新托福阅读复习材料：美国科学文摘精选

小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 新托福阅读复习材料：美国科学文摘精选The Planet EarthThe 2000 Antarctic Ozone Hole Was Largest EverThe 2000 Antarctic Ozone Hole was the largest ever observed. Earth's wetterupper atmosphere may delay global ozone recovery.NASA researchers have found that an increase in water vapor in thestratosphere, stemming partially from greenhouse gases, may delay ozonerecovery and increase the rate of climate change.To check on the long-term stratospheric cooling and ozone depletion, NASA putdata from satellites and other remote sensors into its GISS global climatemodel. It was the first study to link greenhouse gases to increased ozonedepletion over populated areas.Water and ozone. Climate models show cooler stratospheric temperatures happenwhen there is more water vapor present. Water vapor also leads to thebreakdown of ozone molecules.The stratosphere is the dry layer of the atmosphere above the troposphere,where temperatures increase with height.According to satellite data, upper atmospheric temperatures around the world -at altitudes of 20 to 35 miles high -- have cooled between 5.4 and 10.8degrees Fahrenheit over recent decades.Driving forces. NASA found two driving forces behind the change instratospheric moisture:Increased emissions of the greenhouse gas methane are transformed into waterin the stratosphere, accounting for about a third of the observed increasein moisture there.More water is transported from the lower atmosphere. Warmer air holds morewater vapor than colder air, so the amount of water vapor in the lower小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 atmosphere increases as it is warmed by the greenhouse effect. Greenhousegases, such as carbon dioxide and methane, may enhance the transport ofwater into the stratosphere.The increased transport of water vapor to the stratosphere seems likely tohave been induced by human activities.Ozone destruction. Rising greenhouse gas emissions account for all or part ofthe water vapor increase, which causes stratospheric ozone destruction.When more water vapor works its way into the stratosphere, the water moleculescan be broken down, releasing reactive molecules that can destroy ozone. Ifthe trend of increasing stratospheric water vapor continues, it could increasefuture global warming and impede ozone stratospheric recovery.The impact on global warming comes about because both water vapor and ozoneare greenhouse gases, which trap heat leaving the Earth. When they change, theEarth's energy balance changes too, altering the surface climate.Warmer ground. Increased water vapor in the stratosphere makes it warmer onthe ground by trapping heat, while the ozone loss makes it colder on theground.Water vapor has a much larger effect, so that overall the changes increaseglobal warming.Although ozone depletion cools the Earth's surface, repairing stratosphericozone is important to block harmful ultraviolet radiation. Other greenhousegas emissions also need to be reduced.UARS satellite. NASA combined seven years of data from the Upper Atmosphere Research Satellite (UARS) Halogen Occultation Experiment (HALOE) with datacollected on the ground to paint a complete picture of the upper atmosphere.NASA's HALOE was aboard the UARS spacecraft when it was launched September 12, 1991 as part of the Earth Science Enterprise Program. The spacecraft's missionat launch was to improve understanding of stratospheric ozone depletion byanalyzing vertical profiles of ozone, hydrogen chloride, hydrogen fluoride,methane, water vapor, nitric oxide, nitrogen dioxide, and aerosols.Fourteen years of lower stratospheric measurements have revealed largeincreases in water vapor. Though some older studies conflict with lowerstratospheric observations of water vapor trends, new studies agree with the小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 increases, showing they have been taking place for more than four decades.What Is An Ozone Hole?Ozone molecules are made up of three atoms of oxygen. They comprise a thinlayer of the atmosphere that absorbs harmful ultraviolet radiation from theSun.Most atmospheric ozone is found between approximately six miles and 18 milesabove the Earth's surface.An ozone "hole" is what scientists call an "ozone depletion area" of in thatregion of Earth's atmosphere.Really big hole. The largest-ever ozone hole was detected on September 6,2000, by the Total Ozone Mapping Spectrometer (TOMS) aboard a NASA satellite known as Earth Probe (TOMS-EP).The Antarctic ozone hole is three times larger than the entire land mass ofthe United States, making it the largest such area ever observed.The hole had expanded to a record size of 11 million square miles. Theprevious record was 10.5 million square miles in September 1998.Scientists were surprised by