Radiocarbon in methane from waters of the US Atlantic and Pacific margins as collected on R/V Hugh Sharp cruise HRS1713 and R/V Rachel Carson cruise RC0026 in 2017 and 2019 (NCEI Accession 0278599)

Acquisition Description:
Please see the Related Publications (Methods) section for references with additional acquistion and processing details.

Radiocarbon Sample Collection at Sea:
Gases dissolved in seawater were extracted following the procedures outlined in Sparrow and Kessler (2017) and Joung et al (2019 and 2020). As detailed in these prior publications, waters were pumped through a suction hose with a discharge pump and were passed through multiple filters for removing particles. This water was then continuously passed through a gas-permeable membrane to vacuum extract the dissolved gases in the seawater.

Radiocarbon Sample Preparation in the Laboratory:
Extracted gas samples were purified and analyzed using vacuum line procedures and Accelerator Mass Spectrometry as previously detailed in Sparrow and Kessler, (2017). The volume of the whole gas sample needed for processing in the laboratory using vacuum-line techniques was determined by the CH4 concentration and the amount of carbon necessary for the Accelerator Mass Spectrometry (AMS) analysis. Careful monitoring of standards and blanks was done to ensure efficiency of purification and combustion performance.

Dissolved Methane Concentration Measurement:
Dissolved methane concentration measurements were conducted using a headspace equilibration. The concentration of CH4 in the headspace was determined using an Agilent 6850 gas chromatograph with a flame ionization detector (GC-FID), which was then translated into the original dissolved gas concentration using headspace and water volumes, along with the gas solubility.

Instrument notes:
*All apparatus and their detail specifications for the 14C-CH4 measurements can be found in Sparrow and Kessler (2017) and Joung et al. (2019).
*GC-CH4 measurements references are Weinstein et al., (2016) and Leonte et al (2017)
*14C was measured by Accelerator Mass Spectrometer (AMS) at the Keck-Carbon Cycle AMS facility at UC Irvine
*Temperature, salinity, and dissolved oxygen were measured by the CTD sensors