Surface Ocean pH Research Articles

Cloning and Characterization of Four Novel Coral Acid-Rich Proteins that Precipitate Carbonates In Vitro

Biomineralization is a widely dispersed and highly regulated but poorly understood process by which organisms precipitate minerals from a wide variety of elements [1]. For many years, it has been hypothesized that the biological precipitation of carbonates is catalyzed by and organized on an extracellular organic matrix containing a suite of proteins, lipids, and polysaccharides [2, 3]. The structures of these molecules, their evolutionary history, and the biophysical mechanisms responsible for calcification remain enigmatic. Despite the recognition that mineralized tissues contain proteins that are unusually rich in aspartic and glutamic acids [4-6], the role of these proteins in biomineralization remains elusive [5, 6]. Here we report, for the first time, the identification, cloning, amino acid sequence, and characterization of four highly acidic proteins, derived from expression of genes obtained from the common stony coral, Stylophora pistillata. Each of these four proteins can spontaneously catalyze the precipitation of calcium carbonate invitro. Our results demonstrate that coral acid-rich proteins (CARPs) not only bind Ca(2+) stoichiometrically but also precipitate aragonite invitro in seawater at pH 8.2 and 7.6, via an electrostatic interaction with protons on bicarbonate anions. Phylogenetic analysis suggests that at least one of the CARPs arose from a gene fusion. Similar, highly acidic proteins appear to have evolved several times independently in metazoans through convergence. Based purely on thermodynamic grounds, the predicted change in surface ocean pH in the next decades would appear to have minimal effect on the capacity of these acid-rich proteins to precipitate carbonates.

Impact of ocean acidification on metabolism and energetics during early life stages of the intertidal porcelain crab Petrolisthes cinctipes

Absorption of elevated atmospheric CO2 is causing surface ocean pH to decline, a process known as ocean acidification (OA). To date, few studies have assessed the physiological impacts of OA on early life-history stages of intertidal organisms, which transition from habitats with fluctuating pH (intertidal zone) to relatively stable (pelagic zone) pH environments. We used the intertidal crab Petrolisthes cinctipes to determine whether metabolic responses to year 2300 predictions for OA vary among early developmental stages and to examine whether the effects were more pronounced in larval stages developing in the open ocean. Oxygen consumption rate, total protein, dry mass, total lipids and C/N were determined in late-stage embryos, zoea I larvae and newly settled juveniles reared in ambient pH (7.93 ± 0.06) or low pH (7.58 ± 0.06). After short-term exposure to low pH, embryos displayed 11% and 6% lower metabolism and dry mass, respectively, which may have an associated bioenergetic cost of delayed development to hatching. However, metabolic responses appeared to vary among broods, suggesting significant parental effects among the offspring of six females, possibly a consequence of maternal state during egg deposition and genetic differences among broods. Larval and juvenile metabolism were not affected by acute exposure to elevated CO2. Larvae contained 7% less nitrogen and C/N was 6% higher in individuals reared at pH 7.58 for 6 days, representing a possible switch from lipid to protein metabolism under low pH; the metabolic switch appears to fully cover the energetic cost of responding to elevated CO2. Juvenile dry mass was unaffected after 33 days exposure to low pH seawater. Increased tolerance to low pH in zoea I larvae and juvenile stages may be a consequence of enhanced acid-base regulatory mechanisms, allowing greater compensation of extracellular pH changes and thus preventing decreases in metabolism after exposure to elevated PCO2. The observed variation in responses of P. cinctipes to decreased pH in the present study suggests the potential for this species to adapt to future declines in near-shore pH.

Genes related to ion-transport and energy production are upregulated in response to CO2-driven pH decrease in corals: new insights from transcriptome analysis.

