Dissolved methane distribution in the South Pacific and the Southern Ocean in austral summer

Abstract

Oceanic methane (CH4) was examined in the South Pacific and the Southern Ocean from December 2001 to February 2002. From the oligotrophic South Pacific (10S) to the Subtropical Front (STF), we observed the maximum concentrations of CH4 in the subsurface layer which ranged from 2.7 to 3.9 nmol kg-1. South of the STF, higher levels of CH4 were often detected in both the surface and the subsurface layers. Elevated surface CH4 concentrations (3.4-6.1 nmol kg-1) south of the STF tended to be higher than those north of the STF. Higher CH4 were often concomitant with an increase in chlorophyll a levels in the Seasonal Ice Zone (SIZ). Considering that phytoplankton does not generate methane directly, the high CH4 production probably resulted from the grazing processes of Antarctic krill and/or zooplankton fed on phytoplankton and the subsequent microbial methanogenesis in addition to the aerobic CH4 production in the water column. Present results showed a good relation between surface CH4 data (<50 m) and σθ between 10S and the Polar Front (PF), which allow us to provide interpolation/extrapolation schemes for CH4 concentration and sea-air CH4 flux in the wide area ([CH4] = 0.15 σθ - 0.98 (RMS = 0.21 nmol kg -1, r = 0.68, n = 49, p < 0.001). The sea-air fluxes estimated during our survey were from 2.4 to 4.9 mol CH4 km-2 d -1 between 10S and the PF (54S), and from 0.8 to 2.3 mol CH 4 km-2 d-1 south of the PF, where the sea-air CH4 flux was largely affected by the wind speed. © 2011 by the American Geophysical Union.