Causes and timing of recurring subarctic Pacific hypoxia.

Karla P. Knudson,Michelle K. Drake,Tatsuhiko Sakamoto,Ana Christina Ravelo,Christina P. Knudson,Ivano W. Aiello

doi:10.1126/sciadv.abg2906

Abstract

Several North Pacific studies of the last deglaciation show hypoxia throughout the ocean margins and attribute this phenomenon to the effects of abrupt warming and meltwater inputs. Yet, because of the lack of long records spanning multiple glacial cycles and deglaciation events, it is unclear whether deoxygenation was a regular occurrence of warming events and whether deglaciation and/or other conditions promoted hypoxia throughout time. Here, subarctic Pacific laminated sediments from the past 1.2 million years demonstrate that hypoxic events recurred throughout the Pleistocene as episodes of highly productive phytoplankton growth and were generally associated with interglacial climates, high sea levels, and enhanced nitrate utilization-but not with deglaciations. We suggest that hypoxia was typically stimulated by high productivity from iron fertilization facilitated by redox-remobilized iron from flooded continental shelves.

Highlights

As a result of human-influenced climate warming and nutrient inputs, dissolved oxygen content in the open ocean has decreased an estimated 2% in the past 50 years, with the greatest reductions observed in the North and Equatorial Pacific [1]
The aim of our work is to determine (i) whether hypoxia is a regular feature of most or all deglacial warming transitions, (ii) whether the mechanisms underlying hypoxia are consistent throughout time, and (iii) whether the susceptibility of margin environments to the development of hypoxia depends on certain background conditions—including Milankovitch climate cycles, sea level, ocean stratification, and ocean ventilation
Monte Carlo hypothesis tests confirm that U1342 laminated intervals did not occur randomly but were significantly associated with interglacial climates (P ≤ 0.05) and were unlikely to occur during glacial climates (P ≤ 0.05) (Fig. 4, A and B)

Summary

Introduction

As a result of human-influenced climate warming and nutrient inputs, dissolved oxygen content in the open ocean has decreased an estimated 2% in the past 50 years, with the greatest reductions observed in the North and Equatorial Pacific [1]. Models predict a 1 to 7% decline in global ocean oxygen by the year 2100, with a 1000-year trajectory of diminishing oxygen concentrations, due to warming-induced reductions in gas solubility and convection [2, 3]. The North Pacific has the largest OMZ area (Fig. 1A) because of weak overturning circulation, O2-poor source waters, and high rates of phytoplankton productivity that lead to oxygen- consuming respiration at depth [3]

Objectives

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Results