7.03 Formation and Diagenesis of Carbonate Sediments

7.03 Formation andDiagenesis ofCarbonateSediments J.W.Morse TexasA&M University,College Station,TX, USA

7.03.1 INTRODUCTION AND BACKGROUND 67 7.03.1.1 GeneralOverviewofSedimentary MarineCarbonates 67 7.03.1.2 GeochemistryofMajor Sedimentary CarbonateMinerals 68 7.03.1.3 The CO2 SysteminOceanic Waters 69 7.03.2 SOURCES AND DIAGENESISOFDEEP-SEA CARBONATES 70 7.03.2.1 SourcesandSedimentation 70 7.03.2.2 Distribution ofCaCO3 inDeep-sea Sediments 71 7.03.2.3 CaCO3 DiagenesisinDeep-sea Sediments 72 7.03.2.3.1 Generalrelations 72 7.03.2.3.2 Early diagenetic processesindeep-sea sediments 72 7.03.3 SOURCES AND DIAGENESISOFSHOAL-WATER CARBONATE-RICH SEDIMENTS75 7.03.3.1 SourcesofShoal-waterCarbonates 75 7.03.3.1.1 Generalconsiderations 75 7.03.3.1.2 Sourcesofcarbonatemuds 76 7.03.3.1.3 Formation ofcarbonatesands 76 7.03.3.2 Early MarineDiagenesisofShoal-waterCarbonate-richSediments 77 7.03.3.2.1 Pore-waterchemistry 77 7.03.3.2.2 Precipitation ofearly carbonatecements 78 7.03.3.2.3 Dissolution ofcarbonates 80 7.03.3.2.4Carbonatediagenesisassociated withreefs 80 7.03.3.2.5Early dolomiteformation 81 REFERENCES 82

7.03.1 INTRODUCTION AND BACKGROUND areusuallythe most important quantitatively, 7.03.1.1 GeneralOverviewofSedimentary followed byforaminifera.Insediments overlain MarineCarbonates bywaters ofintermediatedepthsuch asmid-ocean ridge crests,aragonitederived frompelagic Carbonateminerals inmodern marinesediments pteropodsandheteropodscanbe found.Calcite canreadily be divided into thosefoundinshoal-to- cementscontainingabundant magnesium canalso shallow anddeep-waterenvironments. Thefactors occur indeep-watersediments,but theyare controllingthe sources,mineralogy,anddiagen- relatively rare. esisofcarbonatesintheseenvironmentsarevery Shallow-watercarbonate-rich sedimentsare different.Withinthe shoal-to-shallowwater largely conﬁned todaytothe subtropic andtropic environment,the sourcesanddiagenesisof climatic zones,but theyarefoundevenathigh carbonatesdiffersubstantiallybetweencarbon- latitudes. Theiroccurrence isstrongly inﬂuenced ate-rich andprimarily siliciclastic sediments. byfactorssuch aswatertemperatureandterrige- Almost all deep-sea carbonate-rich sediments nous input. Thesesediments aregenerally domi- arecomposed ofcalcitelow inmagnesium ( . 99% nated byaragonite, followed bycalcitesrich in CaCO3 ). Thismaterialisprimarily derived from magnesium ( . 4mol.%). Low-magnesiancalcite pelagic skeletalorganisms. Coccolithophores isusually aminor component. Dolomiteoccurs

67 68 Formation andDiagenesisofCarbonateSediments only inspecialenvironments,andeventhenitis equilibrium withrespecttohigh ( , 13 mol.%) generallynot amajor component ofthe sediment. magnesiancalcites. However,dolomiteis Shallow-watercarbonatesareprimarily produced rarely foundinmodern sediments.Sediments bythe disintegration ofthe skeletons ofbenthic containing“recent”dolomitearegenerally found organisms,such ascorals,echinoids,mollusks, associated withexceptionally high salinity enviro- foraminifera, andcorallinealgae.Insome nments. Aragoniteandhigh-magnesiancalcites environments,inorganic precipitatessuch as dominateshoal-to-shallowwatercarbonatesedi- cementsandooïdsarealso abundant. Asdiscussed ments. Seawatermust be , 22 timesormore laterinthischapter,the source ofaragoniteneedle supersaturated withrespecttocalcitebefore mudsremains controversial. pseudo-homogeneous precipitation occurs(e.g., Insiliciclastic sediments,the major source of Morse etal.,1997). However,metastablearago- carbonatesisalsoprimarily derived from benthic niteprecipitates,not calcite, duelargely to the 2 organisms. Theseinclude bivalves,othermol- inhibitingeffects ofMg þ on calciteprecipitation lusks,sea urchins,andforaminifera.Inthese (Pytkowicz,1965;Berner,1975). sediments thereisoftenazoneofconsiderable Dolomiteisoneofthe mostabundant sedimen- undersaturation produced nearthe sediment– tary carbonateminerals. However,afteryears of waterinterface,wherethe oxidation oforganic intensestudyits mode offormation remains matterandbacteriallyproducedsulfidescanresult controversial,andits propertiesunderEarth inalmostcompletedissolution ofsediment surface conditions areless well knownthanfor carbonate.Carbonatesfrom organisms thatbur- most othercarbonateminerals.The primary rowbeneaththiszoneofintensediagenetic reasonfor thisisthatits formation iskinetically activity areoftenwell preserved.Inorganic-rich hindered byits well-ordered structure.Another siliciclastic sediments sulfatereduction maybe probleminunderstandingdolomitemaybe as very extensive, withthe increaseinalkalinity stated byLand(1985):“therearedolomitesand outweighingthe decreaseinpH, resultinginthe dolomitesanddolomites.” Most recently formed precipitation ofcalcium carbonate. marinedolomitesareclassified as“protodolo- Carbonatesinancient sedimentary rocksare mites.” Theyarefarfrom beingperfectly ordered dominantly calciteanddolomite, aspredicted by structurally andusually containafewpercent equilibrium thermodynamics. Thisindicatesthat excess calcium. Attempts to measuretheirsolu- metastableshallow-watersedimentary carbonates bility havebeenfrustrating, but itdoesappearthat haveundergonemajor diagenetic changes. The theycanbe oneor two orders ofmagnitude more distribution ofcalcium carbonateindeep-sea solublethanidealdolomite.Thus,seawatermay sediments indicatesthatextensiveearly diagen- notbe supersaturated withrespectto esishasled to the loss ofmost carbonateminerals protodolomites. from thesesediments. Thisloss appears to be High-magnesiancalcitesarecommoncom- linked to the saturation stateofthe overlying ponentsofshallow marinesediments. Theyare waters.However,kinetic andbiogeochemical derived from the skeletonsoforganisms,such as processesalso playamajor roleincontrolling benthic foraminiferaandsea urchins,andbydirect the distribution ofcarbonateminerals. This precipitation ofmarinecements. Theirsolubility chapterexaminesthe geochemistry ofthe early isstrongly influenced bytheirmagnesium content. diagenesisofcarbonatesinmarinesediments. Thesolubilitiesofbiotic andsynthetic magnesium calcitesdiffersubstantially (Figure1). Insea- water,biogenic magnesiancalcitecontaining 11 mol.% MgCO mayhaveabout the same 7.03.1.2 Geochemistry ofMajor Sedimentary , 3 solubility asaragonite. CarbonateMinerals Berner(1976) reviewed the problems of Beforeproceedingto the “naturalworld,”abrief measuringcalcitesolubility inseawater,anditis commentary on the chemistry ofmajor marine theseproblems,inpart,thathaveled to the useof carbonateminerals isoffered first. Avast literature stoichiometric solubility constantsfor calciteand exists on thistopic: MorseandMackenzie (1990) aragonite(see Section 7.03.3). The mostdifficult canprovideanintroduction. problemisthat,although the solubility ofpure Marinecarbonateminerals havebothbiotic calciteissoughtinexperiments withseawater (dominant) andabiotic (minor)sources. Their solutions,extensivemagnesium co-precipitation formation isoftencontrolled bykinetic factorsor canproduce magnesiancalcites. The solubility of biomediated processesinorganisms. Surface thesemagnesiancalcitesdiffers from thatofpure seawaterismost highly supersaturated (the ion calcite.Thus,itisnot possibleto measurethe activity product(IAP)ismuch greaterthanthe solubility ofpurecalcitedirectlyinseawater. solubility product) withrespecttodolomite Anotherproblemisthe determination of ( , 50 ),followed bypurecalcite( , 6 ), aragonitesolubility inseawater. Astudyofthe thenb£ yaragonite( , 4 ). Itmaybe close£ to solubility ofcalciteandaragoniteinseawater £ Introduction andBackground 69

7.0 information allied withstableisotoperatiodata, which areinﬂuenced bymany ofthe same 7.2 environmentalfactors,hasbecomeamajor area ofstudyincarbonategeochemistry. Much ofthe 7.4 research on co-precipitation reactions withcalcite andaragonitehasbeenreviewed byMucci and 7.6 Biogenic Morse(1990). “best fit” 7.8 IAP 8.0

