The secret ingredient of carbonate mud Kah, Linda C
Proceedings of the National Academy of Sciences - PNAS,
11/2022, Letnik:
119, Številka:
45
Journal Article
Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. The redox sensitivity of cerium ...relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years of Earth’s history, focusing in particular on the mid-Proterozoic Eon (i.e., 1.8 – 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/ Ce* data to estimate the partial pressure of atmospheric oxygen (pO2) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~ 2.4 Ga), followed by invariant pO2 of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).
Molar-tooth structure (MTS) is an unusual carbonate fabric that is composed of variously shaped cracks and voids filled with calcite microspar. Despite a century of study, MTS remains enigmatic ...because it juxtaposes void formation within a cohesive yet unlithified substrate with the penecontemporaneous precipitation and lithification of void-filling carbonate microspar. MTS is broadly restricted to shallow marine carbonate strata of the Mesoproterozoic and Neoproterozoic, suggesting a fundamental link between the formation of MTS and the biogeochemical evolution of marine environments. Despite uncertainties in the origin of MTS, molar-tooth (MT) microspar remains a popular target for geochemical analysis and the reconstruction of Precambrian marine chemistry. Here we review models for the formation of MTS and show how detailed petrographic analysis of MT microspar permits identification of a complex process of precipitation and diagenesis that must be considered when the microspar phase is used as a geochemical proxy.
Molar-tooth fabric is an enigmatic structure in Precambrian sedimentary rocks that is composed of variously shaped cracks and voids filled with carbonate microspar.
Time restriction of this fabric suggests a link between this unusual structure and the biogeochemical evolution of marine environments.
Petrographic analysis of molar-tooth fabric provides insight into fundamental processes of crystallization.
Photic zone euxinia (PZE) is a condition where anoxic, H₂S-rich waters occur in the photic zone (PZ). PZE has been invoked as an impediment to the evolution of complex life on early Earth and as a ...kill mechanism for Phanerozoic mass extinctions. Here, we investigate the potential application of mercury (Hg) stable isotopes in marine sedimentary rocks as a proxy for PZE by measuring Hg isotope compositions in late Mesoproterozoic (∼1.1 Ga) shales that have independent evidence of PZE during discrete intervals. Strikingly, a significantly negative shift of Hg mass-independent isotope fractionation (MIF) was observed during euxinic intervals, suggesting changes in Hg sources or transformations in oceans coincident with the development of PZE. We propose that the negative shift of Hg MIF was most likely caused by (i) photoreduction of Hg(II) complexed by reduced sulfur ligands in a sulfide-rich PZ, and (ii) enhanced sequestration of atmospheric Hg(0) to the sediments by thiols and sulfide that were enriched in the surface ocean as a result of PZE. This study thus demonstrates that Hg isotope compositions in ancient marine sedimentary rocks can be a promising proxy for PZE and therefore may provide valuable insights into changes in ocean chemistry and its impact on the evolution of life.
The Proterozoic Eon hosted the emergence and initial recorded diversification of eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than today's, although ...how much lower is debated. Here, we use concentrations of iodate (the oxidized iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters. To assess the validity of our approach, Neogene–Quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo- and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene–Quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow oxic–anoxic interface. Oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time intervals, are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans. This redox instability may have challenged early eukaryotic diversification and expansion, creating an evolutionary landscape unfavorable for the emergence of animals.
•Proterozoic surface oceans had enough oxygen to sustain simple eukaryotes.•A shallow marine oxycline persisted through the Proterozoic Eon.•The first evaluation of the behavior of iodine during carbonate diagenesis.•Shuram anomaly records seawater iodine and is not a diagenetic artifact.•Proterozoic dolomite can record seawater iodate evolution.
Despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal Neoproterozoic Era, Late Cambrian records point to the persistence of deep-water anoxia and ...potential for development of euxinic conditions. The Late Cambrian SPICE (Steptoean Positive Carbon-Isotope Excursion) event, however, is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere. Here, we present high-resolution carbon and sulfur isotope profiles from Early to Middle Ordovician carbonate rocks from the Argentine Precordillera and Western Newfoundland to constrain oceanic redox conditions in the post-SPICE world. Marine C-isotope profiles record relatively stable behavior (excursions <
3‰) that is characteristic of greenhouse climates. Marine S-isotope profiles record short-term (<
10
6
yr), rhythmic variation superimposed over a longer term (~
10
7
yr) signal. Substantial isotopic heterogeneity between average S-isotope values of different sections (15–25‰) suggests the Ordovician marine sulfate reservoir was not well mixed, indicating a low marine sulfate concentration (likely <
2
mM or less than 10% modern). Short-term variation (7‰ excursions over 1
Myr) is consistent with a small sulfate reservoir size and is best explained by the rhythmic oxidation of a deep-water reactive HS
− reservoir. Greenhouse intervals, such as that represented by the Ordovician ocean, are often associated with deep-water anoxia, and the presence of a persistent, deep water HS
− reservoir that is fed through bacterial sulfate reduction (BSR) is not unexpected. A broadly sympathetic relationship between carbon and sulfur isotope systems over long time scales (~
10
7
yr) suggests that the extent of deep-ocean euxinia was moderated by changes in organic productivity, which fueled BSR and production of reduced sulfide species. By contrast, short-term (<
10
6
yr) sulfur isotope variation appears to be decoupled from the marine carbon-isotope signal. We suggest that this apparent decoupling reflects a combination of elevated pCO
2 during greenhouse times—which acts to dampen C-isotope response—and relatively small-scale fluctuations in organic productivity that affected the position of the marine oxycline and the balance of HS
− production and reoxidation.
► We use carbon and sulfur isotope profiles to model Ordovician marine C–S cycling. ► We examine marine redox conditions after the Late Cambrian C-isotope excursion—SPICE. ► Rapidity of S-isotope change requires smaller than modern marine sulfate reservoir. ► S-isotope variation is best explained by oxidation of reactive hydrogen sulfide. ► Oceanic euxinia persisted into the Ordovician despite the earlier global SPICE event.
Behavior of marine sulfur in the Ordovician Kah, Linda C.; Thompson, Cara K.; Henderson, Miles A. ...
Palaeogeography, palaeoclimatology, palaeoecology,
09/2016, Letnik:
458
Journal Article
Recenzirano
Odprti dostop
Patterns of change in the isotope composition of sulfur-bearing minerals play a key role in reconstructing the marine sulfur cycle. Determining marine sulfate concentration, in particular, is ...critical to understanding the linkages between oxygenation of the Earth's atmosphere and oceans, and the history of life. Much of our current understanding of marine oxygenation relies on a traditional single reservoir model, wherein the isotopic composition of marine sulfate is controlled primarily by burial of sedimentary pyrite. Utility of this model is limited, however, during times of persistent marine euxinia, which marks a fundamental decoupling between oxic and anoxic marine sulfur reservoirs. At these times, short-term fluxes that act between the two reservoirs (e.g., sulfate reduction and sulfide oxidation processes) often dominate over the long term fluxes (e.g., weathering and pyrite burial) that control the single reservoir ocean model.
Ordovician strata from Argentina, western Newfoundland, and South China illustrate how the marine sulfur cycle is affected by the presence of a persistent euxinic reservoir. Regional euxinic reservoirs remain generally stable from the Floian through the Dapingian, despite evidence for periodic short-term oxygenation events. Dramatic reorganization of the marine sulfur cycle in the early Darriwilian, however, reflects dynamic disequilibrium between oxic and anoxic (euxinic) marine reservoirs, driven by ocean ventilation. Ventilation resulted in a rapid change in the isotopic composition of marine sulfate in surface oceans, and ultimately led to the near depletion of hydrogen sulfide within the anoxic reservoir, as marked by formation of superheavy pyrite. Ventilation occurred coincident with a decline in sea surface temperatures and may represent the onset of climatic change that ultimately led to late Ordovician glaciation.
•Ordovician strata illustrate how marine sulfur is affected by persistent euxinia.•Rorganization of sulfur cycle in Darriwilian reflects marine ventilation.•Ventilation results first superheavy pyrite recorded in the Paleozoic.•Ocean ventilation may represent the onset of Late Ordovician climatic change.
Otoliths are frequently used to infer environmental conditions or fish life history events based on trace-element concentrations. However, otoliths can be comprised of any one or combination of the ...three most common polymorphs of calcium carbonate-aragonite, calcite, and vaterite-which can affect the ecological interpretation of otolith trace-element results. Previous studies have reported heterogeneous calcium carbonate compositions between left and right otoliths but did not provide quantitative assessments of polymorph abundances. In this study, neutron diffraction and Raman spectroscopy were used to identify and quantify mineralogical compositions of Chinook salmon Oncorhynchus tshawytscha otolith pairs. We found mineralogical compositions frequently differed between otoliths in a pair and accurate calcium carbonate polymorph identification was rarely possible by visual inspection alone. The prevalence of multiple polymorphs in otoliths is not well-understood, and future research should focus on identifying otolith compositions and investigate how variations in mineralogy affect trace-element incorporation and potentially bias environmental interpretations.
