Aeolian Iron and Its Contribution to Phytoplankton Production in McMurdo Sound, Southwest Ross Sea, Antarctica

10.26686/wgtn.16997761.v1 ◽

2021 ◽

Author(s):

◽

Victoria Holly Liberty Winton

Keyword(s):

Particle Size ◽

Sea Ice ◽

Ross Sea ◽

Accumulation Rate ◽

Phytoplankton Growth ◽

Ice Shelf ◽

Phytoplankton Productivity ◽

Mcmurdo Sound ◽

Mass Accumulation Rate ◽

Mass Accumulation

Each summer the waters in McMurdo Sound (Lat. 77.5ºS; Long. 165ºE), south-western (SW) Ross Sea encounter vast phytoplankton blooms. This phenomenon is stimulated by the addition of bio-available iron (Fe) to an environment where phytoplankton growth is otherwise Fe-limited. One possible source of such Fe is aeolian sand and dust (ASD) which accumulates on sea ice and is released into the ocean during the summer melt season. The amount of bio-available Fe (i.e. the amount of Fe immedately accessible to phytoplankton) potentially supplied to the ocean by ASD depends on a number of factors including; the ASD flux into the ocean, its particle size distribution and Fe content. However, none of these parameters are well constrained in the SW Ross Sea region and, as a result, the significance of this Fe source in the biogeochemical cycle of phytoplankton growth remains to be quantified. This study focuses on an area (7400 km²) of Southern McMurdo Sound, one of the few areas where direct sampling of ASD that has accumulated on sea ice is possible. To evaluate the flux and solubility of Fe contained in ASD into McMurdo Sound, the mass accumulation rate and particle size of 70 surface snow samples and 3 shallow (3 m) firn cores from the nearby McMurdo Ice Shelf covering the period 2000 - 2008 have been analysed. Selected samples were also measured for total and soluble Fe, Sr and Nd isotopic ratios and mineralogy as a guide to Fe-fertilisation potential and provenance, respectively. Mass and particle size data show an exponential decrease in mass accumulation rate (from 26.00 g m⁻² yr⁻¹ to 0.70 g m⁻² yr⁻¹) and a decrease in modal particle size (from 130 to 69 μm) over a distance of 120 km from Southern McMurdo Sound northwards to Granite Harbour. Both these trends are consistent with ASD being dispersed northwards across the sea ice by southerly storms from an area of the McMurdo Ice Shelf, where submarine freezing and surface ablation have resulted in a surface covered with debris from the sea floor, known as the 'dirty ice' or 'debris bands' (Lat. 77.929ºS; Long. 165.505ºE) in Southern McMurdo Sound. This assertion is further supported by the Sr and Nd isotopic signature of ASD matching local source rocks and the presence of vesicular glass of Southern McMurdo Sound in all samples which also points to the debris bands as the origin of ASD in McMurdo Sound. Bio-available Fe is extremely difficult to quantify hence Fe solubility was used as an approximation in this thesis. Analysis of both total (i.e. particulate and soluble) and the percentage of soluble Fe in the 0.4 - 10 μm dust size fraction (i.e. the fraction most likely to become bio-available) by solution ICP-MS shows a narrow range of values; 3.84 ± 1.99 wt % and 9.42 ± 0.70 % respectively. Combining these values with mass accumulation rate estimates for the particles 0.4 - 10 μm in size, gives an annual soluble Fe flux for the region 500 km² north of the debris bands in McMurdo Sound of 0.55 mg m⁻² yr⁻¹ (9.89 μmol m⁻² yr⁻¹), with spatial variability largely determined by differences in mass accumulation rate. These fluxes are at least an order of magnitude greater than predicted in global dust deposition models for the Southern Ocean and measured in snow samples from East Antarctica. Furthermore, these values exceed the Fe threshold, estimated as 0.2 nM (Boyd and Abraham, 2001), required for phytoplankton growth following the simple dust-biota model of Boyd et al. (2010) and assuming the release of captured ASD in snow is instantaneous. Whilst not constrained in the present study, ASD sourced from the debris bands may be sufficiently widely dispersed, particularly during storm years, to contribute to Fe-fertilisation up to 1200 km from Southern McMurdo Sound. Short, ~10 year long, firn core records of mass accumulation and methylsuphonate concentration, a proxy for phytoplankton productivity, shows a close correspondence between the two during particularly stormy years. Whilst not demonstrating a cause-and-effect relationship, this observation suggests coastal ice cores may contain an important record of the interplay between climate, dust supply, Fe-fertilisation of near shore waters and phytoplankton productivity on decadal and longer timescales.

