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1. Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet

   https://doi.org/10.1038/s41561-021-00753-w  

   Hawkings, Jon R. ; Linhoff, Benjamin S. ; Wadham, Jemma L. ; Stibal, Marek ; Lamborg, Carl H. ; Carling, Gregory T. ; Lamarche-Gagnon, Guillaume ; Kohler, Tyler J. ; Ward, Rachael ; Hendry, Katharine R. ; et al ( July 2021 , Nature Geoscience)

   Abstract The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km −2 year −1 ) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km −2 year −1 ). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year −1 ), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year −1 ). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow andmore »ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming.« less
2. Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

   https://doi.org/10.1126/science.abh3767  

   Mason, Rachel E. ; Craine, Joseph M. ; Lany, Nina K. ; Jonard, Mathieu ; Ollinger, Scott V. ; Groffman, Peter M. ; Fulweiler, Robinson W. ; Angerer, Jay ; Read, Quentin D. ; Reich, Peter B. ; et al ( April 2022 , Science)

   BACKGROUND The availability of nitrogen (N) to plants and microbes has a major influence on the structure and function of ecosystems. Because N is an essential component of plant proteins, low N availability constrains the growth of plants and herbivores. To increase N availability, humans apply large amounts of fertilizer to agricultural systems. Losses from these systems, combined with atmospheric deposition of fossil fuel combustion products, introduce copious quantities of reactive N into ecosystems. The negative consequences of these anthropogenic N inputs—such as ecosystem eutrophication and reductions in terrestrial and aquatic biodiversity—are well documented. Yet although N availability is increasing in many locations, reactive N inputs are not evenly distributed globally. Furthermore, experiments and theory also suggest that global change factors such as elevated atmospheric CO 2 , rising temperatures, and altered precipitation and disturbance regimes can reduce the availability of N to plants and microbes in many terrestrial ecosystems. This can occur through increases in biotic demand for N or reductions in its supply to organisms. Reductions in N availability can be observed via several metrics, including lowered nitrogen concentrations ([N]) and isotope ratios (δ 15 N) in plant tissue, reduced rates of N mineralization, and reduced terrestrial Nmore »export to aquatic systems. However, a comprehensive synthesis of N availability metrics, outside of experimental settings and capable of revealing large-scale trends, has not yet been carried out. ADVANCES A growing body of observations confirms that N availability is declining in many nonagricultural ecosystems worldwide. Studies have demonstrated declining wood δ 15 N in forests across the continental US, declining foliar [N] in European forests, declining foliar [N] and δ 15 N in North American grasslands, and declining [N] in pollen from the US and southern Canada. This evidence is consistent with observed global-scale declines in foliar δ 15 N and [N] since 1980. Long-term monitoring of soil-based N availability indicators in unmanipulated systems is rare. However, forest studies in the northeast US have demonstrated decades-long decreases in soil N cycling and N exports to air and water, even in the face of elevated atmospheric N deposition. Collectively, these studies suggest a sustained decline in N availability across a range of terrestrial ecosystems, dating at least as far back as the early 20th century. Elevated atmospheric CO 2 levels are likely a main driver of declines in N availability. Terrestrial plants are now uniformly exposed to ~50% more of this essential resource than they were just 150 years ago, and experimentally exposing plants to elevated CO 2 often reduces foliar [N] as well as plant-available soil N. In addition, globally-rising temperatures may raise soil N supply in some systems but may also increase N losses and lead to lower foliar [N]. Changes in other ecosystem drivers—such as local climate patterns, N deposition rates, and disturbance regimes—individually affect smaller areas but may have important cumulative effects on global N availability. OUTLOOK Given the importance of N to ecosystem functioning, a decline in available N is likely to have far-reaching consequences. Reduced N availability likely constrains the response of plants to elevated CO 2 and the ability of ecosystems to sequester carbon. Because herbivore growth and reproduction scale with protein intake, declining foliar [N] may be contributing to widely reported declines in insect populations and may be negatively affecting the growth of grazing livestock and herbivorous wild mammals. Spatial and temporal patterns in N availability are not yet fully understood, particularly outside of Europe and North America. Developments in remote sensing, accompanied by additional historical reconstructions of N availability from tree rings, herbarium specimens, and sediments, will show how N availability trajectories vary among ecosystems. Such assessment and monitoring efforts need to be complemented by further experimental and theoretical investigations into the causes of declining N availability, its implications for global carbon sequestration, and how its effects propagate through food webs. Responses will need to involve reducing N demand via lowering atmospheric CO 2 concentrations, and/or increasing N supply. Successfully mitigating and adapting to declining N availability will require a broader understanding that this phenomenon is occurring alongside the more widely recognized issue of anthropogenic eutrophication. Intercalibration of isotopic records from leaves, tree rings, and lake sediments suggests that N availability in many terrestrial ecosystems has steadily declined since the beginning of the industrial era. Reductions in N availability may affect many aspects of ecosystem functioning, including carbon sequestration and herbivore nutrition. Shaded areas indicate 80% prediction intervals; marker size is proportional to the number of measurements in each annual mean. Isotope data: (tree ring) K. K. McLauchlan et al. , Sci. Rep. 7 , 7856 (2017); (lake sediment) G. W. Holtgrieve et al. , Science 334 , 1545–1548 (2011); (foliar) J. M. Craine et al. , Nat. Ecol. Evol. 2 , 1735–1744 (2018)« less
3. Revealing biogeochemical signatures of Arctic landscapes with river chemistry

   https://doi.org/10.1038/s41598-019-49296-6  

   Shogren, Arial J. ; Zarnetske, Jay P. ; Abbott, Benjamin W. ; Iannucci, Frances ; Frei, Rebecca J. ; Griffin, Natasha A. ; Bowden, William B. ( September 2019 , Scientific Reports)

