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

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 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 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 characterising two seasonal steady states at the upper trap: suboxic ([O-2] < 25 mu mol kg(-1)) and hypoxic-oxic (15 < [O-2] < 160 mu mol 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 classifled 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 oxygenation 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 carbonate) follows the sa