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Measuring Ocean Acidification: New Technology for a New Era of Ocean Chemistry Robert H. Byrne* College of Marine Science, University of South Florida, St. Petersburg, Florida 33701, United States (H+), bicarbonate (HCO3−), and carbonate (CO32−) ions. Most of the H+ goes into protonation of borate and carbonate ions. The net result is increasing H+, minor increases in HCO3−, and substantially declining CO32− (Figures 1 and 2). At pH 8.0 (typical of surface seawater), >99% of the CO2 molecules added to the ocean undergo reactions that produce H +. The increase in H+ equates, by definition, to an increase in seawater acidity. A sustained increase is termed “ocean acidification”.10,15 Seawater-carbonation16 is the principal cause of global ocean acidification, analogous to the carbonation and resulting acidification of bottled water. Local short-term variations in seawater acidity, resulting from Human additions of carbon dioxide to the atmosphere are photosynthesis, respiration, and other processes (e.g., refs creating a cascade of chemical consequences that will eventually 17−21) are superimposed on the long-term trend (Figure 1). extend to the bottom of all the world’s oceans. Among the bestSince the beginning of the industrial era, global average surfacedocumented seawater effects are a worldwide increase in openocean acidity has increased ∼26%10 and CO32− has decreased ocean acidity and large-scale declines in calcium carbonate ∼16%.22 saturation states. The susceptibility of some young, fast-growing Ocean carbonation and acidification are likely to benefit calcareous organisms to adverse impacts highlights the potential some organisms (e.g., seagrasses) while adversely affecting for biological and economic consequences. Many important aspects of seawater CO2 chemistry can be only indirectly others (e.g., oyster larvae, pteropods).11,23−26 The larval stages observed at present, and important but difficult-to-observe of some economically important calcareous organisms (young changes can include shifts in the speciation and possibly and fast-growing) seem to be particularly susceptible to adverse bioavailability of some life-essential elements. Innovation and impacts.25,27 Biological effects may be due to any of a number invention are urgently needed to develop the in situ of the chemical changes that accompany ocean carbonation instrumentation required to document this era of rapid ocean for example, increasing seawater CO2 or increasing acidity or evolution. decreasing CO32−. The various mechanisms of biological impact are still being worked out.28,29 Ecological, economic, INTRODUCTION and cultural impacts seem likely.25,26 The carbon dioxide (CO2) content of the atmosphere has Ocean alkalinity buffers the effects of oceanic CO2 uptake, increased by ∼40% over the past few hundred years,1,2 due but slowing and eventually reversing the trend of increasing largely to burning of fossil fuels.3 CO2 is an unusually soluble ocean acidity will require increased continental weathering and gas in aqueous solution relative to most other atmospheric dissolution of ocean carbonate sediments.30,31 These processes gases (e.g., nitrogen, oxygen, argon), and it freely exchanges occur over much longer time scales (centuries to millennia) across the air−sea interface. than the gas-exchange and solution-phase processes associated The increase in atmospheric carbon dioxide has resulted in a with CO2 uptake (microseconds to months). net flux of CO2 into the oceanspresently about 9 billion Not so very long ago, during a time remembered by many 4 tonnes per year. This influx changes ocean chemistry in ways present-day oceanographers, studies of ocean chemistry that can have significant ecological consequences, including included questions as basic as “What is the elemental possible shifts in marine biodiversity and ecosystem function 5−13 composition of seawater?” Today’s marine scientists face the According to a recent assessment of the and services. 14 challenge of assessing rapid and widespread chemical changes geological record, the current rapid rate of atmospheric through time. Such needs cannot be fully met by existing change is capable of creating a diversity and intensity of oceanic technology. This article (1) reviews present-day CO2-system geochemical changes “potentially unparalleled in at least the last capabilities and considerations in designing field research ∼300 million years of Earth history”. programs and (2) highlights opportunities for refining CO2 Sustained uptake of CO2 by the oceans results in a sustained system models and advancing high-resolution in situ measuredecline of seawater pH (Figure 1). Carbon dioxide entering the ments of ocean acidification and its chemical consequences. ocean becomes dissolved as aqueous carbon dioxide, CO2(aq). This dissolved CO2 reacts with water to form carbonic acid (H2CO3), which subsequently dissociates, liberating hydrogen Published: April 7, 2014
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Figure 1. CO2 system changes in the upper ocean (wind-mixed layer) at stations in the Pacific (HOT) and Atlantic (BATS) oceans: the carbonate:bicarbonate concentration ratio and pH. Among the many ions in seawater, the two that are most important in controlling seawater pH are CO32− and HCO3−. The declining concentration ratio shown here, in conjunction with similar data at diverse locations around the world, indicates that major global change is occurring in the equilibrium status of chemical reactions in the oceans. The increasing departure of the carbonate:bicarbonate concentration ratio from a value of 1 (i.e., the point of maximum buffer intensity) shows that the ability of the oceans’ surface layer to buffer pH excursions (including those attributable to local processes such as photosynthesis and respiration) is declining. The concentration ratios shown here were calculated from measurements of pH, S, and T. The black line shows the least-squares best-fit slope: −4.444 (±0.383) × 10−4 yr−1.
