Determination of gas bubble fractionation rates in the deep ocean by laser Raman spectroscopy

Abstract

A new d eep-sea l aser Ram an spectrom eter (DORISS -Deep Ocean Raman In Situ Spectromet er) is used to ob serv e the preferenti al dissoluti on of CO 2 into seawater from a 50%-50% CO 2 -N 2 gas mi xture in a set of experimen ts that test a proposed met hod of CO 2 sequestrati on i n the deep ocean. In a fir st set of experiments performed at 300 m depth, an open-bott omed 1000 cm 3 cube was used to contai n the gas mixture; and in a second set of experiment s a 2.5 cm 3 funnel was used t o hold a bubbl e of t he gas mixtur e in front of the sampli ng optic. By ob servi ng th e ch anging ratios of the CO 2 and N 2 Raman bands we were abl e to determi ne the g as flux and the mass tran sfer co efficient at 300 m depth and com pare th em to t heoreti cal calcul ations for air-sea gas ex change. Although each exp eriment had a differ ent config uration, comparable result s were obt ained. As expected, th e ratio of CO 2 to N 2 drops off at an expon ential rate as CO 2 i s preferen tially di ssolved i n seawater. In fitting th e dat a with theoretical gas flux calculation s, the boundary layer thick ness was det ermined t o be ~42 µm for the gas cube, and ~165 µm for the gas funnel reflecting different boundary l ayer t urbulence. The mass tran sfer co efficient s for CO 2 are k L = 2.82 x 10 -5 m/s for the gas cube experiment, and k L = 7.98 x 10 -6 m/s for the gas funnel experim ent.

Laser Raman spectroscopy as a technique for identification of seafloor hydrothermal and cold seep minerals

“…A remote optical head, connected to the laser and spectrometer via fiber optic cables, is capable of using a stand-off optic behind a pressure window with a working distance of up to 15 cm or an immersion optic with a sapphire window and a 6 mm working distance (Brewer et al, 2004). DORISS has been used to identify and analyze gas mixtures (White et al, 2006a), gas hydrates (Hester et al, 2006;Hester et al, 2007), and minerals. Naturally occurring minerals identified in situ by Raman spectroscopy to date include hydrothermally produced barite and anhydrite, calcite and aragonite in shells, and bacterially produced sulfur (White et al, 2005;White et al, 2006b).…”

Mineral–microbe interactions in deep-sea hydrothermal systems: a challenge for Raman spectroscopy

“…In situ measurements of the composition of natural gas venting in Guaymas Basin and along Hydrate Ridge have shown the composition to be primarily CH 4 (Hester et al 2006). Raman spectroscopy has also been used in ocean experiments to measure rates of CO 2 dissolution (White et al 2006a), and to determine the structure of synthetic and natural hydrates and identify the gas molecules they contain (Hester et al 2006(Hester et al , 2007. And though the focus of this paper is on minerals, the ability for Raman spectroscopy to identify CH 4 and CO 2 is in also highly relevant to hydrothermal studies, as emissions from ultramafic-hosted hydrothermal systems are rich in volatile organic compounds and emissions from volcanic-hosted systems are rich in CO 2 (Charlou et al 2002;Lupton et al 2006).…”

“…Thus, deep-sea deployments have already proven the value of in situ Raman systems; but deep-sea Raman applications have mainly been qualitative with the exception of White et al (2006a), who measured CO 2 dissolution rates. While Raman spectroscopy can also be used for quantitative measurements, the approach is unlike traditional methods of analytic chemistry because absolute peak intensity is not a good basis for quantification.…”

“…Such an approach would greatly improve our understanding of (i) the chemical evolution of hydrothermal systems themselves, (ii) the structure and biogeochemical cycling within microbial mats, and (iii) the impact of hydrothermal venting upon global ocean chemistry. Sea-going Raman systems have been built (Battaglia et al 2004;Brewer et al 2004;Schmidt et al 2004) and deployed to manually analyse gases (White et al 2006a); synthetic and natural clathrate hydrates (Hester et al 2006(Hester et al , 2007; and minerals, fluids and bacterial mats at hydrothermal vents (White et al 2006b;White 2009). Laboratory testing has proven the ability to quantitatively, and autonomously distinguish many minerals typically found at deep-sea hydrothermal vents, even minerals with similar chemical compositions (figure 2; Breier et al 2009a).…”

Deep ocean experiments with fossil fuel carbon dioxide: Creation and sensing of a controlled plume at 4 km depth

“…The use of pH sensors alone will not yield a complete signal, and is likely to underestimate the CO 2 signal in the near field. A sensor for the CO 2 molecule itself would be a great advantage, and newly developed in situ laser Raman spectrometers hold great promise for this Pasteris et al, 2004;White et al, 2005). Figure 12.…”