–log 7.03.1.3 The CO2 SysteminOceanic Waters M g- ca l c i t e 8.2 The chemistryofthe carbonic acid systemin seawaterhasbeenoneofthe moreintensely 8.4 studied areasofcarbonategeochemistry. Thisis 8.6 Synthetics becauseavery preciseanddetailed knowledge of thissystemisnecessary to understandcarbon 8.8 dioxide cyclingandthe deposition ofcarbonate 0 20 30 10 sediments inthe marineenvironment.Amajor mol.% MgCO3 concept applicableto problems dealingwiththe behavior ofcarbonic acidandcarbonateminerals Figure1 The equilibrium IAP for magnesiancalcites ofbiogenic andsynthetic originversus magnesium inseawateristhe idea ofa“constant ionic concentration (afterMorseandMackenzie, 1990). medium.” Thisconcept isbased on the observation thatthe salt inseawaterhasalmost constant composition,i.e.,the ratios ofthe major ions are the samefromplaceto placeinthe ocean wasconducted byMorse etal. (1980);theirresults (Marcet’sprinciple). Possibleexceptions can for calcitesolubility wereingoodagreement with include seawaterinevaporativelagoons,pores earlierwork(e.g.,Ingle, 1975). However,for ofmarinesediments,andnearrivermouths. aragonitetheyfoundamuch lowersolubility Consequently,the major ion composition of thanobserved byprevious investigators. Itwas seawatercangenerallybe determined from its demonstrated thatthe primary reason for this salinity.Ithasbeenpossible, therefore, to develop difference wasthatearlierinvestigators had not equationsinwhich the influence ofseawater waited for asufficiently longperiodoftimeto composition on carbonateequilibria isdescribed reachequilibrium andthatstrangely,for equili- simply interms ofsalinity. bration periodsofless thanabout amonth, Intheory,itshouldbepossibleto dealwithall different solubility valuescanbe obtained ina carbonategeochemistry inseawaterbyknowing reversiblemanner. Thisresult points to some the appropriateactivity coefficients andtheir typeofshort-term,solubility controllingsurface responseto changesinhow salinity,temperature, phase.The valueobtained byMorse etal. (1980) andpressureaffectthem. Inpractice, weareonly for aragonitewasingoodagreement withthe now beginningto approachthe treatment of valuepredicted from thermodynamic solubility activity coefficients withsufficient accuracyto products andion activity coefficients inseawater, be useful for mostproblems ofinterest. Thatis andthe valuefor the solubility ratioofaragoniteto why“apparent” andstoichiometric equilibrium calciteinseawaterwas1.51,ingoodagreement constants,which donot involvethe useofactivity withdilute-solution studies. From theseconsider- coefficients,havebeeninwidespread useinthe studyofmarinecarbonatechemistry since the ations,itappears thatthe co-precipitation of early 1970s. Thestoichiometric constants,usually magnesium ( , 8mol.%MgCO )withcalcite 3 designated as K p ; involveonly the useof formingslowly from seawaterdoesnot change concentrations m ; whereasexpressionsfor its solubility beyondthe uncertaintyinthe apparent equilibrðiumÞ constants K containboth measurements,which isestimated to be less 0 concentrations andapparent hyð droÞ genactivities than10%. a )orconcentrations.Examplesofthese Inaddition to magnesium,atleast trace H þ difð ferent typesofconstantsare amounts ofmany componentspresent inseawater canbe incorporated intomarinecarbonates. p stoichiometric constant : K calcite m Ca2 m CO2 2 Concentrations range fromtrace(e.g.,heavy ¼ þ 3 metals),to minor(e.g.,strontium),to major 1 (e.g.,magnesium,see previous discussion). This ð Þ means thatthereispotentially alarge amount of m 2 2 a CO3 H þ information thatcanbe obtained from the study apparent constant : K 20 2 ¼ m HCO2 ð Þ ofcarbonatemineralcomposition. Thistypeof 3 70 Formation andDiagenesisofCarbonateSediments Itshouldbenoted thatinseawaterthe molinity The calcium concentration innormalseawater concentration scale(mol kg2 1 ofseawater) is canbe calculated from the salinity for most oftenused, andcaremust be takentomake certain purposes(accuracyisalmost always betterthan thatstoichiometric andapparent constants areon within1%). Aspreviously noted, caremustbe the sameconcentration scaleasthe measured takeninstudiesofporewaters andinunusual values. areassuch ascoastalwaters,carbonatebanks,and pHvaluesare“apparent” becauseelectrodesdo lagoons wheresignificant deviations from normal not measurehydrogenion activity.Thesurface seawaterconcentrations canoccur. chemistry ofglass electrodesandliquid junction Theapparent constantsarenotthosefor potentials betweenthe reference electrode filling standardseawaterwhereseawatercomposition solution andseawatercontributeto thiscomplex- hasbeensignificantly altered.Composition ity. NBS bufferstandardshaveamuch lowerionic changescanbe especially important inanoxic strengththanseawater; thisfurthercomplicates environments whereextensivesulfateconcen- the problem. Onewayinwhich thislast problem tration changesareproduced.The carbonateion hasbeenattacked istomake up buffered artificial concentration canbe calculated from any two of seawatersolutions andvery carefully determine the four parameters:pH, totalCO2 (TCO2 ),the the relation betweenmeasurements andactual partialpressureofCO 2 p CO2 ),or TA (e.g.,Morse hydrogenion activitiesorconcentrations (e.g.,see andMackenzie, 1990). ð reviewofMillero,2001). Asapracticalmatter, pHvaluesofseawateraregenerallymeasured at , 25 8 Candatmospheric pressure.ThesepH measurementsmustbe corrected for pressure andtemperaturechangesfor application to insitu 7.03.2 SOURCES AND DIAGENESIS conditions. OF DEEP-SEA CARBONATES Anotherpracticalconsideration whendealing 7.03.2.1 SourcesandSedimentation withthe seawatercarbonic acidsystemisthatin 2 addition to carbonatealkalinity,Hþ andOH ,a The primary sourcesofcalcium carbonatein numberofothercomponentscancontributeto deep-sea sediments arepelagic coccolithophores the totalalkalinity (TA). Theseawaterconsti- andforaminiferathatgrow dominantly inthe tuent thatisusually mostimportant isboric acid. photic zone.Thedistribution ofcalcium carbon- Undermostconditions,boric acidcontributes ate-secretingpelagic organismsisprimarily 2 1 , 0.1 mmol L alkalinity; itisusuallytakeninto controlled bythe fertility andtemperatureofthe consideration whenmakingcalculations. Nutrient near-surface ocean. The fertility ofseawateris compounds,such asammonium,phosphate, and largely aresult ofoceancirculation patterns and, silica,whoseconcentrationsinseawaterare inparticular,processesleadingto upwellingof highly variable, canalsoinfluence alkalinity. nutrient-rich waters. Ingeneral,coccolithophores Theymust be takeninto account for very precise arecommon intemperatewaters,but rareinhigh- work.Inanoxic porewaters anumberof latitude coldwaters wherediatomsdominate. compounds,such ashydrogensulfideanddis- Coccolithophoresarenumericallymuch more solved organic matter,canbe significant con- abundant (usually , 104 )thanforaminifera. tributors to alkalinity (e.g.,see Berner etal., Benthic foraminiferacons£ist eitherofaragonitic 1970). or high-magnesiancalcite.Becauseoftheir Oneofthe primary aims inthe studyofthe scarcity indeep-sea sediments,aragonitic pelagic geochemistry ofcarbonatesinmarinewaters is organisms havereceived relatively littleattention. the calculation ofthe saturation stateofthe Pteropodsarethe mostabundant pelagic aragon- seawaterwithrespecttocarbonateminerals. The itic organisms. saturation stateofasolution withrespecttoa Themechanismsandratesoftransferof givenmineralissimply the ratioofthe ion activity biogenic carbonatematerialfrom near-surface or concentration producttothe thermodynamic waters to deep-sea sediments havebeeninvesti- or stoichiometric solubility product(Equation (3)). gated intensely. Major studieshavedealt withthe Inseawaterthe latterisgenerally used and V mineral transition from livingorganism to carbonatetest, isthe symbol used to represent the ratio. If V 1 ; ¼ ratesofsinking, extent ofdissolution inthe water the solid andsolution areinequilibrium;if column andon the sea bottom,andthe relation V , 1,the solution isundersaturated and betweenlife anddeathassemblages. Thesestudies mineraldissolution canoccur,andif V . 1 ; the haveraised the question ofwhereandwhen solution issupersaturated andprecipitation should carbonatestarts.Formally,diagenesisshould occur: probably not be considered to start untilthe deposition on the seafloor,but pelagic carbonates a Ca2 a CO2 2 m Ca2 m CO2 2 þ 3 þ 3 canbe altered considerably betweendeathinthe V calcite or p 3 ¼ K calcite K calcite ð Þ upperwatercolumn anddeposition on the seafloor SourcesandDiagenesisofDeep-sea Carbonates 71 severalkilometers below thisoceansurface (e.g., weightpercent calcium carbonateindeep-sea Milliman etal.,1999). sediments thatcanrange from 0wt.% to over Becauseoftheirsmall size, individualcocco- 90 wt.%. The distribution ofcalcium carbonatein lithssinkslowly (LermanandDacey,1974; sediments withoceandepthhaswide variations. Lerman,1979)andmayspend , 100 yr inthe Inopenoceanbasins,whereratesofdetrital watercolumn. Thislongresidence timeshould sedimentation aremoderateto low,sediments leadto substantialorcompletedissolution of above3,000 mwaterdeptharegenerally high in coccolithsinthe undersaturated part ofthe water calcium carbonate, whereassedimentsbelow column. Theoriginofcoccolithoozeon portions 6,000 mgenerallyhaveavery lowcalcium ofthe seaflooroverlainbyundersaturated waters, carbonatecontent.Betweenthesedepthsthereis therefore, isdifficult to explaininterms ofthe apoorcorrelation betweenthe weightpercent of settlingofindividualparticles(e.g.,Honjo,1975, calcium carbonateanddepth(Smith etal.,1968; 1976). Dataobtained from sediment traps have Archer,1996a). shownthatmost coccolithsreachthe seaflooras Calcium carbonateismoreabundant in aggregatesproducedbyzooplankton grazingand Atlantic Oceansediments,andgenerallyoccurs encapsulation infecalmatter. insignificant amounts to deeperdepthsthanin Anextensiveinvestigation ofplanktonic for- PacificOceansediments. The major reasonfor aminiferainsediment trapsamplesfrom the thisisthatatagivendepthPacific Oceanwaters tropicalPacific andcentralAtlantic oceans was areless saturated withrespecttocalcium conducted byThunell andHonjo(1981).Atboth carbonatethanAtlantic Oceanwaters.Asecon- studysites,the totalforaminiferalflux andthe dary factor leadingto greaterpreservation in carbonateflux tendto decreasewithincreasing Atlantic Oceansediments isthatatapproxi- depth.Inaddition,the flux ofindividualspeciesof mately the samedegree ofundersaturation in planktonic foraminiferavariessignificantly with depth.The numberofsmall,solution-susceptible Atlantic andPacific Oceanwaters,the input of speciesdecreaseswithincreasingdepth.These terrigenousmaterialishigherinthe Atlantic resultsindicatethatthe dissolution ofsmall Ocean. Thisleadstomorerapid burialandto ( , 150 m m) foraminiferaissignificant asthey betterpreservation ofcarbonates(Archer, settlethrough the watercolumn. Mineralcollected 1996b). Higherproductivity inoverlyingwaters, from the sediment–waterinterface directlybelow ofteninupwellingareas,canalsoleadto the Pacific sediment traparraycontained no morerapid carbonatedeposition andto higher planktonic foraminifera, suggestingthatthe resi- carbonateconcentrations insediments. Afinal dence timeofanindividualskeleton on the major factorinfluencingcalcium carbonate seafloorbeforeitdissolvesisextremely short preservation isthe relativecalcium carbonate (Thunell andHonjo,1981). to organic carbon rainrateratio. Thisisimpor- About12% ofthe CaCO3 flux made byBerner tant,becauseoxidation ofsedimentary organic andHonjo(1981) consistsofaragonite.Astudyof mattercandecreasethe saturation ofporewaters sedimentation ofpteropodsandforaminiferain withrespecttoCaCO3 byincreasing p CO2 .This the NorthPacificbyBetzer etal. (1984)using leadstosignificant carbonatedissolution (for sediment traps confirmed thatconsiderabledis- areviewofworkinthisarea andrecent research, solution ofpteropodstakesplace inthe water see Mekik etal.,2002). Thisdiagenetic pro- column. Theyestimatethat , 90%ofthe cess isdiscussed inthe next section ofthis aragoniteflux wasdissolved inthe upper2.2 km chapter. ofthe watercolumn. Itshouldbenoted that Withincreasingwaterdepth, aragonitic ptero- the depthfor totaldissolution ofcarbonatesin podsdisappearfirst,followed bythe small and the watercolumn isconsiderably morethanthe fragiletypesofcalcitic foraminifera, untilinthe aragonitecompensation depth(ACD). Thisis deeperparts ofthe oceans only badly damaged probably dueto the shortresidence timeof tests oflarge, thick-shelled foraminiferaremainin pteropodsinthe watercolumn becauseoftheir the surface sediments. rapid ratesofsinking. Thecarbonatecompensation depth(CCD) occurswherethe rateofcalcium carbonate dissolution isbalanced bythe rateofinfall,and the calcium carbonatecontent ofsurface sedi- 7.03.2.2 Distribution ofCaCO inDeep-sea 3 ments iscloseto 0wt.% (e.g.,Bramlette, 1961). Sediments The CCD hasbeenconfused withthe calcium Diagenesisofcarbonatesinthe deepsea almost carbonate critical depth(sometimesused inter- exclusively involvesthe dissolution ofcalcium changeably withthe lysoclinediscussed next), carbonate, whereonly 20–30% ofthe flux to the wherethe carbonatecontent ofthe surface seafloorispreserved (Archer,1996a). Assuch, it sediment drops below10wt.