Tracking fluctuations in the intensity of the eastern tropical north Pacific oxygen minimum zone using a multi-proxy approach

Abstract:

Naturally occurring oxygen minimum zones (OMZs) are permanent midwater features that are associated with highly productive regions of the ocean, key microbial mediated biogeochemical cycling, and the deposition of organic rich sediments. The intensity of OMZs varies on glacial to interglacial time scales, indicating a relationship between the climate system and the expanse of OMZs, however, high-resolution studies tracking the fluctuations of OMZs on a near annual basis are lacking. In the Gulf of California (Pescadero Slope), the eastern tropical north Pacific OMZ impinges on basin slopes depositing laminated, hemipelagic sediments that record useful proxies of OMZ fluctuations. In an OMZ, low O₂ content promotes water column denitrification, imparting an elevated ¹⁵N/¹⁴N isotopic signature on the residual nitrate in the water column. The elevated ¹⁵NO₃^- is upwelled and incorporated into particulate organic mater (POM) during primary production. The exported POM δ¹⁵N_sed then reflects the intensity of the regional OMZ at the time it was deposited.

The Pescadero Slope record shows distinct temporal fluctuations in the δ¹⁵N_sed with decadal to multi-decadal and centennial cyclicity over the past 1200 years. The record reveals that the OMZ contracts during warmer climatic periods, including the Medieval Climate Optimum and the Industrial Revolution, and expands during cooler climates, specifically the Little Ice Age. The local mechanisms that are influencing fluctuations in the OMZ are investigated through an analysis of productivity proxies, including weight percent organic carbon and weight percent biogenic silica, and indicate OMZ expansion during periods of high productivity. Local productivity is driven by the intensity in the easterly trade winds in the Pacific, and thus the relationship between regional trade wind intensity and fluctuations in the intensity of the OMZ over the past 160 years will be discussed.