N-cycling in marine oxygen deficient zones (ODZs)
One main focus of our research is N-cycling in ODZs in the Eastern Tropical Eastern North and South Pacific (ETNP and ETSP). Few recent projects are described below.
1. Impact of mesoscale processes on chemical fluxes in ODZs
ODZs account for 30-50% of global fixed nitrogen (N) loss in the ocean. We observed the highest N deficits, biogenic N2 concentrations, and δ N of NO3 and NO2 values ever reported for an ODZ in the center of an anticyclonic eddy off the Peru coast in the Southeast Pacific Ocean. We proposed that eddies could transport organic material from the more productive coastal waters and increase N-loss offshore. We used this eddy as a natural tracer experiment to estimate a lower isotope fractionation during N-loss (~14‰) compared to previous studies (20-30‰), which could help reconcile the current imbalance in the global N budget (Bourbonnais et al., 2015). The main PI for this project was Dr. Mark Altabet at UMass Dartmouth.
2. N2O cycling in the coastal Peru Upwelling System associated with an ODZ
We used stable isotope and isotopomer data in conjunction with a 3-D-reaction-advection-diffusion model to evaluate the rates and modes of N2O production and consumption (collaboration with Dr. Robert Letscher, University of New Hampshire, USA). Our results suggest that N2O production by incomplete denitrification or nitrifier-denitrification decoupled from aerobic organic matter remineralization are important, previously overlooked, pathways for extreme N2O accumulation in newly upwelled surface waters off the Peru coast. We estimated a large N2O efflux for this region, representing up to ~20% of current estimates for total open ocean N2O emissions, which agrees with previous independent estimates by Arévalo-Martínez et al. (2014) (Bourbonnais et al., 2017).
3. Temporal changes in O2 distribution and N-loss in ODZs
I am also investigating temporal changes in O2 distribution as an indicator of ODZ expansion, and losses of bioavailable N along the P18 meridional section in the Pacific Ocean. The P18 line crosses both the ETNP and ETNP ODZs and has been occupied three times as part of the U.S Repeat Hydrography Program (WOCE, CLIVAR, GO-SHIP) in 1994, 2007 and 2016.
4. Development and use of autonomous platforms to better elucidate spatial and temporal variations in biogeochemical cycles
I collaborated in the past with Dr. Mark Altabet (UMass Dartmouth), Dr. Craig McNeil and Eric D'Asaro (University of Washington) and helped calibrate a novel gas tension device (GTD) to be deployed on autonomous Lagrangian floats during cruises in 2014 and 2017 in the ETNP ODZ. We used N2/Ar measurements from onboard quadruple and isotope ratio mass spectrometers (Reed et al., 2018) to calibrate float gas tension measurements. I most recently deployed an Argo float equipped with this GTD in the ETNP ODZ during the 2016 GO-SHIP P18 expedition and collected N2/Ar samples to calibrate the float data. The float is still collecting high-quality data nearly one year after deployment and will allow the derivation of regional N-loss rates.
Selected relevant publications:
Dale, A., A. Bourbonnais, M. A. Altabet, K. Wallmann, and S. Sommer (2018), Isotopic fingerprints of benthic nitrogen cycling in the Peruvian oxygen minimum zone, Geochemica et Cosmochimica Acta, submitted April 2018, manuscript #: GCA-D-18-00357.
Reed, A., C. McNeil, E. D’Asaro, M. Altabet, A. Bourbonnais, and B. Johnson (2018), A gas tension device for the mesopelagic zone, Deep-Sea Research I, https://doi.org/10.1016/j.dsr.2018.07.007.
McNeil, C., E. D’Asaro, A. Reed, M. A. Altabet, A. Bourbonnais, and C. Beaverson (2018), Innovative Nitrogen Sensor Maps the North Pacific Oxygen Minimum Zone, Oceanography Supplement, 31, p. 96.
Bourbonnais, A., R. T. Letscher, H. W. Bange, V. Échevin, J. Larkum, J. Mohn, N. Yoshida, and M. A. Altabet (2017), N2O production and consumption from stable isotopic and concentration data in the Peruvian coastal upwelling system, Global Biogeochemical Cycles, 31, doi:10.1002/2016GB005567.
Löscher, C. R., A. Bourbonnais, J. Dekaezemacker, C. N. Charoenpong, M. A. Altabet, H. W. Bange, R. Czeschel, C. Hoffmann, and R. A. Schmitz (2016), N2 fixation in eddies of the eastern tropical South Pacific Ocean, Biogeosciences, 13, 2889-2899.
Hu, H.*, A. Bourbonnais*, J. Larkum, H. W. Bange, and M. A. Altabet (2016), Nitrogen cycling in shallow low oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes, Biogeosciences, 13, 1453-1468.
*These authors contributed equally to this work.
Bourbonnais, A., M. A. Altabet, C. N. Charoenpong, J. Larkum, H. Hu, H. W. Bange, and L. Stramma (2015), N-loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment, Global Biogeochemical Cycles, 29, 793-811.