Since the preindustrial era, the average surface ocean pH has declined by 0.1 pH units and is predicted to decline by an additional 0.3 units by the year 2100. Although subtle, this decreasing pH has profound effects on the seawater saturation state of carbonate minerals and is thus predicted to impact on calcifying organisms. Among these are the scleractinian corals, which are the main builders of tropical coral reefs. Several recent studies have evaluated the physiological impact of low pH, particularly in relation to coral growth and calcification. However, very few studies have focused on the impact of low pH at the global molecular level. In this context we investigated global transcriptomic modifications in a scleractinian coral (Pocillopora damicornis) exposed to pH 7.4 compared to pH 8.1during a 3-week period. The RNAseq approach shows that 16% of our transcriptome was affected by the treatment with 6% of upregulations and 10% of downregulations. A more detailed analysis suggests that the downregulations are less coordinated than the upregulations and allowed the identification of several biological functions of interest. In order to better understand the links between these functions and the pH, transcript abundance of 48 candidate genes was quantified by q-RT-PCR (corals exposed at pH 7.2 and 7.8 for 3 weeks). The combined results of these two approaches suggest that pH≥7.4 induces an upregulation of genes coding for proteins involved in calcium and carbonate transport, conversion of CO2 into HCO3 − and organic matrix that may sustain calcification. Concomitantly, genes coding for heterotrophic and autotrophic related proteins are upregulated. This can reflect that low pH may increase the coral energy requirements, leading to an increase of energetic metabolism with the mobilization of energy reserves. In addition, the uncoordinated downregulations measured can reflect a general trade-off mechanism that may enable energy reallocation.

Decoupled Response of Ocean Acidification to Variations in Climate Sensitivity

Abstract It is now well understood that the global surface ocean, whose pH has been reduced by ~0.1 in response to rising atmospheric CO2 since industrialization, will continue to become more acidic as fossil fuel CO2 emissions escalate. However, it is unclear how uncertainties in climate sensitivity to future CO2 emissions will alter the manifestation of ocean acidification. Using an earth system model of intermediate complexity, this study performs a set of simulations that varies equilibrium climate sensitivity by 1.0°–4.5°C for a given CO2 emissions scenario and finds two unexpected and decoupled responses. First, the greater the climate sensitivity, the larger the surface mixed layer acidification signal but the smaller the subsurface acidification. However, taken throughout the ocean, the highest climate sensitivity will paradoxically cause greater global warming while buffering whole-ocean pH by up to 24% on centennial time scales. Second, this study finds a large decoupling between pH and carbonate ion concentration in surface waters whereby these chemical properties show opposite effects under variable climate sensitivity. For every 1°C increase in climate sensitivity, the surface ocean pH reduction grows by 4%, while surface ocean carbonate ion reduction shrinks by 2%. The chemical and spatial decoupling found here highlights the importance of distinguishing the biological impacts of pH and aragonite saturation and understanding the spatial extent of important calcifying biomes so as to truly understand the long-term impacts of ocean acidification.

Geoengineering impact of open ocean dissolution of olivine on atmospheric CO2, surface ocean pH and marine biology

Ongoing global warming induced by anthropogenic emissions has opened the debate as to whether geoengineering is a ‘quick fix’ option. Here we analyse the intended and unintended effects of one specific geoengineering approach, which is enhanced weathering via the open ocean dissolution of the silicate-containing mineral olivine. This approach would not only reduce atmospheric CO2 and oppose surface ocean acidification, but would also impact on marine biology. If dissolved in the surface ocean, olivine sequesters 0.28 g carbon per g of olivine dissolved, similar to land-based enhanced weathering. Silicic acid input, a byproduct of the olivine dissolution, alters marine biology because silicate is in certain areas the limiting nutrient for diatoms. As a consequence, our model predicts a shift in phytoplankton species composition towards diatoms, altering the biological carbon pumps. Enhanced olivine dissolution, both on land and in the ocean, therefore needs to be considered as ocean fertilization. From dissolution kinetics we calculate that only olivine particles with a grain size of the order of 1 μm sink slowly enough to enable a nearly complete dissolution. The energy consumption for grinding to this small size might reduce the carbon sequestration efficiency by ∼30%.

The effect of carbonic anhydrase on the kinetics and equilibrium of the oxygen isotope exchange in the CO2–H2O system: Implications for δ18O vital effects in biogenic carbonates

