The coming centuries may see more ocean acidification than the past 300 million years. Most carbon dioxide released into the atmosphere as a result of the burning of fossil fuels will eventually be absorbed by the ocean1, with potentially adverse consequences for marine biota2,3,4. Here we quantify the changes in ocean pH that may result from this continued release of CO2 and compare these with pH changes estimated from geological and historical records. We find that oceanic absorption of CO2 from fossil fuels may result in larger pH changes over the next several centuries than any inferred from the geological record of the past 300 million years, with the possible exception of those resulting from rare, extreme events such as bolide impacts or catastrophic methane hydrate degassing.

Oceanography: anthropogenic carbon and ocean pH.

Rising atmospheric carbon dioxide (CO2), primarily from human fossil fuel combustion, reduces ocean pH and causes wholesale shifts in seawater carbonate chemistry. The process of ocean acidification is well documented in field data, and the rate will accelerate over this century unless future CO2 emissions are curbed dramatically. Acidification alters seawater chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts shell-forming marine organisms from plankton to benthic molluscs, echinoderms, and corals. Many calcifying species exhibit reduced calcification and growth rates in laboratory experiments under high-CO2 conditions. Ocean acidification also causes an increase in carbon fixation rates in some photosynthetic organisms (both calcifying and noncalcifying). The potential for marine organisms to adapt to increasing CO2 and broader implications for ocean ecosystems are not well known; both are high priorities for future research. Although ocean pH has varied in the geological past, paleo-events may be only imperfect analogs to current conditions.

/pdf/ocean-acidification-the-other-co-2-problem-4h18mw2cpj.pdf

Ocean Acidification: The Other CO 2 Problem

Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth's history is important for a
reconstruction of the links between climate and radiative forcing of the Earth's surface temperatures. Although atmospheric
carbon dioxide concentrations in the early Cenozoic era (about 60Myr ago) are widely believed to have been higher than at present,
there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most
important for the control of CO2 concentrations over geological timescales. Here we use the boron-isotope ratios of ancient
planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used
to reconstruct atmospheric CO2 concentrations. We estimate CO2 concentrations of more than 2,000 p.p.m. for the late Palaeocene
and earliest Eocene periods (from about 60 to 52 Myr ago), and ®nd an erratic decline between 55 and 40 Myr ago that may have
been caused by reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial. Since
the early Miocene (about 24Myr ago), atmospheric CO2 concentrations appear to have remained below 500 p.p.m. and were more
stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately
15 and 3 Myr ago.

Atmospheric carbon dioxide concentrations over the past 60 million years

Contributing Authors D. Archer, M.R. Ashmore, O. Aumont, D. Baker, M. Battle, M. Bender, L.P. Bopp, P. Bousquet, K. Caldeira, P. Ciais, P.M. Cox, W. Cramer, F. Dentener, I.G. Enting, C.B. Field, P. Friedlingstein, E.A. Holland, R.A. Houghton, J.I. House, A. Ishida, A.K. Jain, I.A. Janssens, F. Joos, T. Kaminski, C.D. Keeling, R.F. Keeling, D.W. Kicklighter, K.E. Kohfeld, W. Knorr, R. Law, T. Lenton, K. Lindsay, E. Maier-Reimer, A.C. Manning, R.J. Matear, A.D. McGuire, J.M. Melillo, R. Meyer, M. Mund, J.C. Orr, S. Piper, K. Plattner, P.J. Rayner, S. Sitch, R. Slater, S. Taguchi, P.P. Tans, H.Q. Tian, M.F. Weirig, T. Whorf, A. Yool

/pdf/the-carbon-cycle-and-atmospheric-carbon-dioxide-41bw4t3lwd.pdf

The Carbon Cycle and Atmospheric Carbon Dioxide

Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean's 'biological pump', the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.

https://faculty.washington.edu/battisti/589paleo2005/Papers/SigmanBoyle2000.pdf

Glacial/interglacial variations in atmospheric carbon dioxide

Boron isotopic composition and concentration in modern marine carbonates

RECORDS of past changes in the pH of the oceans should provide insights into how the carbonate chemistry of the oceans has changed over time. The latter is related to changes in the atmospheric CO2 content, such as that which occurred during the last glacial-interglacial transition1. Previous studies2,3 have shown that the fractionation of boron isotopes between sea water and precipitated carbonate minerals is pH-dependent. This finding has been used to reconstruct the evolution of ocean pH over the past 20 million years by analyses of boron isotopes in the carbonate shells of foraminifera4. Here we use the same approach to estimate changes in ocean pH between the last glacial and the Holocene period. We estimate that the deep Atlantic and Pacific oceans had a pH 0.3±0.1 units higher during the last glaciation. The accompanying change in carbonate ion concentration is sufficient to account for the decrease in atmospheric pco2 during the glacial period1. These results are consistent with the hypothesis5 that the low CO2 content of the glacial atmosphere was caused by an increased ratio of organic carbon to carbonate in the 'rain' to the sea floor, which led to an increase in carbonate ion concentration (and thus in pH) of deep water without a corresponding increase in the lysocline depth.

Evidence for a higher pH in the glacial ocean from boron isotopes in foraminifera

Culture experiments were carried out with the species Orbulina universa at four different pH values (7.70±0.05, 8.15±0.05, 8.60±0.05, and 9.00±0.10) in order to establish the pH-dependence of boron isotope fractionation between seawater and foraminifera. A clear relationship between the boron isotopic composition of the foraminifera and the pH of the seawater culture solutions was found, showing heavier boron isotopic composition at higher pH. This finding supports the viability of boron isotopes as a paleo-pH tool. It is important to note that Orbulina universa cultured in natural seawater, as well as those obtained from coretop samples, have a significantly lighter boron isotopic ratio than Globigerinoides sacculifer from coretop samples, suggesting that at least for this species, a vital effect is active.

N. G. Hemming

Papers

Boron isotopic composition and concentration in modern marine carbonates

Evidence for a higher pH in the glacial ocean from boron isotopes in foraminifera

Oceanic pH control on the boron isotopic composition of foraminifera: Evidence from culture experiments

Assessing scleractinian corals as recorders for paleo-pH: Empirical calibration and vital effects

Mineral-fluid partitioning and isotopic fractionation of boron in synthetic calcium carbonate