Atmospheric deposition of aerosols transported from the continents is an important source of nutrient and pollutant trace elements (TEs) to the surface ocean. During the U.S. GEOTRACES GP15 Pacific ...Meridional Transect between Alaska and Tahiti (September–November 2018), aerosol samples were collected over the North Pacific and equatorial Pacific and analyzed for a suite of TEs, including Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb. Sampling coincided with the annual minimum in dust transport from Asia, providing an opportunity to quantify aerosol TE concentrations and deposition during the low dust season. Nevertheless, peak concentrations of “crustal” TEs measured at ∼40–50°N (∼145 pmol/m3 Fe) were associated with transport from northern Asia, with lower concentrations (36 ± 14 pmol/m3 Fe) over the equatorial Pacific. Relative to crustal abundances, equatorial Pacific aerosols typically had higher TE enrichment factors than North Pacific aerosols. In contrast, aerosol V was more enriched over the North Pacific, presumably due to greater supply to this region from oil combustion products. Bulk deposition velocity (Vbulk) was calculated along the transect using the surface ocean decay inventory of the naturally occurring radionuclide, 7Be, and aerosol 7Be activity. Deposition velocities were significantly higher (4,570 ± 1,146 m/d) within the Intertropical Convergence Zone than elsewhere (1,764 ± 261 m/d) due to aerosol scavenging by intense rainfall. Daily deposition fluxes to the central Pacific during the low dust season were calculated using Vbulk and aerosol TE concentration data, with Fe fluxes ranging from 19 to 258 nmol/m2/d.
Plain Language Summary
Both natural material such as soil dust and industrial emissions such as soot can be transported thousands of miles in the atmosphere as small particles before gradually settling out of the atmosphere or being stripped out by rain. This process can be an important mechanism for delivering material from the continents to surface waters of the open ocean and also introduces elements that are essential for algal growth, but present in the ocean in very low concentrations (known as trace elements). In this study, we measured the concentrations of several TEs on airborne particles during fieldwork in the Pacific Ocean, between Alaska and Tahiti. The observed amounts of TEs were low because the timing of the work coincided with the annual minimum in atmospheric transport of dust from Asia. The deposition rate of TEs on these particles to the ocean was calculated from the activity of a naturally occurring radionuclide, 7Be, that was also measured in atmospheric samples and in the surface ocean.
Key Points
Aerosol trace element (TE) concentrations are reported along a meridional Pacific Ocean transect during the North Pacific "low dust" season
Mineral aerosol loading and varied anthropogenic aerosol sources influenced observed distributions of the different TEs
Regional dust and Fe deposition fluxes derived from beryllium‐7 were at the low end of previous estimates, due to low dust concentrations
This study provides unique insights into the properties of iron (Fe) in the marine atmosphere over the late summertime Arctic Ocean. Atmospheric deposition of aerosols can deliver Fe, a limiting ...micronutrient, to the remote ocean. Aerosol particle size influences aerosol Fe fractional solubility and air-to-sea deposition rate. Size-segregated aerosols were collected during the 2015 US GEOTRACES cruise in the Arctic Ocean. Results show that aerosol Fe had a single-mode size distribution, peaking at 4.4 µm in diameter, suggesting regional dust sources of Fe around the Arctic Ocean. Estimated dry deposition rates of aerosol Fe decreased from 6.1 µmol m
yr
in the areas of ~56°N-80°N to 0.73 µmol m
yr
in the areas north of 80°N. Aerosol Fe solubility was higher in fine particles (<1 µm) which were observed mainly in the region north of 80°N and coincided with relatively high concentrations of certain organic aerosols, suggesting interactions between aerosol Fe and organic ligands in the high-latitude Arctic atmosphere. The average molar ratio of Fe to titanium (Ti) was 2.4, substantially lower than the typical crustal ratio of 10. We speculate that dust sources around the Arctic Ocean may have been altered because of climate warming.
Atmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry ...deposition) in remote regions. In this work, we discuss the upper ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges‐Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large‐scale gradients in mineral dust deposition and multiple‐source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66 ± 60 mg m−2 yr−1) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m−2 yr−1 at 18°S) and in the southern Bay of Bengal (2500 ± 570 mg m−2 yr−1). Our total deposition fluxes, which have an uncertainty on the order of a factor of 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.
Key Points
High‐resolution dissolved Fe and Al from Antarctica to the Bay of Bengal
Mixed layer dissolved Al reflects meridional dust deposition gradients
Atmospheric model and Al‐based estimates of total deposition are compared
The sea surface microlayer (SML) is the boundary interface between the atmosphere and ocean, covering about 70% of the Earth’s surface. With an operationally defined thickness between 1 and 1000 µm, ...the SML has physicochemical and biological properties that are measurably distinct from underlying waters. Recent studies now indicate that the SML covers the ocean to a significant extent, and evidence shows that it is an aggregate-enriched biofilm environment with distinct microbial communities. Because of its unique position at the air-sea interface, the SML is central to a range of global biogeochemical and climate-related processes. The redeveloped SML paradigm pushes the SML into a new and wider context that is relevant to many ocean and climate sciences.
Aerosol deposition is an important pathway for delivering trace elements, including those of anthropogenic origin, into the Arctic. Assessment of this process is difficult in the harsh Arctic ...environment, and limited field studies have forced a reliance on poorly constrained models. Here we use the cosmic ray produced radioisotope, 7Be, to trace the atmospheric deposition of elements within the Arctic water/ice/snow system, and link aerosol concentrations to flux. Seawater, ice, snow, melt pond, and aerosol samples were collected during late summer 2011 as part of the RV Polarstern's ARK-XXVI/3 campaign. From the measured 7Be inventories we determined an average 7Be flux of 109dpm/m2/d, which is consistent with results from previous studies in the region. Snow, ice and melt ponds represent significant reservoirs of 7Be, and the relative 7Be inventory in ice increased through late August, as melt pond inventories decreased with onset of freezing. The total (water/ice/snow system) inventory was relatively constant across our transect, but mixed layer inventories increased towards lower latitudes as ice-free, open water was approached. The latter gradient drives transport of 7Be, and presumably other atmospherically-derived species, towards the ice-covered ocean mixed layer. This is modeled by advective transport along the Transpolar Drift. The average 7Be aerosol concentration was 0.0182dpm/m3. None of the lithogenic aerosol elements showed any significant enrichment above crustal composition, while the pollution-derived elements (Cr, Ni, Cu, Zn, Cd, Sb, Pb) showed varying degrees of enrichment relative to crustal values. Historical aerosol 7Be data was used to derive a seasonal cycle in the 7Be inventory that was calibrated to the inventory measured in this study, using an effective bulk (wet plus dry) deposition velocity of 1350m/day. This deposition velocity was then used to estimate the seasonal atmospheric flux of aerosol trace elements.
•7Be was used to trace atmospheric deposition of elements within the Arctic Ocean.•An average 7Be flux of 109dpm/m2/d was determined.•This flux was partitioned between the ocean, ice, snow and melt ponds.•An effective bulk deposition velocity of 1350m/day was derived from the 7Be inventory.•This deposition velocity was used to estimate the atmospheric flux of trace elements.
