The Geological Record ofOcean AcidificationBärbel Hönisch,1* Andy Ridgwell,2 Daniela N. Schmidt,3 Ellen Thomas,4,5 Samantha J. Gibbs,6

Appy Sluijs,7 Richard Zeebe,8 Lee Kump,9 Rowan C. Martindale,10 Sarah E. Greene,2,10

Wolfgang Kiessling,11 Justin Ries,12 James C. Zachos,13 Dana L. Royer,5 Stephen Barker,14

Thomas M. Marchitto Jr.,15 Ryan Moyer,16 Carles Pelejero,17 Patrizia Ziveri,18,19

Gavin L. Foster,6 Branwen Williams20

Ocean acidification may have severe consequences for marine ecosystems; however, assessingits future impact is difficult because laboratory experiments and field observations are limited bytheir reduced ecologic complexity and sample period, respectively. In contrast, the geologicalrecord contains long-term evidence for a variety of global environmental perturbations, includingocean acidification plus their associated biotic responses. We review events exhibiting evidencefor elevated atmospheric CO2, global warming, and ocean acidification over the past ~300 millionyears of Earth’s history, some with contemporaneous extinction or evolutionary turnover amongmarine calcifiers. Although similarities exist, no past event perfectly parallels future projectionsin terms of disrupting the balance of ocean carbonate chemistry—a consequence of theunprecedented rapidity of CO2 release currently taking place.

Thegeological record is imprinted with nu-merous examples of biotic responses tonatural perturbations in global carbon cy-

cling and climate change (Fig. 1), some of whichcould have been caused by large-scale oceanacidification. By reconstructing past changes inmarine environmental conditions, we can test hy-potheses for the causes and effects of future-

relevant stressors such as ocean acidification onecosystems (1). However, for the fossil record tobe of direct utility in assessing future ecosystemimpacts, the occurrence and extent of past ocean

acidification must be unambiguously identified.In recent years, a variety of trace-element andisotopic tools have become available that can beapplied to infer past seawater carbonate chemis-try. For instance, the boron isotopic composition(d11B) of marine carbonates reflects changes inseawater pH, the trace element (such as B, U, andZn)–to-calcium ratio of benthic and planktic for-aminifer shells records ambient [CO2−

3 ], and thestable carbon isotopic composition (d13C) of or-ganic molecules (alkenones) can be used to es-timate surface ocean aqueous [CO2] (2).

Because direct ocean geochemical proxyobservations are still relatively scarce, past oceanacidification is often inferred from a decrease inthe accumulation and preservation of CaCO3 inmarine sediments, potentially indicated by an in-creased degree of fragmentation of foraminiferalshells (3). However, it is difficult to distinguishbetween the original calcification responses tochemical changes in the surface ocean and post-mortem conditions at the sea floor. For instance,planktic calcifiers may secrete heavier or lightershells (4), but that signal may be modified at thesea floor through dissolution or overgrowth afterdeposition (5, 6). This duality can introduce con-troversy over the identification of causes andeffects, the drivers of biological change, and

