Responses of dissolved trace gases (CH4, N2O, CO, NMHCs, CH3Cl) to phytoplankton bloom during in situ iron enrichment (SEEDII) in the western subarctic Pacific

Biogeochemical processes in ocean surface are known to play important roles in the global circulation of many trace gases: not only CO2 but also non-CO2 gases, such as CH4, N2O, CO, NMHCs and CH3Cl. The mixing ratios of these components in air are highly responsible for controlling global warming, stratospheric and tropospheric O3, tropospheric OH, organic aerosols, and peroxides. While future global changes could alter air-sea fluxes of these components in ocean surface, the detailed processes to produce/consume these components, as well as the range of the variations, are not clarified as yet. The iron fertilization experiment offers us a wonderful chance to quantify the effect of phytoplankton bloom to the production of the trace gases. During 2004 iron-enrichment experiment in the northwest subarctic Pacific (SEEDSII), we monitored mixing ratios of the trace gases (CH4, N2O, CO, NMHCs and CH3Cl) within the phytoplankton bloom. Besides, the stable isotopic ratios were also monitored to clarify the processes that are responsible for the variations. Both the mixing ratios and the stable isotopic ratios of dissolved CH4, N2O and CH3Cl exhibit little changes during the 23 days observation. On the other hand, dissolved CO exhibit remarkable change in the carbon isotopic composition as phytoplankton bloomed. The maximum variation reached to about 25‰ depletion in δ13C. We conclude that the enrichment of the CO precursor must be responsible for the variation. Furthermore, in accordance with the phytoplankton bloomed, the concentration of NMHCs, especially alkane, exhibit remarkable enrichment in the mixing rations: 3 times for ethane, 4 times for propane, 13 times for butane compared with those prior to the bloom. In accordance with the enrichment, the stable carbon isotopic composition of ethane and propane increased, while that of butane decreased. To clarify the source of alkane during the bloom, we determined both concentration and δ13C of alkane emitted from pure phytoplankton cultures in laboratory. As a result, we conclude that most of ethane and propane enriched during SEEDSII were produced from diatoms, while most of butane was produced from cryptophyta and/or dinoflagellates.