Methane sources, distributions and sinks from California coastal waters to the oligotrophic North Pacific gyre

The mechanisms regulating the distributions of dissolved methane in North Pacific Ocean waters were investigated. Water column and sediment trap samples were collected in representative oceanic provinces ranging from the high productivity coastal waters of central California to the oligotrophic ocean near Hawaii. Methane was supersaturated in surface waters throughout the study area with values ranging from 105 to 175%, relative to equilibrium with atmospheric methane. Generally, the greatest supersaturations were found in coastal areas. The methane concentrations in oceanic waters off Hawaii are largely determined by in-situ production and air-sea gas exchange. Near the California coast, sediment and near-bottom inputs become important as potential sources of methane to the water column. Methane measurements in preserved and unpreserved sediment trap solutions documented a sinking particle-associated methane source that was most apparent in the upper 200 m of the water column, and that tended to increase towards the surface and with distance from shore. The exact source of the methane accumulating in the traps could not be determined, but the trap data are consistent with a net in-situ production occurring in surface water microhabitats and, perhaps, macrozooplankton intestinal tracts. In the oligotrophic waters near Hawaii, this novel methane source term may be the most important in-situ process and may help resolve the previously described upper water column methane supersaturation paradox.