%. Asimilarmarker isprimarily reflected inlarge variationsinthe levelindeep-sea sediments isthe ACD,below 72 Formation andDiagenesisofCarbonateSediments which aragoniteisnolongerobserved to accumu- 7.03.2.3 CaCO3 DiagenesisinDeep-sea Sediments lateinsediments. 7.03.2.3.1 Generalrelations The name“lysocline”wasapplied byBerger (1968)tothe region ofrapid increasein Oneofthe most controversialareasofcarbon- dissolution ratethatcouldberecognized in ategeochemistry hasbeenthe relation between surface sediments asthe levelatwhich small calcium carbonateaccumulation indeep-sea thin-shelled foraminiferadisappearorstart show- sediments andthe saturation stateofthe overlying ingstrongsigns ofdissolution. The foraminiferal water. The CCD, FL, R o ,andACD havebeen lysocline(FL)wasdefined byBerger(1968)asthe carefully mapped inmany areas. However,with depthwherethe dominant typeofforaminifera the exception ofcompletedissolution atthe CCD shifts insurface sediments from “soluble”to andACD,the extent ofdissolution thathas “resistant”species(, 50% change inratio). occurred inmost sediments isdifficult to deter- Berger(1970) also defined anotheruseful marker mine.Consequently,itisgenerallynot possibleto levelinpelagic sediments,which issimilarly makereasonably preciseplots ofpercent dissol- based on the change inthe ratioof“soluble”to ution versus depth.Inaddition,the analytical “resistant”speciesofforaminifera.Thisisthe R o chemistry ofthe carbonatesystem(e.g.,GEO- levelatwhich the first significant ( , 10%) change SECS data)andconstants used to calculatethe inthe ratioofthesetwo typesofforaminiferais saturation statesofseawaterhavebeenasource of observed insediments. Laboratory experiments on contention. Oneofthe reasonsfor the controversy sediments to determinehow much carbonatemust regardingthe relation betweenthe extent of be dissolved to produce assemblagesofforamini- carbonatedissolution occurringindeep-sea sedi- ferathatarecharacteristic ofthe FL and R o levels mentsandthe saturation stateofthe overlying wereconducted byBerger(1968). He foundthat wateristhatmodels for the processescontrolling , 80% dissolution isnecessary to produce the FL carbonatedeposition dependstrongly on this assemblageand , 50% dissolution isneeded relation. Hypotheseshaverangedfrom anearly to produce the R o assemblage.Otherattemptsto “thermodynamic”oceanwherethe CCD andACD quantifythe extent ofdissolution required to arecloseto coincident withcalciteandaragonite produce thesesedimentary carbonatemarker saturation levels (e.g.,Turekian,1964;Li etal., horizons andto relatethistothe factors that 1969),to astrongly kineticallycontrolled system control dissolution ratesarediscussed byMekik (e.g.,MorseandBerner,1972) wheremajor etal. (2002). differencesinthe CCD, FL, andsaturation depth The distribution patterns ofcoccolithsand exist. foraminiferadiffersignificantly indeep-sea Calculations such asthosebyMorseand sediments (Schneidermann,1973; Roth etal., Mackenzie (1990) indicatethatthe calcitesatura- 1975). Inmostsedimentsthe moreeasily tion depthisgenerally , 1kmgreaterthan dissolved coccolithsaremissing.Eveninareas proposed byBerger(1977) andthatitismuch wherethe overlyingwaterissupersaturated with greaterthan R o .Itappears only loosely related to respecttocalcite(e.g.,3,000 minthe Atlantic the FL.Inthe equatorialeastern Atlantic Ocean, Ocean; Schneidermann,1973),significant dissolu- FL is , 600mshallowerthanthe saturation depth. tion ofcoccolithshasbeenobserved.Solution- If theseobservations arecloseto correct,the long resistant coccoliths,however,maysurvivein cherishedideaofa“tight” relation between sediments inwhich all foraminiferaltestshave seawaterchemistry andcarbonatedepositional beendissolved.Schneidermann (1973) used these faciesmustbe reconsidered (Mekik etal.,2002). characteristicstocreateadditionalsubdivisions of The influence ofnearinterfacialdiagenetic the region nearthe CCD, andRoth etal. (1975) processesontheserelationships isdiscussed in emphasized thatcoccolithandforaminiferasol- the next section. ution indicescanbe used inacomplementary manner. Coccolithsaregoodindicatorsofdissol- ution abovethe lysocline, whereasforaminifera 7.03.2.3.2 Early diagenetic processes arebetterindicators ofdissolution below the indeep-sea sediments lysocline. The distribution ofaragoniteandmagnesian Numerous models havebeenproposed for the calcitederived fromshallow-waterareasand processesoccurringnearthe sediment–water sediments anddeposited insurroundingdeep-sea interface indeep-sea sediments which lead to a sediments hasalso beeninvestigated.Notable balance betweendissolution andretention of studiesinclude thoseofBerner etal. (1976) on the calcium carbonateinthesesediments.Amajor Bermuda Pedestal,Land(1979) on the north difficultyinstudyingandmodelingthesepro- Jamaicanislandslope, Droxler etal. (1988a)on cessesisthatmany ofthe most important changes the BahamaBanksregion,andDroxler etal. take place overdistancesofonly afewmillimeters (1988b)onthe NicaraguaRise. inahighly dynamic environment. SourcesandDiagenesisofDeep-sea Carbonates 73 Earlymodels for the process ofcalcium organic matterwithdeep-sea sedimentary carbon- carbonatedissolution from deep-sea sediments ates(e.g.,BenderandHeggie, 1984;Emerson (e.g.,Takahashi andBroecker,1977)werebased etal.,1985;Peterson andPrell,1985;Jahnke, on simplediagenetic models,inwhich calcium 1988;Jahnke etal.,1986,1994, 1997;Martin carbonatedissolved into the porewaters ofthe etal.,1986; SaylesandCurry,1988;Archer etal., sediments. The alteration ofconcentrations from 1989,2002; Berelson etal.,1990; Emerson and aninitialsaturation stateofthe overlyingwaterto Archer,1990; Archer,1991;Hales etal.,1994, equilibrium withcalciteresultsindiffusion of 1997; Cai etal.,1995;HalesandEmerson,1996, the reactants andproducts andexchange withthe 1997a,b;MartinandSayles,1996; Wenzho¨fer overlyingwaters. Theprocess isgoverned bythe etal.,2001; R.A.Jahnke andD.B.Jahnke,in difference betweenequilibrium concentrations press). Itisbeyondthe scopeofthischapterto andthosefoundinthe overlyingwaters,the rate present adetailed reviewofthissubstantialand ofdissolution asafunction ofdisequilibrium oftencomplexbodyofscientific literature.Only diffusion rates,andthe thickness ofthe stagnant the major concepts andpoints ofcontention will benthicboundary layerabovethe interface. be presented herefor the discussion ofthe Becausethe dissolution rateofcalcium carbonate diagenesisofshoal-watercarbonate-rich and isrelatively rapid, the depthinthe sediment siliciclastic sediments wheresimilarprocesses calculated for equilibrium hasgenerally beenonly control carbonateaccumulation. afewmillimeters. The approach thathasgenerally beenused isto Amajor process thathad only casually been estimatecarbonatedissolution ratesvia diagenetic considered for its potentialinfluence on benthic models (e.g.,Archer etal.,2002),incorporating carbonateaccumulation indeep-sea sedimentsis organic matteroxidation byoxygen(assuminga the oxidation oforganic matter. The general C:Oratio) thatproducesundersaturated con- reaction for thisprocess is ditions. However,indeepsediments thatcontain moresubstantialconcentrationsofmetaboli- CHOH 106 NH3 16H 3 PO4 138O2 zableorganic matter,suboxic electron accep- ð Þ ð Þ þ 2 124CaCO 16H O 16NO tors,such asnitrateandMnO 2 ,must alsobe þ 3 ! 2 þ 3 2 2 2 2 considered.Arelationshipisthenestablished HPO 124Ca þ 230HCO 4 þ 4 þ þ 3 ð Þ betweenthe calcitedissolution rateandpore-water (Notethatthisreaction overestimatesthe oxygen saturation state.The generalkinetic equation for andwatercontent ofaverage organic matter calciteintroduced byMorseandBerner(1972) (Hedges etal.,1999).) Although the importance for calcitedissolution inseawaterhasbeenmost ofthisreaction incoastalandcontinentalslope commonly used: sediments thatarerelatively richinorganic matter waswidely recognized, its effectwasgenerally R % = d k % = d 1 2 V n 5 ð Þ¼ ð Þð Þ ð Þ ignored indeep-sea sediments,which usually containless than0.2 wt.% organic carbon. With where R isthe rate, k the rateconstant, V the the advent ofsediment traps,however,itbecame saturation state, and n the reaction order. Several apparent thatsignificant amountsoforganic variablesinfluence the relationshipbetweenthe matterarereachingthe sediment–waterinterface rateandthe saturation state, includingtemperature andthatnearinterfacialoxidation oforganic anddissolved components thatactasreaction matterispotentiallyamajor diagenetic process inhibitors. evenindeep-sea sediments. The fundamentalparameterinall models for Amodelfor the influence oforganic matteron calcium carbonatedissolution inthe deepsea is carbonatedissolution indeep-sea sediments was the saturation stateofporewaters. Inorderto introduced byEmerson andBender(1981).In determinethe saturation state, not only mustthe theirmodel,theyemphasized the importance of composition ofthe porewaters be known,but also determiningthe depthdistribution oforganic the solubility ofthe calcium carbonate.Therefore, matteroxidation. If organic matterisrapidly many studiesofcarbonatechemistry indeep-sea oxidized afterarrivalatthe sediment–water sediment porewatershavefocused on the interface, the CO2 generated will havelittle apparent solubility behavior ofcarbonatesin chance to interactwithcalcium carbonate, and thesesediments. the influence ofthe oxidation oforganic matteron The results ofthesestudieshaveshowna dissolution will only be ofsecondary importance. surprisingdegree ofvariability andhavefurther If, alternatively,organic matterisrapidly mixed demonstrated the complexity ofcalcium carbon- into the sediment bybioturbation,the oxidation of ateaccumulation indeep-sea sediments. Several the organic mattercanbe very important. studiesbydifferent groups ofinvestigators Subsequent to the workofEmerson andBender appeared atabout the sametime(Emerson etal., (1981),numerous studieshavequantifiedand 1980; Murray etal.,1980; Sayles,1980).The modeled various aspects ofthe interaction of results ofEmerson etal. (1980) andSayles(1980) 74 Formation andDiagenesisofCarbonateSediments indicated that,inspiteofconsiderablevariability selectingthe propercombination ofrateconstants inthe pore-waterchemistry ofsediments ata andreaction orders for deep-sea sediments. Even numberofdifferent sitesinthe Atlantic and withinagivenstudy,different valuesofapparent Pacific oceans,the porewaters aregenerallyclose solubility products andthe rateconstantshave to equilibrium withcalcite.Higherion concen- beenrequired (e.g.,HalesandEmerson,1997a; 2 tration products inporewaters whereMn þ was Wenzho¨fer etal.,2001). Although therehas measurablewereobserved byEmerson etal. beenaconsiderableamount ofspeculation in (1980),indicatingpossiblecontrol byacarbonate attempts to explaintheselarge variationsinthe phaseotherthancalcite.Subsequently,Sayles kineticsofdeep-sea carbonatedissolution,our (1985)observed similarchangesbelow , 20 cm ability to make predictionsisstill quitelimited. inmany sediments andSayles(1980) alsofound Anumberofparameters maycontributeto the much higherion concentration products inone variability. sediment containingaragonite.Asimilarstudyin Apuzzlingobservation hasrecently beenmade the GuatemalaBasinwasconducted byMurray byR.A.Jahnke andD.B.Jahnke (inpress).They etal. (1980).Therethe sediment contained only a foundthatinsediments abovethe saturation depth fewtenthsofaweightpercent CaCO3 .Asharp thatcontainhigh concentrations ofcalcium drop inpHbelow the sediment–waterinterface carbonate, the ratioofthe calcium carbonate wasobserved asthe result oforganic carbon dissolution rateto the organic matterreminerali- oxidation. Theporewaters wereundersaturated in zation ratewassubstantiallyless thanatother the top fewcentimeters,but camecloseto the typesofsites. Theyhavesuggested thatthismay predicted valuefor saturation atdepth.Halesand be the result ofexchange on carbonateparticle Emerson (1997a)observed different apparent surfacescoupled withparticlemixing, but this solubilitiesinsediments from the CearaRiseand process hasyettobe clearly substantiated. the Ontong-JavaPlateau. Insummary,ourunderstandingofthe Perhaps no aspectofthe diagenesisofdeep-sea diagenetic factors thatcontrol the partialor carbonateshasbeenmorecontroversialthanthe completedissolution ofdeep-sea carbonatescan valuesofrateconstant andreaction orderin largely be considered to be oneofscale.Atthe Equation (5)(Figure2). Most investigators have scaleofthe (for differencesamong)major ocean modeled rateswhich demandrateconstants to be basins,itisclearthatthe primary variableisthe orders ofmagnitude less thanthosemeasured in saturation stateofwateratagivendepth.This : the laboratory (e.g.,Morse, 1978;Keir,1980; islargely controlled byits potential p CO2 A MorseandArvidson,2002). Thevaluesfor the secondary influence isthe deposition rateof reaction orderhaveranged from 1(Halesand siliciclastic sediments. Withinoceanbasins, Emerson,1997b)to4.5(e.g.,Wenzho¨fer etal., variations inthe near-surface primary productivity 2001).Figure3illustratesthe difficulty of andthe ratioofthe rainrateoforganic carbon to calcium carbonateto sediments canexert astrong influence and, atleast insomeareas,lead to 8 substantialdissolution abovethe saturation depthofthe overlyingwater. Thisprocess will 7 be explored furtherinthe discussions of carbonatediagenesisinshoal-watercarbonate- 6 rich andshallow-to-intermediatedepthsiliciclas- )

