Quaternary Sediment Cores from the Southern Fairway Basin on the Northern Lord Howe Rise (Tasman Sea)

Thick packages of Cretaceous and Tertiary sediment with numerous diapirs fill the Southern Fairway Basin (SFB) on the Lord Howe Rise (LHR). A bottom-simulating reflector (BSR) also extends across much of this basin, perhaps indicating substantial amounts of CH4 as gas hydrate and free gas. As part of the ZoNiCo 5 survey, run on behalf of the New Caledonian government, 13 piston cores were taken by the RV L'Atalante in 1999 to assess the gas and petroleum potential of the SFB. Specifically, the cores were recovered to document the nature of sediment, pore water and gas in the shallow sedimentary section. The 13 cores, from 1250 to 2753 m below sea level (mbsl) and between 405 and 758 cm long, contain stiff nannofossil ooze. If average regional sedimentation rates apply (10 m/my), the maximum age at the bottom of cores is less than 800,000 years. The sediment typically grades from greyish orange at the top, to very pale orange in the middle, and then to either yellowish grey, very light grey or white at the bottom. Thin black horizons, presumably composed of pyrite, also occur. The changes in colour are related to variations in magnetic susceptibility (MS) and pore water SO42-. Pale and grey zones generally have low MS punctuated by MS highs, and low pore water SO42- concentrations. Methane was detected in most sediment samples, although at trace levels. The presence of ethane, propane and higher hydrocarbons suggests that gases in the SFB have a thermogenic component. With the available data, the best explanation for colour, MS and SO42- profiles is that Fe has been remobilised under anoxic conditions. Ferric iron in solid oxyhydroxide phases and SO42- in pore waters have been converted to dissolved ferrous iron and sulphide. Some of this iron and sulphur has then re-precipitated as pyrite or magnetite (the MS spikes). The overall process may be driven by CH4 from underlying gas hydrate deposits. Upward fluxes of CH4, perhaps of thermogenic origin, induce anaerobic CH4 oxidation in shallow sediment, a process that consumes SO42-. As a consequence, unexpectedly shallow redox fronts occur in the SFB. However, longer cores with less-oxidised sediment and additional analyses are needed to understand sediment, water and gas in this region.