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1Lamont-Doherty Earth Observatory of Columbia University,Palisades, NY 10964, USA. 2School of Geographical Sciences,University of Bristol, Bristol BS8 1SS, UK. 3School of EarthSciences, University of Bristol, Bristol, BS8 1RJ, UK. 4Depart-ment of Geology and Geophysics, Yale University, New Haven,CT 06520, USA. 5Department of Earth and EnvironmentalSciences, Wesleyan University, Middletown, CT 06459, USA.6Ocean and Earth Science, National Oceanography CentreSouthampton, University of Southampton, Southampton SO143ZH, UK. 7Department of Earth Sciences, Utrecht University,3584 CD Utrecht, Netherlands. 8School of Ocean and EarthScience and Technology, Department of Oceanography, Uni-versity of Hawaii at Manoa, Honolulu, HI 96822, USA. 9Depart-ment of Geosciences, Pennsylvania State University, UniversityPark, PA 16802, USA. 10Department of Earth Sciences, Uni-versity of Southern California (USC), Los Angeles, CA 90089,USA. 11Museum für Naturkunde at Humboldt University, 10115Berlin, Germany. 12Department of Marine Sciences, Universityof North Carolina–Chapel Hill, NC 27599, USA. 13Earth andPlanetary Sciences Department, University of California SantaCruz, CA 95064, USA. 14School of Earth and Ocean Sciences,Cardiff University, Cardiff CF10 3AT, UK. 15Department of Geo-logical Sciences and Institute of Arctic and Alpine Research,University of Colorado, Boulder, CO 80309, USA. 16University ofSouth Florida St. Petersburg, Department of EnvironmentalScience, Policy, and Geography, St. Petersburg, FL 33701, USA.17Institució Catalana de Recerca i Estudis Avançats and Depart-ment of Marine Biology and Oceanography, Consejo Superiorde Investigaciones Científicas, 08003 Barcelona, Catalonia, Spain.18Institute of Environmental Science and Technology, UniversitatAutònoma de Barcelona, 01893 Barcelona, Spain. 19Depart-ment of Earth Sciences, Vrije Universiteit, 1081HV Amsterdam,Netherlands. 20W. M. Keck Science Department of ClaremontMcKenna College, Pitzer College, and Scripps College, Claremont,CA 91711, USA.

*To whom correspondence should be addressed. E-mail:hoenisch@ldeo.columbia.edu

Organic-walleddinocysts

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Fig. 1. Idealized diversity trajectories of the calcareous and organic fossil lineages discussed in the text.Extinction and radiation suggest events of major environmental change throughout the past 300 My.Calcareous plankton is shown in black, calcareous benthos in blue, and organic fossils in green, and theline thickness indicates relative and smoothed species richness. Highlighted events (vertical red lines)have been associated with potential ocean acidification events (Fig. 4). Calcareous organisms were notuniformly affected at all times, suggesting the importance of synergistic environmental factors to ex-tinction, adaptation, and evolution as well as different sensitivity due to physiological factors. Iden-tification of a paleo-ocean acidification event therefore requires independent geochemical evidencefor ocean chemistry changes. Images of organisms are exemplary. References and further informationon the displayed organisms are available in the supporting online material.

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whether past intervals of ocean acidificationare characterized by environmental conditionsrelevant for the near future. Coeval changes inocean circulation will also introduce regionalbiases in proxy records and hence affect globalinterpretations.

Here, we review the factors controlling oceanacidification, describe evidence for the occurrenceof ocean acidification events in the past, and dis-cuss the potential as well as weaknesses of thegeological record in helping us predict future eco-system changes.

Is Ocean Acidification Primarily apH-Decline Phenomenon?The current rate of anthropogenic CO2 releaseleads to a surface ocean environment charac-terized not only by elevated dissolved CO2 anddecreased pH (7) but, critically, decreased satura-tion with respect to calcium carbonate (CaCO3),a compound widely used by marine organismsfor the construction of their shells and skeletons(8). In contrast, slower rates of CO2 release leadto a different balance of carbonate chemistrychanges and a smaller seawater CaCO3 saturationresponse, which may induce differential bioticresponse or even no response at all, invalidating adirect analog. The reason for a smaller saturationresponse to slow CO2 release is that the alkalinityreleased by rock weathering on land must ulti-mately be balanced by the preservation and burialof CaCO3 in marine sediments (Fig. 2), which

itself is controlled by the calcium carbonate sat-uration state of the ocean (9). Hence, CaCO3

saturation is ultimately regulated primarily byweathering on long time scales, not atmosphericpartial pressure of CO2 (PCO2). While weatheringitself is related to atmospheric PCO2 (10), it isrelated much more weakly than ocean pH, whichallows pH and CaCO3 saturation to be almostcompletely decoupled for slowly increasing at-mospheric PCO2.