imply changes in not only the oceanic CO2 system (Figures 1 and 2) but also many other chemicals, their interactions with each other, and their availability to marine organisms.34−36 Shifting acid−base equilibria can also change the chemistry and perhaps the sinking behavior of some ocean particles.37 In other words, because the carbonate system controls the acidity of seawater and because so many types of chemical reactions are H+-dependent, ocean carbonation and acidification have a great many avenues by which to alter the distributions and biogeochemical cycling of a wide range of seawater constituents. Many chemicals dissolved in seawater are partitioned into different forms (“species”; e.g., Figures 2 and 3). When conditions shift toward higher acidities (i.e., lower pH values), the relative abundances of the positively charged species increase while the negative species decrease. A particularly important aspect of this phenomenon38 is the declining concentration of CO32− (Figures 1 and 2). The speciation of many trace elements, some toxic and some essential to marine life, similarly vary with pH. Iron (Figure 3) and copper are examples. The total concentration of each of these trace metals is very low (approximately one million times lower than the concentration of HCO3−), but even small changes in their chemistries (e.g., their species distributions or their complexation with other constituents, including organics) may imply significant changes in their bioavailability. Iron, for example, has a solution chemistry that is complex and highly pH dependent (Figure 3). This element is essential
Figure 2. Modeled concentrations of carbon species in surface seawater (TA = 2200 μmol kg−1) equilibrated with atmospheric pCO2. The speciation of many other chemicals in seawater likewise varies with seawater pH and therefore atmospheric CO2. The “AD” numbers and arrows along the x-axis indicate the years when those atmospheric pCO2 values were realized or are projected to occur. The “RCP” labels indicate the IPCC10 representative concentration pathways that were used for the year-2100 projections.
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SHIFTING ACID−BASE EQUILIBRIA A large number of seawater constituents, dissolved and particulate, participate in reactions involving hydrogen-ion exchange.32,33 Ocean carbonation and acidification thereby 5353
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shallower depths. In light of what we understand so far, it seems reasonable to speculate that ocean acidification could reduce the vertical fluxes of many elements, especially to the deep ocean.
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WATCHING THE OCEAN EVOLVE CO2 System Measurements. Chemists generally investigate the marine inorganic CO2 system by measuring four fundamental parameters (keystone variables): • DIC, the total concentration of all dissolved forms of inorganic carbon (i.e., [CO2] + [HCO3−] + [CO32−]), provides a direct assessment of the dissolved inorganic component of the ocean’s growing carbon inventory (shipboard precision ∼ ± 1−2 μmol kg−1) • TA, total alkalinity, expressed56,57 in terms of the ability of seawater to neutralize strong acids; reflects the presence of not only carbon species but also borate, silicate, phosphate, and organic ions (shipboard precision ∼ ±2−4 μmol kg−1) • pH, the negative log of the total hydrogen ion concentration, provides a direct assessment of ocean acidity (shipboard precision ∼ ±0.0004−0.001)58 • f CO2, carbon dioxide fugacity or “escaping tendency”59, provides a direct assessment of the propensity for surface seawater to export or import CO2 to or from the atmosphere; directly proportional to CO2(aq) (shipboard precision ∼ ±0.1−0.2%) Relationships among the CO2 system variables can be quantitatively stated using an equilibrium computational modele.g.,60 by which any two input variables can be used to calculate all others. It can be helpful to think of pH as a master descriptive variable and the DIC:TA ratio as a master controlling variable. Even small changes in the relative proportions of DIC (increasing under ocean carbonation) and TA (not directly affected by ocean carbonation) create large increases in f CO2 and H+ and large decreases in CO32−. These amplifications, which are associated with the Revelle factor (i.e., buffer factor),18 can profoundly influence ocean biogeochemical cycling. Total alkalinity is the most challenging of the four keystone measurements. Because shipboard titrations require volumetric metering of a strong acid, shipboard TA precision is typically significantly worse than the precision of onshore TA measurements. In addition, accurate interpretation of TA measurements requires that all equilibria involving H+ exchangeboth inorganic and organicbe considered. The use of TA in the computational models therefore requires either ancillary measurements or assumptionsthat is, the noncarbonate inorganic alkalinities (e.g., phosphate and silicate) and the organic alkalinity61−63 must be either measured or approximated. Organic alkalinity is often assumed negligible. In cases where it is non-negligible, the H+-exchange characteristics of the participating organic bases must be quantitatively characterized. This analytical undertaking is formidable and not yet routine. To the extent that particles participate in H+ exchange, acid titrations may in principle yield a different TA for some filtered versus unfiltered samples (e.g., nearshore particle-rich waters). If calcium carbonate saturation state (a measure of the capability of seawater to dissolve CaCO3) were directly measurable, this parameter (Ω) would likely be regarded as a
Figure 3. Relative abundances of inorganic iron (Fe) species in seawater (expressed as a fraction of total iron) as a function of pH. Because pH is expressed on a logarithmic scale, small differences in pH indicate large differences in acidity. Within the current normal pH range of seawater (shown by the light shading), the fraction of iron present as Fe3+ changes by a factor of 100 or more for a one-unit change in pH. The contributions of organic complexes can dominate over inorganic forms but are not shown on this species-composition diagram because the pH-dependent behaviors of organic complexes in seawater are poorly understood.
