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1. Lateral Carbon Exports From Drained Peatlands: An Understudied Carbon Pathway in the Sacramento‐San Joaquin Delta, California

   https://doi.org/10.1029/2020JG005883  

   Richardson, C. M. ; Fackrell, J. K. ; Kraus, T. E. C. ; Young, M. B. ; Paytan, A. ( December 2020 , Journal of Geophysical Research: Biogeosciences)

   Degradation of peatlands via drainage is increasing globally and destabilizing peat carbon (C) stores. The effects of drainage on the timing and magnitude of lateral C losses from degraded peatlands remains understudied. We measured spatial and temporal variability in lateral C exports from three drained peat islands in the Sacramento‐San Joaquin Delta in California across the 2017 and 2018 water years using measurements of dissolved inorganic C (DIC), dissolved organic C (DOC), and suspended particulate organic C (POC) concentration combined with discharge. These measurements were supplemented with stable isotope data (δ¹³C‐DIC, δ¹³C‐POC, δ¹⁵N‐PON, and δ²H‐H₂O values) to provide insight into hydrological and biogeochemical controls on lateral C exports from drained peatlands. Drainage DOC and DIC concentrations were seasonally variable with the highest values in the winter rainy season, when discharge was also elevated. Seasonal differences in the mobilization of dissolved C appeared to result from changing water sources and water table levels. Peat island drainage C contributions to surrounding waterways were also greatest during the winter. Although temporal variability in C cycling processes and trends were generally similar across islands, baseline drainage DIC, DOC, and POC concentrations were spatially variable, likely a result of sub‐island‐scale differences in soil organic matter content and hydrology. This spatial variability complicates system‐wide assessments of C budgets. Net lateral C exports were water year dependent and comparable to previously published vertical C emission rates for this system. This work highlights the importance of including lateral C exports from drained peatlands in local and regional C budgets.

    

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2. The Role of Glacier Erosion in Riverine Particulate Organic Carbon Export

   https://doi.org/10.1029/2023GB007721  

   Behnke, Megan I. ; Fellman, Jason B. ; Nagorski, Sonia ; Spencer, Robert G. M. ; Hood, Eran ( November 2023 , Global Biogeochemical Cycles)

   Biospheric particulate organic carbon (POC_bio) burial and rock petrogenic particulate organic carbon (POC_petro) oxidation are opposing long‐term controls on the global carbon cycle, sequestering and releasing carbon, respectively. Here, we examine how watershed glacierization impacts the POC source by assessing the concentration and isotopic composition (δ¹³C and Δ¹⁴C) of POC exported from four watersheds with 0%–49% glacier coverage across a melt season in Southeast Alaska. We used two mixing models (age‐weight percent and dual carbon isotope) to calculate concentrations of POC_bioand POC_petrowithin the bulk POC pool. The fraction POC_petrocontribution was highest in the heavily glacierized watershed (age‐weight percent: 0.39 ± 0.05; dual isotope: 0.42 (0.37–0.47)), demonstrating a glacial source of POC_petroto fjords. POC_petrowas mobilized via glacier melt and subglacial flow, while POC_biowas largely flushed from the non‐glacierized landscape by rain. Flow normalized POC_bioconcentrations exceeded POC_petroconcentrations for all streams, but surprisingly were highest in the heavily glacierized watershed (mean: 0.70 mgL⁻¹; range 0.16–1.41 mgL⁻¹), suggesting that glacier rivers can contribute substantial POC_bioto coastal waters. Further, the most heavily glacierized watershed had the highest sediment concentration (207 mgL⁻¹; 7–708 mgL⁻¹), and thus may facilitate long‐term POC_bioprotection via sediment burial in glacier‐dominated fjords. Our results suggest that continuing glacial retreat will decrease POC concentrations and increase POC_bio:POC_petroexported from currently glacierized watersheds. Glacier retreat may thus decrease carbon storage in marine sediments and provide a positive feedback mechanism to climate change that is sensitive to future changes in POC_petrooxidation.

    

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3. Anomalous δ 13 C in Particulate Organic Carbon at the Chemoautotrophy Maximum in the Cariaco Basin

   https://doi.org/10.1029/2019JG005276  

   Scranton, Mary I. ; Taylor, Gordon T. ; Thunell, Robert C. ; Muller‐Karger, Frank E. ; Astor, Yrene ; Swart, Peter ; Edgcomb, Virginia P. ; Pachiadaki, Maria G. ( February 2020 , Journal of Geophysical Research: Biogeosciences)

