Shoal Water Carbonate Geochemistry: The Effect Of Global Warming And RisingAtmospheric COz ANDREAs J. ANDERssoN+ AND FRED T. MACKENzIE Dept. of Oceanography,School of Oceanand Earth Scienceand Technology,Univ. of Hawaii Introduction Much attention has been paid in recent years to the behavior of organic carbon in the oceanas a potential sink for anthropogenicCOz. Much less attention has been given the oceanic carbonatecycle despitethe fact that the ultimate sink of anthropogenicCO, in the oceanwill be the dissolution of carbonates and hence increased alkalinity of ocean waters. Storage of carbon from the atinosphereper kilogram of seawateris about twice as much if continuousequilibrium with a metastable carbonate mineral is maintained, as when the reaction is simply hoinogenous solution of increased atmospheric COz, Model predictionssuggest that atmosphericCOz concentration by the end of the 21" century will be close to 700 ppmv comparedto 370 ppmv at present,assuming a "businessas usual" scenario.At this time, the global mean temperaturecould be 2-3'C warmer than present day. Increasedatmospheric COz will subsequentlyfacilitate increaseddissolved inorganic carbon in the mixed layer of the ocean,simply relatedto the increaseduptake of gaseousCOz in seawater accordingto COz+ H,O + CO> 2HCO>.Consequently COz' concentrationwill decrease, therebylowering the saturationstate of seawaterwith respectto carbonateminerals, The saturation stateis determinedby theproduct of theconcentrations of [Ca +] and[CO,'] dividedby the apparentsolubility product K,~*!. In seawatersaturation state is largelydetermined by [CO, ] since [Ca ] can be assumed to be conservative. Decreased saturation state can lead to the dissolution of carbonateparticles both in the water column and in sedimentsof reefs, banks,and other shoal water environments.Ultimately decreasedsaturation state and increasedtemperature inight impair the rate of calcification by marinecalcareous organisms, Since the accumulationand expansionof carbonatereefs necessitatethat CaCOzis depositedin excessof physical,biological, and chemical erosion, concerns have been raised regarding the fate of coral reefs in the future. On the one hand, it has been proposedthat calcareousorganisms will have difficulty calcifying and produceweaker skeletons that will be more vulnerableto erosion.On the other hand, it hasbeen suggestedthat no significant impact will be observedbecause any changesin saturationstate and pH will be restored by the dissolution of metastablecarbonate minerals, i.e. high Mg-calcite, which will buffer the overlying surface water. In an attempt to determinethe effects of global warming and rising atmosphericCOz on shallow ocean water carbonatechemistry, we took a theoreticalapproach and developeda model designedto investigatethe hypothesisthat dissolution of metastablecarbonate minerals will buffer the shoal water environment against rising atmospheric COz. We define the shoal water environment to include coastal zones, reefs, banks and shelves. Shoal Water Model To investigatethe responseof shoal water carbonateminerals to global warming and rising atmosphericCOz, we constructeda simple box model differentiating betweensedimentary calcite, aragonite,and high Mg-calcite Figure 1!. An averagecomposition of 15 mole % MgCO3 was chosen to representthe Mg-calcite reservoir, A separate sedimentaryreservoir was also assignedto river transportedcarbonates mainly calcite! originating from weatheringon land. In addition organic matter delivered via rivers and in situ production by benthic and pelagic organismswere also considered.The sizesof the reservoirsare shownin Table 1. The sedimentary reservoirswere connectedand interrelatedby the presenceof an averagepore water compositional reservoir. Remineralizationof organic matter and dissolution/precipitationreactions of carbonate mineralscontrolled the averagedissolved inorganic carbon cheinistry of the pore water.Carbonate dissolution and precipitation were defined by kinetically derived equations related to saturation stateby the generalform R = k-0!" andR = k Q-I!", respectively,where R is the rate of change, k is the rate constant, K2 is the saturation state with respect to the specific carbonate mineral under 107 consideration,and n is the empirical reactionorder. Dissolution was basedon data adaptedfrom Keir 980! Calciteand Mg-calcitek.q 10", n=4.5,Aragonite: kq, 10 ', n=4.2! whereas precipitation was basedon data from Zhong and Mucci 989! Calcite and Mg-calcite: k= 10 ltmolm' h', n= 2.80;Aragonite: k= 10' ltmol m h', n= 2.36!.The major flux of carbonate minerals to the sedimentsoriginates from biogenic production by calcareousorganisms such as corals, algae, bryozoans, and foraminifera. The relative rate of biogenic calcification was ktnetically related to saturationstate and temperaturebased on data for Srylophorapisrillara from Gattusoet al. 998! R~, 26.5 l-e ' ! !27!! andPorolirhon grtrdineri from Agegian 985! R>, 00 0.00013AT - 1.3AT !!, respectively, GOBS'~df~AIIIIOCPhPII; F~~M1gl. -25.5! FIIIrnIiVan OI'IIIIIII'II! IIIIFIIMI; iillII 77! 