to marine phytoplankton to such an extent that continent-sized areas of the open ocean are infertile due to insufficient iron availability.39,40 Dissolved iron concentrations in seawater are influenced by many factors, including wind-driven dust inputs,41 solubility reactions (pH dependent),42,43 oxidation− reduction reactions (pH dependent),44 and complexation by strong organic ligands45,46 (likely pH dependent). Models of the inorganic chemistry of iron33 would suggest that ocean acidification may increase the solubility and availability of iron to microorganisms. In the ocean, however, iron exists as a complex mixture of inorganic and organic species, and it can be adsorbed onto organic and mineral colloids of biogenic or geochemical origin. Laboratory investigations of iron-limited phytoplankton indicate that decreasing seawater pH can actually lower iron bioavailability.47 Another important class of H+-exchange reactions involves trace-metal interactions with particles (i.e., adsorption and desorption reactions).48−52 In the ocean, dissolved constituents in surface waters may be adsorbed onto biological particles that later sink to the deep ocean.48,53,54 This process of particle “scavenging”the result of chemical sorption (pH dependent) and physical sinking (possibly pH dependent)thereby exports many chemicals from the upper ocean to the deep ocean, leaving a strong imprint on their vertical distributions (e.g., ref 54). Organic-rich particles that include mineral components such as calcium carbonate (CaCO3), a common shell material, sink more quickly than those without. With increasing seawater acidity, the affinity of particle surfaces for dissolved constituents generally decreases.49−52 In addition, at sufficiently high levels of seawater acidity, CaCO3 is prone to dissolution. The overall effect of increasing ocean acidity on mineral-enhanced sinking, though hard to predict,55 could include particle dissolution and decomposition at 5354
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keystone variable as well. Lower values of Ω (which accompany lower [CO32−])10 provide yet another avenue by which ocean carbonation and acidification may impact marine organisms11,23,24 and ecosystems.10 The Value of Overdetermination. Only two keystone parameters are required for comprehensive CO2 system calculations, but “overdetermination” (direct measurement of three or more parameters) has long been used to assess procedural and other types of errors (e.g., refs 64−66). Small but systematic differences between measured and calculated quantities indicate that improvements are needed, either in sample processing and measurement or in the computational framework (equilibrium model). Substantial improvements in measurement quality have been achieved in recent decades. One reason is that certified reference materials (CRMs), essential for evaluating analytical accuracy and preventing systematic methodological errors, are now routinely used for quality control in measurements of DIC and TA.67 Another reason is that purification of the indicator dyes68,69 used for spectrophotometric pH analyses is becoming standard protocol. Still, routine internal consistency (i.e., an absence of systematic deviations between measured and calculated quantities) remains elusive (e.g., ref 70). One factor may be the challenge of measuring TA at sea. Another reason may be the mismatch between measurements of total alkalinity (inorganic + organic) and model characterizations of alkalinity (typically inorganic only). For most marine waters, the assumption of negligible organic alkalinity is appropriate, but where it is not (e.g., estuarine and coastal waters61−63), TAbased calculations are subject to significant systematic errors. Overdetermination is therefore still valuable for assessing data quality within a single data set and for determining whether measurement characteristics are changing through time (e.g., by comparing the consistency characteristics of data sets collected years or decades apart). Assessing internal consistency is especially important because it imposes essential quality control on the calculation of some key parameters that are critical to ocean health (e.g., [CO32−] and Ω). These ocean characteristics are rapidly changing but cannot yet be directly measured. Prioritizing Measurements. If operational constraints (e.g., ship space, manpower) do not allow for measurement of all four CO2 system parameters, then a subset must be selected. The choice will depend in part on the primary purpose of the sampling program. In all cases, measuring DIC is highly advisible. Standard coulometric DIC measurements67 are generally precise, reliable, and accurate. If only two types of measurements are possible, prompt shipboard analysis of either f CO2 or pH should be the second choice. Measurements of the f CO2 of discrete seawater samples is somewhat more demanding than measurements of spectrophotometric pH, and f CO2 sample throughput is somewhat slower. State-of-the-art measurements of the DIC− pH pair or the DIC− f CO2 pair can provide calculated TA values at a precision superior to direct TA measurements.65 If three types of measurements are possible, the third selected parameter should be TA. Differences between measured TA and calculated TA allow insights into the local significance of organic alkalinity. Only when all four measurements are possible should f CO2 and pH both be included in the measurement suite. This duo is generally not the best choice as an input pair for calculating carbonate concentrations.71
The Challenge of In Situ Calculations. Marine chemical systems and biota are experiencing ocean carbonation and acidification under in situ conditions, but in situ measurements are not yet routine. Many measurements are still made by retrieving seawater samples from depth and then analyzing those samples in a laboratory, either on ships or on land. Care must therefore be taken when using off-the-shelf computational models to obtain in situ characterizations from laboratory and shipboard measurements: · DIC Because sample DIC does not change when the temperature (T) or pressure (P) of the sample changes, the laboratory DIC value is the same as the in situ DIC value · TA Because sample TA does not change when the T or P of the sample changes, the laboratory TA value is the same as the in situ TA value · pH Because reported pH values are specific to the T and P conditions under which the measurements were made (e.g., 25 °C), these values cannot be used directly as input for off-the-shelf model calculations of in situ conditions · f CO2 Because reported f CO2 values are specific to the T and P conditions under which the measurements were made (e.g., 20 °C), these values cannot be used directly as input for off-the-shelf model calculations of in situ conditions Without in situ measurements, the calculation of CO2 system variables (e.g., Ω) at in situ conditions therefore requires that the DIC−TA pair (either directly measured or calculated) be provided as input to the computational model. This point is subtle but