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Sedimentology and numerical modelling of aeolian sediment dispersal, McMurdo Sound, southwest Ross Sea, Antarctica

10.26686/wgtn.17005975 ◽

2021 ◽

Author(s):

◽

Jane Margaret Chewings

Keyword(s):

Sea Ice ◽

Ross Sea ◽

Transport Model ◽

Accumulation Rate ◽

Fine Sand ◽

Mcmurdo Sound ◽

Sediment Distribution ◽

Sediment Dispersal ◽

Wind Events ◽

Aeolian Sediment

Large volumes of aeolian sand and dust are deflated from unconsolidated till deposits, and supraglacial debris surrounding McMurdo Sound, Antarctica. This material is transported offshore with windblown snow onto extensive winter-formed sea ice in the southwest Ross Sea, and is subsequently released into the water-column during summer sea ice breakup. Aeolian sediment samples were collected from a ~600 km² area of sea ice in western McMurdo Sound to determine the magnitude of deposition and identify sediment sources. A new 2-dimensional numerical aeolian sediment transport model (NaMASTE) tuned specifically for the McMurdo Sound area, was used to explore the ability of the local wind system to move sediment from source areas to sea ice and to determine the pattern and extent of aeolian sediment dispersal to the southwest Ross Sea. Debris deposits on the McMurdo Ice Shelf debris bands are the most dominant sediment source for the area. Unconsolidated deposits between Cape Bernacchi and Spike Cape, and the Taylor Valley mouth are significant secondary deposits. Mass accumulation rates varied between 0.15 g m⁻² y⁻¹ and 54.6 g m⁻² y⁻¹, equating to a background aeolian sediment accumulation rate, excluding extremely high values, of 1.14 ± 0.59 g m⁻² y⁻¹ for the McMurdo Sound coastal sea ice zone. This is 3–5 orders of magnitude more than global background dust fallout for the Ross Sea. Modal grain size is very-fine sand to coarse silt. Notably, much of this material is distributed in localised, high sand content plumes that are oriented downwind from source, with finer deposits found outside these zones. An average seafloor linear sedimentation rate of 0.2 cm ky⁻¹ is calculated for McMurdo Sound, which is minor compared to biogenic sedimentation for the region. This equates to ~0.7 Gg y⁻¹ aeolian sediment entering McMurdo Sound during sea ice melt. Application of NaMASTE successfully simulated the general aeolian sediment distribution pattern. Testing of model variables suggests that aeolian material is mainly transported during strong (>20 m s⁻¹) wind events. Modelling also suggests aeolian material from McMurdo Sound can be transported north to the Drygalski Ice Tongue, ~250 km from source, but only in very trace quantities.

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Sedimentology and numerical modelling of aeolian sediment dispersal, McMurdo Sound, southwest Ross Sea, Antarctica

10.26686/wgtn.17005975.v1 ◽

2021 ◽

Author(s):

◽

Jane Margaret Chewings

Keyword(s):

