The deposition of aerosols to the open ocean and the mechanisms controlling trace element solubility are important factors in the biogeochemical cycling of biolimiting elements, including iron, with ...implications for the global carbon cycle. During 2004-2006, 24-hour integrated aerosol samples were collected on two Climate Variability and Predictability (CLIVAR)-CO2 Repeat Hydrography cruises in the Pacific Ocean. The cruise sections traversed the North Pacific Ocean along 30 degree N (Section P02) and the eastern Pacific along 150 degree W (Section P16). This dataset includes analyses of aerosol particle chemistry as well as iron, aluminum, and manganese solubility in ultrapure deionized water and iron solubility in filtered surface seawater, measured using a rapid, flow-through extraction technique. The atmospheric concentrations of soluble aerosol iron were not significantly different using these extraction solutions (Wilcoxon signed rank, p=0.076). However, aerosol iron fractional solubility was higher in ultrapure deionized water than in filtered surface seawater (Wilcoxon signed rank, p=0.009). The median fractional solubility of aerosol iron in ultrapure water was 9.2% (3.2-29.1%) and 6.4% (0.5-81.1%) in seawater. Soluble aerosol Fe(II) accounted for 1.7% of the total aerosol Fe and 26.2% of the seawater soluble aerosol iron. The fractional solubility of aerosol iron did not increase with distance from Asian source regions nor was solubility related to the concentration of aerosol Fe in the atmosphere.
Atmospheric input of trace element micronutrients to the oceans is difficult to determine, as even with collection of high‐quality aerosol chemical concentrations, such data by themselves cannot ...yield deposition rates. To transform these concentrations into rates, a method of determining flux by applying an appropriate deposition velocity is required. A recently developed method based on the natural radionuclide 7Be has provided a means to estimate the bulk (wet + dry) deposition velocity (Vb) required for this calculation. Here, water column 7Be inventories and aerosol 7Be concentrations collected during the 2018 US GEOTRACES Pacific Meridional Transect are presented. We use these data together with those from other ocean basins to derive a global relationship between rain rate (m/yr) and bulk deposition velocity (m/day), such that Vb = 999 ± 96 × Rain rate + 1,040 ± 136 (R2 = 0.81). Thus, with satellite‐derived rainfall estimates, a means to calculate aerosol bulk deposition velocities is provided.
Plain Language Summary
Atmospheric input of trace element micronutrients to the global ocean such as iron (Fe), cobalt (Co), and Zinc (Zn) is difficult to determine. Even with collection of high‐quality aerosol chemical concentrations, such data by themselves cannot yield rates of deposition. A recently developed method based on the natural radionuclide 7Be, which is deposited to the surface ocean, has provided a means to estimate the bulk (wet + dry) deposition velocity (Vb) required for this calculation. In this work, water column 7Be inventories and aerosol 7Be concentrations collected during the 2018 US GEOTRACES Pacific Meridional Transect are presented. We use these data together with those from other ocean basins to derive a global relationship between rain rate (m/yr) and bulk deposition velocity (m/day), such that Vb = 999 × Rain rate + 1,040 (R2 = 0.81). Thus, deposition velocity to an ocean region can be estimated from rainfall rate. This information is critical for evaluating limitations on phytoplankton growth and the strength of the biological carbon pump and represents an important input to ocean biogeochemical models.
Key Points
Aerosol input of trace element micronutrients is difficult to determine as aerosol chemical concentration alone cannot yield deposition rate
The natural radionuclide 7Be provides a means to estimate the bulk deposition velocity (Vb) required to calculate aerosol fluxes
We use new 7Be data from the Pacific with data from other ocean basins to derive a global relationship between rain rate and Vb
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.
Aerosol and precipitation sampling as part of the 2003 Climate Variability and Predictability (CLIVAR)-CO2 Repeat Hydrography trace element sampling program has produced an aerosol chemistry dataset ...for a region of the central Atlantic Ocean between 65 degree N and 5 degree S. This dataset includes analyses of aerosol particle chemistry as well as Fe and Al solubility (measured using a rapid, flow-through leaching technique). Several factors thought to influence aerosol Fe solubility including chemical weathering and aerosol source are evaluated as well. Air mass back-trajectories were used to characterize the atmospheric regime of each aerosol sample. Aerosol concentrations varied greatly with the highest concentrations observed between 23 degree N and 8.7 degree N. Aerosol Fe solubility was 9% plus or minus 5% in seawater and 15% plus or minus 8% in ultrapure deionized water. The concentration of soluble aerosol Fe in seawater was estimated with reasonable accuracy from the concentration of soluble aerosol Fe in deionized water by the relationship logFeSW=(0.85 plus or minus 0.039) logFeDI+log (1 plus or minus 1.2), (r super(2)=0.93).
