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
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 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.
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.
Display omitted
•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 processing of iron in dust and combustion aerosols is simulated using an
intermediate-complexity soluble iron mechanism designed for Earth system
models. The solubilization mechanism ...includes both a dependence on aerosol
water pH and in-cloud oxalic acid. The simulations of size-resolved total,
soluble and fractional iron solubility indicate that this mechanism captures
many but not all of the features seen from cruise observations of labile
iron. The primary objective was to determine the extent to which our
solubility scheme could adequately match observations of fractional iron
solubility. We define a semi-quantitative metric as the model mean at points
with observations divided by the observational mean (MMO). The model is in
reasonable agreement with observations of fractional iron solubility with an
MMO of 0.86. Several sensitivity studies are performed to ascertain the
degree of complexity needed to match observations; including the oxalic acid
enhancement is necessary, while different parameterizations for calculating
model oxalate concentrations are less important. The percent change in
soluble iron deposition between the reference case (REF) and the simulation
with acidic processing alone is 63.8 %, which is consistent with previous
studies. Upon deposition to global oceans, global mean combustion iron
solubility to total fractional iron solubility is 8.2 %; however, the
contribution of fractional iron solubility from combustion sources to ocean
basins below 15∘ S is approximately 50 %. We conclude that, in
many remote ocean regions, sources of iron from combustion and dust aerosols
are equally important. Our estimates of changes in deposition of soluble iron
to the ocean since preindustrial climate conditions suggest roughly a doubling due to a combination of
higher dust and combustion iron emissions along with more efficient
atmospheric processing.
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).
GEOTRACES Buck, Clifton S.; Fietz, Susanne; Hamilton, Douglas S. ...
Oceanography (Washington, D.C.),
06/2024, Letnik:
37, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The international GEOTRACES program has allowed unprecedented access to aerosols in the remote marine environment, moved the community toward standardized methodologies, and contributed to expanding ...research capabilities around the globe. A key aspect of GEOTRACES aerosol research is the quantification of deposition fluxes of trace elements and isotopes to the surface ocean as a source for biolimiting nutrients and anthropogenic contaminants. New methods for quantifying flux have been developed that leverage radioisotope tracers in the surface ocean to estimate bulk deposition rates on longer and more relevant timescales. The question as to what fraction of aerosols is soluble remains and is a source of continuing uncertainty. To that end, the community has engaged in coordinated study to address questions regarding the fractional solubility of aerosol trace elements and isotopes through the evaluation of widely used dissolution and extraction techniques. Intercalibration efforts, method standardization, and normalization of open-access data archiving will be the legacy of GEOTRACES-era aerosol research.
Despite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of ...data. Here, we present dissolved Cd and δ114Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ114Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ114Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO4 maximum and is thus a consequence of an integrated regeneration signal of Cd and PO4 at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol−1), rather than being related to in situ removal processes in low‐oxygen waters.
Key Points
Atmospheric inputs of isotopically light Cd play an important role in setting surface δ114Cd when surface Cd concentrations are low
Strong Southern Ocean control on subsurface Cd and δ114Cd distribution; Antarctic Intermediate Water influences δ114Cd of North Pacific intermediate waters
A Cd* minimum at depth in the North Pacific is associated with the PO4 maximum, a consequence of integrated regeneration
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
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