Sea Ice ◽

Ross Sea ◽

Transport Model ◽

Accumulation Rate ◽

Fine Sand ◽

Mcmurdo Sound ◽

Sediment Distribution ◽

Sediment Dispersal ◽

Wind Events ◽

Aeolian Sediment

Large volumes of aeolian sand and dust are deflated from unconsolidated till deposits, and supraglacial debris surrounding McMurdo Sound, Antarctica. This material is transported offshore with windblown snow onto extensive winter-formed sea ice in the southwest Ross Sea, and is subsequently released into the water-column during summer sea ice breakup. Aeolian sediment samples were collected from a ~600 km² area of sea ice in western McMurdo Sound to determine the magnitude of deposition and identify sediment sources. A new 2-dimensional numerical aeolian sediment transport model (NaMASTE) tuned specifically for the McMurdo Sound area, was used to explore the ability of the local wind system to move sediment from source areas to sea ice and to determine the pattern and extent of aeolian sediment dispersal to the southwest Ross Sea. Debris deposits on the McMurdo Ice Shelf debris bands are the most dominant sediment source for the area. Unconsolidated deposits between Cape Bernacchi and Spike Cape, and the Taylor Valley mouth are significant secondary deposits. Mass accumulation rates varied between 0.15 g m⁻² y⁻¹ and 54.6 g m⁻² y⁻¹, equating to a background aeolian sediment accumulation rate, excluding extremely high values, of 1.14 ± 0.59 g m⁻² y⁻¹ for the McMurdo Sound coastal sea ice zone. This is 3–5 orders of magnitude more than global background dust fallout for the Ross Sea. Modal grain size is very-fine sand to coarse silt. Notably, much of this material is distributed in localised, high sand content plumes that are oriented downwind from source, with finer deposits found outside these zones. An average seafloor linear sedimentation rate of 0.2 cm ky⁻¹ is calculated for McMurdo Sound, which is minor compared to biogenic sedimentation for the region. This equates to ~0.7 Gg y⁻¹ aeolian sediment entering McMurdo Sound during sea ice melt. Application of NaMASTE successfully simulated the general aeolian sediment distribution pattern. Testing of model variables suggests that aeolian material is mainly transported during strong (>20 m s⁻¹) wind events. Modelling also suggests aeolian material from McMurdo Sound can be transported north to the Drygalski Ice Tongue, ~250 km from source, but only in very trace quantities.

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Modeling the effect of Ross Ice Shelf melting on the Southern Ocean in quasi-equilibrium

The Cryosphere ◽

10.5194/tc-12-3033-2018 ◽

2018 ◽

Vol 12 (9) ◽

pp. 3033-3044 ◽

Cited By ~ 1

Author(s):

Xiying Liu

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Ross Sea ◽

Global Ocean ◽

Freshwater Flux ◽

Quasi Equilibrium ◽

Ice Shelf ◽

Sea Ice Concentration ◽

Ross Ice Shelf ◽

Ice Concentration

Abstract. To study the influence of basal melting of the Ross Ice Shelf (BMRIS) on the Southern Ocean (ocean southward of 35∘ S) in quasi-equilibrium, numerical experiments with and without the BMRIS effect were performed using a global ocean–sea ice–ice shelf coupled model. In both experiments, the model started from a state of quasi-equilibrium ocean and was integrated for 500 years forced by CORE (Coordinated Ocean-ice Reference Experiment) normal-year atmospheric fields. The simulation results of the last 100 years were analyzed. The melt rate averaged over the entire Ross Ice Shelf is 0.25 m a−1, which is associated with a freshwater flux of 3.15 mSv (1 mSv = 103 m3 s−1). The extra freshwater flux decreases the salinity in the region from 1500 m depth to the sea floor in the southern Pacific and Indian oceans, with a maximum difference of nearly 0.005 PSU in the Pacific Ocean. Conversely, the effect of concurrent heat flux is mainly confined to the middle depth layer (approximately 1500 to 3000 m). The decreased density due to the BMRIS effect, together with the influence of ocean topography, creates local differences in circulation in the Ross Sea and nearby waters. Through advection by the Antarctic Circumpolar Current, the flux difference from BMRIS gives rise to an increase of sea ice thickness and sea ice concentration in the Ross Sea adjacent to the coast and ocean water to the east. Warm advection and accumulation of warm water associated with differences in local circulation decrease sea ice concentration on the margins of sea ice cover adjacent to open water in the Ross Sea in September. The decreased water density weakens the subpolar cell as well as the lower cell in the global residual meridional overturning circulation (MOC). Moreover, we observe accompanying reduced southward meridional heat transport at most latitudes of the Southern Ocean.

