Methane in coastal sea water, sea ice, and bottom sediments, Beaufort Sea, Alaska
This report summarizes data acquired from 1990 to 1994 for the gas-hydrate portion of the USGS project "Permafrost and gas hydrate as possible sources of methane" of the USGS Global Change and Climate History program. The objective of this project has been to test the hypothesis that gas hydrate deposits of the Beaufort Sea continental shelf are destabilized by the ~10°C temperature increase that has resulted from the Holocene transgression of the Arctic Ocean. To test this idea we have selected an area off the north coast of Alaska centered on Harrison Bay. We have measured the concentration of methane in surficial sediments, in the water column when ice is present and absent, and in seasonal sea ice. Our results show that more methane is present in the water when ice is present than when ice is absent, and that methane is also present within the ice itself, often at higher concentrations than in the water. Thus the Beaufort Sea shelf of Alaska is a seasonal source of methane. The primary source of this methane has not yet been defined, but gas hydrate is a reasonable candidate.
The objective of this project has been to test the hypothesis that gas hydrate deposits of the Beaufort Sea continental shelf are destabilized by the ~10°C temperature increase that has resulted from the Holocene transgression of the Arctic Ocean.
The surveys were undertaken during the last week of April and first week of May 1992, the middle of April 1993, the latter portion of April 1994, the second week of September 1993, and the later portion of August 1994.
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The study area showing six north-south transects with the water and ice station locations.
The study area showing six north-south transects with the water and ice station locations.
The project has been conducted in collaboration with the University of Washington and the University of Hawaii. We thank (1) M.D. Lilley, E.J. Olson, and E. McLaughlin, School of Oceanography, University of Washington, for the information on methane concentrations, methane oxidation rates, and nutrient concentrations in seawater; (2) B.N. Popp, F. J. Sansone, and T. Rust, SOEST, University of Hawaii, for the isotopic analyses of methane. We are grateful to (1) P.W. Barnes, U.S. Geological Survey, for guidence in Arctic operations, and (2) E. Reimnitz for help in collecting sediment samples for gas analyses. This work is supported by the USGS Global Change and Climate History Program.
See process descriptions for analytical methods. For more information about attribute accuracy, users are urged to contact the authors.
CTD (conductivity, temperature, depth) casts were made at each station. CTD data was recorded internally by a Seacat 19 CTD probe made by Sea Bird of Seattle Washington. The data was downloaded onto a computer where it was processed using Seasoft Version 4.016. The sub-programs LOOPEDIT, BINAVE, then DERIVE were utilized to process the raw data. The data were averaged into 0.5m intervals. The upcast and downcast were evaluated, and only one cast is plotted. In general the upcast was used because the sensors showed less fluctuation.
Methane oxidation rate is given only for samples collected in 1992 and 1993. Isotopic compositions are given only for samples collected in 1994. Nutrients are given only for samples collected in 1992.
Measurements of sea water.In our survey, we measured methane concentration in sea water under the ice on six primary transects from nearshore to the shelf break at water depths ranging from 2.5 to 88m from Cape Halkett east to Mikkelsen Bay, Alaska. Other surface water samples were taken at various locations where water depths ranged from 2 to 20m within the area between Camden Bay and the Colville River Delta. One oceanic surface water sample was also collected in April 1993 at 72° 38' N. latitude . The surveys were undertaken during the last week of April and first week of May 1992, the middle of April 1993, the latter portion of April 1994, the second week of September 1993, and the later portion of August 1994. At each station in April and May, a 23- cm diameter hole was drilled through the 2-m thick first-year ice. Niskin bottles (1.7 l) were attached at intervals to a line lowered to the seafloor and subsequently tripped to collect water samples for methane determinations. In September 1993 and August 1994, when no ice was present, a boat was used to transit to stations. As each Niskin bottle was recovered, 100 cm3 of water was transferred to a 140 cm3 syringe. A heat lamp to prevent the water from freezing during April and May surveys. In the laboratory at the end of each day's field work, 40 cm3 of ultra-pure nitrogen were added to each of the syringes, and methane was extracted from the water into the nitrogen at room temperature by shaking the syringe, following a method modified from McAullife (1971). A portion of the resulting gas mixture was measured for methane content by gas chromatography. Multiple extractions were used on several samples to determine an empirical extraction efficiency coefficient which was utilized with the remainder of the samples that were extracted only once. Our analyzed gas standard contained 10 ppm ±2% methane in nitrogen. Duplicate samples were analyzed for each water depth with an average standard deviation of ±0.6nM.
