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Fly ash—the residuum of coal burning—contains a considerable amount of fossilized particulate organic carbon (FOC ash ) that remains after high-temperature combustion. Fly ash leaks into natural environments and participates in the contemporary carbon cycle, but its reactivity and flux remained poorly understood. We characterized FOC ash in the Chang Jiang (Yangtze River) basin, China, and quantified the riverine FOC ash fluxes. Using Raman spectral analysis, ramped pyrolysis oxidation, and chemical oxidation, we found that FOC ash is highly recalcitrant and unreactive, whereas shale-derived FOC (FOC rock ) was much more labile and easily oxidized. By combining mass balance calculations and other estimates of fly ash input to rivers, we estimated that the flux of FOC ash carried by the Chang Jiang was 0.21 to 0.42 Mt C⋅y −1 in 2007 to 2008—an amount equivalent to 37 to 72% of the total riverine FOC export. We attributed such high flux to the combination of increasing coal combustion that enhances FOC ash production and the massive construction of dams in the basin that reduces the flux of FOC rock eroded from upstream mountainous areas. Using global ash data, a first-order estimate suggests that FOC ash makes up to 16% of the present-day global riverine FOC flux to the oceans. This reflects a substantial impact of anthropogenic activities on the fluxes and burial of fossil organic carbon that has been made less reactive than the rocks from which it was derived. 

Permafrost degradation is altering biogeochemical processes throughout the Arctic. Thaw‐induced changes in organic matter transformations and mineral weathering reactions are impacting fluxes of inorganic carbon (IC) and alkalinity (ALK) in Arctic rivers. However, the net impact of these changing fluxes on the concentration of carbon dioxide in the atmosphere (pCO₂) is relatively unconstrained. Resolving this uncertainty is important as thaw‐driven changes in the fluxes of IC and ALK could produce feedbacks in the global carbon cycle. Enhanced production of sulfuric acid through sulfide oxidation is particularly poorly quantified despite its potential to remove ALK from the ocean‐atmosphere system and increasepCO₂, producing a positive feedback leading to more warming and permafrost degradation. In this work, we quantified weathering in the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the village of Huslia in summer 2018. Using measurements of major ion abundances and sulfate () sulfur (³⁴S/³²S) and oxygen (¹⁸O/¹⁶O) isotope ratios, we employed the MEANDIR inversion model to quantify the relative importance of a suite of weathering processes and their net impact onpCO₂. Calculations found that approximately 80% of in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover,³⁴S/³²S ratios,¹³C/¹²C ratios of dissolved IC, and sulfur X‐ray absorption spectra of mainstem, secondary channel, and floodplain pore fluid and sediment samples revealed modest degrees of microbial sulfate reduction within the floodplain. Weathering fluxes of ALK and IC result in lower values ofpCO₂over timescales shorter than carbonate compensation (∼10⁴ yr) and, for mainstem samples, higher values ofpCO₂over timescales longer than carbonate compensation but shorter than the residence time of marine (∼10⁷ yr). Furthermore, the absolute concentrations of and Mg²⁺in the Koyukuk River, as well as the ratios of and Mg²⁺to other dissolved weathering products, have increased over the past 50 years. Through analogy to similar trends in the Yukon River, we interpret these changes as reflecting enhanced sulfide oxidation due to ongoing exposure of previously frozen sediment and changes in the contributions of shallow and deep flow paths to the active channel. Overall, these findings confirm that sulfide oxidation is a substantial outcome of permafrost degradation and that the sulfur cycle responds to permafrost thaw with a timescale‐dependent feedback on warming.

 

The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic‐to‐biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ³⁰Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ³⁰Si and δ¹⁸O of sponge spicules and radiolarians from Ordovician–Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150 μM during early Paleozoic time—a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed.

 

The redox state of Earth's surface is controlled on geological timescales by the flow of electrons through the sedimentary rock cycle, mediated largely by the weathering and burial of C‐S‐Fe phases. These processes buffer atmosphericpO₂. At the same time, CO₂influxes and carbonate burial control seawater acid‐base chemistry and climate over long timescales via the carbonate‐silicate cycle. However, these two systems are mechanistically linked and impact each other via charge balance in the hydrosphere. Here, we use a low‐order Earth system model to interrogate a subset of these connections, with a focus on changes that occur during perturbations to electron flow through the sedimentary rock cycle. We show that the net oxidation or reduction of the Earth's surface can play an important role in controlling acid‐base processes in the oceans and thus climate, and suggest that these links should be more fully integrated into interpretive frameworks aimed at understanding Earth system evolution throughout Precambrian and Phanerozoic time.

 

Ooids are a common type of carbonate sand grain that form through a combination of constructive and destructive mechanisms: growth via precipitation and diminution via physical abrasion. Because growth and abrasion obey distinct morphometric rules, we developed an approach to quantitatively constrain the history of growth and abrasion of individual ooid grains using the record of evolving particle shape preserved by their cortical layers. We designed a model to simulate >10⁶possible growth‐abrasion histories for each pair of cortical layer bounding surfaces in an individual ooid. Estimates for the durations of growth and abrasion of each cortical layer were obtained by identifying the simulated history that best fit the observed particle shape. We applied this approach to thin sections of “modern” lacustrine ooids collected from several locations in the Great Salt Lake (GSL), UT, to assess the spatial and temporal variability of environmental conditions from the perspective of individual grains within a single deposit. We found that GSL ooids do not all share the same histories: Clustering ooid histories by a Fréchet distance metric revealed commonalities between grains found together locally within a deposit but distinct differences between subpopulations shared among localities across the GSL. These results support the tacit view that carbonate sedimentary grains found together in the environment do reflect a common history of sediment transport. This general approach to invert ooid cortical stratigraphy can be applied to characterize environmental variability over <1,000 year timescales in both marine and lacustrine ooid grainstones of any geologic age.

 

An irreversible increase in alluvial mudrock occurred with the Ordovician-Silurian evolution of bryophytes, challenging a paradigm that deep-rooted plants were responsible for this landscape shift. We tested the idea that increased primary production and plant organics promoted aggregation of clay into flocs in rivers and facilitated mud deposition on floodplains. In experiments, we observed that clay readily flocculated for organic and clay concentrations common to modern rivers, yielding settling velocities three orders of magnitude larger than those without organics. Using a transport model, we found that flocculation substantially increased mud deposition, resulting in muddier floodplains. Thus, organic-induced flocculation may have been more critical than deep-rooted plants in the proliferation of muddy floodplains.