Dissolved methane concentrations in the southeast Bering Sea, 1980 and 1981

NOAA Data Report ERL PMEL-6

DISSOLVED METHANE CONCENTRATIONS IN THE SOUTHEAST BERING SEA, 1980 AND 1981

Charles N. Katz Joel D. Cline Kimberly Kelly-Hansen

Pacific Marine Environmental Laboratory Seattle, Washington July 1982

UNITED STms DEPARTMENT OF COMMERCE

Malcolm Baldrige. Secratary

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

John V. Byrne. Administrator

Environmental Research Laboratories

George H. Ludwig Director

NOTICE Mention of a commercial company or product does not constitute an endorsement by NOAA Environmental Research Laboratories. Use for publicity or advertising purposes of information from this publication concerning proprietary products or the tests of such products is not authorized.

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Table of Contents

1. Introduction

1

2. Methods

1

2.1 Sampling

1

2.2 Methane Analysis

1

2.2.1 Preconcentration

1

2.2.2 Gas Chromatography

3

2.3 Salinity-Temperature Data

3

3. Organization and Explanation of Data Tables

3

4. Acknowledgments

4

5. References

5

Appendix A: RP-4-SU-80A-IV 13 Aug-07 Sep 1980

6

Appendix B: RP-4-SU-81A-I 26 Jan-04 Feb 1981

67

Appendix C: RP-4-DI-81A-II 11 May-04 Jun 1981

121

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DISSOLVED METHANE CONCENTRATIONS IN THE

SOUTHEAST BERING SEA, 1980 AND 1981

Charles N. Katz, Joel D. Cline and Kimberly Kelly-Hansen

1. Introduction

Dissolved methane was measured in the southeastern Bering Sea (fig. 1) as a part of the Outer Continental Shelf Environmental Assessment Program (OCSEAP) sponsored by the Bureau of Land Management (BLM) and administered by the National Oceanic and Atmospheric Administration (NOAA). The purpose of this program is to identify potential environmental impacts to marine resources prior to the leasing and development of offshore petroleum and gas. These data were collected to assess the spatial and seasonal distributions of methane in order to estimate Lagrangian space scales and the magnitude of diffusive transport processes.

On previous visits to the southeastern Bering Sea, a source of dissolved methane appeared to emanate from Port Moller along the Alaska Peninsula and from the bottom sediments of Saint George Basin (see figure 1). By obtaining the spatial information in conjunction with circulation and microbiological studies, the dispersion of methane from its source could be modeled using appropriate horizontal or vertical diffusion parameters.

This data report summarizes dissolved methane concentration data made on three cruises during 1980 and 1981. Chronologically, the cruise number and inclusive dates covered are: RP-4-SU-80A-IV, 13 Aug-07 Sep 1980; RP-4-SU-81A-I, 26 Jan-19 Feb 1981; RP-4-DI-81A-II, 11 May-04 Jun 1981. A total of 2634 discrete observations of dissolved methane were made.

2. Methods

2.1 Sampling

Discrete water column samples were obtained with 5-L Niskin bottles attached to a General Oceanics Rosette fitted with a Plessey Environmental Systems model 9040 CTD. A 1-L aliquot was taken from the Niskin samplers and stored in a glass-stoppered bottle until analyzed; usually within one hour of sampling.

2.2 Methane Analysis

2.2.1 Preconcentration

The analysis of methane was accomplished routinely in the field using a purge and trap technique adopted from Swinnerton and Linnenbom (1967). The method involves removal of the dissolved gases from a 0.2 L volume of seawater by helium purging. The gases removed from solution passed through

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Figure 1.--Southeast Bering Sea study area.

2

Ascarite@, Drierite@ and Tenax G.C.? traps to remove carbon dioxide, water vapor and heavy hydrocarbons respectively, before being concentrated on an activated alumina trap held at -196?C. After quantitative removal of the

gases from solution (~6 minutes with a purge rate of 100 mL min-I), the

activated alumina trap was warmed to 100?C and the gases backflushed into a gas chromatograph.

2.2.2 Gas Chromatography

Detection and quantitation of methane was carried out on a HewlettPackard 5710A gas chromatograph equipped with dual flame ionization detectors (FID). The column packing used was activated alumina, 60-80 mesh (1.8 m x 0.48 cm o.d.). The column temperature was held at 100?C.

Quantitation of the methane was obtained by comparing the relative detector response of a methane gas standard with that of a sample. Standard gases prepared by Matheson Gas Products were intercalibrated with a standard gas analyzed by the National Bureau of Standards (NBS). Analytical precision was generally less than 1% while accuracy, based on the NBS intercalibration, was 5%. The detection limit of the method, based on a signal-to-noise ratio of 2 is approximately 5 nL L- 1. A detailed dissussion of the method can be found in Katz (1980).

2.3 Salinity-temperature Data

Conductivity, temperature and pressure data were collected using Plessey Systems model 9040 CTD with model 8400 data logger. These systems sampled five times per second for values of temperature, conductivity and pressure.

Data were recorded during the down-cast using a lowering rate of 30 m min-I. Nansen bottle samples were taken on every other cast to provide temperature and salinity calibration. Nominal precision of the salinity and temperature data are to.02 g kg-1 and to.02?C respectively.

3. Organization and Explanation of Data Tables

The data presented in Appendices A-C are organized in a hierarchy of cruise (chronological order), station and depth distribution. Each cruise is presented in a separate appendix. Station data include: Station number; latitude and longitude to the nearest tenth of a degree; local data and time of sampling; Greenwich Mean Time zone; and depth to the bottom in meters. Depth information includes: depth of the sample in meters; water temperature in degrees centigrade; salinity in grams per kilogram; sigma-t; dissolved methane concentration in nanoliters of methane gas per liter seawater corrected to standard temperature and pressure (STP); dissolved methane equilibrium solubility calculated from the equations of Yamamoto et al. (1976) and using the atmospheric mixing ratio of 1.72 parts per million by volume (Heidt et al., 1980); and the methane solubility ratio, C/C*, which is the ratio of the observed methane concentration to the equilibrium solubility methane concentration. Data not available are indicated by a asterisk(*).

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4. Acknowledgements The authors would like to thank Dwight Daniels and Anthony Young for

helping with the analysis of samples. A special thanks also goes to the Captains and crews of the NOAA ships Survegor and Discoverer.

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