Southern Ocean Iron Fertilization near Islands: Fe-Isotope Exchange between Reactive Particles and Seawater

岛屿对海洋铁的施肥是控制海洋浮游植物生产和大气CO2吸收的重要过程。这一过程通常在南大洋中观察到，那里的岛屿上的冰川风化被认为提供了过量的生物活性颗粒铁（pFe）。冰川风化是对全球变暖特别敏感的一个方面。铁的成岩循环和陆架沉积物的再悬浮是在全球海洋中观测到的另一种岛屿海洋施肥机制。我们之前在南极半岛乔治王岛的研究表明，铁同位素可以区分两种来源。然而，整个南大洋的溶解铁（dFe）的铁同位素数据在不同地点之间相差约2‰（δ56Fe）。这表明，要么是局部不同的来源组合，要么是区域不同的铁循环过程，dFe和pFe阶段之间的相互作用高度可变，要么是水柱中区域非常动态的生物铁循环，或者是这三个方面的可变组合。在即将到来的FS Polarstern巡航（PS-133）中，我建议使用Fe同位素追踪两种来源类型的颗粒Fe，以及它们从南乔治亚海岸到公海高生产力区向生物活性dFe的转化，并在dFe-pFe同位素分异的基础上确定Fe施肥过程中dFe和pFe之间的溶解和交换过程。我试图通过一系列实验室和船上实验来确定基于过程的铁同位素分馏因子，这些因子在海洋环境中通常很少受到限制。交换实验包括将纯氢氧化铁和锰氧化物矿物相与人工海水混合，以及将来自岛架和斜坡的经横流过滤富集的天然海洋颗粒与来自岛上游经横流过滤的无颗粒海水混合。同样，上述矿物相将与过滤后的南大洋海水混合。所有的交换实验都使用了同位素尖刺水、长时间间隔的时间控制采样以及三同位素方法，即使交换反应没有进行到不完成，也可以准确地推断出同位素分馏因子。这些实验分馏因子是鉴定自然施肥过程中固有的分子水平dFe-pFe交换反应的基础。我期望在PS-133巡航期间观察和取样从沿海到公海的铁施肥过程。在乔治国王岛附近的沿海和沿海海域进行的早期活动中获得的额外样本将被分析以进行比较，那里的沉积物源端成员的同位素组成已经已知。

Ocean Fe-fertilization by islands is an important process governing marine phytoplankton production and atmospheric CO2 drawdown. The process is commonly observed in the Southern Ocean, where glacial weathering on islands is believed to supply excess bio-reactive particulate Fe (pFe). Glacial weathering is one aspect particularly sensitive to global warming. Diagenetic recycling of Fe and resuspension from the shelf’s sediments is another mechanism of island ocean fertilization that can be observed throughout the global ocean. Our previous work on King George Island – Antarctic Peninsula – has shown that Fe-isotopes allow for a discrimination between the two sources. However, Fe-isotope data in dissolved Fe (dFe) from throughout the Southern Ocean vary by about 2‰ (δ56Fe) between sites. This suggest either locally different combinations of sources, regionally diverse processes of Fe-cycling with highly variable interaction between dFe and pFe phases, a regionally very dynamic biologic Fe-cycle in the water column, or a variable combination of all three aspects. I propose to use Fe-isotopes to trace the two source types of particulate Fe, and their transformation into bio-reactive dFe from the coast of South Georgia into the open ocean high productivity region during an upcoming FS Polarstern cruise (PS-133), and determine the dissolution and exchange processes between dFe and pFe during Fe-fertilization on the basis of dFe-pFe isotope fractionation. I seek to determine underlying process-based Fe-isotope fractionation factors – which are generally poorly constrained in the marine environment – by a series of laboratory and on-board experiments. Exchange experiments comprise mixtures of pure Fe-oxyhydroxide and Mn-oxide mineral phases with artificial seawater, and mixtures of natural marine particles from the island shelf and slope enriched by crossflow-filtration with crossflow-filtered particle-free seawater from upstream of the island. Likewise, the above mineral phases will be mixed with filtered Southern Ocean seawater. All exchange experiments utilize isotope-spiked water, time-controlled sampling over long intervals, and the three-isotope method that allows for an accurate extrapolation of isotope fractionation factors even when the exchange reaction does not run to incompletion. These experimental fractionation factors serve as the basis for identification of molecular-level dFe-pFe exchange reactions inherent to the natural fertilization process. I anticipate to observe and sample the Fe-fertilization process from the littoral to the open ocean during the PS-133 cruise. Additional samples from an earlier campaign to the littoral and coastal sea off King Georg Island, where isotopic compositions of sediment source endmembers are already known, will be analyzed for comparison.