Award: OCE-1234664

Intellectual Merit 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 in anoxic/suboxic 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, but also substantial limitations. 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, the collaborating researchers from Pomona and Claremont McKenna Colleges, and Princeton University evaluated a new proxy for the history of the marine N cycle-- del15N of ON bound within the mineral lattice of deep-sea corals (DSC). Over the last two decades, DSC have arisen as a geochemical archive of Pleistocene oceanographic change: they can be precisely dated with U-Th radionuclides, they record 14C ventilation history of ambient water masses, have broad geographic distribution and form long-lived (tens to hundreds kiloyears) colonies on the seafloor. The incorporated by the corals organic matter that derives from the export production is protected from diagenetic alteration within the carbonate lattice. Simultaneous measurements of the del15N, U-Th and C14 possible within a single coral specimen make DSC unique multi-proxy archive that can link centennial to millennial, regional and global history of variability in the N cycle with other oceanographic processes. Summary of findings A survey of modern coral specimens (Desmophyllum dianthus) from disparate oceanographic environments, each chosen to represent a distinct del15N signature of PON exported from the euphotic zone showed strong statistical correlation between the coral-bound ON (CB-del15N) and del15N of regional export PON (Figure 1 and 2, Wang et al., 2014). With the fidelity of CB-del15N archive established, the research group generated a set of time-resolved records of del15N in fossil D. dianthus from the Subantarctic (south of Tasmania and northern Drake Passage) and Antarctic (southern Drake Passage) regions of the Southern Ocean, spanning 40Ka through the present (Figure 3, Wang et al., 2017). In the modern Southern Ocean, the surface nutrients (including nitrate) are not fully consumed, resulting in leakage of deeply sequestered CO2 to the atmosphere. Incomplete nitrate consumption is manifested in low del15N of exported PON. Wang et al. found that in both oceanographic regions the average CB-del15N during Last Glacial Maximum (LGM) was 4 to 5 permil higher than today (Figure 3), providing a strong proof for a previously proposed hypothesis of the more efficient biological pump in this region, as likely a major mechanism driving lower pCO2 during the ice ages. Stronger vertical stratification in the Antarctic and higher iron supply in the Subantarctic are the two likely mechanisms driving more efficient surface nitrate uptake in the LGM Southern Ocean. The trends defined by the CB-del15N in the Southern Ocean corals were comparable to the previously published del15N records of diatom- and foraminifera-bound ON from both the Antarctic and Subantarctic zones, validating the reliability of the fossil deep-sea corals as paleo-archives (Figure 4). However, higher temporal resolution afforded by the DSC revealed a previously unnoticed feature: the CB-del15N in the Antarctic Zone continued to decrease through the Holocene, pointing to the ongoing decline in the nitrate uptake efficiency. One possible mechanism driving this change is the intensifying overturning of the Southern Ocean, which might have contributed to the rise in atmospheric pCO2 since 8 kyr, the Holocene change that remains to be explained. Broader Impacts The grant supported two early career female scientists and partially supported a Ph.D. thesis. For the latter, the project-based research formed ~40% of the dissertation. 8 undergraduate students were supported directly by the grant, and 5 more participated in this project and related research through senior theses, independent studies, or as RAs additionally funded by the Pomona and/or Claremont McKenna College programs. Students participated in short local research cruises, were trained in laboratory techniques, and presented results at conferences. Participation in both practical and intellectual aspects of climate-focused research extended students? educational experience in the STEM field. Two recent college graduates (both members of URM, one female, one is a recent graduate of the Cal State Los Angeles, the university serving members of the local communities) were employed with a partial support of the grant as post-baccalaurean RAs. This "first after college" job boosted their professional resumes – both RAs have been mentored by the PI in climate-related topics, trained in advanced laboratory skills, and gained experience in mentoring and training the undergraduate students. Last Modified: 03/07/2017 Submitted by: Maria Prokopenko