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Impact of the B-15 iceberg “stranding event” on the physical and biological properties of sea ice in McMurdo Sound, Ross Sea, Antarctica

Antarctic Science ◽

10.1017/s0954102008001284 ◽

2008 ◽

Vol 20 (6) ◽

pp. 593-604 ◽

Cited By ~ 21

Author(s):

J.-P. Remy ◽

S. Becquevort ◽

T.G. Haskell ◽

J.-L. Tison

Keyword(s):

Sea Ice ◽

Ross Sea ◽

Ice Cores ◽

Ice Cover ◽

Biological Properties ◽

Trophic Relationships ◽

Oceanic Circulation ◽

Mcmurdo Sound ◽

Algae Biomass ◽

Second Year

AbstractIce cores were sampled at four stations in McMurdo Sound (Ross Sea) between 1999 and 2003. At the beginning of year 2000, a very large iceberg (B-15) detached itself from the Ross Ice Shelf and stranded at the entrance of the Sound, preventing the usual oceanic circulation purging of the annual sea ice cover from this area. Ice textural studies showed that a second year sea ice cover was built-up at three out of the four stations: ice thickness increased to about 3 m. Repeated alternation of columnar and platelet ice appeared, and bulk salinity showed a strong decrease, principally in the upper part of the ice sheet, with associated brine volume decrease. Physical modification influenced the biology as well. By decreasing the light and space available for organisms in the sea ice cover, the stranding of B-15 has i) hampered autotrophic productivity, with chlorophyllaconcentration and algae biomass significantly lower for second year ice stations, and ii) affected trophic relationships such as the bacterial biomass/chlaconcentration correlation, or the autotrophic to heterotrophic ratio.

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Evolution of supercooling under coastal Antarctic sea ice during winter

Antarctic Science ◽

10.1017/s0954102011000265 ◽

2011 ◽

Vol 23 (4) ◽

pp. 399-409 ◽

Cited By ~ 30

Author(s):

Gregory H. Leonard ◽

Patricia J. Langhorne ◽

Michael J.M. Williams ◽

Ross Vennell ◽

Craig R. Purdie ◽

...

Keyword(s):

Sea Ice ◽

Freezing Point ◽

Circulation Pattern ◽

Late Winter ◽

Ice Shelf ◽

Mcmurdo Sound ◽

The North ◽

Antarctic Sea Ice ◽

Ice Production

AbstractHere we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m deep with a maximum supercooling of c. 25 mK located 1–2 m below the sea ice-water interface. Transitory in situ supercooling events were also observed, one lasting c. 17 hours and reaching a depth of 70 m. In spite of these very low temperatures the isotopic composition of the water was relatively heavy, suggesting little glacial melt. Further, the water's temperature-salinity signature indicates contributions to water mass properties from High Salinity Shelf Water produced in areas of high sea ice production to the north of McMurdo Sound. Our measurements imply the existence of a heat sink beneath the supercooled layer that extracts heat from the ocean to thicken and cool this layer and contributes to the thickness of the sea ice cover. This sink is linked to the circulation pattern of the McMurdo Sound.

