Seasonal dispersal of fjord meltwaters as an important source of iron and manganese to coastal Antarctic phytoplankton

Abstract. Glacial meltwater from the western Antarctic Ice Sheet is hypothesized to be an important source of cryospheric iron, fertilizing the Southern Ocean, yet its trace-metal composition and factors that control its dispersal remain poorly constrained. Here we characterize meltwater iron sources in a heavily glaciated western Antarctic Peninsula (WAP) fjord. Using dissolved and particulate ratios of manganese to iron in meltwaters, porewaters, and seawater, we show that surface glacial melt and subglacial plumes contribute to the seasonal cycle of iron and manganese within a fjord still relatively unaffected by climate-change-induced glacial retreat. Organic ligands derived from the phytoplankton bloom and the glaciers bind dissolved iron and facilitate the solubilization of particulate iron downstream. Using a numerical model, we show that buoyant plumes generated by outflow from the subglacial hydrologic system, enriched in labile particulate trace metals derived from a chemically modified crustal source, can supply iron to the fjord euphotic zone through vertical mixing. We also show that prolonged katabatic wind events enhance export of meltwater out of the fjord. Thus, we identify an important atmosphere–ice–ocean coupling intimately tied to coastal iron biogeochemistry and primary productivity along the WAP.

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Seasonal dispersal of fjord meltwaters as an important source of iron to coastal Antarctic phytoplankton

10.5194/bg-2021-79 ◽

2021 ◽

Author(s):

Kiefer Forsch ◽

Lisa Hahn-Woernle ◽

Robert Sherrell ◽

Joe Roccanova ◽

Kaixan Bu ◽

...

Keyword(s):

Phytoplankton Bloom ◽

Vertical Mixing ◽

Katabatic Wind ◽

Glacial Retreat ◽

Dissolved Iron ◽

Chemically Modified ◽

Ocean Coupling ◽

Particulate Iron ◽

Hydrologic System ◽

Wind Events

Abstract. Glacial meltwater from the western Antarctic Ice Sheet is hypothesized to be an important source of cryospheric iron, fertilizing the Southern Ocean, yet its trace metal composition and factors which control its dispersal remain poorly constrained. Here we characterize meltwater iron sources in a heavily glaciated western Antarctic Peninsula (WAP) fjord. Using dissolved and particulate ratios of manganese-to-iron in meltwaters, porewaters, and seawater, we show that glacial melt and subglacial plumes contribute to the seasonal cycle of bioavailable iron within a fjord still relatively unaffected by climate change-induced glacial retreat. Organic ligands derived from the phytoplankton bloom and the glaciers bind dissolved iron and facilitate the solubilization of particulate iron downstream. Using a numerical model, we show that plumes generated by outflow from the subglacial hydrologic system, enriched in labile particulate trace metals derived from a chemically-modified crustal source, can supply the surface through vertical mixing, and that prolonged katabatic wind events enhance export of meltwater out of the fjord. Thus, we identify an important atmosphere-ice-ocean coupling intimately tied to coastal iron biogeochemistry and primary productivity along the WAP.

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Frazil ice growth and production during katabatic wind events in the Ross Sea, Antarctica

The Cryosphere ◽

10.5194/tc-14-3329-2020 ◽

2020 ◽

Vol 14 (10) ◽

pp. 3329-3347 ◽

Cited By ~ 3

Author(s):

Lisa Thompson ◽

Madison Smith ◽

Jim Thomson ◽

Sharon Stammerjohn ◽

Steve Ackley ◽

...

Keyword(s):

