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Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru

Abstract : The fate of the organic matter (OM) produced by marine life controls the major biogeochemical cycles of the Earth's system. The OM produced through photosynthe-sis 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 O 2 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 O 2 series charac-terising two seasonal steady states at the upper trap: sub-oxic ([O 2 ] < 25 µmol kg −1) and hypoxic-oxic (15 < [O 2 ] < 160 µmol kg −1) in austral summer and winter-spring, respectively. The OMZ vertical transfer efficiency of particulate organic carbon (POC) between traps (T eff) can be classi-Published by Copernicus Publications on behalf of the European Geosciences Union. 5094 M. Bretagnon et al.: Modulation of the vertical particle transfer efficiency fied into three main ranges (high, intermediate, low). These different T eff ranges suggest that both predominant preservation (high T eff > 50 %) and remineralisation (intermediate T eff 20 < 50 % or low T eff < 6 %) configurations can occur. An efficient OMZ vertical transfer (T eff > 50 %) has been reported in summer and winter associated with extreme limitation in O 2 concentrations or OM quantity for OM degradation. However, higher levels of O 2 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 oxy-genation conditions together with high fluxes of sinking particles trigger a shutdown of the OMZ transfer (T eff < 6 %). Transfer efficiency of chemical elements composing the majority of the flux (nitrogen, phosphorus, silica, calcium car-bonate) 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 δ 15 N suggests a complex pattern of 15 N impoverishment or enrichment according to T eff modulation. This sensitivity of OM to O 2 fluctuations and particle concentration calls for further investigation into OM and O 2-driven remineralisation processes. This should include consideration of the intermittent behaviour of OMZ towards OM demonstrated in past studies and climate projections .
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Marine Bretagnon, Aurélien Paulmier, Véronique Garçon, Boris Dewitte, Serena Illig, et al.. Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru. Biogeosciences, European Geosciences Union, 2018, 15 (16), pp.5093-5111. ⟨10.5194/bg-15-5093-2018⟩. ⟨hal-02024179⟩

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