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1. Characterizing the Atmospheric Mn Cycle and Its Impact on Terrestrial Biogeochemistry

   https://doi.org/10.1029/2023GB007967  

   Lu, Louis; Li, Longlei; Rathod, Sagar; Hess, Peter; Martínez, Carmen; Fernandez, Nicole; Goodale, Christine; Thies, Janice; Wong, Michelle Y; Alaimo, Maria Grazia; et al (April 2024, Global Biogeochemical Cycles)

   Abstract The role of manganese (Mn) in ecosystem carbon (C) biogeochemical cycling is gaining increasing attention. While soil Mn is mainly derived from bedrock, atmospheric deposition could be a major source of Mn to surface soils, with implications for soil C cycling. However, quantification of the atmospheric Mn cycle, which comprises emissions from natural (desert dust, sea salts, volcanoes, primary biogenic particles, and wildfires) and anthropogenic sources (e.g., industrialization and land‐use change due to agriculture), transport, and deposition, remains uncertain. Here, we use compiled emission data sets for each identified source to model and quantify the atmospheric Mn cycle by combining an atmospheric model and in situ atmospheric concentration measurements. We estimated global emissions of atmospheric Mn in aerosols (<10 μm in aerodynamic diameter) to be 1,400 Gg Mn year⁻¹. Approximately 31% of the emissions come from anthropogenic sources. Deposition of the anthropogenic Mn shortened Mn “pseudo” turnover times in 1‐m‐thick surface soils (ranging from 1,000 to over 10,000,000 years) by 1–2 orders of magnitude in industrialized regions. Such anthropogenic Mn inputs boosted the Mn‐to‐N ratio of the atmospheric deposition in non‐desert dominated regions (between 5 × 10⁻⁵and 0.02) across industrialized areas, but that was still lower than soil Mn‐to‐N ratio by 1–3 orders of magnitude. Correlation analysis revealed a negative relationship between Mn deposition and topsoil C density across temperate and (sub)tropical forests, consisting with atmospheric Mn deposition enhancing carbon respiration as seen in in situ biogeochemical studies. 

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2. Nutrient management offsets the effect of deoxygenation and warming on nitrous oxide emissions in a large US estuary

   https://doi.org/10.1126/sciadv.adq5014  

   Tang, Weiyi; Da, Fei; Tracey, John C; Intrator, Naomi; Kunes, Moriah A; Lee, Jenna A; Wan, Xianhui Sean; Jayakumar, Amal; Friedrichs, Marjorie_A M; Ward, Bess B (December 2024, Science Advances)

   Many estuaries experience eutrophication, deoxygenation and warming, with potential impacts on greenhouse gas emissions. However, the response of N₂O production to these changes is poorly constrained. Here we applied nitrogen isotope tracer incubations to measure N₂O production under experimentally manipulated changes in oxygen and temperature in the Chesapeake Bay—the largest estuary in the United States. N₂O production more than doubled from nitrification and increased exponentially from denitrification when O₂was decreased from >20 to <5 micromolar. Raising temperature from 15° to 35°C increased N₂O production 2- to 10-fold. Developing a biogeochemical model by incorporating these responses, N₂O emissions from the Chesapeake Bay were estimated to decrease from 157 to 140 Mg N year⁻¹from 1986 to 2016 and further to 124 Mg N year⁻¹in 2050. Although deoxygenation and warming stimulate N₂O production, the modeled decrease in N₂O emissions, attributed to decreased nutrient inputs, indicates the importance of nutrient management in curbing greenhouse gas emissions, potentially mitigating climate change. 

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3. Unexpected anthropogenic emission decreases explain recent atmospheric mercury concentration declines

   https://doi.org/10.1073/pnas.2401950121  

   Feinberg, Aryeh; Selin, Noelle E; Braban, Christine F; Chang, Kai-Lan; Custódio, Danilo; Jaffe, Daniel A; Kyllönen, Katriina; Landis, Matthew S; Leeson, Sarah R; Luke, Winston; et al (October 2024, Proceedings of the National Academy of Sciences)

