High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site

Jansson, Pär; Triest, Jack; Grilli, Roberto; Férré, Bénédicte; Silyakova, Anna; Mienert, Jürgen; Chappellaz, J.

doi:10.5194/os-15-1055-2019

Cited by 18 publications

(17 citation statements)

References 49 publications

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“…Seafloor seepage off western Svalbard and study sites. Gas plumes in the water column have been identified in hydroacoustic data at discrete locations all along the western Svalbard margin 24 : (1) at ∼80 m water depth unrelated to present day gas hydrates 25,26 , (2) at ∼300-400 m water depth near the shelf break and the gas hydrates stability zone pinch-out 18,27-29 and (3) at ∼1200 m water depth on the eastern segment of the Vestnesa Ridge 5,[30][31][32][33] , a contourite drift of ∼100 km length and 30 km width ( Fig. 1) 32 .…”

Section: Resultsmentioning

confidence: 99%

Impact of tides and sea-level on deep-sea Arctic methane emissions

et al. 2020

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Sub-sea Arctic methane and gas hydrate reservoirs are expected to be severely impacted by ocean temperature increase and sea-level rise. Our understanding of the gas emission phenomenon in the Arctic is however partial, especially in deep environments where the access is difficult and hydro-acoustic surveys are sporadic. Here, we report on the first continuous pore-pressure and temperature measurements over 4 days in shallow sediments along the west-Svalbard margin. Our data from sites where gas emissions have not been previously identified in hydro-acoustic profiles show that tides significantly affect the intensity and periodicity of gas emissions. These observations imply that the quantification of present-day gas emissions in the Arctic may be underestimated. High tides, however, seem to influence gas emissions by reducing their height and volume. Hence, the question remains as to whether sea-level rise may partially counterbalance the potential threat of submarine gas emissions caused by a warmer Arctic Ocean.

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Section: Resultsmentioning

confidence: 99%

Impact of tides and sea-level on deep-sea Arctic methane emissions

et al. 2020

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“…In October 2015 it was deployed over a hydrate degassing zone west of Svalbard, highlighting for the first time high variability of dissolved CH4 near the seabed together with a strong diffusivity most probably induced by the di-phasic medium generated by 55 the gas flares (Jansson et al, 2019). In this work we report a successful deployment of the Sub-Ocean sensor in a very different setting, highlighting the reliability and adaptability of the technique to different aquatic environments.…”

Section: Introductionmentioning

confidence: 77%

Continuous In Situ Measurement of Dissolved Methane in Lake Kivu Using a Membrane Inlet Laser Spectrometer

Grilli¹,

Darchambeau²,

Chappellaz³

et al. 2019

Preprint

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Abstract. We report the first high resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented, membrane based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to six orders of magnitude with respect to the original prototype (from nmol L−1 to mmol L−1 detection) to fit the range of concentrations at lake Kivu. The accuracy of the instrument was estimated to ±22 % (2 s) from the standard deviation of eight profiles at 80 m of depth, corresponding to ±112 μBar of CH4 in water or ±160 nmol L−1 at 25 °C and 1 atm. The instrument was able to continuously measure the top 150 m of water depth within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 % at 150 m depth, which at this depth and thermal condition of the lake corresponds to 3.5 mmol L−1. At deeper depth, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results were in good agreement with discrete in situ measurements conducted with the commercial HydroC sensor. The fast profiling feature will be highly profitable for future monitoring of the lake, while the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and which would allow to measure at higher concentrations of CH4.

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“…A full description of the in situ membrane inlet laser spectrometer instrument (Sub-Ocean), together with the experimental setup used for laboratory calibrations can be found in Grilli et al (2018) and Jansson et al (2019). In order to adapt the instrument to the high concentrations of dissolved CH 4 expected in Lake Kivu, the absorption spectrum of the optical spectrometer was set away from the strong CH 4 rotationalvibrational transitions, more precisely at 2238.5 nm, where concentrations inside the optical cavity may reach up to 1.5-2 % CH 4 in air before optical saturation (equivalent to the absorption of 10 −5 -10 −6 cm −1 ).…”

Section: The Sub-ocean Instrumentmentioning

confidence: 99%

“…Methane (CH 4 ) is the second-most-important greenhouse gas contributing to the anthropogenic radiative forcing of the atmosphere, and its atmospheric content has risen 2.5-fold since the Industrial Age. During the last decades, significant efforts have been made to better estimate methane contributions of natural and anthropogenic sources to the global atmospheric budget (Kirschke et al, 2013;Saunois et al, 2019). The development of more advanced techniques allowed the recognition of a larger number of sources, which, coupled with the improvements in the modeling, led to continuous rectifications of this budget (Hamdan and Wickland, 2016).…”

Section: Introductionmentioning

confidence: 99%

Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer

Grilli

Darchambeau²,

Chappellaz

et al. 2020

Geosci. Instrum. Method. Data Syst.

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Abstract. We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol L−1 to mmol L−1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to ±22 % (2σ) from the standard deviation of eight profiles at 80 m depth, corresponding to ±0.112 mbar of CH4 in water or ±160 nmol L−1 at 25 ∘C and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L−1. Deeper down, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and total dissolved gas pressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu.

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High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site

Cited by 18 publications

References 49 publications

Impact of tides and sea-level on deep-sea Arctic methane emissions

Impact of tides and sea-level on deep-sea Arctic methane emissions

Continuous In Situ Measurement of Dissolved Methane in Lake Kivu Using a Membrane Inlet Laser Spectrometer

Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer

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