Sub-Ocean: Subsea Dissolved Methane Measurements Using an Embedded Laser Spectrometer Technology

Abstract

We present a novel instrument, the Sub-Ocean probe, allowing in situ and continuous measurements of dissolved methane in seawater. It relies on an optical feedback cavity enhanced absorption technique designed for trace gas measurements and coupled to a patent-pending sample extraction method. The considerable advantage of the instrument compared with existing ones lies in its fast response time of the order of 30 s, that makes this probe ideal for fast and continuous 3D-mapping of dissolved methane in water. It could work up to 40 MPa of external pressure, and it provides a large dynamic range, from subnmol of CH per liter of seawater to mmol L. In this work, we present laboratory calibration of the instrument, intercomparison with standard method and field results on methane detection. The good agreement with the headspace equilibration technique followed by gas-chromatography analysis supports the utility and accuracy of the instrument. A continuous 620-m depth vertical profile in the Mediterranean Sea was obtained within only 10 min, and it indicates background dissolved CH values between 1 and 2 nmol L below the pycnocline, similar to previous observations conducted in different ocean settings. It also reveals a methane maximum at around 6 m of depth, that may reflect local production from bacterial transformation of dissolved organic matter.

Aquatic and terrestrial cyanobacteria produce methane

“…This concentration is around one order of magnitude higher than concentrations of CH 4 found in seawater that is at atmospheric equilibrium (~2 nM). However, CH 4 in the mean mixed layer of the Atlantic Ocean and surface waters of the Mediterranean Sea have been reported to be often supersaturated, reaching maximum CH 4 values up to 7 and 25 nM, respectively (44,45). This simplified calculation demonstrates that Prochlorococcus alone (aside from other marine cyanobacteria) might contribute substantially to the observed oceanic CH 4 supersaturation.…”

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

“…The magnitude and trend of such a phenomenon are still under debate (e.g. Hong et al, 2018;Ruppel and Kessler, 2016;Andreassen et al, 2017) and accurate methods to measure methane concentrations from its source are needed. At shallow seepage sites, such as the East Siberian Arctic Shelf, CH 4 can potentially reach the atmosphere and amplify global warming (Shakhova et al, 2010(Shakhova et al, , 2014.…”

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

“…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 ).…”

“…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). In the last 3 decades, natural sources contributed ∼ 35-50 % of the total global methane emissions, and fresh water constituted one of the largest fluxes after natural wetland and together with geological sources (including seafloor).…”