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The southwestern margin of the Baja California peninsula display intense coastal upwelling, which promotes high primary productivity and the settling of a great amount of organic matter. The oxidation of this organic matter consumes oxygen at a rate that exceeds supply. This condition and the presence of an oxygen minimum zone (OMZ) help to conserve the exported organic matter and can be used to infer changes in productivity. Here we present results of a study based on records of biogenic opal and ?15N to estimate changes in sedimentary denitrification in the eastern tropical North Pacific in the last 2000 years. A multicore (MAGD-MC02) was recovered during the LONO-09 cruise off the Magdalena margin, western Baja California Sur. This site was located within the eastern North Pacific OMZ at a depth of 680 m. The biogenic opal record showed little variability in the last 2000 years, but increased gradually towards the present. The records showed the same variability during the Medieval Warm Period, while denitrification decreased during the Little Ice Age, suggesting that denitrification in the Medieval Warm Period was synchronous with primary productivity, but during the Little Ice Age, the water column showed intensification in ventilation at intermediate depth without change in primary productivity.

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