CTD (conductivity, temperature, depth) casts were made at each station. CTD data was recorded internally by a Seacat 19 CTD probe made by Sea Bird of Seattle Washington. The data was downloaded onto a computer where it was processed using Seasoft Version 4.016. The sub-programs LOOPEDIT, BINAVE, then DERIVE were utilized to process the raw data. The data were averaged into 0.5m intervals. The upcast and downcast were evaluated, and only one cast is plotted. In general the upcast was used because the sensors showed less fluctuation.
The methods utilized to measure nutrients in this study were adapted from older colorimetric procedures and are described in detail by Whitledge and others, (1981). A basic description of each method, conducted on a Technicon Autoanalyzer System, follows.
Phosphate is determined as phosphomolybdic acid which in its reduced form in the presence of antimony has an absorption maximum at 880 nm. The method is basically an automated version of the procedure of Murphy and Riley (1962).
Orthosilicic acid is determined by its reaction with molybdate in aqueous acidic solution to form silicomolybdic acid. In this procedure, which is basically that of Armstrong and others, (1967), stannous chloride is used to reduce silicomolybdic acid to the heteropolic acid which has an absorption maximum at 820 nm.
Nitrite is determined by the Greiss reaction in which sulfanilamide and N-(1-Napthyl)ethylenediamine dihydrochloride react with nitrite in aqueous acidic solution to form an intensely pink diazo dye with an absorption maximum at 540 nm (Bendschneider and Robinson, 1952). Nitrate, after it is reduced to nitrite by passage through a column of copperized cadmium filings, is determined in an identical manner to nitrite (Wood and others 1967). This analysis gives the sum of nitrate + nitrite, and nitrate is determined by difference.
Ammonium is determined by the Berthelot reaction in which hypochlorous acid and phenol react with ammonium in aqueous alkaline solution to form indophenol blue, an intensely blue chromophore with an absorption maximum at 637 nm. The method utilized is a modification of the procedure reported by Slawyk and MacIsaac (1972).
Measurements of sea ice.The methane content of sea ice was measured at 51 stations in 1993 and 1994. A 1m long by 7.6 cm diameter ice corer with an extension was used to core through the ice to sea water. The ice core was laid out on the sea ice, measured and cut into 10 cm sections. Selected intervals were chosen, then placed in 1-liter friction sealed cans equipped with 2 septa ports, sealed, and kept frozen. At the field laboratory, each sample was weighed, then the headspace within the can was purged with ultra pure nitrogen for 5 minutes at flow rates exceeding 200 ml/min. Tests on the methane content of the purged headspace with the frozen ice core inside yielded concentrations less than 0.1 ppm. The cans containing ice samples were placed in hot water baths and stabilized at approximately 20°C. Each can was shaken by hand for about 30 seconds to partition the dissolved methane between the water and the nitrogen headspace. Next, a syringe containing 30 ml of ultra pure grade nitrogen was injected into the can, then 30 ml of the resulting mixture of headspace and gas was removed and analyzed for methane content in the same manner as described for sea water samples. The volume of headspace in the can was determined by the weight of the ice sample, assuming that once melted the water occupied a volume of 1 ml/gm.
Measurements of sea-floor sediment.Sediment samples were taken in May 1992 operating from the ice canopy. A hole was augured in the ice, through which a 45 x 7.5 cm core barrel with an auger cutting head was lowered. The core barrel was attached to 1.5-m long connecting stems in series up to a maximum length of 15.5 m. Once on the ocean bottom, the core barrel was rotated by hand with the aid of a T-bar until no further penetration was achieved. Upon retrieval, the liner containing sediment was extruded onto the ice. The least disturbed 10-cm long (a volume of about 450 ml wet sediment) section of the core was chosen and placed in a septa-equipped 1-liter sample can. Seawater was added to the brim of the can, then 200 ml of water was removed, creating a 200 ml headspace. About 2-3 grams of sodium azide was added as a biocide. The can was sealed, then purged with helium at a flow rate estimated to be greater then 300 ml/min for 5 minutes. The samples were kept frozen until analysis at our Menlo Park laboratories.
Measurements of methane isotopic composition.A limited number of measurements of the carbon isotopic composition of methane were made at the University of Hawaii. The results reveal a wide range of values from -31.1 to -80.5 o/oo.
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Identifier for the sample locality, composed as YYMsssR where YY is the last two digits of the year, M is the month (no samples were taken from October through December, thus one digit suffices), sss is the site number, and R (literally) is present to indicate that the site was occupied at an earlier date.
Longitude of the sample locality. Negative values are measured westward from the Prime Meridian, positive values are measured eastward from the Prime Meridian.
Calendar date when sample was collected, expressed as a FIPS date using the form YYYYMMDD where YYYY signifies the year, MM the month, and DD the day (month and day beginning with 1).
Depth in meters at which water was sampled, measured from sea-surface.