–1 5 tic sediments,wheremetabolizableorganic matterisathigherconcentrations andwhereit %d 4 exertsevenagreaterinﬂuence on carbonate

Ra t e( diagenesis. 3 Submarinelithiﬁcation andprecipitationof 2 cementsindeep-sea carbonatesedimentsare relatively rareintypicalmajor oceanbasin 1 sediments (Milliman,1974;MillimanandMu¨ller, 1973,1977). The cements consist ofaragonitic 0 00.050.1 0.15 0.2 andmagnesiancalcitemineralogies. Theyare 1–Ω largely restricted to shallow seassuch asthe Mediterraneanandthe Red Sea, andto sediments Figure2 Rateofcarbonatedissolution from deep-sea inthe shallowerparts ofmajor oceanbasins in sediment versus 1 2 V : Solid linefrom Halesand Emerson (1996),dð otted lÞ inefrom Keir(1980),dashed which biogenic aragoniteisalso present. Mucci linefrom Atlantic Ocean,anddotted anddashed line (1987) (see also Garrels andWollast,1978) from Paciﬁc Oceansediment results ofMorse(1978). summarized much ofthe dataon the composition NotethatHalesandEmerson (1996) used adifferent ofmagnesiancalcitecementsindifferent environ- calcitesolubility product. ments. He foundthatmany ofthe shallow-waterand SourcesandDiagenesisofShoal-waterCarbonate-rich Sediments 75

