Sulfur hexafluoride as a transient tracer in the North Pacific Ocean

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L18603, doi: /2006GL026514, 2006 Sulfur hexafluoride as a transient tracer in the North Pacific Ocean John L. Bullister, 1 David P. Wisegarver, 1 and Rolf E. Sonnerup 2 Received 5 April 2006; revised 1 August 2006; accepted 10 August 2006; published 19 September [1] The atmospheric concentration of sulfur hexafluoride (SF 6 ) has increased steadily during the past 30 years, making it potentially a valuable transient tracer of oceanic circulation and mixing processes on decadal timescales. Simultaneous measurements of dissolved SF 6 with chlorofluorocarbons (CFCs), which have longer atmospheric histories but different growth rates, provides additional information over the use of each tracer alone. Concentrations of dissolved SF 6, CFC11 and CFC12 were measured at the Hawaii Ocean Time-Series (HOT) site in Concentrations were highest in the upper water column, with pronounced CFC11 and CFC12 maxima at a depth of 400 meters. Apparent water mass ages calculated from SF 6 concentrations tend to be younger than those calculated from CFC12 concentrations. An isopycnal pipe model is used to estimate the effects of mixing on SF 6 and CFC12 derived ages. Combining SF 6 and CFC12 ages allows improved estimates of ideal ages and oceanic uptake of anthropogenic CO 2. Citation: Bullister, J. L., D. P. Wisegarver, and R. E. Sonnerup (2006), Sulfur hexafluoride as a transient tracer in the North Pacific Ocean, Geophys. Res. Lett., 33, L18603, doi: /2006gl Introduction 1 Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington, USA. 2 Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, USA. Copyright 2006 by the American Geophysical Union /06/2006GL [2] Sulfur hexafluoride (SF 6 ) and the chlorofluorocarbons CFC11 (CCl 3 F) and CFC12 (CCl 2 F 2 ) are anthropogenic compounds. Because of their low chemical reactivity, these compounds have accumulated and spread throughout the earth s atmosphere. Significant production and release of CFC11 and CFC12 into the atmosphere began in the 1940s, while that for SF 6 has occurred only since the 1960s. Atmospheric concentrations of these compounds increased rapidly in the decades following initial production (Figure 1a). In addition to the concentration increases, the CFC12/CFC11 ratio in the atmosphere also increased monotonically during (Figure 1b). Since the mid-1980s, international agreements restricting production and use has slowed the growth of atmospheric CFC11 and CFC12. In contrast, the concentration of SF 6 (Figure 1a) and the SF 6 / CFC12 ratio (Figure 1b) in the atmosphere have continued to increase during the past two decades. [3] SF 6 and CFCs dissolve in surface seawater, and are carried into the interior of the ocean, where they act as passive tracers of ocean circulation and mixing processes. Measurements of dissolved CFCs, together with information on the history of their atmospheric input functions, have been used to estimate the rates and pathways of ocean mixing and circulation processes, to evaluate numerical ocean models, and to help quantify the global oceanic uptake of anthropogenic CO 2 [e.g., Gruber et al., 1996; McNeil et al., 2003; Sabine et al., 2004]. [4] Because surface seawater is usually close to solubility equilibrium with respect to CFCs, dissolved CFC data can be used to calculate CFC apparent ages [Doney and Bullister, 1992] of a water sample. In this method, the dissolved CFC concentration in a sample is converted to a CFC partial pressure (pcfc) value, based on the temperature (T) and salinity (S) of the water. The pcfc is then compared to the atmospheric growth curve (Figure 1a) to determine when the CFC in the sample would have been in equilibrium with the atmosphere. The difference between that date and the date the sample was collected gives the pcfc age of the sample, which is usually used as an indication of the date when a subsurface water mass was last at the surface. pcfc ages can be biased if the gases in the surface layer are not in equilibrium at the time of water mass formation and, because the atmospheric concentrations have not increased linearly with time, these ages can also be biased due to sub-surface mixing. Also, pcfc ages become more uncertain as the growth rates of CFC11 and CFC12 have decreased to near zero or to negative values (Figure 1a). In contrast to CFC11 and CFC12, the concentration of SF 6 in the atmosphere has increased rapidly over the past several decades, offering the potential of a new ocean tracer [Law and Watson, 2001; Vollmer and Weiss, 2002; Watanabe et al.; 2003; Tanhua et al., 2004] and, as a complement to the CFCs, may provide a constraint on the impacts of mixing on tracer ages [Waugh et al., 2002, 2003]. [5] Recently, SF 6 has been injected in kilogram quantities as a deliberate tracer in a number of oceanic locations. The dispersal of these SF 6 spikes have been used to quantify mixing in the ocean interior [Ledwell et al., 1993] and airsea gas exchange. A summary of locations where deliberate SF 6 release into the ocean has occurred is provided by Tanhua et al. [2004]. Because the Hawaii Ocean Time- Series (HOT) site is far from these deliberate tracer release sites, the dissolved SF 6 signal present in this region is likely derived exclusively from atmospheric SF 6 by air-sea gas exchange. 