Surface ocean pH variations since 1689 CE and recent ocean acidification in the tropical South Pacific

Henry C Wu,Arnaud Dapoigny,Florence Le Cornec,Dominique Blamart,Aline Tribollet,Louise Bordier,Claire E Lazareth,Delphine Dissard,Edwige Pons-Branchu,Eric Douville

doi:10.1038/s41467-018-04922-1

Abstract

Increasing atmospheric CO2 from man-made climate change is reducing surface ocean pH. Due to limited instrumental measurements and historical pH records in the world’s oceans, seawater pH variability at the decadal and centennial scale remains largely unknown and requires documentation. Here we present evidence of striking secular trends of decreasing pH since the late nineteenth century with pronounced interannual to decadal–interdecadal pH variability in the South Pacific Ocean from 1689 to 2011 CE. High-amplitude oceanic pH changes, likely related to atmospheric CO2 uptake and seawater dissolved inorganic carbon fluctuations, reveal a coupled relationship to sea surface temperature variations and highlight the marked influence of El Niño/Southern Oscillation and Interdecadal Pacific Oscillation. We suggest changing surface winds strength and zonal advection processes as the main drivers responsible for regional pH variability up to 1881 CE, followed by the prominent role of anthropogenic CO2 in accelerating the process of ocean acidification.

Highlights

Increasing atmospheric CO2 from man-made climate change is reducing surface ocean pH
New Caledonia and the surrounding region of the south-western Pacific are of great interest for investigation because more than 40% of the global anthropogenic CO2 inventory is found in the region between 14°S and 50°S18 and about 60% of the total oceanic anthropogenic CO2 is incorporated in the Southern Hemisphere oceans[4] (Fig. 1)
Ages are presented as both age corrected and age (CE) with the corresponding 2σ-uncertainty aδ234U = ([234U/238U]activity −1) × 1000 bDecay constants are 9.1705 × 10−6 yr−1 for 230Th, 2.8221 × 10−6 yr−1 for 234U and 1.55125 × 10−10 yr-−1 for 238U cδ234U initial was calculated based on 230Th age (t), i.e., δ234Uinitial = δ234Umeasured × eλ234*t, and t is corrected age dAge corrections were calculated using 230Th/232Th activity ratio of 10 ± 3

Summary

Introduction

Increasing atmospheric CO2 from man-made climate change is reducing surface ocean pH. One of the major consequences is the decrease of carbonate saturation state that is crucial for calcifying organisms such as scleractinian corals to precipitate their aragonite skeletons[5] These critical threats of ocean acidification (OA) on such marine organisms and ecosystems have been documented on the Great Barrier Reef (GBR) in Australia that witnessed a 14% decrease in coral calcification since 19906. Due to spatiotemporal constrains, the currently available coral-based pH reconstructions predominantly focus on the GBR and the marginal South China Sea[12,13,14,15,16], whereas longer timescale pH changes of the open surface ocean from the Pacific Ocean remain poorly documented[17]. New Caledonia and the surrounding region of the south-western Pacific are of great interest for investigation because more than 40% of the global anthropogenic CO2 inventory is found in the region between 14°S and 50°S18 and about 60% of the total oceanic anthropogenic CO2 is incorporated in the Southern Hemisphere oceans[4] (Fig. 1)

Methods

Results

Conclusion