2021_16: deoxygenation: what are the biogeochemical implications, are there mitigation options? Supervisors: Yves Plancherel ([email protected]); Prof Katsumi Matsumoto, University of Minnesota, USA Department: Department of Earth Science and Engineering impose stresses on the global marine in at least three major ways: warming, acidification, and deoxygenation (www.ocean-.org). While warming and acidification are detrimental to marine organisms, the ocean heat and carbon reservoirs are big, and that size helps to buffer (or delay) some of the negative impacts. In contrast, the ocean oxygen reservoir is small, finite and rapidly diminishing. Oceanic creatures are strongly dependent on oxygen level on the time scales directly relevant to the survivability of each individual. A lack of oxygen is quickly lethal. Ocean warming is changing ocean stratification and affecting air- gas exchange; agricultural runoffs and anthropogenic cause . Together, these stressors cause marine oxygen levels to decrease, with dangerous consequences for and the biogeochemical cycles of many elements. Moderation of ocean deoxygenation would in principle be possible if surface biological production was to decrease in specific locations. Theoretically, reducing in special ocean regions called “nutrient-traps” could allow some of the nutrients trapped there to leak into the gyres, and thereby not only alleviate local oxygen demand in the subsurface but also fertilize downstream nutrient-poor oligotrophic regions with a knock-on positive impact on ocean desertification as well. The water that upwells in nutrient-traps is rich in nutrients and low in oxygen. Once at the surface, nutrients stimulate the production of organic matter, which ultimately sinks. Respiration at depth releases the nutrients back and consumes oxygen, trapping the nutrients in a closed loop with intense oxygen demand. Down-regulating surface productivity would break the loop and let nutrients leak out, alleviating the subsurface oxygen problem. The idea that decreasing productivity in small specific regions could increase productivity elsewhere and overall is counter-intuitive but could result in win-win scenarios for oxygen, downstream nutrient availability and possibly carbon cycling depending on levels of anoxia, changes in carbon export and influences on nutrient sinks. There are also of course other negative regional effects to consider as well. Since highly productive nutrient trap regions are also systems that sustain major , the implications of decreasing productivity in these specific regions have to be thoroughly evaluated. The objectives are to: • Investigate how existing climate-biogeochemistry model represent the oxygen cycle, quantify biases and uncertainties.

For more information on how to apply to us please visit: www.imperial.ac.uk/changingplanet

• Evaluate how the cycle of other biogeochemical variables (nutrients, carbon, trace elements) relate to climate induced perturbations in the oxygen cycle. • Develop numerical modelling experiments to explore the proposition that down- regulating marine productivity in specific regions could provide relief to deoxygenation and eutrophication. The ideal candidate will have a strong interest in global biogeochemistry, ideally with a background in ocean sciences. Demonstrated experience in computer programming would be an advantage. See http://endian.earth for information about the Environmental Diagnostic and Analysis group. Do not hesitate to get in touch for further details or to discuss other opportunities ([email protected]). Background information: • https://www.ocean-oxygen.org • https://www.pnas.org/content/117/37/22866 • https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GB006601 • https://www.annualreviews.org/doi/abs/10.1146/annurev-marine-010318-095138 • http://elementsmagazine.org/2020/06/01/biogeochemical-controls-on-the-redox- evolution-of-earths--and-atmosphere/ • http://www.geotraces.org • https://www.imperial.ac.uk/grantham/education/science-and-solutions-for-a- changing-planet-dtp/

For more information on how to apply to us please visit: www.imperial.ac.uk/changingplanet