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¹ Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas

The expansion of the oxygen minimum zone (OMZ) associated with global warming has generated interest in itsvariability during the last two millennia. Several oceanographic mechanisms, as advection of dissolved oxygenand depletion of dissolved oxygen by oxidation of exported marine productivity, could explain the variability ofd15N in organic matter as a denitrification indicator of the water column in the Pacific Ocean. Our objective wasto infer local or remote forcing mechanisms that lead to the strengthening or weakening of the OMZ in theEastern Tropical North Pacific. A 42 cm sediment core was recovered at 680 m of depth in the Magdalena marginand sectioned every 1 cm for carbon and nitrogen stable isotope analysis. During the periods 1260–1420 CE and1670–1800 CE, the desynchronization of d15N between the Pescadero and Magdalena margins with respect to theSanta Barbara Basin confirms a decrease in denitrification in the Mexican Pacific. During the Little Ice Age (LIA:1450 to 1800 CE), the d15N indicated a reduction in denitrification in the Magdalena margin and the SantaBarbara Basin, but not in the Pescadero margin. This suggests an intense advection of dissolved oxygen fromtropical waters during the LIA. In the Pescadero margin, denitrification conditions persisted over the sameperiod, while exported productivity maintained a high oxygen demand.

Palabras clave: oxygen minimum zone; nitrogen stable isotopes; Magdalena margin; Exported productivity

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