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.
Early Silurian (∼431 Ma) carbonate rocks record a ca. 4.5‰ positive excursion in the stable isotopic composition of carbonate carbon (δ13Ccarb). Associated with this isotopic shift is a ...macroevolutionary turnover pulse known as the ‘Ireviken Event’. The onset of this carbon isotope excursion is commonly associated with a shallowing-upward facies transition that may have been accompanied by climatic change, as indicated by a parallel positive shift (∼0.6‰) in the stable isotopic composition of carbonate oxygen (δ18Ocarb). However, the relationships among carbon cycle perturbations, faunal turnover, and environmental changes remain enigmatic. Here we present a suite of new isotopic data across the Ireviken Event from multiple sections in Gotland, Sweden. These samples preserve no systematic change in δ18Ocarb but show positive excursions of equal magnitude in both carbonate (δ13Ccarb) and organic (δ13Corg) carbon. In addition, the data reveal a synchronous perturbation in sulfur isotope ratios, manifest as a ca. 7‰ positive excursion in carbonate-associated sulfate (δ34SCAS) and a ca. 30‰ positive excursion in pyrite (δ34Spyr). The increase in δ34Spyr values is accompanied by a substantial, concomitant increase in stratigraphic variability of δ34Spyr.
The relatively constant offset between the δ13Ccarb and δ13Corg excursions throughout the Ireviken Event could be attributed to increased organic carbon burial, or possibly a change in the isotopic composition of CO2 sources from weathering. However, a positive correlation between carbonate abundance and δ13Ccarb suggests that local to regional changes in dissolved inorganic carbon (DIC) during the shallowing-upward sequence may have been at least partly responsible for the observed excursion. The positive excursion recorded in δ34SCAS suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir. An inverse correlation between CAS abundance and δ34SCAS supports the notion of decreased sulfate concentrations, at least locally, consistent with a concomitant increase in pyrite burial. A decrease in the offset between δ34SCAS and δ34Spyr values during the Ireviken Event suggests a substantial reduction in the isotopic fractionations (εpyr) expressed during microbial sulfur cycling and pyrite precipitation through this interval. Decreased εpyr and the concomitant increase in stratigraphic variation in δ34Spyr are typical of isotope systematics observed in modern shallow-water environments, associated with increased closed-system behavior and/or oxidative sedimentary reworking during early sediment diagenesis. While the isotopic trends associated with the Ireviken Event have been observed in multiple locations around the globe, many sections display different magnitudes of isotopic change, and moreover, are typically associated with local facies changes. Due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the Ireviken Event, and their relationship to changes in local depositional environment, we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes.
Global carbon cycle perturbations throughout Earth history are frequently linked to changing paleogeography, glaciation, ocean oxygenation, and biological innovation. A pronounced carbonate ...carbon-isotope excursion during the Ediacaran Period (635 to 542 million years ago), accompanied by invariant or decoupled organic carbon-isotope values, has been explained with a model that relies on a large oceanic reservoir of organic carbon. We present carbonate and organic matter carbon-isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the Sturtian and Marinoan glacial events of the Cryogenian Period (approximately 720 to 635 million years ago). Growth of the organic carbon pool may be related to iron-rich and sulfate-poor deep-ocean conditions facilitated by an increase in the Fe:S ratio of the riverine flux after Sturtian glacial removal of a long-lived continental regolith.
Marine carbonate rocks are composed, in varying abundance, of cement, micrite, abiotic grains and fossils, which can provide information about the physical and chemical environments in which they ...formed. Geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic (including petrographic) information available, resulting in potentially faulty reconstructions of biogeochemical and environmental conditions. These concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies. Here, we have combined X-ray Absorption Near Edge Structure (XANES) spectroscopy and μ-X-ray Fluorescence (μ-XRF) imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in Upper Ordovician carbonates from Anticosti Island, Canada and early Silurian carbonates from Gotland, Sweden, across multiple depositional facies. Lithofacies and fossil communities between Anticosti Island and Gotland are similar, which allows for comparison of changes in the dominant S species and their abundance in separate basins, associated with variations in (glacio)eustatic sea level. Sulfide abundance is greatest in mudstone, wackestone and packstone facies, where interstitial micrite hosts abundant pyrite. Sulfate abundance, as carbonate-associated sulfate (CAS), varies within individual fossil fragments, as well as within the same fossil phylum and is particularly high in unaltered brachiopods. In contrast, sulfate abundance is generally very low in micrite (near the detection limit) and generally arises in situ from sulfide that has been oxidized as opposed to true CAS. In different cement fabrics, sulfate abundance is greatest in drusy, pore-filling cements. Organic sulfur compounds are also detected and, although low in abundance, are mostly found within micrite. The detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization. This approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates, as well as the effect of depositional setting and diagenesis on CAS incorporation, (re)mobilization, and ultimate abundance in sedimentary carbonates. Additionally, this work has implications for the CAS isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous CAS sections throughout Earth history.
Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating ...suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO
was 10
PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO
rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.
•Post-Marinoan marine cements in Central Flinders Ranges record oxic conditions.•Regional transition from red to green shales appears to show Fe2+/Fe3+ redoxcline.•Earliest Ediacaran shales and ...carbonates show evidence of an oxygenation event.
The first appearance of animals during the Ediacaran is arguably related to an increase in oceanic oxygenation during this time. However, there is considerable ambiguity in the global record of Ediacaran oxygenation, making it difficult to assess the potential links between oxygen and metazoan evolution. Here, we examine the earliest Ediacaran Nuccaleena Formation cap dolomite and basal Brachina Formation of the Adelaide Superbasin, South Australia, to determine the redox landscape in which these units were deposited. Red shales are present at the base of the Brachina Formation (lower Moolooloo Siltstone Member) over much of the Adelaide Superbasin but these transition laterally into green shales in the north, correlating with a facies transition into a deeper water setting.
Fibrous dolomite cements within sheet cavities of the Nuccaleena Formation cap dolomite display evidence of a primary marine origin. Cathodoluminescence microscopy and laser ablation ICP-MS trace element analysis of these marine dolomite cements indicates a transition from an oxic environment in the south (with low Fe, Mn, and a Ce anomaly) to an anoxic (ferruginous) setting in the north (with high Fe, Mn and no Ce anomaly). This cap carbonate trace element geochemistry is spatially consistent with the overlying red to green shale transition in the basal Brachina Formation. Together, these data suggest the existence of a deep-water chemocline in this basin, separating an oxic upper water column from a ferruginous deeper water mass. This oxic interval directly post-dates the end-Cryogenian Marinoan Glaciation and is synchronous with an earliest Ediacaran oxygenation event previously described from South China.
This evidence from the Adelaide Superbasin provides direct evidence for an earliest Ediacaran oxic water mass penetrating to a substantial paleodepth. The synchronous development of oxic intervals in both Australia and South China supports the notion of a globally developed oceanic oxygenation event and is consistent with the hypothesis that Ediacaran continental margin settings were periodically bathed in oxic water, conducive to the evolution of metazoans.
Globally, coronavirus disease 2019 (COVID-19) has affected how children learn. We evaluated the impact of Test to Stay (TTS) on secondary and tertiary transmission of severe acute respiratory ...syndrome coronavirus 2 and potential impact on in-person learning in 4 school districts in the United States from September 13 to November 19, 2021.
Implementation of TTS varied across school districts. Data on index cases, school-based close contacts, TTS participation, and testing results were obtained from 4 school districts in diverse geographic regions. Descriptive statistics, secondary and tertiary attack risk, and a theoretical estimate of impact on in-person learning were calculated.
Fifty-one schools in 4 school districts reported 374 coronavirus disease COVID-19 index cases and 2520 school-based close contacts eligible for TTS. The proportion participating in TTS ranged from 22% to 79%. By district, the secondary attack risk and tertiary attack risk among TTS participants ranged between 2.2% to 11.1% and 0% to 17.6%, respectively. Nine clusters were identified among secondary cases and 2 among tertiary cases. The theoretical maximum number of days of in-person learning saved by using TTS was 976 to 4650 days across jurisdictions.
TTS preserves in-person learning. Decisions to participate in TTS may have been influenced by ease of access to testing, communication between schools and families, testing logistics, and school resources. Tertiary attack risk determination became more complicated when numbers of close contacts increased. Minimizing exposure through continued layered prevention strategies is imperative. To ensure adequate resources for TTS, community transmission levels should be considered.
Sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the younger Cryogenian ice age. The presence of ice-rafted debris in the ...basal dolostone implies that at least the lower portion of the cap sequence records deglaciation. An isochronous model proposes that cap dolostones were deposited synchronously around the world regardless of water depth, whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation. The Nuccaleena Formation of the Adelaide Rift Complex (ARC), South Australia, exhibits many of the unique sedimentary features observed in other younger Cryogenian cap dolostones around the world. Some bedforms are the product of wind-driven oscillatory flow, thus constraining post-glacial dolostone deposition to <
400
m. These sedimentary features are absent in the deepest basinal facies in the northern ARC, suggesting that this region was below storm wave base even at the glacial sea level lowstand. In the north Flinders Ranges, there is a distinct relationship between lateral facies variability in the pre-, syn- and post-glacial sediments and the axes of 50
km scale structural folds. The northern fold limbs are characterised by basinal facies, whilst the southern limbs are associated with upper-slope facies. We interpret the abrupt facies transitions to reflect lower slope to upper slope/shelf breaks across a series of linked, south-facing half-grabens. The majority of cap dolostone carbon isotope records show monotonic declines in
δ
13C of <
2‰, with starting points between −
0.5 and −
3.5‰. An isochronous model implies a ∼
3.0‰ lateral gradient from platform to lower slope that varies dramatically on a short spatial scale and non-systematically with palaeobathymetry. If pre- and syn-glacial facies are used to infer palaeobathymetry, and cap dolostones are deposited diachronously as sea level rises during deglaciation, then
δ
13C values becomes progressively lighter with time, implying that cap dolostone deposition tracked the glacioeustatic sea-level rise over a series of half-grabens that deepened to the north. The carbon isotope dataset cannot rule out uniquely isochronous or diachronous models. Given the high frequency spatial variability of
δ
13C values, temperature cannot be the dominant control on the isotopic variability of the cap dolostone.
Background
Person-centered care has been shown to increase desired outcomes for people with dementia, yet informal caregivers’ dissatisfaction with care is often reported. For those living in a ...nursing home, informal caregivers are uniquely situated to provide key insights into the individual’s care. However, little is known of the informal caregivers’ perspective, which hinders efforts to improve their satisfaction with person-centered nursing home care. Thus, we examined the comprehensive experiences, priorities, and perceptions of informal caregivers of nursing home residents with dementia.
Methods
In collaboration with stakeholders, a scoping review of Medline (Ovid), EMBASE.com, CINAHL (EBSCO), the Cochrane Library (Wiley), and PsycINFO (Ovid) databases from January 2000 to July 2020 was conducted. Data were extracted reflecting the experiences, priorities, and preferences of caregivers of people with dementia residing in nursing homes.
Results
We identified 114 articles that revealed nine themes: (1) communication, (2) transition to nursing home, (3) quality of care, (4) quality of life, (5) informal caregiver role, (6) knowledge of dementia, (7) end-of-life preferences, (8) medication use to manage neuropsychiatric behaviors, and (9) finances.
Conclusion
Informal caregivers described aspects of care that led to both positive and negative experiences with and perceptions of nursing home care. The shortcomings in communication were discussed most frequently, indicating a high priority area. While researchers define the identified themes individually, informal caregivers perceive them to be interwoven as they relate to person-centered care delivery. Although we did not assess the quality of included articles, by identifying themes relevant to caregivers’ perspectives of nursing home care, our findings may help to inform efforts to optimize caregivers’ satisfaction with nursing home care for residents with dementia.
Uncertainties in the number and age of glacial deposits within the Port Nolloth Group have hindered both structural and stratigraphic studies in the Neoproterozoic Gariep Belt of Namibia and South ...Africa. These uncertainties are compounded by major lateral facies changes that complicate correlations locally. Herein, we report the results of integrated geological mapping, chemo- and litho-stratigraphic, and sedimentological studies that shed light on the age and stratigraphic architecture of the Port Nolloth Group. Particularly, we have distinguished an additional glacial deposit, herein referred to as the Namaskluft diamictite, which is succeeded by a ca. 635 Ma basal Ediacaran cap carbonate. This interpretation indicates that the stratigraphically lower, iron-bearing Numees diamictite is not Marinoan or Gaskiers in age, as previously suggested, but is instead a ca. 716.5 Ma Sturtian glacial deposit. A Sturtian age for the Numees Formation is further supported by the discovery of microbial roll-up structures in the dark limestone of the Bloeddrif Member that caps the diamictite. A re-evaluation of the age constraints indicates that all Neoproterozoic iron formations may be of Sturtian age, and thus indicative of secular evolution of the redox state of the ocean. PUBLICATION ABSTRACT
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