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Iron in modern and ancient East Antarctic snow : implications for phytoplankton production in the Southern Ocean

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Edwards, Ross(Peter Ross) (1999) Iron in modern and ancient East Antarctic snow : implications for phytoplankton production in the Southern Ocean. PhD thesis, University of Tasmania.

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Abstract

lion (Fe) was measured in present-day and ancient East Antarctic snow to investigate
the atmospheric flux of Fe into the Southern Ocean, the solubility of this atmospheric
iron, and the level of new phytoplankton production it could support in Southern
Ocean waters, given that iron is an essential micronutrient for algal growth. To
investigate the present-day atmospheric Fe deposition, acid-soluble total-dissolvable
Fe (TD-Fe) was measured in present-day East Antarctic snow from inland sites in
Princess Elizabeth Land and marine sites in Prydz Bay, the Dumont d'Urville Sea
and the Ross Sea. To investigate temporal variations in atmospheric Fe deposition, TD-Fe
concentrations were measured in glacial ice-core (i.e., ancient snow) samples of
Holocene, Wisconsin-Holocene transition and Last Glacial Maximum (LGM) age
from Law Dome on the coast of Wilkes Land, East Antarctica. Average TD-Fe
concentrations in modern snow from Prydz Bay, Princess Elizabeth Land and the
Ross Sea were similar, with a range of 612-749 pg Fe g -1 . Average TD-Fe
concentrations in modern snow from the Dumont d'Urville Sea were an order of
- magnitude less (62 pg Fe g 1 ), and comparable to TD-Fe concentrations in Holocene
sections of the Law Dome ice-cores. There are significant variations in the Law
Dome ice-core TD-Fe concentrations, on time scales ranging from seasonal to
glacial-interglacial. Summer TD-Fe concentrations exceed winter by —4x. Average Holocene ice TD-Fe concentration (99 pg Fe g -1 ) was much lower than that for the
Holocene-Wisconsin transition (1100 pg Fe g -1 ) and LGM (6700 pg Fe g-1 ). Soluble
Fe in modem East Antarctic snow was estimated from measurements of TDFe and
total-filterable (0.2 iim) Fe. Soluble Fe in the samples ranged from 10-90% of TDFe,
averaging —40%. Past and present-day atmospheric Fe fluxes were estimated from
average snow and ice TDFe concentrations and estimated snow accumulation rates.
Present-day and late Holocene flux estimates are in the range of 0.02 -0.10 mg Fe II1-2
yfl , with an average of 0.07 mg Fe III-2 yr-1 . The estimated LGM atmospheric Fe
flux onto Law Dome ranges from 0.86 -2.15 mg Fe In-2 yr-I , using minimum and
maximum estimates of the LGM snow-accumulation rate. This is 12-30 times the
average present-day atmospheric Fe flux and 16-41 times the average atmospheric Fe
flux estimated from the late Holocene ice-core samples.
Assuming (1) similar atmospheric Fe fluxes exist over the Southern Ocean (south of
50°S), (2) limitation of algal production in this region by Fe deficiency (i.e., nutrientand
light-replete conditions), (3) 40% of the Fe is bioavailable, and (4) an algal C:Fe
molar assimilation ratio of 33,000-500,000, then the maximum potential algal new
production supported by atmospheric Fe deposition in the present-day is estimated at
0.017-0.25 mol C n12 yfl . Assuming a Redfield algal C:N assimilation ratio and an
upwelling flux of 12 g nitrate M-2 yfl , this estimated new production rate could
consume 0.3-4% of the nitrate upwelled into surface waters of the present-day
Southern Ocean. Similar calculations using estimated LGM atmospheric Fe flux
yield potential new production of 0.2-7.9 mol C In-2 yr-I , which could consume up to 136 % of the nitrate upwelled in the present-day Southern Ocean. Estimates of the
potential increase in surface-water dissolved Fe concentration due to atmospheric Fe
released from melting annual sea ice suggest that —5 riM increases are possible in
thin (-4 m deep) meltwater lenses, increases which are probably sufficient to
alleviate algal Fe deficiency and allow bloom development. However, the potential
total annual new production supported by atmospheric Fe released from melting sea
ice is estimated as 45 Tg C, which is —1% of the estimated total annual Southern
Ocean primary production of 4414 Tg.
In summary, the results presented in this thesis are consistent with the suggestion that
the present-day atmospheric Fe flux into the Southern Ocean is insufficient to
support the use of the upwelled nitrate by phytoplankton, and also that the majority
of algal new production in this region is supported by Fe supplied from other
sources, such as upwelling and shelf sediments. However, these data indicate that
atmospheric Fe inputs may support short-lived high-production events at the edge of
retreating seasonal sea ice. These results are also consistent with the hypothesis that
the atmospheric Fe flux into the Southern Ocean was significantly greater during the
LGM, potentially supporting much greater phytoplankton new production at that
time.

Item Type: Thesis (PhD)
Keywords: Phytoplankton, Iron, Seawater
Copyright Holders: The Author
Copyright Information:

Copyright 1999 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2000. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:05
Last Modified: 11 Mar 2016 05:54
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