Using a global carbon cycle model (2), weshow the progressive coupling between CaCO3

saturation and pH as the rate of CO2 emissionsincreases and sources (weathering) and sinks(CaCO3 burial) of alkalinity are no longer ba-lanced. For rapid century-scale and thus future-relevant increases in atmospheric PCO2, bothsurface ocean pH and saturation state decline intandem (Fig. 3). The projected decrease in oceansurface saturation state—here, with respect toaragonite (Waragonite)—is an order of magnitudelarger for a rapid CO2 increase than for a slow[100 thousand years (ky)] CO2 increase. Ulti-mately, saturation recovers while the pH remainssuppressed, reflecting how changes in the oce-anic concentrations of dissolved inorganic carbon(DIC) and alkalinity make it possible to havesimultaneously both high CO2 and high carbon-ate ion concentration saturation ([CO2−

3 ], whichcontrols saturation), but with the relatively greaterincrease in [CO2] causing lower pH. The key tounlocking the geological record of ocean acid-

ification is hence to distinguish between long-term steady states and transient changes. We usethe term “ocean acidification event” for time in-tervals in Earth’s history that involve both a re-duction in ocean pH and a substantial loweringof CaCO3 saturation, implying a time scale onthe order of 10,000 years and shorter (Fig. 3).

Indications of Paleo-Ocean AcidificationWith these criteria in mind, we review (in reversechronological order) the intervals in Earth’s historyfor which ocean acidification has been hypothe-sized, along with the evidence for independentgeochemical and biotic changes. We confine thisreview to the past ~300 million years (My) be-cause the earlier Phanerozoic (and beyond) lacksthe pelagic calcifiers that not only provide keyproxy information but also create the strong deep-sea carbonate (and hence atmospheric PCO2) buf-fer that characterizes the modern Earth system(9). Our criteria for identifying potentially future-relevant past ocean acidification are (i) massiveCO2 release, (ii) pH decline, and (iii) saturationdecline. We also discuss evidence for the timescale of CO2 release, as well as for global warming.Events are given a similarity index that is basedon available geochemical data (table S1) and areindicated in Fig. 4A.

Late Pleistocene deglacial transitions. Thelast deglaciation is the best documented past eventassociated with a substantive (30%) CO2 rise:189 to 265 matm between 17.8 to 11.6 ky before

pCO2(g)

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Fig. 2. When CO2 dissolves in seawater, it reacts with water to form carbonicacid, which then dissociates to bicarbonate, carbonate, and hydrogen ions. Thehigher concentration of hydrogen ions makes seawater acidic, but this processis buffered on long time scales by the interplay of seawater, seafloor carbonatesediments, and weathering on land. Shown are the major pathways of reducedcarbon (black) and of alkalinity (yellow). Processes leading to ocean acid-ification and/or reduction of CaCO3 saturation are indicated in red, and pro-

cesses leading to ocean alkalinization and/or CaCO3 saturation increases areindicated in blue. Anthropogenic perturbations are marked in italics. Ap-proximate fluxes are printed in parentheses (PgC year−1), whereas reservoirinventory values are shown in brackets [PgC]. Natural carbon cycle fluxes arefrom (70); anthropogenic fluxes for 2008 are from (57), which for the landsink is significantly above its 1990–2000 average of 2.6 PgC year−1 due to the2008 La Niña state (8).

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the present (B.P.) (11). Boron isotope estimatesfrom planktic foraminifers show a 0.15 T 0.05unit decrease in sea surface pH (12) across thedeglacial transition—an average rate of declineof ~0.002 units per 100 years compared with thecurrent rate of more than 0.1 units per 100 years(table S1). Planktic foraminiferal shell weightsdecreased by 40 to 50% (4), and coccolith massdecreased by ~25% (13). In the deep ocean,changes in carbonate preserva-tion (14), pH [from foraminiferald11B (15)] and [CO2−

3 ] [fromforaminiferal B/Ca and Zn/Ca(16, 17)] differed between oceanbasins, reflecting covaryingchanges in deep-water circula-tion and an internal carbon shiftwithin the ocean. The regionalnature of these variations high-lights the general need for carefulevaluation of regional versus glob-al effects in paleo-studies.

