Collaborative Research: What Happens to Terrestrial Organic Matter in the Ocean? Solving the Mystery Behind an Iconic Question

亚马逊河是世界上最大、最重要的河流之一。它不仅带着水穿过亚马逊雨林，还带着泥土——大量的泥土。当河流汇入海洋时，这些泥浆沉淀下来，形成了亚马逊三角洲。海浪和风将这些泥重新悬浮起来，形成泥滩，慢慢地从巴西海岸移动到法属圭亚那、苏里南和圭亚那。这些泥滩不仅塑造了这些地区的海岸线，而且还储存了亚马逊雨林从大气中带走的碳，这些碳落到森林地面，被冲入河流，最终以泥粒的形式输送到亚马逊三角洲。通常情况下，这代表了碳从大气转移到沉积物和岩石记录，将其隐藏数千年至数百万年，并参与大气二氧化碳浓度的自然调节。然而，长期以来，人们一直认为亚马逊河沿岸数百英里长的泥滩会搅动这些碳，并将大部分碳释放回大气中。这个观点已经坚持了几十年，但如果错了，可能会对我们对海洋碳循环的理解产生重大影响。这项研究将使用新的科学技术来改变我们看待海洋中河流碳循环的方式，通过对泥岸、海水和亚马逊河水域进行比以前更彻底的采样，目的是了解和量化亚马逊河近海海洋泥岸沉积物中陆源碳的储存。这项研究将作为两篇博士论文的核心，并将与伍兹霍尔合作伙伴教育计划（PEP）相结合，为代表性不足的少数民族大学生提供获得海洋和环境科学实践经验的机会。该项目还将被纳入圣彼得堡科学节，该活动每年秋季的一个周末将数千名学童带到南佛罗里达大学校园，展示不同的研究项目和倡议。20年前，约翰·赫奇斯（John Hedges）提出了一个与海洋在碳循环中的作用有关的开创性问题：“海洋中的陆生有机物会发生什么变化？”这个问题是在一个难题上提出的——因为河流中的有机物被高度降解，它应该在海洋中受到最小的呼吸作用，然而根据大多数说法，大陆边缘似乎是陆地有机物的焚化炉。然而，这种模式的大部分最初是建立在亚马逊河水下三角洲和圭亚那海岸移动流态化泥滩的生物地球化学研究之上的。然而，亚马逊河和泥滩沉积物中有机碳的初步表征表明，陆地有机碳的一部分实际上是不难降解的，因此在海洋中被迅速有效地氧化，而另一部分是难降解的，实际上被定量地保存在海洋沉积物中。总的来说，这些数据表明，埋葬效率被低估了50-100%。该提案的重点是获得对亚马逊河沿岸海洋中陆地有机碳命运的变革机制和定量理解。这将通过1)对圭亚那海岸近海的海洋省（圭亚那泥滩）以及亚马逊河三角洲进行采样，使用浅水研究船和ii)使用最先进的有机碳地球化学分析-特别是斜坡氧化和化合物特定放射性碳分析-和元素组成，以证明陆地有机碳氧化的选择性，并提供该系统埋葬效率的新估计-对全球海洋陆地有机碳清单的影响。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。

The Amazon River is one of the largest and most important rivers in the world. It not only carries water through the Amazon rain forests, it also carries mud - lots of mud. This mud settles when the river joins the ocean, forming the Amazon delta. Waves and wind re-suspend some of this mud, which forms mud banks that slowly move from the coast of Brazil, to French Guiana, Suriname, and Guyana. Not only do these mud banks shape the coastlines of these areas, but they also serve as a storage for carbon that had been removed from the atmosphere by the Amazon rainforest, dropped to the forest floor and washed into the river, ultimately transported on particles of mud into the Amazon delta. Normally, this represents a transfer of carbon from the atmosphere to the sediment and rock record, hiding it away for thousands to millions of years and participating in the natural regulation of atmospheric carbon dioxide concentration. However, the Amazon's special case of mud banks that travel hundreds of miles along the coast has long been believed to stir up this carbon and release most of it back to the atmosphere. This idea has held for decades, but if wrong, may have a large impact on our understanding of the ocean's carbon cycle. This research will use new science and technology to change the way we look at river carbon cycling in the ocean by sampling the mud banks, ocean waters, and Amazon River waters more thoroughly than has been previously done, with the aim to understand and quantify the storage of land-derived carbon in marine mudbank sediments offshore the Amazon River. This research will serve as the core of two Ph.D. dissertations and will interface with the Woods Hole Partnership Educational Program (PEP) to provide under-represented minority college students an opportunity to gain practical experience in marine and environmental sciences. The project will also be incorporated into the Saint Petersburg Science Fest, an event that brings several thousand school children to University of South Florida campus during one weekend every autumn to showcase different research projects and initiatives.Twenty years ago, John Hedges posed a seminal question pertaining to the role of the ocean in the carbon cycle: "what happens to terrestrial organic matter in the oceans?" That question was set upon a conundrum - because riverine organic matter is highly degraded, it should be expected to suffer minimal respiration in the ocean, yet by most accounts continental margins seem to be incinerators of terrestrial organic matter. However, much of this paradigm was originally built upon biogeochemical studies of the Amazon sub-aqueous delta and the mobile fluidized mudbanks along the Guianas coast. However, preliminary state of the art characterizations of organic carbon in Amazon River and mudbank sediments suggest that a fraction of terrestrial organic carbon is actually not refractory and as such is rapidly and efficiently oxidized in the ocean, whereas another fraction is refractory and is in fact quantitatively preserved in marine sediments. Overall, the data suggest an underestimation of burial efficiency in the realm of 50-100%. This proposal focuses on gaining a transformative mechanistic and quantitative understanding of the fate of terrestrial organic carbon in the coastal ocean offshore of the Amazon River. This will be done through i) sampling the marine province offshore of the Guianas coast (the Guianas mudbanks) as well as the Amazon River delta, using a shallow-draft research vessel and ii) using novel state of the art geochemical analysis of organic carbon - in particular ramped oxidation and compound specific radiocarbon analysis - and elemental composition to demonstrate the selective nature of terrestrial organic carbon oxidation and provide new estimates of burial efficiency of this system - with implications for the global inventory of terrestrial organic carbon in the ocean.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.