The mass-independent minor oxygen isotope compositions (Δ′17O) of atmospheric O₂ and CO₂ are primarily regulated by their relative partial pressures, pO₂/pCO₂. Pyrite oxidation during chemical ...weathering on land consumes O₂ and generates sulfate that is carried to the ocean by rivers. The Δ′17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct O₂ incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation—including oxygen sources—in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ′17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′17O values imply no direct tropospheric O₂ incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17O-enriched sources such as reactive oxygen species. Sulfate Δ′17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ′17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate 017O as a proxy for past pO₂/pCO₂ should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.
At Mistaken Point, Newfoundland, Canada, rangeomorph “fronds” dominate the earliest (579–565 million years ago) fossil communities of large (0.1 to 2 m height) multicellular benthic eukaryotes. They ...lived in low-flow environments, fueled by uptake 1–3 of dissolved reactants (osmotrophy). However, prokaryotes are effective osmotrophs, and the advantage of taller eukaryotic osmotrophs in this deep-water community context has not been addressed. We reconstructed flow-velocity profiles and vertical mixing using canopy flow models appropriate to the densities of the observed communities. Further modeling of processes at organismal surfaces documents increasing uptake with height in the community as a function of thinning of the diffusive boundary layer with increased velocity. The velocity profile, produced by canopy flow in the community, generates this advantage of upward growth. Alternative models of upward growth advantage based on redox/resource gradients fail, given the efficiency of vertical mixing. In benthic communities of osmotrophs of sufficient density, access to flow in low-flow settings provides an advantage to taller architecture, providing a selectional driver for communities of tall eukaryotes in contexts where phototropism cannot contribute to upward growth. These Ediacaran deep-sea fossils were preserved during the increasing oxygenation prior to the Cambrian radiation of animals and likely represent an important phase in the ecological and evolutionary transition to more complex eukaryotic forms.
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•Canopy flow modeling permits reconstruction of paleocommunity nutrition•Ediacaran osmotrophic communities accompany the transition to deep-sea oxygenation•Access to flow velocity gave rangeomorphs a selective advantage over prokaryotes•Resource competition in flow likely drove community tiering and multicellular evolution
Using canopy flow-based reconstructions of velocity, mixing, and uptake in the earliest fossil communities, Ghisalberti et al. show that Ediacaran rangeomorphs grew upward to access flow, explaining the advantage of large multicellular form at the dawn of the animal radiation.
We present 28 multiple sulfur isotope measurements of seawater sulfate (δ34SSO4 and Δ33SSO4) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific ...Ocean. The average measured δ34SSO4 is 21.24‰ (±0.88‰,2σ) with a calculated Δ33SSO4 of +0.050‰ (±0.014‰,2σ). With these values, we use a box-model to place constraints on the gross fraction of pyrite burial in modern sediments. This model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of δ34Spyrite, which is poorly constrained, but instead uses new information about the relationship between δ34S and δ33S in global marine sulfate. Our calculations indicate that the pyrite burial flux from the modern ocean is between 10% and 45% of the total sulfur lost from the oceans, with a more probable range between 20% and 35%.
•High precision measurements of multiple sulfur isotopes from modern marine sulfate.•Exploration of the riverine multiple sulfur isotope flux into the ocean.•Model of the composition and flux of pyrite burial in modern marine sediments.•Constrain the gross proportional pyrite burial flux to between 10% and 45%.
Detailed iron, sulfur and carbon chemistry through the >
742
million year old Chuar Group reveals a marine basin dominated by anoxic and ferrous iron-rich (ferruginous) bottom waters punctuated, late ...in the basin's development, by an intrusion of sulfide-rich (euxinic) conditions. The observation that anoxia occurred frequently in even the shallowest of Chuar environments (10s of meters or less) suggests that global atmospheric oxygen levels were significantly lower than today. In contrast, the transition from ferruginous to euxinic subsurface water is interpreted to reflect basinal control—specifically, increased export of organic carbon from surface waters. Low fluxes of organic carbon into subsurface water masses should have been insufficient to deplete oxygen via aerobic respiration, resulting in an oxic oxygen minimum zone (OMZ). Where iron was available, larger organic carbon fluxes should have depleted oxygen and facilitated anaerobic respiration using ferric iron as the oxidant, with iron carbonate as the expected mineralogical signature in basinal shale. Even higher organic fluxes would, in turn, have depleted ferric iron and up-regulated anaerobic respiration by sulfate reduction, reflected in high pyrite abundances. Observations from the Chuar Group are consistent with these hypotheses, and gain further support from pyrite and sulfate sulfur isotope abundances. In general, Chuar data support the hypothesis that ferruginous subsurface waters returned to the oceans, replacing euxinia, well before the Ediacaran emergence of persistently oxygenated conditions, and even predating the Sturtian glaciation. Moreover, our data suggest that the reprise of ferruginous water masses may relate to widespread rifting during the break-up of Rodinia. This environmental transition, in turn, correlates with both microfossil and biomarker evidence for an expanding eukaryotic presence in the oceans, suggesting a physiologically mediated link among tectonics, environmental chemistry and life in the dynamic Neoproterozoic Earth system.