Sea Ice ◽

Heat Loss ◽

Ross Sea ◽

Vertical Mixing ◽

Katabatic Wind ◽

Production Rates ◽

Frazil Ice ◽

Salt Budget ◽

Wind Events ◽

Ice Production

Abstract. Katabatic winds in coastal polynyas expose the ocean to extreme heat loss, causing intense sea ice production and dense water formation around Antarctica throughout autumn and winter. The advancing sea ice pack, combined with high winds and low temperatures, has limited surface ocean observations of polynyas in winter, thereby impeding new insights into the evolution of these ice factories through the dark austral months. Here, we describe oceanic observations during multiple katabatic wind events during May 2017 in the Terra Nova Bay and Ross Sea polynyas. Wind speeds regularly exceeded 20 m s−1, air temperatures were below −25 ∘C, and the oceanic mixed layer extended to 600 m. During these events, conductivity–temperature–depth (CTD) profiles revealed bulges of warm, salty water directly beneath the ocean surface and extending downwards tens of meters. These profiles reflect latent heat and salt release during unconsolidated frazil ice production, driven by atmospheric heat loss, a process that has rarely if ever been observed outside the laboratory. A simple salt budget suggests these anomalies reflect in situ frazil ice concentration that ranges from 13 to 266×10-3 kg m−3. Contemporaneous estimates of vertical mixing reveal rapid convection in these unstable density profiles and mixing lifetimes from 7 to 12 min. The individual estimates of ice production from the salt budget reveal the intensity of short-term ice production, up to 110 cm d−1 during the windiest events, and a seasonal average of 29 cm d−1. We further found that frazil ice production rates covary with wind speed and with location along the upstream–downstream length of the polynya. These measurements reveal that it is possible to indirectly observe and estimate the process of unconsolidated ice production in polynyas by measuring upper-ocean water column profiles. These vigorous ice production rates suggest frazil ice may be an important component in total polynya ice production.

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Frazil ice growth and production during katabatic wind events in the Ross Sea, Antarctica

10.5194/tc-2019-213 ◽

2019 ◽

Author(s):

Lisa De Pace ◽

Madison Smith ◽

Jim Thomson ◽

Sharon Stammerjohn ◽

Steve Ackley ◽

...

Keyword(s):

Ross Sea ◽

Vertical Mixing ◽

Katabatic Wind ◽

Frazil Ice ◽

Wind Speeds ◽

Terra Nova Bay ◽

Air Temperatures ◽

Terra Nova ◽

Wind Events ◽

Ice Production

Abstract. During katabatic wind events in the Terra Nova Bay and Ross Sea polynyas, wind speeds exceeded 20 m s−1, air temperatures were below −25 ℃, and the mixed layer extended as deep as 600 meters. Yet, upper ocean temperature and salinity profiles were not perfectly homogeneous, as would be expected with vigorous convective heat loss. Instead, the profiles revealed bulges of warm and salty water directly beneath the ocean surface and extending downwards tens of meters. Considering both the colder air above and colder water below, we suggest the increase in temperature and salinity reflects latent heat and salt release during unconsolidated frazil ice production within the upper water column. We use a simplified salt budget to analyze these anomalies to estimate in-situ frazil ice concentration between 332 × 10−3 and 24.4 × 10−3 kg m−3. Contemporaneous estimates of vertical mixing by turbulent kinetic energy dissipation reveal rapid convection in these unstable density profiles, and mixing lifetimes from 2 to 12 minutes. The corresponding median rate of ice production is 26 cm day−1 and compares well with previous empirical and model estimates. Our individual estimates of ice production up to 378 cm day−1 reveal the intensity of short-term ice production events during the windiest episodes of our occupation of Terra Nova Bay Polynya.

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Subglacial brine flow and wind-induced internal waves in Lake Bonney, Antarctica

Antarctic Science ◽

10.1017/s0954102020000036 ◽

2020 ◽

Vol 32 (3) ◽

pp. 223-237

Author(s):

Jade P. Lawrence ◽

Peter T. Doran ◽

Luke A. Winslow ◽

John C. Priscu

Keyword(s):

Water Column ◽

Internal Waves ◽

Open Water ◽

Surface Discharge ◽

Katabatic Wind ◽

Neutral Buoyancy ◽

Cold Temperatures ◽

Glacier Terminus ◽

Wind Events ◽

Taylor Glacier

AbstractBrine beneath Taylor Glacier has been proposed to enter the proglacial west lobe of Lake Bonney (WLB) as well as from Blood Falls, a surface discharge point at the Taylor Glacier terminus. The brine strongly influences the geochemistry of the water column of WLB. Year-round measurements from this study are the first to definitively identify brine intrusions from a subglacial entry point into WLB. Furthermore, we excluded input from Blood Falls by focusing on winter dynamics when the absence of an open water moat prevents surface brine entry. Due to the extremely high salinities below the chemocline in WLB, density stratification is dominated by salinity, and temperature can be used as a passive tracer. Cold brine intrusions enter WLB at the glacier face and intrude into the water column at the depth of neutral buoyancy, where they can be identified by anomalously cold temperatures at that depth. High-resolution measurements also reveal under-ice internal waves associated with katabatic wind events, a novel finding that challenges long-held assumptions about the stability of the WLB water column.

