Variability of the oxygen minimum zone associated with primary productivity and hydrographic conditions in the Eastern North Pacific during the last 1200 years

Abstract

The expansion of the oxygen minimum zone (OMZ) associated with global warming has generated interest in its variability during the last two millennia. Several oceanographic mechanisms, as advection of dissolved oxygen and depletion of dissolved oxygen by oxidation of exported marine productivity, could explain the variability of δ15N in organic matter as a denitrification indicator of the water column in the Pacific Ocean. Our objective was to infer local or remote forcing mechanisms that lead to the strengthening or weakening of the OMZ in the Eastern Tropical North Pacific. A 42 cm sediment core was recovered at 680 m of depth in the Magdalena margin and sectioned every 1 cm for carbon and nitrogen stable isotope analysis. During the periods 1260–1420 CE and 1670–1800 CE, the desynchronization of δ15N between the Pescadero and Magdalena margins with respect to the Santa Barbara Basin confirms a decrease in denitrification in the Mexican Pacific. During the Little Ice Age (LIA: 1450 to 1800 CE), the δ15N indicated a reduction in denitrification in the Magdalena margin and the Santa Barbara Basin, but not in the Pescadero margin. This suggests an intense advection of dissolved oxygen from tropical waters during the LIA. In the Pescadero margin, denitrification conditions persisted over the same period, while exported productivity maintained a high oxygen demand.