Nitrogen isotopic (d15N) composition of carbonate-bound organic nitrogen in Deep Sea Corals: A new, high resolution proxy for N cycle studies

氮循环的历史提供了对过去气候和海洋生物地球化学循环之间联系的深入了解。对N循环历史的解释依赖于沉积“档案”中保存的颗粒有机氮（del15N- pon）的氮同位素组成（del15N），例如：在快速积累的富有机质沉积物中埋藏在缺氧/缺氧条件下的块状有机氮（on）；硅藻体和有孔虫试验中与骨骼结合的ON，以及深海蛋白珊瑚的有机骨骼。与古代用物一样，这些档案具有明显的优势和局限性，后者包括缺氧/缺氧沉积物的地理分布有限，由于含有硅藻和有孔虫的沉积物的生物扰动，时间分辨率有限，以及珊瑚软组织存活的时间有限。因此，为了更好地理解过去N周期变化的模式和原因，多代理方法是非常可取的。为了提高del15N古记录的地理和时间分辨率，波莫纳学院的一位科学家与克莱蒙特-麦肯纳学院和普林斯顿大学的研究小组合作，将评估海洋N循环历史的一个新的代理——深海珊瑚矿物晶格内ON的del15N。深海珊瑚可以记录短期的区域和长期的全球N循环变化。这些生物生长缓慢，年代可以精确到100年，而且地理分布广泛。此外，正如以往的研究表明，碳酸盐晶格中的ON键受到保护，不受成岩蚀变的影响。通过分析来自不同海洋环境的珊瑚样本，每个样本都代表了从光带输出的PON的独特del15N特征，研究人员将确定珊瑚结合的ON的del15N是否反映了从海洋表面输出的PON的N同位素组成。此外，将在确定年龄的化石硬核系深海珊瑚中生成珊瑚结合的del15N的时间分辨记录，并与基于更传统的代用物在十年至千年时间尺度上发表的del15N记录进行比较。目前存在很多不确定性，即出口的PON（悬浮与下沉）池是深海珊瑚的主要食物来源。这个问题将通过比较不同深度的现代珊瑚的del15N与在加利福尼亚边界盆地之一的圣佩德罗盆地进行的几次一日游中收集的悬浮和下沉PON池的del15N来解决。该项目的资金将支持波莫纳学院和凯克科学系（包括克莱蒙特麦肯纳学院、斯克里普斯学院和皮策学院）的新气候科学实验室的两名早期职业女性科学家和几名本科生研究助理。学生将有机会参加前往圣佩德罗盆地的短期研究巡航，接受先进分析技术的培训，并在会议上展示研究成果。这项研究的结果将被用作高级论文和独立研究项目的主题，通过让本科生接触气候研究的实践和知识方面来扩展教育经验。

The history of the nitrogen (N) cycle provides insight into the links between past climate and marine biogeochemical cycles. Interpretations of the history of the N cycle rely on the nitrogen isotopic composition (del15N) of Particulate Organic Nitrogen (del15N-PON) preserved in sedimentary "archives" such as: bulk organic nitrogen (ON) buried under suboxic/anoxic conditions in rapidly accumulating, organic-rich sediments; skeletal-bound ON in diatom frustules and foraminiferal tests, and organic skeletons of deep-sea proteinaceous corals. As is often is the case with paleo-proxies, these archives have distinct advantages and limitations, the latter including the restricted geographic occurrence of suboxic/anoxic sediments, limited temporal resolution due to bioturbation of sediments containing diatoms and foraminifera, and the limited time span for survival of the soft tissues of corals. Therefore, a multi-proxy approach is highly desirable for better understanding the patterns and causes of N cycle variability in the past. To improve upon the geographic and temporal resolution of del15N paleo-records, a scientist from Pomona College, collaborating with research groups from Claremont-McKenna College and Princeton University, will evaluate a new proxy for the history of the marine N cycle-- del15N of ON bound within the mineral lattice of deep-sea corals. Deep-sea corals can record both short-term regional and longer-term global variability in the N cycle. These organisms grow slowly, can be dated with a precision of 100 years, and have broad geographic distribution. Moreover, as demonstrated by previous research, the ON bound within the carbonate lattice is protected from diagenetic alteration. By analyzing coral specimens from disparate oceanographic environments, each chosen to represent a distinct del15N signature of PON exported from the euphotic zone, the researchers will determine whether the del15N of coral-bound ON reflects N isotopic composition of PON exported from the surface ocean. Furthermore, time-resolved records of del15N of coral-bound ON will be generated in fossil scleractinian deep-sea corals of established ages and compared to published del15N records based on more traditional proxies on decadal to millennial time scales. Much uncertainty currently exists as to which of the pools of exported PON (suspended vs. sinking) is the main food source for deep-sea corals. This issue will be addressed by comparing the del15N of modern corals from variable depths to the del15N of both suspended and sinking PON pools collected during several one-day cruises to San Pedro basin, one of the California Borderland basins.Funding for this project will support two early career female scientists and several undergraduate student research assistants from Pomona College and the new Climate Science lab in the Keck Science Department (including Claremont McKenna, Scripps, and Pitzer Colleges). Students will have an opportunity to participate in short research cruises to the San Pedro basin, to be trained in advanced analytical techniques, and present research at conferences. The results of this study will be used as topics for senior theses and independent study projects, extending the educational experience by exposing undergraduate students to both practical and intellectual aspects of climate-focused research.