Interpretations of the primary paleoceanographic information recorded in stable oxygen isotope values (δ18O) of biogenic CaCO3 can be obscured by disequilibrium effects. CaCO3 is often depleted in 18O relative to the δ18O values expected for precipitation in thermodynamic equilibrium with ambient seawater as a result of vital effects. Vital effects in δ18O have been explained in terms of the influence of fluid pH on the overall δ18O of the sum of dissolved inorganic carbon (DIC) species (often referred to as “pH model”) and in terms of 18O depletion as a result of the kinetic effects associated with CO2 hydration (CO2+H2O↔H2CO3↔HCO3−+H+) and CO2 hydroxylation (CO2+OH−↔HCO3−) in the calcification sites (so-called “kinetic model”). This study addresses the potential role of an enzyme, carbonic anhydrase (CA), that catalyzes inter-conversion of CO2 and HCO3− in relation to the underlying mechanism of vital effects. We performed quantitative inorganic carbonate precipitation experiments in order to examine the changes in 18O equilibration rate as a function of CA concentration. Experiments were performed at pH 8.3 and 8.9. These pH values are comparable to the average surface ocean pH and elevated pH levels observed in the calcification sites of some coral and foraminiferal species, respectively. The rate of uncatalyzed 18O exchange in the CO2–H2O system is governed by the pH-dependent DIC speciation and the kinetic rate constant for CO2 hydration and hydroxylation, which can be summarized by a simple mathematical expression. The results from control experiments (no CA addition) are in agreement with this expression. The results from control experiments also suggest that the most recently published kinetic rate constant for CO2 hydroxylation has been overestimated. When CA is present, the 18O equilibration process is greatly enhanced at both pH levels due to the catalysis of CO2 hydration by the enzyme. For example, the time required for 18O equilibrium is nearly halved by the presence of 3.7×10−9M of CA used for the experiments. Despite its significant influence on the oxygen isotope exchange kinetics, the equilibrium oxygen isotope fractionation between individual DIC species and H2O is unaffected by CA. Because many CaCO3-secreting organisms possess active CA, our findings imply that 18O equilibration of the CO2–H2O system is possible within realistic timescales of biogenic calcification.

The planktic foraminiferal B/Ca proxy for seawater carbonate chemistry: A critical evaluation

The ratio of boron to calcium (B/Ca) in the carbonate tests of planktic foraminifers has been proposed as a proxy for surface ocean pH or carbonate ion concentration ([CO32−]), with a possible secondary influence of temperature. In both cultured and wild specimens, B/Ca generally increases with [B(OH)4−/HCO3−], consistent with the proxy's theoretical basis. However, close examination of the available data reveals that calibrations using the empirical boron partition coefficient, KD=[B/Ca]calcite/[B(OH)4−/HCO3−]seawater, can be driven by independent relationships between [B(OH)4−/HCO3−] and other environmental parameters, not by B/Ca itself. If the influence of B/Ca on a calibration is negligible, it follows that combining that calibration with down-core B/Ca can not yield new information about past ocean conditions. In this study, we evaluate existing calibrations and down-core records with the aim of establishing a framework that allows consistent calibration methods to yield accurate proxy reconstructions from the fossil record. While many issues still need to be addressed, B/Ca does respond to seawater chemistry, and does change across some major climate transitions. Because it may provide insight into major carbon cycle perturbations, we make specific recommendations on how to tackle current uncertainties and develop B/Ca into a more robust proxy.

Is the climate response to CO2 emissions path dependent?

Recent studies with coupled climate‐carbon cycle models suggest that global mean temperature change is proportional to cumulative CO2 emissions, independent of the timing of those emissions. This finding has prompted the suggestion that climate stabilization targets, such as the 2°C target adopted by the Copenhagen Accord, can be expressed in terms of cumulative CO2 emissions. Here we examine the simulated response of a range of global and regional climate variables to the same cumulative CO2 emissions (2500 PgC) released along different pathways using a complex Earth system model. We find that the response of most surface climate variables is largely independent of the emissions pathway once emissions cease, with the exception of variables with response timescales of centuries, such as ocean heat content and thermosteric sea level rise. Peak responses of many climate variables, such as global mean temperature, precipitation and sea ice, are also largely independent of the emissions pathway, except for scenarios with cumulative emissions overshoot which require net removal of CO2 from the atmosphere. By contrast, peak responses of atmospheric CO2 and surface ocean pH are found to be dependent on the emissions pathway. We conclude that a CO2mitigation framework based on cumulative emissions is well suited for limiting changes in many impact‐relevant climate variables, but is less effective in avoiding impacts directly associated with atmospheric CO2, whose peak response is dependent on the rate of emissions.