2. Methods [6] The HOT site is located at 23 N, 158 W in the North Pacific subtropical gyre. This site has been visited monthly for the past two decades in a major effort to monitor long-term chemical, physical and biological processes in the oligotrophic North Pacific Ocean [Karl and Lukas, 1996]. Vertical profiles of dissolved CFC11, CFC12 L of5

2 soluble than CFC11 and CFC12 [Bullister et al., 2002] expected equilibrium concentrations of SF 6 are 1 fmol kg 1 (1 fmol = 1 femtomole = pmol). [8] The estimated precision of the dissolved CFC measurements at HOT is 1% or pmol kg 1, whichever is greater. Overall accuracy of the measurements (including errors in calibration scales) for the CFCs was estimated to be 2% or pmol kg 1, whichever was greater. Blank levels (based on analysis of abyssal samples at HOT thought to be tracer free) for the CFCs were pmol kg 1. The precision of the dissolved SF 6 measurements was 2% or 0.02 fmol kg 1, whichever was greater, with an estimated overall accuracy of 4% or 0.04 fmol kg 1, whichever was greater. Blank levels for SF 6 were 0.01 fmol kg Data Figure 1. (a) Concentrations, in parts per trillion (ppt), of CFC11, CFC12 and SF 6 in the troposphere of the Northern Hemisphere (NH) as a function of time. SF 6 concentrations have been multiplied by 100. CFC values from are from Walker et al [2000] and are on the SIO98 calibration scale [Prinn et al., 2000]. Recent ( ) CFC11 and CFC12 values are from NOAA s Global Monitoring Division (GMD) [ hats], normalized to the Walker et al. [2000] SIO98 values in SF 6 values from are from GMD and are on their 2000 calibration scale. SF 6 values from 1940 to 1996 are based on the release estimates of Maiss and Brenninkmeijer [1998], and normalized to the GMD atmospheric value in (b) Ratio of CFC11/CFC12 and SF 6 /CFC12 in the troposphere of the NH as a function of time. SF 6 concentrations have been multiplied by 100. [9] In near surface waters, CFC11, CFC12 and SF 6 concentrations (Figure 2) were scattered within a few percent of those expected from equilibrium with the atmosphere at the measured T ( 26.3 C) and S ( 35.2) of surface seawater samples at the HOT site. There were pronounced subsurface CFC11 and CFC12 maxima at about 400 m (T9.9 C, S34.1). Subsurface CFC maxima were present over a significant portion of the North Pacific during the 1980 s and 1990 s [Warner et al., 1996]. These maxima have been attributed to a combination of the temperature structure of the water column (CFCs and SF 6 are more soluble in colder water), the time history of CFCs, and the characteristic time scales for ventilation along isopycnals in this region. These maxima have been predicted to evolve and SF 6 were collected at HOT in Oct Seawater samples were collected in 10 liter PVC sample bottles, and aliquots for CFCs and SF 6 analysis were immediately transferred into 200 cc glass syringes to minimize contact with air. About 150 cc of water from a syringe was transferred to a purge and trap system and analyzed for CFC11, CFC12 and SF 6 on board ship based on methods described by Vollmer and Weiss [2002]. [7] The concentrations of dissolved CFC11 and CFC12 expected in warm (26.3 C) surface seawater at equilibrium [Warner and Weiss, 1985] with the atmosphere at HOT in Oct are about 1.5 pmol kg 1 (1 pmol = moles) and 3.0 pmol kg 1, respectively. Atmospheric levels of SF 6 in 2005 were about 100 times lower than the CFCs, and because SF 6 is more than an order of magnitude less Figure 2. Potential temperature and concentrations (1 pmol = 1 picomole = mole) of dissolved CFC11, CFC12 and SF 6 versus depth at the HOT station in Oct SF 6 concentrations have been multiplied by CFC concentrations are on the SIO98 calibration scale; SF 6 concentrations are on the GMD 2000 scale. Concentrations at depths greater than 1000 m were at or below the detection limit. Data are available at hawaii.edu/ HOT_WOCE/ftp.html. 2of5