Metabolic models for fractionations produced by sulfate-reducing Bacteria and Archaea derived from experimental observations are the cornerstone of our interpretation of ancient and modern ...biogeochemical cycles. Although recent studies have called into question a traditionally accepted model, experimental evidence has been lacking for such a claim. We present data from all four sulfur isotopes that suggest that the internal fractionations associated with the sulfate reduction network are larger than previous estimates. Models of a traditional sulfate reduction network, as well as a more recent incarnation of the sulfate reduction network (with multiple sulfur intermediates) are constructed to aid in the understanding of new experimental data. These data also allow for the further development of additional minor isotope relationships, one of which is easily measurable in geologic settings and accurately depicts the net effect of an environment, whereas the other is more applicable to modern environments and may better illuminate the specific process(es) controlling the fractionation in those environments. This approach illustrates the uses of systems containing more than two isotopes.
The present review explores the concept of learning within the context of neurorehabilitation after spinal cord injury (SCI). The aim of physical therapy and neurorehabilitation is to bring about a ...lasting change in function—to encourage learning. Traditionally, it was assumed that the adult spinal cord is hardwired—immutable and incapable of learning. Research has shown that neurons within the lower (lumbosacral) spinal cord can support learning after communication with the brain has been disrupted by means of a thoracic transection. Noxious stimulation can sensitize nociceptive circuits within the spinal cord, engaging signal pathways analogous to those implicated in brain-dependent learning and memory. After a spinal contusion injury, pain input can fuel hemorrhage, increase the area of tissue loss (secondary injury), and undermine long-term recovery. Neurons within the spinal cord are sensitive to environmental relations. This learning has a metaplastic effect that counters neural over-excitation and promotes adaptive learning through an up-regulation of brain-derived neurotrophic factor (BDNF). Exposure to rhythmic stimulation, treadmill training, and cycling also enhances the expression of BDNF and counters the development of nociceptive sensitization. SCI appears to enable plastic potential within the spinal cord by down-regulating the Cl− co-transporter KCC2, which reduces GABAergic inhibition. This enables learning, but also fuels over-excitation and nociceptive sensitization. Pairing epidural stimulation with activation of motor pathways also promotes recovery after SCI. Stimulating motoneurons in response to activity within the motor cortex, or a targeted muscle, has a similar effect. It is suggested that a neurofunctionalist approach can foster the discovery of processes that impact spinal function and how they may be harnessed to foster recovery after SCI.
•Spinal cord injury (SCI) enables plasticity by reducing GABA-dependent inhibition•Pain input after SCI induces sensitization, fosters hemorrhage and impairs recovery•Controllable/predictable stimulation increases BDNF and promotes adaptive plasticity•Neurorehabilitative strategies that involve relational learning have a lasting effect•Procedures that introduce periodic (regular) stimulation promote adaptive plasticity
Those studying neural systems within the brain have historically assumed that lower-level processes in the spinal cord act in a mechanical manner, to relay afferent signals and execute motor ...commands. From this view, abstracting temporal and environmental relations is the province of the brain. Here we review work conducted over the last 50 years that challenges this perspective, demonstrating that mechanisms within the spinal cord can organize coordinated behavior (stepping), induce a lasting change in how pain (nociceptive) signals are processed, abstract stimulus-stimulus (Pavlovian) and response-outcome (instrumental) relations, and infer whether stimuli occur in a random or regular manner. The mechanisms that underlie these processes depend upon signal pathways (e.g., NMDA receptor mediated plasticity) analogous to those implicated in brain-dependent learning and memory. New data show that spinal cord injury (SCI) can enable plasticity within the spinal cord by reducing the inhibitory effect of GABA. It is suggested that the signals relayed to the brain may contain information about environmental relations and that spinal cord systems can coordinate action in response to descending signals from the brain. We further suggest that the study of stimulus processing, learning, memory, and cognitive-like processing in the spinal cord can inform our views of brain function, providing an attractive model system. Most importantly, the work has revealed new avenues of treatment for those that have suffered a SCI.
The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions ...involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric CO2 greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate- or iron-reducing bacteria. In both cases, this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO2 concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased C:N of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth.
The triple oxygen isotope composition (Δ'
O) of sulfate minerals is widely used to constrain ancient atmospheric
O
/
CO
and rates of gross primary production. The utility of this tool is based on a ...model that sulfate oxygen carries an isotope fingerprint of tropospheric O
incorporated through oxidative weathering of reduced sulfur minerals, particularly pyrite. Work to date has targeted Proterozoic environments (2.5 billion to 0.542 billion years ago) where large isotope anomalies persist; younger timescale records, which would ground ancient environmental interpretation in what we know from modern Earth, are lacking. Here we present a high-resolution record of the Formula: see textO and Δ'
O in marine sulfate for the last 130 million years of Earth history. This record carries a Δ'
O close to 0o, suggesting that the marine sulfate reservoir is under strict control by biogeochemical cycling (namely, microbial sulfate reduction), as these reactions follow mass-dependent fractionation. We identify no discernible contribution from atmospheric oxygen on this timescale. We interpret a steady fractional contribution of microbial sulfur cycling (terrestrial and marine) over the last 100 million years, even as global weathering rates are thought to vary considerably.
An active microbial assemblage cycles sulfur in a sulfate-rich, ancient marine brine beneath Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet, with Fe(III) serving as the terminal ...electron acceptor. Isotopic measurements of sulfate, water, carbonate, and ferrous iron and functional gene analyses of adenosine 5′-phosphosulfate reductase imply that a microbial consortium facilitates a catalytic sulfur cycle. These metabolic pathways result from a limited organic carbon supply because of the absence of contemporary photosynthesis, yielding a subglacial ferrous brine that is anoxic but not sulfidic. Coupled biogeochemical processes below the glacier enable subglacial microbes to grow in extended isolation, demonstrating how analogous organic-starved systems, such as Neoproterozoic oceans, accumulated Fe(II) despite the presence of an active sulfur cycle.
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