Oligocene–Pliocene.The cli-mate of the Oligocene to Plio-cene [34 to 2.6 million years ago(Ma)] contains intervals of ele-vated temperature and modestdeviations of atmospheric PCO2

from modern values (Fig. 4). Ofparticular interest has been thePliocene warm period [3.29 to2.97Ma (18, 19)], which is char-acterized by global surface tem-peratures estimated to be ~2.5°Chigher than today (19), atmospher-ic PCO2 between 330 to 400 matm(Fig. 4C) (18, 20), and sea surfacepH(T) ~0.06 to 0.11 units lower(18) than the preindustrial. Eco-logical responses to the warminginclude migration of tropical for-aminifer species toward the poles(21), but there are no documentedcalcification responses or increasednannoplankton extinction rates(22). The early tomiddleMiocene(23 to 11Ma) and Oligocene (34to 23 Ma) were also character-ized periods of elevated temper-atures and slightly higher PCO2

compared with preindustrial val-ues (Fig. 4C) but, because of theirlong duration, were not associ-ated with changes in CaCO3 sat-uration (Fig. 3C).

Paleocene–Eocene. Evidencefor rapid carbon injection asso-ciated with the Paleocene–EoceneThermal Maximum (PETM, 56Ma) aswell as a number of smallertransient global warming events(hyperthermals) during the latePaleocene and early Eocene (58to 51 Ma) comes primarily fromobservations of large [up to –4

per mil (‰)] negative d13C excursions (23)associated with pronounced decreases in calci-um carbonate preservation (24). Depending onthe assumed source, rate, and magnitude of CO2

release (25), a 0.25 to 0.45 unit decline in surfaceseawater pH is possible, with a reduction in meansurface ocean aragonite saturation from W = 3down to 1.5 to 2 (1). The calcite compensationdepth (CCD) (8) rose by ~2 km to shallower than

1.5 km in places (24) (compared with >4 kmtoday). Although a pH decrease or PCO2 increaseremains to be confirmed by geochemical proxiesfor any of the hyperthermal events, the amountof carbon injected can be modeled on the basisof consistent carbonate d13C and CCD changes,yielding between ~2000 and 6000 PgC for theonset of the PETM (26, 27). However, as with thelast glacial transition, deep sea geochemistry ap-pears strongly modulated by regional ocean cir-culation changes (28), which adds an additionallayer of complexity to global extrapolation andhighlights the importance of adequate spatial cov-erage of the data.

PETM sediments record the largest extinctionamong deep-sea benthic foraminifers of the past75 My (29), and a major change in trace fossilsindicates a disruption of the macrobenthic com-munity (30). However, the covariation of oceanacidification, warming, and corresponding oxygendepletion (fig. S2) (23) precludes the attribution ofthis extinction to a single cause (1, 29). In shallowwater environments, a gradual shift from calcar-eous red algae and corals to larger benthic foramin-ifers as dominant calcifiers started in the Paleoceneand was completed at the PETMwith the collapseof coralgal reefs and larger benthic foraminiferalturnover (31). This event is recognized as one ofthe four major metazoan reef crises of the past300My (Fig. 1) (32). Inmarginal marine settings,coccolithophore (33) and dinoflagellate cyst (34)assemblages display changes in species compo-sition, but these are interpreted to reflect sensitiv-ity to temperature, salinity stratification, and/ornutrient availability (34, 35), not necessarily acid-ification (fig. S2). In the open ocean, the occur-rence of deformities in some species of calcareousnannoplankton has been described (36), but de-spite a strong change in assemblages, there is nobias in extinction or diversification in favor ofor against less or more calcified planktic spe-cies (37).

