Atmospheric iron delivery and surface ocean biological activity in the Southern Ocean and Patagonian region

Abstract. Iron is a key nutrient for phytoplankton growth in the surface ocean. At high latitudes, the iron cycle is closely related to the dynamics of sea ice. In recent decades, Arctic sea ice cover has been declining rapidly and Antarctic sea ice has exhibited large regional trends. A significant reduction of sea ice in both hemispheres is projected in future climate scenarios. In order to adequately study the effect of sea ice on the polar iron cycle, sea ice bearing iron was incorporated in the Community Earth System Model (CESM). Sea ice acts as a reservoir for iron during winter and releases the trace metal to the surface ocean in spring and summer. Simulated iron concentrations in sea ice generally agree with observations in regions where iron concentrations are relatively low. The maximum iron concentrations simulated in Arctic and Antarctic sea ice are much lower than observed, which is likely due to underestimation of iron inputs to sea ice or missing mechanisms. The largest iron source to sea ice is suspended sediments, contributing fluxes of iron of 2.2 × 108 mol Fe month−1 in the Arctic and 4.1 × 106 mol Fe month−1 in the Southern Ocean during summer. As a result of the iron flux from ice, iron concentrations increase significantly in the Arctic. Iron released from melting ice increases phytoplankton production in spring and summer and shifts phytoplankton community composition in the Southern Ocean. Results for the period of 1998 to 2007 indicate that a reduction of sea ice in the Southern Ocean will have a negative influence on phytoplankton production. Iron transport by sea ice appears to be an important process bringing iron to the central Arctic. The impact of ice to ocean iron fluxes on marine ecosystems is negligible in the current Arctic Ocean, as iron is not typically the growth-limiting nutrient. However, it may become a more important factor in the future, particularly in the central Arctic, as iron concentrations will decrease with declining sea ice cover and transport.

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Impacts of sea ice on the marine iron cycle and phytoplankton productivity

Biogeosciences Discussions ◽

10.5194/bgd-11-2383-2014 ◽

2014 ◽

Vol 11 (2) ◽

pp. 2383-2418 ◽

Cited By ~ 4

Author(s):

S. Wang ◽

D. Bailey ◽

K. Lindsay ◽

K. Moore ◽

M. Holland

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Arctic Sea Ice ◽

The Arctic ◽

Phytoplankton Production ◽

Iron Cycle ◽

Surface Ocean ◽

Antarctic Sea Ice ◽

Arctic Sea ◽

Iron Concentrations

Abstract. Iron is a key nutrient for phytoplankton growth in the surface ocean. At high latitudes, the iron cycle is closely related to sea ice. In recent decades, Arctic sea ice cover has been declining rapidly and Antarctic sea ice has exhibited large regional trends. A significant reduction of sea ice in both hemispheres is projected in future climate scenarios. To study impacts of sea ice on the iron cycle, iron sequestration in ice is incorporated to the Biogeochemical Elemental Cycling (BEC) model. Sea ice acts as a reservoir of iron during winter and releases iron to the surface ocean in spring and summer. Simulated iron concentrations in sea ice generally agree with observations, in regions where iron concentrations are lower. The maximum iron concentrations simulated in the Arctic sea ice and the Antarctic sea ice are 192 nM and 134 nM, respectively. These values are much lower than observed, which is likely due to missing biological processes in sea ice. The largest iron source to sea ice is suspended sediments, contributing fluxes of iron of 2.2 × 108 mol Fe month−1 to the Arctic and 4.1 × 106 mol Fe month−1 to the Southern Ocean during summer. As a result of the iron flux from ice, iron concentrations increase significantly in the Arctic. Iron released from melting ice increases phytoplankton production in spring and summer and shifts phytoplankton community composition in the Southern Ocean. Simulation results for the period of 1998 to 2007 indicate that a reduction of sea ice in the Southern Ocean will have a negative influence on phytoplankton production. Iron transport by sea ice appears to be an important process bringing iron to the central Arctic. Impacts of iron fluxes from ice to ocean on marine ecosystems are negligible in the current Arctic Ocean, as iron is not typically the growth-limiting nutrient. However, it may become a more important factor in the future, particularly in the central Arctic, as iron concentrations will decrease with declining sea ice cover and transport.

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The importance of alkyl nitrates and sea ice emissions to atmospheric NO<sub>x</sub> sources and cycling in the summertime Southern Ocean marine boundary layer

10.5194/acp-2021-519 ◽

2021 ◽

Author(s):

Jessica Mary Burger ◽

Julie Granger ◽

Emily Joyce ◽

Meredith Galanter Hastings ◽

Kurt Angus McDonald Spence ◽

...

