Methodology:

Atmospheric methane concentrations were continuously measured along the entire DY158 expedition (Montevideo to Mare Harbour) using a Los Gatos Research (Mountain View, CA, USA) Ultra-portable Greenhouse Gas Analyzer (UGGA). The inlet of the UGGA was situated on the meteorological mast at the front of the ship (around 17.6 m asl (above sea level)), to be minimally affected by pollution from the exhaust stack at the back of the ship. A 30-metre long inlet tube with internal diameter of 3/8" (dekabon) connected the mast to the UGGA in the met lab. The UGGA took measurements of atmospheric methane, carbon dioxide and water vapour concentrations every second. A KNF pump (type N816.1.2KN.18) pulled air from the inlet down the tube. The residence time was 10 seconds. The air inlet included a funnel pointing down to minimise rain entering and a water trap (Norgren F07 series 40 um G 1/4, part no.: F07-200-A3TG), which was located just downstream of the inlet, was used to trap the majority of the water droplets entering the inlet. There were 2 in line filters (7 and 2 microns) used in the setup to stop particles entering the pump/UGGA which could cause damage. Dryers (a magnesium perchlorate drying tube and a Nafion dryer) were used initially but were removed after a leak was identified in the Nafion on 05/01/2023. After the removal of the dryers the water vapour measured by the UGGA stayed below 1% which is acceptable for operation.

During cruise DY158 water samples were taken at various locations from a CTD with Niskin bottle rosette-casts and water surface samples were taken more frequently using the underway water system on the ship which has its inlet at 5 m below the sea surface. The CTD rosette contained Seabird SBE 9plus temperature and salinity sensors, AquaTracka III Fluorometer (Chelsea Technologies Group) for chlorophyll a detection and twenty-four 20 L Niskin bottles to take samples of seawater from discrete depths. The salinity measured by the Seabird SBE 9plus sensor was calibrated by taking samples from several Niskin bottles for each CTD cast and analysed using a Autosal salinometer. The underway water system on the ship was also calibrated using the same procedure, with samples being taken approximately every 6 hours. Temperatures measured by the CTD were calibrated using a Deep Ocean Standards Thermometer (Seabird SBE 35 DOST) which was mounted on the CTD frame.

Water samples were stored in 60 ml glass bottles. An airtight Tygon tube of the correct diameter for the 60 ml sample bottles was attached to the spout of the Niskin bottle/underway tap and the sample bottle filled. Bottles were rinsed by letting them overflow for 2-3 seconds, and filled until a meniscus was formed at the top of the bottle. Each sample was then poisoned with 60 uL of saturated mercuric chloride solution (7.7 g/100 ml) to stop biological processes. The bottle is then firmly closed with an isobutyl stopper, an aluminium cap is crimped on top of the stopper with a crimper wrench. The sample is stored at room temperature for the remainder of the cruise. During the transit back to the UK the samples were stored in the +4 degrees C refrigerated storage room on RRS Discovery.

Water samples were analysed from 16 CTD casts. Underway water samples were taken more regularly, again at a range of latitudes and off-shelf and on shelf, usually between CTD casts. Water samples were stored for 7 to 8 months at room temperature until analysis.

Measurements of dissolved CH4 concentration in the water samples was carried out at the Department of Astrophysics, Geophysics and Oceanography at University of Liège, Belgium. Samples were analysed using gas chromatography (GC) (SRI 8610 C gas chromatograph) to measure the concentration of dissolved methane. The method involves creating a 20 ml headspace (using nitrogen) in the 60 ml sample bottle and allowing the wat...(10)

Data collection:

Atmospheric methane and carbon dioxide concentration data were collected using a Los Gatos Ultraportable greenhouse gas analyser (UGGA) installed on RRS Discovery.

A Conductivity-Depth-Temperature (CTD) rosette with Niskin bottles was used to take water samples.

Ship underway water sampling system

A SRI 8610 C gas chromatograph was used to analysis the samples for dissolved methane concentrations.

Python 3 is used to analyse the data and create the datasets.

Data quality:

The UGGA is regularly calibrated by measuring gases of known CO2 and CH4 concentrations. The calibration suite consists of three 5 litre cylinders of compressed ambient air (2 calibration gases and 1 target gas, which are traceable to WMO reference scales for CH4 and CO2). The dataset is filtered based on wind direction in order to minimise contamination from pollution from the ship stack; data corresponding with wind directions between 30 degrees and 330 degrees relative to the ship (0 degrees is wind coming directly from the front of the ship and 180 degrees is wind coming directly from behind the ship) are removed. There is not a lot of variation in the data, so we take 2 hour averages to allow us to see any trend in the dataset.

The absolute error on the dissolved methane concentration measurement in the water samples is 0.4 nM (standard deviation).