Cretaceous and Cretaceous-Paleogene. Thewell-known mass extinction at 65 Ma is gener-ally accepted to have been triggered by a largeasteroid impact (38). In addition to potential ter-restrial biomass or fossil carbon burning, the im-pact may have caused the emission of SO2 fromvaporized gypsum deposits at the impact siteand/or nitric acid aerosols produced by shockheating of the atmosphere, which could have ledto acid rain and hence potentially to rapid acid-ification of the surface ocean (38). Althoughplanktic calcifiers exhibited elevated rates of ex-tinction and reduced production (22, 39), reefcorals did not experience a major extinction (32),and benthic foraminifers were not affected in ei-ther shallow or deep waters (29). Because mul-tiple environmental changes covaried and proxydata for marine carbonate chemistry are not yetavailable, unambiguous attribution of the plankticextinctions to any one driver such as ocean acid-ification is currently not possible.

The earlier Cretaceous (K) (Fig. 4A) is gen-erally a time of massive chalk deposition (mainly

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Fig. 3. The trajectories of mean ocean surface pH and aragonitesaturation (Waragonite) become progressively decoupled as the rate ofatmospheric PCO2 change increases. The four panels show the resultsof a series of experiments in an Earth systemmodel (2). (A) Prescribedlinear increases of atmospheric PCO2 (on a log10 scale) from ×1 to ×2preindustrial CO2, with the different model experiments spanninga range of time scales (but experiencing the same ultimate CO2change). (B) Evolution of mean surface pH in response to rising CO2.(C) Evolution of mean surfaceWaragonite. (D) A cross-plot illustratinghowWaragonite is progressively decoupled from pH as the rate of PCO2increase slows, with future-relevant rate of PCO2 increase showing adiagonal trajectory from top left to bottom right, whereas slow PCO2increases result in an almost horizontal trajectory toward lower pHwith very little saturation change. All plots are color-coded from red(“fast”) to blue (“slow”). These model results include both climateand long-term (silicate) weathering feedback. See (2) and fig. S1 forthe role of these and other feedbacks.

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in the form of nannofossil calcite),as well as one of elevated PCO2

(Fig. 4B) and lower pH (Fig. 4D).This association can be miscon-ceived as evidence that marinecalcification will not be impairedunder conditions of low pH in thefuture. However, this reasoning isinvalid because extended periodsof high PCO2 (Fig. 4B) do not nec-essarily result in a suppressed sea-water calcite saturation state (Fig. 3)(1, 40), which exerts an impor-tant control on organisms’ calcifi-cation (41).

Cretaceous and Jurassic oce-anic anoxic events. The Mesozoicoceanic anoxic events (OAEs) (inparticular, OAE 2 ~93 Ma, OAE1a~120 Ma, and Toarcian OAE ~183Ma) were intervals during whichthe ocean’s oxygen minimum anddeep anoxic zones expanded mark-edly (42). The onsets of theseOAEshave been linked to the emplace-ment of large igneous provinces,degassing large amounts of CO2

and associated environmental con-sequences of warming, lower oxy-gen solubility, and possibly oceanacidification (42). Some of theCretaceous OAEs were associatedwith turnover in plankton commu-nities (43). Deformities and someminor size reduction in coccoliths,as well as a massive increase inthe abundance of heavily calcifiednannoconids, have been observed(44, 45). However, similar to morerecent events, there is difficulty inunequivocally attributing observa-tions to surface water acidificationgiven the covariation of environ-mental changes (46).

