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1. Old carbon routed from land to the atmosphere by global river systems

   https://doi.org/10.1038/s41586-025-09023-w  

   Dean, Joshua F; Coxon, Gemma; Zheng, Yanchen; Bishop, Jack; Garnett, Mark H; Bastviken, David; Galy, Valier; Spencer, Robert_G M; Tank, Suzanne E; Tipper, Edward T; et al (June 2025, Nature)

   Abstract Rivers and streams are an important pathway in the global carbon cycle, releasing carbon dioxide (CO₂) and methane (CH₄) from their water surfaces to the atmosphere^1,2. Until now, CO₂and CH₄emitted from rivers were thought to be predominantly derived from recent (sub-decadal) biomass production and, thus, part of ecosystem respiration^3–6. Here we combine new and published measurements to create a global database of the radiocarbon content of river dissolved inorganic carbon (DIC), CO₂and CH₄. Isotopic mass balance of our database suggests that 59 ± 17% of global river CO₂emissions are derived from old carbon (millennial or older), the release of which is linked to river catchment lithology and biome. This previously unrecognized release of old, pre-industrial-aged carbon to the atmosphere from long-term soil, sediment and geologic carbon stores through lateral hydrological routing equates to 1.2 ± 0.3 Pg C year⁻¹, similar in magnitude to terrestrial net ecosystem exchange. A consequence of this flux is a greater than expected net loss of carbon from aged organic matter stores on land. This requires a reassessment of the fate of anthropogenic carbon in terrestrial systems and in global carbon cycle budgets and models. 

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2. Net Community Production and Inorganic Carbon Cycling in the Central Irminger Sea

   https://doi.org/10.1029/2024JC021027  

   Yoder, M F; Palevsky, H I; Fogaren, K E (July 2024, Journal of Geophysical Research: Oceans)

   Abstract The subpolar North Atlantic plays an outsized role in the atmosphere‐to‐ocean carbon sink. The central Irminger Sea is home to well‐documented deep winter convection and high phytoplankton production, which drive strong seasonal and interannual variability in regional carbon cycling. We use observational data from moored carbonate chemistry system sensors and annual turn‐around cruise samples at the Ocean Observatories Initiative's Irminger Sea Array to construct a near‐continuous time series of mixed layer total dissolved inorganic carbon (DIC),pCO₂, and total alkalinity from summer 2015 to summer 2022. We use these carbonate chemistry system time series to deconvolve the physical and biological drivers of surface ocean carbon cycling in this region on seasonal, annual, and interannual time scales. We find high annual net community production within the seasonally varying mixed layer, averaging 9.8 ± 1.6 mol m⁻² yr⁻¹with high interannual variability (range of 6.0–13.9 mol m⁻² yr⁻¹). The highest daily net community production rates occur during the late winter and early spring, prior to the observed high chlorophyll concentrations associated with the spring phytoplankton bloom. As a result, the winter and early spring play a much larger role in biological carbon export from the mixed layer than traditionally thought. 

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3. Carbonate Chemistry and the Potential for Acidification in Georgia Coastal Marshes and the South Atlantic Bight, USA

   https://doi.org/10.1007/s12237-023-01261-3  

   Reimer, Janet J.; Medeiros, Patricia M.; Hussain, Najid; Gonski, Stephen F.; Xu, Yuan-Yaun; Huang, Ting-Hsuan; Cai, Wei-Jun (January 2024, Estuaries and Coasts)

   Abstract In coastal regions and marginal bodies of water, the increase in partial pressure of carbon dioxide (pCO₂) in many instances is greater than that of the open ocean due to terrestrial (river, estuarine, and wetland) influences, decreasing buffering capacity and/or increasing water temperatures. Coastal oceans receive freshwater from rivers and groundwater as well as terrestrial-derived organic matter, both of which have a direct influence on coastal carbonate chemistry. The objective of this research is to determine if coastal marshes in Georgia, USA, may be “hot-spots” for acidification due to enhanced inorganic carbon sources and if there is terrestrial influence on offshore acidification in the South Atlantic Bight (SAB). The results of this study show that dissolved inorganic carbon (DIC) and total alkalinity (TA) are elevated in the marshes compared to predictions from conservative mixing of the freshwater and oceanic end-members, with accompanying pH around 7.2 to 7.6 within the marshes and aragonite saturation states (Ω_Ar) <1. In the marshes, there is a strong relationship between the terrestrial/estuarine-derived organic and inorganic carbon and acidification. Comparisons of pH, TA, and DIC to terrestrial organic material markers, however, show that there is little influence of terrestrial-derived organic matter on shelf acidification during this period in 2014. In addition, Ω_Arincreases rapidly offshore, especially in drier months (July). River stream flow during 2014 was anomalously low compared to climatological means; therefore, offshore influences from terrestrial carbon could also be decreased. The SAB shelf may not be strongly influenced by terrestrial inputs to acidification during drier than normal periods; conversely, shelf waters that are well-buffered against acidification may not play a significant role in mitigating acidification within the Georgia marshes. 

