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1. Chemical Weathering and Physical Erosion Fluxes From Serpentinite in Puerto Rico

   https://doi.org/10.1029/2024JF007776  

   Moore, Angus K; Méndez_Méndez, Kimberly; Hughes, K Stephen; Granger, Darryl E (January 2025, Journal of Geophysical Research: Earth Surface)

   Abstract Weathering of ultramafic rocks emplaced at low latitude during arc‐arc and arc‐continent collisions may provide an important sink for atmospheric CO₂over geologic timescales. Accurately modeling the effects of ultramafic rock weathering on Earth's carbon cycle and climate requires understanding mass fluxes from ultramafic landscapes. In this study, physical erosion and chemical weathering fluxes and weathering intensity are quantified in 15 watersheds across the Monte del Estado, a serpentinite massif in Puerto Rico, using measurements of in situ³⁶Cl in magnetite, stream solute fluxes, and sediment geochemistry. Despite high relief in the study watersheds, erosion fluxes are moderate (22–109 tons km⁻² yr⁻¹), chemical weathering fluxes are large (55–143 tons km⁻² yr⁻¹), and weathering intensities are among the highest yet reported for silicate‐rock weathering (up to 0.88). We use these data to parameterize power‐law relationships between weathering, erosion, and runoff. We interpret the relative importance of climate versus erosion in setting weathering fluxes and CO₂consumption from the best‐fit power‐law slopes. Weathering fluxes from tropical, montane serpentinite landscapes are found to be strongly controlled by runoff and weakly controlled by the supply of fresh rock to the weathering zone through physical erosion. The strong runoff dependence of weathering fluxes implies that, to the extent that precipitation rates are coupled to global temperature, ultramafic landscapes may be important participants in the negative silicate weathering feedback, increasing (decreasing) CO₂consumption in response to a warming (cooling) climate. Thus, serpentinite landscapes may help stabilize Earth's climate state through time. 

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2. Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

   https://doi.org/10.1029/2023JG007638  

   Treat, Claire C; Virkkala, Anna‐Maria; Burke, Eleanor; Bruhwiler, Lori; Chatterjee, Abhishek; Fisher, Joshua B; Hashemi, Josh; Parmentier, Frans‐Jan W; Rogers, Brendan M; Westermann, Sebastian; et al (March 2024, Journal of Geophysical Research: Biogeosciences)

   Abstract Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan‐Arctic permafrost maps, an increase in terrestrial measurement sites for CO₂and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process‐based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO₂sink with lower net CO₂uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO₂sink was located in western Canada (median: −52 g C m⁻² y⁻¹) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m⁻² y⁻¹). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH₄m⁻² y⁻¹). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year‐round CO₂and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non‐growing season emissions and disturbance effects. 

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3. Mercury stocks in discontinuous permafrost and their mobilization by river migration in the Yukon River Basin

   https://doi.org/10.1088/1748-9326/ad536e  

   Smith, Magdalene I; Ke, Yutian; Geyman, Emily C; Reahl, Jocelyn N; Douglas, Madison M; Seelen, Emily A; Magyar, John S; Dunne, Kieran BJ; Mutter, Edda; Fischer, Woodward W; et al (June 2024, Environmental Research Letters)

   Rapid warming in the Arctic threatens to destabilize mercury (Hg) deposits contained within soils in permafrost regions. Yet current estimates of the amount of Hg in permafrost vary by ~4 times. Moreover, how Hg will be released to the environment as permafrost thaws remains poorly known, despite threats to water quality, human health, and the environment. Here we present new measurements of total mercury (THg) contents in discontinuous permafrost in the Yukon River Basin in Alaska. We collected riverbank and floodplain sediments from exposed banks and bars near the villages of Huslia and Beaver. Median THg contents were 49+13/-21 ng THg g sediment−1 and 39 +16 /-18 ng THg g sediment−1 for Huslia and Beaver, respectively (uncertainties as 15th and 85th percentiles). Corresponding THg:organic carbon ratios were 5.4+2 /-2.4 Gg THg Pg C-1 and 4.2+2.4 /-2.9 Gg THg Pg C-1. To constrain floodplain THg stocks, we combined measured THg contents with floodplain stratigraphy. Trends of THg increasing with smaller sediment size and calculated stocks in the upper 1 m and 3 m are similar to those suggested for this region by prior pan-Arctic studies. We combined THg stocks and river migration rates derived from remote sensing to estimate particulate THg erosional and depositional fluxes as river channels migrate across the floodplain. Results show similar fluxes within uncertainty into the river from erosion at both sites (95 +12 /-47 kg THg yr-1 and 26 +154/-13 kg THg yr-1 at Huslia and Beaver, respectively), but very different fluxes out of the river via deposition in aggrading bars (60 +40/-29 kg THg yr-1 and 10+5.3/-1.7 kg THg yr-1). Thus, a significant amount of THg is liberated from permafrost during bank erosion, while a variable but generally lesser portion is subsequently redeposited by migrating rivers. 

