Observed acidification trends in North Atlantic water masses

M Vázquez-Rodríguez,A Velo,H Mercier,F F Pérez,A F Ríos

doi:10.5194/bg-9-5217-2012

Abstract

Abstract. The lack of observational pH data has made it difficult to assess recent rates of ocean acidification, particularly in the high latitudes. Here we present a time series that spans over 27 yr (1981–2008) of high-quality carbon system measurements in the North Atlantic, which comprises fourteen cruises and covers the important water mass formation areas of the Irminger and Iceland Basins. We provide direct quantification of acidification rates in upper and intermediate North Atlantic waters. The highest rates were associated with surface waters and with Labrador Sea Water (LSW). The Subarctic Intermediate and Subpolar Mode Waters (SAIW and SPMW) showed acidification rates of −0.0019 ± 0.0001 and −0.0012 ± 0.0002 yr−1, respectively. The deep convection activity in the North Atlantic Subpolar Gyre injects surface waters loaded with anthropogenic CO2 into lower layers, provoking the remarkable acidification rate observed for LSW in the Iceland Basin (−0.0016 ± 0.0002 yr−1). An extrapolation of the observed linear acidification trends suggests that the pH of LSW could drop 0.45 units with respect to pre-industrial levels by the time atmospheric CO2 concentrations reach ~775 ppm. Under circulation conditions and evolution of CO2 emission rates similar to those of the last three decades, by the time atmospheric CO2 reaches 550 ppm, an aragonite undersaturation state could be reached in the cLSW of the Iceland Basin, earlier than surface SPMW.

Highlights

The ocean acidification due to the increasing atmospheric CO2 is a well-known fact (Bates et al, 2012; Doney et al, 2009; Raven, 2005) but the direct pH observations are sparse (Byrne et al, 2010; Tittensor et al, 2010; Wootton et al, 2008)
The deep convection activity in the North Atlantic Subpolar Gyre injects surface waters loaded with anthropogenic CO2 into lower layers, provoking the remarkable acidification rate observed for Labrador Sea Water (LSW) in the Iceland Basin (−0.0016 ± 0.0002 yr−1)
At 3500 m, the decrease rate of pHSWS25-BA here obtained for lower NADW (lNADW) (0.0002 ± 0.0002 yr−1) has a very low pHSWS25-BA vs. time correlation coefficient (r2 = 0.15; Fig. 3c) and is not significant, yet similar to that given by Gonzalez-Davila et al (2010) (−0.0002 ± 0.0002 yr−1) for the same water mass between 1995 and 2004

Summary

Introduction

The ocean acidification due to the increasing atmospheric CO2 is a well-known fact (Bates et al, 2012; Doney et al, 2009; Raven, 2005) but the direct pH observations are sparse (Byrne et al, 2010; Tittensor et al, 2010; Wootton et al, 2008). Ocean acidification is thought to be the onset for a number of cascading effects throughout marine ecosystems that may leave no time for many organisms to adapt, especially in the case of calcareous organisms (Feely et al, 2008; Doney et al, 2009). It causes a combination of contrasted impacts on the marine environment, from reproductive larval survivorship and growth-related issues in several taxa (Doney et al, 2009) to the reduction of seawater’s sound absorption coefficient (Ilyina et al, 2009). Cold-water scleractinian corals dwelling in intermediate and deep North Atlantic (NA) waters are vulnerable to acidification (Guinotte et al, 2006; Raven, 2005)

Results

Discussion

Conclusion