Coral and mollusc resistance to ocean acidification adversely affected by warming

Increasing atmospheric carbon dioxide (CO2) concentrations are expected to decrease surface ocean pH by 0.3‐0.5 units by 2100 (refs 1,2), lowering the carbonate ion concentration of surface waters. This rapid acidification is predicted to dramatically decrease calcification in many marine organisms 3,4 . Reduced skeletal growth under increased CO2 levels has already been shown for corals, molluscs and many other marine organisms 4‐9 . The impact of acidification on the ability of individual species to calcify has remained elusive, however, as measuring net calcification fails to disentangle the relative contributions of gross calcification and dissolution rates on growth. Here, we show that corals and molluscs transplanted along gradients of carbonate saturation state at Mediterranean CO2 vents are able to calcify and grow at even faster than normal rates when exposed to the high CO2 levels projected for the next 300 years. Calcifiers remain at risk, however, owing to the dissolution of exposed shells and skeletons that occurs as pH levels fall. Our results show that tissues and external organic layers play a major role in protecting shells and skeletons from corrosive sea water, limiting dissolution and allowing organisms to calcify 10,11 . Our combined field and

Impacts of anthropogenic SOx, NOxand NH3on acidification of coastal waters and shipping lanes

[1] The acidification of the ocean by anthropogenic CO2 absorbed from the atmosphere is now well-recognized and is considered to have lowered surface ocean pH by 0.1 since the mid-18th century. Future acidification may lead to undersaturation of CaCO3 making growth of calcifying organisms difficult. However, other anthropogenic gases also have the potential to alter ocean pH and CO2 chemistry, specifically SOx and NOx and NH3. We demonstrate using a simple chemical model that in coastal water regions with high atmospheric inputs of these gases, their pH reduction is almost completely canceled out by buffering reactions involving seawater HCO3− and CO32− ions. However, a consequence of this buffering is a significant decrease in the uptake of anthropogenic CO2 by the atmosphere in these areas.

Cellular pH measurements in Emiliania huxleyi reveal pronounced membrane proton permeability

• To understand the influence of changing surface ocean pH and carbonate chemistry on the coccolithophore Emiliania huxleyi, it is necessary to characterize mechanisms involved in pH homeostasis and ion transport. • Here, we measured effects of changes in seawater carbonate chemistry on the fluorescence emission ratio of BCECF (2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) as a measure of intracellular pH (pH(i)). Out of equilibrium solutions were used to differentiate between membrane permeation pathways for H(+), CO(2) and HCO(3)(-). • Changes in fluorescence ratio were calibrated in single cells, resulting in a ratio change of 0.78 per pH(i) unit. pH(i) acutely followed the pH of seawater (pH(e)) in a linear fashion between pH(e) values of 6.5 and 9 with a slope of 0.44 per pH(e) unit. pH(i) was nearly insensitive to changes in seawater CO(2) at constant pH(e) and HCO(3)(-). An increase in extracellular HCO(3)(-) resulted in a slight intracellular acidification. In the presence of DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid), a broad-spectrum inhibitor of anion exchangers, E. huxleyi acidified irreversibly. DIDS slightly reduced the effect of pH(e) on pH(i). • The data for the first time show the occurrence of a proton permeation pathway in E. huxleyi plasma membrane. pH(i) homeostasis involves a DIDS-sensitive mechanism.

Effects of lower surface ocean pH upon the stability of shallow water carbonate sediments

It is predicted that surface ocean pH will reach 7.9, possibly 7.8 by the end of this century due to increased carbon dioxide (CO 2) in the atmosphere and in the surface ocean. While aragonite-rich sediments don't begin to dissolve until a threshold pH of ~ 7.8 is reached, dissolution from high-Mg calcites is evident with any drop in pH. Indeed, it is high-Mg calcite that dominates the reaction of carbonate sediments with increased CO 2, which undergoes a rapid neomorphism process to a more stable, low-Mg calcite. This has major implications for the future of the high-Mg calcite producing organisms within coral reef ecosystems. In order to understand any potential buffering system offered by the dissolution of carbonate sediments under a lower oceanic pH, this process of high-Mg calcite dissolution in the reef environment must be further elucidated.

Predicted impacts of climate change on New Zealand’s biodiversity.

