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

浮游植物是生活在阳光照射下的海洋表面的微小植物。铁（Fe）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。有机碳沉降到深海或沉积物中，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）。海洋的大部分地区都是铁有限的，那里的铁供应不能支持健康的浮游植物和强大的生物泵。为了了解生物泵的强度，我们需要知道海洋铁的来源。铁最终来自于形成岩石的物质，这些物质可以以灰尘、河流/地下水、融化的冰川、大陆架沉积物或热液喷口的形式进入海洋。然而，测量每一种来源的铁供应速率是具有挑战性的，而且尚未受到限制。我的项目将使用镭（Ra）来确定铁的供应和去除率，以便更好地了解铁在海洋中的循环。我们对铁循环的认识有三个关键的空白：1)有多少铁来自大陆架沉积物，2)有多少铁来自冰川融水，3)铁从热液喷口富含金属的流体中被清除的速度有多快？所有这些过程对于理解铁循环至关重要。镭和铁有共同的来源（形成岩石的物质），但Ra会随着时间的推移以非常精确的速率自然衰变。这种衰变使我们能够在海洋中使用Ra作为时钟。在水源附近，一小块海水的Ra含量会很高，但随着时间的推移，这一小块海水的Ra含量会减少。通过测量Ra的衰变量，我们可以计算出这包水与源接触的时间。同时测量铁，我们可以计算出有多少铁和Ra一起被供应，以及是否有铁丢失。铁不会腐烂，但可以通过两个过程去除：浮游植物光合作用的生物吸收；当它附着在海水中的颗粒上时，它也会清除。被清除的铁随着颗粒下沉，不再被吸收。我将测量Fe和Ra，以确定从南极半岛西部大陆架到公海的铁供应，因为南大洋是世界上最大的铁有限地区。我将验证这样一个假设，即大陆架上的高铁水被输送到近海，并有可能向上混合到地表水中，为浮游植物提供铁。我还将监测该地区冰川融水中的铁和Ra。由于该地区的变暖加速了冰川融化的速度，冰川铁的供应可能正在发生变化。通过使用Ra来计算水接触沉积物的时间，我将评估融水中的铁损失的速度（要么被吸收，要么被清除）。为了比较其他地区的供应和移除率，我将对南极半岛西部和格陵兰岛的其他冰川采用相同的方法，以估计全球冰川融化的铁输入。最后，我将在大西洋中脊热液喷口附近测量铁和Ra。结合这两种元素来确定从喷口流出的流体中去除的铁，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。我的研究结果将解决我们对海洋铁循环的理解中的关键空白。提高我们对这种必需营养素的认识将有助于我们确定海洋系统对当前铁供应的敏感程度，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.