High Frequency Fluctuations in the Eastern Tropical North Pacific Oxygen Minimum Zone During the Last 1200 Years

Oxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially-mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuate naturally, however, the degree to which they fluctuate and a comprehensive understanding of the factors that drive these fluctuations on interannual to centennial time scales is lacking. A high-resolution (near annual) record of δ¹⁵N_sed from the Pescadero Slope in the Gulf of California (eastern tropical north Pacific, ETNP) suggests that the OMZ is self-regulating, capped by maximum (10.5‰) and minimum (8.0‰) δ¹⁵N_sed values which create hard ceilings and floors between which OMZ intensity has varied over the past 1200 years. A comparative analysis of the relationship between δ¹⁵NO₃^- and [O₂] in Pescadero and nearby sites suggests the observed range of δ¹⁵N_sed values is equivalent to a 45 µM fluctuation in O₂ content and that these changes can occur in less than 35 years. Our findings show that the OMZ typically intensifies quickly and contracts gradually; the average rate of OMZ intensification (-1.1µM O₂/year) is twice as fast as the rate of OMZ re-oxygenation. Spectral and qualitative analyses suggests Gleissberg and Suess (deVries) solar cycles, the Pacific Decadal Oscillation, and the El Niño Southern Oscillation all influence the internal variability in the intensity of the OMZ during the late Holocene. Intensity fluctuations are also associated with changes in productivity as evidenced by a significant correlation between δ¹⁵N_sed and weight percent C_org. ETNP OMZ self-regulation has significant implications for nitrogen cycling on a global scale.