EGU2020-5366
https://doi.org/10.5194/egusphere-egu2020-5366
EGU General Assembly 2020
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Temporal water column dynamics control microbial methane oxidation above an active cold seep (Doggerbank, North Sea)

Tim de Groot1, Malika Menoud2, Thomas Röckmann2, Hossein Maazallahi2, Darci Rush1, Chris Mesdag3, Bart Meijninger4, and Helge Niemann1
Tim de Groot et al.
  • 1NIOZ Royal Netherlands Institute for Sea Research, Marine Microbiology & Biogeochemistry, Den Hoorn, The Netherlands
  • 2University Utrecht, Atmospheric physics and chemistry, Utrecht, The Netherlands
  • 3Deltares, Institute for applied science, Delft, The Netherlands
  • 4TNO, Applied natural research institute, Utrecht, The Netherlands

Methane is a potent greenhouse gas with strongly increasing atmospheric concentrations since industrialisation. In the ocean, methane is most dominantly produced in sediments and is of microbial and/or thermogenic origin. Uprising methane may escape from the ocean floor to the overlying water column where it can be oxidized by methane oxidizing bacteria. The aerobic methane oxidation (MOx) is thus an important final barrier, which can mitigate methane release from the ocean to the atmosphere where it contributes to global warming. Nevertheless, there is rather little knowledge on the temporal dynamics of the microbial methane filter capacity in the water column. To gain a better understanding of the dynamics, we conducted two 48 hours’ time-series experiments during highly stratified conditions in summer and and mixed water column conditions in autumn above an active methane seep in the North Sea (Doggerbank, 41m water depth). At Doggerbank, dissolved CH4  δ13C-values (<-65 ‰) indicate a microbial CH4 origin, and seismic data suggest a gas pocket at >50 m sediment depth. Our time series measurement showed that CH4 concentrations were highly elevated with up to 2100 nM in bottom and 350 nM in surface waters under stratified conditions. The maxima showed a ~12h periodicity, indicating that the flux of CH4 from the seep was linked to tidal dynamics with the lowest CH4 concentrations at rising tide and enhanced flux at falling tide. In contrast, during mixed water column conditions we found lower maxima of only up to 450 nM. Yet, during mixed conditions we found that surface water methane concentrations were on average XX-fold higher compared to stratified conditions, suggesting a higher methane efflux to the atmosphere during this time period.  MOx activity showed a similar temporal behaviour suggesting that tidal dynamics are an important control on the efficiency of the microbial CH4 filter in the water column. Under stratified conditions MOx rates were highest in bottom waters (<5.7 nM/day), however we also found high MOx rates in near-surface waters at times of elevated seep activity during stratified (<3.2 nM/day) and mixed water column conditions (<16.2 nM/day). Our results indicate that the efficiency of the microbial filter is affected by temporal dynamics and seasonality.

How to cite: de Groot, T., Menoud, M., Röckmann, T., Maazallahi, H., Rush, D., Mesdag, C., Meijninger, B., and Niemann, H.: Temporal water column dynamics control microbial methane oxidation above an active cold seep (Doggerbank, North Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5366, https://doi.org/10.5194/egusphere-egu2020-5366, 2020.

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Display material version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-5366, Kun Zhang, 06 May 2020

    It's really detailed work. My question is that is there any temporal changes in the plumes in the water column?

    • AC1: Reply to CC1, Tim de Groot, 06 May 2020

      Dear Khun,

      Thank you for your interest and your question. Unfortunately we don't have direct proof of temporal changes in bubble intensity yet. To visualize this, you would ideally want to setup a camera next to the seep for a similar period or investigate bubble acoustics (ADCP). The latter I am currently working as I am processing ADCP data from both cruises from which I hope to find a temporal bubble effect to strengthen our finding. This is still work in progress. However, we know from previous studies that seep intensities are affected by hydrostatic pressure, and our high frequency methane concentrations data clearly show a similar pattern. 

      I hope it answers your question. Kind regards, Tim de Groot (tim.de.groot@nioz.nl)

      • CC2: Reply to AC1, Kun Zhang, 06 May 2020

        Thanks for your reply. It is really good news that there are ADCP data. I am wondering that are there any responses on multibeam water column data, e.g. the height of plumes, the active time of plumes?

        • AC2: Reply to CC2, Tim de Groot, 06 May 2020

          Hi Khun,

          During our 2 days time-series the multibeam was continuesly on to be certain that we were above the seep. To be able to come up with seep intensity data from multibeam might be tricky due to movement of the ship in combination with tidal currents (bubbles drag away). But I agree it is worthwhile to see if the intensity of the bubble jet varies over time and correlates with our other results. 

          Cheers, Tim

          • CC3: Reply to AC2, Kun Zhang, 06 May 2020

            Dear Tim:

            Thanks for your detailed reply. I have followed your profile in Researchgate. Hoping to see more achievements from you. 

            Best wishes to you and your researches.

             

            Kun Zhang

            Ph.D. student of Tongji University

            kunzhang@tongji.edu.cn