More Like this

1. Depth‐Partitioning of Particulate Organic Carbon Composition in the Rising and Falling Stages of the Amazon River

   https://doi.org/10.1029/2023GC011273  

   Rosengard, Sarah Z; Moura, Jose_Mauro S; Spencer, Robert_G M; Johnson, Carl; McNichol, Ann; Steen, Andrew D; Galy, Valier (June 2024, Geochemistry, Geophysics, Geosystems)

   Abstract The Amazon River mobilizes organic carbon across one of the world's largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems such as the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles with Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kg per second) between the rising and falling stages of the Amazon River mainstem. The C/N ratio and stable and radiocarbon signatures of bulk POC are less variable within the cross‐section at Óbidos and suggest that riverine POC in the Amazon River is predominantly soil‐derived. However, smaller shifts in these compositional metrics with depth, including leaf waxn‐alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross‐sectional surveys at Óbidos highlight the importance of depth‐specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river. 

   more » « less
2. Arctic Permafrost Thawing Enhances Sulfide Oxidation

   https://doi.org/10.1029/2022GB007644  

   Kemeny, Preston Cosslett; Li, Gen K.; Douglas, Madison; Berelson, William; Chadwick, Austin J.; Dalleska, Nathan F.; Lamb, Michael P.; Larsen, William; Magyar, John S.; Rollins, Nick E.; et al (November 2023, Global Biogeochemical Cycles)

   Abstract 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. 

   more » « less
3. Modeling Spatiotemporal Patterns of Ecosystem Metabolism and Organic Carbon Dynamics Affecting Hypoxia on the Louisiana Continental Shelf

   https://doi.org/10.1029/2019JC015630  

   Jarvis, Brandon M.; Lehrter, John C.; Lowe, Lisa L.; Hagy, James D.; Wan, Yongshan; Murrell, Michael C.; Ko, Dong S.; Penta, Bradley; Gould, Jr., Richard W. (April 2020, Journal of Geophysical Research: Oceans)

   Abstract The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient‐enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf‐wide carbon production contributing to hypoxia formation. This study applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations reproduced spatial and temporal patterns of carbon production, respiration, and bottom‐water oxygen gradients compared to field observations. We used multiyear simulations to quantify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial displacement of carbon production and respiration in our simulations was created by westward and offshore POC flux via phytoplankton carbon flux in the surface layer and POC flux in the bottom layer, supporting heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results support existing studies suggesting the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are most variable. 

   more » « less
4. Impacts of convective storms on runoff, erosion, and carbon export in a continuous permafrost landscape

   Repasch, Marisa; Arcurie, Josie; Overeem, Irina; Anderson, Suzanne P; Anderson, Robert S; Koch, Joshua C (June 2024, Proceedings of the 12th International Conference on Permafrost Whitehorse, Canada)

   Beddoe, Riley; Karunaratne, Kumari (Ed.)

   Permafrost holds more than twice the amount of carbon currently in the atmosphere, but this large carbon reservoir is vulnerable to thaw and erosion under a rapidly changing Arctic climate. Convective storms are becoming increasingly common during Arctic summers and can amplify runoff and erosion. These extreme events, in concert with active layer deepening, may accelerate carbon loss from the Arctic landscape. However, we lack measurements of carbon fluxes during these events. Rivers are sensitive to physical, chemical, and hydrological perturbations, and thus are excellent systems for studying landscape responses to thunderstorms. We present observations from the Canning River, Alaska, which drains the northern Brooks Range and flows across a continuous permafrost landscape to the Beaufort Sea. During summer 2022 and 2023 field campaigns, we opportunistically monitored river discharge, sediment, and organic carbon fluxes during several thunderstorms. During one notable storm, river discharge nearly doubled from ~130 m3/s to ~240 m3/s, suspended sediment flux increased 70-fold, and the particulate organic carbon (POC) flux increased 90-fold relative to non-storm conditions. Taken together, the river exported ~16 metric tons of POC over one hour of this sustained event, not including the additional flux of woody debris. Furthermore, the dissolved organic carbon (DOC) flux nearly doubled. Although these thunderstorm-driven fluxes are short-lived (hours to days), they play an outsized role in exporting organic carbon from Arctic rivers. Understanding how these extreme events impact river water, sediment, and carbon dynamics will help predict how Arctic climate change will modify the global carbon cycle. 

   more » « less
5. Sediment geochemical data from the Bering Shelf and Yukon Delta, Eastern Bering Sea, June 2023

   https://doi.org/10.18739/A2PG1HQ63  

   Repasch, Marisa (June 2023, NSF Arctic Data Center)

   This dataset contains geochemical and grain size measurements of seabed and suspended sediment samples collected from the Bering Shelf and Yukon prodelta in June 2023. These samples were collected during the 6-12 June 2023 Arctic Chief Scientist Training Cruise, which was a short cruise on the R/V Sikuliaq during its transit from Seward, Alaska to Nome, Alaska. The cruise was sponsored by the University National Oceanographic Laboratory System (UNOLS) Arctic Icebreaker Coordinating Committee and the National Science Foundation. These data were generated as a pilot data set to understand the fate of terrestrial organic carbon in the Yukon River delta and adjacent Bering Sea. Seabed sediment was collected at three locations (MC04, MC06, and MC08) using a multicorer. One seabed sediment sample was collected using a van veen grab sampler. Suspended sediment samples were collected using a hand-deployed niskin bottle and transferred into a clean 10-liter cubitainer for transport and temporary storage. Suspended sediment was filtered on the ship within 24 hours of collection using a Geotech barrel filter with a 0.45 micron Polyethersulfone (PES) membrane filter. Multicores were extruded on deck and subsamples were transferred to sterile whirl-pak sample bags. All sediment samples were frozen and subsequently shipped back to the lab in coolers. Sediment samples were analyzed for grain size, bulk elemental composition via X-ray fluorescence (XRF), and organic carbon and nitrogen concentrations and isotopes via EA-IRMS (elemental analyzer-isotope ratio mass spectrometry). Grain size distributions were measured using a Malvern mastersizer laser diffraction particle size analyzer at the University of Colorado Boulder. Bulk elemental composition was measured on the Rigaku XRF in the Analytical Geochemistry Lab at the University of New Mexico. Total organic carbon (TOC), total nitrogen (TN), and carbon/nitrogen isotopes were measured at the Center for Stable Isotopes at the University of New Mexico. Prior to EA-IRMS analyses, sediment samples were acid-treated to remove inorganic carbon following the method of Galy et al., 2007. References Galy, V., Bouchez, J., & France‐Lanord, C. (2007). Determination of total organic carbon content and δ13C in carbonate‐rich detrital sediments. Geostandards and Geoanalytical research, 31(3), 199-207. 

   more » « less