Novel laser based instruments for fast real-time monitoring of dissolved gases:
from methane 3D mapping towards in situ precise isotopic ratio measurements
Roberto Grilli
Institut des Géosciences de l’Environnement - IGE, Grenoble, France,
In the frame of a challenging ice-core drilling project (SUBGLACIOR), a unique probe for finding ice older than 1 million years in Antarctica has been developed. The probe consists of an embedded optical spectrometer capable to measure in-situ and real-time the composition of the air bubbles trapped in the ice. For testing the drilling probe, a more accessible environment such as the ocean was selected, opening the doors to novel developments dedicated to the monitoring of marine environments.
The technology to extract and analyze dissolved gases in liquids consists of a membrane based gas extraction system coupled to a laser spectrometer and shows several advantages with respect to current available sensors, such as: fast response time (~15 sec), high dynamic range, multiple species detection and isotopic ratio measurements, telemetric deep-sea applications (up to 4000 m) with compatibility for ROV and AUV surveys.
In October 2015, in the framework of the CAGE project, the instrument ‘SubOcean’ has been deployed over a gas-hydrate zone of western Svalbard. Continuous measurements to depths up to 400 m were made over three days, resulting in high-resolution 3D profiles (~100k data points). The very fast response time of the sensor allows to display the in-situ measurements in real-time directly onboard of the ship while underway. Hectometric-scale variations in the dissolved methane concentrations have been observed over an active flare zone, whereas previous studies consider the distribution to be much more uniform at multi-kilometricscale. A bubble based flare model can reproduce these variations, confirming that physical processes in the ocean can be the cause. First evidence of methane enriched fresh water release from dissociating hydrates, which cannot be detected by echo-sounding and is easily missed with discrete sampling, indicates that the overall methane emission budget from the seabed could end to a significantly different scenario.
Today, we are already working on a new generation of spectrometers which would allow us to access isotopic signatures. For instance a sensor for measuring the 13C of methane while simultaneously detecting ethane as already be developed. This will led to a key tool for identifying different sources of hydrocarbons in the ocean and distinguish between thermogenic and biogenic sources. Furthermore, in the next few years we will also focus on the development of water isotope instruments for continuous and in-situ measurements. D and 18O signatures will be exploited for studying ocean/ice-shelf interactions in the southern oceans, identifying the sources of freshwater and the key processes that are interplaying.
[1]R. Grilli, N. Marrocco, T. Desbois, C. Guillerm, J. Triest, E. Kerstel, and D. Romanini, “Invited Article : SUBGLACIOR : An optical analyzer embedded in an Antarctic ice probe for exploring the past climate,” Rev. Sci. Instrum., vol. 85, no. 111301, pp. 1–8, 2014.
[2]O. Alemany, J. Chappellaz, J. Triest, M. Calzas, O. Cattani, J. F. Chemin, Q. Desbois, T. Desbois, R. Duphil, S. Falourd, R. Grilli, C. Guillerme, E. Kerstel, B. Laurent, E. Lefebvre, N. Marrocco, O. Pascual, L. Piard, P. Possenti, D. Romanini, V. Thiebault, and R. Yamani, “The SUBGLACIOR drilling probe: concept and design,” Ann. Glaciol., vol. 55, no. 68, pp. 233–242, 2014.
[3]R. Grilli, J. Triest, J. Chappellaz, M. Calzas, T. Desbois, C. Guillerm, L. Lechevallier, V. Ledoux, and D. Romanini, “SUB-OCEAN : subsea dissolved methane measurements using an embedded laser spectrometer technology,” Accepted for Publication to Limnology & Oceanography: Methods, 2017.