important. DIC is typically directly measured. TA may be either directly measured or calculatedi.e., from measured DIC−pH or DIC−f CO2 pairs. Both calculations are subject to error unless all alkalinity equilibria (including organic alkalinity) are fully characterized with respect to variations in S, T, and P. Measuring either pH in situ or f CO2 in situ to pair with measurements of DIC would provide the most straightforward route to the direct calculation of other in situ quantities. For example, in situ [CO32−] can be calculated from DIC and in situ pH without having to account for organic alkalinity, borate alkalinity, and other noncarbonate alkalinity contributions. The Promise of In Situ Measurements. The oceans are vast, and research expeditions are expensive. Improving the spatial and temporal resolution of ocean sampling programs will require ongoing development of underwater platforms and sensors. For shipboard sampling, sensors can be mounted on the rosettes used to collect seawater samples. Real-time profiles of S, T, P, and oxygen, with vertical resolutions on the order of 1 m are typically collected this way. Vertical profiles of pH72 have been similarly obtained but are not routine. Other in-water platforms include moorings and drifting surface buoys,73−75 autonomous underwater vehicles and gliders,76 “crawlers” that traverse mooring lines,77 autonomous profiling floats that yo-yo through the water column,78,79 and packages or chambers parked on the sea bottom.80 Each platform imposes different constraints on instrument design and performance in terms of measurement frequency, accuracy, and precision, as well as sensor size, power requirements, and endurance. For in situ sensors, desirable characteristics include 5355
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• Autonomous operation, requiring only infrequent servicing • High-frequency measurement capabilities • Inherent calibration or only infrequent field calibration • Good precision and accuracy, plus well-characterized performance • Protection against biofouling • Ease of integration with other sensors • Small size, rugged construction, and low power consumption • Low cost, plus ease of operation and maintenance • Real-time bidirectional communications and adaptivesampling capabilities Some state-of-the-art shipboard and laboratory procedures are not adaptable to underwater deployment. For example, coulometric instrumentation (used to measure DIC) is illsuited to in situ analysis, and potentiometric procedures (suitable for shipboard and laboratory TA analyses) are quite challenging for in situ TA applications. In-water Spectrophotometric CO2 System Measurements. Spectrophotometric techniques possess many of the characteristics needed for underwater instrumentation. As a result, these methods have been used for in situ measurements of all four keystone CO2 system parameters. The general spectrophotometric approach, now used in marine CO2 system analyses for more than two decades, entails measuring the colors (i.e., optical absorbance ratios) of samples or reagents to which sulfonephthalein indicators (dyes) have been added.81,82 Calibration is inherent because the measurements are based on the absorbance ratios of the molecularly characterized indicators. Collectively, these dyes allow for observations over a pH range of approximately 2−9. Purified indicator69 should be used for all procedures. In situ measurements of the DIC− pH and f CO2−pH pairs have been recently reported.80,83 Measurements of pH. This procedure is straightforward and has been used for shipboard and in situ58,66,84 work: dye is added to a seawater sample and the resulting color (absorbance) is measured at specified wavelengths. If a wellcharacterized purified indicator is used, no field calibration is required. Purification procedures have been developed for two indicators appropriate for direct measurements of seawater pH: meta cresol purple (mCP) and cresol red.69 Characterizations of mCP as a function of S, T, and P allow for in situ measurements58,80 throughout the oceanic water column. Measurements of f CO2. In situ spectrophotometric f CO2 measurements84,85 rely on equilibration of CO2 across a semipermeable membrane that separates a natural seawater sample from a synthetic solution of known TA. The postequilibration pH of this reagent is measured spectrophotometrically, and f CO2 is calculated from the TA−pH pair. Such measurements of f CO2 are free of the need for periodic calibration during field deployments.85 Measurements of DIC. Spectrophotometric DIC measurements rely on equilibration of CO2(aq) across a semipermeable membrane that separates an acidified seawater sample from a synthetic solution of known TA.80,86 The seawater is acidified to an extent (pH ≈ 3) that essentially all HCO3− and CO32− are converted to CO2(aq). The postequilibration pH of the synthetic reagent is measured spectrophotometrically, and CO2(aq) is calculated from the TA−pH pair. This CO2(aq) concentration is equal to the seawater DIC. For in situ instruments,80 the acidified seawater offers the important
advantage of effectively preventing biofouling, perhaps the primary impediment to marine sensor endurance. Measurements of TA. When sulfonephthalein dye is dissolved in the acid used for TA titrations, absorbance measurements can provide a direct measure of not only solution pH but also the acid−seawater mixing ratio. Bromocresol green has been used in this way for in situ TA measurements.87 An alternative approach (not yet employed) would be to equilibrate seawater with either an acidified synthetic solution of known DIC or a gas mixture of CO2 and nitrogen. Seawater TA could then be calculated from the measured pH of the sample and the known f CO2 of the synthetic solution or gas. In-water methods to measure organic alkalinity are needed but are likely to be challenging. Other In Situ Approaches. New in situ capabilities are rapidly being developed. In most cases the new technologies do not provide internally calibrated measurements, but they do provide other advantages that should engender their widespread use in ocean acidification studies. ISFETs. Ion selective field effect transistors (ISFETs) provide high-frequency pH measurements with low power consumption88 and very slow rates of measurement drift. Deployed on ARGOS profiling floats, these devices are yielding detailed pH profiles of the ocean’s upper kilometer. The instruments are calibrated once per profile at their point of deepest descent, where pH changes are very small. Mass Spectrometry. Underwater mass spectrometry (UMS)89 is capable of measuring DIC profiles in the ocean’s upper 2 km with a vertical resolution of ∼1 m, on par with spectrophotometric and ISFET pH profile measurements. Deeper deployments may be possible with the use of novel nanocomposite membranes.90 UMS systems have relatively high power requirements but can provide simultaneous, detailed profiles of a variety of dissolved gases (e.g., ref 91). Conductimetry. Conductometric procedures92 appear to be well suited to determinations of DIC when