∆ pH ∆ pH –0.04–0.02 0–0.04–0.02 0 –1

0 η C =1

2 η C =1 c m) 3

D ep t h( η C =4.5 4

6 η C =4.5 7

Ontong-JavaSta.2ACearaRiseSta.G

Figure3 D pHversus depthfor sediments from the Ontong-JavaPlateauandCearaRise. n isthe reaction orderused to fitthe data(afterHalesandEmerson,1996). deep-sea carbonatecementscontain10–15mol.% offecalmaterial. Ooïdsaresphericaltoovoid, magnesium;thereisastrongmaximumin 0.2–1mmgrains withaninternalconcentric or magnesium abundance at , 13 mol.%. radialstructure.Lithoclastsarefragmentsof previouslydeposited,andusuallysomewhat lithified, carbonatesediment. Relictgrains areof olderorigin,havingformed underprevious 7.03.3 SOURCES AND DIAGENESIS OF environmentalconditions. SHOAL-WATER CARBONATE-RICH The skeletalcomponentsofcarbonatesedi- SEDIMENTS ments represent the completeor partialskeleton, or the decomposed anddisaggregated skeletal 7.03.3.1 SourcesofShoal-waterCarbonates remains,oforganismsextant atthe timeof 7.03.3.1.1 Generalconsiderations deposition ofthe sediment. Sedimentary particles canbe producedfrom calcareous organisms by Becauseshelvesintropicaltosubtropical disaggregation oftheirskeletons,bymechanical environmentsarecapableofproducingmajor quantitiesofcarbonates,theyareoftenreferred to means related to waveandcurrent energy,or by as“carbonatefactories.”Amodelinwhich bioerosion ofcarbonatesubstratessuch ascorals, carbonateproduction decreaseswithincreasing mollusks,androcks. The wide diversity ofshoal- waterdepthwasproducedbyBosscherand waterorganisms andtheircomplexcomposition Schlager(1992).Others (e.g.,BowmanandVail, andmorphology,incombination withthe varied 1999) haveincludedthe influence ofdistance processescausingtheirdisaggregation,account from the platformedge asanimportant factor in for the very complexmixtureofbiogenic carbonateproduction inshallow-to-shoalwater carbonategrains inshoal-watersediments. environments. Seawaterinthe upperregions ofthe oceanis Carbonatesediments canbe subdivided into strongly supersaturated withrespecttocalcite, skeletalandnonskeletalcomponents. Nonskeletal aragonite, andidealdolomite.Whythesemin- carbonategrains havebeendivided into fivemajor erals failtoprecipitatedirectlyfrom ithasbeena types:mud, pellets,ooïds,lithoclasts,andrelict. problemofmajor interest.Inexperimental Carbonatemudsarecommon deposits inlow- studies(e.g.,Pytkowicz,1965, 1973; Berner, energyenvironments,such astidalflats and 1975),the magnesium ion hasbeenobserved to subtidalareas. Pellets areformed bythe ingestion be astronginhibitor ofcalciteprecipitation and ofsediment bymarineorganisms andexcretion largely responsiblefor its failureto precipitate 76 Formation andDiagenesisofCarbonateSediments directly from seawater. Thepresence ofmag- algae.Itisnecessary,therefore, to examinethe nesium canalsoelevatethe supersaturation chemistry ofthe needlestodeterminetheirorigin. necessary for aragoniteprecipitation inthe However,eventheirchemicalcharacteristicsare absence ofnuclei, to alevelfarbeyondthat not unambiguously diagnostic (e.g.,Lowenstam likely to be foundinnaturalwaters.Natural andEpstein,1957; Milliman,1974;Bathurst, organic mattercanalso strongly inhibitCaCO3 1975;Loreau,1982). precipitation (e.g.,Berner etal.,1978). Itis Not nearlyasmuch attention hasbeenpaid to unlikely,based on thisexperimentalwork, the production oflimemudfrom calcitic sources, thatconditions appropriatefor direct(homo- but thesemudscanbe the dominant component geneous) nucleation ofcalcium carbonatewill be inperi-reefal(Debenay,1985)andlagoonal foundinnormalmodern seawater(Morseand (Reid etal.,1992)environments where70–90% He, 1993). However,theseexperimentalresults ofthe mudmayconsist ofmagnesium cal- donot preclude the precipitation ofcarbonate cite.Thiscanform from the micritization and minerals on pre-existingcarbonatemineral recrystallization ofskeletalgrains (Reid etal., nuclei. 1992),andthe breakdownofforaminiferaltests (Debenay etal.,1999). 7.03.3.1.2 Sourcesofcarbonatemuds

