Ocean acidification and Decadal Alkalinity Variability in the North Pacific

Chair: James Orr

Jessica N. Cross(1,2), Brendan R. Carter(2), Samantha A. Siedlecki(3), Simone R. Alin(1), Andrew G. Dickson(4), Richard A. Feely(1), Jeremy T. Mathis(5), Richard H. Wanninhkof(6), Alison M. Macdonald(7), Sabine Mecking(8), and Lynne D. Talley(4)

1 NOAA Pacific Marine Environmental Laboratory, Seattle, WA, 98115, USA
2 Cooperative Institute for Alaska Research, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA
3 Joint Institute for the Study of Atmosphere and Ocean, University of Washington, Seattle, 98195, USA
4 University of California San Diego, La Jolla, CA, 92093, USA
5 NOAA Arctic Research Program, Ocean and Atmospheric Research, Silver Spring, MD, 20910, USA
6 Atlantic Oceanographic and Meteorological Laboratory, Miami, FL, 33149, USA
7 Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
8 Applied Physics Laboratory, University of Washington, Seattle, WA, 98105, USA


Background

Recent observations of acidification-driven shoaling of the calcium carbonate saturation horizon in the North Pacific have prompted new interest in carbonate cycling in this region, particularly related to impacts on biogenic calcification and dissolution at the surface layer. Some estimates project that after several decades of declining pH values, the impacts of ocean acidification on alkalinity cycling are now beginning to emerge.

Methods
Total alkalinity concentrations along a meridional transect of the North Pacific (WOCE, CLIVAR, and US GO-SHIP line P16N; 152 °W) have been collected over a period of three decades, allowing for decadal snapshots of alkalinity cycling. Calculations estimating the impact of other ocean chemical processes, such as decadal oscillations, mixing and denitrification, allow for the exploration of potential impacts of acidification on alkalinity cycling occurring in the background.

Findings
The largest source of variability in alkalinity concentrations is related to North Pacific circulation, particularly in the surface mixed layer. Precise normalization of these data reveal some small spatial and temporal variability in the background (on the order of +10 µmol kg-1). A stronger buildup of alkalinity was observed near the subsurface layer of the subarctic boundary (on the order of +20 µmol kg-1).

Conclusions
The subarctic boundary is a region of known sediment, calcium carbonate, and silicate export, indicating that the ocean build-up of alkalinity over time could correspond to increases in the rate of dissolution processes. The greatest build-up of alkalinity was also found near the aragonite and calcite saturation horizons. While the variations observed here are small, these correlations indicate that ocean acidification could be contributing to enhanced carbonate dissolution in this area. Further observation and monitoring in this area will be critical to understanding how these processes unfold over the coming decades.