high-frequency (e.g., hourly) measurements are not required. CO2(aq) in an acidified sample is equilibrated across a membrane that separates ambient seawater from a synthetic inner alkaline solution, lowering the inner-solution concentration of highly mobile OH− and increasing the concentration of much less conductive HCO3−. This approach requires the use of multiple standard solutions, but the device is inherently simple with relatively low power requirements. Infrared Methods. Nondispersive infrared (NDIR) procedures93 allow rapid measurements of in situ CO2 fugacity. CO2(aq) is equilibrated across a membrane that separates ambient seawater from an inner chamber where gas is circulated through an optical cell that includes a temperature-stabilized, single-beam, dual-wavelength NDIR detector. Baseline measurements (CO2-free) are periodically made by scrubbing the gas stream with an internal soda-lime cartridge, thus allowing for compensation of drift in sensor response. A commercial version of the sensor94 allows measurements to full ocean depths with a reported resolution94 of
Measuring Ocean Acidification: New Technology for a New Era of Ocean Chemistry Robert H. Byrne* College of Marine Science, University of South Florida, St. Petersburg, Florida 33701, United States (H+), bicarbonate (HCO3−), and carbonate (CO32−) ions. Most of the H+ goes into protonation of borate and carbonate ions. The net result is increasing H+, minor increases in HCO3−, and substantially declining CO32− (Figures 1 and 2). At pH 8.0 (typical of surface seawater), >99% of the CO2 molecules added to the ocean undergo reactions that produce H +. The increase in H+ equates, by definition, to an increase in seawater acidity. A sustained increase is termed “ocean acidification”.10,15 Seawater-carbonation16 is the principal cause of global ocean acidification, analogous to the carbonation and resulting acidification of bottled water. Local short-term variations in seawater acidity, resulting from Human additions of carbon dioxide to the atmosphere are photosynthesis, respiration, and other processes (e.g., refs creating a cascade of chemical consequences that will eventually 17−21) are superimposed on the long-term trend (Figure 1). extend to the bottom of all the world’s oceans. Among the bestSince the beginning of the industrial era, global average surfacedocumented seawater effects are a worldwide increase in openocean acidity has increased ∼26%10 and CO32− has decreased ocean acidity and large-scale declines in calcium carbonate ∼16%.22 saturation states. The susceptibility of some young, fast-growing Ocean carbonation and acidification are likely to benefit calcareous organisms to adverse impacts highlights the potential some organisms (e.g., seagrasses) while adversely affecting for biological and economic consequences. Many important aspects of seawater CO2 chemistry can be only indirectly others (e.g., oyster larvae, pteropods).11,23−26 The larval stages observed at present, and important but difficult-to-observe of some economically important calcareous organisms (young changes can include shifts in the speciation and possibly and fast-growing) seem to be particularly susceptible to adverse bioavailability of some life-essential elements. Innovation and impacts.25,27 Biological effects may be due to any of a number invention are urgently needed to develop the in situ of the chemical changes that accompany ocean carbonation instrumentation required to document this era of rapid ocean for example, increasing seawater CO2 or increasing acidity or evolution. decreasing CO32−. The various mechanisms of biological impact are still being worked out.28,29 Ecological, economic, INTRODUCTION and cultural impacts seem likely.25,26 The carbon dioxide (CO2) content of the atmosphere has Ocean alkalinity buffers the effects of oceanic CO2 uptake, increased by ∼40% over the past few hundred years,1,2 due but slowing and eventually reversing the trend of increasing largely to burning of fossil fuels.3 CO2 is an unusually soluble ocean acidity will require increased continental weathering and gas in aqueous solution relative to most other atmospheric dissolution of ocean carbonate sediments.30,31 These processes gases (e.g., nitrogen, oxygen, argon), and it freely exchanges occur over much longer time scales (centuries to millennia) across the air−sea interface. than the gas-exchange and solution-phase processes associated The increase in atmospheric carbon dioxide has resulted in a with CO2 uptake (microseconds to months). net flux of CO2 into the oceanspresently about 9 billion Not so very long ago, during a time remembered by many 4 tonnes per year. This influx changes ocean chemistry in ways present-day oceanographers, studies of ocean chemistry that can have significant ecological consequences, including included questions as basic as “What is the elemental possible shifts in marine biodiversity and ecosystem function 5−13 composition of seawater?” Today’s marine scientists face the According to a recent assessment of the and services. 14 challenge of assessing rapid and widespread chemical changes geological record, the current rapid rate of atmospheric through time. Such needs cannot be fully met by existing change is capable of creating a diversity and intensity of oceanic technology. This article (1) reviews present-day CO2-system geochemical changes “potentially unparalleled in at least the last capabilities and considerations in designing field research ∼300 million years of Earth history”. programs and (2) highlights opportunities for refining CO2 Sustained uptake of CO2 by the oceans results in a sustained system models and advancing high-resolution in situ measuredecline of seawater pH (Figure 1). Carbon dioxide entering the ments of ocean acidification and its chemical consequences. ocean becomes dissolved as aqueous carbon dioxide, CO2(aq). This dissolved CO2 reacts with water to form carbonic acid (H2CO3), which subsequently dissociates, liberating hydrogen Published: April 7, 2014
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Figure 1. CO2 system changes in the upper ocean (wind-mixed layer) at stations in the Pacific (HOT) and Atlantic (BATS) oceans: the carbonate:bicarbonate concentration ratio and pH. Among the many ions in seawater, the two that are most important in controlling seawater pH are CO32− and HCO3−. The declining concentration ratio shown here, in conjunction with similar data at diverse locations around the world, indicates that major global change is occurring in the equilibrium status of chemical reactions in the oceans. The increasing departure of the carbonate:bicarbonate concentration ratio from a value of 1 (i.e., the point of maximum buffer intensity) shows that the ability of the oceans’ surface layer to buffer pH excursions (including those attributable to local processes such as photosynthesis and respiration) is declining. The concentration ratios shown here were calculated from measurements of pH, S, and T. The black line shows the least-squares best-fit slope: −4.444 (±0.383) × 10−4 yr−1.
