Download Changes in pH and TA in the Atlantic

Changes in pH and TA in the Atlantic
Recent work (Feely and Talley, 2005) has shown that changes in the apparent
oxygen utilization (AOU) and total inorganic carbon dioxide (TCO2) have occurred in the
North Atlantic over the past ten years. We are interested in examining the changes in pH
and total alkalinity (TA) between the 1993 and 2003 studies made by our group. One
would expect that the changes in TA corrected for salinity will be quite small, but lower
TA could indicate the formation of CaCO3 and higher TA could indicate higher
dissolution rates. This latter effect could be due to the increase in the TCO2 in the waters
due to fossil fuels. It has been shown for example by Chung et al. (2004) that the
saturation state of Aragonite off the coast of Africa is affected by the CO2 added from the
burning of fossil fuels. The changes in pH should be directly affected by the increase in
fossil fuel CO2 and the oxidation of plant material indicated by the increases in AOU.
The oxidation of Redfield organic carbon with an increase in AOU of 40 µM (Feely and
Talley, 2005) would lead to an increase of 30 uM of TCO2 (Millero, 1996). If one
assumes that the TA is constant, an increase of 30 uM in TCO2 would decrease the pH by
~0.2. Since the spectroscopic measurements are precise to 0.002 pH units, one should be
able to see changes in pH 100 times the expected decrease. Changes in the pH of 0.002
are equivalent to changes of 2.6 uatm in pCO2 or 0.5 µM in TCO2 (assuming the TA does
not change significantly). Changes in calculated TCO2 using pH and TA measurements
can be used to compare with the measured changes. Calculations on the CO2 data from
the 2003 A16N cruise (Millero et al., 2004) indicate that the calculated values of TCO2
using pH and TA are in good agreement with measured values (3 µM).
In summary, changes in the pH of North Atlantic waters can be used to examine
chemical and biological changes in the waters with a greater sensitivity. We plan on
carefully examining the changes in pH and TA in the North Atlantic from the cruises we
made in 1993 and 2003. The results should show changes in the carbonate system due to
the dissolution and precipitation of CaCO3 and oxidation of plant material. As shown
below, the surface values of pH and TCO2 on the two cruises are different and follow the
expected changes found by in the increase of AOU and TCO2 shown by Feely and Talley
(2005). The differences between the two cruises will be examined by fitting the results of
one or both cruises to an empirical equation of theta, S, AOU, as done in previous studies
(Lee et al., 1997; Millero et al., 2004).
Chung, S.-N., G.-H. Park, K. Lee, R.M. Key, F.J. Millero, R.A. Feely, C.L. Sabine, and
F.G. Falkowski (2004) Postindustrial enhancement of aragonite undersaturation in
the upper subtropical and tropical Atlantic Ocean: The role of anthropogenic CO2,
Limnol. Oceanogr., 14(2):1-7.
K. Lee, F.J. Millero, and R. Wanninkhof (1997) The carbon dioxide system in the
Atlantic Ocean, J. Geophys. Res., 102(C7), 15,696-15,707.
Millero, F.J. (1996) Chemical Oceanography, CRC Press, page 256.
Millero, F.J., T. Graham, X. Zhu, W. Hiscock, M. Trapp, D. Valdes and V. Koehler,
Global ocean repeat hydrographic study: pH and Total Alkalinity Measurements
in the North Atlantic (A16N, June-August, 2003) (2004), University of Miami
Technical Report, No. RSMAS 2004-05.
Feely, R. A. and Talley, L.D. CLVAR/CO2 repeat hydrography program cruise reveal
large-scale chemical changes in the North Atlantic, EOS submitted.
8.20
pH 20oC
8.15
8.10
8.05
1993
2003
8.00
7.95
-10
0
10
20
30
Latitude
40
50
60
70