   Riverine fluxes of carbon and inorganic nutrients are increasing in virtually all large permafrost-affected rivers, indicating major shifts in Arctic landscapes. However, it is currently difficult to identify what is causing these changes in nutrient processing and flux because most long-term records of Arctic river chemistry are from small, headwater catchments draining <200 km²or from large rivers draining >100,000 km². The interactions of nutrient sources and sinks across these scales are what ultimately control solute flux to the Arctic Ocean. In this context, we performed spatially-distributed sampling of 120 subcatchments nested within three Arctic watersheds spanning alpine, tundra, and glacial-lake landscapes in Alaska. We found that the dominant spatial scales controlling organic carbon and major nutrient concentrations was 3–30 km², indicating a continuum of diffuse and discrete sourcing and processing dynamics. These patterns were consistent seasonally, suggesting that relatively fine-scale landscape patches drive solute generation in this region of the Arctic. These network-scale empirical frameworks could guide and benchmark future Earth system models seeking to represent lateral and longitudinal solute transport in rapidly changing Arctic landscapes.
4. Fluvial Carbon Dynamics across the Land to Ocean Continuum of Great Tropical Rivers: the Amazon and Congo.

   https://doi.org/10.1002/9781119657002.ch20  

   Richey, J.E. ; R.G.M. Spencer ; T. W. Drake ; N.D. Ward. ( January 2022 , Congo Basin Hydrology, Climate, and Biogeochemistry: A Foundation for the Future, Geophysical Monograph 269, First Edition)

   R.M. Tshimanga ; G.D. Moukandi N’kaya ; D. Alsdorf (Ed.)

   Many river systems of the world are super-saturated in dissolved CO2 (pCO2) relative to equilibrium with the atmosphere. Here we compare the coupled organic matter and pCO2 dynamics of the world’s two largest and most organic-rich river systems. The emerging data sets for the Congo River, joint with Amazon River data, enable us to begin to think more generally about the overall functioning of the world’s two largest river basins. Discharge is the primary control on POC and DOC export in both the Amazon and Congo Rivers. TSS yield from the Amazon is twentyfold greater per unit area than the Congo. However, despite low TSS concentrations, the Congo has a POC content approximately five times higher than the Amazon. The organic-rich character of both watersheds is reflected in the DOC export, with the Amazon exporting ~11% and the Congo ~5% of the global land to ocean flux (but care should be taken when describing estimates of TSS and carbon to the ocean since processing and sequestration in tidal and coastal areas can significantly alter TSS and carbon delivery, and last measuring stations are typically hundreds of kilometers from the sea). pCO2 in the Amazon mainstem range from 1,000 to 10,000more »ppm, with floodplain lakes ranging from 20 to 20,000 ppm. Concentrations in the Congo are lower, with high values of 5,000 ppm. The elevated level of pCO2 even as far as the mouth of such major rivers as the Amazon and Congo, up to thousands of kilometers from CO2-rich small streams, poses a most interesting question: What set of processes maintains such high levels? The answer is presumably some combination of instream metabolism of organic matter of terrestrial and floodplain origin, and/or injection of very high pCO2 water from local floodplains or tributaries."« less
5. Enhanced trace element mobilization by Earth’s ice sheets

   https://doi.org/10.1073/pnas.2014378117  

   Hawkings, Jon R. ; Skidmore, Mark L. ; Wadham, Jemma L. ; Priscu, John C. ; Morton, Peter L. ; Hatton, Jade E. ; Gardner, Christopher B. ; Kohler, Tyler J. ; Stibal, Marek ; Bagshaw, Elizabeth A. ; et al ( November 2020 , Proceedings of the National Academy of Sciences)

   Trace elements sustain biological productivity, yet the significance of trace element mobilization and export in subglacial runoff from ice sheets is poorly constrained at present. Here, we present size-fractionated (0.02, 0.22, and 0.45 µm) concentrations of trace elements in subglacial waters from the Greenland Ice Sheet (GrIS) and the Antarctic Ice Sheet (AIS). Concentrations of immobile trace elements (e.g., Al, Fe, Ti) far exceed global riverine and open ocean mean values and highlight the importance of subglacial aluminosilicate mineral weathering and lack of retention of these species in sediments. Concentrations are higher from the AIS than the GrIS, highlighting the geochemical consequences of prolonged water residence times and hydrological isolation that characterize the former. The enrichment of trace elements (e.g., Co, Fe, Mn, and Zn) in subglacial meltwaters compared with seawater and typical riverine systems, together with the likely sensitivity to future ice sheet melting, suggests that their export in glacial runoff is likely to be important for biological productivity. For example, our dissolved Fe concentration (20,900 nM) and associated flux values (1.4 Gmol y⁻¹) from AIS to the Fe-deplete Southern Ocean exceed most previous estimates by an order of magnitude. The ultimate fate of these micronutrients will depend onmore »the reactivity of the dominant colloidal size fraction (likely controlled by nanoparticulate Al and Fe oxyhydroxide minerals) and estuarine processing. We contend that ice sheets create highly geochemically reactive particulates in subglacial environments, which play a key role in trace elemental cycles, with potentially important consequences for global carbon cycling.

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