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4. Comparative Metabolism and Blue Carbon Sequestration of Two Wetland-Dominated Estuaries

   https://doi.org/10.34133/olar.0091  

   Hopkinson, Charles S; Weston, Nathaniel B; Vallino, Joseph J; Garritt, Robert H; Forbrich, Inke (January 2025, Ocean-Land-Atmosphere Research)

   Coastal tidal wetlands and estuaries play important roles in the global carbon budget by contributing to the net withdrawal of CO₂from the atmosphere. We quantified the linkages between terrestrial and oceanic systems, marsh-to-bay carbon exchange, and the uptake of CO₂from the atmosphere in the wetland-dominated Plum Island Sound (MA, USA) and Duplin River (GA, USA) estuaries. The C budgets revealed that autotrophic marshes [primary production:ecosystem respiration (P:R) ~1.3:1] are tightly coupled to heterotrophic aquatic systems (P:R ~0.6:1). Levels of marsh gross primary production are similar in these systems (865 ± 39 and 768 ± 74 gC m⁻²year⁻¹in Plum Island and the Duplin, respectively) even though they are in different biogeographic provinces. In contrast to inputs from rivers and coastal oceans, tidal marshes are the dominant source of allochthonous matter that supports heterotrophy in aquatic systems. Dissolved inorganic carbon (DIC) exported from marshes to the coastal ocean was a major flux pathway in the Duplin River; however, there was no evidence of DIC export from Plum Island marshes and only minor export to the ocean. Burial was a sink for 53% of marsh net ecosystem production (NEP) on Plum Island, but only 19% of marsh NEP in the Duplin. Burial was the dominant blue carbon sequestration pathway at Plum Island, whereas in the Duplin, DIC and organic carbon export to the ocean were equally important. Regional- and continental-scale C budgets should better reflect wetland-dominated systems to more accurately characterize their contribution to global CO₂sequestration. 

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5. Using DIC‐δ ¹³ C Pair to Constrain Anthropogenic Carbon Increase in the Southeastern Atlantic Ocean Over the Most Recent Decade (2010–2020)

   https://doi.org/10.1029/2024JC021586  

   Gao, Hui; Jin, Meibing; Zhao, Hui; Hussain, Najid; Cai, Wei‐Jun (November 2024, Journal of Geophysical Research: Oceans)

   Abstract The southeastern Atlantic Ocean is a crucial yet understudied region for the ocean absorption of anthropogenic carbon (C_anth). Data from the A12 (2020) and A13.5 (2010) cruises offer an opportunity to examine changes in dissolved inorganic carbon (DIC), its stable isotope (δ¹³C), and C_anthover the past decade within a limited region (1∼3°E, 32∼42°S). For the decade of 2010–2020, C_anthinvasion was observed from the sea surface down to 1,200 m based on both DIC and δ¹³C data. The mean C_anthincrease rate (1.08 ± 0.26 mol m⁻² yr⁻¹) during this period accelerated from 0.87 ± 0.05 mol m⁻² yr⁻¹during the previous period (1983/84–2010). The δ¹³C‐based C_anthincrease closely matches the DIC‐based estimation below 500 m but is 26% higher in the upper ocean. This discrepancy is likely due to δ¹³C's longer air‐sea exchange timescale, seasonal variability in the upper ocean, and the chosen ratio of anthropogenically induced changes in δ¹³C and DIC. Finally, column inventory changes based on the two methods also exhibit very similar mean C_anthuptake rates. The paired DIC concentration and stable isotope dataset may enhance our ability to constrain C_anthaccumulation and its controlling mechanisms in the ocean. 

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