► Mesoproterozoic C-isotopes represent distinct feature in Earth history. ► Muted isotopic variability require additional use of stratigraphic/geochronologic information. ► Geochemical data indicate ...distinct water masses, both laterally and vertically. ► Combined dataset indicates no correlation of Atar-Vazante groups, despite Re–Os dates.
Sedimentary strata of the Atar/El Mreiti groups, Taoudeni Basin, Mauritania, are characterized by carbon isotope values that fall largely between −2.5‰ and +4.0‰, and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from ∼1.2Ga to 1.1Ga. Chemostratigraphic results are consistent with recent Re–Os dates of ∼1.1Ga for Atar/El Mreiti group strata (Rooney et al., 2010) and support an emerging dataset that identifies the Late Mesoproterozoic as an isotopically distinctive interval in Earth history. The relatively low-amplitude carbon isotope variability limits the use of chemostratigraphy in detailed correlation during this time interval. In this study, we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of depositional facies to construct a rigorous and testable model for the intrabasinal correlation of Atar and el Mreiti group strata. Data support a depositional model that results in broadly uniform stratigraphic thicknesses across the West African craton, with predominantly stromatolitic facies of the Atar Group (Mauritania) and the Hank and Dar Cheikh groups (Algeria) representing deposition on shallow-water craton margins, and clayey-carbonate and shale facies of the El Mreiti Group (Mauritania) representing deposition on a broad, shallow-water, epicratonic platform. Within this stratigraphic framework, regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely associated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean. Similarly, distinct differences in trace element composition recorded in multilayered marine cements within deeper-water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox-gradients in the water column.
We use this multifaceted approach to test a recent hypothesis that suggests correlation of Atar/el Mreiti group strata with glaciogenic strata of the Vazante Group, São Francisco craton. Critically, preservation of shallow-water carbonate and evaporite strata across more than 1200km of the predominantly epicratonic Taoudeni Basin—with minimal evidence for high-amplitude sea level change—is consistent with inferences of late Mesoproterozoic highstand conditions in a global greenhouse climate. Such characteristics are inconsistent with suggestions of a time-equivalency with glaciogenic strata of the Vazante Group, São Francisco craton, despite broad similarities in Re–Os geochronology and carbon isotope composition. Together, these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution.
•Concretions exhibit morphologic and chemical diversity throughout the Murray fm.•Concretions likely formed in multiple fluid episodes with varying chemistry.•Many concretions likely formed during ...late diagenesis after sediment compaction.
Concretions are prevalent features in the generally lacustrine deposits of the Murray formation in Gale crater. In this work, we document the morphologic, textural, and chemical properties of these concretions throughout 300 m of Murray formation stratigraphy from Mars Science Laboratory observations between Sols 750–1900. We interpret these observations to constrain the timing and composition of post-depositional fluid events at Gale crater. We determine that the overall diversity of concretion morphology, size, texture, and chemistry throughout the Murray formation indicates that concretions formed in multiple, likely late diagenetic, episodes with varying fluid chemistries. Four major concretion assemblages are observed at distinct stratigraphic intervals and approximately correlate with major distinct chemical enrichments in Mg-S-Ni-Cl, Mn-P, and Ca-S, among other local enrichments. Different concretion size populations and complex relationships between concretions and veins also suggest multiple precipitation events at Gale crater. Many concretions likely formed during late diagenesis after sediment compaction and lithification, based on observations of concretions preserving primary host rock laminations without differential compaction. An upsection decrease in overall concretion size corresponds to an inferred upsection decrease in porosity and permeability, thus constraining concretion formation as postdating fluid events that produced initial cementation and porosity loss. The combined observations of late diagenetic concretions and distinct chemical enrichments related to concretions allow constraints to be placed on the chemistry of late stage fluids at Gale crater. Collectively, concretion observations from this work and previous studies of other diagenetic features (veins, alteration halos) suggest at least six post-depositional events that occurred at Gale crater after the deposition of the Murray formation.
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