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.
Arboreal epiphytes, plants that grow on trees, can significantly increase rainwater storage and evaporation (i.e., “interception”) within canopies. Drought conditions may affect this hydrological ...role, as epiphytes' physiological responses change leaf properties that affect water retention. Drought-induced changes in epiphyte water storage capacity could substantially alter canopy hydrology, but have not been studied. We tested the effects of drought on the water storage capacity (Smax) of leaves and leaf properties of two epiphytes with distinct ecohydrological traits: resurrection fern (Pleopeltis polypodioides), and Spanish moss (Tillandsia usneoides). Both species are common in maritime forests of the Southeastern USA, where climate change is expected to decrease precipitation in spring and summer. To simulate drought, we dried leaves to 75 %, 50 %, and ~25 % of fresh weight, and quantified their Smax in fog chambers. We measured relevant leaf properties: hydrophobicity, minimum leaf conductance (gmin; a measure of water loss under drought), and Normalized Difference Vegetative Index (NDVI). We found that drought significantly reduced Smax and increased leaf hydrophobicity for both species, indicating that lower Smax may be due to shedding of droplets. While the overall reduction in Smax did not differ between the two species, they exhibited distinct drought responses. Dehydrated T. usneoides leaves had lower gmin, demonstrating the ability to limit water loss under drought. P. polypodioides increased gmin when dehydrated, consistent with its extraordinary ability to withstand water loss. NDVI decreased with dehydration in T. usneoides but not P. polypodioides. Our results suggest that increased drought may have a dramatic effect on canopy water cycling by reducing the Smax of epiphytes. Reduced rainfall interception and storage in forest canopies could have widespread effects on hydrological cycling, thus understanding the potential feedbacks of plant drought response on hydrology is crucial. This study highlights the importance of connecting foliar-scale plant response with broader hydrological processes.
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•Epiphytes growing in tree canopies capture and store rainwater, but this role may be vulnerable to increasing drought.•We tested the effects of drought (leaf desiccation) on water storage capacity (Smax) and leaf properties of two epiphytes.•Drought significantly reduced Smax and increased leaf hydrophobicity for both species.•Plants’ responses to drought may reduce rainfall storage in forest canopies, potentially altering hydrological cycling.
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
Recent trends of atmospheric lead deposition to the North Pacific were investigated with analyses of lead in aerosols and surface waters collected on the fourth Intergovernmental Oceanographic ...Commission Contaminant Baseline Survey from May to June, 2002. Lead concentrations of the aerosols varied by 2 orders of magnitude (0.1–26.4 pmol/m3) due in part to variations in dust deposition during the cruise. The ranges in lead aerosol enrichment factors relative to iron (1–119) and aluminum (3–168) were similar, evidencing the transport of Asian industrial lead aerosols across the North Pacific. The oceanic deposition of some of those aerosols was substantiated by the gradient of lead concentrations of North Pacific waters, which varied 3-fold (32.7–103.5 pmol/kg), were highest along with the Asian margin of the basin, and decreased eastward. The hypothesized predominance of Asian industrial lead inputs to the North Pacific was further corroborated by the lead isotopic composition of ocean surface waters (206Pb/207Pb = 1.157–1.169; 208Pb/206Pb = 2.093–2.118), which fell within the range of isotopic ratios reported in Asian aerosols that are primarily attributed to Chinese industrial lead emissions.
Aerosol samples were collected in the North Atlantic Ocean during June–August, 2003. Aerosols were divided into nine size fractions ranging from >
18 μm to <
0.056 μm. Total element concentrations ...were measured by energy dispersive X-ray fluorescence to determine the size distribution of biogeochemically important elements. The solubility of Fe and other elements was measured in ultrapure deionized water to investigate the relationship between particle size and aerosol solubility. We found that the majority of soluble aerosol Fe was on particles of ≥
1 μm aerodynamic diameter. Aerosol Fe solubility was somewhat variable but in general, aerosol Fe solubility did not increase with decreasing particle size.
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