Metal dissolution from atmospheric aerosol deposition to the oceans is important in enhancing and inhibiting phytoplankton growth rates and modifying plankton community structure, thus impacting ...marine biogeochemistry. Here we review the current state of knowledge on the causes and effects of the leaching of multiple trace metals from natural and anthropogenic aerosols. Aerosol deposition is considered both on short timescales over which phytoplankton respond directly to aerosol metal inputs, as well as longer timescales over which biogeochemical cycles are affected by aerosols.
Atmospheric iron affects the global carbon cycle by modulating ocean biogeochemistry through the deposition of soluble iron to the ocean. Iron emitted by anthropogenic (fossil fuel) combustion is a ...source of soluble iron that is currently considered less important than other soluble iron sources, such as mineral dust and biomass burning. Here we show that the atmospheric burden of anthropogenic combustion iron is 8 times greater than previous estimates by incorporating recent measurements of anthropogenic magnetite into a global aerosol model. This new estimation increases the total deposition flux of soluble iron to southern oceans (30-90 °S) by 52%, with a larger contribution of anthropogenic combustion iron than dust and biomass burning sources. The direct radiative forcing of anthropogenic magnetite is estimated to be 0.021 W m
globally and 0.22 W m
over East Asia. Our results demonstrate that anthropogenic combustion iron is a larger and more complex climate forcer than previously thought, and therefore plays a key role in the Earth system.
Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high ...aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity.
This work reports on the current status of the global modeling of iron (Fe)
deposition fluxes and atmospheric concentrations and the analyses of the
differences between models, as well as between ...models and observations. A
total of four global 3-D chemistry transport (CTMs) and general circulation
(GCMs) models participated in this intercomparison, in the framework of
the United Nations Joint Group of Experts on the Scientific Aspects of Marine
Environmental Protection (GESAMP) Working Group 38, “The Atmospheric Input
of Chemicals to the Ocean”. The global total Fe (TFe) emission strength in
the models is equal to ∼72 Tg Fe yr−1 (38–134 Tg Fe yr−1)
from mineral dust sources and around 2.1 Tg Fe yr−1 (1.8–2.7 Tg Fe yr−1)
from combustion processes (the sum of anthropogenic
combustion/biomass burning and wildfires). The mean global labile Fe (LFe)
source strength in the models, considering both the primary emissions and the
atmospheric processing, is calculated to be 0.7 (±0.3) Tg Fe yr−1,
accounting for both mineral dust and combustion aerosols. The
mean global deposition fluxes into the global ocean are estimated to be in the range
of 10–30 and 0.2–0.4 Tg Fe yr−1 for TFe and LFe, respectively,
which roughly corresponds to a respective 15 and 0.3 Tg Fe yr−1 for the multi-model ensemble model mean. The model intercomparison analysis indicates that the representation of the
atmospheric Fe cycle varies among models, in terms of both the magnitude of
natural and combustion Fe emissions as well as the complexity of atmospheric
processing parameterizations of Fe-containing aerosols. The model comparison
with aerosol Fe observations over oceanic regions indicates that most models
overestimate surface level TFe mass concentrations near dust source
regions and tend to underestimate the low concentrations observed in remote
ocean regions. All models are able to simulate the tendency of higher Fe
concentrations near and downwind from the dust source regions, with the mean
normalized bias for the Northern Hemisphere (∼14), larger
than that of the Southern Hemisphere (∼2.4) for the ensemble model
mean. This model intercomparison and model–observation comparison study
reveals two critical issues in LFe simulations that require further
exploration: (1) the Fe-containing aerosol size distribution and (2) the
relative contribution of dust and combustion sources of Fe to labile Fe in
atmospheric aerosols over the remote oceanic regions.
The iron cycle is a key component of the Earth system. Yet how variable the atmospheric flux of soluble (bioaccessible) iron into oceans is, and how this variability is modulated by human activity ...and a changing climate, is not well known. For the first time, we characterize Satellite Era (1980 to 2015) daily‐to‐interannual modeled soluble iron emission and deposition variability from both pyrogenic (fires and anthropogenic combustion) and dust sources. Statistically significant emission trends exist: dust iron decreases, fire iron slightly increases, and anthropogenic iron increases. A strong temporal variability in deposition to ocean basins is found, and, for most regions, dust iron dominates the absolute deposition magnitude, fire iron is an important contributor to temporal variability, and anthropogenic iron imposes a significant increasing trend. Quantifying soluble iron daily‐to‐interannual deposition variability from all major iron sources, not only dust, will advance quantification of changes in marine biogeochemistry in response to the continuing human perturbation to the Earth System.
Plain Language Summary
Iron is a limiting micronutrient for marine phytoplankton growth in many ocean basins. A major source of new iron to the open ocean is via atmospheric deposition and until recently was considered to be associated with mineral dust aerosol. However, growing evidence has shown that pyrogenic (fires and anthropogenic combustion) are equally important sources to many basins. Here, for the first time, we quantify the variability (1980 to 2015) in emission and deposition for all three sources across three model versions for robustness. We find that while dust iron dominates the absolute global deposition magnitude, fire iron is an important contributor to daily, seasonal, and interannual variability and that the anthropogenic iron deposition flux to important ocean basins has steadily increased with time. Characterizing more realistic deposition patterns on both short‐term (daily to monthly) and long‐term (annual to decadal) time scales will improve understanding of the biogeochemical response to the continuing human perturbation to iron emissions and their ocean deposition flux.
