Dataset: GN01 Dissolved Methane (CH4)

Name: GN01 Dissolved Methane (CH4)
Published: 2024-12-16
Keywords: methane, GEOTRACES, Arctic Ocean, oceans

View Data: Data not available yet

Bering Sea and Western Arctic Ocean to the North Pole

Project:

GEOTRACES Arctic Section: Methane, vanadium, barium, and gallium as process indicators in the Arctic Ocean (GEOTRACES Arctic Methane V Ba Ga)

U.S. Arctic GEOTRACES Study (GN01) (U.S. GEOTRACES Arctic)

Program:

U.S. GEOTRACES (U.S. GEOTRACES)

Principal Investigator:

Alan M. Shiller (University of Southern Mississippi, USM)

Scientist:

Laura Whitmore (University of Southern Mississippi, USM)

BCO-DMO Data Manager:

Shannon Rauch (Woods Hole Oceanographic Institution, WHOI BCO-DMO)

Version:

Version Date:

2024-12-16

Restricted:

Validated:

Current State:

Data not available

GN01 Dissolved Methane (CH4)

Abstract:

This dataset reports dissolved methane (CH4) concentrations from the US GEOTRACES Arctic Expedition on USCGC Healy (GN01, HLY1502) from August to October 2015. Seawater samples were collected using the ODF rosette. Both GEOTRACES samples and GO-SHIP samples from the same cruise were analyzed. Methane was analyzed using cavity ringdown spectroscopy to determine its concentration in an equilibrated headspace. Arctic waters are a possibly significant source of this Greenhouse Gas to the atmosphere and global change is likely exacerbating its release. Methane is also a potentially valuable indicator of interactions with the shelf as well as of river inputs. Dissolved methane concentrations in this section were highest over the continental shelves and slope, which supports our understanding of the major sources of methane (i.e., from microbes in oxygen-limited sediments, from gas seeps, and from gas-hydrates).

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Methods & Sampling

Samples were collected from the ODF rosette rather than GEOTRACES carousel. The GEOTRACES Cookbook does not specifically address sample collection for methane; however, this rosette system was used during the cruise for sampling various ancillary parameters (e.g., oxygen, nutrients) and thus sampling overall followed Chpt. III (HYDROGRAPHY AND ANCILLARY PARAMETERS) of the Cookbook. Sampling methodology (from the Niskin bottles) followed that of Roberts and Shiller (2015). The dissolved methane samples were obtained from the Niskin bottles immediately after other dissolved gases were sampled.

Dissolved CH4 was determined cavity ringdown spectroscopy (CRDS) following headspace equilibration following the method of Roberts and Shiller (2015). Samples were collected from the Niskin bottles immediately after the other dissolved gas samples, equilibrated with methane-free zero-air (approximately 70 milliliters (mL) water and 70 mL zero air) immediately after samples were collected. Equilibrated head-space methane was determined using a Picarro G2301 Greenhouse Gas Analyzer. Calibration was performed using both methane-free zero air and a 5 ppmv CH4 gas standard (NIST traceable). Variability of dissolved methane in deep ocean samples from the GP15 section suggests a standard deviation of better than 0.06 nanomoles per kilogram (nmol/kg) on samples of ∼0.6 nmol/kg, similar to the results of Roberts and Shiller (2015).

Problems/Issues

No problems noted. SeaDataNet quality flags were used.

Data Processing Description

Gaseous methane concentrations in the equilibrated headspace are converted to dissolved phase methane concentrations using Henry's Law using an Excel spreadsheet template as described in Roberts and Shiller (2015).

Quality Flags: Quality flags were applied following the GEOTRACES policy (https://www.geotraces.org/geotraces-quality-flag-policy/), which recommends the SeaDataNet Scheme:

0 = no quality control;
1 = good value;
2 = probably good value;
3 = probably bad value;
4 = bad value;
5 = changed value;
6 = value below detection; (see attached Supplemental File for detection limits for Be-7 samples)
7 = value in excess;
8 = interpolated value;
9 = missing value;
A = value phenomenon uncertain.

BCO-DMO Processing

currently being processed

More Information about this dataset

Funding Sources

Funding Source	Award Number
NSF Division of Ocean Sciences	OCE-1436312

Deployments

Deployment	Synonyms	Start Date	Platform	Investigator
HLY1502	GN01 U.S. GEOTRACES Arctic 32H120150809	2015-08-09	USCGC Healy	David C. Kadko (Chief Scientist) William M. Landing (Co-Chief Scientist)	data info

Instruments

Niskin bottle

Supplied Description:

Instrument Type

Generic Name: Niskin bottle

Community Identifier: http://vocab.nerc.ac.uk/collection/L22/current/TOOL0412/

Generic Description:

A Niskin bottle (a next generation water sampler based on the Nansen bottle) is a cylindrical, non-metallic water collection device with stoppers at both ends. The bottles can be attached individually on a hydrowire or deployed in 12, 24, or 36 bottle Rosette systems mounted on a frame and combined with a CTD. Niskin bottles are used to collect discrete water samples for a range of measurements including pigments, nutrients, plankton, etc.

Picarro G2301 Greenhouse Gas Analyzer

Supplied Description:

Instrument Type

Generic Name: Picarro G2301 Greenhouse Gas Analyzer

Acronym: Picarro G2301

Generic Description:

The Picarro G2301 is a cavity ringdown spectrometer designed to measure the concentration of methane, carbon dioxide, and water vapor in a gaseous sample.

It provides simultaneous, precise measurement of carbon dioxide (CO2) and methane (CH4) at parts-per-billion (ppb), and water (H2O) vapor at parts-per-million (ppm) sensitivity. Patented Picarro cavity ring-down spectroscopy (CRDS) technology enables an effective measurement path length up to 20 km in a compact cavity. Precision at 5 seconds and 5 minutes is 70 and 25 ppb for CO2; 0.5 and 0.22 ppb for CH4; and 80 and 30 ppm for H2O. Maximum drift at standard temperature and pressure (STP) over 24 hours is 120 ppb for CO2; 1 ppb for CH4; 100 ppm for H2O.

Parameters

None available yet

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Dataset: GN01 Dissolved Methane (CH4)