imply changes in not only the oceanic CO2 system (Figures 1 and 2) but also many other chemicals, their interactions with each other, and their availability to marine organisms.34−36 Shifting acid−base equilibria can also change the chemistry and perhaps the sinking behavior of some ocean particles.37 In other words, because the carbonate system controls the acidity of seawater and because so many types of chemical reactions are H+-dependent, ocean carbonation and acidification have a great many avenues by which to alter the distributions and biogeochemical cycling of a wide range of seawater constituents. Many chemicals dissolved in seawater are partitioned into different forms (“species”; e.g., Figures 2 and 3). When conditions shift toward higher acidities (i.e., lower pH values), the relative abundances of the positively charged species increase while the negative species decrease. A particularly important aspect of this phenomenon38 is the declining concentration of CO32− (Figures 1 and 2). The speciation of many trace elements, some toxic and some essential to marine life, similarly vary with pH. Iron (Figure 3) and copper are examples. The total concentration of each of these trace metals is very low (approximately one million times lower than the concentration of HCO3−), but even small changes in their chemistries (e.g., their species distributions or their complexation with other constituents, including organics) may imply significant changes in their bioavailability. Iron, for example, has a solution chemistry that is complex and highly pH dependent (Figure 3). This element is essential
Figure 2. Modeled concentrations of carbon species in surface seawater (TA = 2200 μmol kg−1) equilibrated with atmospheric pCO2. The speciation of many other chemicals in seawater likewise varies with seawater pH and therefore atmospheric CO2. The “AD” numbers and arrows along the x-axis indicate the years when those atmospheric pCO2 values were realized or are projected to occur. The “RCP” labels indicate the IPCC10 representative concentration pathways that were used for the year-2100 projections.
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SHIFTING ACID−BASE EQUILIBRIA A large number of seawater constituents, dissolved and particulate, participate in reactions involving hydrogen-ion exchange.32,33 Ocean carbonation and acidification thereby 5353
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shallower depths. In light of what we understand so far, it seems reasonable to speculate that ocean acidification could reduce the vertical fluxes of many elements, especially to the deep ocean.
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WATCHING THE OCEAN EVOLVE CO2 System Measurements. Chemists generally investigate the marine inorganic CO2 system by measuring four fundamental parameters (keystone variables): • DIC, the total concentration of all dissolved forms of inorganic carbon (i.e., [CO2] + [HCO3−] + [CO32−]), provides a direct assessment of the dissolved inorganic component of the ocean’s growing carbon inventory (shipboard precision ∼ ± 1−2 μmol kg−1) • TA, total alkalinity, expressed56,57 in terms of the ability of seawater to neutralize strong acids; reflects the presence of not only carbon species but also borate, silicate, phosphate, and organic ions (shipboard precision ∼ ±2−4 μmol kg−1) • pH, the negative log of the total hydrogen ion concentration, provides a direct assessment of ocean acidity (shipboard precision ∼ ±0.0004−0.001)58 • f CO2, carbon dioxide fugacity or “escaping tendency”59, provides a direct assessment of the propensity for surface seawater to export or import CO2 to or from the atmosphere; directly proportional to CO2(aq) (shipboard precision ∼ ±0.1−0.2%) Relationships among the CO2 system variables can be quantitatively stated using an equilibrium computational modele.g.,60 by which any two input variables can be used to calculate all others. It can be helpful to think of pH as a master descriptive variable and the DIC:TA ratio as a master controlling variable. Even small changes in the relative proportions of DIC (increasing under ocean carbonation) and TA (not directly affected by ocean carbonation) create large increases in f CO2 and H+ and large decreases in CO32−. These amplifications, which are associated with the Revelle factor (i.e., buffer factor),18 can profoundly influence ocean biogeochemical cycling. Total alkalinity is the most challenging of the four keystone measurements. Because shipboard titrations require volumetric metering of a strong acid, shipboard TA precision is typically significantly worse than the precision of onshore TA measurements. In addition, accurate interpretation of TA measurements requires that all equilibria involving H+ exchangeboth inorganic and organicbe considered. The use of TA in the computational models therefore requires either ancillary measurements or assumptionsthat is, the noncarbonate inorganic alkalinities (e.g., phosphate and silicate) and the organic alkalinity61−63 must be either measured or approximated. Organic alkalinity is often assumed negligible. In cases where it is non-negligible, the H+-exchange characteristics of the participating organic bases must be quantitatively characterized. This analytical undertaking is formidable and not yet routine. To the extent that particles participate in H+ exchange, acid titrations may in principle yield a different TA for some filtered versus unfiltered samples (e.g., nearshore particle-rich waters). If calcium carbonate saturation state (a measure of the capability of seawater to dissolve CaCO3) were directly measurable, this parameter (Ω) would likely be regarded as a
Figure 3. Relative abundances of inorganic iron (Fe) species in seawater (expressed as a fraction of total iron) as a function of pH. Because pH is expressed on a logarithmic scale, small differences in pH indicate large differences in acidity. Within the current normal pH range of seawater (shown by the light shading), the fraction of iron present as Fe3+ changes by a factor of 100 or more for a one-unit change in pH. The contributions of organic complexes can dominate over inorganic forms but are not shown on this species-composition diagram because the pH-dependent behaviors of organic complexes in seawater are poorly understood.
