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Physical and biogeochemical modulation of ocean acidification in the central North Pacific

  1. David M. Karlb,1
  1. aDepartment of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717; and
  2. bDepartment of Oceanography, University of Hawaii, Honolulu, HI 96822
  1. Contributed by David M. Karl, June 8, 2009 (received for review April 10, 2009)


Atmospheric carbon dioxide (CO2) is increasing at an accelerating rate, primarily due to fossil fuel combustion and land use change. A substantial fraction of anthropogenic CO2 emissions is absorbed by the oceans, resulting in a reduction of seawater pH. Continued acidification may over time have profound effects on marine biota and biogeochemical cycles. Although the physical and chemical basis for ocean acidification is well understood, there exist few field data of sufficient duration, resolution, and accuracy to document the acidification rate and to elucidate the factors governing its variability. Here we report the results of nearly 20 years of time-series measurements of seawater pH and associated parameters at Station ALOHA in the central North Pacific Ocean near Hawaii. We document a significant long-term decreasing trend of ?0.0019 ± 0.0002 y?1 in surface pH, which is indistinguishable from the rate of acidification expected from equilibration with the atmosphere. Superimposed upon this trend is a strong seasonal pH cycle driven by temperature, mixing, and net photosynthetic CO2 assimilation. We also observe substantial interannual variability in surface pH, influenced by climate-induced fluctuations in upper ocean stability. Below the mixed layer, we find that the change in acidification is enhanced within distinct subsurface strata. These zones are influenced by remote water mass formation and intrusion, biological carbon remineralization, or both. We suggest that physical and biogeochemical processes alter the acidification rate with depth and time and must therefore be given due consideration when designing and interpreting ocean pH monitoring efforts and predictive models.


  • 1To whom correspondence should be addressed at:Department of Land Resources and Environmental Sciences, Montana State University, 334 Leon Johnson Hall, Bozeman, MT 59717. E-mail: jdore{at}montana.edu and dkarl{at}hawaii.edu
  • Author contributions: J.E.D., R.L., and D.M.K. designed research; J.E.D., D.W.S., and M.J.C. performed research; J.E.D., R.L., D.W.S., M.J.C., and D.M.K. analyzed data; and J.E.D. wrote the paper.

  • The authors declare no conflict of interest.

  • See Commentary on page 12213.

  • This article contains supporting information online at www.danielhellerman.com/cgi/content/full/0906044106/DCSupplemental.

  • Received April 10, 2009.

Freely available online through the PNAS open access option.

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