   A chemoautotrophy maximum is present in many anoxic basins at the sulfidic layer's upper boundary, but the factors controlling this feature are poorly understood. In 13 of 31 cruises to the Cariaco Basin, particulate organic carbon (POC) was enriched in¹³C (δ¹³C_POCas high as −16‰) within the oxic/sulfidic transition compared to photic zone values (−23 to −26‰). During “heavy” cruises, fluxes of O₂and [NO₃⁻+ NO₂⁻] to the oxic/sulfidic interface were significantly lower than during “light” cruises. Cruises with isotopically heavy POC were more common between 2013 and 2015 when suspended particles below the photic zone tended to be nitrogen rich compared to later cruises. Within the chemoautotrophic layer, nitrogen‐rich particles (molar ratio C/N< 10) were more likely to be¹³C‐enriched than nitrogen‐poor particles, implying that these inventories were dominated by living cells and fresh detritus rather than laterally transported or extensively decomposed detritus. During heavy cruises,¹³C enrichments persisted to 1,300 m, providing the first evidence of downward transport of chemoautotrophically produced POC. Dissolved inorganic carbon assimilation during heavy cruises (n= 3) was faster and occurred deeper than during light cruises (n= 2). Metagenomics data from the chemoautotrophic layer during two cruises support prevalence of microorganisms carrying RuBisCO form II genes, which encode a carbon fixation enzyme that discriminates less against heavy isotopes than most other carbon fixation enzymes, and metatranscriptomics data indicate that higher expression of form II RuBisCO genes during the heavy cruises at depths where essential reactants coexist are responsible for the isotopically heavier POC.

    

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4. Size‐Fractionated Compositions of Marine Suspended Particles in the Western Arctic Ocean: Lateral and Vertical Sources

   https://doi.org/10.1029/2020JC016144  

   Xiang, Yang ; Lam, Phoebe J. ( August 2020 , Journal of Geophysical Research: Oceans)

   We present full water depth sections of size‐fractionated (1–51 μm; >51 μm) concentrations of suspended particulate matter and major particle phase composition (particulate organic matter [POM], including its carbon isotopic composition [POC‐δ¹³C] and C:N ratio, calcium carbonate [CaCO₃], opal, lithogenic particles, and iron and manganese [oxyhydr]oxides) from the U.S. GEOTRACES Arctic Cruise (GN01) in the western Arctic in 2015. Whereas biogenic particles (POM and opal) dominate the upper 1,000 m, lithogenic particles are the most abundant particle type at depth. Minor phases such as manganese (Mn) oxides are higher in GN01 than in any other U.S. GEOTRACES cruises so far. Extremely depleted POC‐δ¹³C, as low as ~ −32‰, is ubiquitous at the surface of the western Arctic Ocean as a result of different growth rates of phytoplankton. Moderate penetration of depleted POC‐δ¹³C to depth indicates active sinking of large particles in the central basin. Lateral transport from the Chukchi shelf is also of significance in the western Arctic, as is evident from increases in biogenic silica to POC ratios and Mn oxide concentrations in the halocline, as well as lithogenic element contents in the deep waters. Our study supports previous suggestions of the near absence of CaCO₃in the Arctic Basin. This study presents the first data set of concentration and composition of suspended particles in the western Arctic Ocean and sheds new light on the vertical and lateral processes that govern particle distribution in this enclosed ocean basin.

    

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5. Carbon cycle dynamics and ecology revealed by the carbon isotopic composition of single organic microfossils during the Late Devonian Biotic Crisis

   https://doi.org/10.1111/gbi.12482  

   Cohen, Phoebe A. ; Junium, Christopher K. ; King Phillips, Ezekiel ; Uveges, Benjamin T. ( December 2021 , Geobiology)

   We apply a new approach for the δ¹³C analysis of single organic‐walled microfossils (OWM) to three sites in the Appalachian Basin of New York (AB) that span the Late Devonian Biotic Crisis (LDBC). Our data provide new insights into the nature of the Frasnian–Famennian carbon cycle in the AB and also provide possible constraints on the paleoecology of enigmatic OWM ubiquitous in Paleozoic shale successions. The carbon isotope compositions of OWM are consistent with normal marine organic matter of autochthonous origins and range from −32 to −17‰, but average −25‰ across all samples and are consistently¹³C‐enriched compared to bulk sediments (δ¹³C_bulk) by ~0–10‰. We observe no difference between the δ¹³C_OWMof leiospheres (smooth‐walled) and acanthomorphic (spinose) acritarch OWM, indicating that our data are driven by ecological rather than taxonomic signals. We hypothesize that the offset between δ¹³C_OWMand δ¹³C_bulkis in part due to a large δ¹³C gradient in the AB water column where OWM utilized relatively¹³C‐enriched dissolved inorganic carbon near the surface. Thus, the organisms producing the balance of the total organic carbon were assimilating¹³C‐depleted C sources, including but not limited to respired organic carbon or byproducts of fermentation. We also observe a systematic decrease in both δ¹³C_OWMand δ¹³C_bulkof 3‰ from shoreward to open‐ocean facies that may reflect the effect of¹³C‐enriched dissolved inorganic carbon (DIC) derived from riverine sources in the relatively enclosed AB. The hypothesized steep carbon isotope gradient in the AB could be due to a strong biological pump; this in turn may have contributed to low oxygen bottom water conditions during the LDBC. This is the first time single‐microfossil δ¹³C_organalyses of eukaryotes have been directly compared to bulk δ¹³C_orgin the deep‐time fossil record.

    

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