In New Zealand, climate change impacts have already been observed, and will increase in future decades. Average air temperature is predicted to warm by 2.1°C by 2090 for a mid-range IPCC scenario (A1B), with larger increases possible for some IPCC scenarios with higher rates of future emissions. Sea-level rise projections range between 0.18 – 0.59 m by 2100, based on six IPCC future emission scenarios excluding future rapid dynamical changes in polar ice-sheet flow. Global surface ocean pH is predicted to decrease by an additional 0.14 – 0.35 units by 2100, with a similar decrease expected in New Zealand waters. Rainfall is predicted to change significantly, with increased precipitation in the west, and reduced precipitation in the east, and more intense rainfall events. Increasing temperature is likely to result in species’ range shifts southward and upward, and mortality during extreme heat events. Ocean acidification is expected to cause declines in carbonate communities, with cold water communities predicted to decline first due to a lower aragonite saturation horizon in cold waters. Sea-level rise is likely to impact on coastal biota, reducing coastal habitats, changing inundation patterns, and increasing vulnerability to storm surges and tides. Changes in storm and rainfall intensity are predicted to increase disturbance to terrestrial and aquatic communities. Areas with increased precipitation will amplify rates of disturbance, erosion and sedimentation into aquatic, estuarine and coastal ecosystems, while areas with low precipitation will experience increased fire risk. In New Zealand, climate change projections are being integrated into management, including increasing protection and improving management of coastal habitats. Contributing to a global reduction in greenhouse gas emissions, New Zealand is the first country to include forestry in their Emissions Trading Scheme, already positively affecting biodiversity by reducing deforestation.

Shallow-water carbonate record of the Paleocene-Eocene Thermal Maximum from a Pacific Ocean guyot

The Paleocene-Eocene Thermal Maximum (PETM; ca. 55.8 Ma) is associated with a rapid and large carbon cycle perturbation, transient warming, and deep-sea acidification. Although this event is widely known from pelagic, hemipelagic, and continental records, the lack of in situ, shallow-water carbonate platform sections inhibits interpretations of whether the PETM had a significant effect on shallow-water carbonate ecosystems. Limalok Guyot (Ocean Drilling Program Site 871) in the Pacific Ocean comprises a volcanic pedestal topped by a Paleogene carbonate platform that drowned in the Middle Eocene. Carbon isotope stratigraphy of the platform carbonate sediments is used, in conjunction with existing biostratigraphy, to refine the stratigraphic framework of the carbonate platform. Although core recovery was poor, the major Late Paleocene–Middle Eocene stratigraphic trends in carbon isotopes are recognized, including a prominent ∼3‰ negative carbon isotope excursion, recording the onset of the PETM. The lithological and paleontological record of the PETM on Limalok Guyot shows no major evidence for a carbonate production crisis, suggesting that the effects of any changes in temperatures or surface ocean pH were relatively short lived or relatively minor.

Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific

Using multiple lines of evidence, we demonstrate that volcanic ash deposition in August 2008 initiated one of the largest phytoplankton blooms observed in the subarctic North Pacific. Unusually widespread transport from a volcanic eruption in the Aleutian Islands, Alaska deposited ash over much of the subarctic NE Pacific, followed by large increases in satellite chlorophyll. Surface ocean pCO2, pH, and fluorescence reveal that the bloom started a few days after ashfall. Ship‐based measurements showed increased dominance by diatoms. This evidence points toward fertilization of this normally iron‐limited region by ash, a relatively new mechanism proposed for iron supply to the ocean. The observations do not support other possible mechanisms. Extrapolation of the pCO2 data to the area of the bloom suggests a modest ∼0.01 Pg carbon export from this event, implying that even large‐scale iron fertilization at an optimum time of year is not very efficient at sequestering atmospheric CO2.

Ocean chemistry and atmospheric CO2sensitivity to carbon perturbations throughout the Cenozoic

We assess the response of atmospheric CO2(pCO2) and ocean chemistry to carbon perturbations, placing modern carbon‐cycle sensitivity in the context of the Cenozoic. We use the carbon‐cycle model LOSCAR to study the effect of perturbations over the past 67 Ma. Our results indicate that atmospheric CO2and surface ocean pH were slightly more sensitive to carbon perturbations during the Miocene (∼15 Ma ago) compared to modern conditions and less sensitive during the earlier part of the Cenozoic. We find that the sensitivity to carbon input at the Paleocene‐Eocene boundary (∼56 Ma ago) was most likely lower compared to modern conditions. Further, we show that the Cenozoic evolution of steady statepCO2and seawater Mg/Ca ratios is critical for the system's sensitivity to a carbon release.

Can ocean iron fertilization mitigate ocean acidification?

