Browsing by Author "Bretagnon, Marine"
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Item Open Access Dynamics of the carbonate system across the Peruvian Oxygen Minimum Zone(Frontiers Media, 2019-10-16) Hernandez-Ayon, Jose M.; Paulmier, Aurélien; Garcon, Veronique; Sudre, Joel; Montes Torres, Ivonne; Chapa-Balcorta, Cecilia; Durante, Giovanni; Dewitte, Boris; Maes, Cristophe; Bretagnon, MarineThe oxygen minimum zone (OMZ) of Peru is recognized as a source of CO₂ to the atmosphere due to upwelling that brings water with high concentrations of dissolved inorganic carbon (DIC) to the surface. However, the influence of OMZ dynamics on the carbonate system remains poorly understood given a lack of direct observations. This study examines the influence of a coastal Eastern South Pacific OMZ on carbonate system dynamics based on a multidisciplinary cruise that took place in 2014. During the cruise, onboard DIC and pH measurements were used to estimate pCO₂ and to calculate the calcium carbonate saturation state ( Ω aragonite and calcite). South of Chimbote (9ºS), water stratification decreased and both the oxycline and carbocline moved from 150m depth to 20–50m below the surface. The aragonite saturation depth was observed to be close to 50m. However, values <1.2 were detected close to 20m along with low pH (minimum of 7.5), high pCO2 (maximum 1,250μatm), and high DIC concentrations (maximum 2,300 μmol kg⁻¹). These chemical characteristics are shown to be associated with Equatorial Subsurface Water (ESSW). Large spatial variability in surface values was also found. Part of this variability can be attributed to the influence of mesoscale eddies, which can modify the distribution of biogeochemical variables, such as the aragonite saturation horizon, in response to shallower (cyclonic eddies) or deeper (anticyclonic eddies) thermoclines. The analysis of a 21-year (1993–2014) data set of mean sea surface level anomalies (SSHa) derived from altimetry data indicated that a large variance associated with interannual timescales was present near the coast. However, 2014 was characterized by weak Kelvin activity, and physical forcing was more associated with eddy activity. Mesoscale activity modulates the position of the upper boundary of ESSW, which is associated with high DIC and influences the carbocline and aragonite saturation depths. Weighing the relative importance of each individual signal results in a better understanding of the biogeochemical processes present in the area.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.