PNNL-13501 - Pacific Northwest National Laboratory

Environmental Fate and Effects of Deep Ocean Carbon Sequestration
Study Control Number: PN00041/1448
Ann Drum Skillman, Michael Huesemann, Eric Crecelius
The emission of man-made sources of CO2 into the atmosphere is considered a major factor in the greenhouse effect
leading to global warming. Several options for capturing and sequestering this CO2 have been proposed. These options
include deep-ocean injection. This study examined the effects of CO2 ocean sequestration on marine nitrification
processes and associated environmental impacts.
Project Description
In an attempt to slow down global warming, it has been
proposed to reduce the rise of atmospheric carbon dioxide
concentrations by disposing CO2 (from the flue gases of
fossil fuel fired power plants) into the ocean. The release
of large amounts of CO2 into mid or deep ocean waters
will result in large plumes of acidified seawater with pH
values ranging from 6 to 8. To determine whether these
CO2-induced pH changes have any negative effect on
marine nitrification processes, surficial (euphotic zone)
and deep (aphotic zone) seawater samples were sparged
with CO2 for varying time durations to achieve a specified
pH reduction and the rate of microbial ammonia oxidation
was measured spectrophotometrically as a function of pH.
For both seawater samples taken from either the euphotic
or aphotic zone, the nitrification rates dropped drastically
with decreasing pH. Relative to nitrification rates in the
original seawater at pH 8, nitrification rates are reduced
by about 50% at pH 7 and by more than 90% at pH 6.5.
Nitrification is essentially completely inhibited at pH 6.
Introduction
The combustion of fossil fuels during the last two
centuries has significantly increased the concentration of
carbon dioxide in the atmosphere and this is predicted to
result in global warming and a wide range of concomitant
negative environmental and social impacts (Houghton
1997; Rayner and Malone 1998). To reduce the future
potential impacts of global warming, carbon dioxide
could potentially be removed from either the atmosphere
or from the flue gases of power plants by sequestering it
in terrestrial ecosystems, geological formations, or
oceans. A number of different ocean disposal strategies
have been proposed including the release of CO2 enriched
seawater at 500 to 1000 m depth, or the injection of liquid
CO2 at 1000 to 1500 m depth. The CO2 injections will
result in large pH plumes around the injection points
(Figure 1) (DOE 1999; Caulfield et al. 1997; Herzog et al.
1996). Modeling results indicate that the pH around the
218 FY 2000 Laboratory Directed Research and Development Annual Report
injection point could be less than 5.8 and the steady-state
plume of seawater with a pH less than 6.6 could be at
least 25 km long and 1000 m wide and after a 20-year
continuous CO2 injection could be more than 1000 miles
long (DOE 1999; Caulfield et al. 1997; Herzog et al.
1996).
Figure 1. Conceptual diagram of several methods of ocean
sequestration of CO 2
Little is known about the potential environmental effects
of CO2 disposal in oceans including impacts to the marine
nitrogen cycle. Figure 2 shows a conceptual model of the
oceanic nitrogen cycle (Libes 1992). Most of the nitrogen
from phytoplankton growing in the euphotic zone is
recycled by bacteria and zooplankton while the remaining
nitrogen sinks as particulate organic material into the
aphotic zone. While slowly sinking toward the ocean
floor, aerobic bacteria first convert the particulate organic
nitrogen into dissolved organic nitrogen and subsequently
cause the release of free ammonium into the seawater.
Ammonium is readily oxidized to nitrite and then to
nitrate in a stepwise process called nitrification. The
resulting nitrate accumulates in the deep ocean but is
eventually up-welled back into the euphotic zone where it
serves as new nitrogen for the growth of phytoplankton.
The import of new nitrogen from the aphotic zone is very
substantial and serves as an important nutrient source for
oceanic primary productivity and world fisheries (Libes
1992; Capone 2000; Falkowski et al. 1998).