Spatial Distribution of Dissolved Methane Over Extreme Oceanographic Gradients in the Subtropical Eastern South Pacific (17° to 37°S). Issue 5 (2nd May 2021)

• Record Type: Journal Article
• Authors: Farías, L.; Troncoso, M.; Sanzana, K.; Verdugo, J.; Masotti, I.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 5(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-02
• Subjects: dissolved methane -- subtropical eastern South Pacific -- CH4 exchange acros air‐sea interface -- mesoscale processes -- spatial CH4 distribution; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016925
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Methane (CH4 ) is one of the most powerful greenhouse gases with the capacity to influence the Earth's radiative budget as well as contribute to atmospheric chemistry. Natural oceanic production makes up to ∼4% of the overall global CH4 emissions, however, there is uncertainty around the accuracy of this value due to a lack of accurate measurements. Such is the case in the Subtropical Eastern South Pacific Ocean (SESP), a region with pronounced chlorophyll‐a and oxygen gradients, which in turn affect the microbial CH4 cycling. This study was conducted during spring‐summer (2014–2016) in the SESP. The region (∼17°–37°S/71°–110°W) is separated into (i) eutrophic, (ii) mesotrophic, and (iii) oligotrophic areas, according to oceanographic and biogeochemical criteria. The SESP presents high CH4 zonal variability with levels ranging from 0.63 to 33.4 nmol L −1, corresponding to 29% and 1, 423% saturation, respectively. High CH4 concentrations (>1, 000% saturation) are observed in the narrow eutrophic area subjected to coastal upwelling. These conditions clearly differ to those observed in the extended oligotrophic subtropical gyre (∼100% saturation). Furthermore, CH4 also tends to accumulate in the mesotrophic area (with upto 1, 423% saturation), where oceanographic conditions as stratification, mesoscale eddies and island mass effect could trigger the presence of a microbial biomass that may be able to induce CH4 regeneration. The CH4 efflux is estimated to be between 0.13 and 19.1 µmol m −2 d −1 (mean ± SD = 4.72 ± 4.67) and the SESP has an emission rate of ∼87.9 Gg CH4 yr −1 . Plain Language Summary: Methane is a potent greenhouse gas that escapes from different natural and anthropogenic sources into the atmosphere and thus accelerates climate change. Atmospheric CH4 concentrations have risen 2.5 times since the beginning of the Industrial age. While much of this increase is attributed to human activities, natural sources can contribute between 35% and 50% of global CH4 emissions, Aquatic environments as estuarine, coastal, and open sea systems make up to ∼4% of the overall global CH4 emissions. Subtropical Eastern South Pacific Ocean (SESP), a region with different coastal (upwelling) and open sea (subtropical gyres) ecosystems and pronounced photosynthetic biomass and oxygen gradients. There, the majority of CH4 is created by microorganisms whose activities depend on both oxygen and organic matter variables. The SESP presents high CH4 spatial variability with levels ranging from 29% to 1, 423% saturation (100% is equivalent to an equilibrium between the ocean and atmosphere). Thus, most of this region exchanges CH4 to the atmosphere at rates between 0.13 and 19.1 µmol m −2 d −1 (mean ± SD = 4.72 ± 4.67) with a regional emission rate of ∼87.9 Gg CH4 yr −1 . This value could highlight the role of the ocean as emitting gases and emphasize same processes at basin scale. Key Points: CH4 spatial distribution of in the subtropical eastern South Pacific shows strong heterogeneity Maxima in CH4 accumulations occur in the mesotrophic area associated with eddies followed by coastal upwelling The subtropical eastern South Pacific region contributes with 87.9 Gg of CH4 per year toward the atmosphere