The originofcarbonatemudisalong-standing 7.03.3.1.3 Formation ofcarbonatesands topic ofconsiderabledebateandcontroversy. The three major potentialsourcesgenerallyconsidered Carbonate-rich sediments thataredominated by are(e.g.,Reid etal.,1992):(i)physicochemicalor sand-sized grains arecommon inshoal-water biochemicalprecipitation,(ii)postmortemdisin- sediments. Inmostcasestheyareassociated tegration ofcalcareous algae, and(iii)physicalor withmoderate- to high-energyenvironments biologicalabrasion ofskeletalmaterial. Research such asbeachesandbehindfringingreefs. They hasgenerally focused on eitherthe aragonitic or areoftencomposed ofcarbonatesofmixed magnesium calcitemudcomponents. Andrews mineralogyfrom avariety ofbiogenic sources etal. (1997) havefoundthatgenerallywithinthe producedbyphysicalabrasion,boring, andother silt andclaysizes(# 63 m m),thereislittle biologicalprocesses. mineralogic, elemental,or stableisotopic Subsphericalcarbonategrains,generally variation. referred to asooïdsbecauseoftheirresemblance Fine-grained carbonate-rich sediments thatare to fisheggs,comprisesandysediments called dominated bysmall (afewmicrometers) needle- oo¨lites. Theyarecommon members ofmodern shaped aragonitegrains arecalled aragoniteneedle shoal-watertropic andsubtropic carbonate-rich muds. Thetwo major sourcesfor the aragonite sediments. Ooïdsarevariableinbothmineralogy needlesaregenerallyconsidered to be acalcareous andstructure.Aragonite, high-magnesiancalcite, algae andabiotic precipitates. Milliman(1974), andcalcitehavebeenobserved to occur inooïds Bathurst (1975),Scoffin(1987),andMorseand (e.g.,Land etal.,1979).Radial,concentric- Mackenzie (1990) havereviewed numerous tangential(laminated),andmicritic structuresare studiesofthe source ofaragoniteincalcareous commonly observed aroundacentralnucleus of muds. Most ofthe interpretations ofits origin non-ooïd material. The majority ofmodern,and arebased on chemicaldata, although needle probably most ancient,ooïdsare(or originated as) morphologyhasalsobeenused. eitheraragonitewithalaminated structureor Attempts atmakingbudgets ofcarbonate high-magnesiancalcitewitharadialstructure. production andaccumulation havealsobeen Thelaminated aragonitic ooïdsarebyfarthe most used inattempts to definethe source ofarago- abundant typeofooïdsin“recent”sediments and niteneedlemuds. Inareassuch asBritish havereceived the majority ofattention. HondurasandFlorida Bay,the supplyofbiogenic Ooïdsarebelieved to be ofnonbiogenic origin materialappears to be sufficient to providethe andrepresent oneofthe mostimportant modesof sediment for the carbonatemuds,although nonbiogenic removalofCaCO3 from the ocean. In someabiotic precipitation cannot be ruled out. addition,ithasfrequently beenpointed outthat However,inotherareassuch asthe Great theirmineralogyandstructurereflectthoseof BahamaBankandthe PersianGulf, the bio- marinecarbonatecements(e.g.,Fabricius,1977; genic supply appears to be insufficient,andan Land etal.,1979). Infact,theygrade into each abiotic source ofaragoniteneedlesisneeded otherinthe caseofBahamiangrapestone.Land (e.g.,Milliman etal.,1993). etal. (1979) stressed the tie betweencarbonate Unfortunately,the precipitation ofaragonite cements andooïds,pointingout thatooïdscanbe from seawaterproducesneedlesofasizeand viewed ascements centrifugally deposited on morphologythatisvery similartothoseproduced grains,whereassubmarinecements canbe viewed bythe breakdown ofcommon codiaceangreen ascentripetally coated pores. SourcesandDiagenesisofShoal-waterCarbonate-rich Sediments 77

Hypothesesfor the processesresponsiblefor 2.5 the existence ofooïdsarebased on bacterial- mechanical,algal,andchemicalmechanismsfor ooïd formation.Someinvestigators have 2.0 suggested models thatinvolvemorethanoneof thesemechanisms. Anextensivereviewand discussion ofthe hypothesesfor ooïd formation 1.5 canbe foundinMorseandMackenzie (1990). w . r t a goni e Supersaturation 1.0 Undersaturation 7.03.3.2 Early MarineDiagenesisofShoal-water

Carbonate-rich Sediments Sa tur a t ion 7.03.3.2.1 Pore-waterchemistry 0.5 The chemistryofsediment porewaters provides important information regardingchemicalreac- 0.0 tionsinsediments. Inmany cases,the reactions 0 20 406080100 betweenporewaters andsolidsarenot obvious %Sulfatereduction from observations ofthe solidsalone, but because the solid-to-solution ratioinsediments issolarge, Figure4 The saturation statewithrespecttoaragonite versus the extent ofsulfatereduction for aclosed major changescanoccur inporewaters during systemcontainingseawater(afterMorseand diagenesis. If equilibration alonewereto take Mackenzie, 1990). placeincarbonate-richsediments betweenthe porewaters andmetastablesolids,precipitation of astablephasewouldoccur untilequilibrium was bioirrigation ratesinmany carbonatesediments, reached.However,the oxidation oforganic matter depthprofilesofdissolved sulfatemaybe poor canalso be animportant process inthese indicatorsofthe importance ofsulfatereduction in sediments. Thecarbon dioxide generated bythis the oxidation ofsedimentary organic matterin process canbe sufficient to causeundersaturation, shoal-watercarbonatesediments. resultingindissolution—not precipitation—asthe Severalinvestigations ofshallow-watercarbon- dominant early reaction. ate-rich sediments havebeenmade to determine Amajor complicatingfactor inthe diagenesis which processescontrol the calcium carbonate ofshoal-watercarbonate-rich sediments isthat IAP.Two major studiesthatinvestigated thistopic much ofthe organic matteroxidation cantake wereconducted byBerner(1966) insouthFlorida placevia bacterially mediated sulfatereduction.If andBermuda andbyMorse etal. (1985)inthe marineplankton-like organic matterisoxidized Bahamas. Berner’s pioneeringstudywasimpor- via sulfatereduction,the nutrients phosphateand tant indemonstratingthrough pore-waterchem- ammonia arealso produced (Equation (6)): istry that,insedimentsexposed to normal seawater,thereislittlediagenetic alteration of 1 2 2 CH2 O 106 NH3 16H 3 PO4 SO4 sedimentary carbonates. Evidence for the dissolu- 53 ð Þ ð Þ þ tion ofmagnesiancalciteswasfoundinpore 2 2 16 CO2 HCO3 HS NH3 waters from sediments inthe northern part of ! þ þ þ 53 Florida Baywhereporewaters arefresher. Based 1 H 3 PO4 H 2 O 6 on carbonateequilibrium constantsavailableat þ 53 þ ð Þ the time, Bernerconcluded thatmostporewaters Becausethe products aregenerated nearlycon- arecloseto equilibrium withlow-magnesian stant ratios,fixed pHcanbe establishedaftera calcite.Calculationsusingnewconstantsindicate moderatedegree ofsulfatereduction inaclosed thatthisisnot generally the case. system(Ben-Yaakov,1973). Duringthe early Anumberofporewaters inbothcoarse- and stagesofreaction inseawaterthe pHdrops to fine-grained sediments from the northern Great , 6.9 andthenremains constant.Thiscanresult in BahamaBankandLittleBahamaBankwere undersaturated porewaters andcarbonatedissolu- studied byMorse etal. (1985). Figure5 tion.Furthersulfatereduction causesthe alka- summarizestheirfindingsfor the pHandTA linity to risewhilethe pHremains constant.At inporewaters fromsedimentsindifferent , 35%sulfatereduction,the porewaters regain environments. Linesarealso giveninthisfigure supersaturation withrespecttocalcium carbonate. for porewaterinequilibrium withcalcite, Furthersulfatereduction canresult incalcium aragonite, andan18mol.% magnesiancalcite, carbonateprecipitation (Figure4). Furukawa etal. usingthe solubility constants ofWalterand (2000) haveemphasized that,dueto the high Morse(1984). Clearly,thereisgreatvariability 78 Formation andDiagenesisofCarbonateSediments

