Collaborative Research: Dissolved organic carbon (DOC) transformations in deep sub-surface sediments and its role as a source of "old" DOC to the water column

有机碳（Corg）矿化率通常是最高的沉积物-水界面附近，并随着深度不稳定的基板和强氧化剂的消耗而降低。然而，在许多海洋边缘沉积物中，在硫酸盐（SO 4 =）耗尽和CH 4浓度开始增加（硫酸盐-甲烷转换; SMT）的深度间隔，SO 4 =还原率通常显示出强烈的次表层最大值，表明局部增强的微生物活动和碳周转。这些SO 4 =还原热点通常归因于SO 4 =对CH 4的厌氧氧化，但一些研究发现，SO 4 =还原超过CH 4氧化，表明SMT中存在主要的额外SO 4 =汇。在这个项目中，来自旧金山弗朗西斯科州立大学、佛罗里达州立大学和Old自治领大学的研究小组将通过结合尖端孔隙水成分分析--甲烷、溶解有机碳和无机碳（DOC和DIC）的del-14 C和del-13 C，以及DOC上的1H-NMR--与数值反应传输模型来研究这种SO 4 =汇的性质。 他们将检验这样一个假设，即SMT不仅是CH 4的氧化前沿，而且也是沉积物柱深处产生的DOC的氧化前沿，并向上输送到SMT中。 他们还将测试这样一种想法，即并非所有的DOC都在SMT中被氧化，并且一些DOC到达了表层沉积物，并代表了海洋中14 C耗尽（老化前）DOC的来源。的前提是，DOC生产从Corg是增强产甲烷沉积物由于解偶联在厌氧食物链之间的终端代谢和发酵反应参与的整体Corg的生物矿化过程。这项工作将侧重于两个海洋边缘地点，圣莫尼卡盆地和圣巴巴拉盆地，尽管它们在地理上很接近，但似乎在深层沉积物中有不同的甲烷动态。智力优势：这项研究应有助于更好地了解表层以下沉积物在整个海底Corg矿化过程中的作用，以及在海底和水柱之间主要元素的交换中的作用。它还将允许测试的假设，海洋沉积物的来源14 C耗尽，docc的上覆水柱，从而解决了一个问题，困扰了几十年的化学海洋学：什么因素控制14 C签名DOC在深海？更广泛的影响：这项工作将以多种方式整合研究和教育。它将允许3个PI继续努力将他们的研究活动纳入他们所教授的课程中，这些课程从高级研究生课程到面向非科学专业的100级本科课程。二是直接影响2位博士的学习经历。2个MS候选人和至少2名本科生，他们将接受拟议工作关键方面的培训，包括现场，实验和建模组件。

Organic carbon (Corg) remineralization rates are typically highest near the sediment-water interface, and decrease with depth as labile substrates and strong oxidants are consumed. However, in many ocean margin sediments, at the depth interval where sulfate (SO4=) is exhausted and CH4 concentrations begin to increase (the sulfate-methane transition; SMT), SO4= reduction rates typically show strong sub-surface maxima, indicating locally-enhanced microbial activity and carbon turnover. These hot spots for SO4= reduction are generally attributed to anaerobic oxidation of CH4 by SO4=, but a number of studies have found an excess of SO4= reduction over CH4 oxidation, indicating the presence of a major additional SO4= sink in the SMT. In this project a research team from San Francisco State University, Florida State University, and Old Dominion University will investigate the nature of this SO4= sink by combining cutting-edge porewater compositional analyses -- del-14C and del-13C of CH4, dissolved organic and inorganic carbon (DOC and DIC), and 1H-NMR on DOC -- with numerical reactive transport modeling. They will test the hypothesis that the SMT is an oxidation front for not just CH4, but also for DOC that is produced deeper in the sediment column, and transported upward into the SMT. They will also test the idea that not all of this DOC is oxidized in the SMT, and that some reaches the surface sediments, and represents a source of 14C-depleted (pre-aged) DOC to the oceans. The premise is that DOC production from Corg is enhanced in methanogenic sediments due to an uncoupling in the anaerobic food chain between terminal metabolism and fermentation reactions involved in the overall Corg remineralization process. The work will focus on two ocean margin sites, Santa Monica Basin and Santa Barbara Basin, which despite their geographic proximity, appear to have different CH4 dynamics in the deep sediments. Intellectual Merit: This study should result in a greater understanding of the role of sub-surface sediments in the overall benthic Corg remineralization process, and in the exchange of major elements between the sea floor and the water column. It will also allow testing of the hypothesis that marine sediments are sources of 14C-depleted, recalcitrant DOC to the overlying water column, thereby addressing a problem that has perplexed chemical oceanography for several decades: what factors control the 14C signature of DOC in the deep oceans? Broader Impacts: This work will integrate research and education in several ways. It will allow the 3 PIs to continue with their effort to incorporate their research activities into the classes they teach, which range from advanced graduate- to 100-level undergraduate courses intended for non-science majors. Second, it will directly impact the learning experiences of 2 Ph.D. and 2 M.S. candidates, and at least 2 undergraduate students who will receive training in key aspects of the proposed work, including the field, experimental and modeling components.