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Biogeochemistry

Take your iron! Seasonally melting snow as an iron supply to the Ross Sea, Antarctica

Article: Winton, V. H. L.; Dunbar, G. B.; Bertler, N. A. N; Millet, M. A.; Delmonte, B.; Atkins, C. B.; Chewings, J. M.; & Andersson, P.: The contribution of Aeolian sand and dust to iron fertilization of phytoplankton blooms in southwester Ross Sea, Antarctica. Global Biogeochemical Cycles, Vol. 28, pp. 423-436 doi:10.1002/2013GB004574, 2014

Background Information

RossSea

The Ross Sea, Antarctica. Picture Credit: 3 News, New Zealand

The Ross Sea is one of the most biologically productive coastal regions in Antarctica. Biologically productive means that large blooms of phytoplankton occur in this region. These blooms provide a valuable food source to this region as well as inject vast quantities of carbon dioxide to the deep ocean. Antarctica is an area where bottom water is formed, so when phytoplankton (and other ocean critters) die and sink, they take all the carbon dioxide they used during photosynthesis with them. However, the productivity of the Ross Sea is seasonally limited by iron.

Iron is an important micronutrient required for phytoplankton growth. Most of the iron in the Ross Sea comes from sand and dust blown by winds from the continents. Continental rocks, such as granite, contain iron minerals. These rocks are eroded and the global winds can carry these minerals as dust for great distances across the ocean, where they can be deposited.

Since the Ross Sea is seasonally limited in iron, it should limit the growth of phytoplankton. However, observations have shown that phytoplankton blooms occur even after the winter iron reserves have been used up! For this to happen, there must be another source of iron to the Ross Sea region.

Previous observations have shown that phytoplankton blooms occur as the seasonal snow and ice layers melt. This has led to the hypothesis that the melting snow and ice could be a secondary source of iron. The idea is this: sand and dust from distant and local sources (from the Antarctic continent) deposit iron on the winter snow and ice layers; when that snow and ice melts each spring, it releases all that accumulated iron into the water.

Winton et al., investigated the importance of melting snow on the regional productivity.

The Approach

Figure 1: Map of the sample locations and wind directions from the McMurdo Sound located in the Ross Sea. A) Location of the McMurdo Sound and East Antarctic Ice Sheet (EAIS). B) Samples taken for this study from the McMurdo Sound on a transect (the red line) moving away from known debris bands. MDV is the McMurdo Dry Valleys and TAM is the Transantarctic Mountains, both potential sources of unconsolidated sediments which contain iron. C) Wind roses displaying the wind direction of storm events at the Pegasus North and Marble Point weather stations.

Figure 1: Map of the sample locations and wind directions from the McMurdo Sound located in the Ross Sea. A) Location of the McMurdo Sound and East Antarctic Ice Sheet (EAIS). B) Samples taken for this study from the McMurdo Sound on a transect (the red line) moving away from known debris bands. MDV is the McMurdo Dry Valleys and TAM is the Transantarctic Mountains, both potential sources of unconsolidated sediments which contain iron. C) Wind roses displaying the wind direction of storm events at the Pegasus North and Marble Point weather stations.

A total of 127 surface snow samples were collected from November 2009 to November 2010 in the McMurdo Sound, which is part of the Ross Sea (Figure 1). Snow was melted at room temperature and measured for iron content and particle size. Particle size of the bulk sand and dust was measured using a Beckman-Coulter Counter.

Not all iron is bioavailable. The soluble iron, or fraction of iron that can be dissolved in water, is typically the portion called bioavailable. Think of it this way: if you threw an iron nail into a bucket of seawater, a phytoplankton could not “eat” the nail. Instead, that phytoplankton could only use the iron that leaches off the nail. So only a small portion of iron actually deposited into the ocean can be used for productivity.

Winton et al. determined the soluble fraction of the iron-containing snow by digesting it in acid, then passing it through a 0.4 µm filter. Any iron that could pass through the filter was called soluble and iron retained on the filter was called particulate. Iron was determined using inductively coupled plasma mass spectrometry. The researchers also measured a few rare Earth metal isotopes, strontium and neodymium, to help determine whether the iron was from the Antarctic continent or distant sources.

The Findings

Figure 2: The physical and chemical properties of iron-containing atmospheric dust in the McMurdo Sound. Note that the transect (X to Y) is from Figure 1 above for reference. A) The mass accumulation rate of the atmospheric dust. B) The fraction of dust that is <10 µm in diameter. C) The weight fraction of total iron. D) The percentage of soluble iron, or the iron that is available to phytoplankton. E) The concentration of the soluble iron. F) The flux of iron to the McMurdo Sound region; black dots represent all of the atmospheric sand fraction and the red dots are only the fine fraction ranging in size from 0.4 to 1.0 µm.

Figure 2: The physical and chemical properties of iron-containing atmospheric dust in the McMurdo Sound. Note that the transect (X to Y) is from Figure 1 above for reference. A) The mass accumulation rate of the atmospheric dust. B) The fraction of dust that is

Winton et al., found that iron released from melting snow could account up to 15% of the primary productivity in the McMurdo Sound. While this is a large fraction of the annual productivity, phytoplankton growth from iron deposited into the water (and not from melting snow) is still the most important iron source in the Ross Sea. A large quantity of the iron deposited on the seasonal snow was from a local Antarctic source.

The amount of dust and iron deposited on the snow decreased exponentially away from the debris bands located on the Antarctic continent (Figures 1 and 2). The iron resulting from this debris band supplied >2 orders of magnitude more dust than the global (not from Antarctica) supply. The snow sampled in this study had a dust supply ranging from 26 to ~ 1 g m-2 yr-1.

The fraction of soluble iron was ~10%; thus only about 10% of the iron on the snow was considered to be bioavailable. Surprisingly, the iron leached from the sand and dust did not depend on grain size. It was previously thought that the finer dust would supply more soluble iron but Winston et al. found that sand and dust both supplied a constant fraction of this bioavailable iron.

Significance

This study demonstrated that seasonally melting snow in the Ross Sea could supply enough iron to support up to 15% of the regional primary productivity. The iron deposited on the snow was mostly from local eroded sediments and deposited by winds. Seasonal changes in storm frequency and intensity could greatly alter the magnitude of this dust supply.

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