Collaborative Research: What controls the marine refractory DOC reservoir?

海洋中的溶解有机碳（DOC）由微观海洋生物群代谢过程中遗留下来的自由漂浮的有机分子组成，类似于陆地土壤中遗留的有机残留物。海洋目前拥有约6600亿吨碳的DOC，使其成为海洋中最大的还原碳储存库；其大小可与大气中的二氧化碳相媲美。该储层约97%由长寿命部分组成，平均放射性碳年龄为4000-6000年，称为难熔DOC （RDOC）。传统上，RDOC被认为是一种相对惰性和缓慢循环的碳储层，但最近的研究表明，它的循环可能比以前认为的更加动态。动态RDOC循环在地球的碳-气候系统中发挥着重要作用，一年仅氧化0.4%的海洋RDOC库就足以抵消当前海洋大气CO2汇。然而，到目前为止，控制这一储层的生物、化学和物理过程在很大程度上无法量化。该项目将在数值模拟合成中使用海洋DOC浓度及其同位素的数据同化，以测试、诊断和量化导致海洋RDOC产生和去除的环境过程。控制海洋RDOC储库的过程随后将在预测地球系统模型中表示，使用未来气候强迫模拟来揭示海洋RDOC碳-气候反馈的标志和强度。关于海洋RDOC储层的标准范例是，几乎所有的海洋DOC都是通过初级生产在海洋表面内部产生的，其中一部分在生物利用过程中产生了化学顽固性有机分子，这一过程被称为微生物碳泵。另一种观点认为，海洋RDOC池是由外部和内部产生的异质混合物组成的，现代和老化的分子具有局部的源/汇和不同的停留时间。例如，最近确定的局部RDOC去除过程已经在海面（海洋气溶胶形成）和地壳/水柱界面附近（热液喷口，地壳含水层）确定。协调许多假设的DOC来源、汇和持续机制，同时综合连贯地理解控制海洋RDOC库的因素，将需要在全球尺度上进行数值模拟。该项目将利用全球海洋DOC同位素（包括14C和13C）测量的最新进展，通过实施一种新的全球DOC循环及其同位素逆模型，诊断和量化控制RDOC水库的源和汇。将在群落地球系统模型的海洋生物地球化学组件（MARBL）中实施相关的RDOC循环过程和机制，包括来自逆模型结果的碳同位素，以便通过未来气候强迫耦合模拟评估海洋RDOC碳-气候反馈的符号和幅度。该项目将资助两名研究生，分别来自UNH和UCSB。联合国大学团队将与高中教师合作，提供海洋碳循环和地球海洋科学课程模块。加州大学圣巴哥分校将招收两名海洋科学研究本科生暑期实习生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Dissolved organic carbon (DOC) in the ocean consists of free-floating organic molecules left over from the metabolisms of microscopic marine biota, analogous to the organic residues left behind within soils on land. The ocean currently holds ~660 gigatons C of this DOC, making it the largest reservoir of reduced carbon in the ocean; its size rivaling that of atmospheric CO2. Approximately 97% of this reservoir is comprised of a long-lived fraction, with a mean radiocarbon age of 4000-6000 years, termed refractory DOC (RDOC). RDOC has been traditionally considered a relatively inert and slowly-cycling C reservoir, but recent research suggests its cycling may be more dynamic than previously thought. A dynamic RDOC cycle could play a significant role in Earth’s carbon-climate system, with oxidation of only ~0.4% of the marine RDOC reservoir in one year sufficient to counterbalance the current oceanic sink for atmospheric CO2. However, the biological, chemical, and physical processes that govern this reservoir have largely eluded quantification thus far. This project will use data assimilation of marine DOC concentrations and its isotopes in a numerical modeling synthesis to test, diagnose, and quantify the environmental processes responsible for the production and removal of marine RDOC. The processes that control the marine RDOC reservoir will then be represented in a prognostic Earth System Model, using a future climate forced simulation to reveal the sign and strength of the marine RDOC carbon-climate feedback.The standard paradigm regarding the marine RDOC reservoir is that nearly all marine DOC is internally produced in the surface ocean by primary production and that a fraction persists for 1000’s of years due to production of chemically recalcitrant organic molecules during biological utilization, a process termed the microbial carbon pump. An alternative view is that the marine RDOC pool is instead comprised of a heterogenous mixture of externally & internally produced, modern & aged molecules that have localized sources/sinks and distinct residence times. For example, recently identified localized RDOC removal processes have been identified both at the sea surface (marine aerosol formation) and near the crustal/water column interface (hydrothermal vents, crustal aquifers). Reconciling the many hypothesized DOC sources, sinks, and persistence mechanisms while synthesizing a coherent understanding of what controls the marine RDOC reservoir will require numerical modeling at global scales. This project will utilize recent advances in global ocean basin coverage of marine DOC isotope (both 14C and 13C) measurements to diagnose and quantify the sources and sinks controlling the RDOC reservoir through the implementation of a novel global inverse model of DOC cycling and its isotopes. The pertinent RDOC cycling processes and mechanisms including carbon isotopes from the results of the inverse model will be implemented within the ocean biogeochemistry component (MARBL) of the Community Earth System Model, in order to assess the sign and magnitude of the marine RDOC carbon-climate feedback with a future climate forced coupled simulation. Two graduate students will be supported by this project, one each at UNH and UCSB. The UNH team will work with high school instructors to deliver ocean carbon cycle and Earth & Ocean science curricula modules. Two undergraduate summer interns in ocean science research located at UCSB will be recruited from HBCU’s.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.