its enormous size. The lowest readings in theAntarctic ozone hole usually are observed in late September or early Octobereach year.Frail layer. The year 2000 observations reinforced concerns about the frailtyof Earth's ozone layer. Although production of ozone-destroying gases had beencurtailed under international agreements, concentrations of the gases in thestratosphere have been reaching their peak.Due to their long persistence in the atmosphere, it will be many decadesbefore the ozone hole is no longer an annual occurrence.Antarctic vortex. The year 2000 saw an extremely intense Antarctic vortex --an upper-altitude stratospheric air current that sweeps around the Antarcticcontinent, confining the Antarctic ozone hole.Variations in the size of the ozone hole and of ozone depletion accompanyingit from one year to the next are not unexpected.小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区NASA instruments have been measuring Antarctic ozone levels since the early1970s. Since the discovery of the ozone hole in 1985, TOMS has been a key toolfor monitoring ozone levels above Earth.TOMS-EP and other ozone-measurement programs are important parts of a globalenvironmental effort of NASA's Earth Science enterprise, a long-term researchprogram designed to study Earth's land, oceans, atmosphere, ice and life as atotal integrated system.To learn more:Goddard Institute for Space StudiesUpper Atmosphere Research SatelliteHalogen Occultation ExperimentLargest ozone hole detected by TOMSTOMS ozone data and picturesNASA Says Wet Upper Atmopsphere Delays Ozone RecoveryNASA Press Release: Wetter Atmopsphere May Delay Ozone RecoveryInner Planets:MercuryVenusEarthMarsOuter Planets:JupiterSaturnUranusNeptunePlutoOther Bodies:MoonsAsteroidsComets The VoyagersEarth's Interior & Plate TectonicsJust as a child may shake an unopened present in an attempt to discover the contents of a gift, so man must listen to the ring and vibration of our Earth in an attempt to discover its content. This is accomplished through seismology, which has become the principle method used in studying Earth's interior. Seismos is a Greek word meaning shock; akin to earthquake, shake, or violently moved. Seismology on Earth deals with the study of vibrations that are produced by earthquakes, the impact of meteorites, or artificial means such as an explosion. On these occasions, aseismograph is used to measure and record the actual movements and vibrations within the Earth and of the ground.Types of seismic waves(Adapted from, Beatty, 1990.)Scientists categorize seismic movements into four types of diagnostic waves that travel at speeds ranging from 3 to 15 kilometers (1.9 to 9.4 miles) per second. Two of the waves travel around the surface of the Earth in rolling swells. Theother two, Primary (P) or compression waves and Secondary (S) or shear waves, penetrate the小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 interior of the Earth. Primary waves compress and dilate the matter they travel through (either rock or liquid) similar to sound waves. They also have the ability to move twice as fast as S waves. Secondary waves propagate through rock but are not able to travel through liquid. Both P and S waves refract or reflect at points where layers of differing physical properties meet. They also reduce speed when moving through hotter material. These changes in direction and velocity are the means of locating discontinuities.Divisions in the Earth's Interior(Adapted from, Beatty, 1990.)Seismic discontinuities aid in distinguishing divisions of the Earth into inner core, outer core, D", lower mantle, transition region, upper mantle, and crust (oceanic and continental). Lateral discontinuities also have been distinguishedand mapped through seismic tomography but shall not be discussed here.Inner core: 1.7% of the Earth's mass; depth of 5,150-6,370 kilometers (3,219 - 3,981 miles) The inner core is solid and unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing which occurs to most liquids when temperature decreases or pressure increases.Outer core: 30.8% of Earth's mass; depth of 2,890-5,150 kilometers (1,806 - 3,219 miles) The outer core is a hot, electrically conducting liquid within which convective motion occurs. This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents known as the Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron, which indicates the presence of lighter elements. Scientists suspect that about 10% of the layer is composed of sulfur and/or oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron. D": 3% of Earth's mass; depth of 2,700-2,890 kilometers (1,688 - 1,806 miles) This layer is 200 to 300 kilometers (125 to 188 miles) thick and represents about 4% of the mantle-crust mass. Although it is often identified as part of the lower mantle, seismicdiscontinuities suggest the D" layer might differ chemically from the lower mantle lying above it. Scientists theorize that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density.Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 kilometers (406 -1,806 miles) The lower mantle contains 72.9% of the mantle-crust mass and is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminum. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites.Transition region: 7.5% of Earth's mass; depth of 400-650 kilometers (250-406 miles) The transition region or mesosphere (for middle mantle), sometimes called the fertile layer, contains 11.1% of the mantle-crust mass and is the source of basaltic magmas. It also contains小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 calcium, aluminum, and garnet, which is a complex aluminum-bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma.Upper mantle: 10.3% of Earth's mass; depth of 10-400 kilometers (6 - 250 miles)The upper mantle contains 15.3% of the mantle-crust mass. Fragments have been excavated for our observation by eroded mountain belts and volcanic eruptions.Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been the primary minerals found in this way. These and other minerals are refractory and crystalline at high temperatures; therefore, most settle out of rising magma, either forming new crustal material or never leaving the mantle. Part of the upper mantle called the asthenosphere might be partially molten.Oceanic crust: 0.099% of Earth's mass; depth of 0-10 kilometers (0 - 6 miles)The oceanic crust contains 0.147% of the mantle-crust mass. The majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000-kilometer (25,000 mile) network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, covering the ocean floor with basalt. Hawaii and Iceland are two examples of the accumulation of basalt piles. Continental crust: 0.374% of Earth's mass; depth of 0-50 kilometers (0 - 31 miles).The continental crust contains 0.554% of the mantle-crust mass. This is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars(metal-poor silicates). The crust (both oceanic and continental) is the surface of the Earth; as such, it is the coldest part of our planet. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).The Lithosphere & Plate TectonicsOceanic LithosphereThe rigid, outermost layer of the Earth comprising the crust and upper mantle is called the lithosphere. New oceanic lithosphere forms through volcanism in the form of fissures at mid-ocean ridges which are cracks that encircle the globe.Heat escapes the interior as this new lithosphere emerges from below. It gradually cools, contracts and moves away from the ridge, traveling across the seafloor to subduction zones in a process called seafloor spreading. In time, older lithosphere will thicken and eventually become more dense than the mantle below, causing it to descend (subduct) back into the Earth at a steep angle, cooling the interior. Subduction is the main method of cooling the mantle below 100 kilometers (62.5 miles). If the lithosphere is young and thus hotter at a subduction zone, it will be forced back into the interior at a lesser angle.Continental LithosphereThe continental lithosphere is about 150 kilometers (93 miles) thick with a low-density crust and upper-mantle that are permanently buoyant. Continents drift laterally along the convecting system of the mantle away from hot mantle zones toward cooler ones, a process known as小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 continental drift. Most of the continents are now sitting on or moving toward cooler parts of the mantle, with the exception of Africa. Africa was once the core of Pangaea, a supercontinent that eventually broke into todays continents. Several hundred million years prior to the formation of Pangaea, the southern continents - Africa, South America, Australia, Antarctica, and India - were assembled together in what is called Gondwana. Plate TectonicsCrustal Plate Boundaries(Courtesy NGDC)Plate tectonics involves the formation, lateral movement, interaction, and destruction of the lithospheric plates. Much of Earth's internal heat is relieved through this process and many of Earth's large structural and topographic features are consequently formed. Continental rift valleys and vast plateaus of basalt are created at plate break up when magma ascends from the mantle to the ocean floor, forming new crust and separating midocean ridges. Plates collide and are destroyed as they descend at subduction zones to produce deep ocean trenches, strings of volcanoes, extensive transform faults, broad linear rises, and folded mountain belts. Earth'slithosphere presently is divided into eight large plates with about two dozen smaller ones that are drifting above the mantle at the rate of 5 