Water column radiotracer methane oxidation rates in the Guaymas Basin, Gulf of California from R/V Atlantis cruise AT42-05 in 2018 and R/V Falkor cruise FK190211 in 2019 (NCEI Accession 0291647)

Acquisition Description:
Sampling and analytical procedures:

CH4:
Samples were collected by filling 160 ml glass serum vials with seawater, stoppering them headspace-free with butyl chloride stoppers, and crimp sealing them. Samples were killed by injecting 0.5 ml 8 M NaOH. Ten ml killed sample was then displaced with a ten ml UHP nitrogen gas headspace. Samples were stored at room temperature until analysis. At the time of analysis, an additional ten ml of nitrogen was added to the vials. Vials were shaken for 2 min before headspace was sampled and injected into a GC FID (Shimadzu GC 2014, Alltech Carbosphere 80/100 6’x1/8” column). This method is adapted from Magen, 2014 (A simple headspace equilibration method for measuring dissolved methane. Limnol. Oceanogr. Methods 12(9): 637-650).

MOx [data in both cruises]:
Four technical replicates were collected from each sampled depth by filling 16.6 mL Hungate tubes headspace-free and sealing with exetainer septa. Tubes were stored at 4 C until injection. One of the four replicates was killed by displacing 1.6 ml sample with 1.6 ml 37% formalin. All samples, including the killed abiotic control, were injected with ~2000000 dpm tritiated methane tracer. Samples were incubated at near-in situ temperature for 1-4 days. Immediately before incubation termination, a 100 ul subsample was removed from each tube and measured on a liquid scintillation counter to determine total dissolved 3H-methane addition. Incubations were terminated by killing with 1.6 ml 37% formalin and purging with air for 1 hour to remove remaining tracer. A 5 ml aliquot of each killed replicate was measured on a liquid scintillation counter to determine the ratio of tracer turnover. The turnover ratio was multiplied by the measured in situ methane concentration to obtain the MOx rate. Killed controls were subtracted and the replicates averaged to obtain the final rate number. See Rogener, 2018 (Long-term impact of the Deepwater Horizon oil well blowout on methane oxidation dynamics in the northern Gulf of Mexico. Elem Sci Anth, 6(1), 73).