Because most old sea floor(~180 Ma or older) is subducted,the sedimentary record of theToarcian OAE is now restricted to former con-tinental margins. Sedimentary organic and inor-ganic carbon deposits display initially negative,followed by positive d13C excursions, which isconsistent with an influx of CO2 into the at-mosphere followed by organic carbon burial(42). The negative isotopic transition occurs indistinct negative d13C shifts, each estimated tooccur in less than 20 ky (47) and possibly in aslittle as 650 years (48). The Toarcian OAE isassociated with a reef crisis that was particularlyselective against corals and hypercalcifyingsponges (animals with a large skeletal-to–organic biomass ratio) (Fig. 4B) (32) and with adecrease in nannoplankton flux (49). Again,these observations could have been a responseto any one or combination of a number of dif-ferent contemporaneous environmental changes.

Triassic–Jurassic. The Triassic–Jurassic (T/J)mass extinction is linked to the coeval emplace-ment of the Central Atlantic Magmatic Province(50). Proxy records across the T/J boundary(~200 Ma) suggest a doubling of atmosphericPCO2 over as little as 20 ky (51, 52), althoughthe absolute PCO2 estimates differ greatly betweenproxies, with leaf stomata suggesting an increasefrom 700 to 2000 matm, whereas pedogenic car-bonates indicate an increase from 2000 to 4400matm (Fig. 4C) (2). Decreased carbonate satura-tion is inferred from reduced pelagic carbonateaccumulation in shelf sediments (53), althoughshallow water carbonate deposition can vary inresponse to many parameters, not only acidifica-tion. A calcification crisis amongst hypercalcify-ing taxa is inferred for this period (Fig. 4B), withreefs and scleractinian corals experiencing a near-

total collapse (32). However, the observation thattropical species were more affected than extra-tropical species suggests that global warming mayhave been an important contributor or even dom-inant cause of this extinction (32).

Permian–Triassic. The Permo–Triassic (P/T)mass extinction (252.3 Ma) was the most severeof the Phanerozoic Era and coincided, at least inpart, with one of the largest known continentaleruptions, the Siberian trap basalts. Recent es-timates for the total CO2 release put it at ~13,000to 43,000 PgC in 20 to 400 ky (54–56)—an an-nual carbon release of ~0.1 to 1 PgC [comparedwith 9.9 PgC in 2008 (57)]. There is some obser-vational evidence for carbonate dissolution inshelf settings (54), but its interpretation is againdebated (58). There is abundant evidence forocean anoxia, photic zone euxinia (enrichment in

Fig. 4. Compilation of data-based[(B) and (C)] andmodel-reconstructed[(C) and (D)] indicators of globalcarbon cycle evolution over thepast 300 My together with candi-date ocean acidification events (A).(A) Summarization of the degree towhich events (table S1) have somesimilarity tomodern oceanacidifica-tion. The similarity index (table S1)is color-coded, where red indicates3/most similar, orange indicates2/partly similar, and yellow indicates1/unlike. (B) Proxy-reconstructedatmospheric PCO2 (2) grouped byproxy: yellow circles indicate paleo-sol d13C, light blue squares indicatemarine phytoplankton d13C, redtriangles indicate stomatal indices/ratios, dark blue inverted trianglesindicate planktic foraminiferal d11B,green five-pointed stars indicateliverwort d13C, purple six-pointedstars indicate sodium carbonates,with 10-My averages shown by graybars. For plotting convenience, es-timates exceeding3000matmare notshown [primarily paleosold13C fromthe uppermost Triassic/lowermostJurassic (2)]. (C) Ocean Mg/Ca ratios(red triangles, left axis), reconstructedfrom fluid inclusions (2) and echino-derm fossil carbonate [red squares(71)] together with the Phanerozoicseawater model of (72) (red line).Also shown (blue circles, right axis)is [Ca2+] from fluid inclusions (2)and models [blue line (72)]. (D)Model-reconstructed changes inmean ocean surface pH at 20-Myintervals [black line (73)].

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hydrogen sulfide) (59), and strong warming (54),but no direct proxy evidence for pH or carbonateion changes. Knoll et al. (59) inferred the prefer-ential survival of taxa with anatomical and phys-iological features that should confer resilienceto reduced carbonate saturation state and hyper-capnia (high CO2 in blood) and preferential ex-tinction of taxa that lacked these traits, such asreef builders (32).

