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）再矿化率通常在沉积物-水界面附近最高，并且随着不稳定底物和强氧化剂的消耗而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=) 耗尽且 CH4 浓度开始增加（硫酸盐-甲烷转变；SMT）的深度区间，SO4= 减少率通常显示出强烈的次表面最大值，表明局部增强的微生物活动和碳周转。 SO4= 还原的这些热点通常归因于 SO4= 对 CH4 的厌氧氧化，但许多研究发现 SO4= 还原超过 CH4 氧化，表明 SMT 中存在主要的额外 SO4= 汇。在该项目中，来自旧金山州立大学、佛罗里达州立大学和奥多明尼恩大学的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 的 1H-NMR）与数值反应输运模型相结合，研究该 SO4= 汇的性质。 他们将测试这样的假设：SMT 不仅是 CH4 的氧化前沿，而且也是沉积物柱深处产生并向上输送到 SMT 的 DOC 的氧化前沿。 他们还将测试这样的想法，即并非所有 DOC 在 SMT 中都被氧化，并且一些 DOC 到达了表面沉积物，并且代表了海洋中 14 C 耗尽（预老化）DOC 的来源。前提是，由于整个科格再矿化过程中涉及的最终代谢和发酵反应之间的厌氧食物链中的解耦，导致产甲烷沉积物中科格的 DOC 产量增加。这项工作将重点关注两个海洋边缘地点：圣莫尼卡盆地和圣巴巴拉盆地，尽管地理位置接近，但深层沉积物中的 CH4 动态似乎不同。智力价值：这项研究应该使人们更好地了解地下沉积物在整个海底珊瑚再矿化过程中以及在海底和水体之间主要元素交换中的作用。它还将允许测试海洋沉积物是上覆水柱中 14C 耗尽、顽固 DOC 来源的假设，从而解决困扰化学海洋学数十年的问题：哪些因素控制深海中 DOC 的 14C 特征？更广泛的影响：这项工作将以多种方式整合研究和教育。这将使 3 位 PI 能够继续努力将他们的研究活动纳入他们所教授的课程中，这些课程范围从针对非科学专业的高级研究生课程到 100 级本科课程。其次，它会直接影响2个博士的学习经历。和 2 硕士候选人，以及至少 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、实验和建模部分。

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.