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Photoinhibition of DCMU-Enhanced Fluorescence in Lake Ontario Phytoplankton

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f87-263 ◽

1987 ◽

Vol 44 (12) ◽

pp. 2144-2154 ◽

Cited By ~ 8

Author(s):

M. Putt ◽

G. P. Harris ◽

R. L. Cuhel

Keyword(s):

Vertical Mixing ◽

Natural Populations ◽

Bright Light ◽

Euphotic Zone ◽

Lake Ontario ◽

Enhanced Fluorescence ◽

Low Light ◽

Light Levels ◽

Phosphorus Status

Measurement of 1-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) enhanced fluorescence (FDCMU) suggested that photoinhibition of photosynthesis was frequently an artifact of in situ bottle incubations in Lake Ontario phytoplankton. In a seasonal study, FDCMU of all populations was depressed by bright light in an incubator. However, when the euphotic zone did not exceed the depth of the mixed layer, vertical transport of phytoplankton into either low-light or dark regions apparently allowed reversal of photoinhibition of FDCMU. Advantages of FDCMU as a bioassay of vertical mixing include rapidity of response time, ease of measurement in the field, and insensitivity of this parameter to changes in phosphorus status of the population. Because of seasonal changes in the photoadaptive response of natural populations, the rate constants and threshold light levels required to cause the response must be determined at each use if the method is to be quantitative.

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Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1421576112 ◽

2014 ◽

Vol 112 (1) ◽

pp. E15-E20 ◽

Cited By ~ 33

Author(s):

Michael J. Ellwood ◽

David A. Hutchins ◽

Maeve C. Lohan ◽

Angela Milne ◽

Philipp Nasemann ◽

...

Keyword(s):

Stable Isotopes ◽

Mixed Layer ◽

Phytoplankton Bloom ◽

Iron Bioavailability ◽

Global Ocean ◽

Minor Role ◽

Surface Mixed Layer ◽

Spring Phytoplankton Bloom ◽

Dissolved Iron ◽

Particulate Iron

The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

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Dissolved iron (II) in the Baltic Sea surface water and implications for cyanobacterial bloom development

Biogeosciences Discussions ◽

10.5194/bgd-6-3803-2009 ◽

2009 ◽

Vol 6 (2) ◽

pp. 3803-3850 ◽

Cited By ~ 4

Author(s):

E. Breitbarth ◽

J. Gelting ◽

J. Walve ◽

L. J. Hoffmann ◽

D. R. Turner ◽

...

Keyword(s):

Baltic Sea ◽

Surface Waters ◽

Cyanobacterial Bloom ◽

Euphotic Zone ◽

Strong Wind ◽

Ferrous Ions ◽

The Baltic Sea ◽

High Concentrations ◽

Wind Events ◽

The Baltic

Abstract. Iron chemistry measurements were conducted during summer 2007 at two distinct locations in the Baltic Sea (Gotland Deep and Landsort Deep) to evaluate the role of iron for cyanobacterial bloom development in these estuarine waters. Depth profiles of Fe(II) were measured by chemiluminescent flow injection analysis (CL-FIA) and reveal several origins of Fe(II) to the water column. Photoreduction of Fe(III)-complexes and deposition by rain are main sources of Fe(II) (up to 0.9 nmol L−1) in light penetrated surface waters. Indication for organic Fe(II) complexation resulting in prolonged residence times in oxygenated water was observed. Surface dwelling heterocystous cyanobacteria where mainly responsible for Fe(II) consumption in comparison to other phytoplankton. The significant Fe(II) concentrations in surface waters apparently play a major role in cyanobacterial bloom development in the Baltic Sea and are a major contributor to the Fe requirements of diazotrophs. Second, Fe(II) concentrations up to 1.44 nmol L−1 were observed at water depths below the euphotic zone, but above the oxic anoxic interface. Finally, all Fe(III) is reduced to Fe(II) in anoxic deep water. However, only a fraction thereof is present as ferrous ions (up to 28 nmol L−1) and was detected by the CL-FIA method applied. Despite their high concentrations, it is unlikely that ferrous ions originating from sub-oxic waters could be a temporary source of bioavailable iron to the euphotic zone since mixed layer depths after strong wind events are not deep enough in summer time.

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