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

有机碳再矿化速率通常在沉积物-水界面附近最高，随着不稳定底物和强氧化剂的消耗而随深度降低。然而，在许多海洋边缘沉积物中，在硫酸盐（SO4=）耗尽和CH4浓度开始增加的深度区间（硫酸盐-甲烷转变；SMT）， SO4=还原率通常表现出强烈的次表层最大值，表明局部微生物活动和碳周转增强。这些SO4=还原热点通常归因于SO4=对CH4的厌氧氧化，但许多研究发现，SO4=还原超过了CH4氧化，表明SMT中存在一个主要的额外SO4=汇。在这个项目中，一个来自旧金山州立大学、佛罗里达州立大学和老道明大学的研究小组将通过结合最先进的孔隙水成分分析——CH4的del-14C和del-13C，溶解的有机和无机碳（DOC和DIC），以及DOC的1H-NMR——和数值反应传输模型来研究这个SO4=汇的性质。他们将测试SMT不仅是CH4的氧化锋的假设，而且是沉积物柱中产生的DOC的氧化锋，并向上输送到SMT。他们还将验证并不是所有的DOC都在SMT中被氧化，有些到达了表层沉积物，代表了海洋中14c耗尽（预老化）DOC的来源。这一研究的前提是，在产甲烷沉积物中，由于在厌氧食物链中，在整个煤再矿化过程中，终端代谢和发酵反应之间的不耦合，煤中DOC的产生得到了增强。这项工作将集中在两个海洋边缘地点，圣莫尼卡盆地和圣巴巴拉盆地，尽管它们的地理位置接近，但在深层沉积物中似乎有不同的CH4动力学。知识价值：这项研究将使人们更好地了解地下沉积物在整个底栖海洋再矿化过程中的作用，以及在海底和水柱之间主要元素的交换。它还将允许测试海洋沉积物是上覆水柱中缺乏14C的顽固DOC的来源的假设，从而解决困扰化学海洋学几十年的问题：是什么因素控制了深海中DOC的14C特征？更广泛的影响：这项工作将在几个方面整合研究和教育。这将允许3名pi继续努力，将他们的研究活动融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。其次，它将直接影响2名博士和2名硕士候选人的学习经历，以及至少2名本科生，他们将在拟议工作的关键方面接受培训，包括实地，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.