Atmospheric Dry and Wet Deposition of Total Phosphorus to the Great Lakes

Quantifying atmospheric loadings of total phosphorus (TP) to freshwater environments is essential to improve understanding of its fate and transport, and to mitigate the effects of excessive levels in freshwater ecosystems. To date, atmospheric deposition of TP in the U.S. is poorly characterized due to the lack of long-term deposition observations. Here, we integrate several historical datasets to develop an estimate of dry and wet deposition to the Great Lakes region. For dry deposition, we use TP concentrations in fine particulate matter (PM2.5) samples from fourteen land-based IMPROVE sites (2013-2020) upwind of the Great Lakes to provide new fine particle phosphorus dry deposition estimates. For wet deposition, we use TP concentrations in wet-only precipitation samples collected at eleven land-based sites (2001-2009) in the Great Lakes region. For both wet and dry deposition, a seasonal cycle is evident with higher concentrations in warmer and wetter months when compared to colder months. Additionally, there is an increasing gradient from north to south in wet deposition, likely driven by both higher precipitation and increased emissions near southern sites. Despite different sampling time periods, these updated observations can provide further constraints on the TP loadings to each of the five Great Lakes. We estimate annual deposition of TP to Lakes Superior, Michigan, Huron, Erie and Ontario at 526, 702, 495, 212, and 185 MTA per year, which is lower than prior estimates for Lakes Superior, Erie and Ontario, comparable for Lake Huron, and about two times greater for Lake Michigan. When considering only the contribution of fine particulate PM to the dry deposition, wet deposition dominated over dry at all lakes except for Lake Huron. However, prior global estimates suggest greater contributions from larger particles (PM10 and PM100), yet observations to validate these estimates over the Great Lakes are not available. Our findings indicate that dry deposition of a range of particle sizes are needed to constrain the total atmospheric deposition of TP over the Great Lakes.

Phases in fine volcanic ash

Volcanic ash emissions impact atmospheric processes, depositional ecosystems, human health, and global climate. These effects are sensitive to the size and composition of the ash; however, datasets describing the constituent phases over size ranges relevant for atmospheric transport and widely distributed impacts are practically nonexistent. Here, we present results of X-ray diffraction measurements on size-separated fractions of 40 ash samples from VEI 2-6 eruptions. We characterize changes in phase fractions with grainsize, tectonic setting, and whole-rock SiO2. For grainsizes < 45 μm, average fractions of crystalline silica and surface salts increased while glass and iron oxides decreased with respect to the bulk sample. Samples from arc and intraplate settings are distinguished by feldspar and clinopyroxene fractions (determined by different crystallization sequences) which, together with glass, comprise 80-100% of most samples. We provide a dataset to approximate glass-free proportions of major crystalline phases; however, glass fractions are highly variable. To tackle this, we describe regressions between glass and major crystal phase fractions that help constrain the major phase proportions in volcanic ash with limited a priori information. Using our dataset, we find that pore-free ash density is well-estimated as a function of the clinopyroxene + Fe-oxide fraction, with median values of 2.67 ± 0.01 and 2.85 ± 0.03 g/cm3 for intraplate and arc samples, respectively. Finally, we discuss effects including atmospheric transport and alteration on modal composition and contextualize our proximal airfall ash samples with volcanic ash cloud properties. Our study helps constrain the atmospheric and environmental budget of the phases in fine volcanic ash and their effect on ash density, integral to refine our understanding of the impact of explosive volcanism on the Earth system from single eruptions to global modeling.

Rooting carbon dioxide removal research in the social sciences

Reports from a variety of bodies have highlighted the role that carbon dioxide removal (CDR) technologies and practices must play in order to try to avoid the worst effects of anthropogenic climate change. Research into the feasibility of these technologies is primarily undertaken by scholars in the natural sciences, yet, as we argue in this commentary, there is great value in collaboration between these scholars and their colleagues in the social sciences. Spurred by this belief, in 2019, a university and a non-profit organization organized and hosted a workshop in Washington, DC, intended to bring natural and physical scientists, technology developers, policy professionals and social scientists together to explore how to better integrate social science knowledge into the field of CDR research. The workshop sought to build interdisciplinary collaborations across CDR topics, draft new social science research questions and integrate and exchange disciplinary-specific terminology. But a snowstorm kept many social scientists who had organized the conference from making the trip in person. The workshop went on without them and organizers did the best they could to include the team remotely, but in the age before daily video calls, remote participation was not as successful as organizers had hoped. And thus, a workshop that was supposed to focus on social science integration moved on, without many of the social scientists who organized the event. The social scientists in the room were supposed to form the dominant voice but with so many stuck in a snow storm, the balance of expertise shifted, as it often does when social scientists collaborate with natural and physical scientists. The outcomes of that workshop, lessons learned and opportunities missed, form the basis of this commentary, and they collectively indicate the barriers to integrating the natural, physical and social sciences on CDR. As the need for rapid, effective and successful CDR has only increased since that time, we argue that CDR researchers from across the spectrum must come together in ways that simultaneously address the technical, social, political, economic and cultural elements of CDR development, commercialization, adoption and diffusion if the academy is to have a material impact on climate change in the increasingly limited window we have to address it.