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4. Volcanic Arc Weathering Rates in the Humid Tropics Controlled by the Interplay Between Physical Erosion and Precipitation

   https://doi.org/10.1029/2023AV001066  

   Moore, Angus K.; Méndez Méndez, Kimberly; Hughes, K. Stephen; Granger, Darryl E. (April 2024, AGU Advances)

   Abstract Volcanic arcs are chemical weathering hotspots that may contribute disproportionately to global CO₂consumption through silicate weathering. Accurately modeling the impact of volcanic‐arc landscapes on the Earth's long‐term carbon cycle requires understanding how climate and physical erosion control weathering fluxes from arc landscapes. We evaluate these controls by examining the covariation of stream solutes, sediment geochemistry, and long‐term physical erosion fluxes inferred from cosmogenic³⁶Cl in magnetite in volcanic watersheds in Puerto Rico that span a ca. 15‐fold gradient in specific discharge. Analysis of this data using power‐law relationships demonstrates that CO₂consumption from arc‐rock weathering in the humid tropics is more strongly limited by physical erosion and the supply of primary minerals to the weathering zone than by temperature or the flux of fresh, chemically reactive waters through the critical zone. However, a positive correlation between long‐term physical erosion fluxes and specific discharge is also observed. This indicates that fresh mineral supply in arc environments may ultimately depend on precipitation rates, which may maintain a coupling between arc‐rock weathering fluxes and climate under principally supply limited weathering conditions. 

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5. Post-fire stabilization of thaw-affected permafrost terrain in northern Alaska

   https://doi.org/10.1038/s41598-024-58998-5  

   Jones, Benjamin M.; Kanevskiy, Mikhail Z.; Shur, Yuri; Gaglioti, Benjamin V.; Jorgenson, M. Torre; Ward Jones, Melissa K.; Veremeeva, Alexandra; Miller, Eric A.; Jandt, Randi (December 2024, Scientific Reports)

   Abstract In 2007, the Anaktuvuk River fire burned more than 1000 km²of arctic tundra in northern Alaska, ~ 50% of which occurred in an area with ice-rich syngenetic permafrost (Yedoma). By 2014, widespread degradation of ice wedges was apparent in the Yedoma region. In a 50 km²area, thaw subsidence was detected across 15% of the land area in repeat airborne LiDAR data acquired in 2009 and 2014. Updating observations with a 2021 airborne LiDAR dataset show that additional thaw subsidence was detected in < 1% of the study area, indicating stabilization of the thaw-affected permafrost terrain. Ground temperature measurements between 2010 and 2015 indicated that the number of near-surface soil thawing-degree-days at the burn site were 3 × greater than at an unburned control site, but by 2022 the number was reduced to 1.3 × greater. Mean annual ground temperature of the near-surface permafrost increased by 0.33 °C/yr in the burn site up to 7-years post-fire, but then cooled by 0.15 °C/yr in the subsequent eight years, while temperatures at the control site remained relatively stable. Permafrost cores collected from ice-wedge troughs (n = 41) and polygon centers (n = 8) revealed the presence of a thaw unconformity, that in most cases was overlain by a recovered permafrost layer that averaged 14.2 cm and 18.3 cm, respectively. Taken together, our observations highlight that the initial degradation of ice-rich permafrost following the Anaktuvuk River tundra fire has been followed by a period of thaw cessation, permafrost aggradation, and terrain stabilization. 

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