Golden Fluids: On the Colour and Origin of Liquids from Celtic Lands and in Methane Charged Sediment Sequences

Pure whisky condenses from wash heated in a still as a clear liquid, the same translucency of pore waters that typically emerge from marine sediment pressed through a squeezer. How then to share and to understand the commonality of golden fluids from Celtic lands and methane-charged sediment sequences on continental margins? Ocean Drilling Program (ODP) Leg 164 investigated gas hydrates at Sites 994, 995 and 997 on Blake Ridge. These sites hold considerable methane as dissolved gas, free gas and gas hydrate in a "diffusion-dominated system", one characterized by carbon and methane cycling within sediment over millions of years. Notes from this expedition indicate that pore waters squeezed from samples collected tens to hundreds of meters below the seafloor and passed through a <45 mm filter exhibit a "whisky-colour", but no further examination occurred. Site 1230 (ODP Leg 201) and Site 1244 (ODP Leg 204) targeted gas hydrates in diffusion-dominated systems on the Peru Margin and Hydrate Ridge. For these locations, "yellowness" was determined on a Cary-100 UV-VIS spectrophotometer at 325 nm relative to a well-known standard: Johnnie Walker Black Label (JWBL). The standard was selected because of its carefully crafted blend of reproducible colour and wide availability. Colour profiles of filtered pore water from sites 1230 and 1244 display maximum yellowness several tens of metres below the seafloor and smooth curvature, consistent with addition and removal of dissolved constituents at depth combined with diffusion. The yellowness profiles have similar shapes to each other and to those of alkalinity, but the depths and values of the subsurface maxima vary. At Site 1230, yellowness and alkalinity increase from 0 JWBL and ~2 mM to peaks of ~0.7 JWBL and ~160 mM at 70 mbsf; at Site 1244, values grow downwards to ~0.2 JWBL and 50 mM at 40 mbsf. Similar results were determined across nine IODP Expedition 346 locations in the marginal sea between Japan and Korea. This work, however, measured intensity at multiple wavelengths, each which generate a slightly different profile. Pore water "yellowness" apparently derives from multiple molecules released at different burial depth and time. Presumably, somewhat like whisky and seawater, continental margin pore waters can contain significant amounts of coloured dissolved organic matter (CDOM), which is generated, along with methane and alkalinity, through microbial decomposition of solid organic carbon and intermediary compounds. Moreover, the specific composition and amount of CDOM changes with depth across locations. Colour, alkalinity and methane concentrations in pore space are related, but complexly, because processes can separate dissolved constituents over space and time. For diffusion dominated systems that evolve over millions of years without advective loss and seepage, methane and alkalinity can reach extreme concentrations, and the colour can become remarkably close to that of JWBL, as determined at 325 nm wavelength. In any case, water colour should be determined routinely on pore waters, as it  helps to constrain the cycling of carbon (including methane) in continental margin sediment sequences.