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

海岛施铁是控制海洋浮游植物生产和大气CO2排放的重要过程。这一过程通常在南大洋观察到，在那里，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岛屿海洋肥化的另一种机制，可以在整个全球海洋中观察到。我们之前在乔治王岛--南极半岛--所做的工作表明，铁同位素允许区分这两个来源。然而，整个南大洋的溶解铁(DFE)中的铁同位素数据在不同地点之间相差约2‰(δ(56Fe)。这表明，要么是局部不同来源的组合，要么是DFE和PFe相之间高度可变相互作用的区域不同的铁循环过程，要么是区域非常动态的水柱生物铁循环，要么是所有这三个方面的可变组合。我建议使用Fe同位素来追踪颗粒态Fe的两种来源类型，以及它们在即将到来的FS Polarstein巡航(PS-133)期间从南乔治亚州海岸到公海高生产力区域的生物活性DFE的转化，并在DFE-PFe同位素分馏的基础上确定DFE和PFE在铁肥过程中的溶解和交换过程。我试图通过一系列实验室和船上实验来确定潜在的基于过程的铁同位素分馏系数--这在海洋环境中通常很难受到约束。交换实验包括纯氢氧化铁和氧化锰矿物相与人工海水的混合物，以及来自岛屿陆架和斜坡的天然海洋颗粒与来自岛屿上游的横流过滤的无颗粒海水的混合物。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掺入同位素的水，长时间间隔的时间控制采样，以及三同位素方法，即使在交换反应没有完成的情况下，也可以准确地外推同位素分馏系数。这些实验分馏系数为鉴定自然受精过程中固有的分子水平的DFE-PFE交换反应提供了基础。我期待着在PS-133巡航期间观察和采样从沿海到远洋的铁肥过程。将对早先一次活动中的其他样本进行分析，以进行比较。这些样本来自乔治王岛附近的沿海和沿海海域，那里的沉淀物来源端元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.