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)的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，再矿化率随着深度的增加而降低。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=还原速率通常显示出很强的次表层峰值，表明局部微生物活动和碳周转增加。SO4=还原的这些热点通常归因于SO4=对CH4的厌氧氧化，但一些研究发现SO4=还原超过CH4氧化，这表明SMT中存在主要的额外SO4=汇。在这个项目中，来自旧金山州立大学、佛罗里达州立大学和老道明大学的一个研究小组将通过将尖端孔隙水成分分析--CH4的del-14C和del-13C、溶解的有机和无机碳(DOC和DIC)以及DOC上的1H-核磁共振--与数值反应传输模型相结合，来研究这种SO4=汇的性质。他们将检验这一假设，即SMT不仅是CH4的氧化前沿，也是DOC的氧化前沿，DOC在沉淀柱中产生更深，并向上输送到SMT中。他们还将检验这样一种观点，即并非所有这些DOC在SMT中都被氧化了，其中一些到达了表层沉积物，代表着14C耗尽(老化前的)DOC的来源进入海洋。前提是Corg的DOC产量在产甲烷沉积物中得到提高，因为在整个Corg再矿化过程中，终端代谢和发酵反应之间的厌氧食物链是解偶联的。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，尽管它们地理上很接近，但在深层沉积物中似乎有不同的CH4动力学。知识价值：这项研究应有助于更好地了解地下沉积物在整个海底珊瑚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。它还将允许检验海洋沉积物是14C耗竭的、顽固的DOC到上覆水柱的来源的假设，从而解决一个困扰化学海洋学数十年的问题：是什么因素控制了深海DOC的14C特征？更广泛的影响：这项工作将以几种方式整合研究和教育。它将允许3名PI继续努力将他们的研究活动纳入他们所教授的班级，这些班级的范围从高级研究生到针对非理科专业的100级本科课程。其次，它将直接影响2名博士生和2名硕士研究生以及至少2名本科生的学习经历，他们将接受拟议工作的关键方面的培训，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.