It is well recognized that the atmospheric deposition of iron (Fe) affects ocean productivity, atmospheric CO2 uptake, ecosystem diversity, and overall climate. Despite significant advances in ...measurement techniques and modeling efforts, discrepancies persist between observations and models that hinder accurate predictions of processes and their global effects. Here, we provide an assessment report on where the current state of knowledge is and where future research emphasis would have the highest impact in furthering the field of Fe atmosphere-ocean biogeochemical cycle. These results were determined through consensus reached by diverse researchers from the oceanographic and atmospheric science communities with backgrounds in laboratory and in situ measurements, modeling, and remote sensing. We discuss i) novel measurement methodologies and instrumentation that allow detection and speciation of different forms and oxidation states of Fe in deliquesced mineral aerosol, cloud/rainwater, and seawater; ii) oceanic models that treat Fe cycling with several external sources and sinks, dissolved, colloidal, particulate, inorganic, and organic ligand-complexed forms of Fe, as well as Fe in detritus and phytoplankton; and iii) atmospheric models that consider natural and anthropogenic sources of Fe, mobilization of Fe in mineral aerosols due to the dissolution of Fe-oxides and Fe-substituted aluminosilicates through proton-promoted, organic ligand-promoted, and photo-reductive mechanisms. In addition, the study identifies existing challenges and disconnects (both fundamental and methodological) such as i) inconsistencies in Fe nomenclature and the definition of bioavailable Fe between oceanic and atmospheric disciplines, and ii) the lack of characterization of the processes controlling Fe speciation and residence time at the atmosphere-ocean interface. Such challenges are undoubtedly caused by extremely low concentrations, short lifetime, and the myriad of physical, (photo)chemical, and biological processes affecting global biogeochemical cycling of Fe. However, we also argue that the historical division (separate treatment of Fe biogeochemistry in oceanic and atmospheric disciplines) and the classical funding structures (that often create obstacles for transdisciplinary collaboration) are also hampering the advancement of knowledge in the field. Finally, the study provides some specific ideas and guidelines for laboratory studies, field measurements, and modeling research required for improved characterization of global biogeochemical cycling of Fe in relationship with other trace elements and essential nutrients. The report is intended to aid scientists in their work related to Fe biogeochemistry as well as program managers at the relevant funding agencies.
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•Unified nomenclature of the Fe-related terminology is proposed for atmospheric and oceanic communities.•A strategic science prioritization matrix is offered for conducting and facilitating new research.•Mechanistic treatments are needed for aerosol Fe interaction with atmospheric and oceanic DOM.
The measurable supply of 232Th to the ocean can be used to derive the supply of other elements, which is more difficult to quantify directly. The measured inventory of an element divided by the ...derived supply yields a replacement time estimate, which in special circumstances is related to a residence time. As a proof of concept, Th‐based supply rates imply a range in the replacement times of the rare earth elements in the North Atlantic that is consistent with the chemical reactivity of rare earth elements related to their ionic charge density. Similar estimates of replacement times for the bioactive trace elements (Fe, Mn, Zn, Cd, Cu, and Co), ranging from <5 years to >50,000 years, demonstrate the broad range of elemental reactivity in the ocean. Here we discuss how variations in source composition, fractional solubility ratios, or noncontinental sources, such as hydrothermal vents, lead to uncertainties in Th‐based replacement time estimates. We show that the constraints on oceanic replacement time provided by the Th‐based calculations are broadly applicable in predicting how elements are distributed in the ocean and for some elements, such as Fe, may inform us on how the carbon cycle may be impacted by trace element supply and removal.
Key Points
Thorium‐232 supply rates produce reasonable ocean replacement times for the rare earth elements
Iron residence time in the North Atlantic may be less than 6 years
Thorium‐232 supply rates may provide an accurate basis for the supply of many other elements for constraining biogeochemical models
The air-sea interface, or sea surface microlayer (1-1000 mu m), is a unique environment with different physical, chemical, and biological properties compared to the underlying water column. It is an ...important, yet often ignored component in the biogeochemical cycling of trace elements in the marine environment due to the lack of trace element clean sampling and analytical methods. A novel technique, a hollow cylinder of ultra-pure SiO2 (quartz glass) with a plastic handle, was developed to sample the microlayer for trace elements. This research also developed and optimized clean trace element techniques to accurately measure nine trace metals (Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) in the dissolved and particulate fractions of the microlayer and underlying water column. Preliminary data from a study in the Western Mediterranean Sea involving a mesocosm (in situ all plastic bag) showed consistent measurements in the trace element concentrations over the course of several days. Microlayer samples that were collected outside the mesocosm showed increased dissolved and particulate trace element concentrations resulting from a wet deposition event.