Sustained climate warming drives declining marine biological productivity

Climate change projections to the year 2100 may miss physical-biogeochemical feedbacks that emerge later from the cumulative effects of climate warming. In a coupled climate simulation to the year 2300, the westerly winds strengthen and shift poleward, surface waters warm, and sea ice disappears, leading to intense nutrient trapping in the Southern Ocean. The trapping drives a global-scale nutrient redistribution, with net transfer to the deep ocean. Ensuing surface nutrient reductions north of 30°S drive steady declines in primary production and carbon export (decreases of 24 and 41%, respectively, by 2300). Potential fishery yields, constrained by lower-trophic-level productivity, decrease by more than 20% globally and by nearly 60% in the North Atlantic. Continued high levels of greenhouse gas emissions could suppress marine biological productivity for a millennium.

The fate of phosphorus fertilizer in Amazon soya bean fields

Fertilizer-intensive soya bean agriculture has recently expanded in southeastern Amazonia, and whereas intensive fertilizer use in the temperate zone has led to widespread eutrophication of freshwater ecosystems, the effects in tropical systems are less well understood. We examined the fate of fertilizer phosphorus (P) by comparing P forms and budgets across a chronosequence of soya bean fields (converted to soya beans between 2003 and 2008) and forests on an 800 km(2) soya bean farm in Mato Grosso, Brazil. Soya bean fields were fertilized with 50 kg P ha(-1) yr(-1) (30 kg P ha(-1) yr(-1) above what is removed in crops). We used modified Hedley fractionation to quantify soil P pools and found increases in less-plant-available inorganic pools and decreases in organic pools in agricultural soils compared with forest. Fertilizer P did not move below 20 cm. Measurements of P sorption capacity suggest that while fertilizer inputs quench close to half of the sorption capacity of fast-reacting pools, most added P is bound in more slowly reacting pools. Our data suggest that this agricultural system currently has a low risk of P losses to waterways and that long time-scales are required to reach critical soil thresholds that would allow continued high yields with reduced fertilizer inputs.

Aerosol indirect effect on biogeochemical cycles and climate

The net effect of anthropogenic aerosols on climate is usually considered the sum of the direct radiative effect of anthropogenic aerosols, plus the indirect effect of these aerosols through aerosol-cloud interactions. However, an additional impact of aerosols on a longer time scale is their indirect effect on climate through biogeochemical feedbacks, largely due to changes in the atmospheric concentration of CO(2). Aerosols can affect land and ocean biogeochemical cycles by physical forcing or by adding nutrients and pollutants to ecosystems. The net biogeochemical effect of aerosols is estimated to be equivalent to a radiative forcing of -0.5 ± 0.4 watts per square meter, which suggests that reaching lower carbon targets will be even costlier than previously estimated.

Covariant glacial-interglacial dust fluxes in the equatorial Pacific and Antarctica

Dust plays a critical role in Earth's climate system and serves as a natural source of iron and other micronutrients to remote regions of the ocean. We have generated records of dust deposition over the past 500,000 years at three sites spanning the breadth of the equatorial Pacific Ocean. Equatorial Pacific dust fluxes are highly correlated with global ice volume and with dust fluxes to Antarctica, which suggests that dust generation in interhemispheric source regions exhibited a common response to climate change over late-Pleistocene glacial cycles. Our results provide quantitative constraints on the variability of aeolian iron supply to the equatorial Pacific Ocean and, more generally, on the potential contribution of dust to past climate change and to related changes in biogeochemical cycles.

Impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system

Fossil fuel combustion and agriculture result in atmospheric deposition of 0.8 Tmol/yr reactive sulfur and 2.7 Tmol/yr nitrogen to the coastal and open ocean near major source regions in North America, Europe, and South and East Asia. Atmospheric inputs of dissociation products of strong acids (HNO(3) and H2SO(4)) and bases (NH(3)) alter surface seawater alkalinity, pH, and inorganic carbon storage. We quantify the biogeochemical impacts by using atmosphere and ocean models. The direct acid/base flux to the ocean is predominately acidic (reducing total alkalinity) in the temperate Northern Hemisphere and alkaline in the tropics because of ammonia inputs. However, because most of the excess ammonia is nitrified to nitrate (NO(3)(-)) in the upper ocean, the effective net atmospheric input is acidic almost everywhere. The decrease in surface alkalinity drives a net air-sea efflux of CO(2), reducing surface dissolved inorganic carbon (DIC); the alkalinity and DIC changes mostly offset each other, and the decline in surface pH is small. Additional impacts arise from nitrogen fertilization, leading to elevated primary production and biological DIC drawdown that reverses in some places the sign of the surface pH and air-sea CO(2) flux perturbations. On a global scale, the alterations in surface water chemistry from anthropogenic nitrogen and sulfur deposition are a few percent of the acidification and DIC increases due to the oceanic uptake of anthropogenic CO(2). However, the impacts are more substantial in coastal waters, where the ecosystem responses to ocean acidification could have the most severe implications for mankind.

Global iron connections between desert dust, ocean biogeochemistry, and climate

The environmental conditions of Earth, including the climate, are determined by physical, chemical, biological, and human interactions that transform and transport materials and energy. This is the "Earth system": a highly complex entity characterized by multiple nonlinear responses and thresholds, with linkages between disparate components. One important part of this system is the iron cycle, in which iron-containing soil dust is transported from land through the atmosphere to the oceans, affecting ocean biogeochemistry and hence having feedback effects on climate and dust production. Here we review the key components of this cycle, identifying critical uncertainties and priorities for future research.