to marine phytoplankton to such an extent that continent-sized areas of the open ocean are infertile due to insufficient iron availability.39,40 Dissolved iron concentrations in seawater are influenced by many factors, including wind-driven dust inputs,41 solubility reactions (pH dependent),42,43 oxidation− reduction reactions (pH dependent),44 and complexation by strong organic ligands45,46 (likely pH dependent). Models of the inorganic chemistry of iron33 would suggest that ocean acidification may increase the solubility and availability of iron to microorganisms. In the ocean, however, iron exists as a complex mixture of inorganic and organic species, and it can be adsorbed onto organic and mineral colloids of biogenic or geochemical origin. Laboratory investigations of iron-limited phytoplankton indicate that decreasing seawater pH can actually lower iron bioavailability.47 Another important class of H+-exchange reactions involves trace-metal interactions with particles (i.e., adsorption and desorption reactions).48−52 In the ocean, dissolved constituents in surface waters may be adsorbed onto biological particles that later sink to the deep ocean.48,53,54 This process of particle “scavenging”the result of chemical sorption (pH dependent) and physical sinking (possibly pH dependent)thereby exports many chemicals from the upper ocean to the deep ocean, leaving a strong imprint on their vertical distributions (e.g., ref 54). Organic-rich particles that include mineral components such as calcium carbonate (CaCO3), a common shell material, sink more quickly than those without. With increasing seawater acidity, the affinity of particle surfaces for dissolved constituents generally decreases.49−52 In addition, at sufficiently high levels of seawater acidity, CaCO3 is prone to dissolution. The overall effect of increasing ocean acidity on mineral-enhanced sinking, though hard to predict,55 could include particle dissolution and decomposition at 5354
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keystone variable as well. Lower values of Ω (which accompany lower [CO32−])10 provide yet another avenue by which ocean carbonation and acidification may impact marine organisms11,23,24 and ecosystems.10 The Value of Overdetermination. Only two keystone parameters are required for comprehensive CO2 system calculations, but “overdetermination” (direct measurement of three or more parameters) has long been used to assess procedural and other types of errors (e.g., refs 64−66). Small but systematic differences between measured and calculated quantities indicate that improvements are needed, either in sample processing and measurement or in the computational framework (equilibrium model). Substantial improvements in measurement quality have been achieved in recent decades. One reason is that certified reference materials (CRMs), essential for evaluating analytical accuracy and preventing systematic methodological errors, are now routinely used for quality control in measurements of DIC and TA.67 Another reason is that purification of the indicator dyes68,69 used for spectrophotometric pH analyses is becoming standard protocol. Still, routine internal consistency (i.e., an absence of systematic deviations between measured and calculated quantities) remains elusive (e.g., ref 70). One factor may be the challenge of measuring TA at sea. Another reason may be the mismatch between measurements of total alkalinity (inorganic + organic) and model characterizations of alkalinity (typically inorganic only). For most marine waters, the assumption of negligible organic alkalinity is appropriate, but where it is not (e.g., estuarine and coastal waters61−63), TAbased calculations are subject to significant systematic errors. Overdetermination is therefore still valuable for assessing data quality within a single data set and for determining whether measurement characteristics are changing through time (e.g., by comparing the consistency characteristics of data sets collected years or decades apart). Assessing internal consistency is especially important because it imposes essential quality control on the calculation of some key parameters that are critical to ocean health (e.g., [CO32−] and Ω). These ocean characteristics are rapidly changing but cannot yet be directly measured. Prioritizing Measurements. If operational constraints (e.g., ship space, manpower) do not allow for measurement of all four CO2 system parameters, then a subset must be selected. The choice will depend in part on the primary purpose of the sampling program. In all cases, measuring DIC is highly advisible. Standard coulometric DIC measurements67 are generally precise, reliable, and accurate. If only two types of measurements are possible, prompt shipboard analysis of either f CO2 or pH should be the second choice. Measurements of the f CO2 of discrete seawater samples is somewhat more demanding than measurements of spectrophotometric pH, and f CO2 sample throughput is somewhat slower. State-of-the-art measurements of the DIC− pH pair or the DIC− f CO2 pair can provide calculated TA values at a precision superior to direct TA measurements.65 If three types of measurements are possible, the third selected parameter should be TA. Differences between measured TA and calculated TA allow insights into the local significance of organic alkalinity. Only when all four measurements are possible should f CO2 and pH both be included in the measurement suite. This duo is generally not the best choice as an input pair for calculating carbonate concentrations.71
The Challenge of In Situ Calculations. Marine chemical systems and biota are experiencing ocean carbonation and acidification under in situ conditions, but in situ measurements are not yet routine. Many measurements are still made by retrieving seawater samples from depth and then analyzing those samples in a laboratory, either on ships or on land. Care must therefore be taken when using off-the-shelf computational models to obtain in situ characterizations from laboratory and shipboard measurements: · DIC Because sample DIC does not change when the temperature (T) or pressure (P) of the sample changes, the laboratory DIC value is the same as the in situ DIC value · TA Because sample TA does not change when the T or P of the sample changes, the laboratory TA value is the same as the in situ TA value · pH Because reported pH values are specific to the T and P conditions under which the measurements were made (e.g., 25 °C), these values cannot be used directly as input for off-the-shelf model calculations of in situ conditions · f CO2 Because reported f CO2 values are specific to the T and P conditions under which the measurements were made (e.g., 20 °C), these values cannot be used directly as input for off-the-shelf model calculations of in situ conditions Without in situ measurements, the calculation of CO2 system variables (e.g., Ω) at in situ conditions therefore requires that the DIC−TA pair (either directly measured or calculated) be provided as input to the computational model. This point is subtle but important. DIC is typically directly measured. TA may be either directly measured or calculatedi.e., from measured DIC−pH or DIC−f CO2 pairs. Both calculations are subject to error unless all alkalinity equilibria (including organic alkalinity) are fully characterized with respect to variations in S, T, and P. Measuring either pH in situ or f CO2 in situ to pair with measurements of DIC would provide the most straightforward route to the direct calculation of other in situ quantities. For example, in situ [CO32−] can be calculated from DIC and