Radium in Changing Environments: A Novel Tracer of Iron Fluxes at Ocean Margins

浮游植物是生活在阳光充足的表层海洋中的微小植物。铁是浮游植物必需的营养物质，浮游植物利用铁进行光合作用，将二氧化碳(CO2)转化为氧气和有机物。沉入深海或沉淀物的有机碳从大气中去除二氧化碳。这被称为生物泵，是调节大气中二氧化碳的重要过程，从而影响全球气候。铁在海洋中的浓度非常低(每10亿个水分子中不到1个铁原子)。海洋的大片区域是铁有限的，那里的铁供应不支持健康的浮游植物和强大的生物泵。要了解生物泵的力量，我们需要知道海洋中铁的来源。铁最终来自造岩物质，可以作为灰尘、河流/地下水、融化的冰川、大陆架沉积物或热液喷口输送到海洋中。然而，测量来自这些来源的铁的供应速率是具有挑战性的，而且还不受限制。我的项目将使用镭(Ra)来确定铁的供应和去除的速率，以便更好地了解海洋中铁的循环。我们对铁循环的认识有三个关键缺陷：1)有多少铁来自大陆架沉积物；2)冰川融水提供了多少铁；3)从热液喷口富含金属的流体中清除铁的速度有多快？所有这些过程对于理解铁循环都是至关重要的。镭和铁有一个共同的来源(致石物质)，但Ra随着时间的推移以非常精确的速度通过自然放射性衰变。这种衰变使我们可以将Ra用作海洋中的时钟。一块海水在水源附近会有很高的Ra，但随着水块的移动，Ra的量会随着时间的推移而减少。通过测量Ra的衰变程度，我们可以计算出这块水与源头接触有多长时间。同时测量铁，我们可以计算出与Ra一起提供了多少铁，以及这些铁是否已经丢失。铁不会腐烂，但可以通过两个过程去除：浮游植物为光合作用进行的生物吸收；或当它粘在海水中的颗粒上时进行清除。我将测量Fe和Ra，以确定从南极半岛西部大陆架到开阔海洋的铁供应，因为南大洋是世界上最大的铁限制区域。我将检验这样一种假设，即来自大陆架的高铁水被输送到近海，并有可能向上混合到为浮游植物提供铁的地表水中。我还将监测这一地区冰川融化水中的铁和氡。冰川铁的供应可能正在改变，因为该地区的变暖正在加速冰川融化的速度。通过使用Ra计算自从水接触沉积物以来已经过去了多少时间，我将评估熔化水中的铁损失的速度(无论是吸收还是清除)。为了比较其他地方的供应和去除率，我将在南极半岛西部和格陵兰的其他冰川上采用同样的方法，估计全球冰川融化带来的铁的输入。最后，我将沿着中大西洋海脊测量热液喷口附近的铁和Ra。将这两种元素结合起来，以确定喷口流体漂离喷口地点时的铁的清除情况，将为评估这一来源对世界海洋中铁总量的贡献提供重要信息。我的结果将解决我们对海洋铁循环理解中的关键差距。提高我们对这种重要营养物质的认识将有助于我们确定海洋系统对当前铁供应的敏感性，以及预测铁供应变化对浮游植物健康、生物泵和全球气候的影响。

Phytoplankton are microscopic plants that live in the sunlit surface ocean. Iron (Fe) is an essential nutrient for phytoplankton, which use Fe for photosynthesis, converting carbon dioxide (CO2) to oxygen and organic matter. Organic carbon that sinks to the deep ocean or sediment removes CO2 from the atmosphere. This is called the biological pump, and is an important process regulating CO2 in the atmosphere, consequently affecting global climate.Iron is present at very low concentrations in the ocean (less that 1 Fe atom per billion water molecules). Large areas of the ocean are Fe-limited, where supply of Fe does not support healthy phytoplankton and a strong biological pump. To understand the strength of the biological pump, we need to know the sources of Fe to the ocean. Fe ultimately comes from lithogenic material, which can be delivered to the ocean as dust, in rivers/groundwater, in melting glaciers, from sediments of the continental shelf, or from hydrothermal vents. However, measuring the rate of Fe supply from each of these sources is challenging and not yet constrained.My project will use radium (Ra) to determine rates of Fe supply and removal, in order to better understand the cycling of Fe in the ocean. Three key gaps in our knowledge of the Fe cycle are: 1) how much Fe comes from continental shelf sediments, 2) how much Fe is supplied by glacial meltwater, and 3) how rapidly is Fe scavenged from the metal-rich fluids at hydrothermal vents? All of these processes are vital to understanding the Fe cycle.Radium and Fe have a common source (lithogenic material), but Ra decays over time by natural radioactivity, at a very precise rate. This decay allows us to use Ra as a clock in the ocean. A parcel of seawater will have high Ra near a source, but the amount of Ra will decrease over time as the water parcel moves away. By measuring how much Ra has decayed, we can calculate how long since the parcel of water was in contact with the source. Measuring Fe at the same time, we can calculate how much Fe was supplied along with the Ra, and if any of that Fe has been lost. Fe does not decay, but can be removed by two processes: biological uptake by phytoplankton for photosynthesis; or scavenging, when it sticks to particles in seawater. Scavenged Fe sinks along with the particle and is no longer available for uptake.I will measure Fe and Ra to determine Fe supply from the continental shelf of the western Antarctic Peninsula to the open ocean, as the Southern Ocean is the largest Fe-limited region in the world. I will test the hypothesis that high-Fe waters from the shelf are transported offshore, and have the potential to mix upwards into the surface water supplying Fe to phytoplankton. I will also monitor Fe and Ra in glacial meltwater in this region. The supply of glacial Fe may be changing, as warming in this region is accelerating rates of glacial melt. By using Ra to calculate how much time has passed since the water contacted sediment, I will assess how quickly the Fe in this meltwater is lost (either to uptake or scavenging). To compare supply and removal rates in other locations, I will follow the same approach at other glaciers on the western Antarctic Peninsula, and in Greenland, to estimate input of Fe from glacial melt globally.Finally, I will measure Fe and Ra near hydrothermal vents along the Mid-Atlantic Ridge. Combining these two elements to determine the removal of Fe from vent fluids as they drift away from vent sites will provide vital information for evaluating the contribution of this source to the total amount of Fe in the world's oceans. My results will address key gaps in our understanding of the marine Fe cycle. Improving our knowledge of this essential nutrient will help us determine how sensitive marine systems are to current Fe supply, as well as predict the impacts of changes in Fe supply on phytoplankton health, the biological pump, and global climate.