Mercury (Hg) concentration in fish of the Gulf of the Mexico (GoM) is a major concern due to the importance of the GoM for U.S. fisheries. The Deepwater Horizon (DWH) oil spill in April 2010 in the ...northern GoM resulted in large amounts of oil and dispersant released to the water column, which potentially modified Hg bioaccumulation patterns in affected areas. We measured Hg species (methylmercury (MMHg) and inorganic Hg (IHg)) concentrations, and light (C, N and S) and Hg stable isotopes in muscle and liver tissues from tilefish (Lopholatilus chamaleonticeps) sampled in 2012 and 2013 along the shelf break of the northeastern GoM. Fish located close to the mouth of the Mississippi River (MR) and northwest of the DWH well-head (47 km) showed significantly lower Hg levels in muscle and liver than fish located further northeast of the DWH (>109 km), where 98% of tilefish had Hg levels in the muscle above US consumption advisory thresholds (50% for tilefish close to the DWH). Differences in light and Hg stable isotopes signatures were observed between these two areas, showing higher δ15N, and lower δ202Hg, Δ199Hg and δ34S in fish close to the DWH/MR. This suggests that suspended particles from the MR reduces Hg bioavailability at the base of the GoM food chains. This phenomenon can be locally enhanced by the DWH that resulted in increased particles in the water column as evidenced by the marine snow layer in the sediments. On the other hand, freshly deposited Hg associated with organic matter in more oligotrophic marine waters enhanced Hg bioaccumulation in local food webs. Comparing Hg isotopic composition in liver and muscle of fish indicates specific metabolic response in fish having accumulated high levels of MMHg.
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•Hg in fish of the Gulf of the Mexico is a concern due to commercial fisheries.•We hypothesized that the Deepwater Horizon oil spill affected Hg availability.•About 90% of tilefish surpassed Hg thresholds of U.S. regulations for food.•Mississippi River inputs combined with oil caused reduced Hg bioavailability.•Hg concentrations and isotopes in fish organs reflect in vivo processes.
The Arctic region is undergoing significant changes in climate, with a notable decrease in summertime sea ice coverage over the past three decades. This trend means an increasing proportion of Arctic ...Ocean surface waters can receive direct deposition of material from the atmosphere, potentially influencing marine biogeochemical cycles and delivery of pollutants to the Arctic ecosystem. Here, we present aerosol concentrations of selected trace elements (Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) measured during the US GEOTRACES Western Arctic cruise (GN01, also known as HLY1502) in August–October 2015. Concentrations of “lithogenic” elements (Al, Ti, V, Mn, Fe, and Co) were similar to those measured in remote and predominantly marine-influenced air masses in previous studies, reflecting the remoteness of the Arctic Ocean from major dust sources. Concentrations of Ni, Cu, Zn, Pb, and Cd showed significant enrichments over crustal values, and were often of similar magnitude to concentrations measured over the North Atlantic in air masses of North American or European provenance. We use 7Be inventory and flux data from GN01 to estimate a bulk atmospheric deposition velocity during the study period, and combine it with our aerosol concentrations to calculate atmospheric deposition fluxes of the trace elements in the Arctic region during late summer. The resulting estimates for mineral dust and Fe deposition fall at the low end of global estimates and confirm the Arctic Ocean as a low-dust environment during the summer months.
This article is part of a special issue entitled: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
•We report bulk aerosol concentrations of multiple trace elements from the western Arctic Ocean during summer 2015.•Deposition fluxes are calculated using a bulk deposition velocity calculated from aerosol and snow 7Be data.•Summertime atmospheric deposition fluxes of mineral dust and Fe to the Arctic are low relative to other oceanic regions.
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