2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 13
Presentation Time: 8:00 AM-4:45 PM

Effects of Increased pCO2 on Aragonite Crystal Morphology in Halimeda Spp

KNORR, Paul O.1, ROBBINS, Lisa L.1 and HARRIES, Peter J.2, (1)U.S. Geological Survey, 600 Fourth Street South, Saint Petersburg, FL 33701, (2)Department of Geology, University of South Florida, 4202 E. Fowler Ave., SCA 528, Tampa, FL 33620, pknorr@usgs.gov

Atmospheric CO2 concentrations are expected to be double pre-industrial levels before the end of this century. The ocean CO2 reservoir is simultaneously absorbing increased quantities of CO2, with concomitant increases in carbonic acid and decreases in pH. Open-ocean pH values have decreased by 0.1 since 1980 and are predicted to decrease 0.3 to 0.5 in the next 80 years. This process of ocean acidification will likely modify the biogeochemical processes of calcification and carbonate sediment production. Members of the genus Halimeda spp., a calcareous green segmented macroalgae, are important producers of carbonate sediments in tropical, shallow-water, carbonate settings. Aragonite crystals begin to precipitate approximately 36 hours after the growth of a new segment and continue until available space within the segment is completely filled. Halimeda spp. were grown in sealed aquaria containing an aragonite substrate, air bubbler, temperature and pH probes, and a CO2-injection apparatus. The water in the aquaria was maintained at pH values of 8.2 (control), 8.0, 7.9, 7.8, and 7.5 by controlling the levels of pCO2 in the water. Scanning electron microscopy was used to study ultrastructural details of aragonite crystal morphology. As levels of pCO2 increased, changes in crystal growth density and individual crystal size were noted. This work should provide the foundation for an index linking atmospheric CO2 concentrations to unaltered Halimeda spp. crystal morphology.