   Anthropogenic activities emit ~2,000 Mg y⁻¹of the toxic pollutant mercury (Hg) into the atmosphere, leading to long-range transport and deposition to remote ecosystems. Global anthropogenic emission inventories report increases in Northern Hemispheric (NH) Hg emissions during the last three decades, in contradiction with the observed decline in atmospheric Hg concentrations at NH measurement stations. Many factors can obscure the link between anthropogenic emissions and atmospheric Hg concentrations, including trends in the reemissions of previously released anthropogenic (“legacy”) Hg, atmospheric sink variability, and spatial heterogeneity of monitoring data. Here, we assess the observed trends in gaseous elemental mercury (Hg⁰) in the NH and apply biogeochemical box modeling and chemical transport modeling to understand the trend drivers. Using linear mixed effects modeling of observational data from 51 stations, we find negative Hg⁰trends in most NH regions, with an overall trend for 2005 to 2020 of −0.011 ± 0.006 ng m⁻³y⁻¹(±2 SD). In contrast to existing emission inventories, our modeling analysis suggests that annual NH anthropogenic emissions must have declined by at least 140 Mg between the years 2005 and 2020 to be consistent with observed trends. Faster declines in 95th percentile Hg⁰values than median values in Europe, North America, and East Asian measurement stations corroborate that the likely cause is a decline in nearby anthropogenic emissions rather than background legacy reemissions. Our results are relevant for evaluating the effectiveness of the Minamata Convention on Mercury, demonstrating that existing emission inventories are incompatible with the observed Hg⁰declines. 

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4. Seafloor primary production in a changing Arctic Ocean

   https://doi.org/10.1073/pnas.2303366121  

   Attard, Karl; Singh, Rakesh Kumar; Gattuso, Jean-Pierre; Filbee-Dexter, Karen; Krause-Jensen, Dorte; Kühl, Michael; Sejr, Mikael K.; Archambault, Philippe; Babin, Marcel; Bélanger, Simon; et al (March 2024, Proceedings of the National Academy of Sciences)

   Phytoplankton and sea ice algae are traditionally considered to be the main primary producers in the Arctic Ocean. In this Perspective, we explore the importance of benthic primary producers (BPPs) encompassing microalgae, macroalgae, and seagrasses, which represent a poorly quantified source of Arctic marine primary production. Despite scarce observations, models predict that BPPs are widespread, colonizing ~3 million km²of the extensive Arctic coastal and shelf seas. Using a synthesis of published data and a novel model, we estimate that BPPs currently contribute ~77 Tg C y⁻¹of primary production to the Arctic, equivalent to ~20 to 35% of annual phytoplankton production. Macroalgae contribute ~43 Tg C y⁻¹, seagrasses contribute ~23 Tg C y⁻¹, and microalgae-dominated shelf habitats contribute ~11 to 16 Tg C y⁻¹. Since 2003, the Arctic seafloor area exposed to sunlight has increased by ~47,000 km²y⁻¹, expanding the realm of BPPs in a warming Arctic. Increased macrophyte abundance and productivity is expected along Arctic coastlines with continued ocean warming and sea ice loss. However, microalgal benthic primary production has increased in only a few shelf regions despite substantial sea ice loss over the past 20 y, as higher solar irradiance in the ice-free ocean is counterbalanced by reduced water transparency. This suggests complex impacts of climate change on Arctic light availability and marine primary production. Despite significant knowledge gaps on Arctic BPPs, their widespread presence and obvious contribution to coastal and shelf ecosystem production call for further investigation and for their inclusion in Arctic ecosystem models and carbon budgets. 

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5. Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

   https://doi.org/10.1029/2023GB007780  

   DeVries, Tim; Yamamoto, Kana; Wanninkhof, Rik; Gruber, Nicolas; Hauck, Judith; Müller, Jens Daniel; Bopp, Laurent; Carroll, Dustin; Carter, Brendan; Chau, Thi‐Tuyet‐Trang; et al (October 2023, Global Biogeochemical Cycles)

   Abstract This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985–2018, using a combination of models and observation‐based products. The mean sea‐air CO₂flux from 1985 to 2018 is −1.6 ± 0.2 PgC yr⁻¹based on an ensemble of reconstructions of the history of sea surface pCO₂(pCO₂products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO₂, which is estimated at −2.1 ± 0.3 PgC yr⁻¹by an ensemble of ocean biogeochemical models, and −2.4 ± 0.1 PgC yr⁻¹by two ocean circulation inverse models. The ocean also degasses about 0.65 ± 0.3 PgC yr⁻¹of terrestrially derived CO₂, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO₂products reconstruct a trend in the ocean carbon sink of −0.61 ± 0.12 PgC yr⁻¹ decade⁻¹, while biogeochemical models and inverse models diagnose an anthropogenic CO₂‐driven trend of −0.34 ± 0.06 and −0.41 ± 0.03 PgC yr⁻¹ decade⁻¹, respectively. This implies a climate‐forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate‐driven variability exceeding the CO₂‐forced variability by 2–3 times. These results suggest that anthropogenic CO₂dominates the ocean CO₂sink, while climate‐driven variability is potentially large but highly uncertain and not consistently captured across different methods. 

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