Key Points
Modeled soluble iron deposition variability (1980–2015) is characterized for dust, fires, and anthropogenic iron for the first time
A new anthropogenic iron transient data set is developed, showing lifetime depends on particle emission size and thus air quality acts
Anthropogenic trends and wildfire variability are important regionally, and daily variability is larger than seasonal or interannual
Iron can be a growth‐limiting nutrient for phytoplankton, modifying rates of net primary production, nitrogen fixation, and carbon export ‐ highlighting the importance of new iron inputs from the ...atmosphere. The bioavailable iron fraction depends on the emission source and the dissolution during transport. The impacts of anthropogenic combustion and land use change on emissions from industrial, domestic, shipping, desert, and wildfire sources suggest that Northern Hemisphere soluble iron deposition has likely been enhanced between 2% and 68% over the Industrial Era. If policy and climate follow the intermediate Representative Concentration Pathway 4.5 trajectory, then results suggest that Southern Ocean (>30°S) soluble iron deposition would be enhanced between 63% and 95% by 2100. Marine net primary productivity and carbon export within the open ocean are most sensitive to changes in soluble iron deposition in the Southern Hemisphere; this is predominantly driven by fire rather than dust iron sources. Changes in iron deposition cause large perturbations to the marine nitrogen cycle, up to 70% increase in denitrification and 15% increase in nitrogen fixation, but only modestly impacts the carbon cycle and atmospheric CO2 concentrations (1–3 ppm). Regionally, primary productivity increases due to increased iron deposition are often compensated by offsetting decreases downstream corresponding to equivalent changes in the rate of phytoplankton macronutrient uptake, particularly in the equatorial Pacific. These effects are weaker in the Southern Ocean, suggesting that changes in iron deposition in this region dominates the global carbon cycle and climate response.
Key Points
Human activity significantly modifies the magnitude and location of atmospheric soluble iron deposition to the oceans
Marine carbon cycle responses to Anthropocene iron flux changes are modest but more sensitive to varying fire than dust iron emissions
Increasing the iron flux produces offsetting patterns in phytoplankton macronutrient uptake and productivity rates at the basin scale
The supply of nutrients is a fundamental regulator of ocean productivity and carbon sequestration. Nutrient sources, sinks, residence times, and elemental ratios vary over broad scales, including ...those resulting from climate-driven changes in upper water column stratification, advection, and the deposition of atmospheric dust. These changes can alter the proximate elemental control of ecosystem productivity with cascading ecological effects and impacts on carbon sequestration. Here, we report multidecadal observations revealing that the ecosystem in the eastern region of the North Pacific Subtropical Gyre (NPSG) oscillates on subdecadal scales between inorganic phosphorus (Pi) sufficiency and limitation, when Pi concentration in surface waters decreases below 50–60 nmol·kg−1. In situ observations and model simulations suggest that sea-level pressure changes over the northwest Pacific may induce basin-scale variations in the atmospheric transport and deposition of Asian dust-associated iron (Fe), causing the eastern portion of the NPSG ecosystem to shift between states of Fe and Pi limitation. Our results highlight the critical need to include both atmospheric and ocean circulation variability when modeling the response of open ocean pelagic ecosystems under future climate change scenarios.
Herein, we present a description of the Mechanism of Intermediate
complexity for Modelling Iron (MIMI v1.0). This iron processing module was
developed for use within Earth system models and has been ...updated within a
modal aerosol framework from the original implementation in a bulk aerosol
model. MIMI simulates the emission and atmospheric processing of two main
sources of iron in aerosol prior to deposition: mineral dust and combustion
processes. Atmospheric dissolution of insoluble to soluble iron is
parameterized by an acidic interstitial aerosol reaction and a separate
in-cloud aerosol reaction scheme based on observations of enhanced aerosol
iron solubility in the presence of oxalate. Updates include a more
comprehensive treatment of combustion iron emissions, improvements to the
iron dissolution scheme, and an improved physical dust mobilization scheme.
An extensive dataset consisting predominantly of cruise-based observations
was compiled to compare to the model. The annual mean modelled concentration
of surface-level total iron compared well with observations but less so in
the soluble fraction (iron solubility) for which observations are much more
variable in space and time. Comparing model and observational data is
sensitive to the definition of the average as well as the temporal and spatial
range over which it is calculated. Through statistical analysis and
examples, we show that a median or log-normal distribution is preferred when
comparing with soluble iron observations. The iron solubility
calculated at each model time step versus that calculated based on a ratio
of the monthly mean values, which is routinely presented in aerosol studies
and used in ocean biogeochemistry models, is on average globally one-third
(34 %) higher. We redefined ocean deposition regions based on dominant
iron emission sources and found that the daily variability in soluble iron
simulated by MIMI was larger than that of previous model simulations. MIMI
simulated a general increase in soluble iron deposition to Southern
Hemisphere oceans by a factor of 2 to 4 compared with the previous
version, which has implications for our understanding of the ocean
biogeochemistry of these predominantly iron-limited ocean regions.
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