Dissolved and dissolvable iron concentrations and ligand characteristics in samples taken during POLARSTERN expedition ARK-XXII/2 to the Arctic Ocean

The speciation of iron was investigated in three shelf seas and three deep basins of the Arctic Ocean in 2007. The dissolved fraction (<0.2 µm) and a fraction < 1000 kDa were considered here. In addition, unfiltered samples were analyzed. Between 74 and 83% of dissolved iron was present in the fraction < 1000 kDa at all stations and depth, except at the chlorophyll maximum (42-64%). Distinct trends in iron concentrations and ligand characteristics were observed from the shelf seas toward the central deep basins, with a decrease of total dissolvable iron ([TDFe] > 3 nM on the shelves and [TDFe] < 2 nM in the Makarov Basin). A relative enrichment of particulate Fe toward the bottom was revealed at all stations, indicating Fe export toward the deep ocean. In deep waters, dissolved ligands became less saturated with Fe (increase of [Excess L]/[Fe]) from the Nansen Basin via the Amundsen Basin toward the Makarov Basin. This trend was explained by the reactivity of the ligands, higher (log alpha > 13.5) in the Nansen and Amundsen basins than in the Makarov Basin (log alpha <13) where the sources of Fe and ligands were limited. The ligands became nearly saturated with depth in the Amundsen and Nansen Basins, favoring Fe removal in the deep ocean, whereas in the deep Makarov Basin, they became unsaturated with depth. Still here scavenging occurred. Although scavenging of Fe was attenuated by the presence of unsaturated organic ligands, their low reactivity in combination with a lack of sources of Fe in the Makarov Basin might be the reason of a net export of Fe to the sediment.

2007年，研究人员针对北冰洋的3个陆架海与3个深海盆地开展了铁形态分析（speciation of iron）。本研究考察了溶解态组分（<0.2 µm）以及<1000千道尔顿（kDa）的粒径组分，同时对未过滤样品进行了分析测试。除叶绿素最大层（占比42%~64%）外，所有站位与观测深度下，74%~83%的溶解态铁均赋存于<1000千道尔顿的组分中。从陆架海向中央深海盆地，铁浓度与配体（ligand）特征呈现出显著的演变趋势：陆架区总可溶铁（total dissolvable iron, [TDFe]）浓度大于3纳摩尔（nM），而马卡罗夫海盆（Makarov Basin）的总可溶铁浓度则低于2纳摩尔。所有站位均观测到颗粒态铁（particulate Fe）向底层水体相对富集的现象，表明铁向深海圈层发生输出。在深水区，溶解态配体与铁的结合饱和度从南森海盆（Nansen Basin）经阿蒙森海盆（Amundsen Basin）向马卡罗夫海盆逐渐降低，具体表现为[过剩配体]/[铁]（[Excess L]/[Fe]）比值升高。这一趋势可通过配体的反应活性得到合理解释：南森海盆与阿蒙森海盆的配体反应活性更高（log α >13.5），而马卡罗夫海盆中铁与配体的来源有限，配体反应活性较低（log α <13）。在阿蒙森海盆与南森海盆中，配体随深度增加逐渐趋于铁饱和，这一条件有利于铁在深海中发生清除作用（scavenging）；而在马卡罗夫深海盆中，配体随深度增加仍处于不饱和状态，但此处仍发生了铁的清除过程。尽管不饱和有机配体的存在会削弱铁的清除作用，但马卡罗夫海盆配体较低的反应活性与铁来源的匮乏，可能是铁净输出至海底沉积物的核心原因。