3 and deepen with time [Mecking and Warner, 2001] as the atmospheric CFC growth rates slow. The 2005 CFC maxima at HOT are deeper than observed in the 1990 s in this region and were located at a density ( = ) near the base of the ventilated thermocline. The subsurface SF 6 maximum is weaker than those of CFC11 and CFC12. The absence of a strong SF 6 maximum at HOT in 2005 is likely due in large part to the year lag in atmospheric growth of SF 6 versus CFC12 and CFC11 (Figure 1a), and the relatively long time scales (10 20 years; see Figure 3) in this region for ventilation of the isopycnal range where the CFC maxima are currently centered. All three tracers exhibit a rapid drop-off to near-blank values between meters, corresponding to the density range = , the deepest density surfaces that outcrop in the open North Pacific Ocean [Talley, 1985]. Since dissolved tracer concentrations in the upper 200 m were within a few percent of equilibrium with the atmosphere at the time of sampling, psf6 apparent ages (psf 6 ages) are close to 0 in this depth range (Figure 3) The near-constant atmospheric CFC12 levels from (Figure 1a) and analytical precision limits for dissolved CFC12 means that CFC12 apparent ages (pcfc12ages) in this depth range can only be constrained as less than 5 years. From about m, ages increase from roughly 10 to 30 years, with psf 6 ages < pcfc12ages at all depths. [10] Because of the slowdown in atmospheric growth rates of CFC12 during the past several decades, mixing of CFC12 in subsurface waters ventilated during this period tend to bias the resulting CFC12 derived ages older than the mean of the individual components (see Figure 2 of Mecking et al. [2006]). Mixing biases tend to be smaller for psf 6 ages during this period when the SF 6 atmospheric growth rates were approximately linear. As a result, pcfc12ages at HOT tend to be older than psf 6 ages due to the effects of mixing on the differing curvatures of their atmospheric growth rates. Below about 700 meters, the psf 6 and pcfc12 ages correspond to the time when the atmospheric levels for the tracers were near zero (Figure 1a). [11] To more quantitatively interpret the relations among the CFC and SF 6 derived ages at this site, a simple isopycnal pipe model was used [Sonnerup, 2001]. Dissolved gases are assumed to be in solubility equilibrium with the atmosphere at the isopycnal outcrop which tracks the atmospheric growth rates of the gases. In addition to CFC11, CFC12 and SF 6, an ideal-age tracer, i.e. a tracer whose age is set to zero at the surface and whose age increases at a rate of one year per year in subsurface waters [Thiele and Sarmiento, 1990], and a tracer with the same atmospheric growth rate as CO 2 were introduced into the model at the outcrop. The model assumes that any ventilation occurs along isopycnals due to a combination of alongisopycnal turbulent diffusion and advection away from the isopycnal @C ¼ þ 2 Here C represents tracer concentration, K is along-isopycnal eddy diffusivity, v is along-isopycnal velocity, t represents time and y is distance from the outcrop. At the deep ð1þ Figure 3. Partial pressure apparent ages (in years) for CFC12 (pcfc12age) and SF 6 (psf 6 age) versus depth at the HOT site. Ages are young in the upper layer, where the compounds are close to equilibrium with the atmosphere at the time of sampling. pcfc12ages approach 60 years where CFC12 concentrations approach = 0; psf 6 ages approach years where SF 6 concentrations approach 0. boundary, no gradient in tracers is assumed. The relative importance of advection (v) and mixing (K) in the model were varied across a wide range (0 1.4 cm s 1 for v, and m 2 s 1 for K) chosen so the model s age relations would span the age relations observed in the HOT data. Ideal ages were run to steady state, while the CO 2 tracer, CFCs and SF 6 were introduced into the model at the start of their anthropogenic increases in the atmosphere. The model was run to [12] The HOT data are used to estimate model advection and mixing ratios that are most representative at this location. Inclusion of SF 6 in the 1-D model places an important additional constraint on inferred ideal ages over the use of CFCs alone. The pcfc12-psf 6 age pair indicates that pcfc12ages 20 years old in 2005 are fairly accurate indicators of ideal ages at HOT (Figure 4a). These waters would have subducted in 1985, during the linear window of the atmospheric CFC12 history ( , Figure 1a). For older samples, whose pcfc12 ages are >35 years old in 2005, the pcfc12age alone can significantly underestimate ideal age (the ratio of ideal age to pcfc12age can be 1.8 or greater; see contours in Figure 4a). For young samples, with pcfc12ages < 10 years in 2005 (corresponding to a time when the atmospheric growth rate of CFCs was near zero), the pcfc12age alone can significantly overestimate ideal age (the ratio of ideal age to pcfc12age can be as low as 0.4). Despite inaccuracies in predicting ideal ages, errors in using pcfc12age to infer anthropogenic CO 2 were relatively small (the pco2 isopleths plot vertically) from , corresponding to ages of years in 2005 (Figure 4b). The similar nearlinear time histories of CO 2 and CFC12 in the atmosphere during this interval lead to accurate back-calculation of anthropogenic CO 2 at this station. However, in waters outcropped since 1985 (pfc12ages< 20 years), the HOT 3of5

