Iron in modern and ancient East Antarctic snow : implications for phytoplankton production in the Southern Ocean
Creators: name: Edwards, Ross
Publisher: name: University Of Tasmania
Year: 2023

description: 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 ‚ÄövÑvÆ4x. 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 ‚ÄövÑvÆ40%. 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¬¨‚àûS), (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 ‚ÄövÑvÆ5 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 ‚ÄövÑvÆ1% 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.