Is There a Geologic Analog for the Future?Anumber of past ocean carbon-cycle perturbationevents share many of the characteristics of an-thropogenic ocean acidification (Fig. 4 and tableS1), with the notable exception of the estimatedrates of CO2 release. In the general absence ofdirect proxy evidence for lower pH and reducedsaturation before the Pliocene, global carbon cyclemodels can be used to infer the magnitude ofcarbon release by fitting observed changes in thed13C of calcium carbonates and organic remnants(60). However, as well as needing information onthe source and isotopic composition of the addedcarbon, the time scale of d13C change is criticallyimportant to the estimation of CO2 fluxes (25).Because of the lack of open-ocean sediments andincreasingly poor temporal and spatial resolutionof the geological record further back in time, it isdifficult to place adequate constraints on theduration and rate of CO2 release. Radiometricdating techniques are not accurate enough toidentify Mesozoic intervals of 10-ky duration,although orbital spectral analysis of highlyresolved isotope and/or sedimentological recordscan help to partly overcome this—for example,if a d13C excursion is shorter or longer than oneprecession cycle [21 ky (51)]. Even for the well-studied PETM, the duration of the main phaseof this carbon injection is still debated (35, 61),and model-inferred peak rates of ≤1 PgC peryear (26, 61) could potentially be an underestimate.

Additional complications arise because car-bon may not have been released at a uniform rateand, in the extreme, may have occurred in theform of rapid pulses. In such cases, the assump-tion of an average emissions rate throughoutthe entire duration of the pulsed release will failto capture the potential for episodes of intenseacidification. For instance, although the totalduration of the CO2 release from the T/J–ageCentral Atlantic Magmatic Province was esti-mated to be ~600 ky, pulses as short as ~20 kyhave been suggested (51, 62). Similarly, the mainphase of OAE1a (excluding the recovery inter-val) was ~150 ky (45) and hence too slow forcarbonate saturation to be significantly affected(Fig. 3), but major volcanic eruptions and thusrapid CO2 release could potentially have producedfuture-relevant perturbations in the carbon cycle.Substantially improved chronologies and higher-resolution records are needed to refine estimatesof rate.

Given current knowledge of the past 300 Myof Earth’s history (Fig. 4 and table S1), the PETMand associated hyperthermal events, the T/J, and

potentially the P/T all stand out as having excel-lent potential as analog events, although the T/Jand P/T are much more poorly constrained be-cause of the absence of deep-sea carbonate de-posits. OAEs may also be relevant but wereassociated with less severe volcanism (CO2 re-lease) than were the older events (P/T and T/J).The last deglacial transition, although charac-terized by temperature and CO2-increase, is twoorders of magnitude slower than current anthro-pogenic change. It is also thought to largely rep-resent a redistribution of carbon within the oceanand to the atmosphere and terrestrial biosphereand hence did not have as potent and globallyuniform an acidification effect as an input fromgeological reserves. Because of the decouplingbetween pH and saturation on long time scales(Fig. 3), extended intervals of elevated PCO2 suchas the middle Miocene, Oligocene, and Cretaceouscan be firmly ruled out as future-relevant analogs.

What Are the Perspectives forUsing theGeologicalRecord to Project Global Change?Only rapid or pulsed CO2 release events canprovide direct future-relevant information. As-sessment of such events critically depends onindependent geochemical quantification of theassociated changes in the carbonate system, spe-cifically seawater-pH andCaCO3 saturation. Geo-chemical proxy estimates are not yet availablefor the Cretaceous and beyond and need to beobtained to verify whether ocean acidificationdid indeed happen. This is challenging, becausein addition to the potential for increasing post-depositional alteration and reduced stratigraphicexposure, uncertainty over the chemical and iso-topic composition of seawater increases and lim-its our interpretation of these proxies (63, 64).Future studies will have to improve and expandgeochemical estimates and their uncertainties ofsurface and deep-ocean carbonate chemistry as-sociated with carbonate dissolution and ecolog-ical changes. This includes finding new archivesto study the secular evolution of seawater chem-istry but also the laboratory study of living proxycarriers under conditionsmimicking past seawaterchemistry. An unfortunate aspect of the geolog-ical record, however, is the lack of deep-sea car-bonates in the Early Jurassic and beyond, whichfurther reduces our ability to reconstruct the car-bonate chemistry of those older events.

