Deoxygenation of the Eastern Tropical North Pacific over the last 1200 years

Abstract

Oxygen minimum zones (OMZ) are characterized by a dissolved oxygen (DO) concentration <0.5 ml L⁻¹. In situ DO measurements suggest that the global OMZ upper limit is shallower than 60 years ago due to global warming. The benthic foraminifera assemblage, trace elements, and biological productivity and denitrification tracers have been used in paleoceanographic research, given that they indicate dysoxic and suboxic changing conditions of bottom water. Studies of benthic foraminifera assemblages in the southwestern margin of Baja California Sur are scarce. Thus, it is necessary to conduct a paleoecological analysis of the benthic foraminifera community on an approximately 1200-year timeline. The sediment core LONO09-MC02 (1200 years old) collected at 680 m depth was examined to analyze benthic foraminifera relative abundances and estimate the DO concentration in the OMZ of Baja California Sur's southwestern margin. The DO concentrations inferred from benthic foraminifera assemblages ranged between 0.09- and 0.17-ml l⁻¹ (4.4 and 8.0 μM). This suggests that the OMZ has maintained dysoxic conditions over the past 1200 years. Moreover, the cluster and principal component analysis defined dysoxic, suboxic, and oxic assemblages. The decoupling between geochemical tracers was associated with biogeochemical cycles of the North Pacific subtropical gyre and varied in response to intermediate water circulation in the Pacific Ocean. The low estimate of dissolved oxygen concentration corresponded with minimal solar activity, favoring La Niña-like oceanographic conditions with high primary productivity.