4 Figure 4. (a) Differences between pcfc12age and psf 6 age (pcfc12age - psf 6 age) vs. pcfc12age in the isopycnal model (dots) for a wide range of mixing scenarios. Advection-dominated, low mixing (high Peclet number) model scenarios cluster in the lower portion of the plot, while mixing dominated scenarios cluster in the upper portion of the plot. Since pcfc12ages and psf 6 ages are biased by mixing, age differences are small in low mixing regimes (pcfc12age - psf 6 age = 0 in advection only scenarios) and increase with increasing mixing. The contour lines show the ratio of ideal age to pcf12age in the model. The ratio of ideal age to pcfc12age tend to be closest to 1 in the low mixing, advection dominated region of the plot. Overlain are the observed pcfc12age - psf 6 age differences vs. pcf12ages measured at HOT (asterisks). The HOT observational data tend to plot within the model s high mixing, low advection regime. These measurements are used to identify the model s ideal age most likely at HOT. All ages are in years. (b) pcfc12age psf 6 age vs. pcfc12age in the isopycnal model (dots), overlain with the HOT data (asterisks) as in Figure 4a. The contours represent model-derived pco 2 values. For an advection only scenario, pcfc12age psf 6 age = 0 as in Figure 4a, and the pco 2 values along this 0 line are simply the equilibrium atmospheric values for water with the corresponding pcfc12age. In higher mixing scenarios (upper portion of plot) the model s pco2 can differ significantly from that estimated from pcfc12ages alone due to mixing. pcfc12age - psf 6 age data overlain on the model results indicate that the pcfc12age alone likely underestimates the waters true anthropogenic CO 2 content. For example, for a 12 yr old sample (determined from CFC12) collected in 2005, the pcfc12age implies an anthropogenic DIC content (calculated assuming full uptake of an atmospheric CO 2 increase from 280 to 364 ppm in waters of T = 20 C, S = 35) of 54 mol kg 1. Using SF 6 concurrently with CFC12 to constrain the CO 2 exposure timescale indicates that this model sample saw a higher atmospheric CO 2 level of 380 ppm, implying a 20% higher anthropogenic DIC content of 64 mol kg 1. In all cases, the CFC12-SF 6 age pair provided improved constraints on the model s ideal ages and CO 2 content over the CFC11-CFC12 age pair attempted by Sonnerup [2001]. For samples younger than 5 10 years (in 2005), when the pcfc12ages become problematic (Figure 1a), psf 6 ages alone should provide a fairly accurate dating tool for quantifying anthropogenic CO 2 levels in the model due to the linear atmospheric trend since 1990 (Figure 1a). 4. Discussion [13] The rapid increase in SF 6 in the atmosphere makes it potentially a valuable transient tracer for ocean studies. The recent ( ) increases in atmospheric SF 6 parallel the increases of CFC12 and CFC11 which occurred several decades earlier, but which have now slowed. The general pathways of SF 6 entry and transport in the ocean in the upcoming decade should resemble those of CFC12 and CFC11 which occurred about two decades ago, and which have been measured extensively as part of WOCE and other global ocean survey programs. When additional SF 6 data sets become available, SF 6 inventories may prove valuable as a method for estimating water mass formation rates as have CFC inventories [e.g., Orsi et al., 1999; Smethie and Fine, 2001; Rhein et al., 2002] in the past. [14] Although highly idealized, the simple isopycnal model presented here simulates some key features of the CFC and SF 6 tracer ages observed at the HOT site. In particular, in both the model and in the data, pcfc12ages are higher than the psf 6 ages in subsurface waters. In the model, the availability of a second age tracer places an additional constraint on the range of estimates for ideal age and CO 2 content. The model results suggest that the pcfc12ages alone at HOT may significantly miss the mark in estimating anthropogenic CO 2. When combined with other tracers, the SF 6 complement to CFCs will certainly prove useful in constraining transit time distributions in the oceans [Waugh et al., 2003]. [15] We are continuing to develop SF 6 analytical techniques and plan to continue to monitor the changes of CFCs and SF 6 at the HOT site, and to study the evolution of tracer ages in this region. SF 6 measurements may be a valuable addition to large scale ocean hydrographic surveys in the future, including the CLIVAR/CO 2 repeat hydrographic program ( Along with CFCs, SF 6 observations may prove to be useful in testing and evaluating OGCM simulations of ocean circulation and mixing processes and in improving estimates of the global oceanic uptake of anthropogenic CO 2. [16] Acknowledgments. This study was supported by the Global Carbon Cycle Program at NOAA s Climate Program Office. We thank 4of5

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