to 10 centimeters (2 to 4 inches) peryear. The eight large plates are the African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American plates. A few of the smaller plates are the Anatolian, Arabian, Caribbean, Cocos, Philippine, and Somali plates.Views of the Solar System Copyright © 1997-2001 by Calvin J. Hamilton. All rights reserved. Privacy Statement.PMELORNL/CDIAC-115Comparison of the Carbon System Parameters at the Global CO2 Survey CrossoverLocations in the North and South Pacific Ocean, 1990-1996As a collaborative program to measure global ocean carbon inventories and provide estimates of the anthropogenic carbon dioxide (CO2) uptake by the oceans, the National Oceanic andAtmospheric Administration and the U.S. Department of Energy have sponsored the collection of ocean carbon measurements as part of the World Ocean Circulation Experiment andOcean-Atmosphere Carbon Exchange Study cruises. The cruises discussed here occurred in the North and South Pacific from 1990 through 1996. The carbon parameters from these 30 crossover locations have been compared to ensure that a consistent global data set emerges from the survey cruises. The results indicate that for dissolved inorganic carbon,fugacity of CO2, and pH, the agreements at most crossover locations are well within the design specifications for the global CO2 survey; whereas, in the case of total alkalinity, the agreement between crossover locations is not as close.小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区1. INTRODUCTIONHuman activity is rapidly changing the trace gas composition of the earth's atmosphere, apparently causing greenhouse warming from excess carbon dioxide (CO2) along with other trace gas species, such as water vapor, chlorofluorocarbons (CFCs), methane, and nitrous oxide. These gases play a critical role in controlling the earth's climate because they increase the infrared opacity of the atmosphere, causing the planetary surface to warm. Of all the anthropogenic CO2 that has ever been produced, only about half remains in the atmosphere; it is the "missing" CO2 for which the global ocean is considered to be the dominant sink for the man-made increase. Future decisions on regulating emissions of "greenhouse gases" should be based on more accurate models that have been adequately tested against a well-designed system of measurements. Predicting global climate change, as a consequence of CO2 emissions, requires coupled atmosphere/ocean/terrestrial biosphere models that realistically simulate the rate of growth of CO2 in the atmosphere, as well as its removal, redistribution, and storage in the oceans and terrestrial biosphere. The construction of a believable present-day carbon budget is essential for the skillful prediction of atmospheric CO2 and temperature from given emission scenarios.The world's oceans, widely recognized to be the major long-term control on the rate of CO2 increases in the atmosphere, are believed to be absorbing about 2.0 GtC yr-1 (nearly 30 to 40% of the annual release from fossil fuels). Our presentunderstanding of oceanic sources and sinks for CO2 is derived from a combination of field data, that are limited by sparse temporal and spatial coverage, and model results that are validated by comparisons with oceanic bomb 14C profiles. CO2 measurements taken on the World Ocean Circulation Experiment (WOCE) cruises, which began in 1990, have provided an accurate benchmark of the ocean inventory of CO2 and other properties. These measurements were cosponsored by the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Department of Energy (DOE) via the U.S. Joint Global Ocean Flux Study (JGOFS) Program. Investigators supported by these funding agencies have collaborated to examine data collected during the WOCE and Ocean-Atmosphere Carbon Exchange Study (OACES) cruises. This report addresses the consistency of oceanic carbon dioxide system parameters during 1990-1996 in the North and South Pacific.The four parameters of the oceanic carbon dioxide system are dissolved inorganic carbon (DIC), fugacity of CO2 (fCO2), total alkalinity (TAlk), and pH. This report compares the carbon system parameters, along with salinity and dissolved oxygen (O2), against sigma theta () where cruises overlapped throughout the Pacific Ocean basin. Similar comparisons have been made for oceanic carbon in the Indian Ocean (Johnson et al. 1998; Millero et al. 1998). In addition, comparisons of nutrient data have been compiled (Gordon et al. 1998). The cruise data for this report will be made available through the OACES and the Carbon Dioxide Information Analysis Center (CDIAC) data management centers.The Pacific Ocean cruises occurred from 1990-1996, and data have been compared at 30 locations小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 where cruises overlapped