Multiple sources of soluble atmospheric iron to Antarctic waters

Abstract

The Ross Sea, Antarctica, is a highly productive region of the Southern Ocean. Significant new sources of iron (Fe) are required to sustain phytoplankton blooms in the austral summer. Atmospheric deposition is one potential source. The fractional solubility of Fe is an important variable determining Fe availability for biological uptake. To constrain aerosol Fe inputs to the Ross Sea region, fractional solubility of Fe was analyzed in a snow pit from Roosevelt Island, eastern Ross Sea. In addition, aluminum, dust, and refractory black carbon (rBC) concentrations were analyzed, to determine the contribution of mineral dust and combustion sources to the supply of aerosol Fe. We estimate exceptionally high dissolved Fe (dFe) flux of 1.2 × 10 À6 g m À2 y À1 and total dissolvable Fe flux of 140 × 10 À6 g m À2 y À1 for 2011/2012. Deposition of dust, Fe, Al, and rBC occurs primarily during spring-summer. The observed background fractional Fe solubility of~0.7% is consistent with a mineral dust source. Radiogenic isotopic ratios and particle size distribution of dust indicates that the site is influenced by local and remote sources. In 2011/2012 summer, relatively high dFe concentrations paralleled both mineral dust and rBC deposition. Around half of the annual aerosol Fe deposition occurred in the austral summer phytoplankton growth season; however, the fractional Fe solubility was low. Our results suggest that the seasonality of dFe deposition can vary and should be considered on longer glacial-interglacial timescales.

Do dust emissions from sparsely vegetated regions dominate atmospheric iron supply to the Southern Ocean?

“…Our improved model results indicate significantly larger Fe input from the dust sources, especially Patagonian dust, to the Southern Ocean in summer by more than a factor of 2, compared to the conventional dust emission scheme (Figure b). Consequently, the dust is the major source of atmospheric soluble Fe to the Southern Ocean in summer, which is consistent with the seasonality measured in Antarctica [ Winton et al, ] (Figure ).…”

“…Our model estimated lower Fe solubility deposited to the Southern Ocean from dust (<2%) than that from biomass burning aerosols (>10%), because of slower Fe dissolution for dust aerosols (Figure ). This is also consistent with the observed background fractional Fe solubility of ~0.7% from mineral dust sources [ Winton et al , ]. In contrast, high Fe solubility (18%) is observed for aerosols influenced by fires over the Southern Ocean [ Bowie et al , ].…”

“…In contrast, high Fe solubility (18%) is observed for aerosols influenced by fires over the Southern Ocean [ Bowie et al , ]. Therefore, the Fe‐containing aerosols affected by fires may be associated with sporadic high Fe solubility, which was measured in Antarctica [ Conway et al , ; Winton et al , ]. Further investigation of the processes of enhanced Fe solubility over the Southern Ocean is needed to improve our understanding of bioavailable Fe supply from sparsely vegetated regions to the oceans and their effects on the marine ecosystems.…”

“…Biological productivity in high-nutrient, low-chlorophyll (HNLC) regions such as the Southern Ocean is often limited by iron (Fe) scarcity [Martin et al, 1990;Jickells and Moore, 2015]. Consequently, atmospheric deposition of bioavailable Fe from arid and semiarid regions might modulate primary marine productivity and thus oceanic carbon uptake in these regions during summer [Boyd et al, 2010;Conway et al, 2015;Winton et al, 2016]. However, significant uncertainties remain regarding the magnitude of the dust emissions and thus the effect of dust deposition on the oceans, especially in the Southern Hemisphere (SH) [Shao et al, 2011;Schulz et al, 2012;Hajima et al, 2014].…”

Combined Effects of Atmospheric and Seafloor Iron Fluxes to the Glacial Ocean

“…Our estimated LGM surface soluble iron fluxes are higher than PI in parts of the SO, to the east of the Patagonian region and around continents (Figure ). Modern measurements of atmospheric dust concentrations and solubility have high uncertainties (Sholkovitz et al, ; Winton et al, ), so the real LGM surface soluble iron fluxes may have been very different from what we estimate. In order to consider this uncertainty we perform an additional sensitivity experiment where LGM soluble flux is multiplied by 10 south of 35 ∘ S. This estimate is in line with measurements from Antarctic ice cores, where LGM soluble iron fluxes were found to be of the order of ≥10 times larger than PI values (Conway et al, ).…”