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Vertical mixing, critical depths, and phytoplankton growth in the Ross Sea

ICES Journal of Marine Science ◽

10.1093/icesjms/fsu234 ◽

2014 ◽

Vol 72 (6) ◽

pp. 1952-1960 ◽

Cited By ~ 19

Author(s):

Walker O. Smith ◽

Randolph M. Jones

Keyword(s):

Water Column ◽

Mixed Layer ◽

Phytoplankton Biomass ◽

Ross Sea ◽

Vertical Mixing ◽

Biomass Accumulation ◽

Phytoplankton Growth ◽

Ice Shelf ◽

Ross Ice Shelf ◽

Mixed Layers

Abstract Phytoplankton growth and biomass accumulation vary spatially and temporally in the Ross Sea, largely as a function of ice concentrations, vertical mixing depths, and iron concentrations. To assess the role of vertical mixing in bloom initiation, we used a high-resolution numerical model to estimate changes in mixed layer depths from October 1 through early December, the period where phytoplankton growth begins and biomass accumulates, and estimate critical depths for this period. Mixed layers in October ranged from the complete water column (>600 m) to ca. 200 m; over a 60-day period, the mixed layers decreased on average by 70%. Estimated critical depths were exceeded in October, but would allow growth to proceed in late October due to shoaling of mixed layer depths, consistent with the known onset of the spring bloom in the Ross Sea. We also analysed a series of stations sampled near the Ross Ice Shelf during January 2012. Mean vertical profiles for the stations indicated deep vertical mixing; mixed layer depths averaged 60 m and ranged up to 96 m. Chlorophyll concentrations within the mixed layer averaged 6.60 µg l−1, and the pigment contributions were dominated by Phaeocystis antarctica. We suggest that this mesoscale region near the ice shelf is elevated in phytoplankton biomass due to frequent mixing events that redistribute biomass to depth and replenish nutrients, which in turn are utilized by an assemblage capable of utilizing low mean irradiance levels. Thus, the deep mixed layers and high biomass concentrations represent growth over long periods under reduced mixing punctuated by short periods of deeper vertical mixing that redistribute biomass. Water column vertical mixing and phytoplankton biomass in the Ross Sea are consistent with the critical depth concept as originally proposed by Sverdrup.

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Late Pleistocene Paleoclimatology of the Central Equatorial Pacific: A Quantitative Record of Eolian and Carbonate Deposition

Quaternary Research ◽

10.1016/0033-5894(87)90001-9 ◽

1987 ◽

Vol 28 (3) ◽

pp. 323-339 ◽

Cited By ~ 86

Author(s):

John M. Chuey ◽

David K. Rea ◽

Nicklas G. Pisias

Keyword(s):

Atmospheric Circulation ◽

Accumulation Rate ◽

Source Region ◽

Marked Change ◽

Equatorial Pacific ◽

Sedimentation Rates ◽

Glacial Cycle ◽

The Past ◽

Mass Accumulation Rate ◽

Mass Accumulation

AbstractDetailed records of δ18O, δ13C, percentage and mass accumulation rate of CaCO3, and eolian percentage, mass accumulation rate, and grainsize generated for core RC11-210 from the equatorial Pacific reveal the timing of paleoclimatic events over the past 950,000 yr. The CaCO3 percentage record shows the standard Pacific correlation of high CaCO3 content with glacial periods, but displays a marked change of character about 490,000 yr ago with older stages showing much less variability. The carbonate mass flux record, however, does not show such a noticeable change. Sedimentation rates vary from about 0.5 to 3.0 cm/1000 yr and, during the past 490,000 yr, sections with enhanced sedimentation rates correspond to periods of high CaCO3 percentage. Eolian mass accumulation rates, an indication of the aridity of the source region, are usually higher during glacial times. Eolian grainsize, an indication of the intensity of atmospheric circulation, generally fluctuates at a higher frequency than the 100,000-yr glacial cycle. The mid-Brunhes climatic event centered at 300,000 yr ago appears as a 50,000-yr interval of low intensity and reduced variability of atmospheric circulation. Furthermore, the nature of this entire record changes then, with the younger portion indicating less variation in wind intensity than the older part of the record. The late Matuyama increase in amplitude of paleoclimatic signals begins 875,000 yr ago in the eolian record, 25,000 yr before the δ18O and CaCO3 percentage amplitude increases about 850,000 yr ago.

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