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.