Patrick Monreal: fine-scale spatial variation in N2O cycling in the Eastern Tropical Northern Pacific

Authors

Patrick J. Monreal, Colette L. Kelly, Nicole M. Travis, Pascale A. Baya, Karen L. Casciotti

Abstract

Although it is less prevalent in the atmosphere than carbon dioxide, nitrous oxide (N2O) is a powerful greenhouse gas with a potency 300 times as great. Oceanic sources account for up to one third of the total flux of nitrous oxide to the atmosphere. In oxygen-deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes — nitrification, nitrifier-denitrification, and denitrification — that are impacted by a variety of oceanographic variables and substrates. In this study, isotopomers of N2O from a 2016 cruise to the ETNP aboard R/V Sikuliaq were analyzed to look at spatial variability in the concentration and isotopic composition of N2O between stations. Three distinct regimes of N2O cycling, delineated by its isotopic composition, were observed in the 12 stations that were included in analysis. The high N2O regime was distinguished by high near-surface [N2O], reaching 126.07 ± 12.6 nmol/kg, corresponding to low-oxygen conditions that expand into near-surface isopycnals. Keeling plot analyses point to a different near-surface N2O source in the high N2O regime, with the intercepts for δ15Nα and δ15Nβ 3-4‰ lower at these stations than the rest. The central ODZ regime was characterized by strong consumption and production signals in the anoxic core of the ODZ, indicated by high δ18O (> 90‰) and low δ15Nβ (< -10‰) values. The “background” regime (closest to the Baja peninsula) exhibited less dynamic cycling than the other two — regression between δ18O and δ15Nα or δ15Nbulk did not follow the relationship expected for reduction. A time-dependent model was used to analyze the drivers of this variability, and preliminary findings suggest some of the stations could be approaching steady state while others do not.

Poster
Video

13 Comments on “Patrick Monreal: fine-scale spatial variation in N2O cycling in the Eastern Tropical Northern Pacific

  1. Hello, Patrick!
    Great presentation!
    I was a little confused about the conclusions of your study. You found that N2O accumulations correspond to shallow ODZs, but, in your definitions section, you state that N2O is an intermediate for denitrification (from NO3- to N2) that is produced then (if complete) consumed. Is the issue, then, that the prokaryotes performing denitrification are wasteful in allowing N2O to be released into the atmosphere before they transform it into N2?

    • Yes, Chris, good point! Basically, the enzyme that reduces N2O to N2 is inhibited by oxygen to a greater extent than the enzyme that produces N2O. So, at the suboxic-anoxic interface of the water column (where O2 is low but nonzero), oxygen poisoning of the N2O reductase enzyme allows N2O to accumulate. That is the idea behind “incomplete” denitrification’s contribution to [N2O].

  2. Hi Patrick, This looks like you did a lot of analysis work! Lots of different data sets to figure out that maybe the middle stations are at steady state. Tell me more what you mean by steady state with nitrogen.
    I may be wrong about the middle stations. It would have been nice to see the stations labeled, on the contour map of the depth profiles (figure on the left).
    – Jenny

    • Hi Jenny, yes, it is the middle stations. By steady state I mean the rate of N2O production coming into balance with the rate of N2O consumption in the anoxic core of the ODZ. As shown in some of my figures, we see strong evidence that both N2O production and consumption is occurring at these stations — it is a matter of whether there is net consumption, net production, or steady state.

  3. Hi Patrick! I have loved watching your project progress throughout the summer… super interesting stuff and awesome findings. Concerning the differences between the stations that you noted, do you think that marine profile differences (ex. T, pressure, salinity, etc.) could play a role?

    • Hi Isabelle! Yes, I do. If you look at my water mass diagram, the water at Stns. 8-11 at density 25 kg/m3 — where the high accumulation of N2O is located — is slightly warmer and saltier than the other stations. I haven’t gotten the chance to look into this too much, but I think it’s an important difference.

  4. Great job, Patrick! It’ll be interesting to see what you find about the conditions that lead to a stable steady-state, vs. net N2O production or consumption. This will be important for better predicting future responses of the ocean to climate change.

  5. Hi Patrick!
    Your project is really interesting! I never knew that N2O was a greenhouse gas that was 300 times more potent, so it’s super cool that you were able to delve into that this past summer.
    I had one quick question. You spoke about how you were excited that Stations 4-7 and 12 were possibly reaching steady state. Can you tell me more about what exactly steady state is and what the implications are for it?

    • Hi there! Yeah, by steady state I mean the rate of N2O production coming into balance with the rate of N2O consumption in the anoxic core of the ODZ — versus net production or consumption. Discerning conditions that lead to steady state or otherwise would be helpful for biogeochemical models.

  6. Hi Patrick!! This is super cool!! You mentioned some of the next steps are happening right now — have you found any interesting results? Also, what was your favorite part of the SESUR experience?

    • Hi Sreya!! Thanks for watching! I am finding that changing the set of isotope effects input into the model significantly affects reaction progress/balance over time, which is important because we often observe a range of isotope effects for a given process. As for SESUR, I’ve really enjoyed getting to know members of the lab!

  7. Awesome work, Patrick! Isabelle’s comment (and Rian’s work) is making me wonder about the implications for water mass differences between stations. Where does that warm, salty TSW flow — and do you think it might be responsible for carrying higher accumulations of N2O elsewhere in the Pacific? Let’s revisit this after we’ve emerged from the depths of model optimization!

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