Overview of CO 2 -induced Changes in Seawater Chemistry Joan Kleypas & Chris Langdon I. Background II. Facts vs Hypotheses III. Future directions 10-Oct-00 9th Int Coral Reef Symp. 1
Effects of CO 2 on Coral Reefs (direct vs indirect) Increased Atmospheric CO 2 Increased Atmospheric Temperature Climatic Changes Altered storm frequency/ intensity Increased dust (Fe fertilization) Reduced [CO 3 2- ] Increased SST Sea level rise Reduced Calcification Increased Bleaching Differential Impacts Increased Breakage & Erosion Reduced Light 10-Oct-00 9th Int Coral Reef Symp. 2
Carbonate Chemistry CO 2 photosynthesis respiration CO 2 aq (CO 2 + H 2 O) H 2 CO 3 Carbonic acid Alkalinity bicarbonate HCO 3 + H + Proportion of HCO 3 Mg 2+ Na + and CO 2 3 adjusts to balance alkalinity K + Ca 2+ carbonate CO 3 2 + H + [CO 3 2 ]= ALK CO 2 CaCO 3 calcification 10-Oct-00 9th Int Coral Reef Symp. 3
Carbonate Chemistry 1xCO 2 2xCO 2 CO 2 280 560 photosynthesis respiration CO 2 aq (CO 2 + H 2 O) H 2 CO 3 Carbonic acid 8 16 Alkalinity bicarbonate HCO 3 + H + 1635 1867 Mg 2+ Na + K + Ca 2+ carbonate CO 3 2 + H + 272 177 1915 2061 TCO 2 CaCO 3 calcification 8.17 7.93 ph 4.3 2.8 Ω-arag 10-Oct-00 9th Int Coral Reef Symp. 4
FACT Atmospheric CO 2 concentration WILL reach double preindustrial levels, even if emissions are stabilized From IPCC, 2000 2xCO2 From Wigley, 2000 10-Oct-00 9th Int Coral Reef Symp. 5
FACT 560 ppmv CO 2 is unprecedented on human time-scales Latest lines of evidence suggest that CO 2 levels have been below 500 ppmv since the Miocene (~24 my ago) 10 6 yr 10 3 yr 10-Oct-00 9th Int Coral Reef Symp. 6
FACT coral/algal reef development increased around 30 my ago Reef building slow to start until after Eocene 0 65 corals algae bivalves millions of years 240 570 from Kiessling et al. 1999 10-Oct-00 9th Int Coral Reef Symp. 7
FACT uptake of CO 2 at the ocean surface is largely thermodynamic This is strongly temperaturedependent equilibrium reached in about 1y 4.3 1xCO 2 2xCO 2 3.1 Warming lessens 2.8 effect by ~15-20% 10-Oct-00 9th Int Coral Reef Symp. 8
[CO 3 2+ ] and Sea Surface Temperature 10-Oct-00 9th Int Coral Reef Symp. 9
Changes in Aragonite Saturation State 1865-2065 10-Oct-00 9th Int Coral Reef Symp. 10
Field evidence supporting CO 2 changes in surface ocean JGOFS/WOCE/DOE CO 2 Survey Cruises 10-Oct-00 9th Int Coral Reef Symp. 11
Field evidence supporting CO 2 changes in surface ocean 10-Oct-00 9th Int Coral Reef Symp. 12
FACT corals and calcareous algae exhibit reduced calcification when [Ca 2+ ], [CO 3 2- ] or CaCO 3 saturation is lowered Multiple studies reviewed by Gattuso (1999) Langdon et al. (2000) increased time-scale of experiments - show no evidence of adaptation to lowered [CO 3 2- ] Other organisms showing reduced calcification Coccolithophores (Riebesell et al. 2000). Forams (Spero et al. 1999) 10-Oct-00 9th Int Coral Reef Symp. 13
Hypothesis: reef CaCO 3 production will decrease by 15 30% under doubled pco 2 conditions. Supporting/refuting evidence Aquarium/mesocosm experiments Coral cores - Lough & Barnes (1997) did not detect longterm decrease in calcification in GBR Porites cores Current reef distribution (saturation state strongly correlated with temperature) Field evidence? (difficult to obtain) Geologic record (corals existed during periods thought to have high pco 2, but did not build reefs) Distribution of inorganic CaCO 3 - marine cements/ooids. 10-Oct-00 9th Int Coral Reef Symp. 14
Hypothesis: dissolution of sedimentary CaCO 3, in response to increased pco 2 and lowered ph, will adequately buffer water column chemistry Evidence Demonstrated (used) in marine aquaria. Flow of water through system enhances dissolution of carbonates and maintains equilibrium Over time scales of deep ocean circulation, dissolution of deep-sea carbonates does buffer ocean system, although equilibrium would not be reached for 5-6 Ky (Archer et al. 1998) Question How rapidly can sediment dissolution buffer surrounding water column? effects of water residence time effects of sediment porosity, grain size, etc 10-Oct-00 9th Int Coral Reef Symp. 15
Hypothesis: increasing pco 2 in ocean will fertilize zooxanthellae, thus increasing coral growth rates Reasoning: Zoox. known to increase coral calcification/production ( super-corals sensu Benson 1984) Anything that enhances zoox. growth should also enhance calcification Evidence: Zoox. in corals use HCO 3- rather than CO 2 Increase in zoox. growth does not necessarily enhance coral growth (e.g. increased nutrients enhances zoox. growth and compete with corals for carbon - Marubini et al. 1999) 10-Oct-00 9th Int Coral Reef Symp. 16
Future strategies Better determination of reef carbonate budgets how much CaCO 3 corals precipitate how much dissolves how much exported,etc Better determination of light/temperature/co 3 2 controls on coral calcification Resolving coral biochemistry questions (ion transport mechanisms?) Field experiments (e.g. need marine equivalent of terrestrial FACE program?) 10-Oct-00 9th Int Coral Reef Symp. 17
E1: Coral Reefs & Global Change - Science Wednesday a.m. Measurements of Coral Calcification & Metabolism Langdon Shneider & Erez Marubini et al. Amat et al. Coral Cores Lough & Barnes Helmle et al. HMC dissolution & other feedbacks in carbonate system Barnes & Cuff Thursday a.m. HMC dissolution & other feedbacks in carbonate system (cont d) Kayanne et al. Halley & Yates Geological Evidence Opdyke & Buddemeier Vecsei Temperature, Marginal Reefs, Recruitment, etc Nadaoka et al. Sartimbul & Koga Schleyer et al. Souter & Turner García & Corredor 10-Oct-00 9th Int Coral Reef Symp. 18
History of Calcification - CO 2 Issue 1973 Zimen & Altenhein (Z. Naturforsch.) Fairhall (Nature) predicted calcite undersaturation of surface ocean by Y2000 1975 Skirrow & Whitfield (Limnol. Oceanog.) Whitfield (Nature) recalculated effects of increased CO 2 using equilibrium solution predicted no undersaturation until 10x increase in pco 2 1979-present Numerous studies illustrating calcification rate dependent on degree of saturation state. 10-Oct-00 9th Int Coral Reef Symp. 19