The sensitivity of ocean chemistry to CO2 re-lease, and the relationship between induced pHand PCO2 changes, vary through time and furthercomplicate the picture. For instance, seawatercalcium and magnesium ion concentrations weredifferent in the past (Fig. 4C). This alters the ocean’scarbonate ion buffering capacity and hence sen-sitivity of the Earth system to carbon perturbation(65) because all other things being equal, higherambient Ca2+ concentrations means that a lowercarbonate ion concentration is required to achievethe same saturation and hence balance weathering.Varying seawater Mg/Ca ratios may potentiallyalso affect the mineralogy of marine calcifiers,

where the more soluble high-Mg calcite predom-inated Neogene reefs and reefs during the Per-mian through Early Jurassic, and more resistantlow-Mg calcite predominated during the LateJurassic through Paleogene (66). Thus, on thismineralogical basis the response of marine cal-cifiers to ocean acidification and seawater geo-chemistry during the P/T and T/J would arguablybe closer to the modern than, for example, dur-ing the PETM (67). Improved estimates of pastseawater–Mg/Ca composition are necessary tobetter evaluate all of this.

Although we have concentrated on the pros-pects for extracting information from the geo-logical record concerning the impact of oceanacidification, we must question whether it reallyis necessary to isolate its effect on marine orga-nisms from other covarying factors (68). In par-ticular, consequences of increasing atmosphericCO2 will also be associated with warming in thesurface ocean and a decrease in dissolved oxy-gen concentration (69). Massive carbon release,whether future or past, will hence share the samecombination and sign of environmental changes.The strength of the geological record thereforelies in revealing past coupled warming and oceanacidification (and deoxygenation) events as an“integrated” analog, with future and past eventssharing the same combination and sign of en-vironmental changes. However, in additionallydriving a strong decline in calcium carbonate sat-uration alongside pH, the current rate of (mainlyfossil fuel) CO2 release stands out as capable ofdriving a combination and magnitude of oceangeochemical changes potentially unparalleledin at least the last ~300 My of Earth history,raising the possibility that we are entering anunknown territory of marine ecosystem change.

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Acknowledgments: Funding for the “Workshop on PaleoceanAcidification and Carbon Cycle Perturbation Events” wasprovided by NSF OCE 10-32374 and Past Global Changes(PAGES). We thank the workshop participants for stimulatingdiscussions and contributions to this manuscript, and the USCWrigley Institute on Catalina Island for hosting the workshop.Particular thanks are owed to Thorsten Kiefer of PAGES, whoinitiated the workshop and supported it at all stages. Thiswork is a contribution to the “European Project on OceanAcidification” (EPOCA). Data presented in Fig. 4 arepresented in tables S2 and S3 (2).

Supporting Online Materialwww.sciencemag.org/cgi/content/full/335/6072/1058/DC1SOM TextFigs. S1 to S3Tables S1 to S3References (74–217)

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www.sciencemag.org sCiEnCE erratum post date 16 marCh 2012

ErratumReview: “The geological record of ocean acidification” by B. Hönisch et al. (2 March, p. 1058). The affiliation for author Carles Pelejero was incomplete. The complete affiliation is: “Institució Catalana de Recerca i Estudis Avançats and Department of Marine Biology and Oceanography, Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, 08003 Barcelona, Catalonia, Spain.”

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