in the North and South Pacific Ocean (Fig. 1). We do not address survey stations in the Pacific where no crossovers occurred. In addition, carbon and hydrographic data collected during some of the Pacific expedition cruises (i.e., P2, P12, and S4I) were not available in time for this report.2. ANALYTICAL METHODSAnalyses of all carbon parameters were performed following the techniques outlined in the"Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water" (DOE 1994). Certified Reference Materials (CRMs) were used on all cruises as secondary standards for DIC, unless otherwise noted. Discussion of the preparation and use of CRMs is available in detail (UNESCO 1991; Dickson 1992; Dickson, Anderson, and Afghan, unpublished manuscript; Dickson, Afghan, and Anderson, unpublished manuscript). Thesematerials consisted of a matrix of natural, sterile seawater. They were bottled in large batches into 500-mL borosilicate glass containers, sealed to prevent contamination, and shipped to theinstitutes participating in this study. These secondary standards were then analyzed at sea over the course of each of the cruises as a means to verify accuracy. Certification of the reference material for DIC is based on manometric analyses in the shore-based laboratory of Charles D. Keeling of Scripps Institution of Oceanography (SIO) over a period of several months (UNESCO 1991; Guenther 1994; Keeling, C. D., personal communication, 1999). Since CRMs were analyzed routinely for DIC during most cruises used in this report, all groups analyzing for TAlk on those cruises subsequently analyzed CRMs as well; this enabled post-cruise corrections to bemade to the TAlk data based on archived samples that were analyzed at Dr. Keeling's laboratory at SIO. CRMs were not available for any other carbon parameter discussed in this report. Analyses of salinity and O2 followed WOCE Hydrographic Program (WHP) protocol (WOCE 1994).3. RESULTS AND DISCUSSION3.1 Statistical MethodsTables 1 and 2 summarize the crossover sites and parameters measured, and Tables 3, 4, 5, and 6 are summaries of the statistical data for each parameter at the crossover locations. Elevenlaboratories from two countries participated in this comparison study that examines crossovers in both the North and South Pacific. At some of the crossover locations, the site was occupied on more than one occasion [i.e., the crossover at 170?W and 10?S was frequented by NOAA on three different cruises (CGC90, EqS92, and P15S), as well as by the Institute of OceanScience (IOS) (P15N) and the University of Hawaii (UH) (P31)]. A total of 30 crossover locations were studied in this analysis and 41 individual crossover comparisons were made. Individual plots of each carbon parameter, along with salinity and O2, were first created for every crossover against using data from the entire water column (Appendix A). Only data sets that showed good agreement in both salinity and O2 data were used for the comparisons. An expanded area within the plot was examined further based on the region of reasonable agreement of the vs salinity plot.小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 In most cases, > 27.0 was used in the expanded regions.A curve-fitting routine was applied to the expanded plots (Appendix A) using a second-order polynomial fit (unless otherwise noted in Tables 3, 4, 5, and 6). The difference between each region of crossover was calculated based on evenly distributed intervals on the axis; the intervals chosen were, on average, 0.04 units apart. In the case where more than one station on a given cruise was computed at a particular crossover location, averages of the resulting fits of the two or more stations for that cruise were determined, and the total mean of the differences over theentire range was compared. This procedure was performed for every carbon parameter measured (Tables 3, 4, 5, and 6). The mean and standard deviation of the differences were computed, along with the mean and standard deviation of the absolute value of the differences. For the DIC data, the results were calculated both uncorrected and corrected using the CRMs as a basis for the corrections.3.2 Cruise ResultsThe most detailed carbon parameter results are for DIC, as this parameter was measured on all of the cruises (Table 3). The next most frequently measured parameter was fCO2, followed by TAlk and pH (Tables 4, 5, and 6), respectively.DIC CRMs were available to the investigators for almost every cruise during the survey. In general, there is excellent agreement between DIC data sets at the crossover locations. At the beginning of the program, the goal was to obtainagreements between cruises that were less than 4.0 µmol/kg. On 31 of 41 crossover comparisons the uncorrected DIC differences were less than this value, and on 24 of the