8.2 thischapter). Carbonateion maybe “pumped” M g O S O down to valuesatsaturation withless soluble C O O O g O O S g O M g phases,asdissolution ofthe moresoluble M C M g C S S g g M S C O C S g materialeventually causesits removal. However, 7.8 C S M g M S g g O S g M g the persistence ofhigh-magnesiancalcitesin M C M g M S S g M g g M C g g C sediments for longperiodsoftimeindicates C g g g M g M g g C g M M Mg 18-calcite thatthisprocessdoesnot involvealarge 7.4 M M g p H M M M g M M g amount ofmass transferundernormalmarine M g g g g g M g conditions. g g g g g g g The influence ofseagrass bedsonfine-grained 7.0 M Aragonite g g sediments nearSanSalvador Islandwasinvesti- Calcite gated byShort etal. (1985),andMorse etal. (1987) studied theirinfluence on pore-water 6.6 2.0 3.04.05.06.07.0 chemistry incoarser-grained sediments nearthe Totalalkalinity (meq kg–1) Berry Islands. Theirinfluence wasless inthe coarse-grained sediments,but eveninthese Figure5 pHversus totalalkalinity for porewaters in sediments,elevated p CO andalkalinity valuesin Bahamiancarbonate-richsediments. M mud; 2 ¼ sediments beneaththe seagrass bedswereappar- S sand;O oo¨lite;C coarse-grained grassbed; ent. Thisisprobably the result ofelevated organic g ¼ fine-grai¼ned grassbed¼ (afterMorse etal.,1985). ¼ matterconcentrations associated withroots and debrisfromthe plants. Jensen etal. (1998) inpore-watercomposition,andmany sediment observed thatthe carbonatematrixisdissolved pore-waters aresubstantially supersaturated with inthe seagrass rhizosphereresultinginareleaseof respecttoall thesephases. p CO2 valuesarehigh phosphorus,which isbelieved to limitseagrass inthesesediments,typicallyatleast 10 times production incarbonatesediments. the atmospheric value.Evenwithinagivencore itwasfoundthatcalcium carbonateIAPswere highly variable(Morse etal.,1985). Because 7.03.3.2.2 Precipitation ofearly carbonate aragoniteisgenerally the most abundant cements carbonatephaseinthesesediments,control of The formation ofcarbonateovergrowthsand the IAP bythe mostabundant phasedoesnot cements iscertainlyoneofthe most important and explaintheseobservations. highly studied aspects ofcarbonatesinsediments Anumberofsolubility experiments,on sedi- andsedimentary rocks. Thissection discussesthe ments from the Bahamasitesatwhich porewaters geochemicalaspects ofthe formation ofearly werecollected, wereperformed byBernsteinand diagenetic cementsandovergrowths. Anexten- Morse(1985)inanattempt to understandthe siveliteratureexists on the occurrence ofearly processescontrollingthe calcium carbonateIAP carbonateprecipitatesinmarinesediments,where inthesesediments. Equilibrium wasapproached theyaregenerally termed cements. Included inthis from bothsupersaturation andundersaturation for literaturearebooksdevoted solely to carbonate different timeperiodsupto50d.Generally good cements(e.g.,Bricker,1971; Schneidermann agreement wasfoundbetweenthe fieldand andHarris,1985)andnumerous reviews laboratory observationsfor fine-grained sedi- (e.g.,Milliman,1974;Bathurst,1974, 1975; ments,indicatingatleast adynamic steadystate Harris etal.,1985;MorseandMackenzie, withinthe sediments betweenthe porewaterand 1990). Many investigationshavebeenlargely somesolid phase.Less agreement wasfoundfor descriptiveandhavefocused primarily on the coarse-grained sediments,whereflow ofwater distribution,mineralogy,andmorphologyofthe through sediments (e.g.,oo¨litebanks) mayresult cements. inaresidence timetooshortfor dynamic Carbonatecements incalcareoussediments equilibrium to be reached.Aninterestingobser- belongto three major groups.Themost common vation wasthatooïds,although composed almost arethosethatoccur invoidsfoundinbiogenic entirely ofaragonite, weresignificantlymore carbonates. Thisgroup isespecially important in solublethanaragonite, inagreement withthe reefs,which arediscussed inaseparatesection of Weyl (1965)hypothesisofamagnesiancalcite thischapter. Cements also occur on the exterior of coating. carbonateparticles,wheretheirintergrowthcan Fieldandlaboratory observations areconsist- causeformation ofhardened pellets,grapestones, ent withthe idea thatdissolution incarbonate crusts,hardgrounds,andbeachrock.Thistypeof sediments canproceed fasterthanprecipitation, cementation mayalsobe important inooïds(e.g., andthatthe porewaters reach steady-stateIAPs Fabricius,1977). Micritic cements associated with closeto thoseofthe most unstablephase boringalgae comprisethe thirdcommon typeof (dissolution processeswill be discussed laterin cement. SourcesandDiagenesisofShoal-waterCarbonate-rich Sediments 79 Oneofthe most interestingquestions about low-energyenvironment andlow sedimentation carbonatecements inthe marineenvironment is rates(e.g.,Taft etal.,1968;Shinn,1969). whytheyarenot moreabundant. Incarbonate-rich However,Dravis(1979) foundthathardgrounds sediments carbonatesurfacesfor overgrowthsare couldform quickly evenunderhigh-energyand abundant,andbothoverlyingseawaterandmany rapid sedimentation rateconditions. He found porewaters aresupersaturated withrespectto thatinsuch environmentsendolithicalgal carbonateminerals.MorseandMucci(1984) filaments playanimportant roleinbindingthe demonstrated thatprecipitation ofcementsfrom sediment andprovidingsitesofcement for- porewaters ofcarbonatesediments isseverely mation. Aragoniteandhigh-magnesiancalcite inhibited bystudyingIcelandsparcalcitecrystals cementscanform undermost environmental thathad beenburiedfor severalmonthsina conditions,but aragoniteisgenerallyfavored variety ofcarbonatesediments inthe Bahamas. underhigher-energyconditions. Givenand Afterrecovery andanalysisusingvery sensitive Wilkinson (1985)suggested thatthisisbecause depth-profilingAugerelectron spectroscopy, precipitation ratesarefasterunderhigher-energy overgrowthsweredetected on only afewcrystals. conditions andthatafasterprecipitation rate Thecalculated growthratesofthe precipitates favors aragonitic cements. weremuch less thanthoseobserved inlaboratory Amajor concern instudiesofthe chemistry of experiments usingseawateratthe samesuper- carbonatecement formation isthe source ofthe saturation state. calcium andcarbonateions necessary to form the Atleast part ofthe reason for theseobservations cements. The obvioussource isseawater,but large mustbe thatprecipitation isseverely inhibited by volumesofseawaterarenecessary if significant organicssuch ashumic acids(Berner etal.,1978). amounts ofcement areto be produced.Cement Mittererandco-authors(e.g.,Mittererand formation isconsequently favored nearthe Cunningham,1985)haveexplored the possible sediment–waterinterface andinhigh-energy roleoforganic matterincement formation. These environments wherewatercanbe flushed through authors suggested that,whereassometypesof porous structuressuch asreefs. The observation organic matterinhibitprecipitation,othertypes, thatcementsusuallyform only inthincrusts near particularly thoserich inaspartic acid, favor the sediment–waterinterface also demonstrates precipitation bycomplexingcalcium. Inhibition the importance ofnormalseawaterfor cement ofprecipitation,coupled withslow transferof precipitation. Furtherevidence for cement for- freshsupersaturated seawaterinto sediment pores, mation innormalseawatercomesfrom stable seems to account for the lack ofextensiveearly isotopes. The d 13Cvaluesofcementsareusually cementation. closeto thosepredicted for carbonatesprecipitat- The factorscontrollingthe mineralogyand ingfrom seawater(e.g.,see GivenandWilkinson, chemicalcomposition ofcarbonatecements inthe 1985). Anotherpossiblesource ofthe ions marineenvironment havealsobeeninvestigated necessary for cement formation isthe dissolution extensively.Bathurst (1975, 1987) summarized ofcarbonatephasesmoresolublethanthe many ofthe observations (e.g.,GloverandPray, cements. High-magnesiancalcitescouldprovide 1971). Insomecasesthe host carbonatemineralis such asource, ashasbeendemonstrated in afactorindeterminingmineralcomposition. periplatformoozesinthe Bahamas(Mullins Usually aragonitegrains havearagoniteover- etal.,1985)andfor coastalcarbonatesediments growths,whereashigh-magnesiancalcitegrains ofthe BayofCalviinCorsica(Moulin etal., havehigh-magnesiancalciteovergrowthsof 1985). Thisprocessisconsistent withthe similarmagnesium content.Insomeinstances, previouslydiscussed chemistry ofmany pore syntaxialformation ofcement isobserved. waters. Although thisexplanation ofhost control isa The precipitation ofcalcium carbonate(usually, “comfortable”one, itdoesnot explainthe but not exclusively,from petrographic evidence common occurrence ofcementsofmixed miner- inthe formofhigh-magnesiancalcite, e.g., alogyorthosethatdifferinmineralogyfrom their Alexandersson andMilliman,1981) isgenerally host grains. inferred from adecreaseindissolved pore-water Environmentalfactorshavealso beenfoundto calcium (e.g.,Thorstenson andMackenzie, 1974; correlateloosely withcement formation,miner- Gaillard etal.,1986).Moststudiesofthe alogy,andmorphology. Theenergy(waves, impactofchemicaldiagenesisonthe carbonate currents,etc.) ofthe environment andthe rate chemistry ofanoxic sedimentshavefocused ofsedimentation aremost oftencited as primarily on the factthatsulfatereduction important inshallow-waterenvironments. The results inthe production ofalkalinity,which energyofthe environment isimportant in cancauseprecipitation ofcarbonateminerals supplyingseawater,from which the carbonate (e.g.,Berner,1971). Carbonateprecipitation can cements canbe derived.Formation ofhard- also occur via methaneoxidation insteadof groundswasoriginally thoughttonecessitatea organic matteroxidation,resultingindistinctly 80 Formation andDiagenesisofCarbonateSediments “light” d 13Cvaluesfor authigenic carbonates established across the cell wall of Desulfovibrio (e.g.,Ritger etal.,1987). desulfuricans, resultingincalcium carbonate Finally,itshouldbenoted thatwhilemost dissolution inthe nearbymicroenvironment. investigators haverejected the idea thatcarbonate Prior to the onsetofsulfatereduction,extensive cementsaredirectlyorevenclosely linked to organic matterdegradation canoccur bybac- biologicalmechanismsofformation,someinves- terially mediated oxygenuseasdescribed for tigators holdthe position thatorganisms maybe deep-sea sediments. Theinfluence ofbenthic very important for cement formation processes. bacterialactivity on carbonatemineraldissolution Marinepeloids(aggregatesofcemented carbonate underaerobic conditions hasbeendemonstrated grains) havereceived considerableattention in nicely byMoulin etal. (1985)for porewaters thisregardbecauseoftheircloseassociation with from sediments ofthe GulfofCalviinCorsica. bacterialclumps (e.g.,Chafetz,1986).Fabricius Otherprocesses,but probably less important, (1977) haspresented alengthybrief infavor ofthe include nitratereduction andfermentation (e.g., formation ofgrapestonecementsandooïdsby Aller,1980). algae. Walterandco-workers (WalterandBurton, 1990; Walter etal.,1993; Ku etal.,1999)have madeextensiveefforts to demonstratethe import- 7.03.3.2.3 Dissolution ofcarbonates ance ofdissolution ofcalcium carbonatein Many studiesofthe impactofchemical shallow-watercarbonatesediments. Upto diagenesisonthe carbonatechemistry ofanoxic , 50% carbonatedissolution canbe drivenby sediments havefocused primarily on the factthat the sulfatereduction–sulfideoxidation process. In sulfatereduction results inthe production of calcium carbonate-rich sediments thereisoftena alkalinity,which cancauseprecipitation of lackofreactiveiron to produce ironsulfide carbonateminerals (see previousdiscussion). minerals. The sulfidethatisproduced bysulfate However,duringthe earlystagesofsulfate reduction canonly be buried indissolved form in reduction ( , 2–35%),thisreaction maynot porewaters,oxidized,or candiffuseout ofthe causeprecipitation,but dissolution ofcarbonate sediments. Inmostcarbonate-rich sediments minerals,becausethe impactofalowerpHis the oxidativeprocess strongly dominatesthe fate greaterthanthatofincreased alkalinity (Figure4). ofsulfide.Figure6(Walter etal.,1993)shows Carbonateion activity decreasesrapidly asitis the strongrelationshipthatgenerally occursin the carbonatemudsofFloridaBaybetween “titrated”byCO2 from organic matterdecompo- sition leadingto adecreaseinpore-water totalcarbon dioxide, excess dissolved calcium saturation state.Thisprocess isevident indata (calcium ataconcentration abovethatpredicted for the Fe-poor,shallow-watercarbonatesedi- from salinity),andthe amount ofsulfatethathas ments ofMorse etal. (1985)from the Bahamas beenreduced.Itisnoteworthythatthe burrowed andhasbeenconfirmed instudiesbyWalterand banksshow much moreextensiveincreasein Burton (1990),Walter etal. (1993),andKu etal. calcium thanthe othermudbanks. Thisisingood (1999) for Florida Bay,Tribble(1990) inChecker agreement withthe observations ofAllerand Reef, Oahu,andWollast andMackenzie (unpub- Rude (1988)thatinLongIslandSoundsiliciclas- lished data)for Bermuda sediments. tic sediments anincreased bioturbation leadsto Three primary processescanlead to under- increased sulfide oxidation andcarbonate saturation,inaddition to the onethatmayresult dissolution. duringthe early stagesofsulfatereduction. These areearly post-deathmicroenvironmentswithin organisms,oxidation oforganic matterbypro- 7.03.3.2.4Carbonatediagenesisassociated cessesprecedingsulfatereduction,andoxidation withreefs ofsulfides. Commonly,theseprocessesaremost important nearthe sediment–waterinterface. Becauseofthe difficultiesinherent inobtaining Early studiesbyHecht(1933) showed that porewaters from the interior ofreefs,the chemical dissolution ofcarbonateinmolluskshells can environment inreefstructureswent largely beginimmediately afterdeath.Inlaboratory unstudied untilthe 1980s. Most ofthe studies studieshe foundshell weightlossesofasmuch ofearly reef diagenesisarethoseofHawaiian as25%intwo weeks. Otherexamplesofthistype andAustralianreefsbySansoneandco-workers ofbehavior comefrom astudybyBerner(1969), (Sansone, 1985;Sansone etal.,1988a,b;Tribble, whofoundcalcium carbonatedissolution during 1990,1993; Tribble etal.,1990,1992). Fichez bacterialdecomposition oftwo typesofclams, etal. (1997) andAndrie´ etal. (1998)have possibly asaresult ofthe production oforganic conducted studiesofchemicalchangesinpore acidsinaddition to CO2 duringearly degradation waters inadeepboreholewithinaTahitian oforganic matter. Birnbaum andWiremen(1984) fringingreef.Thesechangesoccur during havealso pointed out thataproton gradient canbe timeperiodssubstantially longerthanthose SourcesandDiagenesisofShoal-waterCarbonate-rich Sediments 81