Ocean iron fertilization has been proposed as a method to mitigate an- thropogenic climate change, and there is continued commercial interest in using iron fertilization to generate carbon credits. It has been further speculated that ocean iron fertilization could help mitigate ocean acidification. Here, using a global ocean car- bon cycle model, we performed idealized ocean iron fertilization simulations to place an upper bound on the effect of iron fertilization on atmospheric CO2 and ocean acidification. Under the IPCC A2 CO2 emission scenario, at year 2100 the model simulates an atmospheric CO2 concentration of 965 ppm with the mean surface ocean pH 0.44 units less than its pre-industrial value of 8.18. A globally sustained ocean iron fertilization could not diminish CO2 concentrations below 833 ppm or reduce the mean surface ocean pH change to less than 0.38 units. This maximum of 0.06 unit mitigation in surface pH change by the end of this century is achieved at the cost of storing more anthropogenic CO2 in the ocean interior, furthering acidifying the deep- ocean. If the amount of net carbon storage in the deep ocean by iron fertilization produces an equivalent amount of emission credits, ocean iron fertilization further acidifies the deep ocean without conferring any chemical benefit to the surface ocean.

Early Larval Development of the Sydney Rock OysterSaccostrea glomerataUnder Near-Future Predictions of CO2-Driven Ocean Acidification

Anthropogenic emissions of carbon dioxide (CO2) from fossil fuel combustion and deforestation are rapidly increasing the atmospheric concentration of CO2 and reducing the pH of the oceans. This study shows that predicted near-future levels of ocean acidification have significant negative effects on early larval development of the Sydney rock oyster Saccostrea glomerata (Gould, 1850). CO2 was added to seawater to produce pH levels set at 8.1 (control), 7.8, and 7.6 (actual pH values were 8.11, 7.81, and 7.64, respectively). These treatments represent present-day surface ocean pH, as well as upper (Δ pH ≈ -0.3) and lower (Δ pH ≈ -0.5) pH predictions for the surface oceans in 2100. With decreasing pH, survival of S. glomerata larvae decreased, and growth and development were retarded. Larval survival decreased by 43% at pH 7.8 and by 72% at pH 7.6. Antero-posterior measurement (APM) was reduced by 6.3% at pH 7.8 and 8.7% at pH 7.6, and dorso-ventral measurement (DVM) was reduced by 5.1% atpH 7.8 and 7.5% at pH 7.6. The percentage of empty shells remaining from dead larvae decreased by 16% atpH 7.8 and by 90% at pH 7.6 indicating that the majority of empty shells dissolved within 7 days at pH 7.6. Scanning electron microscope images of 8-day-old larvae show abnormalities on the shell surface at low pH suggesting (1) problems with shell deposition, (2) retarded periostracum formation, and/or (3) increased shell dissolution. Larval life-history stages are considered particularly susceptible to climate change, and this study shows that S. glomerata larvae are sensitive to a high-CO2 world and are, specifically, negatively affected by exposure to pH conditions predicted for the world's oceans for the year 2100.

Sensitivity of ocean acidification to geoengineered climate stabilization

Climate engineering has been proposed as a possible response to anthropogenic climate change. While climate engineering may be able to stabilize temperatures, it is generally assumed that this will not prevent continued ocean acidification. However, due to the strong coupling between climate and the carbon cycle, climate engineering could indirectly affect ocean chemistry. We used a global Earth‐system model to investigate how climate engineering may affect surface ocean pH and the degree of aragonite saturation. Climate engineering could significantly re‐distribute carbon emissions among atmosphere, land and ocean reservoirs. This could slow pH decreases somewhat relative to the non‐engineered case, but would not affect the level of aragonite saturation due to opposing responses of pH and aragonite saturation to temperature change. However, these effects are dependent on enhanced carbon accumulation in the land biosphere; without this, climate engineering has little effect on pH, and leads to accelerated declines in aragonite saturation.

Modeled reconstructions of the oceanic carbonate system for different histories of atmospheric carbon dioxide during the last 20 Ma

A box model was used to estimate oceanic carbonate system parameters (total dissolved inorganic carbon, alkalinity, CO2(aq), CO32−, HCO3−, and pH) in the surface and deep ocean over the past 20 Ma. Geological data were used to constrain boundary conditions, including the carbonate compensation depth (CCD) and seawater temperature, salinity, and ionic composition. In addition, ∼14,000 δ13C measurements of planktic and benthic foraminifera were compiled. Sensitivity tests were performed to evaluate potential errors arising from uncertainties in model boundary conditions. Values of surface water pH were modeled assuming four different histories of atmospheric CO2, and compared with proxy‐based estimates of pH. The system is underconstrained, and therefore multiple CO2 histories can be invoked to explain the same pH trend, depending on the assumptions made regarding ocean overturning and biological productivity. Despite this uncertainty, we conclude that if CO2 was strongly coupled to temperature and ice volume over the last 20 Ma, predicted values of surface ocean pH fit poorly with current proxy‐based pH reconstructions.