in situ pH without having to account for organic alkalinity, borate alkalinity, and other noncarbonate alkalinity contributions. The Promise of In Situ Measurements. The oceans are vast, and research expeditions are expensive. Improving the spatial and temporal resolution of ocean sampling programs will require ongoing development of underwater platforms and sensors. For shipboard sampling, sensors can be mounted on the rosettes used to collect seawater samples. Real-time profiles of S, T, P, and oxygen, with vertical resolutions on the order of 1 m are typically collected this way. Vertical profiles of pH72 have been similarly obtained but are not routine. Other in-water platforms include moorings and drifting surface buoys,73−75 autonomous underwater vehicles and gliders,76 “crawlers” that traverse mooring lines,77 autonomous profiling floats that yo-yo through the water column,78,79 and packages or chambers parked on the sea bottom.80 Each platform imposes different constraints on instrument design and performance in terms of measurement frequency, accuracy, and precision, as well as sensor size, power requirements, and endurance. For in situ sensors, desirable characteristics include 5355
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• Autonomous operation, requiring only infrequent servicing • High-frequency measurement capabilities • Inherent calibration or only infrequent field calibration • Good precision and accuracy, plus well-characterized performance • Protection against biofouling • Ease of integration with other sensors • Small size, rugged construction, and low power consumption • Low cost, plus ease of operation and maintenance • Real-time bidirectional communications and adaptivesampling capabilities Some state-of-the-art shipboard and laboratory procedures are not adaptable to underwater deployment. For example, coulometric instrumentation (used to measure DIC) is illsuited to in situ analysis, and potentiometric procedures (suitable for shipboard and laboratory TA analyses) are quite challenging for in situ TA applications. In-water Spectrophotometric CO2 System Measurements. Spectrophotometric techniques possess many of the characteristics needed for underwater instrumentation. As a result, these methods have been used for in situ measurements of all four keystone CO2 system parameters. The general spectrophotometric approach, now used in marine CO2 system analyses for more than two decades, entails measuring the colors (i.e., optical absorbance ratios) of samples or reagents to which sulfonephthalein indicators (dyes) have been added.81,82 Calibration is inherent because the measurements are based on the absorbance ratios of the molecularly characterized indicators. Collectively, these dyes allow for observations over a pH range of approximately 2−9. Purified indicator69 should be used for all procedures. In situ measurements of the DIC− pH and f CO2−pH pairs have been recently reported.80,83 Measurements of pH. This procedure is straightforward and has been used for shipboard and in situ58,66,84 work: dye is added to a seawater sample and the resulting color (absorbance) is measured at specified wavelengths. If a wellcharacterized purified indicator is used, no field calibration is required. Purification procedures have been developed for two indicators appropriate for direct measurements of seawater pH: meta cresol purple (mCP) and cresol red.69 Characterizations of mCP as a function of S, T, and P allow for in situ measurements58,80 throughout the oceanic water column. Measurements of f CO2. In situ spectrophotometric f CO2 measurements84,85 rely on equilibration of CO2 across a semipermeable membrane that separates a natural seawater sample from a synthetic solution of known TA. The postequilibration pH of this reagent is measured spectrophotometrically, and f CO2 is calculated from the TA−pH pair. Such measurements of f CO2 are free of the need for periodic calibration during field deployments.85 Measurements of DIC. Spectrophotometric DIC measurements rely on equilibration of CO2(aq) across a semipermeable membrane that separates an acidified seawater sample from a synthetic solution of known TA.80,86 The seawater is acidified to an extent (pH ≈ 3) that essentially all HCO3− and CO32− are converted to CO2(aq). The postequilibration pH of the synthetic reagent is measured spectrophotometrically, and CO2(aq) is calculated from the TA−pH pair. This CO2(aq) concentration is equal to the seawater DIC. For in situ instruments,80 the acidified seawater offers the important
advantage of effectively preventing biofouling, perhaps the primary impediment to marine sensor endurance. Measurements of TA. When sulfonephthalein dye is dissolved in the acid used for TA titrations, absorbance measurements can provide a direct measure of not only solution pH but also the acid−seawater mixing ratio. Bromocresol green has been used in this way for in situ TA measurements.87 An alternative approach (not yet employed) would be to equilibrate seawater with either an acidified synthetic solution of known DIC or a gas mixture of CO2 and nitrogen. Seawater TA could then be calculated from the measured pH of the sample and the known f CO2 of the synthetic solution or gas. In-water methods to measure organic alkalinity are needed but are likely to be challenging. Other In Situ Approaches. New in situ capabilities are rapidly being developed. In most cases the new technologies do not provide internally calibrated measurements, but they do provide other advantages that should engender their widespread use in ocean acidification studies. ISFETs. Ion selective field effect transistors (ISFETs) provide high-frequency pH measurements with low power consumption88 and very slow rates of measurement drift. Deployed on ARGOS profiling floats, these devices are yielding detailed pH profiles of the ocean’s upper kilometer. The instruments are calibrated once per profile at their point of deepest descent, where pH changes are very small. Mass Spectrometry. Underwater mass spectrometry (UMS)89 is capable of measuring DIC profiles in the ocean’s upper 2 km with a vertical resolution of ∼1 m, on par with spectrophotometric and ISFET pH profile measurements. Deeper deployments may be possible with the use of novel nanocomposite membranes.90 UMS systems have relatively high power requirements but can provide simultaneous, detailed profiles of a variety of dissolved gases (e.g., ref 91). Conductimetry. Conductometric procedures92 appear to be well suited to determinations of DIC when high-frequency (e.g., hourly) measurements are not required. CO2(aq) in an acidified sample is equilibrated across a membrane that separates ambient seawater from a synthetic inner alkaline solution, lowering the inner-solution concentration of highly mobile OH− and increasing the concentration of much less conductive HCO3−. This approach requires the use of multiple standard solutions, but the device is inherently simple with relatively low power requirements. Infrared Methods. Nondispersive infrared (NDIR) procedures93 allow rapid measurements of in situ CO2 fugacity. CO2(aq) is equilibrated across a membrane that separates ambient seawater from an inner chamber where gas is circulated through an optical cell that includes a temperature-stabilized, single-beam, dual-wavelength NDIR detector. Baseline measurements (CO2-free) are periodically made by scrubbing the gas stream with an internal soda-lime cartridge, thus allowing for compensation of drift in sensor response. A commercial version of the sensor94 allows measurements to full ocean depths with a reported resolution94 of
