Project: Collaborative Research: Did the SE Pacific Gyre become a Hot Spot for N2 Fixation during Dusty Glacial Conditions?

NSF Award Abstract
The element nitrogen is a fundamental component of all living things and its cycling through the environment is an important component of Earth's biosphere. As a vital nutrient, the availability of nitrogen in a biologically usable form often "limits" the growth of plants both on land as well as in the ocean. Paradoxically, nitrogen is very abundant as dinitrogen gas (N2) in both the Earth's atmosphere and dissolved in seawater. However in this chemical form, nitrogen cannot be used by most living things. Only a small subset of microbes has the ability to "fix" N2 gas, that is, to convert it into a biologically usable chemical form. Thus, these N2 fixing organisms provide a critical environmental function sustaining life on this planet. In the ocean, N2 fixation is a major control on the total amount of biologically available nitrogen, balancing over the time losses back to N2 gas. The amount of biologically available nitrogen in turn controls the growth (productivity) of photosynthetic organisms (phytoplankton) in the sunlit region of the surface ocean which form the base of the food chain and contribute to oceanic control of the atmospheric levels of greenhouse gases.

This project concerns itself with understanding the fundamental, large-scale controls of oceanic N2 fixation and how they are influenced by climate change over time. N2 fixing microbes themselves appeared to be limited by the availability of other nutrient elements such as phosphorous and iron. While it is known in which parts of the ocean there is at present greater or lesser availability of phosphorous and iron, it remains unclear if either is of overriding importance or if changes in the past produced significant variations in N2 fixation. Past changes in N2 fixation may have been an important feedback on oceanic control of atmospheric greenhouse gases. Understanding these past changes and their controls will provide the knowledge base for improving prediction of how ocean N2 fixation may respond to future changes in climate. This is of great societal relevance as changes in oceanic N2 fixation will ultimately impact marine ecosystems and living resources as feedback on the greenhouse gases driving climate change.

To address these questions, the research team will undertake a study the climate-sensitivity of N2 fixation in the southeast Pacific gyre over the last glacial cycle as well as its plausible "master controls". This oligotrophic region experiences little modern N2 fixation despite proximity to a large supply of excess phosphate from the adjacent Peru-Chile oxygen minimum zone. This is consistent with modern iron limitation due to low aeolian supply that would have been relieved during past dusty conditions. The research team will use the natural experiment of the last full glacial cycle, captured in the foraminiferal-bound N isotopes of gyre sites as well as sites at its southern margin, to probe controls on the marine N cycle exerted by variable dust inputs and changes in N-loss in the adjacent oxygen minimum zone, and relate these to known changes in greenhouse forcing of climate. Through numerical modeling, the research team will also consider whether past variations in N2 fixation in this region may have impacted the global ocean N cycle and budget.

This project will also fund the training of undergraduate and graduate students and support participation of high school students from underrepresented groups in original research. The research team will continue their outreach efforts through established partnerships with elementary, middle, and high schools, engaging a diverse school population and their families with exciting and relevant science.