Keyword(s):

Boundary Layer ◽

Southern Ocean ◽

Sea Ice ◽

Marine Boundary Layer ◽

Weddell Sea ◽

Alkyl Nitrates ◽

Air Mass ◽

Surface Ocean ◽

Alkyl Nitrate ◽

Atmospheric Nitrate

Abstract. Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx = NO + NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling, and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean where pristine conditions serve as a useful proxy for the preindustrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (> 1 μm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5° S to 70° S, and across the northern edge of the Weddell Sea. The δ15N-NO3− decreased with latitude from −2.7 ‰ to −43.1 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid latitudes, and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotopic fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of −22.0 ‰ ± 7.5 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O-NO3− was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O-NO3− values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea ice covered ocean, which is known to enhance peroxy radical production. The observed low δ18O-NO3− is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought, and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

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Characteristics of biogenically-derived aerosols over the Amundsen Sea, Antarctica

10.5194/acp-2019-133 ◽

2019 ◽

Cited By ~ 4

Author(s):

Jinyoung Jung ◽

Sang-Bum Hong ◽

Meilian Chen ◽

Jin Hur ◽

Liping Jiao ◽

...

Keyword(s):

Biological Activity ◽

Organic Carbon ◽

Southern Ocean ◽

Sea Water ◽

Austral Summer ◽

Water Soluble ◽

Fluorescence Excitation ◽

Amundsen Sea ◽

Marine Biological ◽

Carbon Species

Abstract. To investigate the influence of marine biological activity on aerosols, aerosol and seawater samples were collected over the Southern Ocean (43° S−70° S) and the Amundsen Sea (70° S−75° S) during the ANA06B cruise conducted in the austral summer of 2016 aboard the Korean icebreaker IBR/V Araon. Over the Southern Ocean, atmospheric methanesulfonic acid (MSA) concentration was low (0.10 ± 0.002 µg m−3), whereas its concentration increased sharply up to 0.57 µg m−3 in the Amundsen Sea where Phaeocystis antarctica (P. antarctica), a producer of dimethylsulfide (DMS), was the dominant phytoplankton species. Unlike MSA, mean non-sea-salt sulfate (nss-SO42−) concentration in the Amundsen Sea was comparable to that in the Southern Ocean, suggesting significant influences of marine biological activity on atmospheric sulfur species in the Amundsen Sea. Water-soluble organic carbon (WSOC) concentrations over the Southern Ocean and the Amundsen Sea varied from 0.048–0.16 µgC m−3 and 0.070–0.18 µgC m−3, with averages of 0.087 ± 0.038 µgC m−3 and 0.097 ± 0.038 µgC m−3, respectively. For water-insoluble organic carbon (WIOC), its mean concentrations over the Southern Ocean and the Amundsen Sea were 0.25 ± 0.13 µgC m−3 and 0.26 ± 0.10 µgC m−3, varying from 0.083–0.49 µgC m−3 and 0.12–0.38 µgC m−3, respectively. WIOC was the dominant organic carbon species in both the Southern Ocean and the Amundsen Sea, accounting for 73–75 % of total aerosol organic carbon. WSOC and WIOC were highly enriched in the submicron sea spray particles, especially in the Amundsen Sea where biological productivity was much higher than the Southern Ocean. In addition, the submicron WIOC concentration was quite related to the relative biomass of P. antarctica, suggesting that extracellular polysaccharide mucus produced by P. antarctica was a significant factor affecting atmospheric WIOC concentration in the Amundsen Sea. The fluorescence properties of WSOC investigated using fluorescence excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC) revealed that protein-like components were dominant in our marine aerosol samples, representing 69–91 % of the total intensity. Protein-like components also showed positive relationships with the relative biomass of diatoms; however, they were negatively correlated with the relative biomass of P. antarctica. These results suggest that protein-like components are most likely produced as a result of biological processes of diatoms, which play a crucial role in forming the submicron WSOC observed over the Southern Ocean and the Amundsen Sea, and that phytoplankton community structure is a significant factor affecting atmospheric organic carbon species. The results from this study provide significant new observational data on biogenically-derived sulfur and organic carbon species in the Amundsen Sea.

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What drives the latitudinal gradient in open-ocean surface dissolved inorganic carbon concentration?

Biogeosciences ◽

10.5194/bg-16-2661-2019 ◽

2019 ◽

Vol 16 (13) ◽

pp. 2661-2681 ◽

Cited By ~ 2

Author(s):

Yingxu Wu ◽

Mathis P. Hain ◽

Matthew P. Humphreys ◽

Sue Hartman ◽

Toby Tyrrell

Keyword(s):

