual δ34S records reflect temporal changes in the global sulfur cycle; this assumption may be well grounded for sulfate-bearing minerals but is less well established for pyrite-based records. Here, we investigate alternative controls on the sedimentary sulfur isotopic composition of marine pyrite by examining a 300-m drill core of Mediterranean sediments deposited over the past 500,000 y and spanning the last five glacial–interglacial periods. Because this interval is far shorter than the residence time of marine sulfate, any change in the sulfur isotopic record preserved in pyrite (δ34Spyr) necessarily corresponds to local environmental changes. The stratigraphic variations (>76‰) in the isotopic data reported here are among the largest ever observed in pyrite, and are in phase with glacial–interglacial sea level and temperature changes. In this case, the dominant control appears to be glacial–interglacial variations in sedimentation rates. These results suggest that there exist important but previously overlooked depositional controls on sedimentary sulfur isotope records, especially associated with intervals of substantial sea level change. This work provides an important perspective on the origin of variability in such records and suggests meaningful paleoenvironmental information can be derived from pyrite δ34S records.
Extracellular electron uptake (EEU) is the ability of microbes to take up electrons from solid-phase conductive substances such as metal oxides. EEU is performed by prevalent phototrophic bacterial ...genera, but the electron transfer pathways and the physiological electron sinks are poorly understood. Here we show that electrons enter the photosynthetic electron transport chain during EEU in the phototrophic bacterium Rhodopseudomonas palustris TIE-1. Cathodic electron flow is also correlated with a highly reducing intracellular redox environment. We show that reducing equivalents are used for carbon dioxide (CO
) fixation, which is the primary electron sink. Deletion of the genes encoding ruBisCO (the CO
-fixing enzyme of the Calvin-Benson-Bassham cycle) leads to a 90% reduction in EEU. This work shows that phototrophs can directly use solid-phase conductive substances for electron transfer, energy transduction, and CO
fixation.
Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate "clumped" isotope ...paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician-Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by approximately 5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.
Stratified Redox Model for the Ediacaran Ocean Li, Chao; Love, Gordon D; Lyons, Timothy W ...
Science (American Association for the Advancement of Science),
04/2010, Letnik:
328, Številka:
5974
Journal Article
Recenzirano
The Ediacaran Period (635 to 542 million years ago) was a time of fundamental environmental and evolutionary change, culminating in the first appearance of macroscopic animals. Here, we present a ...detailed spatial and temporal record of Ediacaran ocean chemistry for the Doushantuo Formation in the Nanhua Basin, South China. We find evidence for a metastable zone of euxinic (anoxic and sulfidic) waters impinging on the continental shelf and sandwiched within ferruginous Fe(II)-enriched deep waters. A stratified ocean with coeval oxic, sulfidic, and ferruginous zones, favored by overall low oceanic sulfate concentrations, was maintained dynamically throughout the Ediacaran Period. Our model reconciles seemingly conflicting geochemical redox conditions proposed previously for Ediacaran deep oceans and helps to explain the patchy temporal record of early metazoan fossils.
Ocean Anoxic Event 2 (OAE2) was a period of dramatic disruption to the global carbon cycle when massive amounts of organic matter (OM) were buried in marine sediments via complex and controversial ...mechanisms. Here we investigate the role of OM sulfurization, which makes OM less available for microbial respiration, in driving variable OM preservation in OAE2 sedimentary strata from Pont d'Issole (France). We find correlations between the concentration, S:C ratio, S-isotope composition, and sulfur speciation of OM suggesting that sulfurization facilitated changes in carbon burial at this site as the chemocline moved in and out of the sediments during deposition. These patterns are reproduced by a simple model, suggesting that small changes in primary productivity could drive large changes in local OM burial in environments poised near a critical redox threshold. This amplifying mechanism may be central to understanding the magnitude of global carbon cycle response to environmental perturbations.
Rationale
Secondary ion mass spectrometry data collected using electron multiplier detectors are subject to a correction for the quasi‐simultaneous arrival (QSA) effect. Published Poisson statistical ...models indicate that the QSA coefficients, β, should have an invariant value of 0.5, whereas, with one exception, published experimental determinations vary between 0.6 and 1.0, with a mean value of 0.75.
Methods
We developed a more complex model, combining both ion emission and attenuation, that predicts the observed range in measured β and elucidates the mechanism of secondary ion formation. For a given aperture setting, any secondary ion has an equal probability of successful transit to the electron multiplier. Binomial statistics can model pass–fail aperture attenuation but require probability distributions of the quasi‐simultaneously emitted (QSE) ion tally, per primary ion, as input. Assuming (a) that each primary ion impact results in 0, 1, 2,… secondary ion emissions, randomly, with an average Ks and (b) that there is finite probability (P2) of a further emission process dependent on Ks, the required QSE probability distributions were generated via a combined Poisson–binomial statistical model.
Results
The value of β was output as a function of Ks and P2. For values of P2 > 0 and any value of Ks, β always exceeds 0.5. As P2 → 0, β → 0.5; for values of increasing P2 > 0.5 and decreasing Ks < 0.5, β → >1.
Conclusions
Were the emission of one ion not to influence the probability of the formation of a second (i.e. model output for P2 = 0), β should always be 0.5. Yet measurements have never reported this value. Consequently, assuming that published β values are correct, emissions of QSE secondary ions do not occur independently, and it may be inferred that there are linked mechanisms of secondary ion formation as shown here.
Shallow-sea hydrothermal systems, like their deep-sea and terrestrial counterparts, can serve as relatively accessible portals into the microbial ecology of subsurface environments. In this study, we ...determined the chemical composition of 47 sediment porewater samples along a transect from a diffuse shallow-sea hydrothermal vent to a non-thermal background area in Paleochori Bay, Milos Island, Greece. These geochemical data were combined with thermodynamic calculations to quantify potential sources of energy that may support in situ chemolithotrophy. The Gibbs energies (DELTAG.sub.r) of 730 redox reactions involving 23 inorganic H-, O-, C-, N-, S-, Fe-, Mn-, and As-bearing compounds were calculated. Of these reactions, 379 were exergonic at one or more sampling locations. The greatest energy yields were from anaerobic CO oxidation with NO.sub.2 .sup.- (-136 to -162 kJ/mol e.sup.- ), followed by reactions in which the electron acceptor/donor pairs were O.sub.2 /CO, NO.sub.3 .sup.- /CO, and NO.sub.2 .sup.- /H.sub.2 S. When expressed as energy densities (where the concentration of the limiting reactant is taken into account), a different set of redox reactions are the most exergonic: in sediments affected by hydrothermal input, sulfide oxidation with a range of electron acceptors or nitrite reduction with different electron donors provide 85~245 J per kg of sediment, whereas in sediments less affected or unaffected by hydrothermal input, various S.sup.0 oxidation reactions and aerobic respiration reactions with several different electron donors are most energy-yielding (80~95 J per kg of sediment). A model that considers seawater mixing with hydrothermal fluids revealed that there is up to ~50 times more energy available for microorganisms that can use S.sup.0 or H.sub.2 S as electron donors and NO.sub.2 .sup.- or O.sub.2 as electron acceptors compared to other reactions. In addition to revealing likely metabolic pathways in the near-surface and subsurface mixing zones, thermodynamic calculations like these can help guide novel microbial cultivation efforts to isolate new species.
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