Browsing by Author "Vergara, Oscar"
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Item Open Access Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru(European Geosciences Union (EGU), 2018) Bretagnon, Marine; Paulmier, Aurélien; Garçon, Véronique; Dewitte, Boris; Illig, Seréna; Leblond, Nathalie; Coppola, Laurent; Campos, Fernando; Velazco, Federico; Panagiotopoulos, Christos; Oschlies, Andreas; Hernandez-Ayon, J. Martin; Maske, Helmut; Vergara, Oscar; Montes Torres, Ivonne; Martinez, Philippe; Carrasco, Edgardo; Grelet, Jacques; Desprez-De-Gesincourt, Olivier; Maes, Christophe; Scouarnec, LionelThe fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthesis is either preserved, exported towards sediments or degraded through remineralisation in the water column. The productive eastern boundary upwelling systems (EBUSs) associated with oxygen minimum zones (OMZs) would be expected to foster OM preservation due to low O₂ conditions. But their intense and diverse microbial activity should enhance OM degradation. To investigate this contradiction, sediment traps were deployed near the oxycline and in the OMZ core on an instrumented moored line off Peru. Data provided high-temporal-resolution O₂ series characterising two seasonal steady states at the upper trap: suboxic ([O₂] < 25µmolkg−1) and hypoxic–oxic (15 < [O₂] < 160µmolkg−1) in austral summer and winter–spring, respectively. The OMZ vertical transfer efficiency of particulate organic carbon (POC) between traps (Teff) can be classified into three main ranges (high, intermediate, low). These different Teff ranges suggest that both predominant preservation (high Teff > 50%) and remineralisation (intermediate Teff 20 < 50% or low Teff < 6%) configurations can occur. An efficient OMZ vertical transfer (Teff > 50%) has been reported in summer and winter associated with extreme limitation in O₂ concentrations or OM quantity for OM degradation. However, higher levels of O₂ or OM, or less refractory OM, at the oxycline, even in a co-limitation context, can decrease the OMZ transfer efficiency to below 50%. This is especially true in summer during intraseasonal wind-driven oxygenation events. In late winter and early spring, high oxygenation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (Teff < 6%). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same trend as for carbon, with the lowest transfer level being in late winter and early spring. Regarding particulate isotopes, vertical transfer of δ15N suggests a complex pattern of 15N impoverishment or enrichment according to Teff modulation. This sensitivity of OM to O₂ fluctuations and particle concentration calls for further investigation into OM and O₂-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections.Item Open Access Seasonal variability of the oxygen minimum zone off Peru in a high-resolution regional coupled model(European Geosciences Union (EGU), 2016-08-08) Vergara, Oscar; Dewitte, Boris; Montes Torres, Ivonne; Garçon, Veronique; Ramos, Marcel; Paulmier, Aurélien; Pizarro, OscarIn addition to being one of the most productive upwelling systems, the oceanic region off Peru is embedded in one of the most extensive oxygen minimum zones (OMZs) of the world ocean. The dynamics of the OMZ off Peru remain uncertain, partly due to the scarcity of data and to the ubiquitous role of mesoscale activity on the circulation and biogeochemistry. Here we use a high-resolution coupled physical/biogeochemical model simulation to investigate the seasonal variability of the OMZ off Peru. The focus is on characterizing the seasonal cycle in dissolved O₂ (DO) eddy flux at the OMZ boundaries, including the coastal domain, viewed here as the eastern boundary of the OMZ, considering that the mean DO eddy flux in these zones has a significant contribution to the total DO flux. The results indicate that the seasonal variations of the OMZ can be interpreted as resulting from the seasonal modulation of the mesoscale activity. Along the coast, despite the increased seasonal low DO water upwelling, the DO peaks homogeneously over the water column and within the Peru Undercurrent (PUC) in austral winter, which results from mixing associated with the increase in both the intraseasonal wind variability and baroclinic instability of the PUC. The coastal ocean acts therefore as a source of DO in austral winter for the OMZ core, through eddy-induced offshore transport that is also shown to peak in austral winter. In the open ocean, the OMZ can be divided vertically into two zones: an upper zone above 400 m, where the mean DO eddy flux is larger on average than the mean seasonal DO flux and varies seasonally, and a lower part, where the mean seasonal DO flux exhibits vertical–zonal propagating features that share similar characteristics than those of the energy flux associated with the annual extratropical Rossby waves. At the OMZ meridional boundaries where the mean DO eddy flux is large, the DO eddy flux has also a marked seasonal cycle that peaks in austral winter (spring) at the northern (southern) boundary. In the model, the amplitude of the seasonal cycle is 70 % larger at the southern boundary than at the northern boundary. Our results suggest the existence of distinct seasonal regimes for the ventilation of the OMZ by eddies at its boundaries. Implications for understanding the OMZ variability at longer timescales are discussed.