Southern Ocean ◽

High Latitude ◽

Latitudinal Gradient ◽

Inorganic Carbon ◽

Dissolved Inorganic Carbon ◽

Latitudinal Gradients ◽

Total Alkalinity ◽

Global Ocean ◽

Low Latitude ◽

Surface Ocean

Abstract. Previous work has not led to a clear understanding of the causes of spatial pattern in global surface ocean dissolved inorganic carbon (DIC), which generally increases polewards. Here, we revisit this question by investigating the drivers of observed latitudinal gradients in surface salinity-normalized DIC (nDIC) using the Global Ocean Data Analysis Project version 2 (GLODAPv2) database. We used the database to test three different hypotheses for the driver producing the observed increase in surface nDIC from low to high latitudes. These are (1) sea surface temperature, through its effect on the CO2 system equilibrium constants, (2) salinity-related total alkalinity (TA), and (3) high-latitude upwelling of DIC- and TA-rich deep waters. We find that temperature and upwelling are the two major drivers. TA effects generally oppose the observed gradient, except where higher values are introduced in upwelled waters. Temperature-driven effects explain the majority of the surface nDIC latitudinal gradient (182 of the 223 µmol kg−1 increase from the tropics to the high-latitude Southern Ocean). Upwelling, which has not previously been considered as a major driver, additionally drives a substantial latitudinal gradient. Its immediate impact, prior to any induced air–sea CO2 exchange, is to raise Southern Ocean nDIC by 220 µmol kg−1 above the average low-latitude value. However, this immediate effect is transitory. The long-term impact of upwelling (brought about by increasing TA), which would persist even if gas exchange were to return the surface ocean to the same CO2 as without upwelling, is to increase nDIC by 74 µmol kg−1 above the low-latitude average.

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Diffusion controls the ventilation of a Pacific Shadow Zone above abyssal overturning

Nature Communications ◽

10.1038/s41467-021-24648-x ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Mark Holzer ◽

Tim DeVries ◽

Casimir de Lavergne

Keyword(s):

Southern Ocean ◽

Carbon Storage ◽

North Atlantic ◽

North Pacific ◽

The Other ◽

Shadow Zone ◽

Surface Ocean ◽

Pacific Waters ◽

The Pacific ◽

Bottom Waters

AbstractMid-depth North Pacific waters are rich in nutrients and respired carbon accumulated over centuries. The rates and pathways with which these waters exchange with the surface ocean are uncertain, with divergent paradigms of the Pacific overturning: one envisions bottom waters upwelling to 1.5 km depth; the other confines overturning beneath a mid-depth Pacific shadow zone (PSZ) shielded from mean advection. Here global inverse modelling reveals a PSZ where mean ages exceed 1400 years with overturning beneath. The PSZ is supplied primarily by Antarctic and North-Atlantic ventilated waters diffusing from below and from the south. Half of PSZ waters re-surface in the Southern Ocean, a quarter in the subarctic Pacific. The abyssal North Pacific, despite strong overturning, has mean re-surfacing times also exceeding 1400 years because of diffusion into the overlying PSZ. These results imply that diffusive transports – distinct from overturning transports – are a leading control on Pacific nutrient and carbon storage.

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The sensitivity of pCO<sub>2</sub> reconstructions in the Southern Ocean to sampling scales: a semi-idealized model sampling and reconstruction approach

10.5194/bg-2021-344 ◽

2022 ◽

Author(s):

Laique Merlin Djeutchouang ◽

Nicolette Chang ◽

Luke Gregor ◽

Marcello Vichi ◽

Pedro Manuel Scheel Monteiro

Keyword(s):

Machine Learning ◽

Southern Ocean ◽

Seasonal Cycle ◽

Temporal Frequency ◽

Gradient Boosting ◽

Biogeochemical Model ◽

Clear Understanding ◽

Surface Ocean ◽

Temporal Sampling ◽

Model Domain

Abstract. The Southern Ocean is a complex system yet is sparsely sampled in both space and time. These factors raise questions about the confidence in present sampling strategies and associated machine learning (ML) reconstructions. Previous studies have not yielded a clear understanding of the origin of uncertainties and biases for the reconstructions of the partial pressure of carbon dioxide (pCO2) at the surface ocean (pCO2ocean). Here, we examine these questions by investigating the sensitivity of pCO2ocean reconstruction uncertainties and biases to a series of semi-idealized observing system simulation experiments (OSSEs) that simulate spatio-temporal sampling scales of surface ocean pCO2 in ways that are comparable to ocean CO2 observing platforms (Ship, Waveglider, Carbon-float, Saildrone). These experiments sampled a high spatial resolution (±10 km) coupled physical and biogeochemical model (NEMO-PISCES) within a sub-domain representative of the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean. The reconstructions were done using a two-member ensemble approach that consisted of two machine learning (ML) methods, (1) the feed-forward neural network and (2) the gradient boosting machines. With the baseline observations being from the simulated ships mimicking observations from the Surface Ocean CO2 Atlas (SOCAT), we applied to each of the scale-sampling simulation scenarios the two-member ensemble method ML2, to reconstruct the full sub-domain pCO2ocean and assess the reconstruction skill through a statistical comparison of reconstructed pCO2ocean and model domain mean. The analysis shows that uncertainties and biases for pCO2ocean reconstructions are very sensitive to both the spatial and temporal scales of pCO2 sampling in the model domain. The four key findings from our investigation are the following: (1) improving ML-based pCO2 reconstructions in the Southern Ocean requires simultaneous high resolution observations of the meridional and the seasonal cycle (< 3 days) of pCO2ocean; (2) Saildrones stand out as the optimal platforms to simultaneously address these requirements; (3) Wavegliders with hourly/daily resolution in pseudo-mooring mode improve on Carbon-floats (10-day period), which suggests that sampling aliases from the low temporal frequency have a greater negative impact on their uncertainties, biases and reconstruction means; and (4) the present summer seasonal sampling biases in SOCAT data in the Southern Ocean may be behind a significant winter bias in the reconstructed seasonal cycle of pCO2ocean.

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