Carbon isotopic composition (delta-13C) relative to PDB
Rate of microbial oxidation of methane. This "specific oxidation rate" is really just the first order rate constant for methane uptake and has the units (1/day) because the oxidation rate (nM/day) is divided by the concentration (nM).
The first order rate constant for the microbial oxidation of methane.We measure methane oxidation by first removing (via shaking) most of the methane from the collected samples. We then spike the samples with a known amount of labeled methane which is somewhere near the concentration of methane in the natural sample. We measure the rate of methane oxidation by watching labeled carbon dioxide grow in. Some methane is converted to biomass and we get at this part by filtering and counting the amount of labeled particulate carbon.
Once we have these numbers, we calculate what is called the "specific oxidation rate". This is simply the measured rate divided by the concentration of methane in the incubation vials. This "specific oxidation rate" is really just the first order rate constant for methane uptake and has the units (1/day) because the oxidation rate (nM/day) is divided by the concentration (nM).
We can then derive what we call the "ambient oxidation rate" by multipling the "specific oxidation rate" by the concentration of methane actually measured in the samples. This obviously only works if first order kinetics are followed but we have found this to be the case in all of the environments we have studied.
Descriptive information about the water samples or the analyses of them.
Phosphate is determined as phosphomolybdic acid which in its reduced form in the presence of antimony has an absorption maximum at 880 nm. The method is basically an automated version of the procedure of Murphy and Riley (1962).
Orthosilicic acid is determined by its reaction with molybdate in aqueous acidic solution to form silicomolybdic acid.In this procedure, which is basically that of Armstrong and others, (1967), stannous chloride is used to reduce silicomolybdic acid to the heteropolic acid which has an absorption maximum at 820 nm.
Nitrate, after it is reduced to nitrite by passage through a column of copperized cadmium filings, is determined in an identical manner to nitrite (Wood and others 1967). This analysis gives the sum of nitrate + nitrite, and nitrate is determined by difference.
Nitrite is determined by the Greiss reaction in which sulfanilamide and N-(1-Napthyl)ethylenediamine dihydrochloride react with nitrite in aqueous acidic solution to form an intensely pink diazo dye with an absorption maximum at 540 nm (Bendschneider and Robinson, 1952).
Ammonium is determined by the Berthelot reaction in which hypochlorous acid and phenol react with ammonium in aqueous alkaline solution to form indophenol blue, an intensely blue chromophore with an absorption maximum at 637 nm. The method utilized is a modification of the procedure reported by Slawyk and MacIsaac (1972).
CTD (conductivity, temperature, depth) casts were made at each station. CTD data was recorded internally by a Seacat 19 CTD probe made by Sea Bird of Seattle Washington. The data was downloaded onto a computer where it was processed using Seasoft Version 4.016. The sub-programs LOOPEDIT, BINAVE, then DERIVE were utilized to process the raw data.
Salinity of the water as determined from the conductivity measurements recorded by the CTD instrument.
CTD (conductivity, temperature, depth) casts were made at each station. CTD data was recorded internally by a Seacat 19 CTD probe made by Sea Bird of Seattle Washington. The data was downloaded onto a computer where it was processed using Seasoft Version 4.016. The sub-programs LOOPEDIT, BINAVE, then DERIVE were utilized to process the raw data.
Depth of the CTD instrument when temperature and conductivity were recorded. This parameter is derived internally by the CTD instrument by using ambient hydrostatic pressure.
CTD (conductivity, temperature, depth) casts were made at each station. CTD data was recorded internally by a Seacat 19 CTD probe made by Sea Bird of Seattle Washington. The data was downloaded onto a computer where it was processed using Seasoft Version 4.016. The sub-programs LOOPEDIT, BINAVE, then DERIVE were utilized to process the raw data.
Conductivity of water as determined by the CTD instrument.
CTD (conductivity, temperature, depth) casts were made at each station. CTD data was recorded internally by a Seacat 19 CTD probe made by Sea Bird of Seattle Washington. The data was downloaded onto a computer where it was processed using Seasoft Version 4.016. The sub-programs LOOPEDIT, BINAVE, then DERIVE were utilized to process the raw data.
The length of the part of the ice core that was analyzed.
Weight in grams of the container before placing the sample into it.
Difference between variable "Can+sample Wt. gm" and variable "Can Wt. gm".
Concentration of methane in the ice-core sample, as a proportion of the weight of the sample.
Concentration of methane in the ice-core sample, in absolute rather than relative terms.
Carbon isotopic composition of the methane in the ice- core sample (delta-13C) relative to PDB
Distance from top of sediment core to top of the sampled interval.
Distance from top of sediment core to bottom of the sampled interval.
Concentration of methane in the sediment as a proportion of the volume of the wet sediment.
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U.S. Geological Survey Open-File Report 95-70
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Data General AViiON 6220 system running DG/UX version 5.4R3.10 (UNIX)
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