2! 6! FIIIIIIIilr6FS RI.IICtIIIII IIIiFIIN: IIIIlael' IIIIIalII QWllfA@A FlamneerS 3rganiqvm@AAes 5! Frolnaivars ~ Fic I'igttrel. Schematicdiagram nf reservoirsand f1 axes in thepresent shoat water model F!uxesare in l !' motesC Carbonatefluxes of the present model were adaptedfrom Wollast 994!. The initial fluxes were not in a completesteady state,The processof biogenic calcification removed small amountsof alkalinity from the shoal water surfaceocean. The current shoal-waterocean model was incorporatedinto TOTEM Terrestrial OceanaTmosphere Ecosystem Model; Ver, 1998!, a biogeochemicaltnodel which successfullyhas been able to reproducethe CO2concentration in the atmospherefrom 1700 A.D. until present.The current model was initiated in year 1700,prior to the industrial revolution, and simulated until year 2100. Model Predictions: 1700-2100 In the presentmodel, the saturationstate with respectto carbonateminerals was observed to decreasethroughout the entire simulation in both the pore water and the surfacewater Figure 2!. In both reservoirsthe decreasewas initially relatively slow, rapidly increasingafter year 1950. As anticipatedthe saturationstate trend observedin the surfacewater was essentiallythe inverse of the atmosphericCOq trend. Even though,the saturationstate of the surfacewater had decreased by approximately40% by year 2100, it rernlned well abovesaturation with respectto all carbonate phases under consideration. However, the decreased saturation state of the surface water 108 significantly impaired the biogenic rate of calcification by roughly 10% Figure 3!. At year 2083 the pore water becameundersaturated with respectto l5 mole% Mg-calcite, ConsequentlyMg- calclte dissolved and produced alkalinity, which led to a significant decrease in the rate of change in saturation state with respect to carbonatephases within the pore water-sedimentsystem. In other words, dissolution of Mg-calcite acted as a buffer within the pore water. None of this buffering effect was observedin the overlying surface water. Sensitivity analysis of the initial carbonatesaturation state of the pore water indicatedthat the onset of dissolutiontook place at an earlier stage the lower the value of the initial pore water saturation state. Consequentlythe cumulative mass of dissolved carbonates was greater at the end of the simulation the earlier in tilne that undersaturationand subsequentdissolution were initiated. None of the simulated scenariosin the presentanalysis caused any subsequentbuffering of the overlying surfacewater. Table 1. Carbon reservoir sizes in the shoal water ocean environment Reservoir 10 moiesC Reference/Camments Coastalwater Calculatedby Ver 998! fromBraeker and Peng 982!. Votume-weigted averageTCO2 of oceanicwaters with an average T > 16'C = 1983pmol/L. TOTEMcoastal water volume = 0.3x1019 liters, Thus,total coastal water reservoir= 1983pmol/L x 0.3x10 19 liters = 6000xlO12 mole. Coastalorganic matter 367 CalculatedbyVer 998!. Z POC+DOC+Biota!POCfrom Murray 992!; coastalwater POC computed by partitioningtotal organic particulate mass in shallowwater between coastal and open ocean volumes: 920xl0 12 moleC x .3x10 L/3.61210 L!. DOCfrom Williams 975!; 83.3x10 mole/Lx 0.3x10 . Biota2.1x10 mole!adapted from Lerman et al. 989!. Thus, totalorganic matter = 75xI 0 mole+ 250x10 mole+ 42.1xI 0 mole = 367x10 mole Totalcoastal carbonate sediments 72900 Calculatedfrom Millimao 974! assumingI m sedimentthickness, 50% 3 porosityand average carbonate density = 2.83g/cm . Totalcarbonate area =28.32101km 2 .4xlO 12km 2 with anaverage carbonate composition of 80 percentby weight.26.9x10 6km 2 withan average carbonate composition of 15 percentby weight!,Thus, total shoal water carbonate sediments = I.4x10 12 m x0.8!+6.9xl0 rn x0.15!]x I mx0.5x2.83xlO gm x00 gram/mole!= 72900x10 mole RefractivePIC 29800 It is assumedthat accumulation rates of river transportedrefmctive PIC 0 x 10 molesyr ! andin s1'/nproduced CaCO34.5 x 10 molesyr !have Aragonite 27350 beenrelatively constant for thecarbonate sediment reservoir under consideratian m thickness,50% porosity!, Refractive PIC constitute approximately40% of thisreservoir &.4 x 72900!.The other 60% is Calcite constitutedby accumulatioaof in situ producedCaCO, w0.6 x 72900!.Io the latterreservoir, the relative proportion aragonite, calcite and Mg-calcite is calculatedkom observed proportions in recentneritic sediments compiled by Mg-cakite 10350 Land967!