HR: 08:45h
AN: B11B-04 [PDF]
TI: Effects of Climate Change and Anthropogenic Organic Carbon Inputs on the Carbonate Mineral-Pore Water
System
AU: * Andersson, A J
EM: aj@soest.hawaii.edu
AF: Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii,
1000 Pope Rd., Honolulu, HI 96822 United States
AU: Mackenzie, F T
EM: fredm@soest.hawaii.edu
AF: Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii,
1000 Pope Rd., Honolulu, HI 96822 United States
AB:
Future projections suggest that atmospheric CO$_{2}$ and organic matter deposition to the sediments of the coastal zone are
likely to increase because of anthropogenic activities. Consequently, carbonate saturation state of surface ocean waters and
sediment pore waters are likely to decline owing to increased invasion of atmospheric CO$_{2}$ and increased deposition and
subsequent remineralization of organic matter within the sediment-pore water system, respectively. As a result of changes in
the carbon chemistry of surface waters, marine calcareous organisms may have difficulty calcifying. Ultimately decreasing
saturation states could lead to dissolution of calcareous sediments. In fact, dissolution of metastable carbonate minerals
has been proposed as a mechanism to restore changes in surface water saturation state and pH owing to increased invasion of
atmospheric CO$_{2}$, preventing any negative impacts on marine biogenic calcification. In addition, such changes in the
carbon chemistry of the sediment-pore water system could have implications for the carbonate composition and reactivity of
contemporary calcareous sediments. Numerical simulations based on the physical-biogeochemical box model {\it SOCM}
(Shallow-water Ocean Carbonate Model) show that dissolution of sedimentary carbonate minerals could increase during the
21$^{st}$ century, but will not result in the production of sufficient alkalinity to buffer the carbon chemistry of the
surface ocean water against rising atmospheric CO$_{2}$. The carbonate saturation state of the sediment pore water is not
significantly affected by changes in carbon chemistry of the overlying surface water, but is strongly controlled by microbial
oxidation of organic matter within the sediment-pore water system. In the standard simulation of {\it SOCM}, the flux of DIC
owing to remineralization of organic matter increased by 2.4% between 1700 and 2100. During the same period, cumulative
dissolution of magnesian calcite minerals corresponded to approximately 6% of the total magnesian calcite mass present
within the sediments at the initial conditions. Sensitivity analysis indicates that the extent of dissolution is mainly
driven by remineralization of organic matter rather than carbonate reaction kinetics. Numerical results suggest that the
dissolved carbonate ion activity of the pore water is controlled by a metastable equilibrium with the most soluble solid
carbonate phase present within the sediments. Consequently, during early solutional diagenetic modifications on the seafloor
owing to natural or anthropogenic processes, dissolution of carbonate minerals follows a sequence based on mineral stability,
progressively leading to removal of the more soluble phases until the most stable phases remain. In the current simulation,
the composition of magnesian calcite at metastable equilibrium with the pore water changed from 21 mol% Mg to 14 mol% Mg.
Future decrease in average pore water saturation state is likely to alter this metastable equilibrium and could affect the
average composition and rates of precipitation of carbonate cements in contemporary shallow-water sediments.
DE: 1615 Biogeochemical processes (4805)
DE: 1827 Glaciology (1863)
DE: 4805 Biogeochemical cycles (1615)
DE: 4845 Nutrients and nutrient cycling
SC: Biogeosciences [B]
MN: 2003 Fall Meeting