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

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.

Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities

“…Long‐term declines of δ 13 C aq values due to the combination of the Suess effect, vertical mixing, and primary production (residual carbon pool after POC production) have been documented at other monitoring stations, with varying effects according to region and latitude, in particular in the southern regions (King & Howard, ). However, both instrumental and proxy records of δ 13 C aq indicate a consistent average decrease per year of 0.027‰ at five Pacific stations from Hawaii to American Samoa since 1980 (corresponding to an approximately 0.4‰ decline in our 16 year period; Wu, ). Furthermore, Gruber et al () compared the δ 13 C aq trends in several oceanic regions and found that the highest decrease of 0.025‰ was in the subtropical gyres (Bermuda and Hawaii) and the lowest in the equatorial upwelling region of the Pacific (0.015‰), with the Indian Ocean displaying a decrease of 0.020‰ per year.…”

“…Carbon stable isotopes (δ 13 C values or 13 C/ 12 C) have been used to reconstruct the oceanic carbon cycle using direct measurements or marine archives (e.g., marine sediments, corals) from paleoclimates to the current anthropogenic perturbation (Ehleringer, Buchmann, & Flanagan, 2000;Freeman & Hayes, 1992;Keeling, 2017;Wu et al, 2018). Since the Industrial Revolution, the rise in atmospheric CO 2 has been accompanied by a decrease in the carbon isotope ratio of atmospheric CO 2 , known as the Suess effect (Keeling, 1979).…”

Assessment of paleo-ocean pH records from boron isotope ratio in the Pacific and Atlantic ocean corals: Role of anthropogenic CO<sub>2</sub> forcing and oceanographic factors to pH variability

“…3a) i.e. two are from the GBR (D'Olivo et al, 2015;Wei et al, 2009) and another two from the Flinders reef (Pelejero et al, 2005) and New Caledonia reef (Wu et al, 2018). The  11 B records from the GBR vary from 26.27 to 21.06 ‰ and its corresponding pH ranges from 8.40 to 7.67 ( Fig.…”

“…another dominant ocean-climate mode is known to influence physical and biogeochemical conditions and thereby influence ocean pH at decadal time scale (D'Olivo et al, 2015;Pelejero et al, 2005;Wei et al, 2009;Wu et al, 2018).…”

“…However, similar studies on shorter time scale (annual-decadal scale) based on coral records particularly in the time scale of the Anthropocene are sparse. Boron isotopes have been studied in corals from the Pacific and the Atlantic oceans (D'Olivo et al, 2015;Fowell et al, 2018;Goodkin et al, 2015;Kubota et al, 2017;Liu et al, 2014;Pelejero et al, 2005;Shinjo et al, 2013;Wei et al, 2009;Wei et al, 2015;Wu et al, 2018) to reconstruct pH records of the past few decades to several centuries. In most of these studies, it has been suggested that pH variability is mostly controlled by various oceanographic factors such as upwelling, ocean circulation, river run-off and productivity and therefore, making difficult to deconvolute the contribution of anthropogenic pCO2 forcing in modulating ocean pH.…”