4 atereduction mudbanks (m M ) sulf of Sulfide burrowed banks oxidation system

Closed r ed u c

4 2 mudbanks burrowed banks SO

0 2468101214 00.5 1.01.5 2.02.5 2+ (a)(TCO2 (mM) b) Excess Ca (mM)

2 Figure6 Dissolved sulfateversus:(a)TCO2 and(b)excess Ca þ inthe porewaters ofcarbonate-rich Florida Bay sediments (afterWalter etal.,1993). encompassed bythe “early” diagenesisconsidered FeS formation canplayanimportant rolein inthischapter. regulatingpore-waterchemistry. Systems with The studiesofCheckerReef, Oahu,Hawaii moreintenseFeS formation haveatendency (alagoonalpatch reef)andDaviesReef, Great to becomemoresupersaturated withrespect BarrierReef, Australia (aplatformreef),show to aragonite.N-poor organic matterappeared to thatmicrobiallymediated processesofoxic result inmorecorrosiveconditions. respiration,sulfatereduction,and, to alesser extent,nitratereduction andmethanogenesisare important inthesereef structures. The interstitial 7.03.3.2.5Earlydolomiteformation waters ofthesereefsarehighly depleted in The formation ofdolomiteunderconditions oxygen,andexhibitlowerpHvaluesandelevated encountered inmodern marinesediments and concentrations ofdissolved methane, sulfide, sediment burialtomoderatedepthsisstrongly ammonium,phosphate, andsilica relativeto the controlled byreaction kineticsthatareslow even seawatersurroundingthe reef.InCheckerReef athigh supersaturations (e.g.,Lippmann,1973; the degree ofalteration ofinterstitialwaters Morrow,1982).Aplausibleexplanation for the increasesfrom the margintothe centerofthe slow precipitation kineticsofdolomiteisthe reef;inDaviesReef the most altered porewaters requirement thatcation orderingputs amajor limit areatthe marginofthe reef. on the rateatwhich itcanform(e.g.,the The differencesbetweenthesereefsprobably “simplexity” principle, Goldsmith, 1953).Itis reflectdifferencesintheirstructuralframework 2 likely thatthe dehydration kineticsofMg þ also andvariationsinwavecharacteristicsandtidal playarole(e.g.,Lippmann,1973). Becausethe range inthe two environments. Anaerobicreac- precipitation kineticsofdolomitearetoo slow to tionsinreef interstitialwaters maynot progress be studied inthe laboratory atnearEarth-surface farif reef structuresareopenandwell flushed.If, temperatures(Land, 1998),experimentson however,the systems arenearly closed, littlefresh dolomitereaction kinetics(e.g.,Arvidson and reactant will entervia seawaterexchange and Mackenzie, 1999)havegenerallybeenconducted mass transferwill be limited bythe reactants atelevated temperatures(typically between trapped inthe reef interstitialwaters.Animportant 100 8 Cand300 8 C). Considerableinteresting conclusion ofSansone’s studieswasthatthermo- information hasbeengathered from theseexperi- dynamic disequilibrium amongdissolved species ments. However,the applicability ofthe results to 2 2 such asCH4 andSO 4 impliesmicrozonation of processesatlowertemperaturesandslower chemicalreactions. Microzonation resultingin reaction kineticsremains highly questionable. slightdifferencesinreef interstitialwatercompo- Observations ofdolomiteformation innatural sitions mayaccount for the coexistence of systems havebeenused for definingadditional different cement mineralogiesinreef structures. factorsthatmayinfluence the rateofdolomite Apattern ofinitialcarbonatedissolution formation. Theseinclude catalysisbycertainclay followed bycarbonateprecipitation asafunction minerals (e.g.,Wanless,1979)andproduction of ofthe extent ofsulfatereduction occurswithin organic by-products bybacteria (e.g.,Gunatilaka 2 reefsinamannersimilartothatpreviously etal.,1985). Mg þ transport to sitesofdolomite described for sediments (Tribble, 1993). Through- precipitation caninhibitthe reaction inhemi- out thisprocess the interstitialwaters maintain pelagic sediments(e.g.,BakerandBurns,1985). closeto equilibrium compositions witharagonite. However,the trueinfluence ofreaction ratesis Thisbuffers the pHofthe waters.Itwasfoundthat largely speculative, becausethe kinetic factorsare 82 Formation andDiagenesisofCarbonateSediments generallydeduced from the presence or absence of AllerR.C.(1980) Diagenetic processesnearthe sediment– dolomiteindifferent environments. waterinterface ofLongIslandSound: I.Decomposition and nutrient element geochemistry (S, N, P). Adv. Geophys. 22, Most modern dolomiteisformingfrom high 237–350. ionic strengthsolutions thatareusually derived AllerR.C.andRude P.D.(1988)Completeoxidation of from the evaporation ofseawaterorlakesinarid solid phasesulfidesbymanganeseandbacteria in regions. Theseenvironments havebeenstudied anoxic marinesediments. Geochim. Cosmochim. Acta 52 , extensively,becausetheyprovide anopportunity 751–765. 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