Using Uranium Series Radiogenic Isotopes to Trace and Quantify Chemical Fluxes in the Atlantic Ocean

目前，由于多种原因，热液微量元素流向海洋的数量受到很大限制。首先，我们只在我们的海洋中发现了一小部分热液喷口，而且这些地点中的大多数都是相对较大的、成熟的、集中的喷口地点，如TAG(跨大西洋地学导线)、彩虹或沿中大西洋山脊的失落之城。即使在已经得到广泛研究的TAG这样的地点，铁等元素通过化学、生物或颗粒清除而损失或转化的时间尺度也很少受到限制。放射性同位素为理解这种过程的时间尺度提供了一种工具。镭(Ra)以223Ra、224Ra、226Ra和228Ra四种天然放射性同位素存在于海洋中，半衰期分别为11.4d、3.66d、1600y和5.75y，在水柱中起着保守的作用。了解中性浮力羽流中的垂直或水平通量可以通过Ra同位素比率来实现。将短寿命的同位素与长寿命的同位素进行正常化，可以纠正任何Ra的清除或稀释。因此，如果在给定距离处对中性浮力羽流进行采样，并且已知端元同位素比率，则可以利用样品活度的变化来确定样品的年龄。这将提供一个可能与末端成员和样品地点之间的微量元素浓度变化有关的时间尺度(Kipp等人。2017年).海洋镭活度低得令人难以置信；因此需要大量样本。这种样品可以通过使用独立的泵送系统(SAPS)和浸渍氧化锰的聚合物纤维滤芯来获得。水将被泵过滤芯，纤维上的锰氧化物将在水通过时剥离水中的镭，将镭集中在滤芯上(Moore等人，1973年)。然后，可以使用一台镭延迟符合计数器(RaDeCC)(Moore等人，1996)来确定这个弹药筒的活度。实际上，有多个贯穿边缘的喷口，例如中大西洋海脊，以及导致喷口流体混合的物理过程。然而，镭同位素测量提供的时间尺度将是改进目前模型所产生的热液痕量元素通量估计的有力工具。

Currently the magnitude of hydrothermal trace-element flux to the oceans is poorly constrained for a number of reasons. Firstly, we have only discovered a small number of the total hydrothermal vents in our oceans and the majority of these sites are relatively large, well-established, focussed vent sites such as TAG (Trans-Atlantic Geo-traverse), Rainbow or Lost City along the Mid-Atlantic Ridge. Even at sites such as TAG, which have been studied extensively, the timescale of the loss or transformation of elements such as iron via chemical, biological or particle scavenging is poorly constrained.Radio-isotopes provide a tool to understand the timescale of such processes. Radium (Ra) is present in the ocean as four naturally occurring radioactive isotopes: 223Ra, 224Ra, 226Ra and 228Ra, with half-lives (11.4 d, 3.66 d, 1600 y and 5.75 y, respectively) and acts conservatively in the water column. Understanding vertical or horizontal fluxes within the neutrally buoyant plume could be achieved using Ra-isotope ratios. Normalising a short-lived isotope against a long-lived one corrects for any Ra scavenging or dilution. Hence if the neutrally buoyant plume is sampled at a given distance and the end-member isotope-ratio is known the sample can be aged using the change in sample activity. This would provide a timescale that could be related to changes in trace-element concentrations between the end-member and sample-site (Kipp et al. 2017).Oceanic radium activities are incredibly low; therefore large sample volumes are required. Such samples could be obtained via the use of Stand Alone Pumping Systems (SAPS) in conjunction with polymer-fibre filter cartridges, impregnated with manganese oxide. The water would be pumped through the filter cartridge and the manganese oxides on the fibres would strip radium from the water as it passes through, concentrating the radium on the cartridge (Moore et al., 1973). The activity of this cartridge could then be determined using a Radium Delayed-Coincidence Counter (RaDeCC) (Moore et al., 1996).In reality, there are multiple vents that run the length of a margin such as the Mid-Atlantic Ridge as well as physical processes that cause the mixing of vent fluids. However, the timescales provided by radium-isotope measurements would be a powerful tool in improving the hydrothermal trace-element flux estimates produced by current models.