Seasonal cycle of N_2O vertical distribution and air-sea fluxes over the continental shelf waters off central Chile (～36°S)

摘要

The continental shelf off central Chile is subject to strong seasonal coastal upwelling and has been recognized as an important outgassing area for, amongst others, N_2O, an important greenhouse gas. Several physical and biogeochemical variables, including N_2O, were measured in the water column from August 2002 to January 2007 at a time series station in order to characterize its temporal variability and elucidate the physical and biogeochemical mechanisms affecting N_2O levels. This 4-year time series of N_2O levels reveals seasonal variability associated basically with hydrographic and oceano-graphic regimes (i.e., upwelling and non-upwelling). However, a noteworthy temporal evolution of both the vertical distribution and N_2O levels was observed repeatedly throughout the entire study period, allowing us to distinguish three stages: winter/early spring (Stage Ⅰ), mid-spring/mid-summer (Stage Ⅱ), and late summer/early autumn (Stage Ⅲ). Stage Ⅰ presents low N_2O, the lowest surface saturation ever registered (from 64% saturation) in a period of high O_2, and a homogeneous column driven by strong wind; this distribution is explained by physical and thermodynamic mechanisms. Stage Ⅱ, with increasing N_2O concentrations, agrees with the appearance of upwelling-favourable wind stress and a strong influence of oxygen-poor, nutrient-rich equatorial subsurface waters (ESSW). The N_2O build-up creates a "hot-spot" (up to 2426% N_2O saturation) and enhanced concentrations of NO~-_2 (up to 3.97 μM) and NH~+_4 (up to 4.6 μM) at the oxycline (4-28 μM) (～20-40 m depth). Although the dominant N_2O sources could not be determined, denitrification (mainly below the oxycline) appears to be the dominant process in N_2O accumulation. Stage Ⅲ, with diminishing N_2O concentrations from mid-summer to early autumn, was accompanied by low N/P ratios. During this stage, strong bottom N_2O consumption (from 40% saturation) was suggested to be mainly driven by benthic denitrification. Consistent with the evolution of N_2O in the water column over time, the estimated air-sea N_2O fluxes were low or negative in winter (-9.8 to 20 μmol m~(-2) d~(-1), Stage Ⅰ) and higher in spring and summer (up to 195 μmol m~(-2) d~(-1), Stage Ⅱ), after which they declined (Stage Ⅲ). In spite of the occurrence of ESSW and upwelling events throughout stages Ⅱ and Ⅲ, N_2O behaviour should be a response of the biogeochemical evolution associated with biological productivity and concomitant O_2 levels in the water and even in the sediments. The results presented herein confirm that the study area is an important source of N_2O to the atmosphere, with a mean annual N_2O flux of 30.2 μmol m~(-2)d~(-1); however, interannual variability could not yet be properly characterized.