Dissolved greenhouse gases (nitrous oxide and methane) associated with the naturally iron-fertilized Kerguelen region (KEOPS 2 cruise) in the Southern Ocean

[摘要] The concentrations of greenhouse gases (GHGs), such as nitrous oxide(N₂O) and methane (CH₄), were measured in the Kerguelen Plateauregion (KPR). The KPR is affected by an annual microalgal bloom caused bynatural iron fertilization, and this may stimulate the microbes involved inGHG cycling. This study was carried out during the KEOPS 2 cruise during theaustral spring of 2011. Oceanographic variables, including N₂O andCH₄, were sampled (from the surface to 500 m depth) in two transects alongand across the KRP, the north–south (TNS) transect (46°–51° S, ~ 72° E) and the east–west (TEW)transect (66°–75° E, ~ 48.3° S), both associated with the presence of a plateau, polar front (PF) andother mesoscale features. The TEW presented N₂O levels ranging fromequilibrium (105%) to slightly supersaturated (120%) with respect tothe atmosphere, whereas CH₄ levels fluctuated dramatically, being highlysupersaturated (120–970%) in areas close to the coastal waters of theKerguelen Islands and in the PF. The TNS showed a more homogenousdistribution for both gases, with N₂O and CH₄ levels ranging from88 to 171% and 45 to 666% saturation, respectively. SurfaceCH₄ peaked at southeastern stations of the KPR (A3 stations), where aphytoplankton bloom was observed. Both gases responded significantly, but incontrasting ways (CH₄ accumulation and N₂O depletion), to thepatchy distribution of chlorophyll a. This seems to be associated to thesupply of iron from various sources. Air–sea fluxes for N₂O (from −10.5to 8.65, mean 1.25 ± 4.04 μmol m⁻² d⁻¹) and forCH₄ (from 0.32 to 38.1, mean 10.01 ± 9.97 μmol⁻² d⁻¹) indicated that the KPR is both a sink and a source for N₂O,as well as a considerable and variable source of CH₄. This appears to beassociated with biological factors, as well as the transport of water massesenriched with Fe and CH₄ from the coastal area of the KerguelenIslands. These previously unreported results for the Southern Ocean suggestan intense microbial CH₄ production in the study area.

[学科分类] 地球化学与岩石