Collaborative Research: Constraining the source of oceanic dissolved black carbon using compound-specific stable carbon isotopes

火灾后剩下的炭和烟灰一般称为黑碳。当炭与水相互作用时，其中一些会溶解，并被河流带到海洋中。这种碳的可溶性成分被称为“溶解黑碳”（DBC）。最近的研究表明DBC是碳循环的主要组成部分。最值得注意的是，目前已知DBC占河流带入海洋的所有溶解有机碳的10%。DBC一旦进入海洋，就会在那里停留数千年，将原本以二氧化碳的形式存在于大气中的碳储存起来。由于二氧化碳会导致温室效应，大气中二氧化碳含量的增加会导致全球气温升高，因此了解海洋中的DBC来自何处以及它将在深海中被锁多久是很重要的。在这项受资助的工作中，我们着手确定海洋中DBC的来源。根据我们之前的研究显示，河流向海岸输出了大量的DBC，这表明河流是向公海输出DBC的主要来源。然而，在更仔细地观察河流和海洋中的DBC时，我们发现它们在一个关键方面有所不同：它们具有不同的同位素特征。这就排除了它们有相同的来源，表明海洋中的DBC不是来自河流。我们的初步工作只研究了美国乔治亚州的一条小河和近海水域。在这个资助的项目中，我们将对从亚马逊河、密西西比河和育空河等大河以及大西洋和太平洋中部收集的DBC的同位素特征进行全球研究。如果我们发现这些全球主要河流和海水中DBC的同位素没有重叠，我们就可以得出结论，河流不是海洋DBC的主要来源，我们需要寻找新的解释，来解释火产生的分子是如何最终进入深海的。该项目将通过东北大学合作项目，提升职业生涯并继续培训首次研究者，并在现实世界的研究中培训本科生。此外，我们的研究结果将与《儿童科学杂志》合作，用于为高中生制作学习材料。以前在自然水域追踪DBC来源的努力都有很大的局限性，无法在海洋DBC与其热源之间建立明确的联系。光降解是地表水中DBC的重要去除过程，它极大地改变了DBC的分子组成，并消除了DBC化学组成与其来源之间的任何潜在联系。总体稳定碳同位素测量不能明确地确定DOC子组分（如DBC）的来源。因此，该项目旨在通过测量分子标记物（苯聚羧酸，或BPCAs）的化合物特异性稳定碳同位素来限制DBC的海洋来源，这些分子标记物仅来源于DBC。我们的初步数据显示，bpca特定的同位素值在河流和海洋样本之间具有足够宽的动态范围，以测试我们的假设。在目前的提议中，我们的目标是1)同位素表征DBC到海洋（全球河流）的最大量化通量，2)评估DBC稳定碳同位素特征的原位海洋变化，以及3)研究DBC光降解潜在的bpca特异性分馏效应。通过建立一个强大的DBC源示踪剂，我们将能够准确地约束海洋DBC的来源，并进一步研究DBC在陆地-海洋连续体中的生物地球化学循环。我们的研究结果也将有助于回答更大的研究问题，特别是那些关于海洋中陆源DOM的命运的问题，这些问题已经困扰了生物地球化学家几十年。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 char and soot remaining after fire is broadly referred to as black carbon. When char interacts with water, some of it dissolves and is carried away by rivers to the ocean. This soluble component of char is termed "dissolved black carbon" (DBC). Recent research has revealed DBC to be a major component of the carbon cycle. Most notably, DBC is now known to make up 10% of all dissolved organic carbon that rivers carry to the sea. Once in the ocean, DBC remains there for thousands of years, storing carbon that would otherwise be in the atmosphere as carbon dioxide. As carbon dioxide contributes to the greenhouse effect, with higher carbon dioxide in the atmosphere leading to warmer global temperatures, it is important to understand where DBC in the ocean came from and how long it will stay locked away in the deep ocean. In this funded work, we set out to determine the source of the DBC in the ocean. Based upon our previous work that shows rivers export massive amounts of DBC to the coast, it has been suggested that rivers are the main source of DBC to the open ocean. However, in looking more closely at the DBC in rivers and the oceans, we found them to differ in one critical way: they have different isotopic signatures. This precludes them from having the same source, indicating that the DBC in the oceans is not from rivers. Our preliminary work only looked at a small river in Georgia, USA, and in the coastal waters offshore. In this funded project, we will take a global look at the isotopic signatures of DBC collected from large rivers such as the Amazon, Mississippi, and Yukon, and from the middle of the Atlantic and Pacific Oceans. If we find there is no overlap in the isotopes of the DBC from these major global rivers and ocean waters, we can conclude that rivers are not the main source of oceanic DBC and will need to search for new explanations of how molecules produced by fire end up in the deep ocean. The project will enhance the career and continue the training of a first-time investigator and train undergraduate students in real world research through the Northeastern University Cooperative Program. In addition, our findings be adapted to produce learning materials for high school students in collaboration with the Science Journal for Kids.Previous efforts to track DBC sources in natural waters have come with major limitations, preventing definitive connections to be made between oceanic DBC and its pyrogenic source. Photodegradation, a significant removal process for DBC in surface waters, drastically alters the molecular composition of DBC and erases any potential link between DBC chemical composition and its source. Bulk stable carbon isotopic measurements cannot unambiguously identify sources of DOC subcomponents, such as DBC. As such, this project aims to constrain the oceanic source of DBC by measuring compound-specific stable carbon isotopes of molecular markers (benzenepolycarboxylic acids, or BPCAs), derived exclusively from DBC. Our preliminary data show BPCA-specific isotopic values to have a sufficiently wide dynamic range between riverine and oceanic samples to test our hypotheses. In the current proposal, we aim to 1) isotopically characterize the largest quantified flux of DBC to the ocean (global rivers), 2) assess in situ oceanic variation in DBC stable carbon isotopic signatures, and 3) investigate the potential BPCA-specific fractionation effects of DBC photodegradation. In establishing a robust tracer for DBC source, we will be able to accurately constrain sources of oceanic DBC and further investigate the biogeochemical cycling of DBC across the terrestrial-marine continuum. The results of our research will also assist in answering larger research questions, specifically those regarding the fate of terrigenous DOM in the ocean, which have plagued biogeochemists for decades.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.