Dissolved Inorganic Radiocarbon in the North Pacific Ocean And
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ARTICLE IN PRESS Deep-Sea Research I 55 (2008) 451–459 www.elsevier.com/locate/dsri Dissolved inorganic radiocarbon in the North Pacific Ocean and Sargasso Sea Ellen R.M. Druffela,Ã, James E. Bauerb, Sheila Griffina, Steven R. Beaupre´a, Jeomshik Hwanga,1 aDepartment of Earth System Science, University of California, Croul Hall, Irvine, CA 92697-3100, USA bSchool of Marine Sciences, College of William & Mary, Gloucester Point, VA, USA Received 3 July 2007; received in revised form 8 December 2007; accepted 13 December 2007 Available online 5 January 2008 Abstract We present radiocarbon measurements of dissolved inorganic carbon (DIC) in depth profiles from reoccupations of our central North Pacific Ocean and Sargasso Sea (SS) sites. From 1985 to 1999, an increase in D14C values of 6–17% was measured between 1150 and 2400 m depth in the North Central Pacific. Natural changes in deep circulation are likely responsible for variability of D14C signatures in the deep ocean, though we cannot rule out the presence of bomb 14C at this depth range. Bomb 14C had increased in the deep SS from 1989 to 2000; this is the result of southward transport of North Atlantic Deep Water (NADW) to this site. r 2007 Elsevier Ltd. All rights reserved. Keywords: Dissolved inorganic carbon; Radiocarbon; Bomb radiocarbon; Deep circulation 1. Introduction seawater revealed several hundred-year transit times of deep water masses in the world’s oceans (Bien Oceanic dissolved inorganic carbon (DIC) is the et al., 1963; Broecker et al., 1960; Stuiver et al., largest pool of exchangeable carbon on Earth 1983). Global data sets amassed during programs (36,000 GtC). It is important to know the timescales such as the Geochemical Ocean Sections Study of mixing and ventilation of deep ocean waters (GEOSECS) and the World Ocean Circulation because most of the excess CO2 produced by fossil Experiment (WOCE) have been used for quantita- fuel and biomass burning will eventually be stored tive studies of the oceanic carbon cycle (Key, 1996; in the oceans. Early radiocarbon measurements in Stuiver et al., 1983). Since the production of bomb 14C in the late ÃCorresponding author. 1950s and early 1960s, it has been possible to E-mail addresses: [email protected] (E.R.M. Druffel), measure short-term exchange of carbon, e.g. the [email protected] (J.E. Bauer), sgriffi[email protected] (S. Griffin), transfer of CO2 across the air–sea interface. The [email protected] (S.R. Beaupre´), [email protected] level of bomb 14C in a given carbon pool is a (J. Hwang). 1Present address: Department of Marine Chemistry and reflection of the turnover time of the carbon with Geochemistry, Woods Hole Oceanographic Institution, Woods respect to exchange with the atmosphere. By the 14 Hole, MA, USA. 1970s, bomb C was detected to depths of several 0967-0637/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2007.12.007 ARTICLE IN PRESS 452 E.R.M. Druffel et al. / Deep-Sea Research I 55 (2008) 451–459 hundred meters in the non-polar oceans (Ostlund converted to graphite using hydrogen gas and and Stuiver, 1980; Stuiver and Ostlund, 1980). cobalt metal at 550 1C(Vogel et al., 1987). The deep northern North Atlantic contained bomb The D14C measurements from the Avon 1999 14C throughout the entire water column, introduced cruise were made at the National Ocean Sciences during North Atlantic Deep Water (NADW) Accelerator Mass Spectrometry Facility (NOSAMS) formation. By 1991, penetration of bomb 14C of the Woods Hole Oceanographic Institution extended to 1000 m depth in the Pacific (Key, (WHOI), and those from the SarC 2000 cruise were 1997; Stuiver et al., 1996). made at the W.M. Keck Carbon Cycle AMS The D14C data reported here show an increase in Laboratory at the University of California, Irvine values in the deep waters of both the North-central (UCI). The DIC D14C values were reported according Pacific (NCP) and the Sargasso Sea (SS) (Druffel to standard techniques (Stuiver and Polach, 1977) et al., 1992). Change in circulation is the likely and have a total uncertainty determined from mechanism responsible for the higher D14C values in replicate seawater analyses of73.9%.Thed13C the upper part of the deep NCP. Remineralization measurements were performed at NOSAMS with a of bomb-laden particulate organic matter from the total uncertainty of 70.1%. Alkalinity and [DIC] surface is unlikely to have caused a measurable measurements were obtained by closed vessel titration increase in the D14C values of DIC in the deep of large volume (100 ml) samples using an auto- North Pacific or North Atlantic. mated titration system (Bradshaw et al., 1981; Brewer et al., 1986) in the laboratory of D. McCorkle (WHOI). Alkalinity and [DIC] measurements were 2. Methods determined using a nonlinear curve fitting approach (DOE, 1994) and standardized using certified refer- Water samples were collected from the NCP site ence materials obtained from Andrew Dickson (311N, 1591W, bottom depth 5820 m) on the Avon (Scripps Institution of Oceanography). The standard cruise from 26 May to 12 June 1999 and from the deviation of pairs of replicate analyses of culture SS site (311500N, 631300W, bottom depth at 4500 m) water was 4 meq/kg for alkalinity and 6 mmol/kg for on the SarC cruise from 14 to 29 June 2000. [DIC]. The alkalinity results were consistently high by The NCP site is 1000 km north of Hawaii and the SS 25–40 meq/kg compared with other cruises, which we site is 100 km southeast of Bermuda. Radiocarbon attribute to the unusually long storage times of the and d13C measurements of suspended particulate samples prior to analysis (49mos);forthisreason, organic carbon collected during these cruises were the alkalinity results were not reported. reported previously (Druffel et al., 2003), and radiocarbon and abundance data of dissolved 3. Results organic carbon will be reported separately (Bauer et al., in preparation; Loh et al., 2004). The NCP The DIC D14C values of samples collected from and SS sites were occupied earlier, from 6 June to 3 the 1999 NCP site ranged from 95% (at 3 m depth) July 1987 (Eve cruise) and from 29 May to 20 June to À243% (at 3221 m) (Fig. 1a). Values remained 1989 (Hydros-6 cruise), respectively, and isotope low (average À239.072.9%, n ¼ 4) between 2400 results were reported separately (Druffel et al., and 3600 m, and increased to an average of À2157 1992). 6% (n ¼ 5) between 4200 and 5770 m (50 m above Seawater samples were collected in 1999 and 2000 bottom). Values of [DIC] were low in the upper using 12-L or 30-L Go-flo bottles deployed on 100 m (205076 mmol/kg, n ¼ 4), reached a max- a hydrowire. Samples for DIC D14C and d13C imum at 1300 m (2382 mmol/kg) and decreased analyses were filtered through glass fiber filters slightly in deep waters (Table 1). (1 mm effective pore size) directly into 1-L glass The DIC D14C values obtained for samples from containers and poisoned with a saturated solution theSSsitein2000(Fig. 2) decreased from 81% at 3 m of mercuric chloride. Separate samples were filtered to a low of À72% at 1005 m (Antarctic Intermediate and poisoned in the same manner for alkalinity and Water (AAIW)); values increased to a secondary 14 total CO2 ([DIC]) analyses. For the DIC D C and maximum (À32%) at 1513 m, and decreased to d13C analyses, the samples were acidified and À84% at 50 m above bottom. The [DIC] values were sparged of CO2 gas according to published techni- low in the upper 50 m (2099–2107 mmol/kg), highest ques (McNichol et al., 1994). Carbon dioxide was in AAIW (2203–2221 mmol/kg between 850 and ARTICLE IN PRESS E.R.M. Druffel et al. / Deep-Sea Research I 55 (2008) 451–459 453 Δ14C Δ14C -300 -200 -100 0 100 200 -260 -240 -220 -200 -180 0 1000 1000 1500 2000 3000 2000 Depth m Depth m 4000 2500 1999 AVON 5000 1987 Eve 1991 WOCE 31 1973 Geosecs 204 6000 3000 Fig. 1. D14C values (%) of DIC from the NCP site (311N, 1591W) during Avon (June 1999) and Eve (June 1987) cruises (a) for 0–6000 m and (b) for 1000–3000 m. Also shown are the D14C profiles obtained previously during GEOSECS (September 1973, Stn 204, 311N, 1501W) (Ostlund and Stuiver, 1980) and WOCE (March 1991, Stn 31, 301N, 1521W) (Key, 1996; Stuiver et al., 1996). 1300 m) and slightly lower in NADW (2182– nificantly higher in 1999 than those in 1987 (Fig. 1a, 2201 mmol/kg at 1500–3600 m) (Table 2). b). The average D14C difference at depths between The DIC d13C values in the NCP were highest in 600 and 900 m (38721%, n ¼ 3) was greater than surface waters (1.08% at 3 m), decreased to a that between 1150 and 2400 m (871%, n ¼ 4). minimum of À0.84% at 1141 m, and increased with Comparison of our DIC D14C data with profiles depth to 5770 m (0.22%)(Fig. 3a). At the SS site, obtained during GEOSECS in September 1973 (Stn the DIC d13C values were highest in surface water 204, 311N, 1501W) (Ostlund and Stuiver, 1980) and (1.09% at 20 m), decreased to a minimum (À0.21%) WOCE in March 1991 (Stn 31, 301N, 1521W) (Key, at 1005 m, and increased to 0.7–0.9% in deep water 1996; Stuiver et al., 1996) is also plotted with our (Fig. 3b). data in Fig. 1a, b. These sites are located 860 and 680 km east of our NCP site, respectively, and some 4.