The importance of sulfurized sugars for organic carbon burial: Testing the model in Santa Barbara Basin

当有机物沉积在沉积物中时，它会经历一系列的反应，将原来的有机分子浓缩并转化为密度更大、抵抗力更强的化合物，形成一种叫做干酪根的物质：石油就是从这种物质形成的。我们目前对原始有机分子在沉积物中埋藏过程中如何变化和交联（即聚合）形成干酪根的了解很少。据推测，元素硫在交联中起很大作用；然而，到目前为止，由于缺乏能够适当研究硫在这一过程中的作用的分析技术，几乎没有直接证据。本研究使用一种新开发的、新颖的分析技术来测量有机物质中单个分子化合物的硫同位素组成。这将使我们能够确定有机结合硫的起源，并有可能彻底改变我们对天然有机物聚合的理解，从而改变干酪根的形成。该项目的广泛影响包括确定基本的物理和化学关系，以便更好地了解碳循环，这对干酪根和石油的形成有影响；通过发展和完善一种新的、潜在的强大分析技术和方法，为科学建设基础设施；以及研究生培训。它将通过加州理工学院的少数族裔本科生研究奖学金项目吸引代表性不足的少数族裔本科生，以帮助扩大少数族裔学生参与最先进的研究。生物生成的有机物凝结聚合成干酪根是全球碳循环的关键步骤之一，因为它决定了有机物是被再矿化（溶解）还是被保存和掩埋（转化为干酪根）。虽然硫被认为通过交联有机单体在这一过程中发挥了重要作用，但这一过程发生的精确机制细节尚不清楚。本研究采用了一种新开发的气相色谱与多收集器电感耦合等离子体质谱（ICPMS）耦合技术。这样就可以对单个有机化合物的硫同位素进行分析。利用这项技术，可以记录由：(1)沉积物孔隙水中脂质与硫化氢（HS-）的反应在很长时间尺度上产生的化合物，以及(2)碳水化合物与含氧水柱中多硫化物的反应在很短时间尺度上产生的化合物。由于这两种过程的硫同位素（S-34）特征相差近30 peril，因此很容易区分这两种不同的过程。研究人员将对圣巴巴拉盆地的富有机质沉积物进行分析，并将结果与Cariaco盆地的沉积物进行比较，以确定Cariaco盆地富含硫化物的水柱环境的初步研究结果是否适用于更正常的有机沉积中心。研究目标包括：(1)了解硫化作用（有机分子交联形成干酪根的主要过程）是如何工作的，以及它与有机碳保存的关系(2)看看Cariaco盆地的结果是否可以推广到其他有机碳沉积中心，(3)检查二硫结合化合物的丰度是否与水柱exunia的存在有关。(4)确定二硫化物结合的有机硫是否总是比未结合的硫化脂质富集S-34；(5)单个有机硫分子的硫同位素值是否代表整体有机硫。

When organic matter is deposited in sediments, it undergoes a series of reactions that condense and transform the original organic molecules into a denser, more resistant set of compounds that form a material called kerogen: the material from which petroleum is formed. Our present understanding of how the original organic molecules change and crosslink (i.e., polymerize) to form kerogen during burial in sediments is poor. It has been hypothesized that the element sulfur plays a large role in the crosslinking; however, so far there is little direct evidence due to the absence of analytical techniques that can appropriately study the role of sulfur in this process. This research uses a newly developed, novel, analytical technique to measure the sulfur isotopic composition of individual molecular compounds in organic matter. This will allow the identification of the origin of the organic-bound sulfur and has the potential to revolutionize our understanding of the polymerization of natural organic matter, and hence formation of kerogen. Broader impacts of the project include determining fundamental physical and chemical relationships that allow better understanding of the carbon cycle, which has implications for the formation of kerogen and petroleum; building infrastructure for science via the development and refinement of a new and potentially powerful analytical technique and methodology; and graduate student training. It will engage under-represented minority undergraduate students via the California Institute of Technology Minority Undergraduate Research Fellowship program in order to help broaden participation of minority students in state-of-the-art research. The condensation and polymerization of biologically generated organic matter into kerogen is one of the key steps in the global carbon cycle because it determines whether organic matter will be remineralized (dissolved) or preserved and buried (turned into kerogen). Although sulfur is thought to play an important role in this process by crosslinking organic monomers, the precise mechanistic details of how this happens is unknown. This research uses a newly developed technique that couples gas chromatography with multi-collector inductively-coupled-plasma-mass-spectrometry (ICPMS). This allows analysis of the isotopes of sulfur of individual organic compounds. Using this technique it is possible to document compounds resulting from: (1) the reaction of lipids with hydrogen sulfide (HS-) in sediment porewaters over long timescales, and (2) the reaction of carbohydrates with polysulfide in the euxinic water column on very short timescales. Because these two processes have sulfur isotope (S-34) signatures that differ by almost 30 permil, the two different processes can be easily distinguished. Organic-rich sediments deposited in the Santa Barbara Basin will be analyzed and the results compared to those from sediments in the Cariaco Basin to see if the pilot study in the Cariaco, which is a sulfide-rich water column environment, hold true in more normal organic depocenters. Goals of the research include: (1) understanding how sulfurization, a main process in the cross-linking of organic molecules to form kerogen, works and how it relates to organic carbon preservation (2) seeing if the results from the Cariaco Basin are generalizable to other organic carbon depocenters, (3) examining whether the abundance of disulfide-bound compounds relate to the presence or absence of water column exunia, (4) determining whether disulfide-bound organic sulfur is always enriched in S-34 compared to unbound sulfurized lipids, and (5) seeing if the sulfur isotope values of individual organic sulfur molecules are representative of bulk organic sulfur.