Sediment samples: Sediment cores were collected from the Cinder Cones site including a methane seep habitat and vertically sectioned into identified intervals at either 3cm or 1cm intervals depending on the analysis. To quantify methane from sediment plugs, subcores were taken vertically via a syringe with the end cut off to sample 3 cm3 of sediment.  The subcores were preserved in 2ml of 5M NaOH in a serum bottle and caped with a butyl stopper for later analysis. It was stored and shipped upside down. Large grain size led to rapid dewatering of the cores making the concentrations likely conservative estimates of the methane (i.e. the is a chance some of the porewater was lost during sampling).  For 0-1 cm sediment plugs, a core was vertically sectioned and frozen at -80oC until later analysis.

Porewater samples:  Pore water was sampled in situ by placing a vertical barrier in the sediment, excavating one side and inserting a Rhyzon filter (0.2 um) into the intact sediment on the opposite side of the excavation. 1 ml of porewater was discarded as a rinse and then 5ml of pore water was extracted. These were taken at 2 cm intervals starting at 1 cm below the sediment surface to 5cm.  3ml were preserved in serum bottles with 2ml of 5M NaOH for later methane analysis. 1 ml was filtered to remove particles >0.2 um and frozen for ion analysis and 1ml was preserved with 0.25ml of 0.05 M Zinc acetate for hydrogen sulfide concentration for Spectrophotometric analysis following Cline (1969).  

Sampling and analytical procedures:  
Methane: Samples were kept stored cold and inverted for both porewater and sediment plug sampling approaches for methane.  Headspace from each of the serum vials was injected into a Gas Chromatograph with Flame Ionization Detector (GC-FID) after a standard curve was generated based on known standards increasing from 10 to 1000 ppm and the same injection volume (between 50 and 100 ul) as the headspace of the samples.  Adjusted r2 of the calibration curve was  1.0.  Response area was measured using “Peak Simple” software.

Methane Flux: Methane flux was measured in 2016 through the deployment of benthic flux chambers. Chambers were 10cm i.d. cores with a cap containing a septa sealed hole for sampling and an o-ring seal around the cap. The chambers were sealed with the caps after the cores were inserted into the sediment to ensure minimal disturbance to the system. Chambers were not continually stirred but were stirred by using magnetic propeller system that allowed gentle agitation prior to each sample collection. Samples were taken on 12-hour intervals to quantify that release of methane through syringe sampling through the septa; at no point were the chambers opened