A New Method for Assessing the Magnitude and Impact of Shallow Seawater/Pore water Exchange in Salt Marsh Systems

潮汐沼泽是海洋和陆地化学过程之间的关键交汇点。潮汐沼泽沉积物所经历的化学和物理过程决定了颗粒有机碳(POC)在海陆界面的转化和积累的性质。孔隙水通过沼泽沉积物的物理传输极大地影响碳和养分的可获得性，这些碳和营养物质可以在海洋-陆地界面上发生转化反应。过去的研究表明，预测的孔隙水流动速度与示踪剂研究的计算估计值之间存在差异。由于这种流动对陆地-海洋界面沉积物的化学成分至关重要，而陆地-海洋界面是输出到海洋的化学物种的第一个交界点，因此解释预期流速和观测流速之间的差异是很重要的。这项研究将使用已知的基于天然同位素比率(224Radn/228Th)的示踪方法，并将开发一种新的方法(223Radon/227Atdium)来评估潮汐沼泽沉积物中孔隙水流动导致的POC输出，这将对整个海洋碳收支产生更广泛的影响。这项工作的成果还将被纳入讲习班，对海洋学以外的学生和教师进行关于海洋碳和气候研究的教育。还将与南卡罗来纳州博物馆合作制作演示文稿，作为科学咖啡馆系列的一部分。此外，一名研究生和一名本科生将得到这项基金的资助。非常细粒的潮汐沼泽沉积物中孔隙水的预测平流率已被证明与使用示踪剂研究计算的平流率有显著差异。由于潮汐沼泽是海陆界面上的重要化学联系，而平流作用对在这一界面上进行成岩反应的化学物种的可用性有重要影响，因此评估这种差异具有重要意义。颗粒有机碳(POC)和溶解有机碳(DIC)的出口尤其令人感兴趣，因为这对沉积物中的微生物群落有影响。本研究旨在验证224Ra/228th不平衡方法计算孔隙水交换率的有效性。以前使用这种方法的研究已经提供了证据，证明在这种细粒沉积物中比通常预期的孔隙水交换更快，因为这种细粒沉积物的渗透性比较粗颗粒的沉积物要差。此外，本研究还将尝试发展223Ra/227Ac的不平衡方法。研究人员将评估平流率对盐沼化学物种出口的影响。

Tidal marshes are critical junctions between marine and terrestrial chemical processes. The chemical and physical processes that tidal marsh sediments undergo determine the nature of the transformation and accumulation of particulate organic carbon (POC) at the marine to terrestrial interface. Physical transport of pore water through marsh sediments greatly influences the availability of carbon and nutrients that can undergo transformative reactions at the marine to terrestrial interface. Past studies have shown that there is a discrepancy between predicted rates of pore water flow and calculated estimates from tracer studies. Because this flow is crucial to the chemistry of sediment at the terrestrial to marine interface, which is the first junction of the chemical species exported to the ocean, it is important to explain the discrepancy between expected rates of flow and observed rates. This study will use a known tracer method based on natural isotopic ratios (224Radon/228Thorium) and will develop a new method (223Radon/ 227Actinium) to evaluate the export of POC due to pore water flow in tidal marsh sediments, which will have much broader impacts to the overall marine carbon budget. Results from this work will also be integrated into workshops that will educate students and teachers from outside of oceanography about oceanographic carbon and climate research. Presentations will also be developed in collaboration with the South Carolina State Museum as part of the Science Café series. Additionally, one graduate student and one undergraduate student will be supported by this funding.Predicted advection rates of pore water in very fine-grained tidal marsh sediments have been shown to significantly differ from rates calculated using tracer studies. Because tidal marshes are a crucial chemical link at the marine terrestrial interface, and advection has a significant impact on the availability of chemical species to undergo diagenetic reactions at this interface, it is of significant importance to evaluate this discrepancy. Export of particulate organic carbon (POC) and dissolved organic carbon (DIC), in particular, are of interest, due to the impact on microbial communities within sediments. This research aims to validate the method of 224Ra/228Th disequilibrium to calculate pore water exchange rates. Previous studies using this method have provided evidence for more rapid pore water exchange than would generally be expected in such fine-grained sediments, which are less permeable than coarser grained sediment. Additionally, this research will attempt to develop the disequilibrium method for 223Ra/227Ac. The researchers will evaluate the impact of advection rates on the export of chemical species in salt marshes.