RAPID: Quantifying the Great Marsh Sedimentation Event 2018

2017年12月下旬至2018年1月期间，新英格兰沼泽发生了前所未有的事件，足球场大小的沼泽被2至12英寸的浮冰淤泥和沙子覆盖。这是非常重要的，因为这些沼泽通常通过沉积物积累来获得海拔，比当地海平面上升的速度要慢。因为沼泽基本上是平坦的，即使是水位的小幅上升也会影响到大片地区。最终，如果水仍然太深，沼泽将被淹没并转向开阔水域，失去所有生态系统服务和它们提供的沿海保护。假定沼泽草能在其中生长，1月份沉积的大量沉积物将会提高该地区的海拔，使它们更能适应不断上升的水位。这一事件发生在两周的极端寒冷之后，随后出现了高于正常水平的天文潮汐，并伴有一场强大的东北冬季风暴。在长时间的寒流中，潮汐水冻结在小溪和海湾的底部，将底部的沉积物合并到冰中，随着每一次连续的低潮，冰变厚并捕获了更多的沉积物。当风暴潮发生时，冰筏漂浮到沼泽表面，在那里它们被搁浅，最终融化，在沼泽上留下了沉积物。在正常年份，漂流会带来大约5%的年度无机物质沉积，但据估计，这一个风暴层相当于71年的正常堆积。因此，这种罕见的、高沉积的漂流事件可能是维持新英格兰沼泽不可缺少的（就像飓风对美国南部湿地的影响一样）；然而却很少被记录下来。该项目利用航空摄影对沉积物覆盖区域进行测绘，并在实地测量厚度和覆盖范围，从而解决了这一知识空白问题，从而全面量化进入沼泽的沉积物数量及其对沼泽高程的影响。除了量化冬季风暴过程对北部沼泽的重要性之外，这项研究还将为沼泽恢复策略提供见解，例如通过“薄层沉积”人工高坡建筑，将当地疏浚的沉积物喷洒在沼泽表面顶部，以建立新的沼泽或提升现有的沼泽。2018年1月的事件沉积提供了一个自然实验室，研究以米草属植物为主的高沼泽如何应对人工沉积，以及沼泽可以承受多大厚度的沉积物。这项研究扩大了对地球科学研究的参与，因为它由一名早期职业女性研究人员领导，同时也为一名本科生形成了一个独立的研究项目，该本科生将分析沉积物样本，将实地数据转化为地图，并将这些数据与航空摄影相结合，以确定这一事件的影响。本研究的主要目标是评估2018年1月在马萨诸塞州大沼泽发生的极端沉积事件的范围和数量。研究人员将在春草开始发芽并掩盖其覆盖范围之前收集沉积物的厚度和表面范围的测量数据。他们将进行航空测绘，以10平方厘米的分辨率收集整个沼泽地区（约130平方公里）的正射影照片，并将使用自动图像分析、手动数字化和与历史图像比较的组合来绘制新的冰筏沉积物。基于实地的航空照片的地面真实性将涉及至少8个横跨沼泽的陆地到堰洲岛的横断面（步行），以绘制和测量沉积物沉积物。将提取已知体积的多个沉积物塞，用于粒度，有机含量和容重分析。根据航空照片和实地调查确定的沉积物厚度和沉积物的横向范围，将使研究人员能够估计在此事件中沉积的沉积物的总量，从而能够与已知的沉积速率进行比较。本研究收集的数据将为未来的调查提供记录和基线，包括分析冰筏淹没沼泽时的风暴条件，以及运输矢量与风向、水深和沼泽海拔的相关性，以及沉积物事件后草的恢复评估。这项工作将为拟议的“薄层沉积”沼泽恢复实践提供信息，通过确定引入的沉积层的厚度，沼泽生态系统可以容忍在海平面加速上升的情况下作为增加的辅助物。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

An unprecedented event took place on New England marshes between late December 2017 and January 2018 wherein football field-sized patches of marsh were blanketed with 2 to 12 inches of ice-rafted silt and sand. This is hugely significant because these marshes normally gain elevation through sediment accumulation more slowly than local sea level is rising. Because marshes are essentially flat, even a small rise in water level will affect a large area. Ultimately, if the waters remain too deep, marshes will drown and switch to open water, with loss of all ecosystem services and coastal protections they afford. The large amount of sediment that was deposited in January, assuming the marsh grasses can grow through it, will give the areas a boost in elevation, making them more resilient to rising waters. The event occurred when two weeks of extreme cold were followed by higher than normal astronomic tides, combined with a powerful northeast winter storm. During the prolonged cold snap, tidal waters froze to the bottom of creeks and bays, incorporating bottom sediment into the ice, which thickened and trapped more sediment with each successive low tide. When the storm surge occurred, the ice rafts floated up and onto the marsh surface, where they became stranded, eventually melting and leaving the sediment on the marsh. In a normal year, ice-rafting would bring about 5% of the annual inorganic material deposited, but this single storm layer is estimated to be equivalent to 71 years of normal accretion. Such infrequent, high sedimentation ice rafting events may therefore be integral to sustaining marshes in New England (as hurricanes can be to wetlands in the southern US); yet are rarely documented. This project addresses this knowledge gap by mapping the area covered in sediment using aerial photography ground-truthed with measurements of thickness and coverage in the field, to fully quantify the amount of sediment transported onto the marsh and its impact on marsh elevation. In addition to quantifying the importance of winter storm processes to northern marshes, this study will provide insights into marsh restoration strategies, such as artificial elevation building through "thin layer deposition" whereby locally-dredged sediment is sprayed on top of the marsh surface to build new marshes or elevate existing marshes. The January 2018 event sedimentation provides a natural laboratory to study how a Spartina patens-dominated high marsh will respond to artificial sedimentation, and what thickness of sediment the marsh can withstand. The study broadens participation in geoscience research, as it is headed by an early-career woman researcher, and also forms an independent research project for an undergraduate student, who will analyze sediment samples, translate field data into maps, and combine these data with the aerial photography to determine the impact of this event. The primary goal of this study is to assess the extent and volume of the extreme sedimentation event which occurred in January 2018 on the Great Marsh, MA. The researchers will collect measurements of thickness and surface extent of the deposit before the spring grass begins to shoot and obscure its coverage. They will conduct aerial survey mapping to collect ortho-photographs capturing the entire marsh region (~130 km2) at a resolution of 10 cm2 and will use a combination of automated image analysis, manual digitization, and comparison to historical images to map the new ice rafted sediment. Field-based ground-truthing of aerial photos will involve a minimum of eight land to barrier island transects (by foot) across the marsh to map and measure the sediment deposits. Multiple sediment plugs of known volume will be extracted for grain size, organic content and bulk density analysis. Sediment thickness and lateral extent of the deposit, determined from the aerial photographs and field surveys, will allow the researchers to estimate the total volume of sediment deposited during this event, enabling a comparison to known accretion rates. The data collected for this study will provide a record and a baseline for future investigations, including analysis of storm conditions when the ice rafts inundated this marsh, and correlation of vectors of transport to wind direction, water depth and marsh elevation, as well as assessments of grass recovery following the sedimentation event. The work will inform the proposed marsh restoration practice of "thin layer deposition", by determining the thickness of introduced sediment layers that can be tolerated by the marsh ecosystem as aids to accretion under accelerating rates of sea-level rise.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.