comparisons the differences were less than 2.0 µmol/kg.Most of the cruises that did not meet this criteria occurred at the beginning of the program when methods were still being developed, and one comparison was during a strong El Niño event where the upper water column hydrography wassignificantly different from normal (Feely et al. 1995). When the DIC data were corrected for CRMs, 36 of the 41 comparisons were less than 4.0 µmol/kg, and 31 comparisons were less than 2.0 µmol/kg. The mean of the absolute value of the differences was 2.4 ± 2.8 µmol/kg for the uncorrected data and 1.9 ± 2.3 µmol/kgfor the corrected data (Fig. 2). For a mean DIC concentration of approximately 2260µmol/kg in the deep Pacific, this difference is equivalent to an uncertainty of approximately 0.08%. The excellent agreement of the DIC data waslikely due primarily to the use of the coulometer (UIC, Inc.) coupled with a SOMMA (Single Operator Multiparameter Metabolic Analyzer) inlet system developed by Ken Johnson (Johnson et al. 1985, 1987, 1993; Johnson 1992) of Brookhaven National Laboratory (BNL), as well as the use of CRMs as secondary standards during the cruises. The spirit of cooperation and closeinteractions among the scientists and technicians who were responsible for the measurements also contributed to the outstanding quality of the data set.小小马马过过河河X i a o m a .c o m ————专专业业备备考考社社区区 The crossover comparison of fCO2 in seawater is not as straightforward as the comparison of the other carbon parameters because the measurement temperature for fCO2 differs for different cruises. The comparison thus requires a temperature normalization, which is performed by using the carbonate dissociation constants, and measured DIC. For comparison purposes, all values were normalized to 20癈 in this report. The normalization is dependent on the dissociation constant used. In this comparison, we used the constants of Mehrbach et al. (1973) as refitted by Dickson and Millero (1987). An example of the effect of constants on the final comparison is given in Table 7 in which we use typical deep-sea DIC and fCO2 values as found in the southeastern Pacific. Also included in the table are the fCO2@20癈/DIC values inµatm/(µmol/kg to illustrate the sensitivity of discrete fCO2 measurements relative to DIC in deep waters.We analyzed 16 crossover comparisons for fCO2, and observed differences ranging between -28.7 and 34 µatm, excluding the large difference during the 1992 El Niño at 5?N, 110?W. The mean of the absolute value of the difference was 17.6 ± 16.3 µatm. In deep water 10 atm of fCO2 measured at 20癈 is approximately equivalent to an uncertainty of 1.5µmol/kg DIC. Thus, with the possible exception of two or three crossover locations, the systematic differences in the fCO2 data corresponded to a similar uncertainty to that of the majority of the DIC results. Since there were no CRMs available for fCO2 during the Pacific expeditions, the analysts used their own compressed gas standards for the measurements. Some of the differences between the data sets may have resulted from systematic differences etween standards and/or differences between methods employed.The agreement of the TAlk data between the 15 crossover locations is not quite as good as the DIC results. The differences between cruises ranged from -11.5 to 7.8 µmol/kg; generally, the smallest differences correspond to the excellent agreement by the same laboratory on different cruises. As with DIC and fCO2, the largest offsets generally occur during the strong El Nino event in 1992. The mean of the absolute value of the difference was 5.7 ± 3.3 µmol/kg; this corresponds to a mean uncertainty of approximately 0.2%. CRMs were available for TAlk where crossover comparisons were made for this report, and all data have been normalized to the certified values. Three laboratories performed pH analyses, and as a result, only five crossover locations were available to compare the pH results. All comparisons were made on the totalseawater scale. The differences ranged from -0.0005 to 0.0062 and the mean of the absolute value of the difference was 0.0023 ± 0.0025. In the deep Pacific, an uncertainty of 1 µmol/kg DIC is equivalent to approximately 0.003 pH units. These results suggest that the limited amount of pH data in the Pacific were in excellent agreement with each other.The summary data in Tables 3, 4, 5, and 6 should be viewed as one of several indicators of the overall quality of the carbon data from the Pacific. In addition to these results, there also are the shore-based analyses of replicate DIC samples taken during each of the cruises (Guenther et al. 1994) and the interlaboratory analyses of the CRMs (Dickson 1992). These three pieces ofinformation should be used together with thermodynamic models in the process of evaluating the。